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.

REFERENCES

ALMEIDA,F. F. M. DE,LEONARDOS,O. H., VALENÇA,J.,

(1971), Review on Granitic Rocks of NortheastSouth America. Recife, IUOS/UNESCO,41p.

ARCHANJO,C. J., (1993), Fabrique des plutons granitiqueset deformation crustale dans le Nord-Est du Bresil:

une etude par anisotropie de susceptibilitémagnetique de granites ferromagnetiques. Thêse deDoctorat, Université Paul-Sabatier de Toulouse,

France, 167p.

An. Acad. bras. Ci., (1993) 65 (Supl. 1)

BEARD,.J.S.& LoFGREN,O.E., (1991), Dehydration meltingof basaltic and andesitic' grenstones andamphibolites at 1,3, and 6.9 kb. Jour. Petrol., 32:365-401.

BENDER,J. F., HANSON,O.N.,ANOBENCE,A. E., (1982), TheCortland Complex: evidence for large-scale liquidimmiscibility involving granodiorite and dioritemagmas. Earth PlanoSei. Letters, 58: 330-344.

BERZA,T., MACAIET, V., ANDAR,P. ANOUDRESCU,C. (inpress). Retezat granitoid pluton (SouthCarpathians)- a geochemical approach: RomanianJour. Petrol., 76: 8p.

BLUNDY,J. D. & HOLLAND,T. J. B., (1990), Calcicamphibole equilibria and a new amphibole­plagioclasegeothermometer. Contrib. Mineral:Petrol., 104: 208-224.

BRITONEVES,B. B., (1983), O mapa geológico doNordeste Oriental do Brasil, Escala 1/ 1,000,000.

Tese de Livre-Docência, Univ. de São Paulo, 177p.

BREW,D. A., (1992), Origin and distribution of graniticrocks and related rocks in the coast plutonic-meta­morphic complex, N. American Cordillera,southeastern Alaska, U.S.A.. In: Brown, P.E. &ChappelI, B.W. (eds): The Second HuttonSymposium on the Origin of Granites and RelatedRocks. Trans. Royal Soe. Edinb.: Earth Sei., 83,parts 1 and 2: 486.

CULLERS,R. L., ORIFFIN,T., BICKFORD,M.E.,ANOANDERSoN,

L., (1992), Origin and chemical evolution of the1360 Ma San Izabel batholith Wet Mountains,

Colorado: a mid-crustal granite anorogenicaffinities. Geol. Soco Amer. Bull., 104: 316-328.

DALLMEYER,R.D.,SIAL,A. N.,LoNG,L. E. ANOMcMuRRY,J.,

(1987), New evidence for the polyphasetectonothermal evolution of the Brasiliano orogen,Northeast, Brazil. Geol. Soco Amer. Annual

Meating, Phoenix, Arizone, p. 634.

DAWES,R. L. & EVANS,B.W., (1991), Mineralogy andgeothermobarometry of magmatic epidote-bearingdikes, Front Range, Colorado. Geol. Soe. Amer.BulI., 103: 1017-1031.

FARROW,C. E. O. & BARR,S. M., (1992), Petrology ofhigh-Al-hornblende and magmatic epidote-bearingplutons in the southeastern Cape Breton Highlands\Nova Scotia. CanoMineral. 30: 377-392.

CONTRASTING METALUMINOUS MAGMATIC EPIDOTE-HEARING ... 161

FORISZ, 1., VUKOV, M. ANO JOVIC, V., (1989), Petrologiealsignifieanee of primary and seeondary epidote inthe Zeljin pluton, Yugoslavia. 14th Cong. ofCarpatho-Balkan Geol. Assoe., Sofia, Ext. Abs.:202-205.

GALINDO, A. C., (1993), Petrologia dos granitoidesBrasilianos da região de Caraúbas e Umarizal(Oeste do Rio Grande do Norte). Doetoral thesis,Federal Univ. of Pará (UFPA), 886p.

GHENT, E. D., NICHOLS,J., SIMONY,P. S., SEVIGNY,J. H. ANO

STOUT, M. Z., (1991), Homblende geobarometry ofthe Nelson Batholith, southeastem British

Columbia: teetonie implieations. Cano Jour.Earth-Sci., 28 (12): 1982-1991.

GOIST, T. O., (1989), Petrology and geoehemistry of theEmas pluton, Northeastem Brazil. Master ofSeienee thesis, University of Georgia, Athens,USA, 158p.

GUST, D. A. & ARCULUS, R D., (1986), Petrogenesis ofalkalic and eale-alkalie volcanie roeks of Mormon

Mountain volcano field, Arizona. Contrib. Mineral.Petrol., 94: 416-426.

lIAMMARSTROM,1. & ZEN, E-AN, (1983), Possible use of AIeontent in homblende as a geobarometer forplutonie roeks. Geol. Soe. Amer. Annual Meeting,Abstraets with Programs, Indianapolis: 590.

lIAMMARSTROM,J. & ZEN, E-AN, (1986), AIuminum inhomblende: An empirieal igneous geobarometer.Amer. Mineral., 71: 1297-1313.

lIAMMARSTROM, J. & ZEN, E-AN, (1992), Petrologicaleharaeteristies of magmatie epidote-bearing granitesof the western Cordillera of North America: In:

Brown, P.E. & Chappell, B.W. (eds): The S~eondHutton Symposium on the origin of granites andrelated roeks. Trans. Royal Soe. Edinburgh: EarthSei. 83, parts 1 and 2: 490-491.

HELZ, RT., (1973), Phase relations of basalts and theirmelting ranges at PH2o= 5kb as a function ofoxygen fugacity. Part I. Mafic phases. Jour. Petrol.134: 249-302.

HELZ, RT., (1976), Phase relations of basalts and theirmelting ranges at PH20 = 5kb. Part 11. Melteomposition. Jour. Petrol., 17: 139-193.

HOLLISTER,L. S., GRISSON,G. c., PETERS,E. K., STOWELL,H.

H. ANO SISSON, V. H., (1987), Confirmation of theempirical ealibration of AI in hornblende withpressure of solidifieation of eale-alkaline plutons.Amer. Mineral.,72: 231-239.

HOLLOWAY, J. R & HURNHAM, C. W., (1972), Meltingrelations of basalt with equilibrium water pressureless than total pressure. Jour. Petrol., 132: 1-29.

JARDIM DE SÁ & MARTINS DE SÃ, J., (1987), Proterozoiegranitoids in a polyeyclie setting: a field excursionin the Seridó region, NE Brazil. Int. Symp. onGranites and Assoeiated Mineralizations (ISGAM),Salvador (Brazil), p.33-46.

JOHNSON,M. C. & RUTHEFORD,M. J., (1989), Experimentalealibration of the aluminum-in-homblende

geobarometer with applieation to Long Valleyealdera (California) volcanie rocks. Geology, 17:837-841.

JOHNSTON,A. D. & WYLLIE,P. J., (1988), Constraints on theorigin of Archean trondhjemites based on phaserelationships of Nuk gneiss with HzO at 15 kbar.Contrib. Mineral. Petrol., 100: 35-46.

LiMA, E. S., (1987), Evolução termobarométrica dasrochas metapelíticas da região do Seridó, NordesteBrasileiro. Rev. Bras. Geoc., 17(3): 315-323.

LIOU, J. G., (1973), Synthesis and stability relations ofepidote, CazAhFeSi03012(OH). Jour. Petrol., 14:381-413.

MEZGER,K., ALTIlERR,R:, OKRUSCH,M. HENJES-KUNST,F. ANO

KREUZER, H., (1985), Genesis of acidlbasic rockassociations: a case study of the Kalithea intrusivecomplex, Sarnos, Greece. Contrib. Mineral. Petrol.90: 353-366.

MOENCH, R H., (1986), Comment on "Implications ofmagmatic epidote-bearing plutons on crostalevolution in the aecreted terranes of Northwestem

North America" and "Magmatic epidote and itspetrologic signifieance". Geology, 14: 187-188.

MORTAJI, A. & STUSSI, J.M., (1989), Les granitoidesPreearnbriens de Tagraga d' Akka Anti-AtlasOccidental, Maroe. Colloque de Geologie Franco­Maroeain, Strasbourg, Franee.

NABELEK,C. R & LINDSLEY,D. H., (1985), Tetrahedral AIin arnphibole: a potential thermometer for somemafic roeks. Geol. Soe. Amer. Abstraets with

Programs, 17: 673.

NANEY, M. T. & SWANSON,S. E., (1980), The effeet of Feand Mg on erystallization in granitie systems.Amer. Mineral., 67: 639-653.

OWEN, J. V., (1991), Signifieanee of epidote in orbiculardiorite fIom the Grenville Front Zone, eastemLabrador. Mineral. Mag., 55: 173-181.

An. Acad. bras. Ci., (1993) 6S (Supl. 1)

162 ALCIDES NOBREGA SIAL

PIWINSKll, A J. & WYLLIE, P. J., (1968), Experimentalstudies of igneous rocks series: a zoned pluton inthe Wallowa Batholith, Oregon. Jour. Geol., 76:205-234.

REUSSER,E., (1989), The Al-homblende geobarometer: atheoretical base and application to the AlpineBergell tonalite intrusion. TERRA Abstracts 1: 272.

ROGERS, J., (1988), The Arsiukere Granite of SouthemIndia: Magrnatism and metamorphism in apreviously depleted crust. Chem. Geol., 67:155-163.

SAAVEDRA,J., TOSELLI,A J., ROSSIDE TOSELLI,J. N., RAPELA,

C.W., (1987), Role of tectonism and fractionalcrystallization in the origin of lower Paleozoicepidote-bearing granitoids northwestem Argentina.Geology, 15: 709-713.

SCHMIDT, M. w., (1992), Amphibole composition intonalite as a function of pressure: an experimentalcalibration of the Al-in-homblende barometer.Contrib. Mineral. Petrol. 110: 304-310.

SIAL, A N., (1990), Epidote-bearing calc-alkalicgranitoids in Northeast Brazil. Rev,. Bras. Geoc.20(14): 88-100.

SIAL, A N. & FERREIRA,.V. P., (1990), Granitoids innortheastern Brazil: oxygen and sulfur isotopecompositions and depths of emplacement. Jour.South Amer. Earth Sciences, 3: 103-112,.

SIAL, A. N., FERREIRA, V. P. ANO LoNG, L. E., (in

preparation), High 180, high Al-homblende,epidote-bearing granitoids in Northeast Brazil.

STUSSI, J. M. & MORTAJI, A., (1988), Granites a epidotePrecambriens de Tagragra d' Akka, anti-AtlasOccidental, ·Maroc. 12eme Reunion des Sciences deIa Terre ULLE, Soe. Geol. Fr., edit. Paris: 122.

TEpPER, J. H., (1992), Petrological diversity in I-typegranites due to variation in H2 O fugacity duringmelting of amphibolite. In: Brown, P.E. &Chappell, B. W. (eds). The Second HuttonSymposium, the Origin of Granites and RelatedRocks, Trans. Royal Soe. Edinb.: Earth Sei., 83,parts 1 and 2: 501-502.

An. Acad. bras. Ci., (1993) 65 (Supl. 1)

TuLLOCH, A, (1979), Sec~ndary Ca-Al silicates aslow-grade alteration products of granitoid biotite.Contrib. Mineral. Petrol., 69: 105-117.

TuLLOCH,A, (1983), Granitoid rocks of New Zealand - abrief review. Geol. Soe. Amer. Memoir, 159: 5-20.

TuLLOCH, A, (1986), Comment on "Implications ofmagmatic epidote-bearing plutons on crustalevolution in the accreted terranes of northwestem

North America" and "Magmatic epidote and itspetrologic significance". Geology, 14: 186-187.

WATSON,E. B., (1987), The role of accessory mineraIs ingranitoid geochemistry. In: Hutton Conference ofthe origin of granites. Univ. Edinburgh: 209-211.

WHITE, A J. R., (1992), Granite Handbook: description,genesis, some associated ore deposits. Short course,37° Congr. Bras. Geol., São Paulo: 109p.

WONES, D. R., (1981), Mafic silicates as indicators of

intensive variables in granitic magmas: MiningGeology, 31(4): 212-212.

WYLUE, P. J., (1981), Plate Tectonics and magrna genesis.Geologische Rundschau, Bd. 70, Hefi 1: 128-153.

VAN DER LAAN, S. R. & WYLUE, P. J., (1992), Constraintson Archean trondhjemite genesis from hydrouscrystallization experiments on Nuk gneiss at 10-17kbar. Jour. Geol., 100: 57-68.

VYHNAh C. R., MCSWEEN, H.Y. JR., SPEER, JA., (1981),Homblende chemistry in southem Appalachiangranitoids: implications for aluminum homblendethermobarometry and magrnatic epidote stability.Amer. Mineral., 6: 176-188.

ZEN, E-AN, (1985), Implications of magrnaticepidote-bearing plutons on crustal evolution in theaccreted terranes of northwestern North America.

Geology, 13: 266-269.

ZEN, E-AN, (1988), Tectonic significance of high-pressurerocks in the wsetem Cordillera of Nortn America.

In: Emst, W.G., ed. Prentice-Hall; EnglewoodCliffs, New Jersey, p. 42-67.

ZEN, E-AN & liAMMARSTROM,J. M., (1984), Magmaticepidote and its petrologic significance. Geology, 12:515-518.