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?

摘要

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-Al_2O_3 contents (4-5 wt.%) in orthopyroxene and P-T conditions of ～7 kbar and ～890℃, (2) peak UHT stage (M2) comprising a g-opx-cd-bearing coarse-grained assemblage, with high-Al_2O_3 contents (8-9 wt.%) in orthopyroxene and peak conditions of ～8 kbar and ～970 ℃, and (3) post-peak HT stage (M3) containing an oriented opx-bi-sil-bearing assemblage in matrix, with moderate amounts of Al_2O_3 (6-7 wt.%) in orthopyroxene and P-T conditions of 8-9 kbar and ～870 ℃. 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 exten-sional tectonic setting with associated underplating of mantle-derived mafic magma. A SHRIMP zircon U-Pb age of 278 ± 2 Ma obtained from the metapelitic granulite indicates UHT metamorphism in the Altai orogen occurred during the Permian, coeval with spacially associated mantle-derived mafic intrusions (～280 Ma) and the Tarim mantle plume (～275 Ma). 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.