Petrogenesis of the Zhangpu Composite Granite Pluton in Fujian Province: Constraints from Zircon U-Pb Ages,Elemental Geochemistry and Nd-Hf Isotopes

The Zhangpu composite granite pluton,with an outcrop area over 1500 km2,is located in the coastal region of SE Fujian Province.It is a big batholith formed by multi-satge magmatisms.According to the lithological characteristics,this pluton can be divided into three intrusions: the Changqiao(CQ) biotite granite,the Chengxi(CX) alkali-feldspar granite and the Huxi(HX) granodiorite.Zircon U-Pb ages obtained by LA-ICP-MS for the three intrusions are 119 Ma,101 Ma and 96 Ma,respectively.All three intrusions are subalkaline,metaluminous to weakly peraluminous and poor in phosphorus,with A/NKC values less than 1.10 and P2O5 contents lower than 0.20%,thus can be grouped into calc-alkaline I-type granites.They are enriched in Cs,Rb,Th,U,Pb and LREE,and depleted in Ba,Sr,P,Ti,but the CQ and CX granites show higher Rb/Sr and Rb/Ba,and lower K/Rb ratios,and display more pronounced negative europium anomalies relative to the granites in the HX intrusion,indicating that the three intrusions had underwent different degrees of differentiation.All of the analyzed rocks show homogeneous Nd isotopic compositions,with eNd(t) values ranging from-2.43 to-3.24,and tDM2 values,from 1.11 to 14.16 Ga.However,they display highly variable zircon Hf isotopic compositions with eHf(t) values spreading over 6 eHf units(specifically,-8.3 ~ +3.0 for the CQ intrusion,+1.7 ~ +10.2 for the CX intrusion,and-2.5 ~ +3.5 for the HX intrusion,respectively).Such a large variation in zircon Hf isotopic compositions argues strongly that different source components have contributed to magma genesis.Based on a synthesis of petrography,geochronology,elemental and isotopic geochemistry,it is suggested that the granites in all three intrusions were most likely generated via a two-stage process including formation of parental magma by mixing of mantle-derived magma and induced crustal-melted felsic magma in the deep crust,and then underwent variable degrees of differentiation during magma ascent.The distinct differences in rock-forming ages and chemical variation tendency among the three intrusions indicate that the Zhangpu composite pluton could not be generated by fractional crystallization from a common primary magma,rather it was most likely produced by multiple-stage intrusions of crust-mantle mixing magma with variable degrees of differentiation.