Petrogenesis Of The Early Cretaceous Granitoid In Northwestern Zhejiang And Its Tectonic Evolution

Granitoids are as main components of the upper continental crust, and they recoreded abundant information about the formation of continent crust, crust-mantle interaction and evolution of the lithosphere. The Mesozoic tectono-magmatic activity occurred widely in the northwestern Zhejiang, located to the northeastern Jiangshan-Shaoxing tectonic belt, separated the Yangtze and Cathysia blocks in South China.13 Early Cretaceous granitic intrusions of in the northwestern Zhejiang area are systematically studied and some conclusions are as follows.The zircon U-Pb isotopic data and the field relations of the Early Mesozoic granitoids in the northwestern Zhejiang indicated a magmatic activity event ranging from 148 to 125 Ma. Granodiorite and granitic porphyry formed in the early stage (148-135 Ma) while biotite granites formed in the late stage (135-125 Ma).The whole rock geochemistry suggests that the Early Cretaceous granitoids in northwestern Zhejiang area could be divided into four types including I-type, S-type, A-type and highly fractionated granitoids, among which I-S type granites correspond to the granodiorite and granitic porphyry in the early stage. I-S type granites exhibit low contents of SiO2 (average of 68%) and rare earth elements (REE), along with a weakly negative anomaly of Eu and large fractionation of heavy and light REE, and depleted in high field strength elements such as Nb-Ta and Ti though the I-type granites have higher Sr content than those from the S-type granites. They may be related to intracontinental orogenic and crust trucking process in the region since the Early-Middle Jurassic. The A-type granites, being biotite granites, show high SiO2 content (average of 75%) and FeOT/MgO, with strongly negative Eu anomaly and enriched Rb, Th and U, REE and limited fractionation of heavy and light REE, and depleted in Ba, Sr, Ti and P, and were formed in the post-orogenic lithospheric extensional setting.Whole rock Sr-Nd and zircon Hf isotopic components indicate that the Meso-Neoproterozoic metabasements of the Shangxi, Shuangqiaoshan and Shuangxiwu groups should be as magmatic sources of the Early Cretaceous granitiods in the northwestern Zhejiang. The whole rock geochemistry suggests that the I-type granite was formed at a high pressure, and then the S-type granite at a midium pressure and A-type granite was formed at a low pressure condition.Combined with the previous geophysical and regional geology survey, a tectonic evolutional model is suggested. The Early Cretaceous granitoids in the northwestern Zhejiang formed due to the southwestward subduction of the paleo-Pacific plate during 180～150 Ma, and subsequent roll-back of the subduction plate as well as lithosphere delamination, the asthenosphere-derived basaltic magma upwelling along weak tectonic belt. Partial melting of the mafic lower continental crust in the northwestern Zhejiang area cause the formation of the I-type, S-type and highly fractionated granitoids. After 135 Ma, because of the strong extension of lithosphere and crust thinning, large scale of crust anatexis formed the Cretaceous volcanic rocks and A-type granite.