Chemical and Os isotopic study of Cretaceous potassic rocks from Southern Brazil

Abstract

During the late Mesozoic, an unusually broad range of alkalic magma compositions was erupted along the southern border of the Sao Francisco craton of Brazil. This magmatic activity includes carbonatite, kimberlite, lamprophyre, lamproite, syenite and the largest known example of extrusive kamafugite, the Mata da Corda formation. To determine the nature of the sources of this magmatism, and their geochemical history, an Os isotope study along with major and trace element and Sr, Nd and Pb isotope analyses of kimberlitic, lamproitic and kamafugitic rocks from the Alto Paranaiba province of Brazil was undertaken. This complements recent geochemical and isotopic studies of these magmas. The Os isotope data for Alto Paranaiba samples point to a peridotitic lithospheric mantle source for the kimberlites and lamproites that was variably depleted in Re, presumably by melt removal at some time between the late Archean and mid-Proterozoic. These lithospheric peridotites experienced LIL-element enrichment by fluid/melt metasomatism at roughly 1 Ga, most likely during mobile belt formation along the western border of the Sao Francisco craton. Kamafugitic samples have very radiogenic Os, suggestive of mafic (e.g. pyroxenite, websterite, eclogite) source materials that again appear to have been stabilized in the lithospheric mantle of Brazil in the mid to late Proterozoic. The Os isotope evidence for lithospheric sources for the Alto Paranaiba activity, coupled with Sr, Nd and Pb isotopic characteristics that overlap those of the Walvis Ridge hot-spot trace indicate that the EM1 component in South Atlantic ocean island basalts most likely represents the influence of delaminated Brazilian lithospheric mantle mixed into mantle circulation beneath the South Atlantic and is not related to the plume(s) commonly associated with this ocean island magmatism.