Geology, petrography and geochemistry of the acid volcanism of the Paraná Magmatic Province in the Piraju-Ourinhos region, SE Brazil

Abstract

A succession of trachydacitic lava flows over 150 m thick constitutes the lowermost portion of the Serra Geral Fm. along the Paranapanema River valley in the Piraju-Ourinhos region, State of Sao Paulo. The flows have a well-defined zonality: the central portion is made up of grey porphyritic dacites with low glass and vesicle proportion which grade towards the contacts to strongly vesiculated varieties with glassy groundmass which, when in contact with the Botucatu Fm. basal sandstones or with larger bodies of inter-flow sandstone, show a typical chocolate-brown color. The observed compositional variation is small, and in part a reflection of secondary alteration. Independent of texture, mode of occurrence and stratigraphic position, the rocks show similar contents of SiO (sub 2) (65-67 wt%) and all other major oxides, and a ferrous (mg#= 20-27) and metaluminous A/CNK= 0.85-0.95) character; alteration associated to vesicle infilling and color change to chocolate-brown results in lower mg# and higher A/CNK. Mineral and whole rock chemical data indicate high crystallization temperatures around 1000 degrees C (pyroxene thermometry; apatite saturation), consistent with the predominant mode of occurrence as flows.