High-precision U–Pb dating and Hf isotopes of MARID zircons reveals multi-source, prolonged metasomatism of the Kaapvaal sub-continental lithospheric mantle

Abstract

Mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths are unique examples of phlogopite-dominated mantle metasomatism hypothesized to correlate geochemically and petrologically to kimberlite and carbonate-rich olivine lamproite (formerly group II kimberlite) magmatism. Here, we investigate the linkage between MARID metasomatism and alkaline magmatism using multiple analytical techniques: in situ trace element major phase and zircon geochemistry, in situ Hf isotope and U–Pb zircon geochemistry and geochronology, and high-precision isotope dilution U–Pb zircon geochronology. We find that most zircons in the studied MARID suite from Kimberley (South Africa) crystallized over a protracted interval in the Late Cretaceous coeval with regional kimberlite volcanism, with one sample yielding Neoproterozoic and Mesoproterozoic dates that are broadly coeval with Proterozoic tectono-magmatic events. Notably, we observe that older metasomatic compositions and ages in zircon are not obscured by mantle storage or later metasomatic events. Our findings confirm that some MARID-like metasomatism occurred as early as the Proterozoic, evolving to form a viable lithospheric mantle source for carbonate-rich olivine lamproites. Zircon geochemistry and geochronology also document that MARID precursors are infiltrated and modified by later Cretaceous kimberlitic melts, resulting in their diverse geochemical and isotopic compositions.

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