The Sitio Piranhas Anorthositic Association in the Alto Moxotó Terrane, Borborema Province, NE Brazil

Resumo

This paper introduces the Sítio Piranhas Anorthositic Association (SPAA), which is situated in the Alto Moxotó Terrane, Transverse Zone, in the Borborema Province of Northeastern Brazil. The SPAA is a complex of basic rocks comprised mainly of deformed and metamorphosed anorthosite, gabbro, norite, diorite and ultramafic bodies, mainly magnetitites with Fe-Ti oxide mineral and chalcopyrite mineralization. Regionally, this association is placed as part of the Malhada Vermelha Suite, which contains most of the basic intrusions of Alto Moxotó Terrane. The host rocks are part of the orthogneissic migmatitic Floresta Complex. The anorthositic rocks are cut by a dyke swarm composed of apatite-rich Fe-dioritic to monzodioritic rocks that are enriched in zircon, apatite, titanite and allanite. This anorthositic association takes place on the Mulungu Block, which is an archean nucleus in the Alto Moxotó Terrane. A NW-SE thrust fault and the NE-SW Congo-Cruzeiro do Nordeste Shear Zone are tectonic boundaries of the Mulungu Block. The SPAA was deformed and recrystallized by at least two tectonic-metamorphic events that have produced two distinct structural elements. The first one is a penetrative sub-horizontal foliation marked by amphibole-rich bands and the second is characterized by steeply dipping foliation related to the Congo-Cruzeiro do Nordeste Shear Zone. Some of the anorthositic rocks are well-preserved and do not show the main regional foliation, although metamorphic minerals and textures can be recognized. Polished thin-sections of the SPAA rocks were described and their petrographic characteristics, including textural variations and mineralogical compositions, suggest that all of them are probably part of an anorthositic suite correlated with mafic layered intrusions. Although regionally the SPAA is proposed as belonging to the Malhada Vermelha Suite, this last suite includes different signatures for its basic rocks and different economic potential. The distinctive petrographic characteristics of the SPAA may possibly separate it as a new unit of gabbro-anorthosite with its metallogenetic implications in the Alto Moxotó Terrane.