Petrogenetic Aspects of Acid and Basaltic Lavas from the Paran? Plateau (Brazil): Geological, Mineralogical and Petrochemical Relationships

The acid volcanics (Lower Cretaceous) of the Paran? basin coveran area of about 150000 km² and are represented by dominantrhyodacites and subordinate rhyolites. They may be divided intotwo main types, characterized respectively by relatively lowand relatively high contents of Ti, P, and other incompatibleelements (La, Ce, Zr, etc.), i.e. the Palmas acid volcanics(PAY) and Chapec? acid volcanics (CAV), respectively. PAV arewidespread in the southern Paran? basin and are closely associatedwith basaltic and andesitic rock-types similarly characterizedby low Ti, P, and other incompatible elements. In contrast,CAV are dominant in the northern Paran? basin, where they areclosely associated with basalts containing high Ti, P, and otherincompatible elements. The generation of the Palmas and Chapec? acid melts appearsto be in part consistent with crystal fractionation processes,starting from the associated basic rocks and accompanied bycrustal contamination. However the relative absence of intermediaterock-types (‘silica gap’: 54–56 to 63–65wt. per cent), and the confinement of the acid volcanics towardsthe continental margin suggests that a model involving lowercrustal basic material of significantly different compositionin the northern and southern Paran? basin may be a more plausiblealternative. In this preferred model the basic parent materialmay be represented by mafic granulites of different compositions,or by basalts trapped at the crust-mantle discontinuity andcorresponding in composition to the contrasting low- and high-TiO₂basalts that flooded the Paran? basin in Lower Cretaceous times.The melting of these underplated materials may explain the closegeochemical relationships between fissure acid volcanics andthe closely associated basalt types (e.g., Ethiopia, Paran?).The beginning of the major rifting related to continental break-upshould therefore correspond to the stage when the melting processaffected the lower part of the continental crust. ^*Reprint requests to E. M. Piccirillo