contrasting metaluminous magmatic epidote-bearing granitic ... · magmatic epidote (mep) in...
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Contrasting Metaluminous Magmatic Epidote-Bearing GraniticSuites from two Precambrian Foldbelts in Northeast Brazil*
ALCIDES NÓBREGA SIAL
Dept. of Geology, Federal University of Pernambuco, Nucleus for Granite Studies,P.O. Box 7852, 50732-970 Recife, PE, Brazil
ABSTRACT
Magmatic epidote (mEp) occurs in some Brasiliano-age plutons in northeast Brazil in the Cachoeirinha
Salgueiro (CSF) and Seridó (SFB) Foldbelts. These two groups of plutons differ geochemically from each
other and demonstrate that magmatic epidote forms from different types of granitic magmas, under differ
ent pressure and oxygen fugacity conditions.
At the CSF, mEp-bearing plutons are represented by granodiorite and tonalite stocks which share similar
mineralogical and textural characteristics and intruded rather low-grade metamorphic rocks of the CSF. At
the SFB, mEp-bearing plutons are represented by quartz diorites, quartz monzodiorites, tonalites, quartz
monzonites and granites which form batholiths, stocks and dikes, and intruded higher grade metamorphic
rocks (gneisses and migmatites). Homblendes (edenite to Mg-homblendes) in the CSF mEp-bearing plu
tons solidified between 6 and 9 kbar pressure, while in the Seridó plutons (excluding those to the westem
margin of the belt, near Patu), a pressure range of 3 to 5 kbar was estimated based upon Schmidt's (1992)
AI-in homblende barometer calibration. Temperatures for Zr saturation in the CSF plutons are near
liquidus (785-850°C), while homblende-plagioclase pairs yield temperatures in the range between the
liquidus and the excess-water solidus for granodiorite compositions. In the SFB, plagioclase-homblende
pairs yield temperatures in the 650-720oC range.
Magmatic epidotes in the SFB exhibit compositions with27-29 mole% of pistacite and in the CSF, 20-24
mole% pistacite. Therefore, in the SFB 102 between the HM and NB buffers probably predominated during
crystallization, while in the CSF,102 probably followed, or was slightly below the NB buffer.
In thé CSF mEp-bearing plutons, although mineralogically I-type granites, have an oxygen isotope signa
ture typical of the S-type ( +10%0 SMOW)while the Seridó mEp-bearing exhibit lower ÔI80 (6-9%oSMOW).
Initial 87Sr;86Sr ratios for the CSF mEp-bearing plutons is 0.7058 while in the Seridó plutons to the west of
the belt ratios are 0.7070, suggesting greater crustal contribution in their genesis.
Amphibolite inclusions, present in almost alI of the mEp-bearing plutons in the CSF, regarded as xenolithic
in origin, are possible source rock fragments, transported with the host magmas. They are composed of
homblende (edenite to ferro-edenitic homblende), clinopyroxene and brown rnica. Homblendes in these
xenoliths solidified at 670-690oC and 6-7 kbar pressure. ÔI80 in these xenoliths varies from +10 to
+ ll.5%oSMOW.
Oxygen and Sr isotopes signatures support the hypothesis of a hydrated metabasaltic (= amphibolite)
source for the mEp-bearing plutons in the CSF. More isotopic studies are needed to constrain the source
·Contribution nO55 of the Nucleus for Granite Studies
(NEG), Federal University of Pernambuco.
An. Acad. bras. Ci., (1993) 6S (Supl. 1)
142 ALCIDESNOBREGASIAL
for mEp-bearing granitoids in the Seridó Foldbelt which, in the light of the present data, seem to be in partcrostal derived.
Key words: Magmatic epidote, high ÔI80, high Al-hornblende, amphibolite xenoliths
INTRODUCTION
Magmatic epidote (mEp) in granitic rocks is
known since its description by (Mrazec, 1934) but
only in the last decade has its occurrence been re
corded worldwide (North America: Zen, 1985,
1988, Moench, 1986, Owen, 1991, Brew, 1992,
Zen & Hammarstrom, 1984, Dawes & Evans,
1991, Ghent et aI., 1991, Vyhnal et aI., 1991,
Cullers et aI, 1992, Farrow & Barr, 1992, Ham
marstrom & Zen, 1992 ; Europe: Berza et aI. in
press, Forizs et aI., 1989, Reusser, 1989; Africa:
Stussi & Mortagi, 1988, Mortaji & Stussi, 1989;
South America: Almeida et aI., 1971, Saavedra et
al., 1987, Sial, 1990 and references therein; India:
Rogers, 1988; New Zealand: Tulloch, 1983,
1986). The crystallization of mEp in magmas of
granodiorite compositions occurs at 6-8 kbar pres
sure according to Naney's (1983) experiments. For
mEp crystallized outside this compositional range,
except for trondhjemitic systems (Johnston & Wyl
lie, 1988; Van der Laan, 1992), no experimental
data is available. Chemical and textural aspects of
true mEp have been summarized by Tulloch
(1979) and Zen & Hammarstrom (1984). Factors
controlling naturally occurring mEp are not fully
understood considering that plutons of similar
chemical composition, crystallized at similar pres
sure, may or may not contain magmatic epidote.
Coexisting clinopyroxene (cpx) and mEp in
granodiorites have been less documented and, to
our knowledge, only two experiments (Naney &Swanson, 1980; Van der Laan & Wyllie, 1992) re
fer to clinopyroxene together with mEp in grano
diorites/tonalites.
Our purpose is tocontrast mEp-bearing grani
toids in two foldbelts (different continuous segments of the Central Structural Domain in
northeast Brazil; Fig. 1) and to report the oçcur
rence of clinopyroxene phenocrysts in two mEp
bearing plutons in one of these two foldbelts.
An. Acad. bras. Ci., (1993) 65 (Supl. 1)
GEOLOGIC SETTING AND PETROGRAPHY
(a) CACHOEIRINHA-SALGUEIROFOLDBELT(CSF)
Round to elongate mEp-bearing tonalite/gra
nodiorite plutons intrude low-grade metasediments
of the Cachoeirinha-Salgueiro Foldbelt (CSF, Fig.
l-A) in the Borborema province. These plutons are
composed of zoned plagioclase, quartz, calcic am
phibole, biotite, microcline, epidote, and sphene.
lron oxide mineraIs are rare to absent. The grani
toids are mafic, oxidized I-type granites according
to White's (1992) concept (crystallized above the
nickel-bunsenite buffer). They usually lack contact
aureoles and epidote is found in four different tex
tural relationships, two of which indisputably of
magmatic origino Among these plutons, Emas,
Conceição, Boa Ventura (central state of Paraíba)
and Riacho Santo Antônio (state of Pemambuco)
are better known. The largest magmatic epidotes
are found at Emas and Boa Ventura plutons with
euhedral, zoned allanite cores. Sometimes, patches
of homblende are enclosed in epidote suggesting
this phase formed after homblende. Epidote with
embayed or vermicular contact with unweathered
plagioclase is also present.
Rounded to elongate quartz diorite enclaves,
with major axis up to one meter in diameter, are
common and composed of plagioclase, abundant
biotite and Ca-amphibole, quartz, sphene, apatite
and some epidote. Tourmaline and milky quartz
lumps (e.g. Emas pluton) are found within those
enclaves and the latter are also present in the host
granodiorites. In these enclaves, epidote is rare and
usually forms granular rims around homblende in
contact with plagioclase as a result of subsolidus
reaction between these two phases. These enclaves
lack quench morphologies, except in few places
where needle-like and elongate amphiboles are
present (e.g. Conceição pluton).
Two kinds of amphibolite inclusions are ob
served in the mEp's in the CSF. Type I is repre
sented by deep green calcic amphibole aggregates
•'. I!!!!!!!!!!!!~'!'!""• ::±1K!,,"~,'~~',:';'i
''00'.
CONTRASTING METALUMINOUS MAGMATIC EPIDOTE-BEARING ...
t-SeridóB- cochoeirinho-Salpiro
- Rlocho do Ponto I
[Z] Phonerozoic Cover
143
rr;'I EPlDOTE - BEARINGL...±J CALC ALKALIC
4O"W
11I K-CALC ALKALlC
(A)• SHOSHONITIC
CALC'ALKAUC PLUTONS PERALKALIC SHOSHONITIC PLUTONS
[] POTASSlC CALC-ALKALIC
• EPIOOTE-BEARING CALC-ALKALIC
• TRONOHJEMITE
~ SATURATEO PERALKALIC
• OVERSATURATEO PERALKALIC
~ SHOSHONITE
(B)
IKtCRETACEOUS SEOEMENTARYL.JSJ COVER
~ CACHOEIRINHA ClROUP~ (PHYLLITES. SCHISTS)
~ SALGUEIRO GROUP~ (SCHISTS. GNAISSESI
O BASEMENT ROCKS(ClNAISSES, MIClMATITESI
Fig. 1 - (A) Preliminary geologic map of part of the Seridó Foldbelt (modified from Jardim de Sã & Martins de Sã,
1987 and Brito Neves, 1983); (B) generalized geologic map of Cachoeirinha-Salgueiro Foldbelt, northeaast Brazil
(modified from Sial & Ferreira, 1990).
144 ALCIDES NOBREGA SIAL
Fig. 2 - Small fragments of type II amphibolite, included in quartz diorite enclaves ar
in the host granodiorite (Conceição batholith, state of Paraíba).
(fractionated from the host magma) and type 11,
usually finer-grained, by angular, up to 15 cm
long, amphibolite xenoliths (Fig. 2 and 3). Someti
mes, they are concentrated along schlieren zonesor are found as small fragments within quartz dio
rite enclaves. This second type of amphibolite is
regarded here as fragments from the source rock
for the granodiorite/tonalite magmas.
In two granodiorite plutons (Pedra Branca, in
Paraíba and Angico Torto, Pemambuco) that also
intruded CSF low-grade metasediments, elongate
~linopyroxene (diopside-salite) phenocrysts, up to3cm long are present. Except for the presence of
clinopyroxene, these plutons are mineralogically
and texturally identical to the mEp granitoids in
the CSF. Epidote, however, is rare or absent, is
usually included in biotite and has an allanite core.
Secondary epidote is also present. Clinopyroxene
is very elongate, euhedral and locally occupiesover 5% of the volume of the rock and, sometimes,
Fig. 3 - Fragments of fme-grained type II amphibolite included in a small quartz diariteenclave in an unamed stock 7 km north of Serrita town, state of Pemambuco.
An. Acad. bras. Ci., (1993) 65 (Supl. 1)
CONTRASTINGMETALUMINOUSMAGMATICEPIDOTE-BEARING... 145
encloses homblende and biótite. Homblende forms
epitaxiaI overgrowth on clinopyroxene, promoted
perhaps by chemicaI gradients near crystaI-liquid
interfaces. EuhedraI, calcic amphibole is aIso pre
sent in these plutons. Zoned plagioclase, micro
cline, quartz, biotite, sphene, apatite and some
magnetite are the other components. Mafic inclusions are rare to absent and no contact aureole was
observed. These clinopyroxene-bearing plutons are
mafic, oxidized I-type granites according to
White's concept (1992), probably crystallized un
der fOz above the nickeI-bunsenite reaction buffer.
The Pedra Branca pluton is composed of pla
gioclase, quartz microcline, homblende, biotite,
clinopyroxene, epidote, sphene and apatite, in this
order of abundance. Oscilatorilly zoned plagio
clase encloses euhedraI homblende, biotite, apatite
and clinopyroxene. Microcline is observed as Iarge
grains which encloses all of these phases and is
aIso observed interstitially.
(b) SERIDÓFOLDBELT(SFB)
Magmatic epidote-bearing granitoids in the
SFB (Fig. l-B) in the Borborema province aremuch Iess studied than the ones in the CSF. In this
belt, SiaI (1990) recognized three groups of epi
dote-bearing plutons. The first one intruded Jucu
rutu gneisses and Seridó micaschists (e.g. São
RafaeI batholith) composed of quartz monzonites
to granodiorite~ and contain small amphibolite,
schist and gneiss xenoliths and diorite enclaves, 10
cally with stromatic structure. In these plutons,
mEp is in the groundmass that is composed of
zoned microcline, quartz, plagioclase, biotite,
homblende, epidote, apatite, allanite and rare
opaque mineraIs (Fig. 4 and 5).
A second group of epidote-bearing plutons is
represented by quartz diorite to quartz mon
zodiorites (e.g. Serra da Garganta, Tapera, Serra do
Estreito plutons) which contains amphibolite xen?Iiths. Epidote in this group is an accessory phase
and it is of dubious origino Pyroxene partially
uralitized (optically similar to the clinopyroxene
observed in the amphibolite xenoliths), plagio
clase, microcline, quartz, sphene and apatite are
the other phases.
A third group is represented by mEp-bearingtonalite dikes and sheets, Iess than 10m wide, that
intruded Jucurutu gneisses. They contain the Iarg
est amount of epidote among the plutons in the
Seridó Foldbelt referred to here. Epidote usually
Iacks allanite core and is not essentially related tomafic mineraIs.
Galindo (1993) described mEp-bearing quartz
diorites, tonalites and monzogranites near Patu,
Rio Grande do Norte (Tourão, Caraúbas, Prado and
Fig. 4 - Twinned, magmatic epidote (60x magnification, crossed polars). São Rafael
batholith, state of Rio Grande do Norte.
146 ALCIDES NOBREGA SIAL
Fig. 5 - Magmatic epidote with allanite core (60x magnification, crossed polars). São
Rafael batholith, state of Rio Grande do Norte.
Serra do Lima plutons). In relation to movementsalong a major shear zone, they were intruded synkinematically (Caraúbas and Prado) and late-kinematically (Tourão and Serra do Lima) intomedium-grade metamorphic rocks of the CaicóGroup and Jucurutu Formation, at the westem margin of the SFB. Epidote is present in those plutonsin three textural relationships: (a) included in biotite with allanite core, (b) without allanite coreand (c) as secondary grains fiom alteration ofplagioclase.
The mEp-bearing granitoids studied in theSFB differ fiom those in the CSF in two ways: (a)they intruded higher grade metamorphic rocks and(b) most of them are not granodioritic/tonalitic incomposition.
CHEMISTRY AND THERMOBAROMETRY
(a) CHEMISTRY
Almost sixty complete whole-rock analyses(major and trace and some REE) of the mEp-bearing granitoids in the CSF are available, includingsamples from granodiorites/tonalites, quartz dioriteenclaves and amphibolite xenoliths. Enclaves andhosts show an overall Si02 variation fiom 56 to
71% with Al203 around 15.5%, Na20 usually
An. Acad. bras. Ci., (1993) 65 (Supl. 1)
greater than K20, around 4%, and MgO varyingfiom 0.4 to 4%. Most of these plutons aremetaluminous to slightly peraluminous. They areBa-enriched (650-1500 ppm) and show moderateSr (250-500 ppm), intermediate Zr (= 180 ppm)and low Nb ( ppm).
These mEp-bearing granitoids are enriched inREE relative to chondrite abundances, depleted inHREE relative to LREE and display a variablenegative Eu anomaly (EulEu* varies from 0.75 to0.90) and total REE from 116 to 166 ppm. TheREE abundance pattems for host and quartz dioriteenclaves are parallel and almost coincident arguingthat fractional crystallization only happened at limited extent (Fig. 6-a).
Amphibolite xenoliths in the CSF mEp-bearing plutons display an overall Si02 variation of 4853%, Al203 (6-9%), CaO (7-12%), MgO(13.7-15.9%), FeO (11-13%), K20 (1.3-2.6%),Na20 (0.4-0.7). Trace elements (Th, Y, Zr, Nb andSr) are usually low. Ba varies from 99 to 700 ppm,highest values correlating with higher modalamount of biotite. REE chondrite-normalized pattems are LREE- enriched and HREE-depleted witha discrete negative Eu anomaly, and very similar tothe REE pattems of the granodiorite hosts, withslightly lower total REE (Fig. 6-b).
CONTRASTING METALUMINOUS MAGMATIC EPIDOTE-BEARING... 147
The cpx-bearing plutons (e.g. Pedra Branca)are remarkably homogeneous, with SiOz around66.5% and A1Z03 (15%). CaO (3.9%) and MgO(2%) are lower than in the nearby mEp-bearingBoa Ventura pluton (4.8 and 4.0% respectively).FeO and Fez03IFeO (0.24-0.45) are lower than inthe Boa Ventura (FeOlFez03 = 0.48-0.82) plutonalthough the latter is almost virtually devoid of Feoxide mineraIs. Probably most Fe+3 is tied to theepidote structure in the Boa Ventura pluton which,in hand specimen, appears to be less oxidized thanthe Pedra Branca pluton.
These cpx-bearing plutons have total REEfrom 150 to 204 ppm. They exhibit remarkablyparallel, almost coincident LREE-enriched, HREE-
depleted chondrite-normalized patterns, with discrete negative Eu anomaly (EulEu* 0.64 to 0.67).
In the Seridó Foldbelt, the São Rafael batho
lith shows SiOz around 70%, KzO (2.5-3.5%),NazO (4-5%) and MgO around 1%. These rocksare slightly more Sr-enriched (600-1000 ppm) thanmEp-bearing granitoids in the CSF, but Ba is insimilar range. Zr is found in intermediate values(=250 ppm) while Nb is rather low (= 2Oppm). Inthose plutons both, mafic enclaves and hosts, aremetaluminous. Chondrite-normalized REE patternsare similar to those in the granodiorites of the CSF(Fig.6-c).
Magmatic epidote-bearing plutons studied byGalindo (1993) show consistently LREE-enriched
1000
••••I-
§:x:c.> 10.....~c.>og::
ai C GRANODIDRITEHDST
o 02 DIORITE ENCLAVE
1000
b)
••••
1::,00
15zo:x:c.>.....~c.>o 10li:
AMPHIBOLlTE INCLUSIONS
o AMPH-I
* AMPH-2
[J AMPH-4
,L.o C. Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
REE
ILo c. Pr Nd Sm Eu Gd Tb Dy Ho Er TmYb Lu
REE
1000
••••
I::a:ion 'o.....
gli:
c) SÃO RAFAEL BATHOLITH
(SERIDÓ FOLDBELT)
1000
100
....Iã:oz~c.>....• 'o
~oli:
CARAÚBAS, PRADO, TOURÃOPLUTONS (GALlNDO, 1993)
L.o C. Pr Nd Sm Eu Gd Tb Dy Ho Er TmYb LuREE
La C. Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm YII LuREE
Fig. 6 - Whole-roek normalized rare-earth element (REE) data patterns for magmatie epidote-bearing granitoids of theCaehoeirinha-Salgueiro (a-b) and Seridó (e-d) Foldbelts.
An. Acad. bras. Ci., (1993) 6S (Supl. 1)
148 ALCIDES NOBREGA SIAL
pattems, with a negative Eu anomaly, and slight1y
eoneave upward HREE pattems, without mueh
variation (Fig. 6-d). This suggests homblende fraetionation or its retention in the souree of these
magmas.
b) AMPHIBOLE BAROMETRY
Differenees in the AI eontent of ealcie amphi
boles are direet1y related to depth of emplaeement
of their host plutons (Hammarstrom & Zen, 1983,
1985). Later, it was demonstrated (Hammarstrom
& Zen, 1986) that the total AI eontent of homblende in intermediate ealc-aIkalie roeks varies
linearly with erystallization pressure, and an em
pirical barometrie equation was proposed. This ba
rometer was refined by Hollister et aI. (1987) whoredueed the 3kbar error to 1 kbar. These two em
pirical ealibrations are essentially identicaI.
Johnson & Rutheford (1989) and Rutter et aI.
(1989) added. experimental ealibrations to this ba
rometer. Results differ slightly from the empiriealones and uneertainties were redueed to + 0.5 kbar.
More reeent1y, Sehmidt (1992) reealibrated this ba
rometer usi1].g samples of an epidote-bearing
tonalite (59.0 wt% SiOz) and a homblende-bearing
granodiorite (66.7 wt% SiOz), and made it appliea
ble to pressures up to 13 kbar, with a maximum
AI(total) of 3.37 atoms per formula unit. It is worth
mentioning that this experimental ealibration ap
proaehes the empirieal, field-based ealibration of
Hammarstrom & Zen (1986). Therefore, it seemsto be the best one and will be used here to estimate
pressure of homblende solidifieation in the CSF
and SFB mEp-bearing plutons.
Amphiboles from six plutons of the CSF and
from five of the SFB were ehemically analyzed,
usually next to their margins. Almost all analyses
were performed in an ARL microprobe in the
North Carolina State University at Raleigh (USA),
exeept for analyses of amphiboles from the Boa
Ventura pluton which were analyzed in a
CAMECA microprobe at the Federal University of
Bahia, Salvador, BraziI. Galindo (1993) also pro
vided some analyses of homblende of mEp-bear
ing plutons near Patu, state of Rio Grande do Norte
(Caraúbas, Prado and Serra do Lima). In the CSF
An.Acad. bras. Ci., (1993) 65 (Supl. 1)
plutons, amphiboles are edenitic to Mg-hom
blendes. In the amphibolite xenoliths, amphibolesare edenites to ferro-edenitic homblendes.
Pressure estimates (Table I) for the CSF mEp
bearing granitoids (including those with clinopy
roxene) are in the 6 to 9 kbar range. Amphiboles
from two quartz diorite enclaves (Table I) yield
lower pressures (= 6 kbar), probably due to loss of
AI during epidote formation (granular epidote be
tween homblende and plagioclase, formed by sub
solidus reaetion). Homblendes from the Brejinho
mEp-bearing pluton, which intruded roeks of the
basement nearby the eastem CSF boundary,
yielded pressures between 7 and 9 kbar. Unfortu
nately, no P-T data are available for the CSF me
tasediments near mEp-plutons.
AI(t) in homb!ende in the amphibolite xeno
liths (1.84 to 2.08) yields pressures in the 5.6-6.8
kbar range (e.g. Boa Ventura pluton) if Sehmidt'g
(1992) equation is applied. In the same pluton the
granodiorite host yielded pressures between 5 and
7 kbar. It is possible that amphiboles in the xeno
liths equilibrated with the host magma. Therefore,numbers obtained for solidification of homblende
in the xenoliths are minimum pressures only.
Amphiboles of four mEp-bearing plutons
(quartz monzodiorites to quartz monzonites) in the
Seridó Foldbelt yield pressures in the 3.7-5 kbar
range. Pressures obtained from homblendes of the
São Rafael pluton, one of the largest epidote-bear
ing granitoids in this belt, are in agreement with
pressure estimates for nearby metamorphic eountry
roeks (3-4 kbar; Lima, 1987).
Galindo (1993) estimated pressures of solidi
fieation of amphiboles of mEp-bearing plutons
near Patu, Rio Grande do Norte. Using Sehmidt's
barometrie equation, he found avetage pressures of6.4 kbar for the Serra do Lima, 7.2 kbar for the
Prado and 7.6 kbar for the Caraúbas plutons.
e) PLAGIOCLASE-HORNBLENDE THERMOMETRY
Blundy & Holland (1990) based on semi-em
pirieal evaluation of the available experimental
data on plagioclase + amphibole assemblages, pro
posed a thermometer that yields temperatures with
uneertainties of around 75°C for roeks equili-
CONTRASTING METALUMINOUS MAGMATIC EPIDOTE-BEARING ... 149
TABLE I
Bomblende geobarometry of samples from epidote-bearing plutons.
SampIe
Al(t)SiXabAl(1V)P(l)P(2)KT(l)T(2)
A) CACHOEIRINHA-SALGUEIRO FOLDBELT1. Boa Ventura pIutonType I amphiboliteMBV-17B-24
2.026.55 1.455.16.5 756
MBV-17B-25
1.986.56 1.444.96.3 758
MBV-17B-26
2.116.47 1.533.06.9 764
MBV-17b-27
1.876.68 1.324.45.8 741
MBV-17B-28
2.076.46· 1.545.36.7 772
type 11amphiboliteMBV-20A-2
1.916.73 1.264.65.9 686
MBV-20A-3
1.896.72 1.284.65.9 688
MBV-20A-4
1.986.67 1.324.96.3 682
MBV-20A-5
2.096.63 1.374.66.8 674
MBV-20A-6
1.896.78 1.224.65.8 684
MBV-20A-7
1.846.82 1.18.4.35.6 684
MBV-20A-8
1.936.77 1.284.76.1 683
MBV-20A-9
1.966.71 1.294.86.2 681
MBV-20A-10
1.846.80 1.194.35.6 686
granodiorite/tonalite MBV-20-11
2.136.590.651.405.57.01.85722
MBV-20-13
2.046.710.651.295.26.62.10705
MBV-20-15
1.676.960.651.043.64.82.83680
MBV-20-15
1.956.770.651.224.86.12.27699
MBV-19-2
1.766.710.691.294.05.22.10715
MBV-19-3
1.676.720.691.283.64.82.10719
MBV-19-4
2.106.390.691.605.46.91.49754
MBV-19-8
1.756.660.691.344.05.21.99725
MBV-19-9
1.736.520.691.473.95.11.74739
MBV-19-1O
1.816.55"0.691.444.25.41.77744
MBV-19-14
1.896.970.691.024.65.92.91650
MBV-23-18
1.886.390.691.614.55.81.48772
MBV-23-22
1.876.610.661.394.45.81.87735
2. Conceição pIuton tonalites/ granodioritesMC-35-17
1.736.860.671.143.95.12.50691
MC-35-19
1.896.700.681.304.55.82.08711
MC-35-20
1.966.640.681.364.86.21.94717
MC-35-21
1.866.750.681.254.45.72.19703
MC-35-22
1.706.880.681.123.74.92.58686
MC-35-23
1.686.870.681.133.74.92.54689
MC-35-24
1.656.900.681.103.54.72.63686
MC-35-25
1.856.760.681.234.45.72.24699
MC-35-26
1.986.650.681.454.96.31.55714
MCc35-27
1.846.800.681.194.35.62.35692
to be continued
An. Acad. bras. Ci., (1993) 6S (Supl. 1)
150 ALCIDES NOBREGA SIAL
TABLE I (Continued)
Sample Al(t) Si Xab Al(IV) P(l) P(2) K T(l) T(2)
702657693696697697667717709695703703698697693685
780780804811
767747742
732765
724731
714
733743735
1.942.542.252.202.252.502.512.002.48
2.272.122.172.272.232.30
~.38
1.151.471.45
1.421.451.291.26
2.15
1.591.72
1.681.42
1.781.681.68
4.74.74.14.23.83.84.73.83.84.04.34.03.84.84.04.1
9.07.78.5
8.77.3
6.66.36.36.3
5.7
7.57.5
6.46.77.2
5.25.05.05.0
7.05.8
6.06.0
6.646.876.776.756.776.866.866.676.856.786.726.746.786.716.796.82
6.146.386.37
6.356.376.266.23
6.466.53
6.566.506.51
6.516.35
2.542.282.44
2.012.072.18
2.232.24
1.671.471.531.571.561.491.471.551.401.521.581.511.461.671.511.53
2.482.19
2.051.991.991.99
1.85
KSR-8-1KSR-8-2KSR-8-3KSR-8-4KSR-36-1KSR-36-2KSR-36-3KSR-36-4KSR-16KSR-4-1KSR-4-2KSR-4-3KSR-4-4KSR-4-5KSR-4-6KSR-4-7
ITIM-22ITIM-50AITIM-50B
8-148-1810-1110-18
SER-77SER-86
SER-47ENC
SER-45SER-47
PB-33-1PB-33-2PB-33-3
3. Pedra Branca plutongranodiorites
0.66 . 1.44 5.20.66 1.49 5.50.66 1.49 6.0
4. Santo Antonio Creek
granodiorite/tonalites
0.65 1.490.65 1.65
quartz diorite enclave
6.73 0.65 1.27 4.4
5. Penaforte and Ipueiras plutonsgranodiorites
0.65 1.540.65 1.47
6. Emas plutongranodiorites
0.65 1.650.65 1.630.65 1.740.65 1.77
7. Brejinho pluton0.75 1.86 7.30.68 1.62 6.20.66 1.63 6.9
B) SERIDÓ FOLDBELT1. São Rafael batholith
0.79 1.36 3.b0.84 1.13 3.50.80 1.23 3.00.81 1.25 3.00.80 1.23 2.70.80 1.14 2.80.80 1.14 3.50.80 1.33 2.50.77 1.15 2.50.79 1.22 3.00.79 1.28 3.20.79 1.26 3.00.79 1.22 2.70.79 1.29 3.60.79 1.21 3.00.79 1.18 3.0
2. Other plutons
Estreito 1.60 6.62 0.65 1.38 3.3 4.4 1.89 750
Tapera 1.30 6.91 0.65 1.09 2.0 3.0 2.66 714Serra da Garganta 1.65 6.63 0.65 1.37 3.5 4.7 1.92 755Patu-2A 2.05 6.Z7 0.65 1.73 5.3 6.8 1.31 785
Notes: P(l), pressure based upon Johnson & Rutheford (1989) ealibration; P(2), pressure based upon Sehmidt
(1992) calibration T(l), temperature based upon Blundy & Holland (1990) equation and respective pressure, on
Sehmidt's equation; T(2) temperature based upon Nabelek & Lindsley (1985) thermometrie equation for mafie
metamorphie roeks; and K is the equilibrium constant «Si-4)/(8-Si»Xab from Blundy & Holland (1990).
An. Acad. bras. Ci., (1993) 65 (Supl. 1)
CONTRASTING METALUMINOUS MAGMATIC EPIDOTE-BEARING... 151
brated at temperatures in the 500-1000oC range.
This thermometry can be applied only to assem
blages where plagioclase shows less than 7.8 Si at
oms per formula unit.
Average plagioclase compositions (Xab in Ta
ble I) and coexisting amphibole compositions in
CSF plutons, yield temperatures in the 680-81OoC
range, using pressure values estimated through
Schmidt's barometric equation for each sample.
For mEp-bearing plutons in the Seridó Foldbelt,
temperatures in the 660-720oC range are obtained
through this method.
Nabelek & Lindsley (1985) assumed that
Al(IV) in amphibole is a function of temperature,
pressure and bulk composition and found that itcan be a useful thermometer for mafic metamor
phic rocks, provided that Ca-amphibole coexists
with a suitable buffering assemblage: intermediate
plagioclase, Mg-Fe silicate and possibly Fe-Ti ox
ides. Oxygen fugacity must be at or below theFMQ reaction buffer.
Temperatures obtained through their thermo
metric equation for amphibolite xenoliths in the
mEp-bearing plutons in the CSF (e.g. Boa Ventura
pluton) are in the 670-690oC range, typical of am-
phibolite facies rocks. The granodiorite/tonalite
hosts, however, display higher temperature values
(700-720°C) according to Blundy & Holland's
thermometric calibration for plagioclase-hom
blende pairs. This situation is in agreement with
the assumption that these amphibolites are xeno
liths and not a result of segregation from the host
magma.
(d) ZR THERMOMETRY
Watson (1987) proposed that the Zr content of
a granitic rock can be used to estimate the tem
perature when the magma becomes saturated inthis elemento Under certain circumstances, this
simple method can offer an estimate of the tem
perature of the liquidus. The only possible incon
venience is the presence of inherited zircons that
can partially account for the Zr contents detected
in whole-rock analyses, leading to false temperature estimates.
Magmatic epidote-bearing plutons in the CSF
share roughly the same petrographic and minera
logical characteristics and, therefore, should have
experienced similar crystallization histories. The
liquidus temperature in these plutons, at the depth
Cochoeirinho- SolQueiro FoIdbelt
8Amph. Xenol.
~
6L~o
.c~l.LJ
o::4;:) (f)(f)l.LJo:: Seridó Foldbelt11. 2
o650 700 750
TEMPERATURE (OCl
800
Fig. 7 - Pressure and temperature estimates of solidification of Cachoeirinha-SaIgueirogranitoids (e) and amphibolite xenoliths (O), and Seridó (.) plutonic rocks (São Rafael pluton).Pressures estirnated by the Schmidt's (1992) Al-in-homblende geobarometer; temperaturesestirnated by the Blundy and Hol1and's geothermometer (1990).
An. Acad. bras. Ci., (1993) 65 (Supl. 1)
152 ALCIDESNOBREGASIAL
of emplacement, should have varied very little and
so they offer a good opportunity to test the application of the Zr saturation method to estimate it.
These calculations are important, since they
provide the only evidence of minimum liquidus
temperatures that may be comparable to conditions
of melting formation. The inherent assumption isthat most zircons in these rocks is not of restitic or
cumulate origino
Temperatures (785-850°C) obtained by this
method (Table 11), plotted against pressures esti
mated by Schmidt's equation (Fig. 8) show a
reasonable alignment, suggesting, therefore, that
they are primary, near-liquidus temperatures. Since
these magmas are relatively hydrated, this liquidus
estimate does not seem to be totally unrealistic.
Temperatures estimated by Blundy & Hol
land's method (Fig. 8) do not exhibit good align
ment and lie between the liquidus determined
above and the excess-water solidus for grano
diorite composition, experimentally determined by
Piwiinski & Wyllie (1968). This implies that: (a)
most of the analyzed homblendes are not near-soli
dus phases or, (b) analyzed plagioclases may not
be in equilibrium with homblende, or (c) more
likely that the solidus differs slightly fiom pluton
to pluton.
Ao average of nine determinations of pressure
and temperature for amphibolite xenoliths in CSF
mEp-bearing plutons lies very near the am
phibolite solidus (line 3 in Fig. 8; Wyllie, 1981),
an expected behavior if these amphibolites are
fiagments from the source.
(e) EPIDOTECHEMISTRY
Microprobe data indicate that the atomic
Fe +3/(Fe+3 + AI) ratio (pistacite content) of euhe
dral mEp in the São Rafael batholith in the SFB
lies in a narrow range (PSZ7-Z9)without systematicvariation of AI and Fe contents fiom core to mar
gin. The pistacite components are within the range
(25-29%) reported to be typical for mEp (Tulloch,
1976, 1986; Vyhnal et aI., 1991). Galindo (1993)
reported epidotes in the Prado pluton with a nar
row range of composition (ps=28-29%) equivalent
An. Acad. bras. Ci., (1993)65 (Supl.1)
to that observed in epidotes of the São Rafael plu
ton (Table I1I).
In the CSF, mEp exhibits compositions be
tween Pszo and PSZ4a variation equivalent to that
of epidote phenocrysts (PSI9-24) in high K-calc-al
kalic dikes of the Front Range of Colorado (Dawes
& Evans, 1991) considered to be the strongest can
didates for unequivocally magmatic epidotes.
Some examples described by Farrow & Barr
(1992), Rogers (1988) and Owen (1991) also lie in
this range. Typically, the CSF magmatic epidotes
have lower proportions of the pistacite component
and higher Si, AI, Ca, Ti and lower Fe contents
than those of the SFB (Fig. 9).
It seems clear that epidote in the Seridó Fold
belt and CSF crystallized under different oxygen
fugacities. In the first case, the epidote composi
tion lies between curves which represent the stabil
ity limit of epidote + quartz corresponding,
respectively, to nickel-bunsenite (NB) and magnet
ite-hematite (HM) buffers (25 and 33% Ps in epi
dote, respectively) (Liou, 1973). This implies in
that crystallization probably with fOz between
these two buffers. In the CSF, judging fiom epidote
composition, the oxygen fugacity prevailing dur
ing crystallization prob~bly' followed, or was
slightly below the NB buffer curve.
ISOTOPE CHEMISTRY
(a) OXYGEN ISOTOPES
Oxygen isotope analyses were performed at
the stable isotope laboratory of the Federal Univer
sity of Pemambuco (LABISE) and at the Univer
sity of Georgia, USA.
The mEp-bearing plutons in the CSF, includ
ing those with clinopyroxene, exhibit high ô180(+ 11 to +13%0, Table IV). Those with dinopy
roxene (e.g. Pedra Branca) are isotopically very
homogeneous (+11.6 to + 12%oSMOW)in conso
nance with their major and REE-chemistries which
show very limited variation. Quartz diorite en
claves display ô180 in the same range as the gt'll--'nodiorite/tonalite host, suggesting that they were
cogenetic or that they reached isotopic equilibrium
with the host. Amphibolite xenoliths exhibit values
CONTRASTING METALUMINOUS MAGMATIC EPIDOTE-BEARING ...
TABLE nTemperatures for mEp-bearing p1utonsIn tbe CSF and SFB determined from tbe Zr saturatioo.
Sample
Zr(ppm)T"cPluton Rock type
A) CACHOEIRINHA-SALGUEIRO FOLDBELTMBV-I7B
56707Boa Ventura Type I amphibolite
AMPHI-l
44690
AMPH-2
58710Santo Antônio Type 11AMPH-3
49698Creek amphiboliteAMPH-4
46693
MAF-l
54705ConceiçãoMAF-2
64718
MC-l
240830
MC-4
180803
MC-6
240830Conceição Granodiori-
MC-8
240830 te/tonalite
MC-9
240830
MC-1O
240830
MC-35
240830
MC-l
190808Conceição Quartz diorite
MC-E/2
150787 enclave
E-I
200813Emas Granodiorite
E-2
250833
Elenclave
220821Qz dioriteA
enclaveI I
PB-30290853
PB-31
240830
PB-33
260837Pedra Branca
PB-l
280844
PB-2
270841
MBV-16
210817
MBV-17
220821
MBV-18
220821
MBV-19
220821I
'I MBV-2O220821Boa Ventura Granodiori-
MBV-21
240830 te/tonaliteMBV-23
240830
MBV-25
230826
MBV-26
230826
MBV-2
250833
SER-45
188807Santo Antônio
SER-47
188807Creek
GranodioriteSER-77
188807Penaforte
SER-86
220820Ipueiras
to be continued
An. AClId. bras. Ci., (1993) fi (Sl1pL 1)
154 ALCIDES NOBREGA SIAL
TABLE 11 (Continued)
887 Baixio Granodiorite
921
Patu Tonalite
901
Brejo do Cruz
Granodiorite918
Serra Negra do Norte
821
Tourão*
889 Monzogranite875
Caraúbas*
863 818
Prado * Granite
861
B) SERIDÓ FOLDBELT793
833
679
774
758
793
780
713 São Rafael
821
808
711
793
821
830
793
833
785
753
830
Sample Zr(ppm)
KSR-2
160
KSR-3
250
KSR-3B
39
KSR-3C
128
KSR-4
106
KSR-5
160
KSAR-5A
138
KSR-7
60
KSR-8
220
KSR-8A
190
KSR-9
59
KSR-9A
160
KSR-lO
220
KSR-11
240
KSR-12A
160
KSR-13
250
KSR-14
146
KSR-15
100
KSR-15F
240
BAX-l
430
PATU-2A
590
BC-IA
490
SNN-2
570
T-83
221
T-183
438
C-135
383
C-284
339
P-318d
212
P-254
333 T>c Pluton Roektype
Qz monzonite
to granodiorites
* Analyses fiom the Tourão, Caraúbas and Prado plutons from Galindo (1993). Temperatures
ealculated by Watson's method (1987): T>c = -273 + 12900/17.18 -ln(Zr).
from +10.1 to +11.6%oSMOW, an O-isotopic range
predictable for the souree of these magrnas.
The mEp-bearing quartz monzonites in the
SFB (e.g. São Rafael) exhibit lower Ô180 (7.8
8.1%0) than the CSF ones. Quartz diorite inclusions
An. Acad. bras. Ci., (1993) 65 (Supl. 1)
display oxygen isotope ratios equivalent to the
hosts (around +7.9%0) suggesting that thermal or
isotope equilibrium was attained on cooling. The
mEp-bearing tonalite near Patu, which intruded Ju
curutu gneisses, exhibit Ô180 of 6.4%0.
CONTRASTING METALUMINOUS MAGMATIC EPIDOTE-BEARING ...
310
\t9 \2,* Toe (Zr SoU
•*
\\I
• Toe (8 e: H)8 •* ,
~
•*oAmphibolite Xenolith7 * \
(overoge of 9 vo lues )
• ••*
6*\• *-
, .\•
**\... .8 5~ \- a. 4
\\"3
GRANOOIORITE ~"'-
2
SOLlOUS\~""-....AMPHIBOLITE ..>-----SOLlOUS
o
5006007008009001000
T (OC)
Fig. 8 - P-T plot for mEp-bearing grariitoids in the Cachoeirinha-Salgueiro Foldbelt. Curve 1 - obtained
from Zr saturation equation (Watson, 1987) plotted against P, estimated by Schmidt's (1992) barometric
equation. Curve 2 - melting curve for excess H20 granodiorite composition (plwinskii & Wyllie, (1968).
Curve 3 - amphibolite wet solidus (Wyllie, 1981).
155
(b) Sr ISOTOPES
Analyses of Sr isotopes were performed in the
~iversity of Texas at Austin, USA, by Leon E.Long. The initial 87Sr;86Srratios for the mafic en
claves (quartz diorites) and granodiorite/ /tonalite
hosts in the CSF mEp-bearing granitoids are ex
tremely close (=0.7060). In the Conceição pluton,the enclaves have initial Sr ratio of 0.70598, the
hosts 0.70603 and if Rb-Sr data are regressed to
gether the results are t = 633 0.9 Ma (Fig. 10) and
initial 87Sr;86Sr = 0.70598 0.00001 (2) withMSWD= 0.08, attesting that their Rb-Sr systems
are similar. Therefore, Sr isotopes demonstrate that
quartz diorites are products of fractional crystal
lization from the same magma which generated the
tonalites and granodiorite hosts.
Rb-Sr isotope analyses for cpx-bearing plutons (e.g. Pedra Branca) do not provide an acceptable isochron age, since data points are scattered.Model age using a reasonable assumed initial
87Sr;86Sr of 0.706 are very high (800 to 900 Ma).This implies in that either this pluton is much olderthan the others in the CSF, under consideration, or
if it is younger, then it must have had an extraordi
narily high initial ratio. More detailed work isneeded before one comes to a final conclusion.
In the Seridó Foldbelt, Galindo (1993) re
ported Rb-Sr ages of 654 ± 24 Ma, 631 ± 23 Ma
and 600 ± 7 Ma respective1y for the Prado,
Caraúbas and Tourão plutons, alI of them with highinitial ratios (0.707). No Rb-Sr data are available
for the São Rafael and other mEp-bearing plutonsin the Seridó Foldbe1t.
An. Acad. bras. Ci., (1993) 65 (Supl. 1)
156 ALCIDES NOBREGA SIAL
TABLE mRepresentative eleetron microprobe analyses of magmatic epidote from CSF and Seridó Foldbelt plutons.
CACHOEIRINHA-SALGUEIRO FOLDBELT SERIDÓ FOLDBELT
PInton
Boa Ventura São RafaeIPrado*
SampIe
MBV-23MBV-19-BMBV-19
Grain
1210111412123 12
Si02
38.4438.1238.0737.4537.3637.5437.6938.0037.8137.6937.5537.12
Ti02
0.220.150.200.210.160.090.040.050.070.020.000.03
A1203
24.3323.8025.4824.3124.5422.1622.0522.02~2.0721.8122.4121.69
Cr203
0.050.050.000.050.110.000.000.000.000.00
MgO
0.050.040.060.080.030.020.000.000.000.00
Cao
23.8723.9023.9123.9024.1723.3423.3423.2223.7322.3422.5622.12
MnO
0.220.190.160.100.140.100.190.220.110.100.260.15
FeO
10.3810.708.999.929.5313.8513.2113.7514.2413.9712.4612.73
srO
0.110.150.200.150.19
BaO
0.000.030.000.010.05
Na20
0.030.000.020.010.01
K20
0.010.010.000.000.010.000.000.000.000.00
H20
1.861.841.861.83
Total
99.5798.9898.9698.0198.1397.1196.5397.2997.0695.9595.2493.84
Number of cations on the basis of 25 oxygensSi
3.1023.1043.0703.0713.0592.9032.9262.9292.9212.9213.2653.281
Ti
0.0130.0090.0120.0130.0100.0050.0020.0030.0040.0000.0000.002
A1
2.3142.2842.4212.3492.3692.0202.0182.0012.0102.0012.2962.259
Cr
0.0030.0030.0000.0030.007
Mg
0.0050.0150.0070.0100.0030.0020.0000.0030.0000.000
Ca
2.0672.0862.0662.0992.1211.9341.9411.9181.8811.8812.1012.094
Mn
0.0140.0130.0110.0070.0100.0070.0120.0140.0070.0070.0190.011
Fe
0.7000.7250.6070.6800.5150.8070.7730.7990.8290.8300.9060.941
Sr
0.0050.0070.0100.0070.009
Ba
0.0000.0010.0000.0010.001
Na
0.0050.0000.0020.0010.0010.0000.0000.0000.0010.001
K
0.0000.0010.0000.0000.0010.0000.0000.0000.0000.000
Ps232420222228272829292829
Analyses in wt.%; total Fe measured as FeO; * from Galindo (1993).
Homblende and biotite from the Conceição
pluton have intemally discordant 40Arf9 Ar age
spectra, as determined in the K-Ar laboratory of
the University of Georgia, USA, suggesting that
initial post-magmatic cooling through Ar retention
temperatures occurred between 625 Ma (hbl) and
604 Ma (bi) (Dallmeyer et ai., 1987). Such discor
dance in argon-argon ages reinforces that crystal
lization was rather deep, although this pluton
An. Acad. bras. Ci., (1993) 65 (Supl. 1)
intruded green-schist facies metasediments, towhich P-T data are not available.
CONCLUSIONS
From this study, it is concluded that there is a
correlation between high pressure Al-in-hom
blende with the presence of mEpin the granodiorite/tonalites in the CSF. However, estimated
pressures of amphibole solidification (6-9 kbar)
cannot be reconciled with the regional, rather low
grade, metamorphism. In the SFB, this is not the
case, and Al-in-homblende yields pressure for the
CONlRASTING METALUMINOUS MAGMATlC EPIDOTE-BEARING... 157
e
76
(/) Q) Cachaeirinhaa. E5
Salgueiroo plutons(/) - 4o ~Q).a 3E ::JZ 2.alterotionof '\plagioclose
O (
Rafoelplutons
epidote
alteration
of ~biotite \
)
O 10 20 30 40 50
Mole % pistacite (Fe3• / Fe3't + AI)Fig. 9 - Mole% pistacite in magmatic epidote of the São Rafael batholith (Seridó FoldbeIt), CSFmEp-bearing plutons and pistacite compositions of non-magmatic epidote. The ranges of compositiqns ofepidote from alteration of plagioclase and biotite are from Tulloch (1979) and for igneous epidote, fromJohnston & WyIlie (1988).
São Rafael in agreement with those found for
nearby country rocks (3-5 kbar), implying in thatamphibole is a near-solidus phase in these plutons.
Archanjo (1993) proposed that, in theCachoeirinha-Salgueiro Foldbelt, an E-W exten
sional tectonic phase (recorded in magneticsusceptibility studies of plutons of this age) tookplace towards the end of the Brasiliano. This ex
tension, perhaps, facilitated a substantial upward
migration of some CSF plutons afier their originalemplacement.
Quartz diorite enclaves in the CSF plutonswere originated by different pulses of cogenetic
mafic magma injected into the tonalites/ grano
diorites. An altemative hypothesis is the immiscibility of acidic and basic components, during
cooling of a melting of intermediate composition,as proposed by Bender et aI (1982)and Mezger et
al.(1985). However, this second hypothesis lacksfield morphologic evidence and probably is not theanswer for this problem.
Oxygen isotopes in the Pedra Branca cpx
bearing pluton show very little variation (+11.8 ±
0.2%0) that associated with the very limited variation in its major and REE chemistry, suggest littlecompositional variation during crystallization. Altematively, a eutectic melting could have produced
a large amount of magma with a limited compositional range, emplaced subsequently without sig
nificant crystal fractionation. Textural relationshipssuggest that clinopyroxene, homblende and biotite,
at least in part, formed concomitantly. According
to Naney & Swanson (1980), these phases couldcrystallize together from granodiorite magmas at9000C and 8 kbar pressure with H20 6%. Partial
fusion of a source which contains feldspar andhomblende or biotite and one of these two last
phases remains as a residual phase, yields meltswith 4 to 6% H20 content by weight (Wones,1981). If the source rocks for these granodiorite/tonalite magmas were hydrated-basalts, partial
melting would leave amphibolite (hbl + bi + cpx)
An. Acad. bras. Ci., (1993) 65 (SupI. 1)
158 ALCIDES NOBREGA SlAL
TABLE IV
Oxygen isotope data (Õ18Q%o SMOW)
Pluton
Rocktype Ô180
wholerock
quartzfeldsparbiotiteepidote
A) Cachoeirinha-Salgueiro FoldbeltConceição
tonalite+11.5,+11.6+13.6+12.3 +9.1
Serrote da Cachoeiragranodiorite+12.8
Santo Antônio Creekgranodiorite+11.8,+12.4+13.1+11.5 + 8.9
Ipueiras
granodiorite+12.1,+12.9,+12.8
Penafortegranodiorite+11.5,+12.6,+11.2
+12.8,+12.6
Unamed stock, 27 kmgranodiorite+11.2,+12.3,+12.8
north of Serrita
+12.2
Carrnogranodiorite+12.7,+12.9
Brejinho
bimonzo-+10.5,+12.0,+10.8+12.6,+12.0+10.9,+11.3+7.9, +7.3
granite
+11.6,+10.9,+11.1 +11.6
• Brejinho
qz diorite+10.0• Boa Ventura
amph. xenol.+11.6,+10.1,+11.5
• Boa Venturagranodiorite+12.0,+11.3,+11.1
• Pedra Brancagranodiorite+11.8,+11.8,+12.0, +11.8,+11.6
Emas-Olho D'Agua
granodiorite+11.4,+11.6,+11.9+13.7,+13.9+11.5,+11.5+8.4, +8.0+9.7, +9.4
(Goist, 1989)
+11.8,+11.1,+11.2+14.1,+11.8+12.3,+11.8+10.1,+8.9+12.4,
+11.4,+11.6+13.9 +11.6+8.4+11.9,
+11.2,+11.6
• Emas-Olho D'Agua
amphi. xenol.+10.5
• Emas-Olho D' Agua
qz diorite+10.4enclave
B) Seridó FoldbeltSerra Negra do
granodiorite+6.2Norte São Rafael
qz monzonite+7.9,+7.8,+8.1
• São Rafaelqz diorite+7.9
• Baixioqz diorite+7.8
• Patu-2A
tonalite+6.0
• Brejo do Cruz
tonalite,+6.1
* Analyzed at the stable isotope Laboratory (LABISE) in the Federal University of Pemambuco, Brazil. AlI other
analyses perforrned at the Department of Geology, University of Georgia, USA.
as a residue, and magmas with H20 in the 4-6%
range would have been generated.
Experiments on amphibolites (Holloway &
Burnham, 1972, Helz, 1973, 1976, and Beard &
Lofgren, 1991) indicate that moderate degrees of
melting (30% to 50%) produce liquids of tonalite
granodiorite-granite compositions. In several stud
ies, amphibolite has been proposed as a possible
source rock for calc-alkalic magmas (e.g. Tepper,
1992, Gust & Arculus, 1986).
An. Acad. bras. Ci., (1993) 65 (SupI. 1)
Amphibolite xenoliths in the CSF mEp-bear
ing plutons show chemistry, O-isotopic signatures
(+10.1 to +11.6%0), P and T ranges, compatible to
the characteristics that one could predict for the
source (weathered ocean floor basalt) of CSF mag
mas. Therefore, amphibolite anatexis seems to be a
tenable hypothesis for the origin of the CSF mag
mas under consideration.
In the Ab-Or-Qz temary projection (Fig. 11) it
is shown the trends of partial melting of amphibo
lite at PH20 = 5 kbar according to Helz (1976), in
CONTRASTING METALUMINOUS MAGMATIC EPIDOTE-BEARING ...
0.73
159
0.72
0.71
0.70O
oSanto Antõnio .tock ./
t = 633:t 0.9 Ma
~
(not nome0~ost rock ] Conceição lIronodioriteenclave ----- .tock
0.70598:t 0.00001
2
87Rb/86Sr3
Ab
Fig. 10 - Whole-rock Rb-Sr diagram for Cachoeirinha-Salgueiro plutons (from SiaJ et aI.,
in preparation).
11Pe~ra Bronco granodiorite
QzCEmas granodiorite
+Boa Ventura granodiorite
•Qz - dioriteenclaves
O
Conceicão granodiorite
••Amphibolite xenolith
Or
Fig. 11 - Ab-Or-Qz temary projection of some CSF mEp-bearing granodiorites/tonalites. Trends of partiaJ
melting of amphibolite (1 - tholeiitic; 2 - alka1ic composition) aI PH20 = 5 kbar, from Helz (1976), shown forcomparison.
An. Acad. bras. Ci., (1993) 65 (Supl. 1)
160 ALCIDESNOBREOASIAL
where Curve 1 represents a tholeiitic composition
and curve 2, an alkalic one. The CSF mEp-bearing
plutons form a trend parallel to the two curves de
termined by Helz. This suggests that melting
and/or crystallization happened at different water
vapor and lithostatic pressures than that in Helz's
experiments. The fact, however, that these trends
are parallel seem to support the hypothesis that
these magmas formed by melting of an amphibo
lite whose composition was slightly more potassic
than those in the experiments (amphibolite compo
sition in the Fig. 11 plots in the Or field as a conse
quence of its potassic composition).
For mEp-bearing plutons near Patu, to the
west margill of the SFB, Galindo (1993) advocated
a crostal origin (30% melting; e.g. Tourão), hy
pothesis which finds support in the high initial Sr
ratios. He proposed a monzonitic source for those
magmas which in their ascent underwent amphibole + biotite fractionation. The São Rafael batho
lith, the best representative mEp-bearing granitoid
in this belt, still lacks detailed isotopic studies to
allow petrogenetic interpretation of its origino
ACKNOWLEDGMENTS
I am indebted with RY. Fodor (North Caro
lina State Univ., Raleigh, USA), Christopher Fleis
cher (Athens, Georgia, USA) and Manoel J. M.
Cruz (Federal Univ. of Bahia) for some of the mi
croprobe analyses used in this study. I am also
thankful to Prof. Leon E. Long (Univ. of Texas at
Austin, USA) for the Rb-Sr dating. Finally I would
like to thank the PADCT/FINEP program for .lhe
financial support to this research project.
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