Anticlockwise P-T evolution at ~280 Ma recorded from ultrahigh-temperature metapelitic granulite in the Chinese Altai orogenic belt, a possible link with the Tarim mantle plume?

An ultrahigh-temperature (UHT) metapelitic granulite assemblage consisting of garnet(g)-spinel(sp)-orthopyroxene(opx)-sillimanite(sil)-cordierite(cd)-ilmenite(ilm)-biotite(bi)-plagioclase(pl)-quartz(q) occurs within migmatitic paragneiss near Kalasu in the Chinese Altai, NW China. Textural relations, mineral compositions and P-T estimates, indicate three stages of mineral assemblages: (1) pre-peak prograde stage (M1) consisting of a sp-sil-bearing or sp-opx-bearing inclusion assemblage, with low-Al₂O₃ contents (4-5wt.%) in orthopyroxene and P-T conditions of ~7kbar and ~890°C, (2) peak UHT stage (M2) comprising a g-opx-cd-bearing coarse-grained assemblage, with high-Al₂O₃ contents (8-9wt.%) in orthopyroxene and peak conditions of ~8kbar and ~970°C, and (3) post-peak HT stage (M3) containing an oriented opx-bi-sil-bearing assemblage in matrix, with moderate amounts of Al₂O₃ (6-7wt.%) in orthopyroxene and P-T conditions of 8-9kbar and ~870°C. The three discrete stages define an anticlockwise P-T path involving initial prograde heating and post-peak near isobaric cooling. Such a near isobaric cooling anticlockwise P-T path suggests that UHT metamorphism likely occurred in an overall extensional tectonic setting with associated underplating of mantle-derived mafic magma. A SHRIMP zircon U-Pb age of 278±2Ma obtained from the metapelitic granulite indicates UHT metamorphism in the Altai orogen occurred during the Permian, coeval with spacially associated mantle-derived mafic intrusions (~280Ma) and the Tarim mantle plume (~275Ma). Thus, the Permian UHT metamorphism of the Chinese Altai is likely associated with underplating and heating of mantle-derived mafic magma as a result of the Tarim mantle plume.