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 altera- tion are favourable criteria for finding Variscan-type uranium ore deposits. Introduction products generated during continental colli- sion. 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 0377-0273/90/$03.50 © 1990 - Elsevier Science Publishers B.V. 124 M. CUNEY ET AL. 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 ~ ..... I ¸ 120~," 14°~ ~ I 46 ° lr 2l 3~'~ o ~ i"-.1' 40 ° 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