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Lithogeochemistry and geochronology of the subalkaline felsic plutonism that marks the end of the Paleoproterozoic orogeny in the Salvador-Esplanada belt, São Francisco craton (Salvador, state of Bahia, Brazil)

Litogeoquímica e geocronologia do plutonismo félsico subalcalino que marca o final da orogenia Paleoproterozoica no cinturão Salvador-Esplanada, Cráton São Francisco (Salvador, Bahia, Brasil)

Abstracts

Studies conducted over the last decade concerning the rocks that underlie the municipality of Salvador have shown a complex geological history with a great diversity of medium- to high-grade metamorphic lithotypes, deformed in several phases and frequently cut by tabular mafic dykes and irregular granitic bodies. The latter, which were the subject of this study, frequently outcrop along the coastline of Salvador and are classified petrographically as monzo-syenogranites. They are classified as subalkaline and peraluminous according to their geochemical data, and stand out for being enriched in light rare earth elements and having a strong negative Europium (Eu) anomaly. These rocks are produced from anatectic melts or through the interaction of mantle-derived magmas with crustal materials. The negative values of εNd(t) (-6.08) corroborate with the crustal character and in the diagrams of tectonic ambience, they are plotted in the field of post-tectonic granites. The Sm-Nd model age (TDM) around 2.9 Ga indicates a neoarchean source for these lithotypes, whereas their U-Pb zircon age (LA-ICPMS) of 2,064 ± 36 Ma is similar to the U-Pb (SHRIMP) and Pb-Pb (evaporation) ages for late-tectonic granites of the Itabuna-Curaçá-Salvador belt. Considering the results of recent studies in the area of Salvador, the monzo-syenogranites can be interpreted as late-tectonic intrusions, since they are affected by dextral shear zones correlated with the last stage of deformation registered in the granulites of Salvador.

monzo-syenogranites; petrology; geochronology; Salvador; Bahia; Brazil


Estudos realizados ao longo da ultima década nas rochas que compõem o embasamento da cidade de Salvador, no nordeste do Brasil, mostram uma história geológica complexa, com grande diversidade de litotipos metamórficos de médio e alto grau, deformados de modo polifásico e frequentemente cortados por diques máficos tabulares e corpos graníticos irregulares. Estes últimos, objeto deste trabalho, afloram abundantemente na orla marítima de Salvador, sendo classificados petrograficamente como monzo-sienogranitos. Os seus dados geoquímicos permitem classificá-los como subalcalinos e peraluminosos, destacando-se que eles são enriquecidos em ETR leves e apresentam forte anomalia negativa de Eu. Estes granitóides apresentam características geoquímicas de rochas derivadas de material crustal e/ou produzidos pela interação de material da crosta e do manto, com os valores negativos de εNd(t) (-6,08) que corroboram a característica crustal. Em diagramas discriminantes de ambientes tectônicos, estão dispostos no campo dos granitos pós-tectônicos. A idade-modelo Sm-Nd (TDM) em torno de 2,9 Ga indica uma fonte neoaqueana para esses litotipos enquanto que a idade U-Pb zircão (LA-ICPMS) de 2.064 ± 36 Ma é interpretada como sendo de cristalização, sendo similar às idades U-Pb (SHRIMP) e Pb-Pb (evaporação) para os granitos tardi-tectônicos do Cinturão Itabuna-Salvador-Curaçá. Os monzo-sienogranitos em foco podem ser posicionados como granitos tardi-tectônicos, visto que são afetados por zonas de cisalhamento dextrais correlacionáveis com os estágios finais de deformação registrados nos granulitos de Salvador.

monzo-sienogranitos; petrologia; geocronologia; Salvador; Bahia; Brasil


INTRODUCTION

The relationships between magmatic bodies and deformational events are useful to unravel the complex interactions between tectonics and the processes of generation and emplacement of magmas. Thus, granitoid bodies are good tracers of the rheological evolution of host rocks, as well as of stress fields and kinematics (Druguet et al. 2008Druguet E., Czeckb D.M., Carreras J., Castaño L.M. 2008. Emplacement and deformation features of syntectonic leucocratic veins from Rainy Lake zone (Western Superior Province, Canada). Precambrian Research, 163:384-400.). These rocks are present in different crustal levels at various scales, from large granitic plutons to small anatectic granitic veins (leucosome) in migmatitic terrains.

The granulitic rocks that outcrop in Salvador, state of Bahia, Brazil, are located nearby the confluence of two important tectonic macro-units of the São Francisco craton (SFC; Almeida 1977Almeida F.F.M. de 1977 . O Cráton do São Francisco. Revista Brasileira de Geociências, 7:349-364.): the first one, with N45° trends, corresponds to the Salvador-Esplanada belt (SEB) of Barbosa and Dominguez (1996)Barbosa J.S.F. & Dominguez J.M.L. (eds.). 1996. Texto Explicativo para o Mapa Geológico ao Milionésimo. SICM/SGM, Salvador (Edição Especial),` 400 p., and the second one, oriented N10°, corresponds to the Itabuna-Salvador-Curaçá belt (ISCB) of Barbosa and Sabaté (2002, 2004)Barbosa J.S.F. & Sabaté P. 2002. Geological features and the Paleoproterozoic collision of four Archaean Crustal segments of the São Francisco Craton, Bahia, Brazil. A synthesis. Anais da Academia Brasileira Ciências, 74(2):343-359. (Fig. 1). Both units show a complex evolutionary history (Barbosa and Dominguez 1996Barbosa J.S.F. & Dominguez J.M.L. (eds.). 1996. Texto Explicativo para o Mapa Geológico ao Milionésimo. SICM/SGM, Salvador (Edição Especial),` 400 p., Barbosa and Sabaté 2002Barbosa J.S.F. & Sabaté P. 2002. Geological features and the Paleoproterozoic collision of four Archaean Crustal segments of the São Francisco Craton, Bahia, Brazil. A synthesis. Anais da Academia Brasileira Ciências, 74(2):343-359., 2004Barbosa J.S.F. 1990. The granulites of the Jequié complex and Atlantic mobile belt, southern Bahia, Brazil: an expression of Archean-Proterozoic plate convergence. In: Vielzeuf D. & Vidal P.. (eds.), Granulites and Crustal Evolution. Springer-Verlag, France, p. 195-221., Delgado et al. 2002Delgado I.M.., Souza J.D.., Silva L.C.., Silveira Filho N.C.., Santos R.A.., Pedreira A.J.., Guimarães J.T.., Angelim L.A.A.., Vasconcelos A.M.., Gomes I.P.., Lacerda Filho J.V., Valente C.R.., Perrotta M.M.., Heineck C.A.. 2002. Escudo Atlântico. In: Bizzi L.A., Schobbenhaus C., Vidotti M., Gonçalves J.H. (eds.), Geologia, Tectônica e Recursos Minerais do Brasil. texto, mapas & SIG. CPRM - Serviço Geológico do Brasil, Brasília, 692 p.), which makes it difficult to establish precise geotectonic models and the connection between these two units. The granitoids that occur in the ISCB can be classified as (i) syntectonic, contemporary to the formation of the belt and to the crustal thickening (~2.1 Ga), and (ii) post-tectonic, associated to sinistral transcurrent faults related to the peak of granulitic metamorphism and orogenic collapse (~2.07 Ga) (Barbosa et al. 2008Barbosa J.S.F. 1990. The granulites of the Jequié complex and Atlantic mobile belt, southern Bahia, Brazil: an expression of Archean-Proterozoic plate convergence. In: Vielzeuf D. & Vidal P.. (eds.), Granulites and Crustal Evolution. Springer-Verlag, France, p. 195-221.).

Figure 1
(A) Sao Francisco craton with the main tectonic units of its basement and the mobile belts of Neoproterozoic age (adapted from Alkmim et al. 1993Alkmim F.F., Neves B.B.B., Alves J.A.C. 1993. Arcabouço tectônico do Cráton do São Francisco - Uma revisão. In: Dominguez J.M.L. & Misi A. (eds.). O Cráton do São Francisco. SBG-NBA/SE, SGM, CNPq, Salvador, p. 45-62.). (B) Simplifi ed geological map of the area where Salvador is located, showing the main geotectonic units (adapted from Dalton de Souza et al. 2003Dalton de Souza J., Kosin M., Melo R.C., Santos R.A., Teixeira L.R., Sampaio A.R., Guimarães J.T., Vieira Bento R., Borges V.P., Martins A.A.M., Arcanjo J.B., Loureiro H.S.C.., Angelim L.A.A. 2003. Mapa Geológico do Estado da Bahia - Escala 1:1.000.000. Salvador: CPRM, 2003. Versão 1.1. Programas Carta Geológica do Brasil ao Milionésimo e Levantamentos Geológicos Básicos do Brasil (PLGB). Convênio de Cooperação e Apoio Técnico-Científico CBPM-CPRM.). The square near Salvador corresponds to the location of Fig. 2.

This study had the objective of placing the granitic bodies and veins that outcrop in Salvador within the regional tectonic context. In addition, the petrographic, petrochemical, geochronological, and isotopic data of these rocks are presented and discussed, aiming to contribute to the knowledge of their tectonic environment.

ANALYTICAL PROCEDURES

Twelve whole-rock granite analyses of major and trace elements were carried out at the laboratories of GEOSOL and Geology and Surveying Ltd., and are reported in Tab. 2. Major (SiO2, Al2O3, FeO (t), MgO, CaO, TiO2, P2O5, and MnO) and trace (V, Rb, Ba, Sr, Ga, Nb, Zr, Y, and Th) elements were analyzed by X-ray fluorescence, and rare earth elements were determined using inductively coupled plasma mass spectrometry (ICP-MS). Na2O and K2O contents were determined using atomic absorption spectrometry.

Table 2
Chemical analyses of the rock samples of monzo-syenogranitic bodies and veins of Salvador

The procedures described by Peucat et al. (1999)Peucat J.J., Ménot R.P., Monnier O., Fanning C.M. 1999. The Terre Adélie basement in the East-Antarctica Shield: geological and isotopic evidence for a major 1.7 Ga thermal event: comparison with Gawler craton in South Australia. Precambrian Research, 94:205-224. were followed to analyze the Nd whole-rock isotopic compositions. The values were adjusted to the pattern of Nd AMES standard, which provided a mean 143Nd/144Nd ratio of 0.511896 ± 7, with an error of 0.0015%. The model ages (TDM) were calculated using values of εNd +10 for the current depleted mantle and 147Sm/144Nd ratio of 0.2137, assuming a radiogenic linear growth starting at 4.54 Ga.

Several U-Pb (zircon) analyses were carried out for the monzo-syenogranite sample SG-10G at the Laboratoire Magmas et Volcans - Université Blaise Pascal in Clermont-Ferrand, France, using the in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) method as described by Hurai et al. (2010)Hurai V., Paquette J.L., Huraiová M., Konecny P. 2010. Age of deep crustal magmatic chambers in the intra-Carpathian back-arc basin inferred from LA-ICPMS U-Th-Pb dating of zircon and monazite from igneous xenoliths in alkali basalts. Journal of Volcanology and Geothermal Research, 198:275-287.. The error measured for each analysis (ratios and ages) is presented at the level of 1σ. For the calculation of 207Pb/206Pb weighted mean ages, a confidence limit of 95% was considered. The errors in the discordant intercept ages are presented at the level of 2σ and were calculated using the software Isoplot (Ludwig 2001Ludwig K.R. 2001. User's manual for Isoplot/Ex. Version 2.49. A geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center. Special Publication 1.).

The 207Pb/206Pb dating method through evaporation of lead from zircon monocrystals, developed by Kober (1986)Kober B., 1986. Whole-grain evaporation for 207Pb/206Pb age investigations on single zircons using a double-filament thermal ion source. Contributions to Mineral Petrology, 93:482-490., was used for the same SG-10G sample by means of successive heating stages in a thermal ionization mass spectrometer (TIMS), using the Finnigan MAT 262 mass spectrometer of the Laboratoire de Géosciences Rennes-CNRS, France.

All ages were calculated using decay constants and abundance of the isotopes listed by Steiger and Jäger (1977)Steiger R.H. & Jäger E. 1977. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earth Planetary Science Letters, 36:359-362..

REGIONAL GEOLOGICAL SETTING

In the portion of the SFC that outcrops in the state of Bahia, high-grade metamorphic rocks occur in the area of Itabuna-Ilhéus, in the south, and in the area of Curaçá, in the north, comprising the Itabuna-Salvador-Curaçá block (ISCB) of Barbosa and Sabaté (2002, 2004)Barbosa J.S.F. & Sabaté P. 2002. Geological features and the Paleoproterozoic collision of four Archaean Crustal segments of the São Francisco Craton, Bahia, Brazil. A synthesis. Anais da Academia Brasileira Ciências, 74(2):343-359.. These rocks, at the current level of erosion, are the roots of an orogenic belt, with N-S orientation and Paleoproterozoic age of around 2.1 Ga (Peucat et al.2011Peucat J.-J., Barbosa J.S.F., Pinho I.C. de A., Paquette J.-L., Martin H., Fanning C.M., Menezes Leal A.B., Cruz S.C.P. 2011. Geochronology of granulites from the south Itabuna-Salvador-Curaçá Block, São Francisco Craton (Brazil): Nd isotopes and U-Pb zircon ages. Journal of South American Earth Sciences, 31:397-413.) (Fig. 1A). The southern part of this belt consists of at least four groups of tonalite/trondhjemite, three of them with Archean ages between 2.6 and 2.7 Ga (Peucat et al. 2011Peucat J.-J., Barbosa J.S.F., Pinho I.C. de A., Paquette J.-L., Martin H., Fanning C.M., Menezes Leal A.B., Cruz S.C.P. 2011. Geochronology of granulites from the south Itabuna-Salvador-Curaçá Block, São Francisco Craton (Brazil): Nd isotopes and U-Pb zircon ages. Journal of South American Earth Sciences, 31:397-413.), and one Paleoproterozoic group (~2.2 Ga) (Barbosa and Sabaté 2004Barbosa J.S.F. & Sabaté P. 2002. Geological features and the Paleoproterozoic collision of four Archaean Crustal segments of the São Francisco Craton, Bahia, Brazil. A synthesis. Anais da Academia Brasileira Ciências, 74(2):343-359., Peucat et al. 2011Peucat J.-J., Barbosa J.S.F., Pinho I.C. de A., Paquette J.-L., Martin H., Fanning C.M., Menezes Leal A.B., Cruz S.C.P. 2011. Geochronology of granulites from the south Itabuna-Salvador-Curaçá Block, São Francisco Craton (Brazil): Nd isotopes and U-Pb zircon ages. Journal of South American Earth Sciences, 31:397-413.) (Tab. 1). Subordinately, charnockitic bodies of Archean age also occur, as well as bands of supracrustal rocks and gabbros, and basalts related to ocean floor or back-arc basins (Teixeira 1997Teixeira L.R.. 1997. O Complexo Caraíba e a Suíte São José do Jacuípe no Cinturão Salvador-Curaçá (Bahia, Brasil): petrologia, geoquímica e potencial metalogenético. Tese de Doutorado, Instituto de Geociências, Universidade Federal da Bahia, Salvador, 243 p.). In addition, intrusions of granulitized monzogranites of shoshonitic affinity are also found (Barbosa 1990Barbosa J.S.F. 1990. The granulites of the Jequié complex and Atlantic mobile belt, southern Bahia, Brazil: an expression of Archean-Proterozoic plate convergence. In: Vielzeuf D. & Vidal P.. (eds.), Granulites and Crustal Evolution. Springer-Verlag, France, p. 195-221.). The northern area of the ISCB essentially consists of tonalite-trondhjemite-granodiorite (TTG) orthogneisses of 2.7 Ga (Figueiredo 1989Figueiredo M.C.H. 1989. Geochemical evolution of eastern Bahia, Brazil: a probable early Proterozoic subduction-related magmatic arc. Journal of South American Earth Sciences, 2:131-145., Silva et al. 1997Silva L.C., McNaughton N.P. , Melo R.C., Fletcher I.R. 1997. U-Pb SHRIMP ages in the Itabuna-Caraíba TTG high-grade Complex: the first window beyond the Paleoproterozoic overprint of the eastern Jequié Craton, NE Brazil. In: International Symposium on Granites and Associated Mineralization (ISGAM). Salvador, Abstracts. p. 282-283.) (Tab. 1), with interbedded aluminous gneisses, calc-silicate rocks, metacarbonates, and quartzites (Melo et al. 1995Melo R.C., Loureiro H.S.C. Pereira L.H.M. 1995. Programa Levantamentos Geológicos Básicos do Brasil. Serrinha. Folha SC-24-Y-D. Escala 1: 250.000. MME/CPRM/SUREG-SA. 80 p.), as well as mafic-ultramafic rocks that form the so-called São José do Jacuípe Suite, also of Archean age (Silva et al. 1997Silva L.C., McNaughton N.P. , Melo R.C., Fletcher I.R. 1997. U-Pb SHRIMP ages in the Itabuna-Caraíba TTG high-grade Complex: the first window beyond the Paleoproterozoic overprint of the eastern Jequié Craton, NE Brazil. In: International Symposium on Granites and Associated Mineralization (ISGAM). Salvador, Abstracts. p. 282-283.). The whole ISCB is intruded by syenites dated at 2.08 - 2.09 Ga (Conceição et al. 2003Conceição H., Rosa M.L.S., Macambira M.J.B., Scheller T., Marinho M.M., Rios D.C. 2003. 2,09 Ga idade mínima da cristalização do Batólito Sienítico Itiúba: um problema para o posicionamento do clímax do metamorfismo granulítico (2,05-2,08 Ga) no Cinturão Móvel Salvador-Curaçá, Bahia. Revista Brasileira de Geociências, 33(3):395-398., Oliveira et al. 2004Oliveira E.P., Windley B.F., McNaughton N.J., Pimentel M., Fletcher I.R. 2004. Contrasting copper and chromium metallogenic evolution of terranes in the Palaeoproterozoic Itabuna-Salvador-Curacá orogen, São Francisco craton, Brazil: new zircon (SHRIMP) and Sm-Nd (model) ages and their significance for orogen-parallel escape tectonics. Precambrian Research, 128(1-2):143-165.), and syn- and post-tectonic granites intrusions dated around 2.06 Ga (Silva et al. 2002, Barbosa et al. 2008Barbosa J.F.S., Peucat J.J., Martin H., da Silva F.A., de Moraes A.M., Corrêa-Gomes L.C., Sabaté P., Marinho M.M., Fanning C.M.F. 2008. Petrogenesis of the late-orogenic Bravo granite and surrounding high-grade country rocks in the paleoproterozoic orogen of Itabuna-Salvador-Curaçá block, Bahia, Brazil. Precambrian Research, 167: 35-52.). This entire crustal segment was strongly affected by Paleoproterozoic tectonics with all its lithotypes plunged into granulite facies metamorphism (Barbosa & Sabaté 2002Barbosa J.S.F. 1990. The granulites of the Jequié complex and Atlantic mobile belt, southern Bahia, Brazil: an expression of Archean-Proterozoic plate convergence. In: Vielzeuf D. & Vidal P.. (eds.), Granulites and Crustal Evolution. Springer-Verlag, France, p. 195-221., 2004Barbosa J.S.F. & Sabaté P. 2002. Geological features and the Paleoproterozoic collision of four Archaean Crustal segments of the São Francisco Craton, Bahia, Brazil. A synthesis. Anais da Academia Brasileira Ciências, 74(2):343-359.).

Table 1
U-Pb and Pb-Pb (zircon) ages of the Itabuna-Salvador-Curaçá block and of the Salvador-Esplanada belt (modified from Peucat et al. 2011Peucat J.-J., Barbosa J.S.F., Pinho I.C. de A., Paquette J.-L., Martin H., Fanning C.M., Menezes Leal A.B., Cruz S.C.P. 2011. Geochronology of granulites from the south Itabuna-Salvador-Curaçá Block, São Francisco Craton (Brazil): Nd isotopes and U-Pb zircon ages. Journal of South American Earth Sciences, 31:397-413.)

The SEB of Barbosa and Dominguez (1996)Barbosa J.S.F. & Dominguez J.M.L. (eds.). 1996. Texto Explicativo para o Mapa Geológico ao Milionésimo. SICM/SGM, Salvador (Edição Especial),` 400 p. consists of high-grade metamorphic rocks, which are roughly N45° aligned (Fig. 1B). These lithotypes underlie Salvador, in Bahia, and extend up to Boquim, in the state of Sergipe. A large portion of the northeastern part of the belt is covered by Tertiary deposits of the Barreiras Formation and, by Quaternary sediments, and in the southwestern part, by the sedimentary rocks of the Recôncavo-Tucano Mesozoic basin.

The SEB consists of migmatitic orthogneisses of alkaline to subalkaline affinity, and tonalitic, charnoenderbitic, and charnockitic orthogneisses with calc-alkaline affinity. There are also orthogneisses with felsic tonalitic-granodioritic terms of 2.9 Ga (Silva et al. 2002) (Tab. 1), as well as amphibolized gabbros with tholeiitic affiliation and granites with alkaline tendency (Delgado et al.2002Delgado I.M.., Souza J.D.., Silva L.C.., Silveira Filho N.C.., Santos R.A.., Pedreira A.J.., Guimarães J.T.., Angelim L.A.A.., Vasconcelos A.M.., Gomes I.P.., Lacerda Filho J.V., Valente C.R.., Perrotta M.M.., Heineck C.A.. 2002. Escudo Atlântico. In: Bizzi L.A., Schobbenhaus C., Vidotti M., Gonçalves J.H. (eds.), Geologia, Tectônica e Recursos Minerais do Brasil. texto, mapas & SIG. CPRM - Serviço Geológico do Brasil, Brasília, 692 p.).

In the region of Salvador, these rocks are plunged into granulite facies (Fujimori & Allard 1966Fujimori S. & Allard G.O. 1966. Ocorrência de safirina em Salvador, Bahia. Bololetim da Sociedade Brasileira de Geologia, 15:67-81., Fujimori 1968Fujimori S. 1968. Granulitos e charnockitos de Salvador (Ba). Anais da Academia Brasileira Ciências, 40:181-202., 1988Fujimori S. 1988. Condições de P-T de formação dos granulitos do Farol da Barra, Salvador, Bahia, Brasil. Revista Brasileira de Geociências, 18:339-344., Barbosa et al. 2005Barbosa J.S.F., Corrêa-Gomes L.C., Dominguez, J.M.L., Cruz S.A.S., Souza, J.S. 2005. Petrografia e Litogeoquimica das Rochas da Parte Oeste do alto de Salvador Bahia. Revista Brasileira de Geociências, 35 (4 Supl): 9-22., Souza et al. 2010Souza J.S. de, Barbosa J.S.F., Correa-Gomes L.C. 2010. Litogeoquímica dos granulitos ortoderivados da cidade de Salvador, Bahia. Revista Brasileira Geociências, 40:339-354., among others), although northwards, according to Oliveira Júnior (1990)Oliveira Junior T.R.. 1990. Geologia do extremo nordeste do Cráton do São Francisco, Bahia. Dissertação de Mestrado, Instituto de Geociências, Universidade Federal da Bahia. 126 p., granulitic rocks grade to rocks of the amphibolite facies. These are cut by mafic dyke swarms (Mestrinho et al. 1988Mestrinho S.S.P.., Linhares P., Carvalho, I.G. 1988. Geoquímica de elementos principais e traços do dique de diabásio da praia de Ondina, Salvador, Bahia. In: Congr. Bras. Geol., 32, Anais, 4:1862-1877., Corrêa-Gomes et al. 1996Corrêa Gomes L.C., Tanner de Oliveira M.A.F., Motta A.C., Cruz M.J.M. (eds.). 1996. Província de Diques Máficos do Estado da Bahia Mapa, estágio atual do conhecimento e evolução temporal. SICM/SGM, Salvador (Edição Especial), 144 p., Menezes Leal et al. 2012Menezes Leal A.B., Corrêa-Gomes L.C., Guimaraes J.T. 2012. Diques Maficos. In: Barbosa J.S.F. (Coordenação Geral). Geologia da Bahia. Pesquisa e Atualização. Salvador, Volume 2, p. 199-231.) and by the granitic bodies (Celino et al. 1984Celino J.J., Conceição H., Corrêa-Gomes L.C. 1984. Monzogranito porfirítico: magmatismo ácido tardio no Cinturão Granulítico Atlântico, Salvador, Bahia. In: SBG, Congr. Bras. Geol., 33, Bol. Res., 157-158.), which have motivated this study.

Geologically, the area of Salvador (Fig. 2) was subdivided by Barbosa and Dominguez (1996)Barbosa J.S.F. & Dominguez J.M.L. (eds.). 1996. Texto Explicativo para o Mapa Geológico ao Milionésimo. SICM/SGM, Salvador (Edição Especial),` 400 p. into three major domains: (i) the Alto de Salvador, which is a horst of granulitic rocks (Barbosa et al. 2005Barbosa J.S.F., Corrêa-Gomes L.C., Dominguez, J.M.L., Cruz S.A.S., Souza, J.S. 2005. Petrografia e Litogeoquimica das Rochas da Parte Oeste do alto de Salvador Bahia. Revista Brasileira de Geociências, 35 (4 Supl): 9-22.); (ii) the Recôncavo sedimentary basin, which is limited, eastwards, by the Salvador fault system; and (iii) the Atlantic Coastal Margin, composed of Tertiary and Quaternary deposits of unconsolidated sediments (Dominguez et al. 1999Dominguez J.M.L., Martin L., Bittencourt A.C.S.P., Testa V., Leão Z.M.A.N., Silva C. de C. 1999. Atlas Geoambiental da Zona Costeira do Estado da Bahia, Convênio UFBA/SME.).

Figure 2
Simplified geological map of the area with the location of the studied samples.

Despite the lack of outcrops, the studies carried out by Barbosa et al. (2005)Barbosa J.S.F., Corrêa-Gomes L.C., Dominguez, J.M.L., Cruz S.A.S., Souza, J.S. 2005. Petrografia e Litogeoquimica das Rochas da Parte Oeste do alto de Salvador Bahia. Revista Brasileira de Geociências, 35 (4 Supl): 9-22., focusing on the western area of the horst, showed a great diversity of ortho- and para-derived metamorphic lithotypes of high- and medium-metamorphic grade, which are polydeformed and frequently cut by mafic dykes and irregular monzo-syenogranitic bodies. Souza et al. (2010Souza J.S. de, Barbosa J.S.F., Correa-Gomes L.C. 2010. Litogeoquímica dos granulitos ortoderivados da cidade de Salvador, Bahia. Revista Brasileira Geociências, 40:339-354. and references therein) grouped these rocks into four units: (i) the predominant ortho-derived granulites, with granulitized ultramafic and mafic enclaves; (ii) the para-derived granulites; (iii) the monzo-syenogranitic bodies and veins; and (iv) the mafic dykes (Fig. 2).

Regarding the ductile deformations, at least three stages of continuous deformation were recorded by Barbosa et al. (2005)Barbosa J.S.F., Corrêa-Gomes L.C., Dominguez, J.M.L., Cruz S.A.S., Souza, J.S. 2005. Petrografia e Litogeoquimica das Rochas da Parte Oeste do alto de Salvador Bahia. Revista Brasileira de Geociências, 35 (4 Supl): 9-22. on the granulitic lithotypes, which are the host rocks of the monzo-syenogranitic bodies and veins in question. The main structures of the first stage comprised recumbent folds with subhorizontal axes, which were isoclinally refolded during the second phase. These isoclinal folds have subvertical axial planes and subhorizontal axes. The third phase includes transcurrent dextral shear zones, subparallel to the axial surfaces of the isoclinal folds, which are coeval with the second deformational phase and produce mineral stretching lineations parallel to their axes. U-Pb monazite ages (in situ LA-ICPMS) indicate that the third deformational phase occurred at 2,064 ± 9 Ma (Souza 2013Souza J.S. de. 2013. Geologia, Metamorfismo e Geocronologia de Litotipos de Salvador-Bahia. Tese de Doutorado, Instituto de Geociências, Universidade Federal da Bahia, 153 p.).

Numerous faults and fractures cut the granulitic rocks. The most significant are those oriented N60° - N90°, associated with the intrusion of mafic dykes, and those oriented N120° - N160°, where tabular bodies and monzo-syenogranitic veins were placed. In addition to these, faults oriented N30° have also been registered, and the Iguatemi and Jardim de Alah faults (Fig. 2) exhibit a general N40° orientation.

Although Souza et al. (2010Souza J.S. de, Barbosa J.S.F., Correa-Gomes L.C. 2010. Litogeoquímica dos granulitos ortoderivados da cidade de Salvador, Bahia. Revista Brasileira Geociências, 40:339-354. and references therein) recently presented important results on the granulitic rocks, various geological problems still need to be solved. One of them, which shows the generation and age of the felsic magmatism of the SEB, will be dealt with in this study.

PETROGRAPHICAL AND GEOCHEMICAL ASPECTS

The monzo-syenogranitic dykes and veins outcropping along the coast of Salvador (Fig. 2) fill fractures in various directions. Two dominant directions present are as follows:

1. Fractures oriented N60° - N90°:The dykes exhibit fine- to medium-grained texture, are moderately deformed, and present folds and boudinage when affected by the late dextral shear zones correlated to the third stage of deformation of Barbosa et al. (2005)Barbosa J.S.F., Corrêa-Gomes L.C., Dominguez, J.M.L., Cruz S.A.S., Souza, J.S. 2005. Petrografia e Litogeoquimica das Rochas da Parte Oeste do alto de Salvador Bahia. Revista Brasileira de Geociências, 35 (4 Supl): 9-22.. Mafic dykes were observed with the same orientation. In the outcrop of the Paciência Beach (SG-10F) for example, interpenetrations of mafic material in felsic material and vice versa were identified, characterizing a mingling-type heterogeneous physical mixing of basaltic and granitic magma (Walker & Skelhorn 1966Walker G.P.L. & Skelhorn R.R. 1966. Some associations of acid and basic igneous rocks. Earth Science Reviews, 2:93-109., Wiebe 1991Wiebe R.A. 1991. Commingling of contrasted magmas and generation of mafic enclaves in granite rocks. In: Didier J. & Barbarin B. (eds.), Enclaves and Granite Petrology. Elsevier, Amsterdam, p. 393-402.) (Figs. 3A and B).

Figure 3
Association of the monzo-syenogranitic veins with mafi c dykes in fractures and faults with orientation of N60° - N90°. (A) Panoramic view of Paciencia Beach. (B) Detail of a physical mixing of basaltic and granitic magma (mingling). (C) Verticalized monzo-syenogranitic body with pegmatoid texture and abrupt contacts with its host rocks.

2. Fractures oriented N40° - N70°: The dykes present medium- to coarse-grained texture and occur as vertical and subvertical bodies, with thickness ranging from 0.5 to 2 m and keeping abrupt contacts with their host rocks (Fig. 3C). Irregular bodies are also found, with various thicknesses and diffuse contacts. On the edge of some of the granitoid dykes, in contact with the host rocks, pegmatitic facies were observed, probably related to late magmatic-hydrothermal processes.

From a petrographic point of view, these felsic bodies and veins are classified as monzo-syenogranites. They show quartz (30 - 40%), microcline (30 - 40%), biotite (15%), and plagioclase (5 - 10%) as major minerals. Apatite, opaque minerals, and zircon occur as accessory minerals.

The geochemical data of the major elements allowed classifying them as subalkaline and peraluminous (Fig. 4A and B), characterizing their origin as formed by the partial fusion of a crustal source (Chappell & White 1974Chappell, B.W. & White, A.J.R. 1974. Two contrasting granite types. Pacific Geology, 8:173-174., White & Chappell 1977White A.J.R. & Chappell B.W. 1977. Ultrametamorphism and granitoid genesis. Tectonophysics, 43:7-22.). They are acidic rocks, with SiO2 contents ranging from 69 to 73% (Tab. 2). The Harker (1909)Harker A. (ed.) 1909. The Natural History of Igneous Rocks. Macmillan Publishers, New York, p. 384. type binary diagrams show magmatic differentiation trends, in which TiO2, Na2O, MgO, Sr, Zr, and Ba are compatible with fractional crystallization whereas K2O and Rb are incompatible (Fig. 5). Furthermore, as shown in Fig. 6, these lithotypes are enriched in light rare earth elements (LREE) (136 < LaN < 602) and present a strong negative Europium (Eu) anomaly. The geochemical data of their trace elements, represented in discrimination diagrams of tectonic environment, are not conclusive. Most of them are located in the fields of intraplate and syncollision granites of Pearce et al. (1984)Pearce J.A., Harris N.B.W., Tindle A.G. 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25:956-983., but some samples plot in the field of post-collision granites of Pearce (1996)Pearce J.A. 1996. Sources and settings of granitic rocks. Episodes, 19:120-125. (Figs. 7A and 7B).

Figure 4
(A) The (Na2O + K2O) versus SiO2 diagram of Irvine and Baragar (1971)Irvine T.N. & Baragar V.R.A. 1971. A guide to the chemical classification of common volcanic rocks. Canadian Journal of Earth Sciences, 8:523-548., showing the subalkalinity of the monzo-syenogranitic bodies of Salvador. (B) The Al2O3/(Na2O + K2O) versus Al2O3/(Na2O + K2O +CaO) diagram of Shand (1950)Shand S.J. (ed.). 1950. Eruptive Rocks Their Genesis, Composition, Classification and Their Relation to Ore Deposit, 4th edn., Thomas Murby, London, 488 p., characterizing their peraluminous character.

Figure 5
Chemical variation diagrams of Harker (1909)Harker A. (ed.) 1909. The Natural History of Igneous Rocks. Macmillan Publishers, New York, p. 384. for major and trace elements of the monzo-syenogranitic bodies of Salvador.

Figure 6
Chondrite-normalized rare earth elements patterns for the monzo-syenogranitic bodies. The chondrite values are from Evensen et al. (1978)Evensen N.M., Hamilton P.J., O'Nions R.R. 1978. Rare earth abundances in chondritic meteorites. Geochimica et Cosmochimica Acta, 42:1199-1212..

Figure 7
(A) Rb versus (Nb + Y) diagram, discriminating tectonic environments for granites (Pearce et al. 1984Pearce J.A., Harris N.B.W., Tindle A.G. 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25:956-983.). (B) Nb versus Y diagram, discriminating tectonic environments for granites (Pearce et al. 1984Pearce J.A., Harris N.B.W., Tindle A.G. 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25:956-983.). The ellipse corresponds to the fi eld of post-collision granites of Pearce (1996)Pearce J.A. 1996. Sources and settings of granitic rocks. Episodes, 19:120-125..

ISOTOPIC GEOCHEMISTRY AND GEOCHRONOLOGY

The SG-10G sample of a monzo-syenogranite vein corresponds to a representative part of the felsic portion of the mingling-type mixture observed at Paciência Beach (Fig. 3B).

One sample (SG-10G) of the monzo-syenogranitic bodies was selected for U-Pb zircon dating. This sample was intrusive into the ortho-derived granulites, at the Rio Vermelho district in the city of Salvador (Figs. 2 and 3). Dated zircon grains are large (spot size in Fig. 8 is ca. 30 µm) and they are mainly clear, euhedral, elongated, and without any visible inherited core (see TL images of grains 1.1 and 2.1 in Fig. 8). They are typical of high-temperature types (S19-S24, Fig. 8, after Pupin (1980)Pupin J.P. 1980. Zircon and granite petrology. Contributions to Mineralogy and Petrology 73: 207-220.). The protolith of these granulites were dated at 2,561 ± 7 Ma and the granulite facies were 2,089 ± 11 Ma by U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon ages (Silva et al. 1997).

Figure 8
Concordia diagram with U-Pb data in zircon crystals from the SG-10G sample obtained through LAICPMS. Images of zircon crystals under transmitted light. The circles correspond to the spots (20 µm).

U-Pb zircon ages obtained using ICP-MS data are concordant to slightly discordant and define an intercept age at 2,064 ± 36 Ma, with the mean of 207Pb/206Pb ages of 2,067 ± 13 Ma in 21 analyses. These ages are similar to the Pb-Pb zircon TIMS evaporation age also obtained in this study (i.e., 2,064 ± 6 Ma) and are interpreted as the age of crystallization of the granites. Three grains that exhibit zoned cores (see grains 8.1 and 19.1 in Fig. 8) showed 207Pb/206Pb ages of ca. 2.3 - 2.4 Ga, which were interpreted as an inheritance from the surrounding Archean basement.

Whole-rock Sm-Nd elemental and isotopic data obtained from the same sample were as follows: Sm = 22.9219 ppm; Nd = 134.6957 ppm; 147Sm/144Nd = 0.1028; 143Nd/144Nd = 0.5110. These data provided a model age (TDM) of 2.859 Ga, and εNd(0) = -30.9. Considering the 2.07 Ga crystallization age, the parameter εNd(2.07Ga) results in the value of -6.08, suggesting an important participation of a crustal component in the formation of these granites.

DISCUSSIONS AND CONCLUSIONS

The data obtained in this study from the monzo-syenogranitic bodies and veins intrusive into the granulites of Salvador allow us to classify them as subalkaline and peraluminous, indicating a possible origin from the partial fusion of a crustal source, a hypothesis that is corroborated by the negative values of εNd(t) (-6.08). The rocks are enriched in LREE and present strong negative Eu anomalies.

The Sm-Nd model age (TDM) around 2.9 Ga seems to indicate a Neoarchean source for these lithotypes. The U-Pb zircon ages of 2,064 ± 36 and 2,064 ± 6 Ma obtained in this study are similar to the U-Pb (SHRIMP) and Pb-Pb (evaporation) zircon ages of the syenites and post-tectonic granites from the ISCB, where ages ranged from 2.06 to 2.09 Ga (Peucat et al. 2011Peucat J.-J., Barbosa J.S.F., Pinho I.C. de A., Paquette J.-L., Martin H., Fanning C.M., Menezes Leal A.B., Cruz S.C.P. 2011. Geochronology of granulites from the south Itabuna-Salvador-Curaçá Block, São Francisco Craton (Brazil): Nd isotopes and U-Pb zircon ages. Journal of South American Earth Sciences, 31:397-413. and references therein) (Tab. 1).

On the basis of the present results and the recent studies performed by Souza (2013)Souza J.S. de. 2013. Geologia, Metamorfismo e Geocronologia de Litotipos de Salvador-Bahia. Tese de Doutorado, Instituto de Geociências, Universidade Federal da Bahia, 153 p., the monzo-syenogranites can be positioned as late- to post-tectonic granites, since they exhibit deformation related to the late dextral shear zones dated at 2,064 ± 9 Ma (in situ LA-ICPMS monazite) of Paleoproterozoic orogenesis in the SEB.

Moreover, taking into account the age of 2,083 ± 4 Ma attributed to the metamorphism of the ISCB (Peucat et al. 2011Peucat J.-J., Barbosa J.S.F., Pinho I.C. de A., Paquette J.-L., Martin H., Fanning C.M., Menezes Leal A.B., Cruz S.C.P. 2011. Geochronology of granulites from the south Itabuna-Salvador-Curaçá Block, São Francisco Craton (Brazil): Nd isotopes and U-Pb zircon ages. Journal of South American Earth Sciences, 31:397-413.), the geochemical and geochronological data found in this study are compatible with the geodynamic model proposed by Barbosa and Sabaté (2002, 2004)Barbosa J.S.F. 1990. The granulites of the Jequié complex and Atlantic mobile belt, southern Bahia, Brazil: an expression of Archean-Proterozoic plate convergence. In: Vielzeuf D. & Vidal P.. (eds.), Granulites and Crustal Evolution. Springer-Verlag, France, p. 195-221., as well as with the tectonic evolution of the northeastern part of the ISCB proposed by Oliveira et al.(2010)Oliveira E.P., McNaughton N.J., Armstrong R. 2010. Mesoarchaean to palaeoproterozoic growth of the northern segment of the Itabuna-Salvador-Curaçá orogen, São Francisco Craton, Brazil. In: Kusky T.M.., Zhai M.G.., Xiao W.. (eds.), The Evolving Continents: Understanding Processes of Continental Growth. Geological Society of London, Special Publications, 338:263-286.. Similar to our conclusion, these geodynamic models include some peraluminous types as late-tectonic granites, which are undeformed or rather weakly deformed and cut through the older granulitic rocks and exhibit negative values of εNd(T) between -13 and -5.

ACKNOWLEDGMENTS

We wish to acknowledge the financial resources provided by CNPq for the field work, the Doctorate degree grant of the first author by CAPES, and the financial support for the reciprocal visits of the second author in Brazil and of the third author in France at the Université Blaise Pascal (Project No. 624/09) by CAPES-COFECUB. We also acknowledge the financial resources provided for laboratorial work by CBPM and CPRM.

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Publication Dates

  • Publication in this collection
    Apr-Jun 2014

History

  • Received
    09 Apr 2014
  • Accepted
    14 May 2014
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