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?

Abstract

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.