Carboniferous Highly Fractionated I‐type Granites from the Kalamaili Fault Zone, Eastern Xinjiang, NW China: Petrogenesis and Tectonic Implications

Abstract

Carboniferous magmatism is one of the most important tectonothermal events in the Central Asian Orogenic Belt (CAOB). However, the final closure time of the Kalamaili Ocean between East Junggar and Harlik Mountain is still debated. Early Carboniferous (332 Ma) and late Carboniferous (307–298 Ma) granitic magmatism from Kalamaili fault zone have been recognized by LA‐ICP‐MS zircon U‐Pb dating. They are both metaluminous highly fractionated I‐type and belong to the high‐K calc‐alkaline. The granitoids for early Carboniferous have zircon εHf(t) values of –5.1 to +8.5 with Hf model ages (TDM2) of 1.78–0.83Ga, suggesting a mixed magma source of juvenile material with old continental crust. Furthermore, those for late Carboniferous have much younger heterogeneous zircon εHf(t) values (+5.1 to +13.6) with Hf model ages (TDM2=1.03–0.45 Ga) that are also indicative of juvenile components with a small involvement of old continental crust. Based on whole‐rock geochemical and zircon isotopic features, these high‐K granitoids were derived from melting of heterogeneous crustal sources or through mixing of old continental crust with juvenile components and minor AFC (assimilation and fractional crystallization). The juvenile components probably originated from underplated basaltic magmas in response to asthenospheric upwelling. These Carboniferous highly fractionated granites in the Kalamaili fault zone were probably emplaced in a post‐collisional extensional setting and suggested vertical continental crustal growth in the southern CAOB, which is the same or like most granitoids in CAOB. This study provides new evidence for determining the post‐accretionary evolution of the southern CAOB. In combination with data from other granitoids in these two terranes, the Early Carboniferous Heiguniangshan pluton represents the initial record of post‐collisional environment, suggesting that the final collision between the East Junggar and Harlik Mountain might have occurred before 332 Ma.