Geochronology and geochemistry of the Sikongshan intrusion in the Dabie Orogen, Central China: Implication for Mesozoic geodynamic background

The Sikongshan (SKS) intrusion, the largest pluton situated in the Central Dabie unit of the Dabie Orogen (DBO), is composed of monzogranite and quartz diorite. A study combining whole-rock major and trace element analysis, zircon U-Pb dating, as well as zircon in situ Lu-Hf isotopes was conducted for the SKS intrusion. Zircon dating yielded U-Pb ages of 136 +/- 1.6 Ma and 134.6 +/- 1.7 to 130.4 +/- 1.7 Ma for quartz diorites and monzogranites, respectively, both types belonging to the early stage of the Mesozoic magmatism in the DBO. The quartz diorites exhibited high Na2O (Na2O/K2O = 1.60-1.86) and Sr (1,185-1,365 ppm) and low Y (11.6-14.2 ppm) contents, and therefore high Sr/Y (83.45-108.19) ratios. They also had high ratios of (Dy/Yb)(N) (48.42-54.91) and (Dy/Yb)(N) (1.47-1.53). The monzogranites were enriched in K2O (Na2O/K2O = 0.64-0.99) and were also characterized by low Y (5.6-14.7 ppm) contents and high Sr/Y ratios with a geochemical affinity similar to that of the adakites. However, the Sr (<358 ppm) and Al2O3 (3.31-14.92%) contents were much lower than those in typical adakites and quartz diorites. Both the quartz diorites and monzogranites had trace element signatures similar to that of continental crust with enriched levels of Rb, Ba, Th, U, and LREE and depleted levels of Nb, Ta, P, and Ti, indicating their crustal origin. Following an integrated review of these and other data from previous studies, it was concluded that the Sikongshan quartz diorite originated from the thickened lower crust of the DBO, with a composition similar to that of the Huangtuling granulite, which is the exposed lower crust of the DBO. The monzogranite was generated by plagioclase + amphibolite fractional crystallization of the adakitic Tiantangzhai melt. The adakitic rocks in the DBO originated in the western part during 143 to 134 Ma and then spread extensively throughout the whole orogen (134 to 130 Ma). The collapse of the thickened mountain root in the DBO at an extensional setting may be related to the roll-back of the subducted Paleo-Pacific slab.