CONSTRAINTS FROM GRANITIC PLUTONISM ON PROTEROZOIC CRUSTAL GROWTH OF THE TRANSVERSE ZONE, BORBOREMA PROVINCE, NE BRAZIL

Abstract

The transverse zone in Borborema Province is a mosaic of terranes situated between the Patos and Pernambuco lineaments, which were assembled through two orogenic cycles: an early, Grenville age, the Cariris Velhos event (from 1.1 to 0.95 Ga) and a later Brasiliano event (from 0.75 to 0.54 Ga). It is composed by Meso- and Neoproterozoic terranes, viz. Piancó-Alto Brígida (PBT), Alto Pajeú (APT), Alto Moxoto (AMT) and Rio Capibaribe (RCT), where the granitic plutonism is a noteworthy phenomenon and presents a pattern coherent with the terrane arrangement. The Cariris Velhos plutonism is controlled by a flake tectonics regime. It is absent or unknown in PBT, but abundant in the fold-and-thrust APT, in the arc-related AMT, and also probably occurs in RCT. No Brasiliano-age granite occurs in AMT, the APT/AMT limit lacking late-kinematic shoshonitic and ultrapotassic granites and syenites that are typical of other terrane boundaries in the region. The Cariris Velhos crustal collisional granites of the APT contrasts with the little evolved granites and trondhjemites of the AMT, while Paleoproterozoic and Mesoproterozoic gabbro-anorthositic and alkali-granitic intrusions are probably restricted to AMT and RCT. The occurrence of juvenile mafic-ultramafic rocks and geochemical and isotopic data, especially εNd signatures of volcanic rocks and granites in APT, AMT and RCT, indicate that accretionary processes indeed had an important role in segments of the Cariris Velhos belts. The Brasiliano-age granitic activity was a much more voluminous phenomenon, having developed major plutonic belts in the terranes and along their boundaries. A high-K to normal calc-alkalic super-suite and a trondhjemitic suite are characteristic in the PBT and can represent an evolutionary trend similar to the calc-alkalic-trondhjemitic series of SW Finland. The high-K branch of this super-suite is widespread in APT and RCT, sometimes associated with peraluminous leucocratic granites. Probable inherited zircons with ca.750 Ma age in APT plutons suggest that this is the age of the precursor shoshonitic mafic magmas of this K-rich calc-alkalic suite. Data from the literature indicate taht these granites were emplaced in the 630-620 Ma age interval and, with exception of the high-K calc-alkalic suite, contain moderate initial 87 Sr/ 86 Sr ratio and low negative εNd, indicating a hybrid crustal and mantle-derived source. The late-kinematic peralkalic ultrapotassic and shoshonitic super-suite occurs characteristically along and adjacent to transtensional shear zones separating some terranes, the biggest plutonic belt marking the PBT/APT limit. Data from the literature points to 566 Ma as the age of emplacement of the ultrapotassic suite. The latest major granitic pulse (512 Ma) is the post-kinematic A-type granite suite occurring in the APT/AMT boundary. This suite is subalkalic to slightly alkalic and presents geochemical patterns similar to anorogenic granites. The Brasiliano-age granitic plutonism is related to an extensional and transtensional regime, rather implying in strike-slip dispersion than substantial accretionary processes.