Petrogenesis of Early Paleozoic I-type granitoids in the Wuyi-Yunkai Orogen, South China: Implications for the tectono-magmatic evolution of the Cathaysia Block

摘要

The tectonic setting of the Early Paleozoic granites in the South China Block (SCB) remains enigmatic, and it is uncertain as to whether they are related to intracontinental orogenic processes or oceanic subduction. Previous studies mostly focused on Early Paleozoic S-type granitoids in the SCB, and relatively little attention has been paid to Early Paleozoic I-type granitoids. Here we present whole-rock major and trace elemental data and LA-ICP-MS zircon U-Pb age and Lu-Hf isotopic data for hornblende monzogranites, monzogranites, and mylonitic granites of I-type granites affinities that comprise the Daning and Guiling plutons in the southwest part of the Wuyi-Yunkai orogen in the SCB. These rocks contain zircon with U-Pb ages of 422-418 Ma, taken as the age of emplacement. They are rich in LREEs and moderately depleted in Eu (delta Eu = 0.49-0.64), with negative Ba, Sr, Nb, and Ti anomalies, and positive in Rb, Th, U, and Pb anomalies. Our studies of these granites and their abundant enclaves indicate that they formed via complicated petrogenetic processes, including the partial melting of a mixed source of meta-sedimentary rocks and meta-basalts in the lower crust, magma mingling and mixing of crust-mantle materials, and fractional crystallization. Mantle-derived magma played an important role in the formation of these Early Paleozoic granites. Although an ancient and enriched mantle source might have made a significant contribution to these Early Paleozoic granites, the input of juvenile mantle-derived magma might have been insignificant. Taking into account all the other geological evidence from the Wuyi-Yunkai region, we suggest that the tectono-magmatic events of the Early Paleozoic took place during intraplate orogenesis rather than oceanic subduction and collision, and that these events were triggered by a far-field tectonic response to the assembly of northern East Gondwana.