Genesis of Mafic Microgranular Enclaves in the Shaocunwu Granodiorite, Southern China, and their Implications: Evidence from Zircon and Whole-rock Geochemistry

Magmatic microgranular enclaves (MMEs) are widely developed in the Shaocunwu granodiorite at the northeast margin of the eastern Jiangnan orogenic belt. Field geology showed that the MMEs occur as irregular ellipsoids near the edge of the intrusion, and consist of diorite, dominantly composed of amphibole, biotite, and plagioclase grains, with minor acicular apatite. Zircon U-Pb dating showed the ages of the host granodiorites and MMEs are 145.9 +/- 1.1 Ma and 145.6 +/- 2.5 Ma, respectively, indicating both originated during coeval late Jurassic magmatism. Whole-rock geochemical results show that the host granodiorite and MMEs have similar rare earth and trace element partition curves in spider grams, and similar Sr-87/Sr-86, and Nd-147 /Nd-144 isotope ratios, and their zircon Hf-177/Hf-176 isotopic ratios are similar. Geochemical studies indicate that both the host granodiorite and MMEs formed by mixing of coeval magma. Zircon Ti thermometers and oxygen fugacity of the host granodiorite and the MMEs show high oxygen fugacity, similar to that of W-Cu (Mo) mineralized granitoids in the eastern Jiangnan orogenic belt. A similar magma mixing process was probably one of the mechanisms that generated the W-Cu (Mo) fertile melts.