Jurassic post‐collisional extension in the Songpan–Ganze Terrane, eastern Tibetan Plateau: Evidence from weakly peraluminous A‐type granites within the Zheduo–Gongga Massif

Abstract

The Mesozoic–Cenozoic Zheduo–Gongga Massif, eastern Songpan–Ganze Terrane, can provide important clues for the crustal evolution in response to syn‐ and post‐orogeny. Here, we document petrological and geochemical characteristics of Early Jurassic granites (181.0 ± 2.2 Ma) within the massif along the Xianshuihe Fault in the eastern Tibetan Plateau. The granites have high contents of SiO2, Na2O + K2O, FeOT/MgO, 10,000 × Ga/Al, REE (except Eu) and HFSE (e.g., Nb, Ta, Zr, Hf), low CaO, TiO2, P2O5 and negative Eu, Sr, and Ba anomalies. Zircon REE shows a similar pattern with the whole‐rock REE. Our new data demonstrate weakly peraluminous A‐type affinity with A/CNK >1.0 and alkali‐calcic, ferroan features. Zircon saturation temperatures and geochemical features (e.g., high SiO2 and low MgO contents, high Rb/Ba and Rb/Sr ratios) indicate that the granites resulted from high‐temperature (>800°C) partial melting of Triassic flysch rocks in relatively shallow crust (<500 MPa or <20 km). Zircon εHf(t) values range from 0.55 to 2.91 and the corresponding two‐stage TDMC are older than the neighbouring Kangding complex and Yangtze margin, implying possible contribution of mantle materials to the parent magma. Considering the tectonic events and the regional geology, the magma was generated in a post‐collisional setting, which was possibly driven by the upwelling of hot mid–lower crustal materials due to the break‐off of subducted Palaeo‐Tethys Ocean slab. Under this situation, the magma may move upwardly into the pre‐existing Xianshuihe Fault during the post‐collisional extension of the thickened crust. Thus, slab break‐off played a key role in the genesis of the Zheduo–Gongga Massif studied herein.