Petrogenesis of continental flood basalts in eastern Parnaiba basin, Brazil: A singular sill occurrence with low- and high-TiO2 tholeiites

Abstract

The intracratonic Parnaiba basin, in northeast Brazil, records episodes of tholeiitic magmatism characteristic of Large Igneous Provinces (LIP's). They are represented by the Mosquito (Triassic-Jurassic) and Sardinha (Cretaceous) formations related to tension and rupture processes of Pangea and Gondwana, respectively. The Mosquito (mostly basaltic flows) and Sardinha (mostly diabase sills) formations include low- and high-TiO2 suites. This paper presents the results related with the petrogenesis of a 43 m-thick diabase sill sampled from the 1UN-26 borehole within the eastern portion of the Parnaiba basin. This stratified magmatic body comprises three petrographic facies: fine grained basalt (chilled margin); medium-grained diabase (intermediate portion) and coarse-grained gabbro (inner portion of the sill). The rocks are tholeiitic basalts and andesitic basalts which define two chemically distinct groups: high-TiO2 tholeiites (TiO2 > 2 wt %, inner body portion) and low-TiO2 tholeiites (TiO2 < 2 wt %, upper body portion). The former is more evolved (lower MgO and higher Fe2O3t, SiO2, and alkalis) and enriched in a broad spectrum of LILE (Large Ion Lithophile Elements), HFSE (High Field Strength Elements) and REE (Rare Earth Elements) compared to the latter. Geochemical modeling indicates that the different groups are not cogenetic with each other by fractional crystallization or AFC (assimilation with fractional crystallization) processes, having been generated by different mantle sources as depicted by their trace element and Sr–Nd isotope initial ratios. Negative εNdi suggests at least a contribution of the subcontinental lithospheric mantle (SCLM) in these magma genesis. The La/Ybn ratios of least evolved compositions in both suites can be explained by 15–16% partial melting from either a peridotite with 5% of residual garnet and chondritic La/Yb ratio or a peridotite with 2% of residual spinel and a La/Yb ratio twice chondritic values. Small differences in Sr–Nd isotope ratios of representative samples of the two basaltic suites preclude the involvement of rather different mantle sources but may indicate the participation of a differently enriched, vertical SCLM in the petrogenesis of these basalts. As such, local scale, vertical mantle heterogeneities in addition to the lateral heterogeneity normally observed in LIP's elsewhere may be relevant for geodynamic models involving the Parnaiba basalts.