Petrology of kamafugites and kimberlites from the Alto Paranaiba Alkaline Province, Minas Gerais, Brazil

Mafic rocks representative of the alkaline magmatism of the Alto Paranaiba Province in southwestern Minas Gerais, Brazil were studied by means of petr ography, mineral, whole-rock and isotope geochemistry with the objective of better understanding this Cretaceous magmatism and the characteristics of the magma sources. Because of the variety and complexity of lithotypes exam ined in this research and the paucity of detailed studies of these Brazilia n rocks in the literature, this study also attempts to establish parameters that allow for a clear distinction between kimberlite and kamafugite. Fift y-two occurrences are described and classified as kimberlite or kamafugite. Among the kamafugites, both ugandite (characterized by the presence of leu cite) and mafurite (with kalsilite) end members have been characterized. Mi neral compositions were found to be efficient in distinguishing between kim berlites, mafurites and ugandites in the province, primarily on the basis o f phlogopite composition. The Re-Os isotope systematics permitted a better understanding of the relation of the sublithospheric mantle source to the m agmatism in the region. Kimberlites, mafurites and ugandites have different Os-187/Os-188 ratios (0.117 to 0.129, 0.127 to 0.145 and 0.142 to 0.147, r espectively). The Rb-Sr and Sm-Nd isotope systematics failed to indicate fi rst-order differences between kamafugites and kimberlites, whilst Pb-206/Pb -204 ratios for the kimberlites are higher than those for the other rock ty pes. Kimberlite and kamafugite isotopic compositions appear to be related, to the mixture of at least two dominant mantle components: one with an isot opic signature similar to that of lithospheric peridotites, i.e., with Os-1 87/Os-188 ratios of the order of 0.118, similar to those observed in mantle -derived xenoliths entrained in kimberlites intruded in the Kaapvaal, Wyomi ng, and Siberian cratons; another with higher Os-187/Os-188 ratios of the o rder of 0.135, within the range of ratios reported for pyroxenite veins in alpine-type peridotites and ocean island basalts. Different melting depths of heterogeneous lithospheric sources by a mantle plume are suggested to ex plain the isotopic characteristics ofthe Alto Paranaiba Alkaline Province a lkaline rocks.