Early Cretaceous Fengshan and Caishan high-Mg# adakitic rocks from the Xuzhou-Huaibei area, central China : Interaction between mantle peridotite and melt derived from partial melting of delaminated lower continental crust

Whole-rock geochemistry, Sr-Nd-Hf isotopes and mineral chemistry of the garnet for the Fengshan granodioritic porphyrys and Caishan quartz dioritic porphyrites in Xuzhou-Huaibei area are carried out in present study, which are of significance for understanding the petrogenesis of the Early Cretaceous high-Mg-# adakitic rocks and tectonic evolution in the southeastern margin of North China Craton. Geochemically, these rocks have high SiO2 (60.98% similar to 67.88%) and Al2O3 contents (14.37% similar to 15.04%), high Na2O/K2O ratios (1.58 similar to 2.24) and high Mg-# values (57 similar to 66). They are enriched in large iron lithophile elements (e.g., Rb, Ba, and Sr) and light rare earth elements, depleted in high field strength elements (e.g., Nd, Ta, and Ti) and heavy rare earth elements, and display prominent positive Pb anomalies and weakly Eu anomalies. The above geochemical characteristics, combined with their high Sr contents (579A_10(-6) similar to 778A_10(-6)), high Sr/Y (33 to 69) and (La/Yb)(N) ratios (8.63 to 13.7), low Y (10.5A_10(-6) similar to 17.8A_10(-6)) and Yb contents (0.74A_10(-6) similar to 1.17A_10(-6)), suggest these rocks are high-Mg-# adakitic rocks. Additionally, these adakitic rocks have the (Sr-87/Sr-86)(i) values of 0.7079 similar to 0.7086, have the epsilon(Nd)(t) values and t(DM2) ages of -10.77 similar to -7.18 and 1504 similar to 1793Ma, have epsilon(Hf)(t) values and t(DM2) ages of -14.2 similar to -5.3 and 2101 similar to 2898Ma, respectively. The epsilon(Hf)(t) values of the Liguo, Banjing and Jiagou high-Mg-# adakitic rocks in Xuzhou-Huaibei area range from -13.4 to -7.0, -13.4 to -7.9 and -15.9 to -4.5, and the corresponding t(DM2) values range from 2248 to 2825Ma, 2331 to 2824Ma and 2030 to 3048Ma, respectively. The above isotopic features and the existence of Almandine residual crystals in Fengshan granodiorite porphyrys, combined with their high Pb isotopes compositions and U-Pb geochronology of inherited zircons, imply that the Fengshan and Caishan high-Mg-# adakitic rocks were derived from partial melting of a delaminated garnet pyroxenite phase lower continental crust and subsequent interaction with the mantle peridotites, geodynamically related to extensional environment in response to lithospheric thinning.