Geochemistry and geochronology of orthogneisses across a major transcurrent shear zone (East Pernambuco shear zone, Borborema Province, Northeast Brazil): Tectonic implications

摘要

The East Pernambuco shear zone (EPSZ) is a dextral shear zone bounding the Rio Capibaribe and Pernambuco Alagoas domains of the Neoproterozoic Borborema Province (NE Brazil). This study reports new geological data from orthogneisses across the central segment of the EPSZ. Two samples from the Bezerros orthogneiss to the north of the EPSZ yielded Pb-207/Pb-206 weighted mean ages of 2111 +/- 7 Ma and 2104 +/- 8 Ma. The c. 2.1 Ga age is taken as the age of crystallization of the protolith. It is undistinguishable from the age of the 2.13-2.09 Gaold Vertentes Complex, which is the main Paleoproterozoic unit of the Rio Capibaribe Domain, and of orthogneisses that occur immediately south of the EPSZ. Like the Vertentes Complex, the Bezerros orthogneiss has a subduction-zone geochemical signature (e.g., pronounced negative Nb-Ta anomalies), suggesting intrusion in a magmatic arc setting. South of the EPSZ, a migmatized granitic orthogneiss and the Encruzilhada de Sao Joao orthogneiss yielded Pb-206/U-238 weighted mean ages of 645 +/- 3 Ma and 639 +/- 5 Ma, respectively. Ages of 615 +/- 17 Ma and 618 +/- 12 Ma obtained at the rims of zircon grains from Encruzilhada de Sao Joao and migmatized granitic orthogneisses, respectively, are interpreted as dating the acquisition of the low-angle gneissic fabric. The age of the EPSZ is bracketed by the 591 +/- 4 Ma age of a monazite grain from the Bezerros orthogneiss and by the of 575 +/- 17 Ma age of a young zircon grain in the Encruzilhada de Sao Joao orthogneiss. Rather than being a terrane boundary, the EPSZ more likely represents an intracontinental shear zone that nucleated at preexisting crustal heterogeneities.