Chronology,Geochemistry And Tectonic Setting Of Late Triassic Igneous Rocks In Ageteng Area,Qimantage,Qinghai

The Ageteng area is located in the western part of the East Kunlun orogenic belt,the combination of Qaidam block and Middle East Kunlun block,which belongs to the Qimantage-Dulan Variscan iron,cobalt,copper,lead,zinc,tin,wollastonite?antimony,bismuth?metallogenic belt.The area has experienced multi-cycle multi-stage orogenic movements,with faulted tectonic development and strong magmatic activity.There are different degrees of magmatic activity from the Caledonian,Variscan,to Indosinian periods,among which the Indosinian period is the strongest.A lot of research work has been done on the Indosinian rock mass in the region,but the research on the geochemistry and chronology of the Late Triassic igneous rocks in the Ageteng area is relatively weak and no corresponding article has been published as well as many controversies about the closure time of Paleo-Tethys Ocean.Therefore,this stady took the Late Triassic igneous rocks in the Ageteng area as the research object,carried out LA-MC-IPC-MS zircon U-Pb chronology and elemental geochemical research.Petrogenesis was analyzed,and the diagenetic tectonic background was inferred in order to provide constraints for the tectonic evolution of the East Kunlun orogenic belt.The volcanic rocks in the Ageteng area are mainly the volcanic breccias,breccia tuff,dacite and rhyolite of the Elashan Formation,and the intrusive rocks are mainly syenite,porphyritic monzogranite,monzogranite and granodiorite.The zircon U-Pb dating results of rhyolite and granite were 215.37±0.56Ma,220.75±0.57Ma,220.71±0.47Ma,and220.6±1.4Ma,respectively,indicating that the formation age was Late Triassic.The results of rock geochemical analysis showed that the SiO₂ content of rhyolite in Ageteng area was 70%⁷7.37%,Na₂O=2.45%⁴.08%,K₂O=4.25%⁴.58%,K₂O/Na₂O average was 1.38,Al₂O₃=11.41%¹4.55%,the aluminum saturation index A/CNK was between 0.98 and 1.35.The SiO₂ content of granite in Ageteng area was between 66.46%and 75.42%,the content of Na₂O was 2.45%⁴.74%,K₂O=3.08%⁵.15%,the average value of K₂O/Na₂O was 1.16,Al₂O₃=12.65%¹5.73%,the index A/CNK was between0.95 and 1.2.The rhyolite and granite in the Ageteng area belonged to the high-potassium-calcium-alkaline series,which were quasi-aluminum to weakly peraluminous rocks.The distribution patterns of rare earth elements were basically the same,and the distribution curve was characterized by right-dip light rare earth enrichment with obvious negative europium anomaly.In terms of trace elements,they had similar distribution curves on the original mantle standardized cobweb diagram.Both of them were enriched with large ion lithophile elements such as Rb,Th,K and light rare earth elements,with obvious loss of Nb,Ta,P,Ti and other elements with high field strength.The rhyolite ¹⁴³Nd/¹⁴⁴Nd was 0.512294,?_Nd?t?=-4.63,T_DM 1405.46Ma;granite¹⁴³Nd/¹⁴⁴Nd=0.512277,?_Nd?t?=-4.66,T_DM was 1308.23Ma.Both of them showed very similar Nd isotopic characteristics,indicating that they were mainly derived from the remelting of ancient crustal materials and similar characteristics in terms of Pb isotopes.The Late Triassic granites in the Ageteng area were closely related to the spatial distribution of rhyolites,and the formation time was similar.The rare earths and trace elements,Nd isotopes and Pb isotopes had similarities or consistency.Therefore,they were homogenous magma products in the same period and the tectonic environment.The Late Triassic igneous rocks in the Ageteng area were rich in large ion lithophile elements such as Rb,Th,K,and deficient in high field strength elements Nb,Ta,P,Ti.Zr/Y=4.49¹2.95,which were basically within the range of continental margin arc.Combined with the geological background of the area,the author believes that it was formed in the arc environment of the active continental margin.The fluid was produced by dewatering of the subduction plate with water-rich,large-ionic lithophile elements,and the loss of high-field-strength elements.The fluid metasomatism the overlying ground wedge to cause partial melting,and the formed magma kept upwelling,bringing enough heat to partially melt of the crust material,and eventually formed the igneous rock in Ageteng area.The Late Triassic igneous rocks in the Ageteng area had obvious subduction magmatic characteristics from the geochemical characteristics and the degree of crustal thickening in the area,indicating that the continent-continent collision had not yet begun.Therefore,the author believes that the Paleo-Tethys Ocean has not yet complete closed in this period.