Low- and high-TiO 2 flood basalts of southern Brazil: origin from picritic parentage and a common mantle source

Abstract

The Serra Geral (Paraná) continental flood-basalt province of southern Brazil has two main basalt types: low-TiO 2 (∼ 1 wt.%) basalts occupy the southern portion, and high-TiO 2 (> 3 wt.%) basalts are largely in the northern part. Low-Ti basalts are less evolved (Mg# 60) and more radiogenic (e.g., 87Sr/ 86Sr∼ 0.708) than high-Ti basalts (Mg# 35; 87Sr/ 86Sr∼ 0.705). This is consistent with a model that invokes variable melting of a single mantle source to produce picritic magmas that have relatively lower and higher incompatible element contents. Varying percentages of melting can be related to varying proximity to the early Tristan da Cunha hotspot. The Mg-rich magmas fractionated 60–75% olivine, clinopyroxene, and plagioclase to yield low- or high-Ti flood basalts, assimilating more or less crust in the process. The extent of fractionation and assimilation depended on crustal “warmth” (also tied to location relative to hotspot): (1) above zones of ∼ 25% melting, warm crust relatively easily contaminated crystallizing picritic magma that originated by a high degree of melting (i.e., magma with lower incompatible element contents); additionally, high degrees of melting sustained replenishment of magma with low-Ti magma characteristics; (2) above ∼ 10% melting zones, cooler crust comparatively restricted assimilation during crystallization (of magma with higher incompatible element contents) and permitted magma evolution to high-Ti derivatives; lesser degrees of melting also limited replenishment magma and thereby allowed greater evolution of existing magma. This model refers all diagnostic geochemical and isotopic features of Serra Geral basalts to percentages of partial melting of an essentially homogeneous mantle material.