A major neoproterozoic crustal boundary in the Borborema province of ne Brazil

Abstract

The Borborema Province represents a collage of tectonic blocks amalgamated to Gondwana during the Neoproterozoic. The Pernambuco -Alagoas Domain constitutes the southern part of the Borborema Province, which comprises three subdomains (Garanhuns, Água Branca, and Palmares) separated by shear zones. Investigation of an area across the boundary between the Palmares and Garanhuns subdomains revealed contrasting aerogeophysical fabrics and geological setting with igneous and metasedimentary rocks of Paleoproterozoic and Neoproterozoic ages. The Palmares shear zone divides the studied area into 2 subdomains: 1) Palmares to the south and 2) Garanhuns to the north. Nd isotopic studies show contrasting crustal signatures between the Garanhuns and Palmares subdomains, with Nd TDM model ages ranging from 1.8 to 2.70 Ga for the Garanhuns subdomain and from 0.90 to 1.60 for the Palmares subdomain. The Palmares shear zone constitutes a major crustal boundary, and in pre drift reconstructions may have continuity in Africa as the Tcholliré-Banyo shear zone. The data suggest that the area evolved during two distinct orogenies: 1) Rhyacian-Orosirian (2095 Ma to 1987 Ma), which involved mainly the reworking of Archaean crust, and 2) Cryogenian-Ediacaran (630 Ma to 585 Ma), the so-called Brasiliano-Pan African Orogeny. Our findings, i.e. contrasting isotopic subdomains separated by an important shear zone, suggest that Brasiliano Orogeny, at least in the Pernambuco Alagoas domain, had an accretionary nature.