PETROGENESIS OF HIGH-TI/Y MAGMA TYPES OF THE WESTERN PARANÁ FLOOD BASALT PROVINCE

The high Ti/Y basaltic magma types (Pitanga and Paranapanema) found in the north and western parts of the Early Cretaceous Paraná flood basalt province comprise c. 50% of the total erupted volume of magmatism and yet have not been studied extensively, in part due to the poor exposures inland. For this study, a suite of surface samples were analysed for high-precision trace elements and Sr-Pb isotopes in order to better understand the petrogenetic processes involved in forming these two magma types. Computer modeling using COMAGMAT and MELTS shows that the major element trends of both magma types are dominated by shallow level (< 6 km) fractional crystallisation of olivine, plagioclase and clinopyroxene.

The Pitanga & Paranapanema magma types share broadly similar radiogenic isotopes and trace element ratios (and a more restricted range compared to other Paraná basalts), suggesting derivation from similar magma sources. In detail, Paranapanema samples have slightly higher 206Pb/204Pb, Th/Ta and La/Nb than Pitanga samples, consistent with a greater influence from crustal assimilation. The main compositional differences are in trace element ratios influenced by melting. Pitanga samples have higher La/Yb, Ti/Y and Dy/Yb than Paranapanema samples. These data are consistent with Pitanga samples representing smaller-degree melting with residual garnet, while the Paranapanema samples represent larger-degree melting of shallower mantle. Data from boreholes show that the Pitanga magma type was erupted prior to the Paranapanema magma type. Thus, there was a progressive shallowing of the melt generation region from the Pitanga to the Paranapanema magma type. This might have been caused by plume-related thermal erosion of the lithosphere in the inland regions away from the developing rift near the coast.