santos et al 2000 gondwana research
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Amazon Craton; South America;TRANSCRIPT
Gondwana Research, V: 3, No. 4, p p . 453-488. 0 2000 International Association for Gondwana Research, Japan. ISSN: 1342-937X
A New Understanding of the Provinces of the Amazon Craton Based on Integration of Field Mapping and U-Pb and Sm-Nd Geochronology To50 Orestes Schneider Santosl, L6o Afraneo Hartmann2, Henri Eugene Gaudette3, David Ian Groves4, Neal Jesse Mcnaughton5 and Ian Robert Fletcher6
lnstituto de GeociCncias, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves no 9.500, Porto Alegre, Rio Grande do Sul, 91500-000, Brazil, E-mail: [email protected] Companhia de Pesquisa de Recursos Minerais, Rua Banco da Provincia, 105, Porto Alegre - Rio Grande do Sul, 90840-030, Brazil, E-mail: [email protected] lnstituto de GeociCncias, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves no 9.500, Porto Alegre, Rio Grande do Sul, 91500-000, Brazil, E-mail: [email protected] Department of Earth Sciences, University of New Hampshire, Durham, New Hampshire, 03824, USA, E-mail: [email protected]. Centre for Strategic Mineral Deposits- C S M D -Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia, 6907, Australia, E-mail: [email protected] Centre for Strategic Mineral Deposits- CSMD -Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia, 6907, Australia, E mail: [email protected] Centre for Strategic Mineral Deposits- CSMD -Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia, 6907, Australia, E-mail: [email protected]
(Manuscript received September 24,1999; accepted March 26,2000)
Abstract
New conventional and sensitive high-resolution ion microprobe zircon U-Pb dating has led to a new understanding of the subdivision and evolution of the Amazon Craton during Precambrian time, with major improvements and changes made to the previous Rb-Sr based model. The interpretation of U-Pb and Sm-Nd isotopic data identifies eight main Precambrian tectonic provinces in the Craton, with ages ranging from 3.1 to 0.99 Ga. Some of the provinces were generated by accretional, arc-related processes (Carajas, Transamazonic, Tapaj6s-Parima and RondBnia-Juruena) and others by recycling of continental crust (Central Amazon, Rio Negro and Sunsas). The exposed Archean crust is restricted to the east (Carajas and south Amapa in Brazil) and north (Imataca in Venezuela) of the craton, indicating that the Amazon Craton is largely a Proterozoic crust. The Carajas-Imataca (3.10-2.53 Ga) and Transamazonian (2.25-2.00 Ga) Provinces are composed predominantly of granite-greenstone terranes. The Tapaj6s-Parima (2.10-1.87 Ga) and RondBnia- Juruena (1.75-1 47 Ga) Provinces represent new crust added as orogenic belts, while the Rio Negro (1.86-1.52 Ga) and Sunsas (1.33-0.99 Ga) Provinces originated mainly by magmatic-tectonic recycling of the above two orogenic belts. The only zone with a prominent northeast trend is the poorly known K'Mudku Shear Belt, characterized by a -1.20 Ga shear zone which deforms the rocks of at least three different provinces (Rio Negro, Tapaj6S-Parima and Transamazonic). The Central Amazon Province comprises mostly Orosirian volcano-plutonic rocks (Uatum5 Magmatism) and is a terrane in which the exposed crustal structure and deformation are pluton-related. The Sm-Nd T,, model ages and E~~ suggest that the Central Amazon Province was generated by the partial melting of Archean continental crust (Carajls Province?), perhaps related to underplating that began at the end of the Tapajos-Parima Orogeny (1.88-1.86 Ga).
Key words: Amazon Craton, South America, crustal growth, Transamazonic, U-Pb geochronology.
Introduction the main tectonic units in South America (around 4,500,000 h2), is covered by Phanerozoic basins in the northeast (Maranhgo), central (Amazon), south (Xingu- Alto Tapajbs), southwest (Parecis) and west (SolimGes)
The Amazon Craton is one of the largest and least- known Archean-Proterozoic areas in the world. One of
454 J.O.S. SANTOS ET AL.
areas. It is limited to the west by the Andes Orogenic Belt, and to the east and southeast by the Neoproterozoic Araguaia Fold Belt. There are two main models [or the subdivision of the Craton into tectonic-geochronological provinces. One model describes the Craton as a mosaic of twelve main blocks or paleo-plates, Archean (or Paleoproterozoic) in age, which have granite-greenstone characteristics (Costa and Hasui, 1997). In this model, the blocks' margins are marked by nineteen Archean- Paleoproterozoic collisional or shear belts which were reactivated many times, including in the Phanerozoic. This model based on geophysical data (the gravimetric map of South America and the magnetic map of Brazil), does not effectively utilize the available geochronologic data, and considers only collisional processes, continent against continent, during the craton evolution. The isotopic data show that many of those twelve blocks are much younger than Archean-Paleoproterozoic (Rio Negro and Rondbnia- Juruena blocks, for example), and that the collisional process is dominant only in the Sunsas Province and in the K'Mudku shear belt. Costa and Hasui (1997, Fig. 1, p. 23) indicate several areas with granulite rocks, located in the shear belts, which delimit the twelve main blocks. These granulite facies rocks, if present, would be important in identifying high-grade terrains related to the collisional processes. However, the systematic mapping undertaken in the 1990's by the Brazilian Geological Survey in those areas, collecting more than 14,000 rock samples (Alto Rio Negro, Imeri, Tapajos, Nova Brasilhdia, Roraima Central and Caracarai Projects) resulted in either:
i) Lack of evidence for granulite facies rocks - Rio Negro and Tapaj6s-Parima Provinces or
ii) Evidence that orthopyroxene bearing rocks are mainly igneous, post-tectonic charnockites - RondBnia- Juruena (Jaru area) and North Tapaj6s-Parima (Caracarai area) Provinces.
The other model (Tassinari et al., 1996; Tassinari, 1996; Tassinari and Macambira, 1999) is based on the Cordani et al. (1979) proposal, and is a more mobilistic approach, strongly based on geochronologic data. In this model, the
Craton is divided into six main provinces, including four "mobile belts" accreted to an older nucleus (Central Amazon Province): i) AmazBnia Central - older than 2.30 Ga; ii) Maroni-Itacaiunas - 2.20 - 1.90 Ga; iii) Tapaj6s- Ventuari - 1.90-1.80 Ga; iv) Rio Negro-Juruena - 1.80 - 1.55 Ga; v) RondBnia-San Igndcio - 1.50 - 1.30 Ga; and vi) Surisds - 1.25-1.00 Ga.
The Tassinari et al. (1996) model is based mainly on Rb-Sr isotope data. The limitations of the Rb-Sr method are well known (Dickin, 1995, p. 51-53), being enhanced in polydeformed, polymetamorphosed or high metamorphic-grade areas, where the system could be easily reset. All of the above provinces must be reinterpreted using more robust U-Pb and Sm-Nd data. The new U-Pb and Sm-Nd results, together with new available geological maps from the Brazilian Geological Survey, indicate that significant changes to the previous Rb-Sr-based model must be made, as shown in Table 1.
Samples and Analytical Techniques
The isotopic data presented in this paper correspond to samples from four main provinces: Sunsds (one sample), Rio Negro (5), RondBnia-Juruena (6) and Tapajos-Parima (14). There are three Rb-Sr (Rio Negro and RondBnia), seven Sm-Nd (Tapaj6s and Rio Negro), 14 conventional U-Pb and three U-Pb SHRIMP results. Table 2 shows the analysed samples, methods, locations and rock types. The Sm-Nd data are integrated with previously published Sm-Nd data in Table 3, where the main source was Sat0 and Tassinari (1997). The Rb-Sr and U-Pb data are listed in Tables 4 (Rb-Sr), 5 (U-Pb), 6 (U-Pb) and 7 (U-Pb-SHRIMP).
In addition, the following U-Pb and Sm-Nd isotopic data from other areas., were integrated: i) Imataca: Sidder and Mendoza (1995); ii) Carajds: U-Pb and Pb-Pb results and references are listed in Table 8; iii) Amapa: McReath and Faraco (1997), Lafon et al. (1998); iv) Roraima: Santos and Olszewski (1988); Gaudette et al. (1996); v) South Venezuela: Gaudette and Olszewski (1985);
Table 1. Major tectonic, structural and isotopic characteristics of the Provinces of the Amazon Craton.
Province Main Tectonic Trend Dominant Process T,, (Gal# U-Pb ages#
1.33 - 0.99 1.93-1.52 Sunsas N 40" W collisional -4.46 - +6.25 RondBnia-Juruena N 70" W / E-W juvenile - 1.65 / + 3.81 2.18 -1.68 1.76 - 1.47
N 40" W collisional - 5.00 / + 2.98 2.42-1.88 1.86 - 1.52 1.88 - 1.70 Central Amazon NNW underplating - 14.39 / +1.52
Tapajos-Parima N 30" W juvenile - 2.38 / + 3.51 2.26-2.06 2.10 - 1.87
Rio Negro N-S
2.85-2.41
2.32-2.07 Transamazonic N 50°-700 W juvenile + 0.20 / + 3.83 3.10*-3.06" 2.25 - 2.00 Carajks 70" W juvenile - 1.25 / + 5.30 3.10 - 2.51 3.10 - 2.53
# - Detail on Table 3 ; n.a.= not available.; (") Cupixi Domain.
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U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 455
Table 2. Location of samples discussed in the text.
Sample 11 Rock type Regional Unit Location - State" Province
GR-66 PT-1A
PT- 14 PT- 1 5
WO-63 WO-52
PT- 12
WO-74
MQ-96 AF-01 JO-IMA HC-477 HC-492 HC-508 HC-512 MS-63 50-51 50-52 50-54 50-57 50-66 50-68 50-69 JO-99 50-102 50-174 AL-9b 50-3 50-174 MM-36
24s l r 42-lr l r 2r 42 42 42 16s 42 32 l r 5m-lr l r l r 11s 32 52 42 52 I n 42 I n 42 42 I n I n I n I n I n
Paragneiss (quartzite) Charnockite Metapelite Charnockite Charnockite Metarhyolite Protomylonite Charnockite Dacite Biotite titanite granite Quartz sandstone Gabbro Gabbro Gabbro Gabbro Laterite Tonalite Quartz schist Monzogranite Siltstone Metandesite Quartz wacke Gabbro Monzogranite Tonalite Monzogranite Sandstone Alaskite Monzogranite Rhyodacite
Nova Brasillndia Met. Suite Jaru Charnockites RondBnia Basement Jaru Charnockites Jaru Charnockites Beneficente Group Serra da Provid@ncia Intr. Suite Jaru Charnockites Roosevelt Group UaupCs Intrusive Suite Ima Formation - Tunui Group Tapuruquara Complex Tapuruquara Complex Tapuruquara Complex Tapuruquara Complex Cauaburi Complex Cuiu-Cuiu Complex Jacareacanga Group Late-Parauari Intrusive Suite Abacaxis Formation Undetermined Sequeiro Formation Ingarana Gabbro Maloquinha Intrusive Suite Parauari Intrusive Suite Maloquinha Intrusive Suite Palmares Group Maloquinha Int. Suite Maloquinha Intrusive Suite Iriri Group
Nova Brasillndia - RO BR364 Highway - RO BR364 Highway - RO BR364 Highway - RO BR364 Highway - RO BR364 Highway - RO BR364 Highway - RO BR364 Highway - RO Roosevelt River - MT SBo Gabriel - AM Caparro Mountain -AM Santa Isabel -AM Santa Isabel -AM Santa Isabel - AM Santa Isabel -AM SBo Gabriel - AM ConceiqBo Mine - PA Sai-Cinza Village - PA Rosa de Maio Mine - AM Abacaxis Mine - AM Mamoal Mine - PA Sequeiro Mine - AM David Mine - PA Santa Rita Mine - PA Tropas River - PA Jamaxim River - PA BR-230 Highway - PA Jardim Our0 Mine - PA Jamaxim River - PA Amana River - AM
Sunsas
RondBnia-Juruena
Rio Negro
Tapaj6s-Parima
Central Amazon
(n) = number of analysis; z= U-Pb conventional; s=U-Pb SHRIMP; r=whole-rock Rb-Sr; m=mineral Rb-Sr and n=whole-rock Sm-Nd "Brazil States = Amazonas (AM), Par5 (PA), RondBnia (RO) and Mato Grosso (MT).
Goldstein et al. (1997); Tassinari et al. (1996); vi) French Guyana: Vanderhaeghe et al. (1998); Milesi et al. (1995); vii) Guyana: Gibbs and Olszewski (1982), Norcross et al. (1998); viii) RondBnia: Bettencourt et al. (1999); Payolla et al. (1998); Tassinari et al. (1996); Rizzotto et al. (1999); ix) Rio Negro: Almeida et al. (1997), Dall'Agnol and Macambira (1992); Tassinari et al. (1996); and x) Mato Grosso: Van Schmus et al. (1997); Van Schmus et al. (1998); Geraldes et al. (1999).
The conventional analyses of U-Pb isotopes in zircons were carried out in two different laboratories. The first group of analyses (Table 5) was processed at the University of New Hampshire mass spectrometer laboratory. This group includes the samples from the Rio Negro (AF-01 and IMA) and Rond8nia-Juruena Provinces (PT-12, WO- 74, WO-52, WO-63). The heavy minerals were separated from weathered rock by in situ panning, using the techniques described by Gibbs and Olszewski (1982). Zircons were separated from the heavy mineral concentrates in the laboratory, using heavy liquids and magnetic separation. The zircon concentrates were then split into magnetic and size fractions. Zircons were spiked
and dissolved, and U and Pb were separated using the techniques of Krogh (1973). Lead and U isotopes were analysed on the 23 cm 60" solid-source mass spectrometer at the University of New Hampshire. Errors for the z07Pb/ 235U and z06Pb/23sU ratios are both 1.0%, with a correlation of 0.90. Fractionation of Pb analyses was monitored by repeated analyses of NITS standard 983, yielding an average fractionation of 0.10 If: 0.03% per a.m.u.
The second group of U-Pb analyses (Table 6) corresponds to the Tapaj6s-Parima Province samples (JO- 51, 50-54, 50-57, 50-59, 50-68, 50-102 and JO-199). The zircons were analysed at the Massachusetts Institute of Technology, using standard procedures (Bowring et al., 1993) for dissolution, Pb and U separation, and isotopic analysis. All zircon fractions were air abraded (Krogh, 1982). Sample weights were estimated using a video monitor with a grided screen and are known to be within 40%. Common Pb corrections were calculated using the model of Stacey and Kramers (1975) and interpreted the age of the sample. For analyses with less than 2.0 pg common Pb, the total common Pb was assumed to be blank. The U blank was 0.5 + 0.05 pg. A mass
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456 J.O.S. SANTOS ET AL.
Table 3. Sm-Nd data from the Amazon Craton, listed from the younger to the older Provinces, as defined in this study.
Sample Rock Location Sm (ppm) Nd (ppm) 147Sm/'44Nd i43Nd/'44Ndi 'Nd(i7 'I Age"' TD,'V Ref"
MESOPROTEROZOIC COLLISIONAL SUNSAS PROVINCE PT- 19 diorite RO-Vilhena 0.498 1.292 0.232 PT-2A basalt RO-Vilhena 3.936 14.665 0.1623 PT-4.1 amphibolite RO-Guapore 6.295 21.314 0.1786 PT-3 amphibolite RO-Vilhena 1.786 4.65 0.23226 GR-5 gabro RO 3.21 9.7 0.19979 GR-lob amphibolite RO 5.61 18.65 0.18172 GR-lOa monzogranite RO 13.28 63.41 0.11790 GR-18 gabro RO 3.11 10.59 0.17733 GR-20 granite RO 9.97 51.20 0.11769 GR-20a paragneiss RO 6.54 34.13 0.11587 GR-a1 paragneiss RO 4.09 21.03 0.11748 GR-20c paragneiss RO 5.80 29.33 0.11950 GR-23 granite RO 17.52 89.84 0.11790 MESOPROTEROZOIC ACCRETIONAL RONDONIA-JURUENA PROVINCE PT-51 dacite AM-Roosevelt 6.305 39.099 PT-14f-2 charnockite AM-Siriquiqui 28.377 185.28 PT-14F-3 trondhjemite AM-Siriquiqui 27.665 135.31 PT-15f charnockite RO-Abuni - PT-143 granulite RO-AbunB - PT-537/10 charnockite MT-Aguapei 4.074 28.58 PT-39" granite RO-Providencia 11.845 69.098 PT-21-1 granite RO-Madeirinha 12.523 62.941 PT-61 granulite RO-Machadinh 8.893 41.327 PT-72 granulite RO-Jaru 7.243 32.537 PT-74 basalt RO-Candeias 7.53 33.982 537/79 granite MT- Jauru 5.853 36.314 PO05 tonalite MT-Jauru 5.110 25.858 MESOPROTEROZOIC RIO NEGRO GRANITIC PROVINCE MRRL-31 gneiss AM-Rio Negro 2.953 PT33ASU monzogranite AM-Uaupes 14.973 PT33ASW monzogranite AM-Uaupes 17.167 UA6W monzogranite AM-Uaupks 14.688 5532 tonalite VE 11.397 PASP35 tonalite AM-TiquiC 2.087 MIAB16 granodiorite AM-Rio Negro 2.675 MT1036 gneiss VE 14.133 BA28 gneiss VE 3.850 H508A gneiss VE 7.611 PALEOPROTEROZOIC CENTRAL AMAZON PROVINCE 50-174 granite PA-Jamaxim 6.009 MM-36 rhyodacite 11.40 50-3 granite PA-BR-165 1.08 PL-5 charnockite RR-BR-230 - PL-18 charnockite RR-BR-230 - RB-24 charnockite RR-Caracarai 7.63 MP-39 charnockite RR-Caracarai 7.59 PL-24 charnockite RR-Caracarai 6.15 CR-16 granite PA-Crepori 10.411 CR-09 granite PA-Crepori 8.366 PT-29-3 rhyolite PA-Curud 6.305 AL-90 rhyodacite PA-Tapaj6s 6.903 AP-133c granite PA-Tapaj6s 19.670 XMV-78L rhyodacite PA-Iriri 6.705 XMV-78M rhyodacite PA-Iriri 8.651 470pbk2 granite PA-Inajd 1.155 424bc granite PA-Xingu 5.834
27.898 86.604 93.060 81.796 69.104 9.526 21.485 85.912 23.698 44.857
36.035 58.93 5.84
38.16 38.79 30.60 67.359 56.761 39.099 40.605 113.33 39.935 50.908 7.616 34.05
0.11 1464 0.092614 0,123635 0.100000 0.1700 0.086198 0.103660 0.120314 0.130123 0.134611 0.13399 0.097464 0.119499
0.06400 7 0.104547 0.111550 0.108585 0.099730 0.132480 0.075288 0.099477 0.098240 0.102601
0.1008 0.11695 0.11139 0.15 0.10 0.1209 0.1183 0.1216 0.093462 0.08968 0.097512 0.102801 0.105098 0.101528 0.102759 0.09171 0.10361
PALEOPROT~ROZOIC OROGENIC BELT - TAPNOS-PARIMA PROVINCE MA-5 tonalite PA-Crepori 9.020 45.121 0.120884 MA-13 tonalite PA-Crepori 9.365 46.355 0.122166 PT-25 granite AM-Uatumi 6.289 31.873 0.119316
0.513187 0.512639 0.512832 0.513209 0.512881 0.512881 0.512112 0.512754 0.511992 0.511856 0.511873 0.5 11 860 0.512097
0.511902 0.511715 0.511815 0.511763 0.512342 0.511547 0.5 1 1772 0.511898 0.5 1 185 7 0.511964 0.512274 0.511593 0.511841
0.511026 0.5 11 624 0.511671 0.51 165 1 0.511415 0.511771 0.511 123 0.511610 0.511575 0.511656
0.51 1135 0.511588 0.511443 0.512053 0.511541 0.51114 0.51143 0.51112 0.511281 0.5 1080 1 0.511192 0.511165 0.511236 0.5 11 121 0.511115 0.51113 0.51 11 18
0.51 1773 0.5 1 1749 0.5 11 758
+5.60 +0.48 +6.25 +5.35 +4.41 +2.75 -0.75 +4.66 -1.82 -4.01 -3.91 -4.46 -0.86
+3.81 t 3.02 +0.01 +2.44 -0.12 +1.05 +1.86 +0.90
+0.52 +1.40
+1.65
+ 1.52 -1.22 -1.74 -1.52 -1.05 -2.43 -5.00
-0.63
-1.65
+2.83 i-2.44 +2.98
-6.67 -1.37 -2.84 +1.52 -6.65 -11.85 -5.55 -12.40 -2.04 -1 1.69 -4.76 -6.57 -5.73 -7.12 -7.54 -4.58 -14.39
+ 1.83 +1.08 +1.37
1,050 1,059 180 1,244
1,200 1,235 1,200 1,242
1,846 1,09817 1,649 1,110 1,353
1,120 1,865 1,120 1,870 1,120 1,931
99517 1,523
1,11017
1,100'7 1,685
1,690° 1,724 1,570° 1,684 1,7556 2,098 1,570° 1,729 1.660° 2,516 1.570° 1,800 1.5806 1,774 1.5806 1,886 1.7556 2,186 1.7556 2,095
1,47014 1,914 1,97014 1,962
1,78015 1,880 1,520° 1,996 1,520° 2,063 1,520° 2,034 1,86015 2,196 1,8306 2,416 1,520° 2,124 1,86015 1,925 1,86015 1,951 1,86015 1,916
1,1007 1,493
1,870' 2,597 1,870 2,454 1,870 2,535 1,827'- 2,409 1,827'- 2,552 1,827'- 3,178 1,827'- 2,605 1,827'- 3,239 1,8701 2,250 1,870' 2,850 1,870' 2,446 1,870' 2,603 1,870' 2,558 1,870' 2,634 1,870' 2,673 1,870' 2,406 1,8803 3,188
1.9604 2,104 1.960, 2,175 1,9205 2,093
PT-26 granodiorite AM-Uatumi 4.441 38.277 0.070159 0.511081 -0.20 1,9205 2,114
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4 4 4 4 16 16 16 16 16 16 16 16 16
4 4 4 18 18 4 4 4 4 4 4 4 4
4 4 4 4 4 4 4 4 4 4
0 0 0 8 8 2 2 2 4 4 4 4 4 4 4 4 4
4 4 4 4
U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 457
Table 3. Contd.
Sample Rock Location Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd' 'Nd(T1 I' Age"' T D , 1 V Ref
PT- 19 gneiss MT-Aripuanl PT- 7 gneiss MT-Aripuanl 50-69 gabbro PA-David 50-66 metandesite PA-Mamoal AL9b subarkose AM-Amana M68 granodiorite RR-Rio Branco PT13-2 rhyolite MT-Peixoto Az. PALEOPROTEROZOIC GREENSTONE BELTS -
5.745 30.717 0.113097 4.767 24.521 0.117557 2.257 12.608 0.1083 22.586 164.341 0.0831 5.30 29.87 0.10731 6.954 37.569 0.111662 9.189 48.415 0.114770
. TRANSAMAZONIAN PROVINCE 310 qz-monzodiorite GU-Omai 5.9 349b qz-monzodiorite GU-Omai 4.7 351 qz-monzodiorite GU-Omai 6.0 AD3
D10 H 1 419 B10 B4 c1 349 359 S166 S183 TlOO M l l l T160 T188 L200 L365 8204 EG83 EG18 EGO2
QD2 granodiorite diorite diorite hornblendite basalt basalt basalt basalt andesite andesite amphibolite metatuff komatiite komatiite komatiite komatiite komatiite andesite granulite granite tonalite tonalite
GU-Omai 5.3 GU-Omai 7.0 GU-Omai 5.5 GU-Omai 3.3 GU-Omai 3.0 GU-Omai 2.0 GU-Omai 4.1 GU-Omai 2.2 GU-Omai 1.7 GU-Omai 1.6 AP-Vila Nova 1.94 AP-Vila Nova 3.38 FG-Paramaca 1.656 FG-Paramaca 1.878 FG-Paramaca 1.81 1 FG-Paramaca 2.539 FG-Paramaca 4.69 FG-Paramaca 4.16 GU-Kanuku 10.75 1 AP-Cupixi 0.464 AP-Cupixi 7.143 AP-Cupixi 7.180
ARCHEAN GREENSTONE BELTS - CARAJAS PROVINCE AS1683 granodiorite PA-Mogno 3.086 AH513 trondhjemite PA-Mogno 1.994 AH796-4 granodiorite PA-Rio Maria 3.610 AH796-3 granodiorite PA-Rio Maria 3.177 PPO5A gneiss PA-Carajas 5.553 PP05H tonalite PA-Carajis 1.175 PT-09A granite PA-Inajd 9.270 GB102 meta-rhyolite PA-CarajL 2.70 GB104 meta-rhyolite PA-CarajL 2.90 GB67 metabasalt PA-CarajBs 2.70 GB82A metabasalt PA-CarajBs 3.55 GB82B metabasalt PA-CarajBs 3.55 GB85 metabasalt PA-CarajBs 13.61 GB86 metabasalt PA-CarajBs 2.76 GB87 metabasalt PA-Carajiis 2.94 GB93 metabasalt PA-Carajls 3.03 GB94 metabasalt PA-Carajds 3.25
8100 granulite VE-Imataca 2.479 ARCHEAN GRANULITES - IMATACA PROVINCE
33.7 27.0 33.9 31.6 34.5 26.2 13.9 11.9 6.3 14.8 6.8 7.8 7.3 6.06 12.92 6.54 7.45 6.84 10.54 27.65 20.04 65.663 2.054 52.174 49.680
23.81 13.696 22.63 17.85 50.242 7.919 62.03 11.86 12.99 11.97 12.23 11.82 75.73 12.34 12.84 13.58 15.82
15.862 - 7494 2 ran u 1 it e VE-Imataca 3.817 33.225
0.1048 0.1054 0.1060 0.1014 0.1224 0.1270 0.1426 0.1532 0.1965 0.1675 0.1960 0.1335 0.1342 0.194213 0.158979 0.1536 0.1534 0.1611 0.1467 0.1033 0.1264 0.0990 0.136602 0.082788 0.087394
0.078375 0.088038 0.096463 0.107626 0.066835 0.089724 0.090370 0.13776 0.13700 0.13642 0.17551 0.18134 0.10866 0.13524 0.13869 0.13507 0.12437
0.0941 0.0695
0.5 11602 0.511757 0.5 11506 0.511114 0.51 1665 0.511636 0.51 1688
0.511394 0.511490 0.511499 0.511432 0.511675 0.511741 0.5 12002 0.512233 0.5 12848 0.512309 0.512733 0.511895 0.511876 0.512701 0.512205 0.512182 0.512162 0.512302 0.512070 0.5 1 1440 0.511722 0.511400 0.511921 0.5 10443 0.510508
0.510393 0.5 108 10 0.510862 0.510956 0.510149 0.510641 0.510938 0.511789 0.511483 0.511781 0.512540 0.512060 0.511286 0.511711 0.511544 0.51 1634 0.511352
0.51087 0.51060
-2.00 +0.04 -0.87 -2.38 +3.51 +0.86 +1.11
+0.20 +1.92 +1.94 +1.87 +0.94 +0.98 +1.87 +3.62 +3.83 +1.20 +1.71 +2.39 +1.82 +1.57 +2.20 +2.65 +2.45 +3.08 +2.48 +1.92 +1.18 +2.12 +1.74 -0.63 -3.05
-0.72 +3.90 +1.81 -0.47 -1.25 -0.05 +0.59 +4.09 -0.96 +4.39 +5.30 -6.31 +4.58 +3.43 -1.00 +2.03 +0.32
+2.16 +5.79
1,7304 1,7304 1,900° 1,900° 1,9001 1,9205 1,9001
2,0948 2,0948 2,0948 2,0948 2,0948 2,0948 2,0948 2,1208 2,1208 2,1208 2,1208 2,1208 2,1208 2,2649 2,2649 2,110'0 2,110'0 2,110'0 2,110'0 2,11010 2,110'0 2,120 2,0604 2,0604 2,0604
2,87011 2,87011 2,87412 2,874" 2,8503 2,8503 2,5733 2,7603 2,7603 2,7603 2,7603 2,7603 2,7603 2,7603 2,7603 2,7603 2,7603
2.80013
2,201 2,056 2,240 2,264 2,124 2,118 2,105
2,325 2,202 2,201 2,202 2,306 2,313 2,262 2,073
2,486
2,207 2,264
2,382 2,201 2,226 2,147 2,212 2,215 2,306 2,198 2,227 3,054 3,213
3,009 2,730 2,862 3,036 3,024 2,980 2,695 2,550 3,081 2,523 2,068? 5,699? 2,573 2,626 3,102 2,770 2,923
2,793 2,80013 2.598
4 4 0 0 0 4 4
17 17 17 17 17 17 17 17 17 17 17 17 17 9 9 10 10 10 10 10 10 19 4 4 4
4 4 4 4 4 4 4 20 20 20 20 20 20 20 20 20 20
19 19
I = normalized to 146Nd/144Nd=0.7219; 111 = Ages in Ma. O=This work, U-Pb; 1-Santos et al. (1999), U-Pb; 2-Gaudette et al. (1996), U-Pb; 3-Machado et al. (1991), U-Pb; 4-Sato and
Tassinari (1997), Rb-Sr; 5-Santos and Reis Net0 (1982), Rb-Sr; 6-Tassinari et al. (1996), U-Pb; 7-Teixeira and Tassinari (1984), K-Ar; 8- Norcross et al. (1998), U-Pb; 9-McReath and Faraco (1997), Sm-Nd; 10-Gruau et al. (1985), Sm-Nd; 11-Dall'Agnoll et al. (1998), U-Pb; 12- Macambira and Lancelot (1996), U-Pb; 13-Sidder and Mendoza (1995),U-Pb; 14-Sadowski and Bettencourt (1996), U-Pb; 15-Gaudette et al. (1996), U-Pb; 16-Rizzotto et al. (1999), U-Pb;
I1 = normalized to CHUR: 143Nd/144Nd=0.512655, 147Sm/'44Nd=0.19665
IV = Model age in Ma, relative to depleted mantle (DM). 147Sm/144Ndo, = 0.21353 and 143Nd/'44Nd,, =0.512655 V = References to the Sm-Nd data. Same numbers as in I11 and: 17 - Voicu et al. (1999); 18 - Vignol (1987); 19 - Othman et al. (1984),
20-Olszewski et al. (1989).
Gondwana Research, V. 3, No. 4,2000
458 J.O.S. SANl ros ET AL.
Table 4. New Rb-Sr isotope data from selected Provinces of the Amazon Craton.
Sample Rb Sr" 87Rb 87Sr fractions (ppm) (mm) % Bhpr
RondGnia-Juruena Province Charnockite - Jaru Charnockite PT-O1A 149.38 177.18 2.452 PT-14 4.43 245.64 0.052 PT- 15 88.82 198.17 1.300 PT- 1 5A 55.88 161.64 1.002 RondGnia-Juruena Province Paragneiss - Basement PT-12 80.18 80.80 2.891 Rio Negro Province Gabbro - Tapuruquara Complex, whole-rock HC-477 11.10 1134.3 0.02831 HC-492 12.21 1336.2 0.02642 HC-508 20.26 957.0 0.06123 HC-512 14.53 274.6 0.15311 Rio Negro Province Gabbro - Tapuruquara Complex, HC-492 minerals Plag. 10 2974.0 0.01457 Plag. + biot. 11.8 2314.2 0.01480 Hornblende 16.1 535.6 0.09363 01. + px. + acc. 2.7 60.9 0.12555 whole rock 12.0 1307.7 0.02656
0.76065 0.71159 0.73590 0.72996
0.78058
0.704545 0.704448 0.704999 0.707555
0.704368 0.704329 0.705 779 0.706807 0.704718
fractionation correction of 0.15% per a.m.& * 0.04% a.m.u. was applied to all Pb analyses. All errors are reported as 2 sigma. Data reduction and error analyses were accomplished using the algorithms of Ludwig (1991).
For the third group of U-Pb determinations (samples GR-66, MQ-96 and MS-63), the zircon analyses were carried out on the SHRIMP I1 at the Curtin University of Technology (Table 7). The fresh rocks were crushed and milled, and the heavy minerals were separated from the rock powder using heavy liquids (TBE-tetrabromoethane and Di-iodo methylene) and a Frantz magnetic separator. Handpicked zircons were placed in an epoxy mount, which was polished to expose a cross-section of the crystals. The crystals were photographed and imaged (back-scattered and charge contrast images), using facilities at the Centre for Microscopy and Microanalysis at the University of Western Australia. The images provide important information about zircon morphology and internal structure, allowing the selection of the best sites for spot analyses, and they also represent an important aid in the interpretation of the results. The reference standard was CZ3 zircon (564 Ma: 20fiPb/238U=0.0914). Decay constants used are those recommended by Steiger and Jager (1977). The common Pb correction for the total Pb analysed was done by eliminating the initial 206Pb, 207Pb and 20sPb, using the measured 204Pb and the Pb isotopic composition of Broken Hill galena.
Table 5. Zircon U-Pb analysis of rocks from the West Amazon Craton.
AF-01 (RNP) Monzogranite - Uaupes Intrusive Suite 1 -1 M 56.69 208.15 3.2998 0.2544 2 -1 M 56.40 215.72 3.2982 0.2529 3 OM 63.85 252.05 3.1711 0.2445 4 - l N M 51.87 198.17 3.2776 0.2526 JO-IMA (RNP) Quartz sandstone - Ima Formation -Tunui Group 1 -1 M 328.2 84.8 0.23909 3.7127 2 OM 459.4 105.5 0.21470 3.2300 3 + l M 555.3 112.7 0.18912 2.8195
Ouro Preto Paragneiss - Jaru Complex 1 4 M 1227 205 0.1694 2.1850 2 1 M 889 209 0.2329 3.1968 3 1 M 899 226 0.2523 3.4406 4 OM 822 203 0.2464 3.4035 WO-74 (RJP) Meta tuff (rhyolitic) - Beneficente Group 1 5 M 633 97.7 0.14574 1.7923 2 3 M 868 135.9 0.14800 1.9018 3 1 M 43 1 86.9 0.19363 2.6455 4 O M 322 75.8 0.22555 3.0450
Milonite Gneiss - Serra da Providsncia Intr. Suite 1 O M 1043.5 199.7 0.1874 2.3347 2 1 M 1054.7 188.9 0.1755 2.1527 3 3 M 1561.0 223.2 0.1405 1.6246 4 5M 1882.4 248.6 0.1296 1.4443 wo-52 (SP) Ariquemes Granite - Early Sunsas? 1 1 0 M 2993 269 0.0941 0.9643 2 5 M 2378 309 0.1354 1.4934 3 1 M 1828 343 0.1952 2.2758 4 OM 1900 383 0.2085 2.4651 Letters in brackets represent the following Provinces: RNP=Rio Negro; RJP=RondGnia-Juruna; and SP=Sbnsas. (") Radiogenic Pb.
Zircons analysed at UNH using standard procedures for dissolution, Pb and U separation, and isotopic analysis (Krogh, 1973). Pb and U isotopes were analysed on the 23 cm, 60"solid-source mass spectrometer at the University of New Hampshire. Errors for the 2n7Pb/235U and 206Pb/2'8U ratios are both 1.0%, with a correlation of 0.90. Decay constants are those recommended by Stacey and Jager (1977). Fractionation of Pb analyses was monitored by repeated analyses of NITS standard 983, yielding an average fractionation of 0.10 f 0.03 per a.m.u. First sample number indicates the side slope of the Franz Magnetic Separator. Forward slope was always 20' and the magnet current was 1.5 A. Common Pb corrections were calculated using the model of Stacey and Kramers (1975) and interpreted the age of the sample. Data reduction and error analysis were accomplished using the algorithms of Ludwig (1991).
PT-12 (RJP)
WO-63 (RJP)
The Rb-Sr analyses (Table 4) were carried out using the techniques described in Olszewski et al. (1989a) using the University of New Hampshire mass spectrometer. Replicate analyses of whole-rock powders yielded replication errors of 1.0% for 87Rb/86Sr and 0.06% for 87Sr/ ?Sr. Error correlation was 0.01 and the decay constant
Gondwana Research, V. 3, No. 4,2000
Tabl
e 6.
Zi
rcon
U-P
b da
ta fr
om th
e Ta
pajo
s-Pa
rima
Prov
ince
Sam
ple
Wei
ght
U
Pb"
Isot
opic
ratio
s A
ges
Cor
r. Pb
B
lank
3
.> s
Lu
bb
'b
Z07p
b A
20
7pb
A
z06P
b 2a
7Pb
207P
b co
ef.
(pg)
(p
g)
frac
tions
m
g (p
pm)
(ppm
) Z0
6Pb#
zn
sPb"
20
6PbA
~
__
__
_
~ ~
__
~
204p
b zO
6pb
z3
8u
Y
oerr
23sU
%
err
z06P
b %
err
238U
23
5U
*06P
b
50-5
2, S
ai-C
inza
Sch
ist -
Jac
area
cang
a G
roup
22
0.
010
724
237
766
0.05
0 0.
2847
7 2
3
0.01
0 59
27
50
08
0.23
2 0.
3884
1 24
0.
010
44
25
162
0.17
1 0.
3828
3 zl
a 0.
010
253
103
3942
0.
103
0.38
025
z2a
0.01
0 29
2 53
14
231
0.07
9 0.
1841
6
(0.2
6)
8.09
386
(0.1
9)
7.06
865
(0.3
9)
6.86
146
(0.2
2)
6.84
651
(0.1
1)
1.94
201
(0.8
1)
6.19
013
(0.2
7)
6.11
384
(0.4
0)
5.46
348
(0.2
7)
0.20
614
(0.2
1)
0.13
199
(0.5
3)
0.12
999
(0.2
3)
0.13
059
(0.1
3)
0.07
648
(0.8
3)
0.12
342
(0.2
9)
0.12
394
(0.4
2)
0.12
277
(0.0
5)
(0.0
9)
(0.3
3)
(0.0
5)
(0.0
7)
(0.1
5)
(0.1
1)
(0.1
0)
1615
.3
2115
.4
2089
.5
2077
.5
1089
.7
2000
.0
1971
.6
1803
.2
2241
.6
2120
.1
2093
.7
2091
.8
1095
.7
2003
.1
1992
.2
1894
.9
2875
.4
2124
.6
2097
.8
2105
.9
1107
.7
2006
.2
2013
.7
1996
.8
22
0.01
0 12
7 44
61
20
23
0.01
0 28
3 67
70
72
zl
0.01
0 54
3 13
5 17
36
24
0.01
0 22
9 53
31
34
50-5
7, S
iltst
one - A
baca
xis F
orm
atio
n z5
0.
007
40
15
1804
z2
0.
007
31
11
1410
26
0.
004
68
21
1851
23
0.
003
80
23
815
zl
0.00
2 99
27
30
75
50-6
8, Q
uart
z-w
acke
- Se
quei
ro F
orm
atio
n zl
a 0.
010
1167
47
1 27
744
23
0.
010
24
9 35
60
21
0.
010
39
14
2243
22
0.
010
57
22
6320
0.09
6 0.
3349
9 (1
.13)
5.
3083
8 (1
.04)
0.
1149
3 (0
.08)
18
62.5
18
70.2
18
78.8
0.
109
0.22
509
(2.0
1)
3.49
489
(2.0
1)
0.11
261
(0.0
5)
1308
.7
1526
.1
1841
.9
0.11
4 0.
2287
2 (0
.28)
3.
4916
8 (0
.29)
0.
1107
2 (0
.07)
13
27.8
15
25.3
18
11.3
0.
137
0.21
288
(0.2
0)
3.27
408
(0.2
2)
0.11
155
(0.0
9)
1244
.1
1474
.9
1824
.7
0.15
0 0.
3311
7 (0
.33)
5.
2938
9 (0
.34)
0.
1159
4 (0
.06)
18
44.0
18
67.9
18
94.5
0.
181
0.32
789
(0.4
4)
5.25
551
(0.4
4)
0.11
625
(0.0
7)
1828
.1
1861
.7
1899
.3
0.20
9 0.
2656
2 (0
.45)
4.
2632
8 (0
.48)
0.
1164
1 (0
.17)
15
18.5
16
86.3
19
01.8
0.
140
0.25
245
(0.6
3)
4.06
799
(0.6
4)
0.11
687
(0.1
1)
1451
.1
1647
.9
1908
.9
0.21
9 0.
2458
8 (0
.10)
3.
1138
3 (0
.14)
0.
0918
5 (0
.09)
14
17.2
14
36.1
14
64.2
0.14
3 0.
3688
9 0.
13)
6.48
850
(0.1
4)
0.12
757
(0.0
4)
2024
.2
2044
.3
2064
.7
0.16
7 0.
3457
7 (0
.16)
5.
5378
9 (0
.21)
0.
1161
6 (0
.13)
19
14.4
19
06.5
18
98.0
0.
140
0.34
064
(0.4
3)
5.46
521
(0.4
4)
0.11
636
(0.0
8)
1889
.7
1895
.1
1901
.1
0.20
6 0.
3412
3 (0
.18)
5.
4568
0 (0
.22)
0.
1159
8 (0
.13)
18
92.6
18
93.8
18
95.2
50-1
02, T
ropa
s Riv
er T
onal
ite - P
arau
ari I
ntru
sive
Sui
te
22
0.00
5 63
24
17
16
0.11
0 0.
3623
3 (0
.53)
6.
1632
3 (0
.55)
0.
1233
7 (0
.15)
19
93.2
19
99.2
20
05.5
2
3
0.00
7 16
3 62
66
72
0.19
7 0.
3342
3 (0
.17)
5.
3453
0 (0
.18)
0.
1159
9 (0
.08)
18
58.9
18
76.1
18
95.3
24
0.
051
84
27
2343
2 0.
170
0.29
126
(0.0
7)
4.68
454
(0.0
8)
0.11
665
(0.0
4)
1647
.8
1764
.5
1905
.5
zl
0.01
6 17
4 53
77
53
0.10
0 0.
2889
1 (0
12)
4.58
783
(0.1
3)
0.11
517
(0.0
5)
1636
.1
1747
.1
1882
.6
bl"
0.00
8 17
1 63
45
81
- 0.
3400
0 0.
09
5.44
6 0.
10
0.11
617
0.04
18
86.7
18
92.1
18
98.2
0.98
0 0.
912
0.78
4 0.
975
0.85
6
0.98
3 0.
925
0.96
9
0.99
7 1.
000
0.97
4 0.
913
0.98
2 0.
986
0.93
6 0.
986
0.73
9
0.95
4 0.
789
0.98
4 0.
814
0.96
4 0.
912
0.86
8 0.
939
0.93
6
151.
5 3.
0 63
.3
14.4
2.
4
9.1
11.7
25
.1
4.4
5.7
42.2
9.
3
3.2
3.2
2.4
3.7
0.8
9.3
1.5
3.8
2.0
3.8
3.8
3.3
7.0
6.3
3.5
3.0
3.5
3.5
2.4
3.5
3.5
3.5
3.5
3.5
3.5
3.5
2.0
2.0
2.0
2.0
2.0
3.5
1.5
3.5
2.0
2.0
2.0
2.0
2.0
(*)
Rad
ioge
nic
Pb -
(#)
Mea
sure
d ra
tio c
orre
cted
for s
pike
and
frac
tiona
tion
only
. Mas
s fra
ctio
natio
n co
rrec
tion
of 0
.15?
h/am
u k 0
.04%
am
u w
as a
pplie
d to
all
Pb a
naly
ses.
A
C
orre
cted
for
fr
actio
natio
n, s
pike
, bla
nk a
nd c
omm
on P
b. A
ll er
rors
are
repo
rted
as 2
sigm
a. Z
ircon
s ana
lyse
d at
MIT
usi
ng s
tand
ard
proc
edur
es f
or d
isso
lutio
n, P
b an
d U
sep
arat
ion,
and
isot
opic
ana
lysi
s (B
owrin
g et
al.,
199
3). N
umbe
r of
zirc
ons
in e
ach
frac
tion
show
n in
par
enth
esis
aft
er f
ract
ion
nam
e. A
ll zi
rcon
frac
tions
are
air
abr
aded
(K
rogh
, 198
2). S
ampl
e w
eigh
ts a
re e
stim
ated
usi
ng
a vi
deo
mon
itor
with
a g
ridd
ed s
cree
n an
d ar
e kn
own
to w
ithin
40%
. Com
mon
Pb
corr
ectio
ns w
ere
calc
ulat
ed u
sing
the
mod
el o
f Sta
cey
and
Kra
mer
s (1
975)
and
inte
rpre
ted
the
age
of t
he
sam
ple.
For
ana
lyse
s with
less
than
2.0
pg
com
mon
Pb,
the
tota
l com
mon
Pb
was
ass
umed
to b
e bl
ank.
U bl
ank=
0.5 f 0
.5 p
g. D
ata
redu
ctio
n an
d er
ror a
naly
sis
wer
e ac
com
plis
hed
usin
g th
e al
gori
thm
s of
Lud
wig
(19
91).
(x)
sam
ple
bl, r
esul
ts f
rom
Geo
logi
cal S
urve
y of
Can
ada.
Tabl
e 7.
Is
otop
ic d
ata
and
calc
ulat
ed a
ges
for
zirc
on U
-Pb
SHR
IMP
anal
ysis
of
rock
s fr
om th
e A
maz
on C
rato
n.
Gra
in-
U Th
Th
2
m
4f20
6 ZO
Spb*
Z0
6pb”
* Z0
7pb”
20
8Pb*
co
ncor
d.
207p
b *
206p
b”
__
U
20
6Pb
YO 20
6pb”
23
8U
235U
23
2Th
YO 20
6Pb
238U
spot
(m
m)
(mm
)
MS-
63 (
RN
P) S
eis L
agos
Car
bona
tite
Hos
t Roc
k- C
auab
uri C
ompl
ex
f.1-
1 18
4 10
7 0.
583
0.00
00
0.06
5 f 1
0 f.
10-1
18
3 94
0.
514
0.00
00
0.03
9f
9 f.
11-1
28
2 25
1
0.89
1 0.
0000
0.
OO
Of
6 f.1
2-1
231
187
0.81
2 0.
0002
0.
268f
10
f.
13-1
14
2 65
0.
462
0.00
08
1.29
9f
19
f.2-1
20
8 12
1 0.
585
0.00
00
0.05
0 f 8
f.4
-1
123
116
0.94
5 0.
0000
0,
000 f 1
0 f.4
a-1
177
176
0.99
4 0.
0000
0.
037 f 1
0 f.
5-1
146
61
0.41
8 0.
0000
0
.04
4i
10
f.7-1
12
0 55
0.
462
0.00
00
0.06
6 f 1
2 f.9
-1
256
126
0.49
3 0.
0000
0.
000 f 7
MQ
-96
(RJP
) R
oose
velt
Dac
ite -
Roo
seve
lt G
roup
c.
21-2
10
8 63
0.
582
0.00
05
0.76
4f
16
c.21
-1
146
93
0.64
0 0.
0000
0.
024f
9
c.38
-1
176
202
1.15
1 0.
0000
0.
028 f 9
c.
46-1
14
9 16
1 1.
082
0.00
03
0.44
2f
12
c.46
-2
219
235
1.07
3 0.
0002
0.
243 f 9
c.
41-1
84
75
0.
890
0.00
01
0.O
OO
f 9
c.23
-1
217
251
1.15
4 0.
0001
0.
220 f 8
c.
24-1
18
0 85
0.
474
0.00
00
0.02
4f
7 c.
25-1
21
3 16
0 0.
748
0.00
01
0.13
0 f 7
c.
26-1
16
1 17
2 1.
072
0.00
00
0.00
0 f 7
c.
27-1
28
5 21
9 0.
769
0.00
00
0.00
7 f 6
c.
28-1
17
7 15
2 0.
861
0.00
00
0.00
7 f 1
0 c.
29-1
20
8 21
4 1.
032
0.00
00
0.01
5f
7 c.
30-1
20
7 15
9 0.
766
0.00
00
0.00
0 f 7
c.
31-1
23
2 11
8 0.
507
0.00
00
0.00
0 k 7
c.
32-1
14
8 10
2 0.
689
0.00
01
0.18
7 f 1
2
0.16
88 f 1
9 0.
1479
f 1
9 0.
2587
f 1
6 0.
2388
i 24
0.
1291
f 43
0.
1671
I 16
0.
2716
f 2
7 0.
2888
f 2
6 0.
1187
f 1
8 0.
1300
f 2
4 0.
1392
f 1
1
0.16
73f
37
0.18
31 f 2
1 0.
3292
f 2
4 0.
3185
f 3
1 0.
3138
f 23
0.
2492
i 2
3 0.
3217
f 2
1 0.
1386
f 1
3 0.
2131
f 17
0.
3192
f 2
3 0.
2223
f 1
5 0.
2502
f 2
4 0.
3010
f 1
8 0.
2212
f 1
6 0.
1456
f 13
0.
1963
f 2
8
0.33
26 f 2
9 0.
3231
f 2
8 0.
3264
f 2
4 0.
3281
f 2
6 0.
3340
5 32
0.
3337
f 27
0.
3305
f 3
5 0.
3299
i 30
0.
3387
f 3
1 0.
3224
f 3
2 0.
3418
i 27
0.28
57 f 4
3 0.
3211
i 48
0.
3232
f 4
7 0.
3055
i 4
5 0.
2980
f 42
0.
3119
f 4
9 0.
3157
i 4
5 0.
3270
f 4
7 0.
3232
f 4
6 0.
3087
i 4
6 0.
3256
f 4
6 0.
3241
f 4
7 0.
3301
f 48
0.
3221
f 4
7 0.
3326
f 48
0.
3404
f 5
3
5.13
1 f 6
7 4.
975 f 6
4 4.
945
+_
48
4.99
8 f 64
5.07
9 f 1
10
5.13
2 f 5
9 4.
990 f 7
4 5.
001 f 6
8 5.
187 f 7
0 4.
876 f 7
7 5.
229 I 5
5
4.15
0f
96
4.67
0 f 8
5 4.
680k
82
4.51
0 k 8
8 4.
381 f 7
6 4.
571
k 8
5 4.
589
k 77
4.
977 f 8
2 4.
728 f 7
9 4.
558 f 7
8 4.
819 f 7
7 4.
767f
86
4.
867f
80
4.
718 f 7
8 4.
956
5 83
4.
905 i 1
01
0.09
62 f 1
5 0.
0929
f 1
5 0.
0948
f 1
0 0.
0965
f 1
3 0.
0934
f 3
3 0.
0954
k 13
0.
0950
f 16
0.
0959
f 1
4 0.
0963
5 18
0.
0908
f 2
0 0.
0965
f 1
2
0.08
22f
22
0.09
19 f 1
8 0.
0925
f 16
0.
0899
f 1
7 0.
0872
f 1
4 0.
0873
f 17
0.
0880
k 1
4 0.
0956
f 17
0.
0921
f 1
6 0.
0919
f 1
6 0.
0941
f 1
5 0.
0942
f 1
7 0.
0963
f 1
6 0.
0930
f 1
6 0.
0955
f 1
7 0.
0970
f 2
1
101 99
101
101
103
102
103
102
103
100
104
94
104
105 98
97
101
103
101
104
99
104
1 04
105
104
105
111
Age
(Ma)
1831
f 1
6 18
27 I 1
5 17
97 f 1
0 18
07 f 1
7 18
04 f 3
2 18
25f
13
1791
f 1
7 17
98 i 1
6 18
17 f 1
6 17
94 f 2
0 18
15f
11
1720
f 2
9 17
23 f 1
6 17
15 f 1
5 17
50 f 2
0 17
42 f 1
5 17
37f
15
1722
i 1
3 18
06f
11
1733
i 1
3 17
51 f 1
2 17
55 f 1
1 17
43 i 1
7 17
47 f 1
1 17
36 i 1
1 17
67f
12
1706
f 2
1
Age
(Ma)
1851
f 14
18
05f
13
1821
% 1
2 18
29f
13
1858
f 15
18
56f
13
1841
f 1
7 18
385
14
1880
f 15
18
01f
16
1895
f 13
1620
f 22
17
95 k 2
3 18
05f
23
1718
f 22
1
68
2i
21
1750
f 24
17
69 i 2
2 18
24 f 2
3 18
06 f 2
2 17
34 f 2
3 18
17 f 2
2 18
10 _
+ 23
18
39f
23
1800
f 23
18
51 f 2
3 18
88f
26
PI
L: $1
Tabl
e 7.
Co
ntd U
Th
Th
Z
04Pb
4f
206
ZOSp
b*
206p
b**
Z07p
b-t
207p
b *
206p
b*
Z08P
b*
conc
ord.
-
~ ~
~ _
__
~
~ ~
U z0
6Pb
YO 20
6pb-
t 23
8U
235U
z3
2Th
%
206P
b 23
8U
5 sp
ot
(P
P~
)
(ppm
) .+ I
Gra
in-
.; I
8
GR
-66
(SP)
Par
agne
iss -
Nov
a B
rasi
lhnd
ia M
etam
orph
ic S
uite
d.
75-1
d.
73-1
d.
71-1
d.
70-1
d.
69-1
d.
67-1
d.
65-1
d.
64-1
d.
63-1
d.
62-1
d.
61-1
d.
60-1
d.
59-1
d.
58-1
d.
57-1
d.
56-1
d.
54-1
d.
60-1
d.
61-2
d.
65-1
d.
69-1
d.
74-1
d.
76-1
d.
77-1
244
226 99
672
274
113 71
103 73
61
140
845
466 64
25
5 42
2 44
7 17
9 17
1 76
291
288
237
215
95
65
56
332
207 37
43
62
31
21
90
162
98
34
39
185
100
37
75
52
123
93
95
185
0.38
9 0.
285
0.57
1 0.
495
0.75
6 0.
326
0.60
4 0.
598
0.42
6 0.
350
0.64
6 0.
192
0.21
0 0.
535
0.15
3 0.
438
0.22
3 0.
209
0.44
0 0.
683
0.42
2 0.
321
0.39
9 0.
858
0.00
01
0.00
00
0.00
00
0.00
00
0.00
01
0.00
02
0.00
00
0.00
01
0.00
03
0.00
00
0.00
00
0.00
00
0.00
00
0.00
00
0.00
00
0.00
01
0.00
01
0.00
04
0.00
00
0.00
01
0.00
00
0.00
00
0.00
01
0.00
00
0.00
0 f 9
0.
059 f 1
2 0.
069 f 1
9 0.
024 f 6
0.
147 f 1
0 0.
378 f 2
9 0.
075 f 2
1 0.
238 f 1
9 0.
405 f 3
4 0.
000 f 1
7 0.
016 f 1
2 0.
000 f 5
0.
070 f 7
0.
062 f 2
8 0.
002 f 1
2 0.
093 f 8
0.
085 f 8
0.
616 f 2
1 0.
008f
13
0.
108 f 2
3 0.
050 f 1
0 0.
068 f 1
2 0.
093f
11
0.
047 f 1
1
0.10
20f
16
0.08
40 f 2
3 0.
1675
4 4
4 0.
1438
f 1
2 0.
1019
f 1
8 0.
0936
f 6
3 0.
1718
f 4
6 0.
1712
f 44
0.
1184
f 7
5 0.
1066
f 3
2 0.
1787
f 2
3 0.
0576
_+
6 0.
0618
f 1
2 0.
1583
f 6
4 0.
0460
f 20
0.
1300
f 1
7 0.
0535
f 1
3 0.
0559
f 4
5 0.
1318
f 2
2 0.
1959
f 5
3 0.
1130
f 1
8 0.
0933
f 2
4 0.
1211
f 2
2 0.
2491
f 2
6
0.18
17 f 3
6 0.
2163
f 4
3 0.
2151
f 4
6 0.
1995
f 3
8 0.
2943
f 5
8 0.
2265
f 4
8 0.
2797
f 6
1 0.
2197
f 4
7 0.
2001
f 4
5 0.
2002
f 4
6 0.
3666
f 75
0.
2430
f 4
6 0.
2375
f 4
6 0.
2128
f 48
0.
2315
f 4
6 0.
2165
f 4
2 0.
2039
f 2
9 0.
2429
f 3
9 0.
3781
f 5
9 0.
2709
f 5
2 0.
3191
f 4
8 0.
1940
f 2
9 0.
2244
f 3
5 0.
3432
f 5
3
1.99
1 f 4
8 2.
406 f 6
3 2.
292 f 7
8 2.
217 f 4
7 4.
265 f 9
7 2.
644 f 1
11
3.61
9 f 1
19
2.53
2 f 8
4 2.
105 f 1
09
2.20
0 f 7
4 6.
288 f 1
50
3.01
3 f 6
1 2.
916 f 6
4 2.
326 f 1
02
2.72
9 f 7
0 2.
453 f 5
6 2.
277 f 4
2 2.
929 f 9
0 6.
746 f 1
31
3.68
7 f 1
17
4.70
6 f 8
7 2.
281 f 5
0 2.
648 f 5
7 5.
515 f 1
05
0.04
76 k
12
0.
0637
f 2
2 0.
0631
f 22
0.
0580
f 1
2 0.
0396
f 11
0.
0650
f 4
6 0.
0796
f 2
8 0.
0629
f 2
2 0.
0556
f 3
8 0.
0609
f 2
4 0.
1015
f 25
0.
0729
f 1
6 0.
0699
f 2
0 0.
0630
f 3
0 0.
0696
f 3
3 0.
0643
i 1
5 0.
0489
f 1
4 0.
0649
f 5
3 0.
1133
f 2
7 0.
0777
f 2
7 0.
0854
f 2
0 0.
0564
f 1
7 0.
0681
t 1
7 0.
0996
f 2
0
91
104
111
97
97
101
106
100
107
99
100 98
98
105
101
101
98
102 99
97
102
87
98
100
Age
(Ma1
1184
f 2
2 12
13 f 2
9 11
28 f 4
8 12
11 f 1
4 17
16 f 1
7 13
08 f 6
6 15
05f
41
1283
f 4
4 11
03 f 8
8 11
89 i-
43
2020
f 1
7 14
24 f 1
0 14
05 f 1
6 11
79 f 6
9 13
27 f 2
7 12
50 f 1
9 12
22 f 2
0 13
70 i-
47
2090
f 1
7 15
99 f 4
3 17
48 f 1
6 13
22 f 2
7 13
29 f 2
5 19
04 f 1
7
Age
(Ma)
1076
f 2
0 12
62 f 2
3 12
56 f 2
4 11
73f
21
1663
k 29
13
16f
25
1590
5 31
12
80%
25
1176
f 24
11
77f
25
2013
f 35
14
022
24
1373
f 24
12
44f
26
1342
f 24
1
26
3i
22
1196
f 16
14
02f
20
2068
f 28
15
46f
26
1785
f 23
11
43f
16
1305
f 18
19
02f
25
-
4f20
6 =
(co
mm
on 2
0bPb
) / (t
otal
mea
sure
d 20
6Pb)
base
d on
mea
sure
d z0
4Pb.
Pb
iso
tope
ratio
s are
for r
adio
geni
c com
pone
nts o
nly
(*).
Com
mon
lead
cor
rect
ion
used
f20
6 an
d th
e B
roke
n H
ill c
ompo
sitio
n.
List
ed u
ncer
tain
ties a
re l
o, d
eter
min
ed b
y co
untin
g st
atis
tics (
incl
udin
g pro
paga
tion
of c
omm
on le
ad c
orre
ctio
n) an
d ap
ply
to th
e la
st d
igits
quo
ted.
U
ncer
tain
ties i
n Pb
/U a
re d
omin
ated
by s
pot-
to-s
pot r
epro
duci
bilit
y; c
oncu
rren
t ana
lyse
s ca
libra
ted
to th
e cz
3 st
anda
rd, w
ith a
lo
scat
ter.
conc
ord.
= c
onco
rdan
ce, a
s 100{t[206Pb*/z3aU]/t[207Pb*/zo6Pb*]}.
Dat
a re
duce
d us
ing
Kril
l sof
twar
e.
462 J.O.S. SANTOS ET AL.
used for 87Rb was that recommended by Steiger and Jager (1977).
Sm-Nd analyses were carried out at the geochronologic research centre of the University of S5o Paulo (samples 50-174, JO-69 and JO-66) and at Curtin University of Technology, Australia (samples MM-36,J0-3 and AL-9b). Standard used for control was BCR-1 and the 143Nd/'44Nd ratio errors are lo. The replication errors were 0.20% for 147Sm/144Nd and 0.020!0 for 143Nd/144Nd. The decay constant used for 147Sm was 6.54 x y r ' (Wasserburg et al., 1981). The model ages were calculated referenced to the following present day values (De Paolo, 1988):
CHUR: 147Sm/144Nd = 0.19665 and 143Nd/144Nd = 0.512655.
MORB: 147Sm/144Nd = 0.21353 and 143Nd/144Nd = 0.513168.
The Sm-Nd data are shown on Table 3, with six new results (this work), and including more than 103 samples, analysed between 1984 and 1999 by nine different authors, using different values for the 147Sm decay constant and the present day 147Sm/'44Nd values. All data were recalculated according to the values mentioned above.
Definition, Age and Evolution of the Provinces of the Craton
According to the data available (Table l ) , seven main geological provinces and one shear belt are recognized: Carajds and Imataca - 3.10-2.53 Ga, juvenile; Transamazonic (Guianas) - 2.25-2.00 Ga, juvenile; Tapaj6s-Parima - 2.10-1.87 Ga, juvenile; Central Amazon - 1.88-1.70 Ga, underplating; Rio Negro - 1.86-1.52 Ga, collisional; RondGnia-juruena - 1.76-1.47 Ga, juvenile; and Sunsas - 1.33-0.99 Ga, collisional (including the K'Mudku Shear Belt - 1.10-1.33 Ga).
Each of the Provinces is described below with an evaluation of the geochronological constraints on their ages and evolution. Emphasis is placed on those provinces where there are new geochronological data. The present model is transient and certainly will change when new data are available, particularly from the large areas of the Amazon Craton where the geochronological information is scarce (Tumacumaque, Iriri-Xingu and Imeri-Parima areas).
Carajds province
The craton has been considered to be predominantly Archean, as shown in the Geological Map of Brazil, 1984 edition(Sch0bbenhaus et al., 1984) and in Costa and Hasui (1997). Tassinari et al. (1996) restricted the Archean to the Central Amazon Province, which included the Carajds region. In this paper, the Archean is restricted not
only to the Carajas (and Imataca) granite-supracrustal terrains, but is extended to the south of Amapa where Archean U-Pb ages have been obtained in the Cupixi Granulite in Amapd State (MacReath and Faraco, 1997; Lafon et al., 1998). The Central Amazon Province, despite its Archean crustal origin, is much younger and truncates the west-northwest Carajds tectonic trend. Thus, the exposed Archean rocks comprise only <12 Yo of the Craton, which is thus predominantly Paleoproterozoic and Mesoproterozoic in age.
The Carajds Province is located in the east-southeast region of the Craton, in Pard State and southwest Amapd State, Brazil. The previously state-owned and now private
Table 8. Caraj6s Province selected geochronological data.
Rock or unit Method M Age R Velho Guilherme Granite U-Pb Z 1873f 13 1 Pojuca Granite U-Pb Z 1874f 2 2 Carajas Granite c U-Pb Z 1880f 2 2 Cigano Granite U-Pb Z 1883f 2 2 Musa Granite U-Pb Z 18832 5 2 Salobre Group amphibolite U-Pb T 2497k 5 2 Xingu Complex amphibolite U-Pb T 2519f 5 2
2 25272 34 3 Salobre Group BIF U-Pb M 2551k 2 2 Salobre Group amphibolite U-Pb Z 2555f 4 2 Old Salobre Granite U-Pb Z 2573k 2 2 Salobre Group -granite U-Pb T 25842 5 2 Salobre Group amphibolite U-Pb Z 2732f 2 2 Pojuca Group amphibolite U-Pb Z 2732f 3 2
Grao Para Group Metarhyolite U-Pb Z 2 7 5 8 i 39 5 Grao Para Group rhyodacite U-Pb Z 2759k 2 2 Salobre Group amphibolite U-Pb Z 2761f 3 2 Salobre group Cu-Au ore Pb leaching Cc 2762 k 180 6 Luanga anorthositic gabbro U-Pb Z 2763+ 6 2 Salobre Group Iron ore Pb leaching Mt 2776 f 240 6 Xingu Complex gneiss U-Pb Z 2851k 4 2 Salobre Group gneiss Gneiss U-Pb Z 2 8 5 1 i 4 2 Xingu Complex amphibolite U-Pb Z 2856f 3 2 Xingu Complex granite U-Pb Z 28592 2 2 Pium Complex metamorphism SHRIMP Z 2861 f 12 7 Mogno Trondhjemite U-Pb T 2870 8 Xinguara Granitoids A Pbisochron Z 2870 8 Mata Surrso Monzogranite U-Pb Z 2 8 7 2 i 10 1 Rio Maria Granodiorite U-Pb Z 2874f 10 9 Andorinhas Supergroup SHRIMP Cz 2943f 18 7 Arc0 Verde Tonalite U-Pb Z 2960 8 Xingu Complex gneiss U-Pb Z 2971f 29 9 Lagoa Seca Metagraywacke U-Pb Z 2971k 18 9 Pium Enderbite protolith SHRIMP Z 3002f 14 7 Pium Complex Pbisochron Wr 3050k 114 1 Rio Fresco Group Quartzite U-Pb Cz 31892 22 9 A = First orogenic cycle; B = Second orogenic cycle; C= Paleoproterozoic
anorogenic granites; M = mineral; Z= zircon; Cz = clastic zircon; T = titanite; M = monazite;
Cc = chalcocite; Mt = magnetite; Wr = whole rock. R = references: 1 - Rodrigues et al. (1992); 2 - Machado et al. (1991);
3- Barros et a1 (1992); 4 - Wirth et al. (1986); 5- Olszewski et al. (1989b); 6 - Mellito and Tassinari (1998); 7 - Pidgeon et al. (2000); 8 - Dall - Agnoll et al. (1998); 9 - Macambira and Lancelot (1996).
Estrela Granite U-Pb
Salobre Group rhyolite U-Pb Z 2740 4
Gondwana Research, V. 3, No. 4,2000
U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 463
company, Vale do Rio Doce, has been exploring the area since 1968, after the discovery of large iron deposits. There are several greenstone belts, trending 110" to east-west, cut by two main granite suites, the Old Salobre Granite (2573 f 2 Ma) and the Serra dos Carajas Granite Suite (1880 Ma). Metabasalts, BIFs and metasiltstones dominate the greenstone successions. There are some individually named greenstones: Andorinhas, Inaja, Cumaru, Carajas, Serra Leste, Serra Pelada and Sapucaia.
The geochronological data (Table 8) indicate two cycles of crustal evolution, the first with a 3002 -t 14 Ma (Pium Complex, Rodrigues et al., 1992; Pidgeon et al., 2000) to 2960 f 20 Ma (Arco Verde Tonalite, Macambira and Lancelot, 1996) old supracrustal protolith. These older metavolcano-sedimentary sequences are cut by late- intrusive granitoids, with ages of 2874 f 10 Ma (Rio Maria Granodiorite; Macambira and Lancelot, 1996) and 2870 Ma (Mogno Trondhjemite; Dall'Agnol et al., 1998). Both were affected by regional metamorphism at 2861 k 12 Ma. Some mineralizations, for example the Au-Cu ore from Bahia Creek, may be related to the end of this cycle, as indicated by the Pb-Pb age of 2850 k 65 Ma obtained by Mougeot et al. (1996) for sulfides from this deposit.
The second cycle started with volcanism (2759 f 2 Ma - Gr2o Para Group rhyodacite; 2732 k 3 Ma, Salobre Group amphibolite), followed by metamorphism (2573 f 2 Ma) and granitoid intrusions (Estrela, 2527 k 34 Ma; Old Salobre, 2573 f 2 Ma), according to ages published by Machado et al. (1991). There was a complete lack of major Transamazonian tectonic activity in the province, but at the end of the Paleoproterozoic there was intense intracratonic Uatumg magmatism. The Velho Guilherme (1873 Ma), Pojuca (1874 Ma), Carajas (1880 Ma), Cigano (1883 Ma) and Musa (1883 Ma) granites are examples of Uatumii magmatism in the Carajhs Province.
The Archean Carajas Province is extended to the north (Bacaja region), until southwest Amapa State in Brazil, where Jorge Jog0 and Marinho (1982) describe older crust, dominated by tonalites and trondhjemites (Quata, Cobra, Tartarugal Grande and Ananai units).
The Imataca province
The Imataca Belt, a 60"-70" northeast trending fold belt in the extreme north of the Craton, parallel to the lower Orinoco River in Venezuela, is dominated by orthogneisses and amphibolites (amphibolite facies) followed by quartzites, paragneisses, marble and BIF sequences metamorphosed under granulite facies, which host large iron deposits (e.g. Cerro Bolivar and El Pao). The east- northeast Imataca trend cuts the west-northwest Transamazonian trend along Guri fault. Since the 1970's, its age has been considered to be Lower Archean (Montgomery and Hurley, 1978; Bellizia, 1974), based
on U-Pb whole-rock dating. The protolith ages are ca. 3.10 Ga and might be as old as 3.70 Ga (Montgomery and Hurley, 1978). ). These authors determined the age of the granulite metamorphism at 2.00-2.20 Ga (Pb-Pb and Rb-Sr) and Onstott et al. (1989), using Ar-Ar dating confirmed the effects of the Transamazonian metamorphism and detected younger deformations at 1.70 and 1.10 Ga.
Teixeira et al. (1999) determined Nd model ages (T,,) from 3230 to 2930 Ma, indicating the presence of two major mantle-differentiation events (3230-2930 and 2820-2600). These two groups show good correlation to the two main Carajas cycles (see above). The extension of the exposed Archean rocks in the Imataca belt, as well as the ages of the metamorphic overprint, are not totally defined by the available Ar-Ar, Pb-Pb and Rb-Sr data. The Archean Carajds Province, for example was believed to be Transamazonian, based on Rb-Sr (Gomes et al., 1975) and K-Ar (Amaral, 1974) data.
Transamazonic (Guianas) province
The granite-greenstone terrain of northern South America is a 2000 km long belt trending west-northwest, from Amapa and Para states in Brazil, to Bolivar State in Venezuela, crossing French Guiana, Surinam and Guyana. This province was named Maroni-Itacaiunas province (Tassinari et al., 1996, Teixeira et al., 1989), but Transamazonic is a more used and accepted name (following Hurley et al., 1968). Maroni-Itacai6nas is not an appropriate name, because the Itacai6nas River and the Itacai6nas Mountains are sited inside the Archean Carajas Province and the Maroni River is double named (Maroni in French Guiana and Marowijn in Suriname).
The volcano-sedimentary sequences show less ultramafic and accretionary sequences and more clastic sedimentary units, when compared to classical Archean greenstone belts (Bertoni, 1998). These sequences have received different regional names, Vila Nova and Ipitinga in Brazil, Paramaca and Bonidoro in French Guiana, Armina-Rosebel in Surinam, Barama-Mazaruni in Guyana, and Pastora-Carichapo in Venezuela.
The classic Transamazonic orogenic belt evolved between 2.25-2.00 Ga (Riacian), and is strongly correlated with the Birimian belt in West Africa. Almost the entire belt represents juvenile Paleoproterozoic crust, as indicated by the Sm-Nd T,, model ages in the 2.11-2.29 Ga range, very close to the U-Pb ages obtained for magmatic zircons (2.00-2.25 Ga, Gibbs and Olszewski, 1982; Norcross et al., 1998; Milesi et al., 1995; Vanderhaeghe et al., 1998). The initial oceanic volcanism (back arc extension) occurred at 2174 f 7Ma in French Guiana (Vanderhaeghe et al., 1998). The early calc-alkalic volcanic-plutonic arcs are dated at 2120 f 2 Ma in Guyana
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464 J.O.S. SANTOS ET AL.
(Wenot Lake, Norcross et al., 1998); 2144 k 6 Ma (North French Guiana, Ile de Cayenne Complex, Vanderhaeghe et al., 1998) and 2130 Ma (Volcanic Paramaca Series, French Guiana, MilPsi et al., 1995). There are two main
calc-alkalic batholithic granitic suites, one being related to the first, syn-volcanic magmatic arc, with ages of 2120 to 2160 Ma in French Guiana (Milhsi et al., 1995) and 2094 Ma in Guyana (Norcross et al., 1998).
Carajas and lmataca
RondGnia-Juruena
Transamazonic Tapajbs-Panma
Araguaia Orogenic Belt Andes Orogenic Belt 1 1 Phanerozotc, indiscriminated
Fig. 1. Major Provinces of the Amazon Craton in North South America, as defined in this study. GC=Garzon Complex, Sunsas-Grenvillean. AC=Arequipa Complex, Sunsas-Grenvillian. Rectangles locate Figs. 2, 10 and 16.
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U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 465
The second magmatic arc is almost exclusively plutonic and comprises several batholiths with ages close to 2.00 Ga. The regional evolution and timing of the gold mineralization were established by Norcross et al. (1998) in the Omai deposit in Guyana and by Milesi et al. (1995) in the north French Guiana gold deposits. These authors recognize an earlier gold mineralizing event at around 2.15 Ga and a late main gold mineralizing event at 2014 Ma (Pb-Pb). A Pb-Pb isochron on hydrothermal titanite
and rutile from the Omai ore gives an age of 2001 f 4 Ma (Norcross et al., 1998).
There is one known older remnant block inside the Transamazonic belt, in central Amapa, Brazil, where Lafon et al. (1998) detected Neoarchean rocks, like the Cupixi granulites (2.49 - 2.55 Ga, Pb-Pb ages). The protolith of Cupixi Complex is Paleoarchean, according to Sat0 and Tassinari (1997) model ages, which are in the 3.06-3.10 Ga range. The Cupixi Archean survivor inside the
!W m'w 57O OO'W 5O 00's
.......... ......... ......... ......... .........
6' 00s
........ ........
........... ...........
............ ............ 7> 00's
.................
......... ..........
granites and volcanics
B Village
Sampling site CuiD-Cuiu Suite, \Main shear tonalites and amphibolites 1 o .......... .....-*' Road
50 km - Provinces Boundary Jacareacanga Group, River metaturbidites, metabalt
Fig. 2. Geological map of the Tapaj6s Province, showing sample locations. Map based on mapping by Companhia de Pesquisa de Recursos Minerais, Tapajos Project, unpublished.
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466 J.O.S. SANTOS ET AL
Transamazonic Province may be explained by the proximity to the Carajds Province located in southwest Arnapa State (Fig. 1).
The presence of large areas of granite-greenstone terrain, hosting gold mineralizations in the northern and southeastern Tocantins State in Brazil is one indication that at least part of the Araguaia Belt (Fig. 1) may be paleoproterozoic and an extension of the Transamazonian Province.
Tapajds-Parima province
The western part of the Central Amazon Province was recognized by Tassinari (1996) as a geochronologic province, generated during 1.95 - 1.80 Ga and named Ventuari-Tapajos Domain. Based on its re-interpreted tectonic and geochronologic characteristics this domain is reinterpreted as a Paleoproterozoic orogenic belt (Fig. l ) , extending from southeast (Peixoto de Azevedo region, Mato Grosso State) to northwest (Parima region, Roraima and Venezuela) and constituted by four domains: Peixoto Azevedo, Tapajos, Uaimiri and Parima. The Ventuari region is younger and is not related to this province (see Rio Negro basement chapter). In the Tapaj6s domain, the belt trends north-northwest and includes geological units which range in age from - 2.10 Ga to 1.87 Ga. Some important features, such as the tectonic trend, similar geology and gold metallogeny, indicate that the Tapaj6s belt extends to the northwest (Parima region, Roraima State) and to the southeast (Alta Floresta-Peixoto de Azevedo region, Mato Grosso State). The U-Pb results (Table 5) from this Province are from the Tapaj6s Belt, and Fig. 2 shows sample locations.
Sai-Cinza Meta-turbidite, Jacareacanga group - 50-52
The Jacareacanga Group is a supracrustal unit, trending north-northwest, tightly folded and metamorphosed under greenschist to upper amphibolite facies. It is the oldest exposed unit of the Tapaj6s Orogenic Belt. There are several outcropping areas of Jacareacanga Group less than 50 km from the Cachimbo plateau (Paleozoic- Proterozoic cover), as shown in Fig. 2. Based on further investigation of the outcrops, along the Tapaj6s River and BR-230 Highway, the group is composed of turbidites, chert, BIF, oceanic and magnesian basalts. Sample 50-52 was collected at Sai-Cinza village, where the turbiditic sequence, normally pelitic, contains arenites more favorable for clastic zircons. Only a few zircons were obtained and four crystals were analyzed (Table 6). One result is discordant and Archean (2875 Ma) while three are concordant, with ages (zo7Pb/206Pb) around 2.1 Ga (2098, 2106 and 2125 Ma) - Fig. 3. The 2.10 Ga zircons are interpreted as related to primitive basaltic to andesitic magmatism, developed in the early stages of rifting and
subsidence of the ocean crust. The Archean zircon could have been derived from the continental Archean crust to the East (Central Amazon Province).
ConceipTo tonalite, CuiZi-CuiZi complex - 50-51
The older granitic rocks in the Tapajos area have been grouped into the Cuiu-Cuiu Complex (Pessoa et al., 1977), which is the regional basement. The Parauari and Maloquinha Granite Suites cut the complex, although their relationship with the Jacareacanga Group remains uncertain. The Cuiu-Cuiu rocks, which are dominantly granodioritic to tonalitic, enclose amphibolite fragments and local trondhjemitic veins. Although strong northwest to north-northwest banding is common, isotropic monzogranites occur in some places, such as Porquinho (JH-29) and Baixo Jamaxim (50-174). This complex represents a primitive, calc-alkalic magmatic arc, which hosts several areas of gold mineralization. Three groups of zircons were separated from sample 50-51, a tonalite from Conceiqso Mine. The concordia plot (Table 6, Fig. 4) shows an upper intercept a t 2011 f 2 3 Ma (MSWD=5.5), confirming that the complex is older than the Parauari and Maloquinha Suites (younger than 1.9 Ga), and indicates that it is possibly younger than the Jacareacanga Group. Other important regional implications are: i) the basement in the Tapajos area, besides other dissimilarities, is much younger than the Archean basement in the Caraj 6s Region (Xingu Complex). Hence, the name Xingu Complex (Leal et al., 1978; Santos et al., 1975) must be geographically restricted to Carajas province, 1800 km to the east, and ii) the basement U-Pb age is - 100 m.y. (Parauari Rb-Sr age) or 300 m.y. (Itaituba Gneisses Rb-Sr age) older than the Rb-Sr ages assumed for the basement (Tassinari et al., 1996) and it is - 400 m.y. younger than the 2.45 Ga maximum age proposed by Lima (1999) for the Parima-Tapaj6s Mobile Belt.
Tropas Granodiorite, Parauari intrusive suite - JO-102
The Parauari Suite represents a second calc-alkalic magmatic arc in the Tapaj6s Orogenic Belt, comprising granitoids, mainly monzogranitic, but ranging from tonalite to syenogranite. The granitoid bodies are batholiths, with irregular shape and well-defined stratigraphic position, which are intrusive into the Jacareacanga and Cuiu-Cuiu units, and are cut by the Maloquinha granites. The Rb-Sr ages for the Parauari Suite are 1964 Ma (Tassinari, 1996) and 1955 Ma (Santos and Reis Neto, 1982). Sample 50-102, a tonalite from the Tropas River, 8 km upstream from its mouth, shows banding which has been interpreted as metamorphic banding, and correlated and mapped as Cuiu-Cui6 suite (Bizzinella et al., 1980). Nevertheless, during sampling
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U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON
I 2 0 7 0 1
467
0.32
0 390
3 0.386
hl \
a a W 0 hl
0 382
/ I 4 - / , , P I , I , I
I JACAREACANGA GROUP SAI-CINZA SCHIST - 50-52
0.34
0 32
3 W m @4
\ 0.30
a a , .
(D
028-
0.26r
2110 2,125 M /- CLASTIC ZIRCONS
- ABACAXIS FORMATION SILTSTONE - JO-57 CLASTIC ZIRCONS
- 0.36
-
MSWD = 3.4 -
2,098 Ma
2.106 Ma
6 - ,/
3 6 6 6 7 6 8 6 9 7 0
207Pb I 235U I ' I ' I ' I ' I i
0 36 - PARAUARI INTRUSIVE SUITE TROPAS TONALITE - JO-102
034 -
I> N.
a
co m
a 0 3 2 -
W 0 N
Pb I 235U 207
0 38
3 00 m N . a036 a
(D 0 N
0 34
SEQUEIRO FORMATION QUARTZ-WACKE - JO-68 CLASTIC ZIRCONS
7 5 2 5 6 6 0 6 4 68
0 32 I/, , I , I , I I I
4 8
'"Pb I 235U
' 1 ' 1 ' 1 '
CUIU-CUIU COMPLEX CONCEICAO TONALITE - 50-52
0.37 1 0.36 t
3 - co N rn 0.35 - \
d 0 3 4 -
(D
0 N
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MSWD=55
I
. ROSA-DE-MA10 MONZOGRANITE - JO-54 0.34- LATE PARAUARI INTRUSIVE SUITE
0.30-
3
\ / / 1 W (* N - /'
1'
/' 1879 f 11 Ma 0 N ,/ MSWD=054
I / I I I 2.0 3.0 4.0 5.0
207Pb I 235U Figs. 3 to 8. U-Pb Concordia plots for zircons in rocks from the Tapaj6s Province. From top left to bottom right: J=Sai-Cinza Schist (50-52, Jacareacanga
Group), 4=Concei@o Tonalite (50-51, Cuid-Cuiri Complex), 5=Tropas Tonalite (50-102, Parauari Intrusive Suite), 6=Abacaxis Formation Siltstone (J0-57), 7=Sequeiro Formation Quartz-wacke fJO-68), and 8=Rosa de Maio Monzogranite (50-54, Late-Parauari Intrusive Suite).
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468 .J.O.S. SAN TOS ET AL.
SANTA RITA MONZOGRANITE - JO-199 MALOQUINHA INTRUSIVE SUITE t
- I
0 3 0 t
i
l , < y L o w e r intercept = 1211 f 57 Ma
I 2459k 11 Ma MSWD = 4 4
1
Pb I 235U 207
Fig. 9. Santa Rita Monzogranite (JO-199, Maloquinha Intrusive Suite).
along 20 km of the river channel, the banding appears to be magmatic and the rocks lack evidence of metamorphism in thin section. Five groups of zircons were separated from 50-102 and the U-Pb results (Table 6) plotted on Fig. 5. Three populations lie on a line that intercepts the concordia curve at 1897 k 6 Ma (MSWD=3). Two darker populations are older, representing inherited zircons with a 2010 Ma age. This age is very close to the 50-51 sample age (Cui6-Cuiu Complex), and may indicate that there was an important crustal contribution from Cuiu-Cui6 tonalites to the source of Parauari granitoid melts. The Sm-Nd data from Parauari samples show Nd TDM model ages around 2.1 Ga and E,,
values between -0.87 and + 1.83 (T=1.898 Ma). The crustal formation ages are close to the Cui6-Cui6 Complex age (2.02 Ga). In the same belt, granitoids correlated to the Parauari Suite to the southeast (Juruena Granodiorite, Alta Floresta region) and to the northwest (Agua Branca Adamellite, Amazonas and Roraima States) show similar behavior (Table 2). This indicates that the Parauari Arc was derived from a source containing little or no older crust, or was derived from a slightly older juvenile crust (Cuiu-Cui6 Arc).
Abacm's formation - 50-57
This unit occurs only in the extreme west of the Tapaj6s Province, in the Abacaxis and Palha Branca Mines where it hosts the gold mineralization. It is represented by pinkish siltstones, interbedded with rare lenses of fine sandstone and claystone. Sample 50-57 was collected from fine sandstone layers in the Abacaxis mine, where the sequence is sub-vertical and deeply weathered. The sample is zircon poor and only five clastic zircons were analysed. The results are discordant (Table 6), but four points lie on a
line, which intercepts the concordia curve at 1895 k 9 Ma (MSWD= 3.4). This result (Fig. 6) allows correlation between the Abacaxis and Sequeiro Formations, which are younger than the Parauari Granite Suite. These units could represent fore-arc sedimentation in the Tapaj6s Orogenic Belt. Recent information, from the Golden Star drilling program at Abacaxis Mine, shows that the Abacaxis Formation is cut by a granodiorite correlated to the Maloquinha Suite. This limits the Abacaxis (and probably the Sequeiro) Formation age to between 1895 and 1879 Ma.
Sequeiro formation - 50-68
In the western part of the Tapaj6s Province, in the area between the Tapajbs, Parauari and Abacaxis rivers, there are several exposures of unmetamorphosed immature sedimentary rocks. The Sequeiro Formation is dominated by wackes and quartz-wackes and surrounds the Espirito Santo Mine. Quartz veins with pyrite and gold cut the unit at Sequeiro Creek where it is well exposed. This sequence was previously thought to be related to the Jacareacanga Group, representing zones of less deformation and metarhorphism. Four clastic zircons were analysed (Table 6), giving concordant results (Fig. 7). One crystal has an age of 2065 Ma and may be derived from the Jacareacanga Group. Three others have younger ages (207Pb/206Pb) around 1.90 Ga (1902,1898 and 1895 Ma), indicating that the Sequeiro Formation is younger than the Jacareacanga Group. The ages close to 1.90 Ga are very similar to the U-Pb zircon age of the Tropas Granite - Parauari Intrusive Suite (1897 Ma, Fig. 5). This indicates that the Parauari Suite, which occurs south, west and east of the Sequeiro outcrop, was the main source of detritus for the Sequeiro Formation.
Rosa-de-Maio Monzogranite, Late-Parauari intrusive suite
Rosa-de-Maio Monzogranite is a batholith that dominates the Rosa-de-Maio gold district, cropping out in the region of the Rosa-de-Maio, Anta, Bandeirantes, Perez and Serra Morena Mines (Parauari River Basin). The granitoid has been related to the Parauari Intrusive Suite by Bizzinella et al. (1980), but without any isotopic evidence. From the analysed zircons (Table 6), three populations are discordant and one is concordant (Fig. 8). The concordant 207Pb/206Pb age is 1878.8 Ma and the four populations are aligned, with the upper intercept at 1879 k 11 Ma (MSWD=0.54). This indicates that the Rosa-de-Maio Monzogranite may be slightly younger than the Tropas Tonalite, representing a late-Parauari, more evolved magmatism, compared to the Penedo (1883 -t 4 Ma, Santos et al., submitted) and Cumaru granites (1883 k 8 Ma, Brito et al., 1999), but older than the anorogenic
- 50-54
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U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 469
Maloquinha granites (1872 f 4 Ma, Santos et al., submitted). Many syenogranites and monzogranites were previously mapped as part of the Maloquinha Suite, but reinterpretation is required as they are more likely linked to the arc-related late-Parauari Suite.
Santa Rita granite, Maloquinha intrusive suite - JO-199
This granite crops out in the southernmost part of the province, in the Mi6do and Santa Rita mines region. The heavy mineral concentrate was sampled in the field, by panning the weathered granite from the bottom of the Santa Rita pit. Four zircon fractions were analysed yielding two very concordant results of 2849 and 2460 Ma (207Pb/ 206Pb age), and two discordant results (Fig. 9). These two results, together with one concordant point, lie on the same line that intercepts the concordia at 2459 f 11 Ma (MSWD=4.4). This age is older than the Santa Rita Granite host rock (Cui6-Cui6 tonalites, 2.02 Ga), and does not reflect the granite magmatic age. It is interpreted as the age of inherited zircons, derived from an Archean crustal source. The Archean crustal source for the Maloquinha Suite was also detected by the Sm-Nd T,, model ages, as discussed below and by Santos (1999).
Central Amazon province
The Central Amazon Province has been considered as an Archaean nucleus (Tassinari et al., 1996, Teixeira et al., 1989) on to which several younger, Proterozoic mobile belts were accreted. However, known Archaean rocks are exposed exclusively in the Imataca (Venezuela) and Carajas (southeast Para State, Brazil) areas. As the western part of the Central Amazon Province is constituted by the Tapaj6s-Parima Orogenic Belt, the province area is now reduced by - 45% in area. The resultant area is dominated by a cratogenic rock association composed of: i) acid to intermediate volcanic rocks (Iriri, Surumu, Burro-Burro, Caicara and IricoumC Group/Formation); ii) A type granites (Maloquinha, Mapuera and Saracura Intrusive Suites); iii) fluvial, clastic, platform sedimentary rocks (Palmares, Roraima and Urupi Groups); and iv) tholeiitic sills and dykes (Avanavero, Crepori and Quarenta Ilhas Intrusions). The Central Amazon Province rocks lack regional metamorphism and compressional folding, being characterized by block-faulting tectonics. The acid to intermediate volcanic rocks and A-type granites were generated during the Uatumi Magmatism (Santos and Reis Neto, 1982).
The evolution of this intracratonic sequence extended from 1.88 to 1.70 Ga. Some important features of these rocks are the very low E,, (-7.10 to -12.38, the lowest in all the Amazon Craton), and the old Sm-Nd T,, model ages (2.44 - 2.85 Ga, calculated for t=1.870 Ma), which indicate an Archaean continental crustal source. The
Archaean source fingerprint is detected also in the zircons from the Maloquinha (Santos et al., submitted) and the Santa Rita Monzogranite (Fig. 9). In these granitoids, the old ages obtained are older than their host rocks ages and do not represent the magmatic age. They are interpreted as the ages of the crustal sources. Previous SHRIMP work (Santos et al., submitted) detected the presence of two main zircon populations in sample MA- 32 (Maloquinha Granite type-area), one being magmatic (ages of 1870 f 4 Ma) and the other inherited (2680 f 18 Ma).
The isotopic data indicate Archean crust at depth, but there are no known exposed Archean rocks in the Central Amazon Province. There are large areas poorly known in this province (Tumucumaque in the north and Iriri-Xingu in the south), which may explain the apparent lack of Archean rocks on surface.
Rio Negro province
This province is located in the northwest region of the craton (extreme northwest of Brazil, southwest of Venezuela and southeast of Colombia), and is one of the least-inhabited and least geologically known areas in the world. It is dominated by granitic rocks and local remnants of thick quartz-sandstone sequences (Roraima Group to the east and Tunui Group to the west), covered by extensive Cenozoic sedimentation (south, west, and northwest), and limited to the east by the older Tapaj6s- Parima Orogenic Belt (Fig. 10).
Basement
The Rio Negro basement comprises banded or foliated granitic rocks, which are the host rocks of the Iqana and Uaupks intrusive granitoids. The distribution of these older rocks in the province is still poorly understood and, hence, all available regional maps of the area are different (Pinheiro et al., 1976; Lima and Pires, 1985; Dall'Agnol and Macambira, 1992; Melo and Vilas Boas, 1993; and Sidder and Mendoza, 1995). Moreover, the region has the lowest amount of field data available in the Amazon Craton, making it very difficult to distinguish the basement rocks from the UaupCs Granites. The basement is composed essentially of calc-alkalic granitoids (monzogranites to tonalites), with a chemistry and mineralogy similar to those from UaupCs Suite Granites (biotite + hornblende + titanite). As the UaupCs granitoids are mainly syntectonic, they have been folded and sheared together with the host rocks, and were again locally deformed and sheared during the K'Mudku event (1.20 Ga). Therefore, no consistent structural and compositional criteria have been established to distinguish between the Rio Negro basement and the UaupCs rocks.
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470 J.O.S. SANTOS ET AL.
Fig. 10. Simplified geologic map of the Rio Negro Province (Brazil, Venezuela and ColBmbia border region). Light Gray=Cenozoic cover; Black and white stripes=Piraparana Formation; White=Proterozoic Granitoids (1860 to 1520 Ma); Black=Tunui Group (younger than 1916 Ma) and Dark Grey=Roraima Group. Sample location (0) and results in Ma: (A) Data from this work, (B) Tassinari et al. (1996), (C) Gaudette et al. (1996), (D) Sato and Tassinari (1997), (E) Almeida et al. (1997), (F) Pinheiro et al. (1976), and (G) Priem et al. (1989). SHRIMP U-Pb (11, conventional U-Pb (2), Sm-Nd T,, model-ages (3), Pb-Pb evaporation (4), Rb-Sr (5) and K-Ar (6). Thick lines are the boundaries between the Sunsas (SP), Rio Negro (RNP), Tapaj6s-Parima (TPP), and Central Amazon Provinces (CAP).
The basement is better exposed in Venezuela, in the northern and northeastern zones of the Rio Negro Province. In the South Amazonas Territory of Venezuela, Gaudette and Olszewski (1985) dated several basement samples, including Macabana Gneiss (1847 f 65 MaZ; 1823 k 15 MaW), Minicea Gneiss (1859 k 47 MaZ), Atabapo Gneiss (1793 k 98 MaW), Cassiquiare Granite (1783 f 35 MaW) and the Padamo Granite (1805 f 60 MaW). W = whole-rock Rb-Sr isochron; Z =U-Pb zircon concordia upper intercept. Zircons from another Venezuelan combined sample (6580-6085, Cassiquiare tonalites) yielded a weighted-mean SHRIMP zo7Pb/206Pb age of 1834 k 24 Ma (Tassinari et al., 1996).
Sample MS-63, a niobium ore from the Seis Lagos Carbonatite Complex, is a laterite rich in Nb, Ti, REE and clastic heavy minerals, including zircon. The sample was collected to determine the age of the main clastic sources
to the cover over the Seis Lagos Complex. A relatively small fraction of MS-63 (-500 g) was crushed and processed in the heavy minerals laboratory, producing hundreds of zircons. All zircons appear to belong to only one population, comprising clear and light pink prismatic crystals, 80 pm - 400 pm long, with rounded edges indicating clastic transport. There were no metamict crystals or metamict cores in this population, nor any cores and rims. In some grains (eg. Fig. 4), two SHRIMP spots analyses were made on different internal crystal positions to test the presence of younger rims. However, all the results group into one age population, as also indicated by a small variation in U content (from 120 to 280 ppm) and Th/U ratios (0.42 to 0.99). All eleven U-Pb analyses (Table 7) group on concordia, with a concordant 207Pb/ 206Pb age of 1810 & 9 Ma and a x2 of 0.80 (Figs. 11 to 16).
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U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 471
1 2 3 4 5 6
'07Pb / 235U
I I I I
TAPURUQUARA COMPLEX ,!$~c-512 1 1 GABBRO 0.707c WHOLE ROCK ISOCHRON / -I
MSWD = 0.31 (i) = 0.70374 +0.00029
"'O4v 1 0.703 I I I
0 0 04 0.08 0.16 0.20 87~b/ 86s ;
0 27
3 0 2 6 m m N
1
a 025
W 0 N
0.24
UAUPES INTRUSIVE SUITE SAO GABRIEL GRANITE - AF-1
MSWD = 3.0
2 70 2 90 3 10 3 30 3 50 3 70
207Pb I 235U
IA-MIRIM MONZOGRANITE
1810+9Ma (n=l l ) 22 = 0.80
IA-MIRIM MONZOGRANITE
1810+9Ma (n=l l ) 22 = 0.80
r 1780 fl
4 8 4 9 5 0 5 1 5 2 5 3 207Pb / 2 3 5 u
0 707
0 706
8 \ G O 705 b W
0 704
0 703
I I I I / TAPURUQUARA COMPLEX OLIVINE GABBRO - HC-492 MINERAL ISOCHRON
Olivine and pyroxene
/ Hornbl;y;7 f 189 Ma Plagioclase
I
Plagioclase and biotite
MSWD = 0 13 (I) = 0 70411 r 0 00019
14 I I I I
0 0.04 0.08 0.12 0.16 0.20
87Rbl 86Sr
Figs. 11 to 15. Results of U-Pb zircon (SHRIMP and conventional) and Rb-Sr geochronology from the Rio Negro Province: ll=Tunui Group clastic zircons (IMA, U-Pb); 12 = I&-Mirim Monzogranite (MS-63, SHRIMP U-Pb); 13=Tapuruquara Mafic-ultramafic Suite (HC-492, Rb-Sr whole rock isochron); 14=Tapuruquara Mafic-ultramafic Suite (HC-492, Rb-Sr mineral isochron); and 15=Siio Gabriel Monzogranite (AF-1, U-Pb).
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472 J.O.S. SANTOS ET AL
12"s
62;W
Phanerozoic Basins
Post Sunsas Units - Aguapei I, Pacads Novos. Palmeiral - 4.00Ga.
Sunsas Orogeny - Sunsas, Nova BrasMndia Aguapel II units - >I.lOGa.
I 62'W
The four U-Pb results (1859 f 47; 1834 f 24; 1823 k 15; and 1810 k 9 Ma) indicate that the basement in the Rio Negro Province is younger than that in the Tapaj6s- Parima Province (2.10-2.00 Ga), and is older than that in the RondBnia-Juruena Province (1.70-1.80 Ga). Most of the Gaudette and Olzsewski (1985) data, presented above, come from the Ventuari River basin, indicating that this region does not belong to the Tapaj6s-Parima Province as proposed by Tassinari (1996).
Six Sm-Nd results are available (Sato and Tassinari, 1997) and the crustal formation ages (TDM) have been recalculated, according to the rock ages discussed above (Table 2). Four samples have crustal formation ages in the range 1916 to 2062 Ma, slightly older than the rock crystallization ages obtained by zircon U-Pb analysis (1801 to 1906 Ma, considering the uncertainty ranges). This indicates that the calc-alkalic granitic magmas were generated from a source with no major contribution from continental crust. Two other results trace older sources (2.40 and 2.20 Ga), suggesting a greater crustal contribution. These are the westernmost samples (Fig. lo), and are probably not related to the Transamazonic and Tapaj6s-Parima orogenic belts, because their crustal formation ages are older.
Ima Formation, Tunui'group - M A
The Tunui Group is a folded and metamorphosed sedimentary sequence which crops out in four main mountain areas in the Rio Negro Province, in Brazil (Tunui
- 12"s Juruena Orogeny - Roosevelt and mn Beneficente Groups - 1.70-1 '74 Ga
0 Other Proterozoic Units.
Fig. 16. Simplified geological map of the SW Amazon Craton, showing sample sites (0) and U-Pb zircon ages in Ma. Previous and actual (proposed) positions for the main Sunsas collision front are shown. Based on Scandollara et al. (1996) Rondhia State geological map.
Mountains), or along the Brazil-Col6mbia border (Traira, Caparro-Naquh and Onqa-Caranacoa Mountains), where it hosts all the known gold deposits in the region. The sequence is composed mainly of quartzites (fluvial-braided quartz-sandstones) and conglomerates (lower unit, Maimachi Formation), quartzites and graphite phyllites (Caiio-Loco Formation) and quartzites (top unit, Ima Formation). The Group is more than 2,000 m thick and its sedimentary structures and depositional systems have been compared with those of the Roraima Group (Menezes and Melo, 1994). The minimum age of the Tunui Group is that of two intrusive granitic suites (Iqana and Uaupks), with ages of 1521 (Almeida et al., 1997) and 1518 Ma (this work) respectively, although its depositional and metamorphic ages have not been determined. Pinheiro et al. (1976) describe acid subvolcanic dikes intrusive into the Tunui Group in the Traira Mountains, with an age (Rb-Sr whole-rock isochron) of 1496 k 30 Ma, and dated two muscovite schists, by the K-Ar method (samples PT6 and LP4.1) with ages of 1045 f 19 Ma and 1293 k 18 Ma. Some authors have proposed a Paleoproterozoic or Archean age for the Tunui Group, and interpreted the unit as a greenstone belt (Melo and Vilas Boas, 1993).
To establish the maximum age (source age of the Group) one sample was collected from the upper unit (Ima Formation), in the Caparro Mountains, on the border between Brazil and ColBmbia. The rock is a slightly weathered, coarse quartz-sandstone, which was crushed and panned in the field. The zircons obtained are very
Gondwana Research, V. 3, No. 4,2000
U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 473
heterogeneous, and could not be grouped in well-defined populations. Three groups of grains were analysed (conventional U-Pb), and the age from the concordia plot (upper intercept) has a large uncertainty (57 Ma) and MSWD (5.17), which indicates that the analysed populations are heterogeneous. The age of 1916 k 57 Ma (Fig. 12), despite its high uncertainty, shows that the Tunui Group is not Archean and limits its deposition between 1973 and 1518 Ma (Uaupes Suite age). Santos et al. (unpub. data) have obtained a maximum age of 1950 Ma for the Roraima Group in the Araca Mountains. A similar source age, and sedimentary composition and structures (Menezes and Melo, 1994), allow correlation between the Roraima and Tunui Groups. The latter could represent the Roraima Basin to the west, which was folded and metamorphosed during the Rio Negro collisional event (1.65-1.50 Ga). In support of this concept, the open- folded Serra do Padre may represent the transition in metamorphic grade and deformation between the Neblina flat-lying sandstones to the east and the strong-folded Tunui quartzites to the west (Fig. 10). The Tunui Group rocks may have contributed to the source of the S-type Iqana Intrusive Suite, as discussed by Almeida et al. (1997), based on the geochemical and compositional characteristics of the granitoids and the abundant Tunui Group xenoliths in the S-type granitoids. An imprecise muscovite K-Ar age of 1045 Ma (Pinheiro et al., 1976) reflects regional tectonic reactivation, presumably during the K’Mudku event.
Tapuruquara Gabbro complex
This complex represents three circular intrusions dominated by gabbros, with some anorthosites and ultramafic rocks (websterite, lherzolite). The intrusions are located in the eastern part of the Rio Negro Province, north of the Rio Negro, in the Inambu River basin, Santa Isabel County. This is the westernmost occurrence of a group of 14 gabbro-complexes, which are located mainly in the Tapajos-Parima Province. The main exposed rocks are olivine gabbros and hornblende gabbros, with anorthosite and ultramafic rocks detected in drill cores. The gabbros are very rich in A1,0, (18 - 23%), with low Na,O+K,O and FeO/MgO ratios (Santos and Araujo Neto, 1978).
Previous attempts to date the complex by K-Ar were unsuccessful, due to the very low K,O and radiogenic Ar content of the rocks, leading to meaningless results, with uncertainties greater than 1 billion years. Before the availability of isotopic data, the Tapuruquara Complex was interpreted to be Archean to Paleoproterozoic (Melo and Vilas Boas, 1993; Costi, 1985). Despite the low K and Rb content, four drill-core samples were analyzed for Rb-Sr and Sm-Nd isotopes (HC477,492,508 and 512,
Table 2), resulting in a Rb-Sr isochron with an age of 1607 k 346 Ma. The whole-rock samples were re-analyzed, giving a slightly less imprecise isochron of 1705 f 237 Ma (Fig. 13). Despite the very high uncertainty, the MSWD is low (0.31), indicating that this is a reasonable isochron. The low initial ratios (87Sr/86Sr = 0.7037 f 0.0003) are typical of mantle-derived rocks. In an attempt to improve the Rb-Sr results, four mineral-separates from sample HC492 were analyzed. These separates consist of a plagioclase dominated fraction (floats on bromoform), a mix of plagioclase and biotite (floats on acetylene tetrabromide), a hornblende dominated fraction (floats on methylene iodide) and a fraction of olivine + pyroxene + accessory minerals (sinks in methylene iodine). The Rb-Sr data yield an isochron with an age of 1427 f 189 Ma (Fig. 14) and a MSWD of 0.13. This age is apparently slightly younger than the whole rock age, but both are in the same uncertainty range. The hiatus covered by the two isochrons falls in the 1616-1468 Ma range, or 1542 k 74 Ma. These values, despite the high uncertainties and low reliability of the Rb-Sr method, constrain the minimum age of the basement in this part of the Rio Negro Province. The ages also indicate some temporal correlations with the collisional UaupCs Intrusive Suite to the west, and with the anorogenic anorthosite-rapakivi- charnockite association to the east-northeast (Santos et al., 1999).
Scio Gabriel Monzogranite, Uaupb intrusive suite - AF-1
The most common rocks in the Rio Negro Province are granitoid batholiths related to the Iqana and UaupCs Suites (Dall’Agnol and Macambira, 1992). The Iqana Suite comprises per-aluminous, two-mica (and tourmaline) leucogranites classified as S-type (Almeida et al., 1996) and the UaupCs suite is composed of metaluminous, biotite-titanite-hornblende monzogranites (I-type) . For the Uaupes Suite Dall’Agnol and Macambira (1992), proposed magma generation during continental collision, following the England and Thompson (1986) model.
The Rb-Sr whole-rock age of the Uaupbs Suite is 1459 k 32 Ma, with an 87Sr/86Sr initial ratio of 0.70631 f 0.00117, suggesting a dominantly crustal source for the UaupCs magma. Tassinari et al. (1996) published a U-Pb age of 1521 f 13 Ma for a Papuri River biotite-granite sample. Without the exact location of the sample, it is impossible to define whether the dated sample is related to the UaupCs or Iqana Suite.
In the northern part of the Rio Negro Province, in Venezuela, Gaudette and Olszewski (1985) dated, using Rb-Sr, two granitic bodies, which can be correlated to the UaupCs Suite, the Atabapo Granite (1617 f 90Ma) and the San Carlos Granite (1567 k 25 Ma).
Gondwana Research, V. 3, No. 4,2000
474 J.O.S. SANTOS ET AL.
The Santa Rosa Granite (Ifana Suite) was mapped by Almeida et al. (1997) close to the Papuri River. It is a two-mica granite, with a minor amount of tourmaline and cordierite. The zircons from this rock were dated at 1521 k 32 Ma by Pb-Pb evaporation. Tassinari (1996) reported a Rb-Sr isochron age of 1268 k 23 Ma for the Ifana Suite, which may represent the regional tectonic reactivation during the K’Mudku event.
The Sgo Gabriel Granite sited in the eastern part of the main Uaupks Suite batholith, crops out in S5o Gabriel da Cachoeira town (Fig. 10). Sample AF-1 was collected in a quarry in the town. The rock is a monzogranite with a northwest-southeast protomylonitic foliation produced by supposed old mylonitization (Melo and Was Boas, 1993). The zircons obtained were grouped in four populations, with different magnetic susceptibilities. The U-Pb data lie on a line (Fig. 15) that intercepts concordia at 1518 k 25 Ma (MSWD=3.0), consistent with much of previous Rb-Sr data (Dall’Agnol and Macambira, 1992; Tassinari et al., 1996).
The high 87Sr/86Sr initial ratio and the available Sm- Nd data (four results) from Uaupks Suite rocks, with crustal formation ages (TD,) from 2124 Ma (MIAB16) to 1996 Ma (PT33ASU) indicate that the Uaupes magma originated from a source dominated by an older crustal component (-500 m.y. older). The Tapaj6s-Parima Belt, to the east of the Rio Negro Province, may have been part of the crustal source, because it evolved between 2.10 and 1.90 Ga.
Rondbnia-Juruena province
The basement rocks in the neighboring RondGnia and Juruena areas show distinct Rb-Sr whole-rock isochron ages of 1.45-1.30 Ga and 1.80-1.55, respectively (Tassinari, 1996). Based on this distinction, these areas have been considered different units in the evolution of the craton (Cordani et al., 1979; Tassinari et al., 1996). The recent availability of U-Pb data (conventional and SHRIMP), however, has shown similar ages in both basement areas, in the 1.74-1.54 Ga range (Payolla et al., 1998; Geraldes et al., 1999; this work) indicating that the RondGnia and Juruena regions probably belong to the same province. There are no reliable ages around 1.45 Ga in Rondbnia, to characterize a continental orogenic belt (Rondhia-San Ignacio Orogeny) . The rocks with “San Ignacio” ages are restricted to the Santa Helena Terrain, in northwest Mato Grosso State (Van Schmus et al., 1997; Geraldes et al., 1999).
Roosevelt dacite, Roosevelt group - MQ-96
Sample MQ-96 is a dacite from the Juruena Block, Roosevelt Group. The group is characterized by a sequence of interbedded sedimentary and volcanic rocks,
metamorphosed at greenschist facies and tightly folded with N-S trending fold axes. The volcanic rocks are dacites, rhyolites, rhyodacites, andesites, ash-tuffs, lapilli-tuffs and breccias. Sample MQ-96, collected from a dacite flow, has a small zircon population, with only 135 crystals recovered and placed in the mount. Most zircons in this sample are short and well-defined prisms, and range from 20 ym to 60 ym in length; all lack cores.
Sixteen SHRIMP analyses on 15 crystals indicate a U content of 84 ppm (zircon c.41) to 285 ppm (c.27). All crystals, except c.24, group in one population, giving a 207Pb/206Pb age of 1740 k 8 Ma, with x2 = 1.19 (Table 7, Fig. 17). The c.24 zircon is slightly older, with an age of 1805 k 11 Ma. The magmatic age above (1740 f 8 Ma) correlates well with the basement tonalite age in Rondbnia (1700 k 00 Ma; Payolla et al., 1998) and with the Jauru Complex (Alianqa Gneiss and Cabaqal Meta-tuff) in Mato Grosso (1747 4 13 and 1767 f 24 Ma; Geraldes et al., 1999).
Teleron hill rhyolite, Beneficente group - WO-74
Teleron Hill, near the BR-364 highway in central RondBnia, is a 180 m thick volcano-sedimentary sequence. The lowermost sequence is dominantly volcanic (rhyolites, acid ash-tuffs) and the uppermost sequence is mainly sedimentary (quartzites, hematite quartzites). Scandollara et al. (1996) correlate the Teleron Hill sequence to the Beneficente Group, which is well exposed along the Serra da Providhcia, 15 km to the north. Sample WO-74 is a weathered rhyolitic tuff, which was panned in the field to separate the heavy minerals. The sample is zircon poor, and the zircons are heterogeneous and may represent more than one population. A single population was picked, selecting short, euhedral, translucent to light brown crystals, interpreted to have a magmatic morphology. The U-Pb results are presented in Table 4. The upper intercept age on the concordia diagram (Fig. 18) is 1691 k 73 Ma. This indicates that the rhyolite is older than the surrounding rocks, represented by the Serra da Providencia Granite (mylonitized) . In this case, Teleron Hill may represent a roof pendant in the younger granitoid intrusion (sample WO-63 with a 1,569 Ma U-Pb age - see below). The Teleron Hill Rhyolite age is correlated to the Roosevelt and Dardanelos volcanic sequences, to the east, which have ages in the same uncertainty range.
Jaru paragneiss and Jaru charnockite
Charnockitic rocks are the dominant lithology in central RondBnia State between Ariquemes and Our0 Preto d’Oeste; they are well exposed along 90 km of the BR- 364 highway. These charnockites are spatially related to Serra da ProvidCncia Granitoids, but field evidence of the relationship between the two units is lacking. The
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U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 475
0.30
0.26
3 co m N
.0.22 -0 a .
W 0 N
0.16
0.36 3 I I v I I 3 I I I I I I I I I I I I I 8 I I I I I I I I
- ROOSEVELT GROUP DACITE - MQ-96
- COMEMORACAO GROUP METARHYOLITE - WO-74
-
-
1691 k 73 Ma -
MSWD = 13 1
cx3 m cu 0.32 \
12 co %30 0 r\l
0.10
1740 ?r. 8 Ma (n=15)
, I 8 ,/'
// , ' I I
, 2 8 1 L/
0 30-
0 28 3
00 m 0 2 6 -
Q -
Wn 0 2 4 -
\
0 N
17-1
RONDONIA BASEMENT PARAGNEISS - PT-12
-
-
1400 - L c/) 0.760
(D 00
I 022b / ,,,,,,, /" MSWD 1657 = f 3 80 16 Ma 1200
,/ 0 20
-
/"
0.18 1 l 1 0 y ,,,/"
160 200 2 4 0 280 320 360 400 440
207Pb I 235U
0 28
0 24
3 . 0 2 0
d
co m N
n W
N o 0.16
0 12
0 08 0 1 2 3
'07Pb / 235U 4
I I I
/'
0.14 1 //,,,p" 4 2 3
Pb I 235U 207
0300 > JARU CHARNOCKITE WHOLE ROCK ISOCHRON PT-12
0.780 rn
/ PT-1A
PT-15 1429 k 137 Ma MSWD = 0 19 (i) = 0.7099 k0.0205
0 720
i-/ PT-14 20 0 7 0 0 ' ' ' ' I ' I " I ' I ' I
0 0 4 0 080 120 1 6 0 200 2 4 0 2 8 0
87Rb/ 86Sr l ' l ' l ' l ' l ' l ' l
024 - ARIQUEMES GRANITE - WO-52
0.20 -
0.16 -
3 0 1 2 - co m N \ 4 0.08 -
8 r N 1 MSWD = 0 93
22 I O V ' I ' I ' I ' ' ' ' ' I ' I 0 040 080 120 160 200 240 280
207Pb I 235U Figs. 17 to 22. Results of zircon (SHRIMP and conventional) and Rb-Sr geochronology from the Rondbnia-Juruena Province: 17=Roosevelt Group
Dacite (GR-66, SHRIMP U-Pb); 18=Beneficente Group Metarhyolite (WO-74, U-Pb); 19=Rondbnia Basement Paragneiss (PT-12, U-Pb); 20=Jaru Charnockite (four samples, Rb-Sr whole rock isochron); 21 =Serra da ProvidCncia Mylonite (WO-63, U-Pb) and 22=Ariquemes Granite (WO-52, U-Pb).
Gondwana Research, V. 3, No. 4,2000
476 J.O.S. SANTOS ET AL.
charnockites are often confused with granulites (Costa and Hasui, 1997) or migmatites (Scandollara et al., 1996) where they were overprinted by the Grenvillian-Sunsas mylonitization event. Sample PT-12, a paragneiss surrounded by charnockites (samples PT-1, PT-15, PT-14 and PT-15a), is interpreted as a megaxenolith, with a minimum diameter of 200 meters, and is exposed southeast of Jaru, along BR-364. The field relationship between paragneiss and charnockites is well exposed in a large quarry 8 km southeast of Ariquemes, where fragments of paragneiss, with dimension of 1-2 m, are surrounded by charnockite. In the same quarry, there is an 80 cm thick charnockite dyke cutting the paragneiss. Both paragneisses are correlated and interpreted as part of the basement of the Rondbnia-Juruena Province. Sample PT-12 produced a zircon concentrate, in which the main population of small, brownish and slightly rounded crystals was selected for U-Pb analysis. The upper intercept on the concordia plot (Fig. 19) corresponds to an age of 1657 f 16 Ma (MSWD=3.8), which is the age of the main clastic source of the paragneiss.
Four charnockites (Table 2) plot on a Rb-Sr whole-rock errorchron, which yields an age of 1429 k 137 Ma (87Sr/86Sri= 0.7099). The paragneiss plots above the isochron, indicating that it is older (Fig. 20). The minimum age of the Jaru Charnockite is 1429 Ma, but assuming that it is related or cut by the Serra da ProvidCncia Granite, its magmatic age may be at least 120 m.y. older. Payolla et al. (1998) reported a slightly older zircon U-Pb age (1477 f 14 Ma) from a charnockitic rock collected in the same region (Ariquemes-Jaru). There were not enough zircons in the charnockite samples to be analyzed by
Our0 Preto granite - Serra da Provid2ncia intrusive suite
U-Pb.
- WO-63
This sample, an augen-gneiss in the southern and deformed zone of the Serra da Providencia Batholith, named the Our0 Preto Granite, was collected 2 km southeast of Our0 Preto, along the BR-364 highway, Central RondBnia. The Brazilian Geological Survey (Scandollara et al., 1996) showed that the Serra da Providencia Granite (anorogenic rapakivi) was strongly affected by shear belts, and was transformed into mylonite gneiss. Previously, all occurrences of mylonitized post- tectonic granites in RondGnia were interpreted as pre-Serra da Providencia, basement rocks.
The zircons from sample WO-63 are homogeneous, consisting of a single population of light to dark brown, subhedral to euhedral zircons, with length to width ratios ranging from 1: l to 3:l. This morphology implies an igneous source. The U-Pb results are presented in Table 3 and the concordia diagram in Fig. 21. The upper intercept
yields an age of 1569 f 18 Ma, with an MSWD of 0.51. This age is correlated to other Serra da ProvidCncia Batholith U-Pb ages (1566 f 3 Ma, Bettencourt et al., 1999; 1544 f 4 Ma, Payolla et al., 1998; 1588 k 16 Ma, Tassinari et al., 1996) and to post-tectonic granites in the Jauru Block (extreme southwest of the Craton, Mato Grosso State), dated at 1550 f 7 Ma (Geraldes et al., 1999). Correlations are made to other rapakivi granites in the Amazon Craton, such as Surucucus (1551 Ma, Santos et al., 1999), Mucajai (1544 Ma, Santos and Olszewski, 1988) and Parguaza (1540 Ma, Gaudette et al., 1996).
Ariquemes granite. Early-Sunsas assemblage - WO-52
An evolved biotite syenogranite crops out southeast of Ariquemes town along BR-364 Highway, from which four zircon populations were picked. The zircons are U rich (1900-3000 ppm), cloudy short prisms, with very few crystals suitable for analysis. The four populations show discordant results, which align in an intercept at 1352 f 8 Ma, with MSWD of 0.93 (Fig. 22). There is a correlation of this age to the zircon U-Pb ages (1346 f 5 Ma and 1338 f 5 Ma) obtained by Bettencourt et al. (1999) for the Candeias Intrusive Suite (SW from Ariquemes).
Santa Helena terrain
The Santa Helena terrain in western Mato Grosso State is a Precambrian area limited by extensive sedimentary covers to the west (Guapor6 Cenozoic basin), south (Pantanal Cenozoic basin) and northeast (Parecis phanerozoic basin) and constituted by granitic rocks. Both granites and crust were formed during Mesoproterozoic, according to Geraldes et al. (1999). Magmatic ages were determined by conventional U-Pb dating and correspond to the ages of 1463 k 4 Ma (Lavrinhas Tonalite), 1488 f 11 Ma (Pau-a-Pique Tonalite), 1442 f 15 Ma and 1450 f 13 Ma (Trihgulo Farm Granodiorite) . The E,, (t) values are positive (+ 2.9 to + 4.6) and the Nd T,, ages (1491, 1500, 1532, 1550, 1563 and 1567 Ma) are close to the magmatic ages, indicating little or no crustal component in the granitoids generation (Geraldes et al., 1999). The above isotopic data are evidence for a juvenile Mesoproterozoic crust in southwest Amazon Craton. However, the Santa Helena Terrain may be an allochthonous terrane accreted to the Amazon Craton during the Grenvillian-Sunsas collision. There is a good correlation to the Eastern Granite-Rhyolite Province (1480- 1420 Ma) in Laurentia (USA). Plutons of this age also occur throughout the Laurentian margin belt in Ontario.
Sunsas province Rocks with Late Mesoproterozoic (Estenian) ages have
been recognized in the extreme southwest of the Craton
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U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 477
since Priem et al. (1971), when the tin-granites of RondGnia were dated by Rb-Sr (101 1 Ma) and related to Grenvillian orogeny in Laurentia. Grenvillian rocks generated during the Sunsas Orogeny are known in ColGmbia as Garzon Complex (Priem et al., 1989), Yaca- Yaca Rhyodacite (Priem et al., 1982), Tijereto Granophyre and Piraparana Formation (Galvis et al., 1979). The importance of this orogeny in the Proterozoic of Venezuela was detected by Goldstein et al. (1997) analysing clastic zircons from lower Orinoco River sand, where the second statistical population (15 zircons) groups around the age of 1.12 Ga.
The narrow westernmost part of the Amazon Craton has been included in Rodinia continent reconstructions (Renne et al., 1989; Dalziel, 1992), in which the Rodinia continent resulted from the collision and fusion of AmazGnia and Laurentia around 1.20-1.10 Ga. Only the western part of this orogeny appears to be juvenile, with some granulite restites cropping out close to the contact of the Eastern Andes Belt with the Amazon Cenozoic cover, known as the Colombian Garzon Complex and the Peruvian Arequipa Massif (Fig. 1). The effects of this collision over the Amazon Craton are shearing and thrusting, which affected mainly the RondGnia-Juruena rocks, producing mylonites and rejuvenated Rb-Sr ages. The K’Mudku Shear Belt, in the north-central zone of the Craton, may be related to this collision. Litherland et al. (1989) proposed the existence of an orogenic belt in Bolivia, called Sunsas, with a -1000 Ma age and northwest trend, that can be correlated with the Aguapei belt in Brazil (Fig. 16). More recently, Tassinari et al. (1996) grouped the Aguapei and Sunsas orogenic belts into one mobile belt (1.25-1.00 Ga). However, the ages of the Sunsas Belt post-tectonic younger rocks (tin- granites) are slightly younger than 1.00 Ga (0.99 Ga, Rio Pardo Granite, Rizzotto et al., 1999; 0.99 Ga, Maqangana and Sgo Carlos granites, Bettencourt et al., 1999), leading to move the end of the Sunsas Cycle from 1.00 to 0.99 Ga.
The juvenile part of the orogeny is sited more than 1,500 km to the west (perpendicular to the belt, Fig. 1) and it is possible that, in the craton, collisional deformation of older rocks dominates the Sunsas Belt. There are ten available Sm-Nd T,, model ages from this belt (Table 3), including the youngest in the craton (1 180 Ma, 1140 (2) and 1060 Ma; Sat0 and Tassinari, 1997). These four results were obtained in mafic rocks, which may represent the post-tectonic Nova Floresta and Siriquiqui magmatism, unrelated to a juvenile crust. The post-tectonic, post-Grenvillian rocks correspond to the rapakivi granite suite (part of the classic Rondonian tin- granites), alkaline pipes (Teotanio), alkali basalts (Nova Floresta and Siriquiqui) and a sedimentary cover (Pacaas
Novos Formation). All these units have ages (Rb-Sr and K-Ar) between 1.10 and 1.00 Ga (Tassinari, 1996; Priem et al., 1971). Bettencourt et al. (1999) reported U-Pb ages between 974 k 6 Ma and 1082 k 5 Ma for six Rondonian tin-granites (Siio Carlos, Maqangana, Pedra Branca, Oriente NOVO, Santa Clara and Manteiga). Another post- Sunsas granite, the Rio Pardo Suite, has a similar zircon U-Pb age (995 k 15 Ma, Rizzotto et al., 1999).
The Grenvillian-Sunsas collision overprinted juvenile (1.75-1.70 Ga) and post-tectonic (1.55 Ga) rocks from the RondGnia-Juruena Orogenic Belt, and totally or partially rejuvenated Rb-Sr ages, which now may be anywhere between 1.75 and 1.10 Ga.
Paragneiss, Nova Brasiliindia metamorphic suite - GR-66
Sample GR-66, from Central RondGnia State, is a paragneiss from the Nova Brasildndia Metamorphic Suite, a volcano-sedimentary sequence metamorphosed to predominantly amphibolite facies. Because of the high metamorphic grade, suite is preferred to sequence (cf. Scandollara et al., 1996). The rock types are mica-quartz schist, sillimanite schist, biotite paragneiss, amphibolite, quartzite, metabasite, meta-tuff, tremolite-schist, phyllite and hematite quartzite. The Nova Brasildndia Suite is widespread in the central, south and southeast areas of RondGnia State (Fig. 16). Sample GR-66 was collected in the type-area of the Nova Brasildndia Suite, in RondGnia State, close to the town of Nova Brasildndia do Oeste (GR66, UTM co-ordinates: 63-604741; 08691091).
Sample GR66 is a meta-sandstone and produced abundant zircons, with approximately 400 placed in an epoxy mount for SHRIMP analyses. The zircons are up to 60 pm long, and normally in the 20 pm-40 pm range. This population is heterogeneous, with variable shapes and colors, reflecting several sources, so 24 crystals were analyzed, with one analysis on each grain (Table 7). The concordia plot (Fig. 23) shows that the ages spread from 2090 Ma to 1103 Ma. The 207Pb/206Pb age is more precise for the older zircons (> 1700 Ma) and 206Pb/238U ages were selected for the individual ages of younger crystals (< 1700 Ma). The younger zircons were grouped into three populations (Fig. 24), with ages of 1417 k 35 Ma (x2 = 0.89), 1320 & 20 Ma (x2 = 0.21) and 1211 & 18 Ma (x2 = 1.23).
The oldest zircons from the paragneiss (2090 and 2020 Ma) have ages comparable to the oldest rocks in the Tapaj6s-Parima Orogenic Belt, located 400 km to the east. The age of 1904 Ma possibly correlates to the youngest rocks in the Tapaj6s-Parima Province (Parauari Intrusive Suite). Some clastic contribution from the Roosevelt volcano-sedimentary belt is indicated by zircons with ages of 1748 and 1716 Ma. The Serra da Providkncia rapakivi- type granites are considered to be part of the Nova
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478 J.O.S. SANTOS ET AL.
0 40
0 36
3 03 0 3 2 m N \ a 0 28
W a.
0 2 4
0 20
I I I I I I ’ I I I 1 21001
NOVA BRASlL66~NDIA METAMORPHIC PARAGNEISS - GR-66 CLASTIC ZIRCONS
I5O0 \ f l 7 I 6 1 5 9 9 i 4 2 M a l7 Ma
DETAIL ON FIGURE 24 __
2090 * 17 Ma
-
-
-
23 - I I I I I I I 1 I I
2 0 3 0 4 0 5 0 6 0 7 0
207Pb / 2 3 5 u
0 26 I I I
PARAGNEISS. GR-66
1 8 2 0 2 2 2 4 2 6 2 8 3 0 3 2
Figs. 23 and 24. Results of U-Pb zircon SHRIMP geochronology from the Sunsas Province: 23=Nova Brasilhdia Paragneiss (GR-66, SHRIMP U-Pb, all zircon grains) and 24=Nova Brasildndia Paragneiss (GR-66, SHRIMP U-Pb, younger populations).
Brasildndia sedimentation source, as suggested by the zircons with ages of 1546 and 1540 Ma. The youngest zircon population constrains the maximum age of the Nova Brasildndia sedimentation at 1211 f 18 Ma. The minimum age is constrained by the 11 10 f 15 Ma age of the granite generated by partial melting of the paragneiss (Rizzotto et al., 1999). The sedimentation and metamorphism of sample GR66 is then limited to the 1211-1110 Ma time interval. The two main populations in GR-66 have ages of 1320 f 20 Ma (five spots) and 1215 f 20 Ma (seven spots). This indicates intensive magmatism prior to sedimentation, and suggests that the collisional orogenic process lasted at least -105 m.y. from 1320 to 1215 Ma. The 1320 Ma age correlates well with the granulite metamorphism age determined by Tassinari et al. (1999) at 1331 f 8 Ma and to the early-K’Mudku age (1317 f 4 Ma) determined in Roraima Group
muscovites by Ar-Ar (J.O. Santos, unpub. data). These ages are comparable to the Composite Arc belt age in Laurentia and suggest that the Sunsas Orogenic Cycle started prior to 1.25 Ga, around 1.33 Ga.
K’Mudku shear belt
The K’Mudku Shear Belt is an approximately 200 km wide, NE-trending shear belt, which has produced mylonitization and local rock melting in four provinces of the Amazon Craton at ca. 1.20 Ga, affecting rocks from 2.20 Ga to 1.52 Ga in age. The belt (Fig. 1) represents the zone most affected by shearing, being composed of several parallel to subparallel individual shear zones. The main belt, as indicated in Fig. 1, is known as K‘Mudku (Brazil and Guyana, since Barron, 1966) and Nickerie (Priem at al., 1971). Other northeast trending 1.20 Ga shear belts, named as Jari-Falsino (Amap6 State, Brazil, Lima et al., 1974), Orinoquense (Venezuela, Bellizzia, 1974; Restrepo- Pace et al., 1997) and Madeirense (RondGnia, Amaral, 1974) are correlated to the main belt, but not shown in Fig. 1. Within the main belt there are strong variations in strain, and KMudku shearing may actually extend beyond the limit of the main belt. Rock units in this belt show strong mylonitization along a NE trend, and have been recognized and named in Surinam as Bakhuis and Falawatra, in Guyana as Kanuku and Rupununi, and in Brazil as Rupununi, Serra da Lua and Kanuku. Due to the intensity of the deformation, these rocks were interpreted as the oldest units in the northern part of the Amazon Craton and mapped as Archean (Lima, 1999; Melo and Vilas Boas, 1993; Costa and Hasui, 1997).
According to Berrangk (1977), the intensity of KMudku metamorphism in Guyana reaches high metamorphic grade and produced local rock melting. This indicates that granitic bodies generated by the K’Mudku melting may be present along the belt. The great majority of the granulites mapped in the center-north zone of the Craton appear to be Mesoproterozoic igneous charnockites deformed by the K’Mudku shear zones under amphibolite facies. This applies to other “granulites” in north Amazonas State (Camanau, Pardo and Curiuad “granulites”; Santos et al., 1974) and south (Jufari and Jauaperi; Santos et al., 1974) and central Roraima State (Lua, Cigana, Caracarai, Prata and Barauana “granulites”). Relationships should be tested also in the Kanuku (Guyana) and Bakhuis (Surinam) “granulites”.
Shear-belt age and evolution remain uncertain, although Fraga and Reis (1995a) have indicated the predominance of oblique thrusts, with principal stresses from the NW, as the main tectonic feature in the belt. Several rocks affected by the KMudku shearing have K- Ar and Rb-Sr ages around 1.20 Ga in Roraima (Amaral, 1974), Suriname (Priem et. al., 1971), Rio Negro
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U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 479
(Tassinari, 1996), ColBmbia (Pinson et al., 1962), Amapa (Lima et al., 1974) and in Guyana (Barron, 1966). There are 62 K’Mudku “rejuvenated” K-Ar and Rb-Sr ages (1.35 Ga - 0.98 Ga) available in the bibliography above cited. On the other hand, as the K’Mudku event produced mylonite zones in both Uaup4s Suite granitoids in the Rio Negro Province (1520 Ma) and in the Mucajai Rapakivi Granite in Roraima (1,544 Ma; Gaudette et al., 1996), 1.52 Ga would be the maximum age for the shear belt. This indicates that the hypothesis that the KMudku belt was generated by reactivation of older (Archean?) structures (Melo and Vilas Boas, 1993) is less plausible. A new Ar-Ar study, dating micas generated during K’Mudku mylonitization (Santos et al., ZOOO), has determined ages between 1317 k 4 Ma (Roraima Group, Araca Mountain), 1244 k 5 Ma (SR-189) and 1198 k 4 Ma (SR-144, Urariquera Complex, Roraima). The KMudku (and other locally named k 1.20 Ga shear belts) is interpreted as the intracratonic reflex of the continental Grenvilian-Sunsas collision in the western-northwestern Amazon Craton.
Discussion and Regional Implications
According to the data available, seven main geological Provinces, and the main processes, which produced them, are recognised (Fig. 1, Table 1). In some cases, the relationship between neighboring provinces is clear, based on new data (mapping, U-Pb geochronology, airborne geophysics), however in many cases the relationship remains speculative, due to the extensive sedimentary cover and the lack of, or limited, mapping and geochronological constraints. It is clear that the Sunsas Belt overthrusts the RondBnia-Juruena Belt, and that this last orogeny consumed the southernmost zone of the Tapaj6s-Parima Orogeny. The Central Amazon rocks clearly cut the west-northwest structure of the Carajis greenstone belts. Nevertheless, the relationship between the Rio Negro and Rondbnia-Juruena Provinces, as well as between the Rio Negro and Tapaj6s-Parima, remain uncertain. The Amazon Craton was originated mainly during Paleo and Mesoproterozoic times, with an Archean area smaller than previously considered (Teixeira et al., 1989; Tassinari, 1996).
Imataca province
The Imataca Complex evolution will be better understood only when U-Pb isotopic data are available. The Paleoarchean Imataca block may be an allochthonous terrane pasted to the Craton along the Guri Shear Zone (Teixeira et al., 1999). The Imatac’a belt cuts the main Transamazonian structures indicating that the amalgamation process is late-Transamazonian (ca. 2.00
Ga), or more probably post-Transamazonian. The only known continental collisional event after 2.00 Ga is the Grenvillian-Sunsas Orogeny, indicating that the Imataca accretion may be as young as 1.10 Ga. Based on Ar-Ar plateaus, Onstott et al. (1989) suggested tectonic activity at 1.10 Ga and the Grenville-Sunsas presence close to Imataca region was detected by Goldstein et al. (1997), which found 15 zircons in Orinoco River sand with ages in the 1338 Ma - 962 Ma range (grouped around 1.12 Ga on a histogram). Other possibility is that Imataca and the other two older remnants (Cupixi in Amapa and Pium in Carajds) were linked during the Paleoarchean (> 3.10 Ga), composing the Amazon Craton precursor.
Carajh province
The distribution of Archean crust
The craton has been considered predominantly Archean, as shown in the Brazil geological map, 1984 edition (Schobbenhaus et al., 1984) and in Costa and Hasui (1997), although this study shows that it is restricted to the Carajas (extended to the south Amap6 and Bacaja areas) and Imataca provinces (i.e. less than 12 Yo of the craton area). The Central Amazon Province, despite its precursor Archean crust, is much younger and truncates the Carajas west-northwest tectonic trend. Thus, the exposed Archean rocks comprise only <12 YO of the Craton, which is predominantly Paleoproterozoic and Mesoproterozoic.
The Xingu complex problem
The great majority of the granitic rocks that surround the strongly folded greenstone sequences have been mapped as Xingu Complex, and interpreted as older basement (Aracjo et al., 1994). Some of these granitoids are late-tectonic and were dated by U-Pb in zircon, such as the Estrela and Old Salobre Granites, and are younger than the greenstones. This indicates that the Xingu Complex concept should be revised to include only pre- greenstone rocks. As the Complex is older than 2.85 Ga, the Xingu prefix should not be used to designate the Proterozoic basement in the Tapajds, Rondbnia and Juruena regions.
Transamazonic province
The names Transamazonian (Cycle) and Transamazonic (Province) are widely used in the geology of other provinces in Brazil. In some cases, Transamazonian is used, as a synonym for Paleoproterozoic but this practice should be abandoned. The misuse of the name is requiring clarification. As the classic Transamazonian Orogeny is almost entirely Riacian, correlations to the Transamazonian should be restricted to rocks formed
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approximately between 2.25 and 2.00 Ga. Comparisons related to the nature of the terrain should be limited to the granite-greenstone style of orogenic rocks.
Tapajds-Parima province
The evolution of the orogenic belt
The geochronologic constraints and results from the new mapping program in the Tapaj6s Province have led to a more detailed timing of events for the Tapaj6s-Parima Orogeny. The U-Pb age of 2.1 Ga obtained from clastic zircons from the Sai-Cinza turbidite sequence is tentatively related to oceanic primordial magmatism (Buiuy ocean basalts, not dated). Trench and back-arc sedimentation (Sai-Cinza turbidites) occurred between 2.10 - 2.03 Ga, being younger than the Buiup ocean basalts and older than, or contemporaneous with, the earliest calc-alkalic magmatic arc (Cui6-Cuiu Complex), which formed at 2.20 - 2.00 Ga. The second calc-alkalic magmatic arc (Parauari Suite) formed later at 1.92 - 1.88 Ga. Fore-arc to intra-arc sedimentation is represented by the Sequeiro and Abacaxis Formations, formed at 1.89 - 1.87 Ga. An intracratonic, post-tectonic volcano-plutonic assemblage (Maloquinha Suite and Iriri Group) was emplaced between 1.87 - 1.86 Ga. Some of the gold mineralization is constrained at 1.87 -1.85 Ga (Santos et al., submitted).
Continental, platformal sedimentation of the Palmares Group is interpreted to have developed at 1.80-1.75 Ga. Finally, continental tholeiitic magmatism (Crepori Gabbro- Dolerites) and intrusion of a rapakivi-granite suite (Teles Pires Granite) probably occurred at 1.76- 1.70 Ga.
There is an overall zonation within the belt, from the metabasalts in the western area, to the cratonic rocks in the east, as follows: I) metabasalts, ii) turbidites, iii) first magmatic arc, iv) fore arc and intra-arc sedimentary rocks, v) second magmatic arc, and vi) post-tectonic magmatism.
The western limits of Tapajds-Parima orogeny
The Buiup ocean basalts are restricted to four outcrops in the westernmost part of the Tapaj6s-Parima Orogenic Belt. The poor development of oceanic and accretionary sequences may be explained in two ways: i) the more primitive zone of the TPOB underlies the extensive Cachimbo Graben; or ii) the western part of the TPOB (and of the Amazon Craton) was removed during an interpreted continental break-up at the end of Paleoproterozoic (1.78 Ga?), which may be reflected by tholeiitic magmatism represented by thick sills and the giant diabase-gabbro dike swarm. In the north (Roraima- Venezuela-Guiana), this widespread magmatism is termed the Avanavero Suite and, in the south, the Crepori Suite (Bizinella et al., 1980).
The southeast and northwest correlations
Santos and Reis Net0 (1982) considered the Juruena Granite from the Alta Floresta area comparable in chemistry and age to the Parauari Suite in the Tapaj6s region. Recent geological mapping in the Alta Floresta Domain (Valente, 1998) identified the same main units present in the Tapaj6s Province: Jacareacanga Group (Fabinho and Domingos mine pits); Cuiu-CuiG Complex (Rato, Fazenda Mogno, Paraiba, Melado, Gaspar and Naiuram) ; and Parauari Suite (Vila Guarita, Armando, Sede and Levi). The Tapajos geology may be correlated with units to the northwest and southeast, extending the orogenic belt to Roraima (Parima Domain) and Mato Grosso (Alta Floresta Domain). This correlation is indicated by a number of features, including: i) orogenic rocks dominated by fine grained metasedimentary rocks and minor metabasalts; ii) large volume of calc-alkalic, magmatic-arc volcano-plutonic rocks; iii) similar northwest tectonic trends, with 0, of the orogenic stress fields at - 250"-230"; iv) hundreds of alluvial gold deposits; and v) orogenic lode-type and porphyry-type gold mineralization. Correlation of the Tapaj6s Province with the Ventuari region of Venezuela (Tassinari, 1996) is unlikely, because there are many geologic, isotopic and metallogenic dissimilarities between the two regions, the most important is the age of the Tapaj6s basement, which is 250 to 290 m.y. older than the Ventuari basement. The Ventuari region is included below in the Rio Negro Granitic Province, based on available U-Pb (Gaudette et al., 1996) and Sm-Nd data (Table 3).
Central Amazon province
The origin of the Uatuma magmatism
The Central Amazon Province has the same regional north-northwest trend as the Tapaj6s-Parima Province (Fig. l ) , and clearly truncates the west-northwest Carajas tectonic trend. The Uatuma rocks occur in an anorogenic setting and show the main characteristics of A-type rocks, such as high F, Nb and Y contents, bird-wing REE plots, high magma temperature and very high crustal-level of emplacement. Partial melting of I-type granites is the probable source for A-type rocks (Clemens et al., 1986). Potential sources for this magmatism are the neighbouring provinces, Carajas to the east and Tapaj6s-Parima to the west, which are both largely dominated by calc-alkalic, I- type granitoids. The distribution of the Uatumii A-type rocks over 1.1 million km2 indicates high temperature magmatism at very shallow depths over a wide area, in a continental-scale process. A model to explain the source of the Uatumii Magmatism is proposed below.
The Tapaj6s-Parima Orogeny is interpreted as an ocean- continent orogeny, with 0, of the far-field stress at about
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250". Low-angle slab subduction is considered to have started at about 2.10 Ga beneath the Archean continental crust, which was the extension of the Carajas Province to the west. At the end of the generation of the second magmatic arc (Parauari Arc) at 1.90 Ga, the descending slab, now dipping at a very low angle, is interpreted to have migrated to the discontinuity between the continental crust and the lithosphere. The hydrated fertile slab is considered to have moved - 900 km along the discontinuity and heated the overlying zone, allowing partial melting of the calc-alkalic crust to produce the high-temperature (> 800") and low-pressure A-type magma. For example, Douce (1997) shows that such generation of A-type melts may occur at pressures as low as 4 kilobars. It is assumed too, that the magma produced was highly fractionated and with a low-viscosity, possibly due to its high F content, as indicated by high F in the UatumZi rocks.
Rio Negro province
The separation of the Juruena domain from the Rio Negro province
The Rio Negro and Juruena Domain were previously interpreted to be linked (Rio Negro-Juruena Mobile Province), based mainly on Rb-Sr data and structural similarities (Cordani et al., 1979, Tassinari et al., 1996). The whole-rock Rb-Sr isochrons in both areas indicate ages in the range 1.80-1.55 Ga, and both areas were proposed to be in a mobile belt with a NW trend. The new U-Pb data, in conjunction with new geological maps (Scandollara et al., 1996; Valente, 3998), show that the Juruena and Rio Negro Provinces each had a distinct evolution at different times during the Proterozoic and probably are not related. The main structural trend in the Juruena Province is related to WNW to E-W folding in the supracrustal sequences (Roosevelt, Beneficente and Teles Pires volcano-sedimentary sequences), which is evident from the LANDSAT images and the new maps (Scandollara et al., 1996; Valente, 1998). This trend dominates a large area, with an along-strike length of 600 km, from Peixoto Azevedo in Mato Grosso State to northeast Rondbnia State. In the Rio Negro Province, there are no dominant trends, and the fold belts appear to be related to the emplacement or displacement of several batholiths (Uaupks and Iqana Suites). The trends are highly variable, as shown in five fold belts: i) Padre Mountain - N70°W, ii) Tunui Mountain - N30°E; iii) Traira Mountain - N3OoW; iv) Caparro-Maimachi Mountain - NIOOE; and v) Onqa- Caranacoa Mountain - N-S; N45 W.
The Rb-Sr isochrons from the Juruena Province yield ages comparable to the ages of the UaupCs and Iqana Granite Suites in the Rio Negro Province. Earlier samples
gave Rb-Sr ages of 1.56 Ga for the Roosevelt Formation and 1.65 Ga for the Teles Pires volcanic rocks. The new U-Pb ages of volcanic rocks in the Roosevelt and Teles Pires sequences are roughly 200 m.y. older than the previous Rb-Sr results. The U-Pb ages from the basement rocks are mainly 1.75-1.70 Ga in the Juruena Domain and 1.86-1.79 Ga in the Rio Negro Province.
Supracrustal volcano-sedimentary sequences are common in the Juruena Domain, but rare, and exclusively sedimentary, in the Rio Negro Province (Tunui Group). The Rio Negro Province is composed essentially of collisional granites (Iqana and Uaupes Suites), which cover more than 80 percent of the exposed area. In the Juruena and Rondbnia Domains, rapakivi granites and charnockites are dominant as intrusive rocks but these rock-types are completely absent in the Rio Negro Province. The presence of large collisional batholiths (Iqana Suite, S- type; UaupCs Suite, I- type; Almeida et al., 1997), with U-Pb ages in the range of 1550 to 1520 Ma, indicate a continental collisional process in the northwestern part of the Amazon Craton. The age of this collision, possibly equivalent to the granite ages of 1.55- 1.52 Ga, remains uncertain, due to the strong deformation of the collisional granites by the ENE-trending K'Mudku shear belt, which has a proposed age (K-Ar data, Barron, 1966) of around 1.20 Ga.
Rondbnia-Juruena province
Regional implications
The tectonic boundary between the Juruena and Rondbnia areas, as delineated by Cordani et al. (1979) and Teixeira and Tassinari (1996), was questioned by Bettencourt et al. (1987) and by Payolla et al. (1998), who suggested its re-evaluation. The U-Pb and Sm-Nd data from the basement samples from both areas are similar, with U-Pb ages in the range 1.97 to 1.69 Ga. The dominant tectonic regime in both areas is the same (WNW to E-W) as that shown in the new Rondbnia State (Scandollara et al., 1996) and the Alta Floresta region (Valente, 1998) geological maps, released recently by the Brazilian Geological Survey. The trend is evident in the LANDSAT images, especially in the Roosevelt River area, where Rizzotto et al. (1996) recognized a volcano- sedimentary association. A dacite from the Roosevelt Formation has a 1.74 Ga magmatic age, with no inherited zircons (Santos et al., 1999), and similar results have been obtained to the east, in the AripuanZi River (Dardanelos), Mato Grosso State (J. Leite, verbal comm., 1999). The E,, of the Rondbnia-Juruena Province rocks plot between the depleted mantle reservoir growth curve (De Paolo, 1988) and CHUR. The initial ratios for these rocks correspond to cNd values between $1.65 and +3.81 (for
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482 J.O.S. SANTOS ET AL. ~
T= 1100 to 1970 Ma), including the post-tectonic rapakivi granites at 1570 Ma and the tholeiites at 1100 Ma. The range in E~~ values indicates that the calc-alkalic magmas were derived from a source with very limited interaction with the continental crust. These juvenile volcano- sedimentary sequences (Teleron Hill Rhyolite, Jaru Metapelite, Roosevelt Dacite, Dardanelos Dacite, Mutum- Parana Tuff) form a 900 km long belt, which extends from northwest Mato Grosso through southeast Amazonas and central RondBnia to east Acre States, and represents new crust accreted to the Parima-Tapaj6s Belt in the east. The belt is named the Guapork Orogenic Belt, because the majority of the streams draining the belt flow into the Guapork River. Correlations to the Jauru block (Mato Grosso State) are made to the Cabaqal Meta-tuff and to the Alianqa Gneiss, with U-Pb ages of 1767 k 24 and 1747 f 13 Ma (Geraldes et al., 1999).
The orogenic cycle was followed by the post-tectonic Serra da Providhcia rapakivi granite suite; the Jaru Charnockites, with U-Pb ages of 1.57-1.55 Ga; and by continental sedimentation and basaltic volcanism (PacaBs Novos Formation and Nova Floresta Basalt, with K-Ar ages of 1100 Ma). The granitoids are strongly affected by Grenvillian-Sunsas thrusting (1.20-1.10 Ga?), and are locally mylonitized (Scandollara et al., 1996). Sample WO-63, collected south of Ariquemes, along the BR364 highway, is an example of the mylonitization of the Serra da Providhcia-type granite. Most rocks previously classified as granulites in central RondBnia (Jaru and Our0 Preto areas), are actually 1.57 Ga charnockites mylonitized by the Grenvilean-Sunsas collision.
Geraldes et al. (1999) reported equivalent U-Pb ages for some granitoids in the Jauru block (Mato Grosso), as the Cachoeirinha Granite (1536 f 11 Ma; T,,= 1754 Ma), Cabagal Tonalite (1540 k 16 Ma; T,,=1773 Ma), Santa Cruz Gneiss (1556 f 2 Ma), Agua Clara Graniodiorite (1567 f 6 Ma), Alvorada (1522 f 12 Ma; T,,=1743 Ma) and Araputanga granites (1522 f 12 Ma; T,,= 1777 Ma). Payolla e t al. (1998) record zircons from syenogranitic gneiss with a U-Pb age of 1526 f 12 Ma. In the same sample, the monazite U-Pb age is 1200 Ma, corresponding to the Sunsas metamorphic age. The Nd model ages indicate the juvenile volcano-sedimentary sequences above described as the more probable source for these granitoids. Both the orogenic rocks (1.74-1.70 Ga) and the post-tectonic charnockites and granites (1.57- 1.53 Ga) have a variety of Rb-Sr ages between 1.70-1.57 and 1.10 Ga. The younger, scattered and reset Rb-Sr ages from the RondBnia Block can be explained by their greater proximity to the Sunsas Collisional Belt (Grenvillian) in the southwest.
In some areas, the T,, model ages in the Rondbnia- Juruena Province are in the 2.10-1.90 Ga range. This may
indicate some crustal contribution, possibly from the Tapajos-Parima Province, because the tectonic trend (E- W) of the RJP truncates the Tapaj6s-Parima Province (North Mato Grosso, Alta Floresta region; Valente, 1998) in the easternmost part of the Rondbnia-Juruena Province.
Implications for Uatumii magmatism
All the Proterozoic calc-alkalic volcanic rocks from both the RondBnia-Juruena and Central Amazon Provinces were grouped previously in the UatumB Supergroup. Redefinition of the age and distribution of the Uatum5 Supergroup magmatism ( Santos, 1982) is required. Basei (1977), based on Rb-Sr data grouped the UatumB volcanic rocks into four main units: Iriri (1765 f 16 Ma), IricoumC (1790 k 20 Ma) Teles Pires (1680 !c 13 Ma) and Surumu (1860 f 28 Ma). The more precise U-Pb ages in zircons (Santos et al., submitted) are: 1870 f 8 Ma (Iriri), 1862 f 7 Ma (Iricoumk), 1740 f 12 Ma (Teles Pires) and 1960 f 6 Ma (Surumu). The Uatum5 type-area is enclosed by the Iricoumk Group, at UatumB River, Amazonas State and the Uatum5 Magmatism appears to be restricted to this area and to the correlated Iriri Group to the south. The zircon U-Pb ages above suggest a correlation between Iriri and Iricoumk volcanics and indicate that the Surumu volcanism is - 100 Ma older and the Teles Pires volcanism is -120 Ma younger.
The UatumB magmatism is the product of an anorogenic process that has occurred around 1880-1870 Ma, involving the partial melting of an Archean crustal source. The rocks with these characteristics correspond to the Iriri, IricoumC and part of the Surumu Groups, and are restricted to the Central Amazon Province. Other calc- alkalic volcanic rocks (Roosevelt, Rio Branco, Teles Pires, Dardanelos and Mutum-Parana volcanics), with a very distinctive age and origin, are related to two other orogenies, the Tapaj6s (2.10-1.90 Ga) and Guapork (1.74- 1.69 Ga) orogenies, and do not belong to the Uatum5 Magmatism. Therefore, the name Iriri should not be used to define the calc-alkalic volcanic rocks in the RondBnia- Juruena Province, as is common practice in the north Mato Grosso State (Moura and Botelho, 1998; Lima, 1999).
Sunsas province
The Earlier Sunsas granites?
There is a suite of granitoids in RondBnia State which have ages around 1350 Ma. For example, sample WO-52 has an age of 1352 f 8 Ma (Fig. 22), and an important clastic zircon population in sample GR-66 has an age of 1320 f 20 Ma. Bettencourt et al. (1999) recognize two comparable tin-granite suites, Alto Candeias (1346 f 5 Ma) and SBo Lourenqo (1314 k 15 Ma). The two main recognized tin-granite suites in the region are rapakivi-
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U-Pb AND Sm-Nd GEOCHRONOLOGY OF THE AMAZON CRATON 483
type and post-orogenic, one being post-Juruena-RondBnia Orogeny (1.56 Gal and the other post-Sunsas Orogeny (<1.10 Ga). The existence of a third tin-granite suite, with an age around 1.35 Ga (Bettencourt et al., 1999), is not well understood. This suite could be related to another intermediate (1.45-1.35 Ga) orogeny (Rond6nia or San Ignacio), but there are no orogenic rocks yet detected in Rondbnia, in that time range. Alternatively, the suite could represent Serra da Providhcia-type granites (1.56 Ga), which were partially or totally remelted in the earlier stages of the Sunsas Orogeny. This hypothesis can be tested by a detailed geochemical-geochronological study on this granitoid group.
The Sunsas front location
The presence of a large belt of meta-volcano- sedimentary rocks (Nova Brasilhdia Suite), with a Grenvillian age, shows that Sunsas orogenic rocks are widespread in the central Rondbnia region, - 400 km northeastwards from the boundary of the Sunsas Belt in Bolivia (Litherland et al., 1989; Tassinari, 1996). This suggests that the main front of the Sunsas orogeny is -400 km northeast from the original position proposed by Litherland et al. (19891, as shown in Fig. 16. The Paragua Craton in Bolivia is sited between two major Sunsas fronts, and possibly represents a zone less affected by the orogeny. The small Santa Helena Arc in the extreme southwest of the craton (western Mato Grosso State), displays juvenile ages around 1.45 Ga (Van Schmus et a1.,1997) and represents another local area not or less affected by the Sunsas collision.
In the northwest, the Grenvillian front may be located close to the Brazilian border, as indicated by the 1.20 Ga Piraparana Formation and Tijereto Granophyre (Galvis et al., 1979), the 0.92 Ga Yaca-Yaca Metarhyodacite in ColBmbia, and by 15 Grenvillian zircons detected in Venezuela (Goldstein et al., 1997).
Collisional or accretionary?
The Sunsas assemblage in the investigated area is represented by three main units: i) a metamorphosed, quartzose, supracrustal sedimentary assemblage (Nova B r a s i l h d i a Metamorphic Suite); ii) variably metamorphosed, tholeiitic gabbros and dolerites; and iii) anatectic, lenticular, S-type granitoids. This association is characteristic of a collisional assemblage, with a complete lack of juvenile rocks (calc-alkaline volcanic- plutonic suite, oceanic basalts, or accretionary sequences). Both the U-Pb SHRIMP results from sample GR-66 and the Sm-Nd data from six samples (Rizzotto et al., 1999) indicate a much older crustal source for the Sunsas rocks in RondBnia. S-type granites, derived from the Nova BrasilAndia Suite have a magmatic U-Pb age of 1100 Ma,
and a Sm-Nd T,,model age of 1630 Ma. Five other Sunsas rocks have Sm-Nd model ages between 1.91 and 1.63 Ga (Table 1). These data confirm that, in the southwest Amazon Craton, the Sunsas-Grenville orogeny was not juvenile. The juvenile remnants of this continental orogeny in South America (Fig. 1) are located -2000 km southwest (Arequipa Complex, Peru) and in eastern Colombia (Garzon Complex).
Regional nomenclature and correlations
The western part of the RondBnia-Juruena Province and the Sunsas Province enclose several sedimentary and volcano-sedimentary sequences, which form an elongate, northwest to east-west belt, from Mato Grosso to Acre, crossing Amazonas, Acre and northeast Bolivia (Fig. 16). These sequences are locally termed Comemoraqiio, Nova Brasilbndia, Mutum-Parana, Beneficente, Roosevelt, Aguapei, Huanchaca and Abunii. Although some of these are now related to the RondBnia-Juruena (Beneficente and Roosevelt) and Sunsas (Nova Brasilhdia Suite) Orogenic Belts, as indicated by U-Pb ages, the others are of unknown age. The name Comemoraqiio, following Scandollara et al. (1996), must be abandoned, as the type- area (Comemoraqiio River) is entirely composed of Palaeozoic sedimentary rocks.
Two different sequences have been correlated with the Aguapei Group, one folded and metamorphosed (for example the host rock in the Santa Elina gold mine, in northwest Mato Grosso State) and the other lacking deformation and metamorphism (for example the Huanchaca plateau in Bolivia). These sequences could represent the same unit with different degrees of deformation or could be distinctive basins, deposited under different tectonic environments in different epochs. In some cases, as in the Pacabs-Novos and Huanchaca plateaus, the presence of an erosive unconformity between folded and unfolded sedimentary sequences is evident even in satellite images. Although, in some areas, the relationship between the two main sequences remains uncertain, it is proposed that the names Aguapei (Mato Grosso State and Bolivia) and Nova Brasilhndia (RondBnia State) be limited to the metasedimentary units related to the Sunsas Orogeny. It is preferable to retain the names Pacaas-Novos (Leal et al., 1978) and Huanchaca (Litherland et al., 1989) for the post-collisional, continental, horizontal sedimentary cover.
The postsunsas assemblage
The youngest rocks in the southwest Amazon Craton appear not to be affected by the Sunsas deformation, and are now included in a post-Sunsas assemblage. The youngest Sunsas rocks are the late S-type granites that are intrusive into the Nova BrasilAndia Suite, having ages
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484 J.O.S. SANTOS ET AL.
of 1,098 f 10 and 1,100 k 8 Ma (Rizzotto et al., 1999). There is a variety of rocks younger than 1,100 Ma, related to a more stable tectonic environment. Included in this group are: i) Rondbnia tin Granites (995 Ma); ii) Nova Floresta alkali-basalts (980 Ma, K-Ar); iii) TeotGnio Alkaline Pipes; and iv) post-Nova Floresta sedimentary cover (Guajara-Mirim, Pacaas-Novos, Huanchaca) .
Conclusions
1) The ages obtained from geologically well- constrained samples from the Amazon Craton samples using the U-Pb method (conventional and SHRIMP), with a few exceptions, are 100 to 400 m.y. older than the Rb- Sr whole-rock isochron ages, making it essential to carefully review the existing regional stratigraphy.
2) Seven main Provinces are recognized, each with its own geological, structural, magmatic and isotopic characteristics: Carajas and Imataca, Transamazonic (Guianas), Tapaj6s-Parima, Central Amazon, Rio Negro, RondGnia-Juruena and Sunsas. The K'Mudku Shear Belt affected four Provinces.
3) Interpretation of U-Pb rock formation ages, combined with Sm-Nd model ages, indicates that four provinces (Carajas, Transamazonic, Tapaj6s-Parima and Rondbnia-Juruena) represent juvenile crust. Two are more likely to be related to continental collisional processes (Rio Negro, and Sunsas/K'Mudku effect), and one is proposed to be the product of reworking of the Archean crust during underplating (Central Amazon).
4) The exposed Archean crust is restricted to the Imataca and Carajas Provinces, corresponding to less than 12 percent of the exposed area of the Craton, which was largely formed during the Paleoproterozoic and Mesoproterozoic. This shows that considerable revision is required for existing regional geological maps, including the map of Brazil.
5) Large areas of the Craton still have no geological maps or geochronological data (Tumucumaque and Imeri Mountains, South Surinam, South Guyana and the Iriri River Basin, for example). This is being remedied by further geologically constrained isotope research, in order to test and further extend the tectonic models presented here.
6 ) The model presented here is transient and will change as new data are obtained.
Acknowledgments
This research was supported by the Conselho Nacional do Desenvolvimento Cientifico e Tecnolhgico - CNPq and by CPRM - Brazilian Geological Survey. Zircon analyses were carried out on a Sensitive High-mass Resolution Ion
Micro Probe mass spectrometer (SHRIMP II), operated by a consortium consisting of the University of Western Australia (UWA), Curtin University of Technology and the Geological Survey of Western Australia, with the support of the Australian Research Council. Sincere thanks to Companhia de Pesquisa de Recursos Minerais geologists for additional sampling and discussions, and to Rebecca Ford for initial revisions in the text. The senior author wrote the paper while a sandwich doctoral student in the Centre for Strategic Mineral Deposits at UWA.
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