Early Paleozoic granitoids from South China: implications for understanding the Wuyi-Yunkai orogen

摘要

The early Paleozoic Wuyi-Yunkai orogeny involves voluminous magmatism and represents the first major tectonothermal event in the South China block since the Neoproterozoic break-up of the Rodinia supercontinent. Here we investigate three representative granitic plutons in Guangxi province of southwest China with a view to understand the magmatic response of the Wuyi-Yunkai orogeny. We present the results of major and trace elements, Sr-Nd isotopes, zircon U-Pb ages and Hf isotope. LA-ICPMS zircon U-Pb geochronology reveals that these plutons emplaced during the early Silurian (441-432 Ma), overlapping with the reported ages of the early Paleozoic magmatism in this orogen. Among the three plutons investigated in this study, the Qinjia pluton is characterized by weakly to strongly peraluminous (A/CNK = 1.0-1.2) nature with SiO2 content varing from 70.6 to 72.3 wt.%. Combined with the relatively high epsilon(Nd)(t) values (-4.9 to -4.2) and epsilon(Hf)(t) values (-2.9 to +6.1), the Qinjia pluton is thought to derive from partial melting of metasedimentary and metabasic rocks and there may exsit juvenile components in magma source. The Damingshan porphyritic granites (SiO2 = 74.8-76.5 wt.%) and Yingqiao granites (SiO2 = 76.5-76.6 wt.%) are strongly peraluminous (A/CNK = 1.1-1.6) and they have negative zircon epsilon(Hf)(t) values (-6.8 to -4.2, -12.8 to -6.1, respectively) and whole-rock epsilon(Nd)(t) values (-6.3 to -6.2, -8.4, respectively). These signatures indicate that the Damingshan and Yingqiao plutons were derived from partial melting of ancient metasedimentary rocks. Combining our data with those from previous studies on the magmatic, sedimentary and metamorphic records of this area, we suggest that the early Paleozoic magmatism is likely associated with lithospheric delamination and the Wuyi-Yunkai orogen is an intracontinental orogen linked to the far-field response to the assembly of the South China block with the Gondwana in the early Paleozoic.