The Petrological And Geochemical Study Of Syenite In Changling Of Luonan County, Shaanxi Province

North Qinling exposed the NW-SE distribution with alkaline rocks, have been paid more attention by scholars,because of Long and complex evolution history. However, scholars still have different views on the formation of rock and environment. This paper takes the Syenite of Changling in Luonan County, Shaanxi province as studying object. The research on characteristics of petrology,mineralogy,geochemistry of the Syenite,is made by using microscopic, electron probe( EPMA), Inductively coupled plasma atomic emission spectroscopy(ICP-OES), Inductively coupled plasma mass spectrometry(ICP-MS), Laser ablation inductively coupled plasma mass spectrometer(LA-ICP-MS) and Uranium thorium lead method(U-Th-Pb). The result shows:1. The Syenites of Changling are Quartz syenite and aegirine syenite. Two types of syenites are fine-grained structure, massive structure. rock-forming minerals in aegirine-augite syenite are K-feldspar(70%), aegirine(10%), albite(3%), quartz(3%),arfredsonite(2%), biotite(1%). rock-forming minerals in quartz-syenite are K-feldspar(80%), quartz(10%),albite(4%), aegirine(3%), arfredsonite(2%)and biotite(1%).Accessory minerals are monazite, xenotime, ilmenite, magnetite, rutile, barite, apatite, titanite, zircon, calcium iron garnet stone, calcite.2. Aegirine-augite syenite and quartz-syenite are homologous evolution of the product, and the direction of the evolution of quartz syenite to aegirine syenite. The rocks have the characteristics of total quantity of REE is low,while the content of LREE is enrichment and the content of HREE is deficit, and the chondrite-normalized REE pattern is right deviation. Feature has enriched Th, U,Ba,Rb, K, loss of P, Ti and other elements.3. Zircon U-Pb age is 265 Ma, The area syenites are formed in the Hercynian, but the scale is small.4. The feldspar of syenite in Changling have higher Ba content, endless belt phenomenon is obvious. The biotite is iron black mica with the characteristics of crustal sources. The monazite as an accessory mineral is Ce monazite.The rutile contains Nb. The zircon forms in the magmatic environment. In the rock, the types of the accessory minerals are complex, and the component is generally rich in the crust source element such as Ba、K, which implies that the source area is the lower crust.5. When the magma in turbulent environment feldspar annulus formed. The temperature change range 693℃ to 713℃,Decide to separate crystallization, formed in a partial tension environment.6. The alkaline rocks forms along the second-order faults, due to the partial tension effect makes the earth’s crust cracking, pressure decreases, and part of the mantle material upwelling, partial melting in the lower crust material.