Late Carboniferous to early Permian partial melting of the metasedimentary rocks and crustal reworking in the Central Asian Orogenic Belt: Evidence from garnet-bearing rhyolites in the Chinese South Tianshan

Abstract

Peraluminous granitic rocks provide important insights on crustal reworking processes. Here we study the garnet-bearing rhyolites of the Early Permian Xiaotikanlike Formation exposed in the Heiyingshan region located in the South Tianshan region in Xinjiang, NW China. Two layers of garnet-bearing rhyolites were recognized, which discordantly overlie the strongly deformed Late Carboniferous strata. The rocks generally display porphyritic texture with alkali-feldspar, quartz, plagioclase, biotite and garnet as the major phenocryst phases. Zircon LA–ICP-MS U–Pb analyses yield ages of 281.5±0.7Ma for the upper layer of rhyolite (ULR) and 299.9±1.9Ma for the lower layer (LLR). The ULR shows SiO2 in narrow range of 71.08 to 72.39wt.%, and Fe2O3 from 0.99 to 1.69wt.% with Fe2O3/FeO ranging from 1.68 to 2.68. The normalized trace element patterns are characterized by prominent troughs for Nb, Ta, Sr, P and Ti and relatively sloping rare earth element patterns. They have radioactive Sr–Nd isotopic compositions [(87Sr/86Sr)i=0.7128 to 0.7179 and εNd=−13.09 to −12.44] and negative εHf(t) values of −9.97 to −1.27. The LLR exhibits similar trace and rare earth element patterns, εNd (−12.68 to −11.70) and εHf(t) values (−7.56 to −2.6) to the ULR, but have higher (87Sr/86Sr)i=0.7166–0.7269 and lower Fe2O3/FeO ratios (0.17–1.73). Geochemical and isotopic characteristics indicate that these rhyolites are dominantly alkali-calcic and strongly peraluminous, which were most likely derived from muscovite dehydration melting of metagreywacke. We correlate this with the tectonic processes following the collision between the Central Tianshan–Yili Block and the Tarim Craton. Both the Zr saturation temperatures and Al2O3/TiO2 ratios suggest an increase in temperature of the source during 300Ma to 280Ma, which may be attributed to heat input from the underplating of mantle-derived basaltic magmas. The field relation between the rhyolite and the Late Carboniferous strata favors the model that the final collision culminated during Late Carboniferous before 300Ma. In combination with the occurrence of felsic lavas in other regions such as the Xiaotikanlike Formation from Wensu, Laohutai and Boziguo'er region, we define a >200km peraluminous magma belt in the central part of Chinese South Tianshan, which provides new insights into the crustal reworking process in the Central Asian Orogenic Belt.