Age, geochemistry and mantle source of the Alto Diamantino basalts: Insights on NW Paraná Magmatic Province

The Paranà-Etendeka is one of the largest and most thoroughly investigated large igneous provinces in the world formed in a very narrow time-range (i.e., 1.6–3.0 Ma) between 135 and 132 Ma. The South American portion of this province, i.e. the Paranà Magmatic Province, is mainly characterized by a mafic magmatism of coeval low and high-Ti magmatic terms. This bimodal activity has been explained by different degrees of melting of a homogenous mantle source or by plume activity and metasomatism of the sub-continental mantle. Despite the large body of literature, the true origin of the mantle source and processes leading to this bimodal magmatism are still debated. In this work, we present the first geochronological, geochemical and isotopic data of basalts from Alto Diamantino (southern Mato Grosso, Brazil). These basalts have tholeiitic affinity and can also be classified in two different suites based on TiO2 content (L-Ti < 3 wt%; H-Ti > 3 wt%) and Ti/Y (450 ppm), similarly to other volcanics from the Paran´a Magmatic Province. Other geochemical parameters as Zr/Y and Nb/Yb ratios range from 3.78 to 8.60 and from 3.82 to 8.31, respectively. The L-Ti and H-Ti series have significant geochemical similarities, including 87Sr/86Sr ratios, with the Ribeira and Pitanga basalts of the Paran´a Magmatic Province, respectively. The 40Ar/39Ar ages of the two series are statistically equivalent ranging between 134.4 ± 1.6 and 132.9 ± 2.0 Ma. Our data suggest that the Alto Diamantino basalts constitute the northwestern most expression of the Paranà Magmatic Province. Sr and Nd isotope ratios suggest that the original source of the Alto Diamantino basalts was a EM-I metasomatized mantle characterized by small amounts of different components (DMM and EM-II). Positive Ba and lack of Pb anomalies, together with K/Rb ratio, indicate the presence of amphibole in the mantle source of the Alto Diamantino basalts. Th/Nb and TiO2/Yb ratios suggests that the compositional difference between the L-Ti and H-Ti basalts could be explained by different degree of melting at different depths of a heterogeneously metasomatized mantle source. The H-Ti group of basalts is compatible with low degrees of partial melting at depth while the L-Ti suite records larger degrees of melting. The metasomatic event occurred after the Mesoproterozoic, possibly during the Neoproterozoic formation of the Gondwana supercontinent.