Idades U/Pb em zircão e idades modelo (Sm/Nd ) de ortognaisses e enclaves metamáficos da área de Barro Vermelho-Pe, Terreno Alto Moxotó, Província Borborema, Nordeste do Brasil

Abstract

The Barro Vermelho area is located in the border between municipalities of Custodia and Sertânia – PE, Pajeu-Paraiba Fold Belt, Borborema Province. Geological mapping at 1/25.000 scale allowed to distinguish two metamorphic domains respectively built up of orthoderivated rocks and paraderivated ones. The orthoderivated domain is formed mainly by augen gneisses more or less migmatized of granitic, monzogranitic, granodioritic, tonalitic and quartz-dioritic composition inside of which are found metamafic enclaves of leucogabbros, gabbronorites, gabbros/diorites, and anorthosites, apart from banded amphibolites, with a small occurrence of Fe-Ti ore enclosed by some of these enclaves. In addition, inserted in the orthogneisses are found also others enclaves and intrusions (some apparently concordant and others certainly discordant in relation to the prominent foliation of the area, Sn) of metric to hectometric dimentions, built up of amphibolites/metadiorites, metaplite, calcissilicate rocks of mafic ultramafic protholiths, weakly deformed granites and diorites, and two hectometric bodies of olivine diabase to troctolite. Field relations and similarities in terms of composition, texture and lithogeochemistry allowed to place the lithotypes of the orthoderivated domain in the following groups, considering them in a relative sequence of events from the older to the newest ones: a) anorthositic-gabbros xenoliths; b) tonalite (protolith of the orthogneisse of equal composition); c) enclaves/ dikes of amphibolites/metadiorites, synplutonic in relation to tonalite and comagmatic to the xenoliths; d) granitic orthogneisses formed from migmatization of tonalite; e) enclaves/dikes of amphibolites/metadiorite, synplutonic in relation to migmatization of tonalites; f) granites and diorites late to post migmatization and g) olivine diabase to troctolite post the last tectonic-metamorphic event recorded in the area. Concordia diagrams U/Pb with colinear regression of three zircon fractions to tonalitic and granitic orthogneisses, respectively, display ages of cristalization 2.44 Ga and 2.01 Ga to their protoliths. Model ages Sm/Nd (T DM ) were determined in seven different lithotypes. However, only two yield meaningful ages, 2.30 and 2.73 Ga, respectively. These data suggest that some older Archean crustal material was incorporated during the genesis of the 2.44 Ga tonalitic gneisses, while the mafic enclaves appear to have been derived from a Paleoproterozoic source region.