The Petrogenesis,geochemistry And Genesis Of Volcanic Rock In Zhujiayuan Of Pingli County, Shaanxi Province

South Q inling-Northern Daba Mountain as an important region of the Q inling belt, have attracted many scholars’ attention at home and abroad, because of its long and complex evolution history. However, different geologists hold different perspectives on the petrogenesis and tectonic background. This paper takes the volcanic rock of Zhujiayuan in Pingli County, Shaanxi province as the research object. By means of field geological survey, polarizing microscope, electron probe, ICP-MS analysis methods, such as the petrography, mineralogy and rock geochemical aspects are studied, as well as, the causes and tectonic environment ha ve carried on the preliminary discussion. The results are as follows:1. The volcanic rocks in Zhujiayuan include biotite trachyte and quartz trachyte. They are all fine- grained cryptocrystalline texture, trachytic texture and massive structure. The main rock-forming minerals are K-feldspar(40%~50%), albite(40%~45%), biotite(3%~10%), and a small amount of quartz and hornblende. The accessory minerals are monazite, allanite, aeschynite, rutile, ilmenite, apatite, and a small quantity of sphene, zircon and calcite.2. The two types of trachyte in Zhujiayuan are the products of the comagmatic evolution. The sequence of magmatic evolution is from biotite trachyte to quartz trachyte. The rocks are characterized by high contents of total REE, rich LREE, and depleted HREE, and the REE distributive patterns of the rocks showe steep right dip due to the clear differentiation between LREE and HREE; The characteristics of trace elements is rich in high field strength elements of Nb、Ta、Zr、Hf, but poor in Sr、P、Ti.3. In the trachytes in Zhujiayuan, the biotite is magnesian biotite, formed in the orogenic background. The accessory mineral monazite is featured with high Ce and magmatic origin. The aeschynite is N ioboaeschynite of alkaline rock. The rutile contains Nb. The ilmenite is Mn- ilmenite. The zircon formed in the alkaline rock contains Nb. The accessory minerals are diversity, but they generally contain the crust elements. It implies the rocks are associated with the components of its original rocks.4. The magma of trachytes in Zhujiayuan derives from the upper crust or crust- mantle transitional region. During the process of the formation of trachyte evolution, the fractional crystallization plays a main role on. The trachytes in Zhujiayuan should be formed in a post-collisional orogenic environment. It is associated with the ultrabasic-basic dykes of Ziyang and Langao County. They are all limited by the fracture of the region.5. The trachytes in Zhujiayuan have great mineralization potential of rare and rare elements.