Early Permian mantle–crust interaction in the south-central Altaids: High-temperature metamorphism, crustal partial melting, and mantle-derived magmatism

Abstract

Early Permian mafic magmatism, the partial melting of the crust, and high-temperature metamorphism in the Chinese Altai, south-central Altaids, provide an excellent case study of exchange of energy and mass between the mantle and crust. Field and petrographic observations, together with microprobe mineral analyses, have, for the first time, allowed us to identify scapolite-bearing calc-silicate granulites along the southern margin of the Chinese Altai. The conditions of metamorphism were 680–800 °C and 6–7 Kbar, based on mineral phase re- lationship and compositions, as well as the results of previous studies. New LA-ICP-MS zircon U-Pb geochronol- ogy demonstrates that the high-T metamorphism in the Chinese Altai was accompanied by the emplacement of leucogranites at approximately 295 Ma. SHRIMP II dating results, combined with previously published data, re- veal a mafic magmatic event at around 275 Ma in a large region of northern Xinjiang, NW China. This mafic magma was derived from N-MORB-like depleted mantle, as deduced from our new bulk geochemical and zircon Hf isotopic data. Synthesizing available geochemical (including isotopic) and geochronological data, we propose a two-stage model of mantle–crust interaction to explain the early Permian geology of northern Xinjiang. The early stage of interaction involved high-temperature metamorphism and the coeval partial melting of crustal rocks, indicating solely a heat exchange between the mantle and crust; in contrast, the later stage involved the exchange of both mass and energy between the mantle and crust.