Petrography and Geochemistry of the acid metavolcanic rocks from the Estrada Real, Rio de Contas (BA).

Abstract

Along the “Estrada Real”, in the Rio de Contas County(BA), outcrop acidic metavolcanic rocks representing the Novo Horizonte Formation, Espinhaço Supergroup. Those rocks were formed in the Espinhaço sin-rift phase. The rocks were submitted to intense deformation and metamorphism processes that modified their original features. This work aims to understand the acid magmatism genesis of Espinhaço sin-rift and the metamorphic and hydrothermal processes that affected the region, based on field, petrographic and whole rocks geochemical data. Acid metavolcanic rocks show mineralogy consisting of quartz blast-porphyries, andalusite and kyanite porphyroblasts, garnet pseudomorphs, feldspars, biotite, muscovite, sericite, chlorite, epidote, monazite, zircon and opaque minerals immersed in a quartz-feldspar matrix. The sericitization and muscovitization indicate the occurrence of  hydrothermal alterations. According to the petrographic and geochemical characteristics, acidic metavolcanic protoliths were classified as rhyolites. The Harker diagrams show scattered values of the larger elements Ca, Na and K and of the LILEs, indicating a probable mobility of these elements. Negative anomalies of Nb-Ta, Sr, P and Eu are visualized in the trace-elements and ETRs spider diagrams, in addition to enrichment of ETRL in relation to ETRP. The formation of these rocks in a continental rift environment is suggested, starting from a crustal melting process with the interaction of a mantle previously modified by zone of subduction (mantle metassomatizado).