Magma mixing for the origin of granodiorite: Geochemical, Sr-Nd isotopic and zircon Hf isotopic evidence of dioritic enclaves and host rocks from Changshannan granodiorite in the Jiaodong Peninsula, eastern China.

A large number of directional microgranular dioritic enclaves occur in Changshannan granodiorite from Kunyushan granitoids in the Jiaodong Peninsula, eastern China. Microgranular dioritic enclaves, commonly showing ellipsoidal and spindled shapes, display igneous textures and range from meter- to centimeter-sized fragments. The enclaves are darker-colored and finer-grained than their hosting granodiorite, and exhibit a very similar mineralogy to those of their granitic hosts. The enclaves have higher contents of ferro-magnesian phases and plagioclase, and lower contents of quartz and K-feldspar than those found in the hosting granodiorite. The chemical composition of biotite and hornblende within the dioritic enclaves is mostly similar to those of the same phases found in the host granodiorite, but the mineralogy display obvious disequilibrium structure. Acicular apatite is common in the enclaves. Those observations suggest that the enclaves are derived from a hybrid-magma formed as a result the intrusion of a mafic magma into the base af a felsic magma chamber. Geochemical data show dioritic enclaves and hosting granodiorite are enriched in LILEs and LREEs and depleted in HFSEs. Enclaves and hosting granodiorite, similar in Sr- and Nd- isotopic compositions, have low initial 87Sr/86Sr ratios (0.70784-0.70818) and negative eNd(t) values (- 15.0 - - 15.5), and display a trend characterized by a linear correlation between major elements. Petrological and zircon Hf isotopic features indicate the occurrence of magma mixing between mafic magma and felsic magma in the original processes of Changshannan granodiorite. During the magma mixing, the Sr and Nd isotope and later crystallized minerals can much quickly homogenize, but major element and Hf isotope of high temperature mineral such as zircon have preserved some information of the primary magmas. The results indicate that although magma mixing has produced similar Sr and Nd isotopic compositions in the enclaves and the hosting granodiorite, the zircon Hf isotope has not easily reached equilibrium and has more effectively traced the hybrid-magma sources and the magma mixing processes.