Age and origin of the Neoproterozoic Brauna kimberlites: Melt generation within the metasomatized base of the S?o Francisco craton, Brazil

摘要

The diamondiferous Brauna kimberlite field is situated in the northeast part of the S?o Francisco craton, Bahia State, Brazil, and ongoing exploration revealed three complex pipe-like bodies and nineteen dykes oriented along a N30W trend. Uranium-Pb perovskite age determinations of hypabyssal kimberlite material yielded a high-precision emplacement age of 642. ±. 6. Ma (2-sigma uncertainty). This age provides the first evidence for Neoproterozoic kimberlite magmatism in Brazil, and the Brauna kimberlites thus represent the oldest known primary source of diamonds in the S?o Francisco craton.Primary kimberlite mineralogy at Brauna comprises olivine, spinel, ilmenite, phlogopite, perovskite, apatite, as well as late-stage serpentine and carbonate. Furthermore, abundant olivine, Cr-diopside, and pyrope garnet xenocrysts are present. The most common xenoliths are wall-rock granodiorites, mantle eclogites and peridotites. The Brauna kimberlites do not fit into the classic subdivision of kimberlites and orangeites (formerly Group-II kimberlites) based on South African type occurrences. Their geochemical and isotopic signatures, with negative initial ε_(Nd) values of -5.8 to -8.1 and variably enriched ~(87)Sr/~(86)Sr ratios (0.7045-0.7063), suggest that Brauna kimberlite magmas were derived from a mantle source region that was strongly metasomatized prior to kimberlite magmatism at ~642Ma. In particular, the incompatible element distribution suggests that Brauna kimberlite magmas originate from the metasomatized base of the S?o Francisco craton, with only minor input from the convecting upper mantle. The Brauna kimberlites mineralogically and geochemically resemble the slightly older "anomalous" mica kimberlites from Guaniamo in Venezuela (712±6Ma). This may suggest that both mica-rich kimberlite occurrences formed from similarly enriched subcratonic mantle lithosphere during Late Neoproterozoic extensional tectonics related to the breakup of the supercontinent Rodinia.