Geochronology,Geochemical Characteristics Of Triassic Granites In The Gouli Area,Dulan County,Qinghai Province,and Their Geological Significance

The East Kunlun orogenic belt is located in the northern margin of the Tibetan Plateau,adjacent to the Qaidam Basin in the north,the Bayankhara orogenic belt in the south,bounded by the Altun fault in the west and the West Qinling orogenic belt in the east.The region has experienced several tectonic movements,including Protethian evolution(Late Proterozoic to Early Paleozoic)and Paleo Tethian evolution(Late Paleozoic to Early Mesozoic)since Phanerozoic.Complex tectonic movements led to frequent magmatic activities and formed a large number of magmatic rocks,especially granitic rocks in Indosinian period.Since there are still great disputes on the tectonic evolution and magmatic source properties of East Kunlun area,this paper selects the Triassic granodiorite and monzogranite in Gouli area as the research objects.Based on the field geological survey,Thin section identification,principal and trace components testing,LA-ICP-MS zircon U-Pb dating,zircon Lu-Hf isotope tracing and biotica electron probe analysis were carried out to discuss the genesis,provenance properties,tectonic setting and geological significance of these two granites combined with previous research results.The petrographic study suggests that the two granites are finally named light gray medium-fine granodiorite and light flesh red medium-coarse-grained monzonitic granite.The U-Pb ages of granodiorite and monzonitic granite zircon LA-ICP-MS in Gouli area are 247.8±2.0Ma and 237.3±1.7Ma,respectively,and they were formed in Early and Middle Triassic,respectively.The major elements of the whole rock show that granodiorite is quasi-aluminous calc-alkaline rock and monzonitic granite is quasi-aluminous high-k calc-alkaline rock.According to the REE analysis,both granites are relatively enriched in light REE and depleted in heavy REE,and europium has a negative anomaly(δEu=0.78~0.90).According to the results of trace elements,the two granites are relatively enriched in Ba,K,Th,Zr,Hf and deficient in U,Ta,Nb,S and both have the characteristics of type I granites.The average oxygen fugacity of monzonitic granite biotite is estimated to be 10-14 bar,the crystallization temperature is715℃~745℃,the pressure is 106~140MPa,and the emplacement depth is 4.0~5.3km.Rb/Sr ratios of monzonitic granite and granodiorite samples range from 0.04 to0.12,and La/Nb ratios range from 0.69 to 3.04,both of which are between the mean values of crust and mantle.Lu-Hf isotope results of zircon show εHf(t)values in the range of-5.82 to-2.26.The second-stage Hf model age is 1411~1643Ma,belonging to the Mesoproterozoic era.Combined with the biotite electron probe test results,it is concluded that the sample magma originated from the partial melting of the lower crust basic rocks in the mesoproterozoic era,and a small amount of mantle source material was involved.The granodiorite was formed in the tectonic environment of late subduction of oceanic crust,which defined that the Paleo-Tethys Ocean was not closed at 247 Ma.The monzonitic granite was formed in a collisional orogenic environment,indicating that the Paleo-Tethys Ocean had ended subduction and entered the syncollisional stage at237 Ma.They provide magmatic evidence for the evolution of the Paleo-Tethys Ocean during the Triassic.The formation of gold deposits in Gouli area is closely related to the Triassic granite both in time and in space.The magmatic activity of Triassic provides heat source and ore-forming materials for the formation of gold deposits.The evolution of Paleo-Tethys provides favorable tectonic conditions for the formation of orogenic gold deposits such as Walega and Ashar.