Chronology And Geochemistry Of Intrusive Rocks From Xiaoyao Area, South Anhui Province, SE China, And Its Tectonic Significance

Xiaoyao country situated in Anhui Province, southeast China, is located on the southeastern margin of Yangtze plate, with its south rim adjacent to Jiangnan Orogenic belt. The area is lack of systematic study due to the transport problems.This paper reports the SHRIMP zircon U-Pb ages for granodiorite and granodiorite prophyries in Kaobeijian intrusion, Xiaoyao country, south Anhui province. The two weight mean ages of 151.9±1.1 Ma and 147.7±1.3 Ma indicate that Kaobeijian intrusion was formed during late Jurassic, and imply a intensive magmatic activity in south Anhui province during 146-151 Ma. Hydrothermal alterations are well developed in Kaobeijian intrusion, indicating that pervious 40Ar/39Ar age of biotite from Kaobeijian intrusion is more likely to document a tectonic-thermal event, rather than the formation age of the intrusion.Whole-rock major-and trace-element analysis have shown that SiO2 of the granitoid range 58.76-69.75%, K2O=2.07-5.80%,Na2O=2.15-3.85%, A12O3=12.28-15.69%; CaO=1.64-5.94%; A/CNK=0.72-1.06. The quartz syenites belong to high-K calc-alkaline and metaluminous series. They are also enriched in LILE, but depleted in HFSE. The Nb/Ta ratio is similar to that of upper crust showing Dongyuan intrusion may be generated due to the partial melting of crust. The REE content is between 69.48～192.90ug/g, with(La/Yb)N ratios of 3.41～21.03. (La/Sm)N ratio ranging from 1.84～5.06 and (Gd/Yb)N ranging from 1.06～3.71 show the fractional differentiation of LREE is more intensive than HREE. Samples in REE Chondrite normalized diagram is characterized by right skewed curves with slight Eu anomalies.All the data points fall in the field of VAG of Rb-Y-Nb and Rb-Yb-Ta diagram. Combining the Zircon ages with the tectonic environment, this paper suggests that three intrusions in Xiaoyao deposit are likely generated in a plate-subduction-related compressional volcanic arc environment.