Geochemical Characteristics,Ages And Geological Significance Of Triassic Chalageng Composite Granite In The Lancai Area,Tongren County Of The West Qinling Orogenic Belt

The Chalageng composite granite is distributed in the Lancai area,Tongren County of southeastern Qinghai province,with an area of about 200km2 and a tectonic position in the west Qinling orogenic belt,the petrological,geochemical and isotopic geochronology of the granite will provide important information for Mesozoic tectonic magmatic evolution and regional Hot-Dry-Rock exploration in the west Qinling orogenic belt.Through the field investigation,the Calageng composite granite intrusion into the lower middle Triassic series Longwuhe group and the middle Triassic series Rinaore Group,which consisted of monzonitic granite,porphyry monzonitic granite,potash feldspar granite and porphyry potash feldspar granite,with pulsant contact between the granite bodies.By Zircon LA-ICP-MS U-Pb dating,monzonitic granite U-Pb weighted average age is 220.5±2.7Ma,the porphyry monzonitic granite U-Pb weighted average age is 218.9±2.4Ma,the potash feldspar granite U-Pb weighted average age is 213.5 ± 2.8Ma and the U-Pb weighted average age of the porphyry potash feldspar granite are 210.5 ±2.3Ma,211.2±2.2Ma,determine the period of emplacement is the late Triassic epoch.The results of rock chemical analysis showed that the SiO2content of Calageng composite granite is 61.31-77.99%,total alkali K2O+Na2O content 7.29-9.85%,Rittman index ?=1.64-3.57,Aluminum Supersaturation index A/CNK=0.99-1.19,the K2O/Na2O is 0.99-2.51,which belongs to the calcium-alkaline over aluminum acid granite;the composite curve of rare earth elements of the Calageng composite granite is ? type,which has the characteristics of light REE enrichment,heavy REE loss,and?Eu are negative anomalies,from monzonitic granite to porphyry monzonitic granite to potassium feldspar granite to porphyry potassium feldspar granite,the ?Eu loss degree has an increasing tendency,which reflects the characteristics of magma evolution has undergone obvious crystallization differentiation.The results of trace elements analysis showed that the Calageng composite granite was characterized by a large ionic nucleophilic element Ba,Sr,Nb,P,Ti loss and high field strength elements U,Th,Zr,Hf and other elements of enrichment,trace elements of the original mantle standardization cobweb map to the right type,Nb loss may imply that the crust material involved in magma evolution.Tectonic environment discrimination showed that in the Nb-SiO2 discriminant diagram,the Calageng composite granite basically falls into the range of I type granite,and a few samples fall outside the range of A type and I type granite,which may be related to the characteristics of crust-mantle mixing in the source magma of the region;in the Rb/30-Hf-3Ta discriminant diagram,the Calageng composite granite falls into the plate and collides with the tectonic background environment,which may be formed in the two tectonic environments after the collision and collision;TFeO/[w(TFeO)+w(MgO)]&SiO2 diagram,R1-R2 granite Genesis classification diagram and Nb-Y discriminant diagram show,the Calageng composite granite is an arc-collision-after the collisional granite tectonic environment.Sum,combined with regional geological background analysis of the friction and tension of the Calageng composite granite formed in the central orogenic belt after the crustal thickening effect of the lithosphere of the tectonic environment.The content of U,Th and K of35 rock samples tested by the ICP-MS showed that Th content 25.50-43.1ppm,U content 3.35-37.1ppm,K content 2.93-6.31%,by microscope observation and electron microprobe spectroscopy analysis,the existence of uranium thorium-stone single minerals was found in the porphyry potassium feldspar granite,it is shown that the porphyry potassium feldspar granite in the Calageng composite granite is a high radioactive granite,the rock radioactive element U,Th and K content are 2-5 times of the average in world granite,and its radioactive heating production rate is higher,therefore,the Calageng composite granite high U,Th,K element content has important geological significance in searching for U and Th radioactive deposit,and the high radioactive heating production rate of the rock may provide the main heat source for the formation of hot springs in this area,and provide important information for finding the radioactive decay type Hot-Dry-Rock in this area.