When Did the Emeishan Mantle Plume Activity Start? Geochronological and Geochemical Evidence from Ultramafic-Mafic Dikes in Southwestern China

SHRIMP zircon U-Pb dating of an ultramafic-mafic dike intruded into Lower Devonian limestones from southwestern China gives a mean 238U/206Pb age of 262 ±3 Ma (n = 12, MEWD = 5.7). This is the oldest age hitherto reported for the Emeishan large igneous province (ELIP). These and related dikes have OIB-like trace-element signatures (e.g., low LILE/HFSE, LREE/HFSE, and Zr/Nb ratios) and strong geochemical and Sr-Nd isotopic similarities to the least contaminated ELIP high-Ti basalts. A compilation of published Ar/Ar and SHRIMP zircon U-Pb ages for the ELIP shows that plume activity probably started at ~262 Ma, and attained its maximum at 253-251 Ma as a result of continuous plume-lithosphere interaction. The ~10 m.y. interval of the ELIP magmatic episode favors a plume-impaction model, wherein the plume head impinged on the base of the lithosphere over a time scale predicted by laboratory experiments.