The 2 Ga Peraluminous Magmatism of the Jacobina- Contendas Mirante Belt (Bahia) Brazil)" Major and Trace-Element Geochemistry and Metallogenic Potential

Journal of Volcanology and Geothermal Research, 44 (1990) 123 - 141 123 Elsevier Sicence Publishers B.V., Amsterdam

The 2 Ga peraluminous magmatism of the Jacobina- Contendas Mirante Belt (Bahia) Brazil)" Major and trace-element geochemistry and metallogenic potential

Michel Cuney a, Pierre Sabat6 b, Philippe Vidal c, Moacyr M. Marinho d and Herbet Conceiqao d a GREGU and GS CNRS-GREGU, BP 23, 54501, Vandoeuvre Cedex, France b ORSTOM, France and UFBA, CP 4021, Barra, Salvador, Bahia, Brazil CUA 10 CNRS, 2 Rue Kessler, Clermont Ferrand, France dCBPM, CAB, Salvador, Bahia, Brazil

(Received August 12, 1989; revised and accepted March 5, 1990)

ABSTRACT

Cuniey, M., Sabat6, P., Vidal, Ph., Marinho, M.M. and Conceiqao, H., 1990. The 2 Ga peraluminous magmatism of the Jacobina - Contendas Mirante belt (Bahia-Brazil): major- and trace-element geochemistry and metallogenic potential. In: P. ke Fort, J.A. Pearce and A. P~cher (Editors), Collision Magmatism. J. Volcanol. Geotherm. Res., 44:123 - 141.

The Jacobina - Contendas Mirante belt represents a Transamazonian (2 Ga), N - S, 500-km long, elongated orogenic domain in the central part of the S~o Francisco craton, BalSa state. Numerous syntectonic to post-tectonic peraluminous leucogranites were emplaced along the major structures of the belt. Their mineralogical and geochemical and some of their metallogenetic characteristics are very similar to their Hercynian and Himalayan equivalents. However, their average peraluminous index varies from one granitic pluton to another and biotite is, on average, slightly more magnesian in the Transamazonian leucogranites. Higher oxygen fugacity is indicated by the general occurrence of magnetite, the stability of allanite and sometimes epidote in most of the plutons and by biotite chemistry. The peraluminous magmatism of the Jacobina - Contendas Mirante belt results from crustal partial melting during a continental collision event at 2 Ga. Trace-element geochemistry implies variable source composition and/or melting conditions for the different granitic plutons and some different facies within the same plutonic unit. The scarcity of ilmenite, the general occurrence of magnetite, and the relatively low peraluminous index of some of these granites suggest that graphite-beating sediments are not a significantly source material. From their mineralogical and geochemical characteristics, acid meta-igneous rocks such as the Sete Voltas TTG suite of presumed Archaean age, seem to represent a suitable source for these granites. Sn, W, Li, F and Be enrichment of most Transamazonian leucogranites is much weaker than in the mineralized Variscan equivalents. The Caetano-Alian~a and Riacho das Pedras granites represent the most specialized granitic bodies. Beryl (emerald), molybdenite and scheelite mineralizations are related to some of these granites which intrude ultrabasic formations: the Campo Formoso and the Carnaiba granites. In many of these granites, uranium content is comparable to values measured in mineralized Hercynian leucogranites. The occurrence of hexavalent uranium minerals, mineralization and episyenitic alteration are favourable criteria for finding Variscan-type uranium ore deposits.

Introduction products generated during continental collision. These highly differentiated melts result Continental crustal thickening typically from low degrees of melting of metamorphic allows the generation of large amounts of material. Associated intermediate to basic crustal melts without the direct involvement of plutonic rocks are typically lacking. In the Her- mantle material. Peraluminous leucogranites cynian and Himalayan belts, peraluminous represent the most characteristic magmatic leucogranites form relatively narrow belts

elongated over several hundreds of kilometres still a matter of considerable debate. The Tran- parallel to the major thrust planes. Peralumi- samazonian (2 Ga) leucogranites of the Jaco- nous magmatism may also occur at passive bina - Contendas Mirante belt, which forms an margins but in much more limited volumes and elongated belt over 500 km long in the central is mainly represented by volcanic tuffs (Picha- part of the Bahia state (Brazil), represents a vant et al., 1988a, b). rather unique domain for studying this pro- Although the absence of well-developed blem (Figs. 1 and 2). peraluminous magmatism is insufficient to re- As peraluminous leucogranites result from ject a continental collision setting, the presence partial melting of continental crust, the dif- of large volumes of syn- to late-tectonic ferences in their mineralogical and chemical peraluminous leucogranites may be used in compositions reflect the composition of the conjunction with other geological evidence to various igneous and/or sedimentary sources signify the presence of ancient suture zones bet- that have contributed to magma generation. ween continental masses in old orogenic belts. Thus, the geochemical signature of the Tran- The existence of modern plate tectonic regimes samazonian leucogranites is an important tool during Archaean and early Proterozoic times is for evaluating the composition of the Archaean

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Fig. 1. Simplified geological map of the Bahia state (Brazil) 1 = Undifferentiated formations; 2 = Contendas Mirante, Jacobina and Itapicuru Transamazonian volcanosedimentary belts; 3 = Salvador Curag~i and Atlantic coast belts; 4 = Jequi6 Archaean crustal domain; 5 = Gaviho Archaean crustal domain; 6 = major thrust faults. MAGMAT1SM OF THE JACOBINA-CONTENDAS MIRANTE BELT (BAHIA-BRAZIL) 125

380 i -o

ii 21 31 41 5 GAMELEIRA

RIACHO DAS PEDRAS

CA ETANO I ALLAN

LAGOA GRANDE LAGOINHA

Fig. 2. Geological map of the Transamazonian Jacobina - Contendas Mirante belt with the location of the major granitic plutons. 1 = Contendas Mirante Jacobina and Itapicuru Transamazonian volcanosedimentary belts; 2 = Transamazo- nian granites; 3 = Jequi6 Archaean crustal domain and Salvador Cura~fi-Atlantic coast belts; 4 = Gavi~o Archaean crustal domain; 5 = major thrust faults. crust, or the crust at the Archaean-Lower Transamazonian leucogranites have been Proterozoic transition, depending of the age of characterized and compared with their Hercy- the source material. This geochemical nian and Himalayan equivalents. signature also has direct implications for the evaluation of the metallogenic potential of Regional geology these granites. To achieve these objectives, the mineralogy, The Jacobina - Contendas Mirante volcano- major- and trace-element geochemistry of the sedimentary belt is located in the eastern part of 126 M. CUNEY ET AL, the S~o Francisco craton (Fig. 1). This part of most of these granitic bodies is supported by the craton is free of tectonothermal events of their shape, which is elongated parallel to the Brasiliano age, unlike the western part major thrust planes, and by the magmatic folia- (Espinh~tgo belt, Turpin et al., 1988). The Sho tion, parallel to ductile shear zones that are Francisco craton represents one of the largest generally developed within the pluton at its pieces of Archaean and lower Proterozoic crust eastern margin, and parallel to the foliation of in South America and is mainly exposed in the the surrounding metamorphic rocks. state of Bahia. The lower Proterozoic Jacobi- na-Contendas Mirante belt, onented N-S The Jacobina belt and about 500 km long, lies between two major Archaean crustal domains (Figs. 1 and 2): the In the Jacobina area (Fig. 1), the Archaean Jequi6 domain, with the Atlantic coast mobile basement, composed of high grade metamor- belt to the east, which is over 700 km in length phic rocks (migmatites, itabirites, quartzites, from north to south; and the Gavi~o domain to calcsilicates and metabasic to ultrabasic rocks) the west. is overthrust from east to west by the Serra de The Jequi6 domain and the Atlantic coast Jacobina series. The first phase of deformation mobile belt are composed of high to very high was followed by an E-W shortening with grade volcanosedimentary formations, char- strike-slip tectonism leading to vertical fold nockites and enderbites (Mascarenhas, 1973, axes and the horizontal structures were 1976; Babosa, 1986); the protoliths of the Je- transformed into steep N-S trending belts. qui6 formations have given Rb/Sr dates of late The Jacobina series forms a narrow (6- 12 km Archaean age (2.7. Ga, Cordani et al., 1985) wide), elongate belt over 200 km in length, and, in the western part, metamorphosis of age composed of thick metaquartzite units, meta- Transamazonian (Wilson, 1987). conglomerates, mica-schists, banded iron for- The Gavi~o domain is composed of a mations and intercalated meta-ultrabasic slices migmatitized gneiss-amphibolite complex 100- 300 m thick. According to Couto (1978), corresponding to a volcanosedimentary asso- the micaschists may correspond to volcanic ciation. It represents the basement of the formations. The total thickness of the Jacobina Jacobina-Contendas Mirante supracrustal series may reach 8 kin. In the micaschists, the sequences. Several large Archaean domes, metamorphic paragenesis varies from north to elongated north-south, composed of a south from andalusite-kyanite to cordie- trondhjemite-granite association similar to rite - anthophyllite - sillimanite - garnet. the classical Tonalite - Trondhjemite - Grano- Seven leucogranitic plutons have been recog- diorite (TTG) suites, are tectonically emplaced nized in the Jacobina area (Couto, 1978; Celino within the supracrustal sequences in the and Sabat6, 1988; Rudowski and Fonteilles, southern part of the Contendas Mirante belt: 1988). Two of them have been sampled, the these include the Lagoa do Morro, Sete Voltas Campo Formoso and the Carnaiba plutons and Boa Vista domes. They probably represent (Fig. 2). the oldest lithologies in the S~o Francisco craton (3.1 and 3.5 Ga Rb/Sr age for the Lagoa The Contendas Mirante belt do Morro and Boa Vista domes, respectively, Cordani et al., 1985). Two lithostratigraphic units have been re- Over twenty syn- to late-tectonic leucograni- cognized (Marinho et al., 1979, 1980): a lower tes intrude either the Gavi~o basement or the unit composed of basalts and intermediate Jacobina-Contendas Mirante supracrustal volcanics with intercalations of clastic and sequences. The syntectonic emplacement of chemical sediments, and an essentially detrital MAGMATISM OF THE JACOBINA-CONTENDAS MIRANTE BELT (BAHIA-BRAZIL) 127 upper unit. Metamorphism increases towards of early-emplaced granites in later intrusions. the Jequi6 block from low to medium grade. Basic, microgranular enclaves have never been Two major phases of deformation have been observed. recognized (Sabat6 et al., 1980): westward overthrusting was followed by an E-W Carnpo Formoso shortening which rotated the previous structures towards the vertical. The horizontal The Campo Formoso composite pluton is structures generated during the first phase were a circular, two-mica leucogranite, with a dia- progressively transformed into steep N-S meter of about 25 km, mainly emplaced in trending elongated belts. The Jequi6 block is the Archaean basement. The southern and overthrust onto the Gavi&o block and the Con- eastern part of the granite intrudes serpen- tendas Mirante belt (Sabat6 et al., 1988). tinites which form the basal contact of the At least fifteen leucogranitic plutons have Jacobina overthrust formations. Two main been recognized in the Contendas Mirante area units have been recognized in the granite (Marinho et al., 1979, 1980; Marinho and (Rudowski and Fonteilles, 1988; Rudowski, Sabat6, 1982; Concei~o, 1986). Six of them 1989). An early external fine to coarse grained have been sampled in the Contendas Mirante unit, emplaced at the southern and eastern belt, from the Gameleira pluton in the north margins of the pluton, is characterized by an through the Riacho das Pedras, Alianya, abundance of large muscovite megacrysts. The Caetano and Lagoa Grande plutons to the central unit, which represents the main part of Lagoinha pluton in the south. the pluton, is porphyritic, richer in biotite and Deformed and undeformed leucogranitic poorer in muscovite. Late garnet-bearing plutonic rocks of the Sete Voltas Archaean granites and aplopegmatites are widespread dome have been also studied. They represent throughout the pluton. Only one emerald de- one possible source material for the Tran- posit occurs in the metasomatic zones devel- samazonian leucogranites. No isotopic data are oped at the contact between granitic pegmatites presently available on these rocks, but they are and ultrabasic rocks (Rudowski et al., 1987). A of presumed Archaean age as they present the Rb/Sr whole rock dating of the external fine same lithological and tectonic characteristics as grained unit (Torquato et al., 1978) gave an the Lagoa do Morro and Boa Vista domes. age of 1.97 _ 0.02 Ga with an initial ratio of 0.708 _+ 0.001. A more recent Rb/Sr whole Petrography of the granites rock dating for samples taken from both granitic units (Sabat6 et al., 1990) gave the same The granites are generally equigranular, fine age (1.97 _+ 0.03 Ga), with an initial ratio of to medium grained (1 - 5 mm); they are rarely 0.706 _ 0.003. porphyritic, unlike part of the Campo For- moso pluton. Subsolidus deformation may be Carna179a important in some plutons. Three types of enclaves have been recognized: (1) enclaves of The Carnaiba granite represents a smaller, enclosing metamorphic rocks mostly located in circular (4 km in diameter), homogeneous, and the vicinity of the contact, but also as roof pen- fine grained two mica leucogranite, emplaced dants, such as the serpentinite enclave (about in the Archaean basement (Rudowski and Fon- 100 m wide) in the centre of the Carnaiba teilles, 1988; Rudowski, 1989). The granite out- pluton (Couto, 1978); (2) small (about 1- 10 crops in a tectonic window in the Jacobina cm) biotite-rich layered enclaves most probably overthrust formations. Late garnet-musco- representing restitic material; and (3) enclaves vite granitic dykes are also widespread 128 M. CUNEY ET AL. throughout the pluton (Rudowski et al., 1987). belong to the same intrusion. Biotitic __ garnet Numerous areas of emerald mineralizations are schlieren and muscovite + biotite + garnet _+ present in the ultrabasic bands, both in the tourmaline pegmatites are common. Metamor- basement and the overthrust cover. Rb/Sr ra- phic quartz rods are observed locally in the diometric measurements give an age of biotitic schlieren. Hexavalent uranium 1.88 + 0.09 Ga with an initial ratio of mineralization occurs along horizontal joints 0.733 _+ 0.018 (Sabat6 et al., 1990). in the Alianga granitic body. Some muscovite- rich layers have a higher radioactivity than the Gameleira homogeneous granite. The pegmatites tend to be aligned along a N25°E- N40°E foliation. The Gameleira pluton is a nearly circular (20 km in diameter), biotite-rich, two mica Lagoa Grande leucogranitic body, emplaced in the Contendas Mirante supracrustals and syntectonic with the The Lagoa Grande pluton (10 x 3 km), last phase of deformation (Sabat6 et al., 1980). located close to the southern margin of the Sete Thermal metamorphism has formed cordierite Voltas Archaean dome intrudes the Contendas and andalusite in the surrounding rocks. Rb/Sr Mirante supracrustal rocks and migmatites radiometric measurements give an age of from the Gavi~o basement. This granite is poor 1.95 + 0.05 Ga. with an initial of 0.707 + in muscovite and very homogeneous. The main 0.004 (Sabat6 et al., 1990). magmatic foliation, oriented N160°E, is parallel to the elongation of the pluton and to Riacho das Pedras the regional foliation of the surrounding rocks. This foliation is underlined by biotite + garnet The Riacho das Pedras pluton represents a schlieren. Subvertical ductile deformation with small, posttectonic and leucocratic garnet- the same orientation increases eastwards, in the bearing, two mica granite, with muscovite direction of the Sete Voltas dome. Mylonitic always more abundant than biotite. It is textures occur on the eastern margin of the elongate and extends 10 km in a north - south pluton. direction. It was emplaced by magmatic stop- ing in the Contendas Mirante supracrustals and Lagoinha is associated with numerous tourmaline- bearing aplopegmatites. Cordierite nodules The very small Lagoinha pluton (4 × 1.5 have formed in the contact metamorphism kin) is one of the southernmost leucogranitic aureole. Rb/Sr radiometric measurements give bodies intruding the Contendas Mirante an age of 1.93 + 0.02 Ga with an initial ratio supracrustal rocks close to a dome of Archaean of 0.748 _+ 0.013 (Sabat6 et al., 1990). basement. Biotite and/or muscovite-rich schlieren and muscovite + biotite + tour- Caetano and Alian(a maline pegmatitic pockets are widespread. A ductile vertical foliation oriented N 145 °E tends The Caetano and Alianga plutons are to align the biotite-rich schlieren and the elongated 20 km north-south and are only pegmatites. As in the Lagoa Grande pluton, separated by a small isthmus of Contendas this deformation increases eastwards. Mirante supracrustal rocks east of the Sete Voltas Archaean dome. These two bodies of Sete Voltas TTG suite two mica (+ garnet) leucogranite, together with a few smaller ones to the north, probably The Sete Voltas TTG suite, which represents MAGMATISM OF THE JACOBINA-CONTENDAS MIRANTE BELT (BAHIA-BRAZIL) 129 the largest Archaean dome outcroping in the ferent granitic plutons lies in how basic the Contendas Mirante belt, is composed of band- plagioclase is and the ratio of it to K-feldspar. ed grey gneiss with incipient anatexis and weak- The Sete Voltas Archaean plutonic rocks ly deformed intrusive porphyritic granite. The have the highest plagioclase content with the porphyritic granite is in the central part of the highest anorthite content (up to An35). Their dome. composition varies from trondhjemitic, with less than 10%0 K-feldspar and up to 56% Mineralogy of the granites plagioclase, to granitic. In the banded grey gneiss, mineral proportions may vary consider- The proportion of the major mineral phases ably within the same outcrop. The porphyritic has been calculated from a biotite mesonorm granites have a more homogeneous mineralo- (Fig. 3). The main variation between the dif- gical composition corresponding to the average composition of the foliated grey gneiss. In the Transamazonian leucogranites, the average proportion of plagioclase and its anorthite content decreases from Lagoa Grande to the Campo Formoso, Gameilera, Carnaiba v v v v ~i and Lagoinha plutons (down to 25% plagioclase). The average proportion of quartz increases slightly from the Lagoa Grande (26.5 ./ ,',, L wt.%) to the Campo Formoso (27.5 wt.%) Carnaiba (28 wt. %), and Lagoinha (32 wt. %) granites. The Riacho das Pedras granite is the C v v v ~' v richest in albite. Ab Or In the Transamazonian leucogranites, the biotite content is always below 7%. Only some II Archaean plutonic rocks of the Sete Voltas 3~ dome contain up to 9% biotite. The aluminium content in biotite of the leucogranites (15.7 to 19.0 wt.% A1203 is slighly lower on average than in the biotite of Hercynian peraluminous granites (Fig. 4). Their composition lies on the boundary between the calcalkaline and the aluminopotassic field defined for the French Hercynian granites. The biotite inclusions in

Ab 30 60 Or quartz give the lowest Al-content. Such an A1- content is similar to that of the Sete Voltas Ar- Fig. 3. Q- Ab- Or and An- Ab- Or diagrams for the chaean trondhjemites and may represent mine- Transamazonian leucogranites and the Sete Voltas Ar- chaean rocks from biotite mesonormative calculations. 1 ral inclusions preserved from the source = Lagoa Grande; 2 = Carnaiba; 3 = Campo Formoso; material. Most of them are also richer in 4 = Caetano-Alianga; 5 = Lagoinha; 6 = Riacho das magnesium compared to Hercynian leucogra- Pedras; 7 = Gameleira; 8 = Sete Voltas porphyritic; 9 = nites. Their fluorine content never exceeds 1.2 Sete Voltas banded; T = Trondhjemite field and G = wt.%. granite field from O'Connor (1965) defined from catanorm. As the biotite content of rocks studied is low, Muscovite contents vary from 1 to 14 vol.%0. the difference between catanormative and mesonor- The largest amounts of muscovite have been mative calculations for Or do not exceed 3%. observed in some samples of the Lagoinha and 130 M. CUNEY ET AL,

1 L i z ,\ f I i i i z ed with the main foliation and belonging to the

| o metamorphic paragenesis.

2 o Spessartine-rich garnet is a common mineral

3 G in the latest intrusions of the Campo Formoso o 4 ~, and Carnaiba plutons and in associated aplo- 5 pegmatites (Rudowski, 1989). Exceptionally,

3.0 ~ ~, ./,, ~"~ 6 D cordierite may be present in the aplopegma-

7 [] tites.

8 + Tourmaline is common in the pegmatites and in quartz veins, but rare as disseminated , "--?..~ , 2.6 I i I J I I crystals in the granites. Beryl and, exceptional- 0.6 1.2 1 8 2.4 Mg ly, molybdenite occur in some aplopegmatites of the Campo Formoso and Carnaiba granites. Fig. 4. A1-Mg diagram from Nachit et al. (1985) for Hydrothermal alteration with K-metasoma- selected biotite analysis from the Transamazonian leucogranites and the Sete Voltas Archaean rocks. The tism and quartz leaching similar to the feld- compositional field for aluminopotassic (leucocratic spathic Hercynian episyenites (Leroy, 1978; muscovite-biotite granite and mesocratic biotite-cor- Cathelineau, 1986) has been observed in the dierite Gu6ret-type peraluminous granites), calcalkaline Campo Formoso pluton. and subalkaline Hercynian granites are represented. A1 Chloritization is common in most of the and Mg are in cations per structural formula unit. 1, 2 and 3 = Sete Voltas banded rocks; 4 and 5 = Lagoinha; 6and granites studied but its development is general- 7 = Campo Formoso; 8 = Alianga. Fields (data from ly very limited. Chlorite is more common in the Rudowski, 1989): dotted line = Campo Formoso biotite most differentiated intrusives. from the matrix; stippled line = biotite inclusions in Besides apatite and zircon, allanite and epi- quarts; solid line = Carnaiba biotites. dote, generally as epitaxic overgrowths on allanite crystals, have been observed in most Transamazonian leucogranites. Magnetite is Campo Formoso plutons. Primary, euhedral the main iron oxide in most of the granites. II- and secondary muscovite exist. Euhedral menite crystals have only rarely been observed muscovite is generally the most abundant. As in some samples of the Campo Formoso granite observed in Hercynian leucogranites (Monier, generally associated with magnetite. Monazite 1987), three types of secondary muscovite may is rare, except in the Riacho das Pedras granite, be distinguished: symplectitic overgrowths on where large crystals with a corona of small euhedral crystals; muscovite developed in automorphic allanite crystals are common. A1- fissures; and muscovite laths in plagioclase. lanite, epidote and magnetite are also very com- Their fluorine contents never exceed 0.6 wt. °7o. mon in the Sete Voltas Archaean rocks. The evolution of the fluorine content from the core of the euhedral crystals to the symplectitic Major element geochemistry muscovite margins does not show significant fluorine enrichment trends as observed in The A - B diagram (Fig. 5) of Debon and Le mineralized Hercynian leucogranites (Monier, Fort (1982) shows clearly that all these granites 1987). are peraluminous, although the peraluminous Muscovite is also present in some of the index varies largely between and within the dif- samples of the Sete Voltas banded gneiss. ferent granitic plutons. In the Sete Voltas Ar- Muscovite is clearly secondary in some of these chaean plutonic rocks, the most leucocratic rocks, having symplectitic textures. Muscovite samples are the most peraluminous. The por- also occurs as well-developed crystals associat- phyritic facies is weakly peraluminous. MAGMATISM OF THE JACOBINA-CONTENDAS MIRANTE BELT (BAHIA-BRAZIL) 131

> LEUCOGRANITES < between those of the Lagoa Grande and the Lagoinha pluton. The Campo Formoso pluton ~3 has the largest differentiation range, whereas / \\ the Carnaiba pluton is homogeneous and leu- C,~ I 6 f_ 7 cocratic. The Riacho das Pedras granite is also so " \t ', 2', leucocratic and shows a large variation in the z oj' I, / ' peraluminous index without any significant ' ' c, variation in biotite content. Two sub-groups are distinguished in the A - B diagram for the ee ,'~.", T L , , L i I Riacho das Pedras pluton: one having a low 50 100 B Fo+Mg+T[ and rather constant peraluminous index; the other a high and variable peraluminous index. Fig. 5. A -B diagram from Debon and Le Fort (1982) showing the leucocratic and peraluminous character of These two groups also exhibit distinct trace ele- the Transamazonian leucogranites and the Sete Voltas ment compositions. This second trend is iden- Archaean rocks. A and B in millications. 1 = Lagoinha; tical to that of the Himalayan Manaslu 2 = Campo Formoso; 3 = Carnalba; 4 = Lagoa Grande; peraluminous leucogranite. 5 = Sete Voltas banded; 6 = SeteVoltas porphyritic; 7 = Apart from the Lagoinha and the second Caetano-Alianga; 8 = Riacho das Pedras; 9 = Gameleira. Light stippled line = composition field for the sub-group in the Riacho das Pedras granites, Hercynian Saint Sylvestre granite (unpublished data); the Transamazonian peraluminous leucograni- heavy stippled line = composition field for the tes are distinctly less peraluminous than their Himalayan Manaslu granite (from Le Fort et al., 1987). Hercynian or Himalayan equivalents. This re- The arrows indicate the differentiation trends in a single sult is consistent with the lower Al-content of pluton. the biotite in the Transamazonian peraluminous leucogranites.

In most Transamazonian leucogranites, the Trace element geochemistry peraluminous index (A parameter) clearly increases with the degree of differentiation of the Rare earth elements granite. In the A - B diagram (Fig. 5) the degree of differentiation is indicated by the decrease in The REE have been analyzed by ICP -AES the B parameter which mainly represents the (Govindaraju and Mevelle, 1987) in selected amount of biotite + magnetite in the rock. samples of the different leucogranitic plutons This trend is typical of Hercynian peralumi- and in two samples of the Sete Voltas TTG suite nous leucogranite. The Lagoa Grande pluton is (Table 1, Fig. 6). one of the most leucocratic granites but is the The REE pattern of the most trondhjemitic least peraluminous and has a very homogene- sample of Sete Voltas is typical of Archaean ous composition. In contrast, the nearby La- TTG suites with a strong and relatively regular goinha pluton is the most peraluminous and ex- fractionation of all the REE, an extremely hibits a very large variation in its peraluminous weak europium anomaly and a very low HREE index, although it is not very leucocratic. The content. The more potassic samples have lower Lagoinha pluton has a composition similar to LREE content but higher HREE and Y con- that of the Saint Sylvestre granite in the French tents and an extremely weak europium anoma- Massif Central, which is a typical example of ly. Hercynian peraluminous leucogranites. The The Lagoa Grande and Gameleira samples, Caetano - Alian9a, Campo Formoso and Car- and the least differentiated sample of the Car- naiba plutons have intermediate characteristics naiba and Campo Formoso plutons give REE TABLE 1

Chemical and mesonormative compositions of the Transamazonian granites

SSY Mana- Gain RdP CA5 CA10 CF6 CF48b CF49 LG54hc LG58hc Lh9hcb Lhl0hc Cae-All CAE16b CAE64 n 40 slu 13 13 1 1 1 1 1 1 1 1 1 2 1 1

SiO 2 71.96 73.64 71.65 74.59 73.80 74.59 71.70 72.32 72.23 74.01 72.59 72.52 70.92 73.75 71.47 74.29 A1203 14.80 14.87 14.86 13.85 13.74 14.25 14.62 15.58 14.64 14.55 14.8 14.66 15.2 14.08 15.41 13.91 Fe203 1.62 1.35 1.73 0.96 1.42 0.51 1.85 1.30 1.69 0.90 1.1 1.42 1.47 0.94 2.48 1.03 MnO 0.03 0.03 0.03 0.03 0.01 0.14 0.01 0.02 0.04 0.02 0.01 0.04 0.04 0.07 0.03 0 MgO 0.35 0.11 0.40 0.35 0.29 0.07 0.45 0.38 0.36 0.16 0.25 0.47 0.49 0.13 0.47 0.11 CaO 0.40 0.47 1.31 0.71 0.71 0.43 1.27 0.62 1.10 1.06 1.14 0.78 0.51 0.51 2.13 1.12 Na20 3.13 4.05 4.13 4.47 3.61 5.38 3.81 4.13 3.76 4.31 4.51 3.46 3.71 3.57 5.17 4.61 K20 5.43 4.56 4.58 4.30 5.23 3.92 5.06 4.55 5.08 4.58 4.43 4.8 5.63 5.1 2.06 3.78 TiO z 0.29 0.10 0.21 0.03 0.14 0.00 0.22 0.11 0.18 0.11 0.15 0.12 0.13 0.05 0.25 0.1 P205 0.30 0.13 0.18 0.16 0.11 0.14 0.16 0.18 0.17 0.08 0.09 0.41 0.45 0.31 0.13 0.08 I.L. 1.10 0.84 0.61 0.55 0.74 0.42 0.62 1.05 0.92 0.56 0.68 1.12 1.24 0.82 0.75 0.57 F 2000 1020 691 431 650 340 790 740 930 440 720 410 310 790 440 100 Rb 467 286 260 254 361 420 250 378 317 218 183 257 281 344 55 73 Li 244 170 101 27 129 39 90 104 84 83 27 43 37 149 62 15 Cs 39 42.4 20 38 16.3 12.5 7.7 12.6 8.1 5.5 3.8 6.3 5.5 - 4.5 1.2 Nb 33 - 19.6 32.5 13 37 13 11 11 7 7 7 7 30 5 5 Sn 35 19 2.9 9.4 5 2.5 1 2 2.5 1 1.5 1 1 8 1 1 U 16.0 8.0 7.7 14.8 6.4 6.9 6.2 40.3 6.9 22.4 3.9 4.5 5.8 13.7 0.4 6.4 Be 23.2 - 4.0 4.6 4.0 7.4 4.5 3.5 3.6 3.0 3.7 2.4 2.0 12 1.0 1.5 W 4.0 4.4 0.6 2.7 0.9 0.7 0.3 0.8 0.8 0.1 0.1 1.2 0.8 - 0.3 0.1 Sr 76 76 147 10 85 20 168 42 128 170 239 76 80 46 702 226 Ba 226 213 570 55 343 36 635 120 518 577 966 398 471 198 632 351 Th 19.0 6.0 32.1 3.4 26.1 2.0 31.9 5.3 25.3 10.5 17.0 10.4 11.4 10.4 8.1 12.1 Zr 86 - 141 36 118 35 177 38 132 72 113 91 100 45 172 76 n 25 65 3 2 1 1 1 1 1 1 1 1 1 1 1 1 La 25.8 11.5 49.9 3.2 88.4 2.1 79.5 8.8 68.2 17.6 41.8 19.1 16.8 9.9 35.9 23.6 Ce 51.5 19.0 84.8 9.0 203.3 6.3 159.2 18.1 108.8 30.3 65.8 40.4 42.2 24.7 73.3 28.9 Nd 24.7 8.0 27.3 2.6 78.0 2.6 55.5 7.2 37.1 10.3 21.0 16.3 15.3 8.7 18.6 12.1 Sm 5.1 2.0 4.5 1.1 12.0 0.9 8.6 1.9 5.9 2.3 3.6 4.1 4.4 2.8 2.8 2.7 Eu - 0.05 0.88 0.09 1.71 0.09 1.29 0.26 0.78 0.46 0.69 0.6 0.74 0.26 0.57 0.61 Gd 3.80 2.00 3.25 0.81 5.63 0.70 4.60 1.45 3.68 1.57 2.32 3.46 3.91 2.73 1.67 2.11 Dy - 2.40 1.79 1.78 2.01 0.76 1.64 1.33 1.87 0.87 1.04 3.07 3.52 3.50 0.68 1.44 Er - 1.00 0.94 1.26 1.01 0.43 0.77 0.68 0.82 0.43 0.49 1.38 1.49 1.97 0.35 0.71 Yb 0.70 1.00 0.81 2.22 0.85 0.49 0.67 0.77 0.69 0.34 0.31 1.17 1.34 1.88 0.26 0.58 Lu - 0.14 0.12 0.34 0.17 0.03 0.12 0.12 0.10 0.12 0.02 0.16 0.23 0.29 0.06 0.08 Y 12.7 14.5 12.0 13.9 11.6 5.7 8.7 9.5 10.9 5.2 6.0 21.40 21.1 25.3 3.5 9.7 Biotite mesonorm Q - 26.3 29.9 29.8 25.7 26.4 28.2 27.4 27.4 25.5 31.8 32.3 31.5 27.5 29.0 Or - - 24.8 24.8 28.5 22.4 27.6 25.1 28.0 26.0 24.8 26.5 32.1 29.6 8.9 21.4 Ab - - 37.6 40.9 33.0 48.3 34.7 38.0 35.7 38.8 40.9 31.7 25.2 32.8 46.7 41.9 An - - 4.6 0.7 2.4 1.2 4.6 1.5 3.8 4.4 4.6 0.8 0.0 0.5 8.9 4.7

Bi - - 4.0 1.6 3.1 1.3 4.3 3.3 3.8 1.9 2.5 3.9 3.8 1.9 5.3 1.9 Cor - - 1.6 1.7 1.4 0.8 1.4 3.7 1.8 1.0 1.0 3.9 5.3 2.8 1.6 0.6

Gain = Gameleira; RdP = Riacho das Pedras; CA = Carnaiba, CF = Campo Formoso; LG = Lagoa Grande; Lh = Lagoinha; Cae-All = Caetano- Alianca; CAE = the Sere Voltas Archaean rocks SSY = Hercynian Saint Sylvestre granite; French Massif Central values, unpublished data from M. Cuney; Manaslu granite, data from Himalaya, N6pal France-ganord and Le Fort (1988); n = number of samples. 134 M. CUNEY ET AL.

I~00 1000 1000 Caetano-Allianz Sete Voltas Carnaiba

100 100 100 -= ~3

10 e~ I0

, i ~ i i i i i i J , , , , , ta Ce Pr NdPmSm Eu Gd Tb By Xo [r Tm Yb Lu La Ce Pr NdPmSm [u Gd ]b Oy Ho Er Tm Yb lu La Ce Pr NdPmSm EU Gd ~ Dy Ho Er Tm Yb LU

1,000 [~ Lagoinha .... I000 1000 [1 5 Gameilera .... Campo Formoso Riacho das Peclras __ 8 Lagoa Grande __ 3 16 i,. ]00 ,oot\ 10( -r~

-a

1 i , i i i i , i i i i i i 1 i I I i J I i ] i i i I t i ta Ce Pr Nd Pm Sm Eu Gd To Dy Xo [r Tm Yb Lu La £e Pr Nd PmSm [u Gd Tb Oy No [r Tm Yb lu la Ce Pr NdPmSm £u Gd To Dy Ho [r Tm Yb Lu

Fig. 6. REE pattern of the Transamazonian leucogranites and the Sete Voltas Archaean rocks. 1 = CA5, 2 = CA10, Car- naiba; 3 = CF6, 4 = CF49, 5 = CF48b, Campo Formoso; 6 = CAE16B, 7 = CAE64, Sete Voltas; 8 = G58HC, 9 = G54HC, Lagoa Grande; 10 = G09HCBC, 11 = G10HC, 12 = CMS 88- 3, Lagoinha; 13 = CMS 88- 16, 14 = CMS 88 - 17, Caetano-Alianqa; 15 = DMM92, 16 = DMM95, 17 = DMM96, Riacho das Pedras; 18 = DMM92, 19 = DMM 99L, Gameleira.

patterns very similar to the most trondhjemitic and an important europium anomaly. samples of the Sete Voltas dome, suggesting The most differentiated samples from a derivation from a source material of similar single pluton (Riacho das Pedras) and the most composition. The Lagoinha, and the more dif- differentiated plutons (Riacho das Pedras and ferentiated samples of Carnaiba and Campo Caetano-Alian~a granites) have very low Formoso plutons give REE pattern similar to REE contents, "sea-eagle" shaped patterns those of Hercynian and Himalayan granites and the strongest europium anomaly. The (Vidal et al., 1982, Bernard-Griffiths et al., decrease of the LREE content with differentia- 1985) with a weaker fractionation of the REE tion is similar to that observed in more recent MAGMATISM OF THE JACOB1NA-CONTENDAS MIRANTE BELT (BAHIA-BRAZIL) 135 leucogranites but mainly corresponds to alla- granitic composition. Sr and Ba have high nite fractionation in the Transamazonian values characteristic of such rock types. The leucogranites instead of monazite. most trondhjemitic sample has a more fractionated REE pattern but a similar total REE Other trace elements content. Most of the Transamazonian leucogranites The other trace elements have been nor- exhibit very similar patterns. They have a much malized to a typical Hercynian peraluminous lower incompatible element content than the leucogranite from the French Massif Central Saint Sylvestre granite, especially in Sn and W, (Fig. 7) associated with major uranium de- but are as enriched in Sr and Ba as the TTG posits and numerous Sn-W mineralizations gneisses. REE patterns are generally more frac- (Saint Sylvestre). Incompatible elements are tionated for the Transamazonian leucogranites plotted on the left of the figure, whereas com- apart from the Lagoinha and Riacho das patible elements are plotted on the right of the Pedras granites and the Caetano-Alian~a figure. plutons. The Riacho das Pedras and the The two samples of the Sete Voltas Archaean Caetano- Alianqa granites represent the most gneiss dome are extremely poor in all incom- differentiated Transamazonian leucogranites patible elements, except uranium for the most with very low Sr, Ba, Ti, Zr, Th and REE contents similar to the Himalayan Manaslu I0' peraluminous leucogranite and the same Cs, Nd and U enrichment as the Hercynian Saint I0 °' ...... ,.. / ...... Sylvestre peraluminous leucogranite. Despite , . . ,'~ , , "E the highly fractionated character of the Riacho ~I0 1 • ",.,'..,.,.,. , \I das Pedras granite, F, Li, Sn and W concentra- 'V \ tl ...... Er49/s~v ~ I ...... LG/SSY V ...... Gam/SSY2 tions remain very low. 10"2 The different granitic massifs, and even Rb Li C$ Nb Sn U Be W Sr Ba Ti Th Zr La Ce Nd Sm Gd Yb

I0 i several units with in the same granitic body, as in the Gameleira granite, can be also character-

I0 ° ized by their trace element ratios such as Zr/Sr ..... "-/~"~.. /%." , L.,.;":.~--.-../ ~'~" / ..... 250

~.~ |0 ! \/ ' I ...... tae-R./SSV I ...... RDPISSY i i i i i i r i i i i [ ...... Manaslu/SSY I 0 -2 Rb ti CS Nb Sn U Be W Sr Ba Ti Th Zr ta Ce Nd Sm Gd Yb Y O I0 ~ 150

I0 a O 100 O 00

5o o

10 2 Rb Li Cs Nb Sn U Be W Sr Ba Ti Ih Zr ta Ce Nd Sm Gd Yb 2;0 4~o 6;0 Fig. 7. Graphs of the Transamazonianleucogranites, the Sr HimalayanManaslu graniteand the Sete Voltas Archaean Fig, 8. Zr/Sr diagram for the Transamazonianleucogran- rocks normalizedto the HercynianSaint Sylvestre(SSY) ites and the Sete Voltas Archaeanrocks. Same symbols as peraluminous leucogranite.(Same abbreviations as Table Figure 3 excepted: dotted field for Riacho das Pedras 1). granite and hatched field for Gameleiragranite. 136 M. CUNEYET AL.

results from a pre-metamorphic alteration 250 ~o 'w and/or deformation episode, but we have no 200 0 definite argument to support either hypothesis. What is certain is that some of the Archaean O0 [] • .o(t rocks were already peraluminous before (or at I50 least became peraluminous during) the Tran- •/_~. ~ A~.° samazonian orogeny. The alteration of initially 100 o~ o "~ ~--~" meta-aluminous rocks into peraluminous rocks ~Im []° o $~+s during deformation and metamorphism by

50 / [~~ alkalis and/or calcium leaching is a well-known //~0 |, 'l. phenomenon (Marquer et al., 1985). i~ i • I Most of the Transamazonian leucogranites tO 20 Y of the Jacobina-Contendas Mirante belt Fig. 9. Zr/Y diagram for the Transamazonian follow the typical evolution trend of Hercynian leucogranites and the Sete Voltas Archaean rocks. Same peraluminous leucogranites, namely a strong symbols as Figure 3. increase in the peraluminous index with decreasing mafic mineral content. This trend and Zr/Y (Figs. 8 and 9). In the Zr/Y diagram, differs greatly from the trends in other two sub-groups can be clearly distinguished in peraluminous granites. In the S-type suites the Gameleira and in the Riacho das Pedras (White and Chappel, 1977), a decrease in the plutons. Two parallel differentiation trends peraluminous index with decreasing mafic have also been distinguished in the Campo For- mineral content, corresponding well to the moso pluton from trace element data (Rudov- restite unmixing model, is observed. Two ski, 1989). The composition of the Sete Voltas granites present different trends: the most Archaean rocks encompasses most of the com- peraluminous group of the Riacho das Pedras position field defined by the Transamazonian granite shows the same evolution as the leucogranites with the exception of the Lagoin- Himalayan Manaslu granite, in which biotite ha and Riacho das Pedras granites in the Zr/Y content remains constant with the increase in diagram. the peraluminous index. The Lagoa Grande granite and the less peraluminous group of the Interpretation and conclusions Riacho das Pedras granite are weakly peraluminous and may have formed either from Granitoid types direct partial melting of a weakly peraluminous source or from the extreme fractionation of a The Sete Voltas rocks represent an unusual calcalkaline suite. The lack of associated in- petrographic association for an Archaean TTG termediate or basic plutonic rock favours the suite. Their composition varies from trondh- first hypothesis. jemitic to rather potassic granites, and some of Most of the Transamazonian leucogranites them are strongly peraluminous. The strongly are less peraluminous than their Hercynian or peraluminous character is marked by the Himalayan equivalents (Table 1). Biotite-rich presence of muscovite. Part of this muscovite is samples are less peraluminous and biotite com- clearly secondary and related to the last defor- position is less aluminous. The characteristics mation episode corresponding to the emplace- of the source material from which these gra- ment of the earliest generation of leucograni- nites derive may explain these features. Highly tes. Some of the muscovite may also reflect peraluminous sediments are less abundant in either a primary peraluminous composition or Archaean crust whereas meta-aluminous meta- MAGMATISM OF THE JACOBINA-CONTENDAS MIRANrI~E BELT (BAHIA-BRAZIL) 137

igneous rocks are abundant. Therefore the Transamazonian leucogranites. However two main source material may consist of meta- samples from the Campo Formoso pluton with igneous plutonic or volcanic formations similar very distinct eNd(t) support the theory of to the Sete Voltas Archaean TTG suite. Evi- heterogeneous source material within the same dence for such a source is further supported by intrusion. the mineralogical characteristics of the granite discussed below. A metapelitic component The source problem may, however, be important in the Riacho das Pedras and Lagoinha granites. One of the major characteristics of the Tran- samazonian peraluminous magmas is the oc- Genetic model currence of the magnetite-allanite accessory mineral association instead of the ilmenite- The trends defined by the trace elements, monazite assemblage generally observed in mainly the decreasing content of incompatible most Himalayan and Hercynian leucogranites. elements with decreasing titanium content in The presence of allanite instead of monazite the Campo Formoso and Carnaiba pluton, is generally related to the high Ca content have been explained in terms of fractional crys- (CaO >_ 1 wt. %) of the silicate melt (see Cuney tallization of a meta-aluminous source (Ru- and Friedrich (1987) for a review of this pro- dowski and Fonteilles, 1988). However, the in- blem), but most of the Transamazonian leuco- crease in the peraluminous character, which is granites have a CaO content well below 1 wt. %. a major feature of many of these granites and Oxygen fugacity is another important parame- is also reflected by the increasing Al-content of ter controlling epidote stability (Liou, 1973). biotite with decreasing abundance of compati- Although no experimental data on allanite are ble elements (Rudowski and Fonteilles, 1988), available, we should expect the stability of this cannot be explained by this model and is the op- mineral to be very close to that of epidote. Un- posite of the "restite unmixing model" of der the reducing conditions (close to or below White and Chapel (1977). The evolution ob- the Ni- NiO buffer) commonly prevailing in served in the peraluminous leucogranites re- peraluminous granites (Ishihara, 1977) and for quires the fractionation of a meta-aluminous a pressure of crystallization below 3 kbar, epi- mineral phase such as pyroxene or amphibole, dote is stable below 600°C and therefore can- which has never been observed in this type of not crystallize at the magmatic stage. Higher granite. The lack of any associated mafic or in- oxygen fugacity closer to the haematite - mag- termediate enclaves or plutonic units also does netite buffer allows epidote crystallization at not support the fractional crystallization model 640 - 680°C for a pressure of 2- 3 kbar. High and/or mixing with a basic igneous compo- oxygen fugacity in the Transamazonian leuco- nent. granitic magmas is also indicated by the scarci- The different trends observed within and be- ty of ilmenite and the general occurrence of tween the different intrusions are assumed to magnetite. The relatively magnesian character result from the heterogeneity of the source ma- of the biotite compared to equivalent Hercy- terial. The heterogeneity of the source material nian granites can probably be explained in the within the same pluton has been clearly sup- same way, because the Fe/Fe + Mg ratio of ported by data on the radiogenic and stable iso- biotite decreases with oxygen fugacity (Wones topes from the very recent Manaslu peralumi- and Eugster, 1965). Such features directly re- nous leucogranite (Vidal et al., 1982; France- flect the lack or the low abundance of graphite- Lanord et al., 1988) but such a demonstration bearing sediments in the source material of is more difficult on such an old system as the these granites and corroborate the pre-domi- 138 M CUNEY El" AL. nantly meta-igneous character of this source Metaliogenic potential material. This represents a major difference to the predominantly metasedimentary source The Jacobina-Contendas Mirante peralu- material generally proposed for Hercynian and minous leucogranites have low concentrations Himalayan leucogranites (Le Fort et al., 1987; of the incompatible elements, Li, F, Cs, and Vidal et al., 1982, 1984; Bernard-Griffiths et Be, and metals such as Sn and W compared to al., 1985). mineralized Hercynian granites (Table 1 and The major and trace element composition Fig. 7). The Carnaiba granite associated with field of the Sete Voltas Archaean TTG suite is one of the most important emerald mineraliza- very large and encompasses the compositional tions is not the richest in incompatible ele- field of the porphyritic granites outcropping in ments. Despite the occurrence of beryl and the Sete Voltas dome and also that of most molybdenite locally in the aplopegmatites, and Transamazonian leucogranites, apart from the of molybdenite + scheelite associated with Lagoinha pluton. The variation in the pera- emerald in the metasomatic veins, the Be, Mo luminous index and the accessory mineral para- and W contents of the Carnaiba granite remain genesis characterized by magnetite and allan- very low. As proposed by Rudowski (1989), the ite/epidote also corresponds to that of the importance of these mineralizations results Transamazonian leucogranites. Mineralogical mainly from the efficiency of the metasomatic and chemical signatures therefore support the trap. Although the Riacho das Pedras granite idea that the Archaean TTG suites may repre- shows much differentiation, its enrichment in sent a possible source material for the Tran- incompatible elements remains relatively low, samazonian leucogranites. except for Cs, Nb, and U; perhaps reflecting The parallel or divergent trends existing in the primitive nature of the crustal material that the different plutons which led to the evolution underwent partial melting during the Tran- of peraluminous character, and the fractiona- samazonian orogenesis. tion of major and trace elements, reflect varia- Uranium enrichment in the Riacho das tions in the source composition and the melting Pedras and Caetano - Alianga granites is simi- conditions. lar to that of the Hercynian Saint Sylvestre The high strontium initial ratios (0.7065- peraluminous leucogranite associated with ma- 0.7331) and the low eNd values (-4.9 to jor uranium deposits. They represent the most -13.1) measured on some of these granites favourable granitic bodies for the occurrence (Sabat6 et al., 1990) support the idea that they of uranium deposits. Uranium mineralizations originated from the partial melting of con- have been observed in the Caetano-Alianga tinental crust of probable Archaean age and pluton which is the richest in incompatible that the source composition varied. However elements of all the Transamazonian leucogra- Nd model ages (2.42-3.17 Ga), tend to rule nites. The recognition of episyenites in the out the Boa Vista and Mata Verde TTG suites Campo Formoso pluton similar to those des- (3.1 -3.5 Ga, Cordani et al., 1985) as a source cribed in the Hercynian leucogranites, where for the Transamazonian granites. Isotopic data they represent a trap for many economic ura- favour the Gavi~o medium grade terrains as a nium deposits, is another favourable criterion possible source for these granites (Sabat6 et al., for proposing the possible existence of uranium 1990). Isotopic data for the Sete Voltas dome mineralization in the Transamazonian pera- are necessary to give further constraints on the luminous leucogranites. possible source of the granites. MAGMATISM OF THE JACOBINA-CONTENDAS MIRANTE BELT (BAHIA-BRAZIL) 139

The geotectonic model circular diapirs, most of them are lenticular bodies with internal magmatic structures and A continental collision regime at 2 Ga be- an elongation parallel to the major thrust tween two Archaean crustal domains, in the planes, as in the High Himalaya belt. They are S~o Francisco craton, is supported by several generally located in the vicinity of the dishar- features. Regional scale westward over- monic boundary between the Archaean infra- thrusting is well known in the Jacobina area structure, the Gavi~.o block, and a superstruc- and is becoming well-documented in the Con- ture, the Jacobina - Contendas Mirante supra- tendas Mirante area. Intrusive igneous rocks crustal rocks. This relationship is similar to that observed in the Jacobina- Contendas Mirante observed in the central part of the Himalayan belt share many characteristics with Hercynian chain, where the infrastructure is the Tibetan and Himalayan syncollision magmatism. They slab and the superstructure the High Himalaya form a 500 km elongate belt of typical syn- Tethyan sedimentary sequence (Le Fort, 1981). kinematic to postkinematic peraluminous Thus, if such a comparison is valid, the equiva- leucogranites parallel to the major thrust lent of the Main Central Thrust (MCT) should planes. The high Rb content of these granites be searched far below the Archaean granitog- for a relatively low Nb + Y content are consis- neissic unit further to the west. It may corres- tent with a syncollision tectonic environment pond to the tangential structure identified by during their genesis according to the Pearce et Sabat6 et al. (1988) in the Aracatu dome in the al. (1984) classification (Fig. 10). Characteristi- Gavi~o block, the northern extention of which cally, associated intermediate to basic pluto- is unfortunately concealed by a late Protero- nism is totally lacking in these plutons. Al- zoic sedimentary basin. though some of the granites are emplaced as Acknowledgments

I000 Syn-Co; G The results presented in this paper constitute a contribution of the CNRS-INSU program o ~. ",:,.. "Dynamique et Bilan de la Terre" and of the °"=°. m~l'~,] t ° project "Granit6ides da Bahia: geologia e metalogen~se" which receives financial sup- ~: I00 port through agreements FINEP (PADCT)/ UFBA, CNPq/ORSTOM, SGME-SME (Ba)/ ORG UFBA and CNEN/UFBA. The authors thank H. Martin, G. Giuliani and two anonymous reviewers for constructive comments and revi- I0 r sion of the English text. 10 100 1000 Y+Nb

Fig. 10. Rb/Nb+Y diagram from Pearce et al. (1984) References showing the syn-collision geochemical characteristics of the Transamazonian leucogranites and some of the Sete Barbosa, J.S., 1986. Constitution lithologique et Voltas Archaean rocks. Most samples from the Sete m6tamorphique de la r6gion granulitique du sud de Voltas have compositions very poor in Y and Nb and plot Bahia. Ph.D. Thesis, Univ. Paris, 401 pp. (unpub|.). in the volcanic arc - granite field. CF = Campo Formoso; Bernard-Griffiths, I., Peucat, J.J., Sheppard, S.M.F. Car = Carnaiba; Gain = Gameleira; L.G. = Lagoa and Vidal, P., 1985. Petrogenesis of Hercynian leuco- Grande; Lgh = Lagoinha; RdP = Riacho das Pedras; granites from the southern Armorican Massif: con- SVF = Sete Voltas banded; SVP = Sete Voltas por- tribution of REE and isotopic (Sr, Nd, Pb and O) phyritic. Syn-Col G. = syn-collision granites. geochemical data to the study of source rocks charac- 140 M. CUNEY ET AL.

teristics and ages. Earth. Planet. Sci. Lett., 74:235 - analysis. J. Anal. At. Spectrom., 2: 615-621. 250. Ishihara, S., 1977. The magnetite-series and the ilmenite Cathelineau, M., t986. The hydrothermal alkali series in granitic rocks. Min. Geol. Spec., 27: metasomatism effects on granitic rocks: quartz 293 - 305. dissolution and related subsolidus changes. J. Petrol., Le Fort, P., 1981. Manaslu leucogranite: a collision 27 (4): 945- 965. signature of the Himalaya, a model for its genesis and Celino, J.J. and Sabat6, P., 1988. O Macico de Jaguarari: emplacement. J. Geophys. Res., 86 (Bll): uma intrus~o granitica de vocag~o crustal na margem 104545- 10568. do cintur~o transamazonico da Serra de Jacobina Le Fort, P., Cuney, M., Deniel, C., France - Lanord, C., (Bahia, Brasil). Congr. Lat. Am. Geol., 7th (B61em, Sheppard, S.M.F., Upreti, B.N. and Vidal, P., 1987. Brasil), p. 437. abstr. Crustal generation of the Himalayan leucogranites. Conceiq~ao, H., 1986. 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