Studies On The Petrology And Geochemistry Of Late Mesozoic Xiaozhushan Intrusions In Qingdao, Shandong Province

The late Mesozoic Xiaozhushan granite pluton,located in the eastern Shandong Province,is composed of alkaline granite and syenogranite.In order to probe their genesis,Petrographic and geochemical studies on these granites have been carried out by XRF and ICP-MS.Analyze results show that the arfvedsonites-bearing alkaline granite is characterized by high contents of SiO₂?66.05-76.24%?,Zr+Nb+Ce+Y,low contents of CaO,enrichment in FeO_T,Ga,deplete in Sr,P,high Ga/Al ratio,typical REE distribution pattern,and high zircon saturation temperatures(T_Zr,843-905?).Those suggest that the alkaline granite are A₁-type granite.Contrasted to the alkaline granite,the hornblende-bearing syenogranite is characterized by higher contents of SiO₂?75.48-77.88%?,Rb,Lower contents of FeO_T,Zr,Zr+Nb+Ce+Y,higher Differentiation Index?DI?,lower Ga/Al,Zr/Hf ratios,and lower T_Zr?777-823??.Those suggest that the syenogranite is highly fractionated I-type granite.Integrating geochemical data,characters of enclaves in the syenogranite and results of electron probe analysis in feldspar zoning structure,assisted with Bayes discriminant method,we argue that the granites generated via a process including formation of parental magma by mixing of a mantle-derived basaltic magma and a crustal felsic magma in the deep crust and subsequently extensive magmatic differentiation of the parental magmas.The granite pluton,whose generations are associated to the rollback of subducting paleo-Pacific,could be a key clue for the decipher of late Mesozoic tectonic conversion in eastern Shandong province.