THE MAUÁ GRANITIC MASSIF, CENTRAL RIBEIRA BELT, SÃO PAULO: PETROGRAPHY, GEOCHEMISTRY AND U-Pb DATING

Abstract

The Maua granitic massif is an elongated body dominated by a porphyritic biotite monzogranite which grades, in its southwestern extremity, to lighter equigranular granite and greissenized (tourmaline)-biotite-muscovite leucogranite. Abundant enclaves can be divided in three types: grey micro granular enclaves, with rounded shapes and igneous textures, are compositionally similar to the enclosing porphyritic granites; dark, rounded, micaceous enclaves have high K/Na, and may correspond to highly assimilated metasedimentary xenoliths; and angulous gneiss xenoliths seem to be fragments of an unexposed type of country rock. The primitive magmas that formed the massif were Zr, P and LREE-saturated, and became progressively enriched in U, Cs, Y, HREE, F and possibly Ta. Geochemical data show that most of the observed compositional variation can be a reflection of crystal fractionation at the level of emplacement. However, other processes such as magma mixing, contamination and post-magmatic alteration seem to respond for local chemical variations. U-Pb monazite dating point to a crystallization age of 588 + or - 2 Ma which is ca. 20 myr. younger than those of nearby crust-derived syn-orogenic granites.