Sources of basalt suites in the Campos and Pelotas basins (South-Southeast of Brazil) and geodynamic breakup models for Western Gondwana

Abstract

Based essentially on geochemical data, this work aims at defining constrain to the petrogenesis of the Eocretacious basalts from the Campos and Pelotas marginal rift basins (South-Southeast Brazil). Geochemical modeling, including quantification of mantle sources and binary mixing methods, were performed in order to contribute to the elaboration of geodynamic models related to the initial stages of Gondwana breakup. Basalts from Campos and Pelotas basins comprise two transitional series with tholeiitic affinities. All basalts from the Campos Basin can be assigned to a low-TiO2 suite (TiO2 = 1.20 ± 0.12 wt %; Ti/Y = 272); whereas basalts from the Pelotas Basin comprise a low-(TiO2 = 1.19 ± 0.02 wt%; Ti/Y = 288) and a high-TiO2 suites (TiO2 = 2.10 ± 0.19 wt %; Ti/Y = 387). Non-modal batch partial melting modeling showed that the La/YbN = 5.78 generation ratio of the Campos Basin suite was obtained from 21% of partial melting of lherzolite garnet. The same model required a source with small-er amounts of garnet (melted at 28%) to generate the La/YbN generation ratio of the low-TiO2 suite of the Pelotas Basin. Larger amounts of grenade and less partial melting (22%) were necessary to generate the La/YbN ratio of the high-TiO2 suite of the Pelotas Basin. The simple binary model shows that parent compositions of Campos and Pelotas cannot result from the mixture of TC (Tristão da Cunha) and SCLM (Subcontinental Lithospheric Mantle). The best results obtained suggest the participation of component N-Morb in the generation of basalt suites of the Campos and Pelotas basins (respectively 61% and 93% of partial melting). Tristão da Cunha seems to have been an important component to the genera-tion of the basalt suite of the Campos Basin (at 39%of partial melting). In Pelotas, the model indicates restricted contribution of SCLM reservoirs.