Zircon Dating And Petrogenesis Of The Taoling-Duanxin Belt Granites In The Eastern Jiangnan Orogen

The Taoling-Duanxin magmatic belt was located in the eastern Jiangnan Orogen,composed by six east-west distributed Yanshanian granitic intrusive bodies.LA-ICPMS zircon U-Pb dating yielded weighted mean ages of 131.2±1.9 Ma,131.3±1.9 Ma,132.0±1.8 Ma,129.7±1.6 Ma,149.1±1.8 Ma and 152.4±2.6 Ma for six samples from the six intrusions,respectively.The results revealed that these granites could be subdivided into two active stages.The early stage granites were emplaced during the Late Jurassic?152.4-149.1 Ma?,with rock types of dominant two-mica granite and two-mica monzogranite.The late stage granites were formed during the Early Cretaceous?132.0-129.7 Ma?,with rock types of two-mica monzogranite and biotite monzogranite.Regardless of the Dexing area,the Yanshanian magmatism in the eastern Jiangnan Orogen could also be divided into two active stages?155-137 Ma and 137-120Ma?.The early magmatism was closely related to W-Mo polymetallic mineralizatio,while the later magmatism had no observation with significant mineralization.In terms of geochronology,the Yanshanian early and late stage granites from the Taoling-Duanxin belt were correspond to that of the eastern Jiangnan Orogen.The early stage granites in Taoling-Duanxin belt had SiO₂ contents ranging from 66.6%to 68.9%,Na₂O+K₂O ranging from 6.98%to 7.17%,K₂O/Na₂O ranging from 0.92 to 1.36,Al₂O₃ranging from 14.80%to 15.48%,with A/CNK=1.041-1.136,which belonged to the high potassium calc-alkaline I-type granites.While the late stage granites in Taoling-Duanxin belt had higher SiO₂?71.66-75.62%?and total alkali?7.61-8.84%?contents,relatively rich in K with K₂O/Na₂O=1.27¹.84 and lower Al₂O₃?12.2-14.8%?contents with A/CNK mostly ranging from 1.10 to 1.21,belonging to the high potassium calc-alkaline series and highly fractionated S-type granites.Isotopically,the early stage granites in the Taoling-Duanxin belt exhibited relatively uniform whole-rock?_Nd?t??-10.3 to-7.6?and zircon in situ?_Hf?t?values?-8.2 to-5.3?,while the late stage granites in had large range of?_Nd?t??-9.2 to-3.8?and?_Hf?t??-8.5 to 0.2?.Integrated studies of the major elements,trace elements and Sr-Nd-Hf isotopes indicated that,the granites of early stage in the Taoling-Duanxin belt had similar geochemical characteristics to those of the early stage granites in southern Anhui Province,both of which were originated from partial melting of the Meso-Neoproterozoic accretionary thickened lower crust.While the late stage granites in the Taoling-Duanxin belt were originated from partial melting of the Mesoproterozoic metasedimentary sources?such as the Shuangqiaoshan Group?with the certain thickening cause.Regional comparison indicated that the diverse sources and formation environment might be the main reasons for the different geochemical characteristics between the early and late stage granites in the Taoling-Duanxin belt and granites related to mineralization in the Dahutang and Zhuxi areas.Similar to those ore-bearing granites in the Dahutang and Zhuxi areas,the late stage granites from the Taoling-Duanxin belt also had a magma source riched in tungsten and had F-rich and low oxygen fugacity features,speculated that it might also have certain metallogenic potential.The decreasing pressure and the increasing temperature from the early stage to the late stage magmatisms in the Taoling-Duanxin Yanshanian area might occur in a environment of Mesozoic regional lithospheric extension and thinning.