Early Cambrian Intra-Oceanic Subduction of the Paleo-Shangdan Ocean: New Evidences from the Oceanic Plagiogranite in the Guanzi Town Ophiolite Melange

A series of quartz granodiorite-granodiorite rock assemblage has been identified in Heiwan area of the Guanzi Town ophiolite mélange. In order to determine the formation mechanism of them, the petrology and whole rock geochemistry of gneissic granodiorite and metamorphic tonalite were conducted, and the zircon U-Pb dating, Lu-Hf isotope, Rb-Sr and Sm-Nd isotope of gneissic granodiorite were conducted. The results show that gneissic granodiorite and metamorphic tonalite have low contents of rare earth and trace elements, and metamorphic tonalite has obvious Eu positive anomaly, and both of them have the characteristics of oceanic plagiogranite. The age of granodiorite obtained is 512.6±2.0 Ma, and the ε_Hf(t) value varies from +8.9 to +13.8, indicating that its magma source area is depleted mantle. The I_Sr value of granodiorite ranges from 0.7046 to 0.7053, and the ε_Nd(t) value ranges from +2.55 to +3.08. Combined with the regional tectonic evolution, this set of ocean plagiogranite was formed by the emplacement of acidic magma formed by partial melting of basaltic rocks in the mantle at low temperature (602.9～707.4 ℃) and low pressure (＜ 0.2 GPa), and was formed in the island arc environment related to the subduction of Shangdan Ocean in the early Middle Cambrian.