Early Permian slab breakoff in the Chinese Tianshan belt inferred from the post-collisional granitoids

We present U-Pb-Hf isotopic and elemental data for granitoid intrusions formed during the early Permian in Eastern Tianshan (eastern part of Chinese Tianshan), with the aim of deciphering their magma sources and geodynamic evolution. These granitoids were formed at ca. 300-270 Ma and generally exhibit alkali-calcic to alkali compositions, and metaluminous and high-K calc-alkaline to shoshonitic characteristics. Commonly, steep REE patterns with significant enrichment of LREE relative to HREE showing (La/Yb)(CN) (4.4-43), distinctive negative Eu anomalies and depletion of Nb, Ta and Ti are noteworthy features of these granitoids, akin to the typical I-type granitoids implying an influx of mantle-derived magma. The tectonic setting of these granitoids in combination with the regional geology, suggests that the granitoids were generated in a post-collisional setting. The epsilon(Hf)(t) values from -10 to +14 (with four peaks at ca. -4, +5, +8.5 and +12) and T-DM(C) model ages (with four populations at ca. 1500, 970, 720 and 500 Ma) imply that the reworking of early Mesoproterozoic ancient crust related to Columbia and Neoproterozoic crust associated with Rodinia along with the early Permian input of juvenile material were variably sourced during the generation of the magmas studied herein. During the post-collisional process, the slab breakoff played a key role in the genesis of the granitoids and involved the reworking of ancient crust and mantle-derived sources, consistent with the Hf isotopic compositions. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.