Chemical and isotopic relationships between peridotite xenoliths and mafic-ultrapotassic rocks from Southern Brazil

摘要

Peridotite xenoliths from the late-Cretaceous Alto Paranaiba and Goids mafic-alkalic magmatic provinces of central and southeast Brazil reveal the existence of compositionally and temporally distinct lithospheric mantle beneath these areas. Garnet and spinel-1herzolites and spinel-harzburgites from the Alto Paranaiba province are generally depleted in Ca, Al and Re, which indicates that they are residues of melt extraction. Old Re-depletion model ages (average 2.4 Ga) for these peridotites show that this area is underlain by the early-Proterozoic to late-Archean melt-depleted lithospheric mantle of the S5o Francisco Craton. In contrast, spinel-1herzolites from the Goiis alkalic province, located 500-600 km northwest of the Alto Paranaiba province, have major- and trace-element compositions similar to modem fertile mantle. Most GoiAs peridotites have fertile mantle Os-187/Os-188 (0.1261-0.1292), but two samples with lower Os-187/Os-188 define Re-depletion model ages of similar to 1.2 Ga. The compositional distinction between the lithospheric mantle samples is mirrored in the kamafugitic rocks of these two provinces, which suggests that the composition of these magmas was strongly influenced, or determined entirely, by the lithospheric mantle. Most of the kamafugitic rocks have Os-187/Os-188 higher than observed for peridotite, which suggests that the source of these magmas is a mixture of lithospheric peridotite of varying age, veined and/or interlayered with various olivine-poor components. The relatively limited range in Sr, Nd, and Hf isotopic composition of the mafic-alkalic magmas indicates that the olivine-poor component was added regionally, overprinting the incompatible-element characteristics of the lithospheric mantle beneath these provinces. The age of metasomatism in the lithospheric mantle of this area is poorly constrained, but the isotopic characteristics of the mafic-alkalic magmatism suggest that this event may have occurred during the mid- to late-Proterozoic assembly of the Brasilia mobile belt. (c) 2007 Elsevier B.V. All rights reserved.