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

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 SiO(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 SiO(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 zirconε(t)values(-18.4 to-21.1),suggesting they were from partial melting of the mafic lower continental crust.The granites have relatively higher zirconε(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.