Geochemistry, Zircon U-Pb Age and Hf Isotope of the Huilanshan Granitoids in the North Dabie Terrane: Implications for Syn-Collapse Magmatism of Orogen

Abstract

Syn-collapse magmatism is a critical issue for evolution of the continental orogen. The Dabie Orogen is a typical orogen which was suffered from a complete collapse. Two kinds of granitoids, namely, the coarse-grained diorite and the fine-grained granite, are recognized at the center of the Luotian extensional dome, providing an opportunity to decipher the syn-collapse magmatism in the Dabie Orogen. The diorites (125±3 Ma) are high K calc-alkaline rocks, with low SiO2 (51.9 wt.%–56.6 wt.%) and high MgO (3.5 wt.%–4.0 wt.%) contents. They are enriched in LREE and LILEs (e.g., Ba, K, Rb) and depleted in HFSEs (e.g., Ta, Nb, and Hf) with low ratio of Sr/Y (30.82–46.89). The granites (118±2 Ma) are shoshonite series rocks, with relatively high SiO2 (68.9 wt.%–72.6 wt.%) and low MgO (0.32 wt.%–0.66 wt.%) contents. They are also enriched in LREE and LILEs with weakly negative Eu anomalies (δEu=0.81–0.85), and are depleted in HFSEs with low Sr contents (338 ppm–477 ppm) and Sr/Y ratios (23.80–33.13). Therefore, the two kinds of granitoids have no geochemical characteristics of adakitic rocks, suggesting that they were generated from a normal or thinned crust level. The diorites have quite negative zircon eHf(t) values (-18.4 to -21.1), suggesting they were from partial melting of the mafic lower continental crust. The granites have relatively higher zircon eHf(t) values (-14.4 to -18.1). The granites also contains a series of old inherited zircon cores, such as two upper intercept ages of 2 628±41 and 1 840±37 Ma, and a concordant age of 807±9 Ma. All these features suggest that the granites were generated from partial melting of the felsic middle-lower continental crust. Thus, the Huilanshan Early Cretaceous granitoids coupled with the Luotian extensional dome revealed the collapsed process of the Dabie Orogen.