HIGH-TI AND LOW-TI MAFIC POTASSIC MAGMAS - KEY TO PLUME-LITHOSPHERE INTERACTIONS AND CONTINENTAL FLOOD-BASALT GENESIS (VOL 136, PG 149, 1995)

The role of mantle plumes in the genesis of continental flood-basalts (CFB) remains controversial, primarily due to our limited knowledge of the composition of the subcontinental lithospheric mantle (SCLM). In this study we use the widespread Cretaceous mafic potassic magmatic ro cks, emplaced around the margins of the Parana sedimentary basin, to p robe large-scale compositional variations of the SCLM beneath southern Brazil and Paraguay. On the basis of Ti contents, together with major -, trace-element and isotopic ratios, these mafic potassic rocks may b e subdivided into high-Ti and low-Ti groups. The former have relativel y high average TiO2 (4.64), CaO/Al2O3 (1.74) and Nd-143/Nd-144(i) (0.5 1232), together with low La/Nb (1.1) and Sr-87/Sr-86(i) (0.7050). The latter are characterised by much lower average TiO2 (1.77), CaO/Al2O3 (0.72) and Nd-143/Nd-144(i) (0.51182), together with higher La/Nb (2.0 1) and Sr-87/Sr-86(i) (0.7068). These high-Ti and low-Ti groups are sp atially separate and their distribution correlates with tectonic setti ng; the low-Ti magmas are associated with cratonic regions, whereas th e high-Ti magmas are in Proterozoic mobile belts. The distribution of the subgroups of mafic potassic magmatic rocks correlates closely with the geochemical provinciality of the Early Cretaceous high-Ti and low -Ti Parana flood-basalts. This is the first reported occurrence of ext ensive low-Ti mafic potassic magmatism associated both spatially and t emporally with the low-Ti region of a major CFB province. Our study fu rther reveals similar relationships between tectonic setting and the g eochemical provinciality of mafic potassic magmas and continental floo d-basalts across Gondwana. We use the bulk-rock compositions and radio genic isotopic ratios of both the high-Ti and low-Ti mafic potassic ma gmatic rocks as end members in models of CFB genesis. Mixing calculati ons involving Sr and Nd isotopic ratios indicate that the flood-basalt s may contain up to 50% of mafic potassic lithosphere-derived melts. O verall, the results of our geochemical modelling agree with geophysica l arguments that the convecting asthenosphere is the predominant sourc e of CFB magmas.