Aetas II South American Symposium on Isotope Geology , j 'I "; r»

GEOCHEMISTRY AND GEOCHRONOLOGY OF CALC-ALKALINE AND THOLEIITICDYKE SWARMS FROM THE TANDILIA SYSTEM, ARGENTINA.

\ ,lP.P. Pinese' , W. Teixeira' , V.A V. Girardi', H. Echeveste'', A. Ribot3, R. Fernandez',

M. Iacumin", E.M. Piccirillo4 and P.R Renne"

(1) University of Londrina, Department of Geoscience. PO Box 6001 (86051-990) Londrina (PR), Brazil.ginese@npd.uel.br2) University of Sao Paulo , Institute of Geosci ence. PO Box 11348 (05422-970) Sao Paulo (SP) , Brazil.wteixeir@usp.br; girardi@usp.br(3

l University of La Plata, INREMI and LEMIT. Calle 52, PO Box 128. 1900 La Plata, Argentina.rfernand@isis.unlp.edu.ar; aribot@isis.unlp. edu.ar .(4) University of Trieste, Dipartimento di Scienze della Terra. Via Weiss, 8 (34127) Trieste, Italy.jacumin@univ.trieste.it; picciril@univ.trieste.it .(S) Berkeley Geochronology Center . 2455 Ridge Road , Berkeley, CA 94709, USA.

K eywords: Geochemistry, Geochronology, Dyke Swarms, Tandilia.

The Precambrian crust of Sierras Septentriona les (Echeveste et al., 1997; Iacumin et al., this volume).known as Tandilia System, Buenos Aires Province Geochemically, averages of major, minor and(Argentina), crops out in the southernmost part of Rio trace elements, including REE, reveal significantde La Plata Craton (Terruggi et al., 1988). This var iation among the tholeiitic and calc-alkalinesystem comprises metamorphic and granitic basement, groups. In general , the two groups of dykes presentwhich predominantly belongs to the Transamazonian differences in terms of MgO contents (wt%), butCycle (e.g. Varel a et al., 1988: Dalla Salda et al., within the tholeiitic group (B dykes), internal1992), intruded by two distinc t dyke swarms: one variations have ' been observed for similar MgObasic (B), and other intermediate (I) and acid (A) in contents. The tholeiitic dykes with Ti02 higher thancomposition. These dykes have mean width s from I to 1.7 (wt%) are more enriched in P20S, FeOt, K20 , Ba,80 meters and until 5 kilometers long. Sr, La, Ce, Nd, Zr, Y and Nb relative to those with

The basic dykes (B) are characterized by ophitic Ti02 less than 1.7. The calc-alkaline group (I and Ato subophitic, sometimes intergranular textures. dykes) have major and trace elements characteristicsCommon minerals are plagioclase [labradorite (An distinctive from the tholeiitic group. ' Moreover I and52-53)] , olivine and pyroxene. The latter is more A dykes show higher mean values of Si02 (56 to 76commonly augite, subordinately pigeonite and/o r wt%), K20 (1.0 to 4.8 wt%) , Na20, Ba and Rb thanorthopyroxene. Intermediate dykes (I) exhibit those of the tholeiitic types , that otherwise haveporphyrytic texture, composed of andesine plagioclase higher mean contents of Ti02, FeOt, CaO, Ni and Cr.and Ti-augite. Acid dykes (A) show porphyryt ic fabric The calc-alkaline dykes are also characterized by REEand are constitued by plagioclase, alca li-feldspar and patterns with [La/Lu]n ratios of 9.0 to 40.0, whereashornblende. the tholeiitic dykes have [La/Lu]n values of 1.3 to 2.8.

According to geochemical classification of De The geochemical data therefore demonstrate that BLa Roche et al. (1980), modifi ed by Bellieni et al. . dykes are genetically different from the I and A(1981), the B dykes are predomin antly tholeiitic dykes. .basalts and subordinately andesi -basalts. The I dykes Rb-Sr geochronology was carried out, byplot in the andesite and andesi-basalts fields. The A isotope dilution technique, on eighteen whole rocksdykes are Classified as rhyoli tes. In the AFM diagram, from the tholeiitic dykes and ten whole rocks from theB dykes plot in the tholeiitic field whereas both I and calc-alkaline dykes (five samples of each 1 and AA dykes plot in the calc-alkaline field. The available dykes). The latter yield an errorchron of 2,112 ± 169geochemical data and field relations suggest that I and Ma (Ic), Ro = 0.70290 ± 0.05843, whereas theA dykes together, are related to single evolution trend tholeiites yield an errorchron of 1,363 ± 167 Ma (Io),

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Financial support by FAPESP [Brazilian Agency(grant n? 97/0640-5)] and CNR and MURST (ItalianAgencies).

dykes are derived from an enriched and depletedmantle sources comparing to the Bulk Earth. Inaddition, this indicates a heterogeneity of thelithospheric mantle sources independently of theadopted age.

To summarize, available dates indicate that thecalc-alkaline dykes clearly emplaced at 2,000 Ma ago,as supported by 40Ar/39Ar and Rb-Sr 'determinations.Tectonically they emplaced during a transextensionalphase of the Transamazonic Orogeny, as alreadysuggested by Echeveste et al. (1997). The tholeiiticdykes either late Paleoproterozoic (1.73 Ga ?) oryounger (0.8 Ga ?), as suggested from Rb-Sr and40Ar/39Ar,determinations and regional correlations, butadditional data are needed for testing these ideas. Inanycase the unprecise emplacement ages suggest thatlarge scale extensional tectonics occurred prior to theBrasiliano Orogenic Cycle, as suggested in WesternUruguay (Teixeira et al., 1999; Bossi et al., 1993) andin the Nico Perez terrane, Eastern Uruguay (Rivalentiet al., 1995; Girardi et al., 1996) .

Actus II South American Symposium on Isotope Geology

Ro = 0.70820 ± 0.05883. The extremely high errorsshowed in both linear arrays are due to the large datascatter, interpreted to be related from heterogeneousisotopic disturbance of the Rb-Sr whole rocksystematic. However, we consider the unprecise Rb-Sr"ages" obtained to be indicative of minimumemplacement age of both dyke groups. .

Interpretation of three 40Ar;J9AI' and two K-Ardates on plagioclase, biotite and whole rocks from thecalc-alkaline and tholeiitic dykes of the TandiliaSystem, provides additional constraints on the dykeemplacement ages and the regional tectonothermalhistory. The emplacement age of the calc-alkalinedykes are estimated from two 40Ar/39Ar dates ofoutgassed biotites from baked country rocks that weresampled in sharp contact with the dykes. Both biotitesamples yield step-heating plateau ages of 2,020 ± 24and 2,007 ± 24 Ma : (20-) [Iacumin et al.,unpublished]. K-Ar dates on whole rocks samples ofthe tholeiitic dykes in the region, range from 803 ± 14to I ,193± 18 Ma [Pinese et al., unpublished], and40Ar/39Ar data on plagioclase yield an age of 8II ± 36Ma (20') [Iacumin et aI., unpublished], but, could be ACKNOWLEDGEMENTS :related to argon loss in accordance with the geologicalsetting of the Tandilia System which rocks were partlydeformed and metamorphosed during theNeoproterozoic (Dalla Salda et al., 1992). On theother hand, it is noteworthy that the Uruguayan dyke REFERENCES. .swarm, Rio de La Plata Craton emplaced at 1,727 ±10 Ma ago, as supported by detailed 40ArP9Ar and Rb-S h I ( T · · I 1999) Th' Bellieni, G., Piccirillo, E.M., and Zanettin, B., 1981

r geoc rono ogy e.g. eixerraet a ., . IS age Int. Union Geoi. Sci., Subcomm Systematic Igneoussupports the idea that the Rb-Sr linear array herein Rocks Contrib.87: 1-19.obtained for the Tandil tholeiitic dykes (see above) is

. . I Bossi, 1., Campal, N., Civetta, L., Demarchi, G.,a minimum emp acement age.

Ten Sm-Nd whole rock determinations were Girardi, V.A.V., Mazzucchelli, M., Negrini, L.,carried out in calc-alkaline dykes (five samples) and Rivalenti, G., Fragoso Cesar, A.R.S., Sinigoi, S.,

Teixeira, W.; Piccirillo, E.M. and Molesini, M., 1993tholeiitic dykes (five samples), by isotope dilution Chemical Geology. 106: 263-277.technique. The Sm-Nd (TDM) model ages of the calc-lk I· d k (2 3 263 G ) 'd . Dalla Salda, L., Franzese, J .R. and Posadas V.G.,

a a me y es .5 to. a , provi es a restrict 1992. Basament Tectonics,7: 161 -174.time period for the rock protholiths. The ·eNd (2.0Ga) De La Roche, H., Leterrier, P., Grandc1aude, P. andparameters yield slightly negative values between - Marchal, M. 1980 Chemical Geology. 29: 183-210.4.83 to -3 .19, supporting that the calc-alkaline rocks h HiRib AT ' . W F d REc eveste, ., Ribot, ., eixeira, ., ernan ez, .,derived by means of partial melting of a probably G' d' V A V P' . '11 EM B II' . G dIraI' I, . . ., ICClrI 0 , . ., e tent , . anmetasomatic (lithospheric) mantle source. However, Iacumin, M., 1997 South-American Symposium onthe tholeiitic dykes provided highly variable Sm-Nd Isotope Geology, Campos de Jordao, S. Pablo.(TDM) model ages between 660 to 2,513 Ma, as we Extended Abstracts: 107-109.expected because their isotopic composition compared G' di V A V Mill' M MI " Mirar I, ..., azzucc ie 1, ., 0 eSIl1I, .,to the mantle array. We have tentatively modelled the C' L P . . R B . J C I N Tei .ivetta, ., etrini, ., OSSI, ., ampa, ., eixetra,eNd for to=1.73 Ga (-5.82 to +12.93) and 10=0.81 Ga (- W. and Correia, C.T., 1996 Journal of South15.63 to +7.54). The values suggest that the tholeiitic American Earth Science. 9(3/4): 243-249.

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Aetas II South American Symposiumon Isotope Geology

Rivalenti, G., Mazzucchelli, M., Molesini , M., Petrini,R., Girardi , V.A.V. , Bossi , J. and Campal, N., 1995Mineralogy and Petrology . 55: 239-263.Teixeira,W., Renne, P. R., Bossi , J., Campal, N. andD'Agrella Filho , M. S., 1999. Precambrian Research.93: 153-180.

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Teruggi, M.E., Leguizam6n, M.A. and Ramos ,V.A.,1988. Revista Asociacion Geol6gica Argentina.18(3): 366-374.Vare la, R., Cingolani, C. and Dalla Salda, L., 1988.Aetas Il? Jornadas Geologicas Bonaerenses. 291-305.