Generation of Northern Parana High Ti/Y Basalts By Progressive Lithospheric Thinning Above a "Gough"-like Mantle Source.

Abstract:

Stratigraphic and geochronologic data show that the high Ti/Y magma types (Pitanga & Paranapanema) of NW Paraná are the youngest magmatic phase in the Paraná-Etendeka flood basalt province and comprise ~50% of the total erupted volume. They are more homogeneous than low Ti/Y basalts in SE Paraná and the Etendeka, with a restricted range in Sr-Nd-Pb isotope composition (⁸⁷Sr/⁸⁶Sr 0.7055-0.7063; εNd –5 to –3; ²⁰⁶Pb/²⁰⁴Pb 17.7-18.2). Subtle differences between Pitanga and Paranapanema (Th/Ta, ²⁰⁶Pb/²⁰⁴Pb) are consistent with minor crustal assimilation. Pitanga show greater incompatible element enrichment compared to Paranapanema (Ti/Y 440-590 vs. 325-510; La/Yb 9.3-12.2 vs. 5.9-9.0), and have greater MREE/HREE enrichment (Dy/Yb 2.1-2.5 vs. 1.8-2.1). Boreholes and surface profiles reveal a consistent temporal transition from Pitanga to Paranapanema lavas, and the decrease in Dy/Yb requires a shallowing of the mean depth of melting, consistent with lithospheric thinning. Pitanga and Paranapanema lavas show Dupal characteristics (elevated Δ7/4Pb 8-13), distinct from Tristan hot-spot and S Atlantic MORB compositions, but similar to the EM-I endmember composition from Walvis Ridge DSDP Site 525A. Previous workers suggested a common origin for Parana high Ti/Y magmas and DSDP Site 525A in continental lithospheric mantle. However, recent comprehensive sampling of the Tristan - Gough - Walvis Ridge - Rio Grande Rise hotspot track has revealed spatial geochemical zonation with a northern "Tristan"-track and a southern "Gough"-track, and the "Tristan" component (Δ7/4Pb 3-6) is only found in samples < 70 Ma (Hoernle et al. 2015). The early hotspot track history is dominated by the "Gough" component (Δ7/4Pb 6-13), inferred to be derived from the African LLSVP, and this material has the compositional features (Sr-Nd-Pb-Hf isotopes, elevated La/Nb and Th/Nb) required for the mantle source for the Pitanga and Paranapanema magma types of the 135 Ma Paraná flood basalt province.