PICRITIC PORPHYRITES AND ASSOCIATED BASALTS FROM THE REMNANT COMEI LARGE IGNEOUS PROVINCE IN SE TIBET: RECORDS OF A MANTLE PLUME ACTIVITY

Picritic porphyrites and associated basalts are found in the Lower Cretaceous slates within the recently identified remnant Comei large igneous province (LIP; ca. 132 Ma), SE Tibet, providing an opportunity to explore the mantle potential temperature of this LIP and the genetic correlation between the picrite porphyrites and associated basalts. The picritic porphyrites exhibit porphyritic-like texture, consisting of abundant olivine phenocrysts and fine-grained plagioclase and glass groundmass. The whole-rock Re-Os isochron of the picritic porphyrites and associated basalts gives an age of 127.8 ± 6.1 Ma. The high amount MgO and CaO in olivine phenocrysts in picritic porphyrites indicate that these olivines are crystallized from high-Mg magmas rather than mantle xenocrysts. By deducting the accumulated olivines, ca. 20 wt.% MgO of parental magma are obtained for the picritic porphyrites, corresponding to a high mantle potential temperature of >1550°C for the parental magma. Such a high temperature provides solid evidence for the mantle plume origin of the remnant Comei LIP. Parallel trace-element patterns and similar ε_Nd(T) and (¹⁷⁷Os/¹⁷⁸Os)_T values between the associated basalts and the picritic porphyrites suggest a common hot mantle source region for their generation.