Petrology,geochemistry And Zircom U-Pb Chronology Of The Chazangcuo Granite Within Shenzha County,Tibet

The Chazangcuo Granite within Shenzha County in Tibet is located in the northern part of the Gangdise orogenic belt.Its tectonic structure belongs to the southeastern margin of the Cuoqin-Shenzha volcanic magmatic arc,the secondary tectonic unit of the Gangdise-Nianqing Tanggula plate.The Chazangcuo granite body is distributed in NE direction,with a maximum length of 7 Km in N-S direction and a maximum width of 6 km in E-W direction.Its outcropping area is about 40 Km~2,and it is horseshoe-shaped.The outcropping area of the granite body is relatively large and its lithology is relatively single.It is mainly composed of alkali feldspar granite and biotite monzonite,with transitional contact relationship between facies zones.Alkali feldspar granite is light incarnadine,medium fine-grained and porphyry-like-structured.Most of phenocrysts are quartz and striped feldspar,while a small numbers of them are orthoclase phenocrysts.With medium-fine-grained granite structure,the biotite monzonite granite is grayish-white or grayish-black.The main mineral components are potash feldspar,plagioclase,quartz and some dark minerals.The main and trace element analysis of the whole rock shows that the alkali feldspar granite in the study area is rich in alkali,high silicon and poor in magnesium and manganese,weak peraluminous subalkaline granite,biotite monzogranite is rich in alkali,high silicon and poor in magnesium,and weak peraluminous subalkaline granite.The variation trend of biotite monzogranite samples is consistent,they are all light rare earth enriched,heavy rare earth depleted,large ion lithophile elements Rb,Th and light rare earth elements enriched,Zr,Ho and other high field strength elements depleted strongly.In addition,biotite monzogranite is rich in Pb element,which shows the difference in Zr element.Alkali feldspar granites are characterized by right tilt distribution of light rare earth enrichment and heavy rare earth depletion.Large ion lithophile elements Rb,Th and light rare earth elements are relatively enriched,and high field strength elements such as Zr and Ho are relatively deficient.It can also be seen that alkali feldspar granite shows differentiation and enrichment of Pb element in Zr element.The study shows that Chazangcuo granite body belongs to I-type granite with high differentiation.According to the discriminant map of the comprehensive element tectonic environment,the Chazangcuo granite body intrudes into the same collisional tectonic environment,which indicates that the Late Triassic Chazangcuo area experienced the process of collisional tectonic movement.The age data of zircon U-Pb dating by LA-ICP-MS is 204.0±1.3 Ma for alkali feldspar granite and 203.5±1.4 Ma for biotite monzogranite.The age of alkali feldspar granite and biotite monzogranite is Late Triassic,which indicates that the emplacement age of Chazangcuo granite body magma should be Late Triassic.According to the genetic type,tectonic environment and chronological data of Chazangcuo granitic body,it can be inferred that in the Late Triassic,the Chazangcuo area experienced syn collisional tectonic movement,resulting in crustal uplift,structural fracture,magma intrusion and cooling crystallization to form the Chazangcuo granitic body.