Source mantle heterogeneity and its role in the genesis of Late Archaean–Proterozoic (2.7–1.0 Ga) and Mesozoic (200 and 130 Ma) tholeiitic magmatism in the South American Platform

Abstract

A comparative study of Precambrian and Mesozoic tholeiitic basalt magmatism from the South American Platform reveals that both Late Archaean–Proterozoic and Mesozoic tholeiites are characterized by high TiO 2 (>2 wt.%; HTi) and low TiO 2 (<2 wt.%; LTi), and high and low incompatible elements, respectively. The Early Jurassic (ca. 200 Ma) Central Atlantic Magmatic Province (CAMP) and the Early Cretaceous Paraná (ca. 130 Ma) HTi and LTi tholeiites have major and trace element contents and Sr–Nd isotopic compositions similar to their Precambrian analogues, particularly for the tholeiites belonging to the same craton. Both the Precambrian and Mesozoic LTi and HTi tholeiites are characterized by “depleted” (low LREE/HREE) to “enriched” (high LREE/HREE) features. The “enriched” tholeiitic basalts have negative Nb anomaly (relative to K and La), while the “depleted” ones may show a slight positive Nb spike. Sr and Nd isotopes of both the HTi and LTi Precambrian tholeiites, as well as most of HTi Mesozoic ones, fit the mantle array trend, while the LTi tholeiites of Mesozoic age trend towards EM II. The Precambrian and Mesozoic tholeiites from the South American Platform (SAP) with high Sr and low Nd isotopic compositions cannot be easily explained by AFC and mixing processes between crustal and mantle components if a basaltic major element composition should be preserved. This suggests that the Precambrian and Mesozoic SAP tholeiitic melts reflect a heterogeneous source mantle, including EM I (e.g. fluids, small volume melts) and EM II (e.g. ancient subduction-related metasomatism and/or silicic melt veining) components. All the data indicate that mantle source heterogeneity was well established at least since Late Archaean times, as documented by the Precambrian and Mesozoic SAP tholeiites which have similar compositional features, particularly the tholeiites cropping out on the same craton. The concentration of the Precambrian and Mesozoic SAP tholeiitic magmatism towards craton/mobile belt boundaries suggests that an important role in its genesis was played by upper mantle “edge drive convection” geodynamics.