Genesis,Geological Significance Of Granitic Rock In Quandongshan-mazhuangshan,Gansu Province

The Beishan orogenic belt has undergone the collision and closure of paleo-oceanic crust,causing extensive magmatism and mineralization.Located in the north of the Beishan orogenic belt,Quandongshan-Mazhuangshan widely developed granitic rock,Which provide reliable material records for the Beishan orogeny,and provide a basis for further understanding the relationship between its formation and the closure of paleo-oceanic basins and the genetic relationship with gold deposits.Based on systematic analysis about petrology,petrography,geochemistry and the U-Pb ages of zircon of granitic rock in the Quandongshan-Mazhuangshan area,the paper not only place constriants on physicochemical conditions of the formation of mineral and the host rock,he type of rocks and the characteristics of their genesis,but also Provide important insights into understanding Beishan orogenic process.The geochemical characteristics of the rocks indicate that the granitic rock is rich in silicon,potassium and depleted iron,phosphorus and titanium,belonging to metaluminous to weakly peraluminous granitic rock of high Potassium and calc-alkaline series.The feldspar in the monzogranite are albite and orthoclase,while in the granodiorite is andesine.Electron microprobe analysis(EMPA)shows that the biotites is rich in iron,biotites chemistry analysis reveals that the crystallization temperature for biotites is 709~749? and the solidification pressure is 113.85~156.67 MPa,corresponding to solidification depth is 4.3~5.9 km,and its oxygen fugacity is-14.2~-11.5;The hornblende in the rock are pargasite,the chemical characteristics of the pargasite are rich in Calcium,magnesium,silicate and depleted alkali.The granitic rock of this area have a high rare earth gross,and a right-hand curve distribution modle with LREE enrichment.The main characteristics of rare earth elements of monzogranite is obviously negative Eu anomaly,indicating the monzogranite of this area have high degree of magmatic differentiation.Trace element analysis show that the rock mass concentration of K,Rb,Ba,and other large ion lithophile elements,relatively depletion P,Nb,Ti and other high field strength elements.Characteristics of trace element indicate that the source region is subjected to intense crustal assimilation and contamination.The zircon U-Pb age of granitic rock are 317.2 ±1.3Ma(MSWD=0.54)and 309.3 ±1.4Ma(MSWD=0.053).They are formed before Mineralization.These features suggest that the granitic rock is typical I-type granitic rock and it derived from a crust-mantle mixing source.the granitoids biotites composition suggest that granitic rock is formed in a relativeby high temperature and oxygen fugacity environmente.The high-K calc-alkaline and esite-dacite-rhyolite suite and highly fractionated I-type granitic rock indicate the granitic rock formed from mature crust.Considering the rock association and the reach results of discrimination diagram for tectonic settings,we speculate that the granitic rock formed in post-collision extrusion to stretch transformation tectonic setting.According to high oxygen fugacity of Granitic rock and the characteristics of deep source sulfur isotopes of the Mazhuangshan gold deposit,the origin of hydrothermal has close genetic relationship with magmatic activities.The comprehensive analysis shows that there is a good potential for mineralization in the Nanpozi-Mazhuangshan area.