The Discovery and Tectonic Setting of the Early Cretaceous High-Mg Diorites in the Jiaodong Peninsula

A large number of Early Crataceous granitoids with crust-mantle mixed source were reported in the Jiaodong Peninsula, however, mantle-derived plutons have not been reported so far. In this paper, we present new data from the Liulinzhuang and Xiahecheng plutons in the Jiaodong area. These two plutons were previously taken as Triassic diorites, however, based on detailed field work, lithogeochemistry together with geochronology and isotopic geochemistry analyses, we proposed that they were formed in the Early Cretaceous with high-Mg diorite affinity. The SiO2 contents of the collected samples vary from 53. 29% to 62. 54%, MgO from 3. 60% to 8. 10%, and Mg-# mainly between 0. 47 and 0. 68. They are characterized by enrichments of Ba, Rb and K, and depletions of Nb, Ta and Zr, together with the initial Sr-87/Sr-86 values of 0. 7082 similar to 0. 7083, which are similar to the values of the mantle. Their epsilon(Nd) (t = 113 Ma) values are very low (- 20. 5 similar to - 16. 8), which is similar to mafic dykes in the Jiaodong Peninsula. Compared with the Xiahecheng pluton, samples from the Liulinzhuang pluton are rich in K2O, Fe2O3T, REE, LILE and HFSE, but low in MgO, Ni and Cr. LA-ICPMS U-Pb zircon data from three samples yield weighted mean ages of 120. 1 +/- 1. 6Ma, 118.3 +/- 1. 7Ma and 122.3 +/- 4. OMa, respectively. These ages are similar to that of the Weideshan type granitoid and the ages of gold deposits in Jiaodong Peninsula. Therefore, we propose that they were formed in a unified geodynamic setting. In addition, geochemical characteristics of the high-Mg diorites in the Jiaodong Peninsula are similar to the high-Mg adakitic and sanukite, closer to Piip type high-Mg andesite, indicating that the high-Mg diorites are most likely formed in a mantle wedge due to the subduction of the Pacific Plate. So, the primary magma of the high-Mg diorites in the Jiaodong Peninsula was derived from partial melting of the enriched lithospheric mantle. In details, the Xiahecheng pluton was formed as the amphibole-rich enriched mantle, whereas the Liulinzhuang pluton was most likely formed as phlogopite-rich mantle and modified by the ancient lower crustal materials.