Geochemical and isotopic signatures of Proterozoic granitoids in terranes of the Borborema structural province, northeastern Brazil

Abstract

Two large E–W trending megastructures, the Patos and Pernambuco shear zones, subdivide the Borborema Province (BP), northeastern Brazil, into three main domains: Northern (ND), Transverse Zone (TZD) and Southern (SD). These domains evolved through reworking and amalgamations, during the Brasiliano cycle (0.70–0.50 Ga), of several tectonostratigraphic terranes and major crustal blocks previously deformed during Meso and Paleoproterozoic orogenic cycles. Petrological and geochemical characteristics of granitoids and syenitoids in these domains allow their classification into nine groups. These groups are enriched in K and Ba and have low Nb (usually <20 ppm), which seems to be a peculiar feature of the lithosphere in the BP. As an isolated criteria, the characteristics of the granitoid groups do not allow a clear individualization of different terranes, in most cases, but point neverthless to important contrasts among the major tectonic domains. Calc-alkalic magmatic epidote (mEp)-bearing granitoids in the ND display low δ 18O (+6 to +8‰ SMOW), magnetic susceptibility (MS)>1.0×10 −3SI, ε Nd(0.6 Ga) from −15 to −20, and t DM model ages>2.0 Ga. Within the TZD, in contrast, calc-alkalic and high-K calc-alkalic mEp-bearing granitoids, respectively in the Cachoeirinha–Salgueiro and Alto Pajeúterranes, display lower MS (0.4×10 −3SI), high δ 18O (+10 to +13‰ SMOW), lower ε Nd (−1 to −4), and younger t Nd (1.1–1.4 Ga). In high-K mEp-free granitoids, ε Nd values cluster around −12 in the Granjeiro terrrane and from −12 to −15 in the Capibaribe terrane. The Alto Pajeúterrane in the TZD is characterized by intrusions of peralkalic ultrapotassic syenitoids that show negative ε Nd (−15 to −19), t DM from 2.1 to 2.4 Ga, and MS from 0.7 to 1.0×10 −3 SI, and are regarded as derived from a metasomatized lithospheric mantle source. Granitoids in the SD are isotopically more complex. Three t DM Nd model-age intervals (1.0–1.5, 1.8–2.2 and 2.4–2.5 Ga) are found in the Pernambuco–Alagoas terrane, the largest one within this domain. MS values are rather low, approaching zero in leucocratic granitoids, and up to 15×10 −3, in high-K metaluminous syenitoids. Calc-alkalic mEp-bearing plutons in the Macururéterrane are similar to those in the TZD in all respects and distinguish the Macururéterrane from the neighbouring terranes. Altogether, the geochemical and isotopic characteristics of the different granitoid types suggest that: (1) the ND is rather homogeneous, at least vertically, in spite of lateral differences in terms of composition of source rocks and magmatic processes that produced the granitoids; (2) the TZD has lateral and vertical heterogeneities, except for the Cachoeirinha–Salgueiro terrane, which is very homogenous; (3) the SD is even more heterogenous than the TZD. The SD had a long and complex accretionary history, as suggested by the presence of a large number of in situ, leucocratic melts associated with large areas of migmatization, both northern and southern boundaries of the Pernambuco–Alagoas terrane, coupled with a large volume of high-K calc-alkalic granitoids emplaced in several pulses.