Petrogenesis And Geological Significance Of Late Mesozoic Granites In Ningwu And Liyang Volcanic Basins

Late Mesozoic granitoids associated with voluminous volcanic rocks in are widely distributed in eastern China,but petrogenesis and tectonic setting of the granitoids remain unclear.This study mainly focuses on the least studied granitoids from Ningwu and Liyang basins in the Middle-Lower Yangtze River.On the basis of previous research results and field observations,petrology,zircon geochronolgy,and elemental-isotopic geochemistry of the granitoids had been studied to obtain the following understandings:1.The geochronological results show that the granitoids in Ningwu and Liyang basins was mainly formed at 127.1±3.2?130.2±2.1 Ma and 125.2±3.4?130.1±1.9 Ma,which are the products of the second stage of Late Mesozoic magmatism in the Middle-Lower Yangtze River.Moreover,Proterozoic inherited zircons have been found in the granitoids of the Ningwu and Liyang basin,which provides information for the existence of Proterozoic basement in the Middle-Lower Yangtze River.2.The Ningwu granitoids mainly consist of K-feldspar granite,rhyolitic tuff and granite porphyry,which belong to high-K calc alkaline peraluminous S-type granite.The granites have high K₂O/Na₂O(1.90-2.71 wt.%)ratio and variable Mg#(20.9-80.4%)value,and are characterized by enrichment of Rb,U,Th,K,depletion of Ba,Nb,P,Eu,Ti,and enrichment of Sr-Nd-Hf isotopes.These geochemical characteristics show that the Ningwu granitoids were formed by the mixture of crust and mantle,followed by the crystallization differentiation.The comprehensive analysis indicates that the Early Cretaceous magmatism in the Ningwu region had possibly formed in the period of lithospheric thinning and extension under the post-collision tectonic setting.3.The Liyang granitoids are mainly composed of granodiorite porphyry,syenite porphyry and K-feldspar granite.The Liyang granitoids(Si O₂=70.8-77.1wt.%)are mainly high-K calc-alkaline peraluminous granites,which are divided into S-type(granodiorite porphyry)and A-type granites(K-feldspar granite and syenite porphyry).They exhibit negative whole-rock?_Nd(t)(-9.79 to-6.2)and zircon?_Hf(t)(-17.6 to-3.1)values,and high radiogenic Pb isotopic compositions.The comprehensive interpretation of geochemical data shows that the Liyang granitoids have different genesis:granodiorite porphyry is mainly derived from partial melting of Paleo-Mesoproterozoic crust;syenite porphyry is mainly derived from crystallization differentiation of intermediate acid magma formed by contemporaneous metasomatic enriched mantle in the shallow region of the lower crust(<30 km);K-feldspar granite may be the product of mixing of mantle derived magma and Paleo-Mesoproterozoic crust.A comprehensive discriminant analysis shows that the Liyang granitoids were formed in the Paleo-Pacific plate roll-back and intensive lithospheric extension setting under low oxygen fugacity condition.