Geochronology,Geochimistry And Its Geological Significance Of Rhyolite In Nanjing Basin,South Jiangxi Province

Nanjing basin is located in the middle of the Wuyi-Yunkai orogenic belt and the eastern part of the Nanling tectonic belt.The volcanic rock series is well developed in eastern Nanling region,consists of weak metamorphic light gray dacite,gray andesite,greyish white rhyolite and pyroclastic rocks,be classified as Nanjing Formation.The volcanic rock series is conformably contact with the Huangzhudong Formation,underlies unconformable Middle Devonian Yunshan Formayion.Zircon SHRIMP U-Pb dating suggests that the rhyolite of Nanjing basin was emplaced at 443.9±2.8Ma(N=15,MSWD=1.4)and 436±2.4Ma(N=17,MSWD=0.89),indicating a Late Ordovician-Early Silurian magmatic event.The greyish white rhyolite at Nanjing basin have porphyritic,flow structure and massive structure.Rock-forming minerals are quartz,plagioclase,feldspar and biotite.The content of SiO₂range of 74.14%?74.56%(the average is 74.35%),Al₂O₃range of13.78%?14.05%(the average is 13.94%),Na₂O range of 2.11%?2.86%(the average is 2.71%),K₂O range of 3.43%?4.53%(the average is 3.60%),(K₂O+Na₂O)range of6.21%?6.64%(the average is 6.31%),CaO range of 1.9%?2.7%(the average is2.54%),TFe O range of 1.84%?2.07%,A/CNK range of 1.05?1.19.CIPW standard mineral corundum(C)content is 0.69%?2.16%(the average is 0.97%<1%).The rhyolite of Nanjing basin belongs to the series of volcanic rocks with weak peraluminum high potassium calc-alkaline and has the characteristics of I type granite.The rhyolite is characterized by relative enrichment in LILEs(e.g,Rb,K)and LREEs and pronounced depletion in HFSEs(e.g,Ta,Nb and Ti)and HREEs.Such geochemical features are typical of Subduction-related magmatic rocks.The total amount of REE is relatively low,showing distinct rich LREE,(La/Yb)_Nvalues are8.65?12.23,Eu/Eu*=0.6?0.65,negative Eu anomaly is obvious,suggesting that they were derived from the partial melting of the lower crust,and suffered from the fractional crystallization of plagioclase.The corresponding(⁸⁷Sr/⁸⁶Sr)_iis0.7149?0.7156(the average is 0.7153),?_Nd(t)is-10.3?-11(the average is-10.7)and the single stage Nd model ages(t _DM2)vary from 2070 to 2009 Ma(the average is2043Ma).In?_Nd(t)-t(Ma)diagram,the rhyolite is located Proterozoic crustal evolution area in South China and suggest that it originated from the partial melting of crustal material.In Zr-Y,Ti O₂/Al₂O₃-Zr/Al₂O,Ta/Yb-Th/Ta and Yb-Th/Ta diagrams,the projection points fall within the range of active continental margin magmatic rocks.Through field geological characteristics and zircon U-Pb SHRIMP age of rhyolite,combined with the geochemical characteristics of main,trace elements and isotope analysis,it is confirmed that the rhyolite in Nanjing basin is Caledonian volcanic rock.During the early Paleozoic period,underplating of mantle derived magma resulted in the remelting of Paleoproterozoic crustal materials,and crystallization differentiation during the ejection of the surface to form rhyolite in the Nanjing basin.The rhyolite shows the characteristics of active continental margin magmatic rock,which suggest exit Ocean basin and ocean subduction in South China during Early Paleozoic time.