Ree Signatures in pyroxenes from estatherian Passira meta-anothositic complex , NE Brazil: futher evidences for massif type origin

Resumo

The Passira Meta-anorthosite Complex (PAC) in NE Brazil is a massiftype complex mainly composed of anorthosite with minor gabbro and lenses of pyroxenite rich in Fe-Ti oxide minerals, surrounded by a dyke complex of apatite-rich ferrodiorite. It is situated in the Rio Capibaribe terrain, north of the Pernambuco Lineament, Borborema Province. It was formed at 1.7Ga, and was subjected to two major tectono-metamorphic events. The PAC has a sigmoid shape, is hosted by gneiss-migmatite rocks, and was controlled by movements along the Paudalho and Limoeiro shear zones, branches of the Pernambuco Lineament. Most present textures developed during retro-metamorphism of granulite facies assemblages, but practically unmodified primary mineral assemblages are identified. Augite (sub-calcic augite in older terminology) is identified in the gabbros, and enstatite (bronzite) in the anorthosites. REE patterns in augite have strong light REE enri- chment with LaN/YbN > 10, with high total REE contents (SREE) and are therefore different from those expected for augite crystallized from common basalt magmas. Bronzite from anorthosite has a flatter pattern with LaN about twice chondritic values and slight HREE enrichment. The patterns observed for pyroxene are attributed to crystallization from a LREE – enriched jotunitic magma, which may have been formed from enriched mantle, or by melting of mafic lower crust.