Geochemistry and Nd-Sr isotopic studies of Late Mesozoic granitoids in the southeastern Hubei Province, Middle-Lower Yangtze River belt, Eastern China: Petrogenesis and tectonic setting

摘要

A geochemical and isotopic study was carried out for the Mesozoic Yangxin, Tieshan and Echeng granitoid batholiths in the southeastern Hubei Province, eastern China, in order to constrain their petrogenesis and tectonic setting. These granitoids dominantly consist of quartz diorite, monzonite and granite. They are characterized by SiO2 and Na2O compositions of between 54.6 and 76.6 wt.%, and 2.9 to 5.6 wt.%, respectively, enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE), and relative depletion in Y (concentrations ranging from 5.17 to 29.3 ppm) and Yb (0.34-2.83 ppm), with the majority of the granitoids being geochemically similar to high-SiO2 adakites (HSA). Their initial Nd (ε _(Nd)=-12.5 to -6.1) and Sr ((~(87)Sr/~(86)Sr)_i=0.7054-0.7085) isotopic compositions, however, distinguish them from adakites produced by partial melting of subducted slab and those produced by partial melting of the lower crust of the Yangtze Craton in the Late Mesozoic. The granitoid batholiths in the southeastern Hubei Province exhibit very low MgO ranging from 0.09 to 2.19 wt.% with an average of 0.96 wt.%, and large variations in negative to positive Eu anomalies (Eu/Eu~*=0.22-1.4), especially the Tieshan granites and Yangxin granite porphyry (Eu/ Eu~*=0.22-0.73). Geochemical and Nd-Sr isotopic data demonstrate that these granitoids originated as partial melts of an enriched mantle source that experienced significant contamination of lower crust materials and fractional crystallization during magma ascent. Late Mesozoic granitoids in the southeastern Hubei Province of the Middle-Lower Yangtze River belt were dominantly emplaced in an extensional tectonic regime, in response to basaltic underplating, which was followed by lithospheric thinning during the early Cretaceous.