Isotopic (Sr, Nd, Pb, and Os) composition of highly magnesian dikes of Vestfjella, western Dronning Maud Land, Antarctica: A key to the origins of the Jurassic Karoo large igneous province?

Abstract

There is a substantial debate on the mantle sources and ultimate origins of continental flood basalts (CFBs), e.g., whether they are related to deep-seated thermal upwellings (i.e. mantle plumes) in the upper mantle or not. Here we present high-precision isotopic (Sr, Nd, Pb, and Os) whole-rock data on some primitive dike rocks (ferropicrites, meimechites, picrobasalts, and basalts) associated with the Antarctic extension of the Jurassic (~ 180 Ma) Karoo CFB province. The isotopic data together with previously published trace element data show that the parental melts of the studied rocks sampled two distinctive geochemical reservoirs in the deep sub-Gondwanan mantle. The isotopic signatures of the relatively depleted types show evidence of extensive melt extraction in the past and are indistinguishable from those of mid-ocean ridge basalts (MORBs) of the SW Indian Ridge. On the other hand, the relatively enriched type isotopically resembles modern oceanic island basalts (OIBs) and may sample pyroxenitic sources either formed by melt infiltration in the upper mantle or by reaction of peridotite with recycled oceanic crustal components (with up to 15% of sediment material). The depleted types have previously been associated with anomalously hot mantle sources (> 1600 °C) and mantle plumes, but their MORB-like isotope compositions are unusual for plume-derived rocks. Instead, our findings are more concordant with a recent model [Coltice, N., Bertrand, H., Rey, P., Jourdan, F., Phillips, B.R., Ricard, Y., 2009. Global warming of the mantle beneath continents back to the Archean. Gondwana Research 15, 254–266] that suggests the generation of the Karoo CFBs in an extensive melting episode caused by internal heating of the upper mantle beneath the Gondwana supercontinent.