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The Transversal Zone of the Borborema province is marked by two major episodes in the Neoproterozoic Brasiliano cycle: (1) collisional period characterized by intrusion of granitoids at 650-620 Ma; and (2) a late to post-collisional period at 590-570 Ma. The end of the Brasiliano cycle in this domain is marked by intrusion of peralkalic syenitic and rare A-type granitic magmas, at 540-512 Ma (not discussed here). The oldeste magmatism is characterized by calc-alkalic (CA) and high-K calc-alkalic (HKCA) magmatic epidote-bearing (mEp)- granitoids.
Amphibole-rich clots are common and widespread in all CA plutons, and regarded as fragments from the source rock for the tonalite magmas. The HKCA granitoids present magmatic structures indicative of convection during the formation of the magma chamber. The 590-570 Ma-old plutons are mEp-free high-K calc alkalic granitoids (mEp-free HKCA), peralkalic potassic to ultrapotassic syenitoids, and metaluminous high-K syenites. The mEp-free HKCA granitoids display features of magma mingling with dioritic magma; mixing was more intense in some plutons and favoured the expressive development of hybrid granodiorites.
The ultrapotassic plutons consist of alkali feldspar syenites and alkalic pyroxenite co-magmatic inclusions and syn-plutonic and late-stage dikes, and are interpreted as formed by liquid immiscibility from a mafic syenite magma. Initial Sr ratios for the oldest granitoids are variable, ranging from 0.70598 to 0.710621 (CA), from 0.70839 to 0.71002 (HKCA). Values of εNd (0.6Ga) for the CA granitoids are lower and with a restricted range (from ca. -1 to -2) than those for the HKCA granitoids (-2.8 to - 10.1). Younger mEp-free HKCA granitoids have initial Sr values, around 0.706. The peralkalic potassic/ultrapotassic syenitoids have high initial Sr ratios (∼ 0.710), which together with strongly negative εNd values (ca. -19) are interpreted as inherited from a metasomatized upper mantle source. The values of εNd for plutons of all ages are negative, but there is a tendency for younger suites to have more negative values. The 650-620 Ma old granitoids present younger Nd model ages (< 2.0 Ga) and less negative εNd (ca. -2 to -14) than the youngest ones (TDM = ∼ 1.5 to 2.5 Ga; εNd = ∼ -8 to -20). The highest and more variable initial Sr values, on the other hand, are observed in the oldest granitoids. These isotopic data indicate an important change of type of magma source with time.
The oldest granitoids have isotopic data suggestive that they represent a mixture of Neoproterozoic, juvenile, mantle-derived material with magma derived from melting of Paleoproterozoic continental crust. Data for the youngest plutons are compatible with magmas formed by remelting of Paleoproterozoic lower continental crust. The only exception are the ultrapotassic syenitoids, which present high initial Sr ratios and very negative εNd values, interpreted as derived from an incompatible element enriched mantle source.
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