Abstract
Dominantly Mesozoic to Proterozoic xenocrystic zircons extracted from Tiébaghi chromitite coexist with a well-defined population (30%) of Eocene magmatic grains (U-Pb: 48.2 ± 1.1 Ma). Since Eocene zircons are absent in the peridotite host rock and are accompanied by rare rutiles with the same age range, it is suggested that both are derived from the circulating melt from which the chromite ore was generated. This hypothesis is strengthened by a similar 40Ar/39Ar age of 46 ± 2 Ma on amphibole that is closely associated with chromitite. These data confirm the Eocene age and supra-subduction setting of the Tiébaghi podiform chromitite, which thus was formed by interaction of peridotite and mafic melt in the final stage of the forearc magmatic event that occurred between 55 Ma and 47 Ma. Modelling the melt associated with chromite formation using zircon trace-elements and REE-zircon partitioning coefficients suggests forearc basalt as a likely candidate and the felsic by-product of peridotite-melt interaction a source for early Eocene magmatic zircons.
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