LA-ICP-MS zircon U-Pb dating and geochemical characteristics of granodiorite in Rilonggou area, Xinghai County, Qinghai Province

Located at the junction between the Eastern Kunlun Mountains and the western Qinling Mountains, the Sertang-Rilonggou area is a polymetallic orefield in Qinghai province, related mainly to copper mineralization. In this area, acidic intrusive bodies are extensively distributed and have a deep effect on the mineralization there. Through the research on the LA-ICP-MS zircon U-Pb dating and geochemistry of biotite granodiorite, the authors found that the rock mass of the Sertang-Rilonggou area was formed in the Triassic, and it also has the features of relatively poor silicon (the content of the SiO2 is 62.24%~63.67%), strong alkali (the rate of the alkalinity is 1.73~1.88), and excessive aluminum (the A/CNK is 2.37~2.70). On the diagram of SiO2-(K2O+Na2O), graphic sample is pointed to the alkaline series of granodiorites zone. And in this polymetallic orefield, the amount of the rare earth elements is low (∑REE=111×10-6~164×10-6), and the rare earth element distribution curve shows that the light rare earth elements are of enrichment (LREE/HREE=9.83~16.95) with the curve right-oblique. The average (La/Yb)N is 21.13; the Eu and Gd have the feature of a weak negative anomaly (δEu=0.79~0.98, δGd=0.73~0.81), while the element of Tb has the feature of a positive anomaly (δTb=1.23~1.28). An analysis of the trace elements shows that Rb, Ba, Th, U and some other large ion lithophile elements exhibit relative enrichment, some high field-strength elements, such as Nb, Ta, Dy, Y, Ho, Yb, and Lu are deficient, and the values of some elements like Pb, Sn are relatively high. This rock mass is mainly composed of plagioclase, quartz and hornblende. Under the microscope, two groups of plagioclase and poikilitic texture can be observed. The synthetic judgment of these features has led the authors to believe that this rock mass is of type I granite, resulting from intermediate-volcanic rock melt in Permian. Combined with the field investigation, It is held that the Sertang-Rilonggou orefield might have experienced at least two very contiguous magma events.