Petrology And Geochemical Characteristics Of Laojunshan Granite In Southeast Yunnan And Its Tectonic Signigicance

Laojunshan area of Eastern Yunnan is located on the boundary of Yangtze plate and Cathaysian plate. It is our country’s important polymetallic mineralized concentration area. This paper chose to study the first phase granite intrusive body from Laojun Mountain on petrology, main elements and trace elements, zircon U-Pb age,Hf isotope and so on, to analyze magma material source, formation mechanism and tectonic environment. the first phase granite intrusive body from Laojun Mountain is entitled medium-coarse grained adamellite. the rock has a relative high content of SiO2(71.26%～73.44%), rich potassium (K2O5.27%～5.78%), and total alkali content (K2O+Na2O) up to (7.35%～8.84%), A/CNK value between1.56～1.99, which shows that it is peraluminous. And according to trace elements, Nb/Ta ratio (4.84～6.23) is within the scope of high differentiated granite (2.3～9.9), and K/Pb ratio less than150(66.57～92.29), which shows that it is high differentiated granite. In addition, as SiO2～P2O5relationship basically remain unchanged, Y and Rb have negative correlation, and P2O5and Rb are positively correlated, the rock is estimated as S type granite. Rare earth distribution pattern presents a rightward tilt and light rare earth enrichment type with obvious negative Eu anomaly. The first phase granite intrusive body’ zircon LA-ICP-MS U-Pb age is85.88±0.76Ma～88.0±2.0Ma, in combination with cathodoluminescence (CL) images, it is estimated as the result of magmatic activity in late Yanshanian, late Cretaceous. Zircon’s Hf isotope research result shows: εHf(t)=-10.4262～-4.61534. The Hf isotopic model age (TDMC) of its two-phase is mainly1335～1701ma, reflecting that the source area of Laojun Mountain granitic intrusive rock is mainly from lower crust remelting. Combining the regional tectonic background, this paper argues that in Yanshanian epoch, there occurred an interpolate subduction between Indosinian plate and South China plate, the extrusion of which caused the partial melting of the lower crust materials, after the magmatic crystallization and differentiation, the magma upwelled and formed Laojunshan granite intrusive body.