Petrogenesis Of The Granitoids In Qiashui Area,Guangdong Province:Constraints From Mineralogical,Geochronology,Geochemistry And Nd-Hf Isotopes

A large amount of?100 Ma magmatic rocks were developed in the coastal area of southeast China,but the magmatism of this period in the interior of South China has been rarely reported.The tectonic environment of its formation is still controversial.The granites in Qiashui area,Guangdong province,including the Baishuizhai,Jilongling and Jiangjuntou plutons and the southern part of Lianyang pluton,are located near the Chenzhou–Linwu fault in the western nanling range,based on the systematic study of the granites,it is concluded that they were all formed at about 100 Ma.In order to discuss the petrogenesis,magmatic evolution and tectonic setting of the granitoids in Qiashui area,Guangdong province,the petrographic analysis,electronic probe analysis,geochemistry analysis and Nd–Hf isotope characteristics analysis of these rock bodies were compre-hensively analyzed,which provides new evidence for the analysis of the?100 Ma tectonic environment of interior South China.According to the lithological characteristics and their intrude relationship,the granitoids in Qiashui area,Guangdong province can be divided mainly into five intrusive times,including coarse–medium–grained porphyritic hornblende bearing biotite monzo-granite,medium–fine–grained porphyritic hornblende bearing biotite monzogranite,medium–fine–grained porphyritic biotite monzogranite and fine–grained porphyritic biotite syenogranite,respectively.Among them,the first and the second time intrusive rocks are commonly found with mafic microgranular enclaves(MMEs)of magma mingling origin.Zircon LA–ICP–MS U–Pb dated yield the ages of the first to fifth intrusions with101.8?97.1 Ma,which belong to the turn of the Early and Late Cretaceous(?100 Ma).Therefore,this study believes that the Lianyang granite pluton was formed at about 100Ma.Petrographic analysis and electronic probe analysis showed that the mineral grain size and the content of dark minerals decrease gradually from the early intrusion to the late intrusion in Qiashui area,Guangdong province.The results showed that the content of biotite and the MF value were decreased gradually.Furthermore,the type of biotite changed from ferrobiotite to iron biotite,hornblende from existence to absence,and plagioclase number(An value)decreased gradually.In general,the rocks showed high Si O₂,K₂O+Na₂O,poor Ca O and Mg O characteristics,which were metaluminous to peraluminous(A/CNK=0.91?1.05).The rocks enriched in Rb,Th,U,Pb,K and LREE,and strongly depleted in Ba,Sr,P,Ti and Eu(?Eu=0.12?0.65),and slightly depleted in Nb and Ta,relatively.The results showed that the granitoids in Qiashui area were the evolution products of consanguineous magma with genetic relationship.It was concluded that the granitoids is a transitional type granite between I-type and A-type.The Nd-Hf isotopic characteristics showed that the granites have relatively high?_Nd(t)values(-10.6?-4.2),young T_DM2(Nd)values(1.79 Ga?1.24 Ga)and wide range of?_Hf(t)values(-9.0?-0.4).It is suggested that the source area is mainly felsic rocks from mayuan group of continental basement in middle proterozoic era,and has some contribution of mantle source.Based on a comprehensive analysis of the magmatic events at?100 Ma in the inland area of South China,suggested that the magmatic events were related to the back arc or intracontinental lithosphere extension caused by the roll-back of the subducted Paleo–Pacific plate,which resulted in the reactivation of the Chenzhou–Linwu and other ancient deep faults,led to the upwelling of the asthenosphere and thinning of the lithosphere,as well as the strong mantle and crust magmatism.