Characteristics And Geological Significance Of Middle Basic Magmatic Rocks In The Kagong Area Of The North Lancangjiang Tectonic Belt

The Kagong area is located in the north section of Lancang River,the east section of Tethys structural domain,the junction of Gondwana and Lauria?Yangtze block?.As an important dividing line between the division of tectonic units and the study of plates in the Qinghai Tibet Plateau,Lancang River structural belt is one of the ideal areas for studying the evolution of East Tethys.Due to the precipitous terrain,inconvenient transportation and shallow research in the eastern Tibet,people's understanding of the north extension of Lancangjiang structural belt and its formation and evolution is affected.In this paper,the petrology,whole rock geochemistry and zircon U-Pb geochronology of the middle basic magmatic rocks exposed in the Kagong area of the North Lancangjiang structural belt are studied to explore the tectonic environment and geological significance of their formation.In this paper,we have studied the intermediate intrusive rocks?the granodiorite in mori Village and the quartz diorite in jinduo Village?and the crystal tuff in the migmatite of the karong rock formation.The Si O₂content of crystalline tuff is31.61%?49.37%,Ca O content is 1.09%?21.3%,Na₂O content is 0.70%?3.66%,K₂O content is 0.29%?3.38%,total alkali content?Na₂O+K₂O?is 3.56%?6.04%,and the rocks are characterized by calc alkaline tholeiite.The content of Si O₂in diorite is 57.48%?60.83%,and the content of Al₂O₃,Na₂O,Ca O and K₂O is higher,while the content of K₂O is lower.The total alkali content?Na₂O+K₂O?is 6.36%?7.89%,showing the characteristics of high alkali.K₂O/Na₂O value is 0.27-0.50,A/CNK value is 0.77-1.01.The Riemann index???of diorite is between 2.09?3.43,which is calc alkaline quasi alumina weak peraluminous diorite.In the REE distribution map,both of them have relative enrichment of LREE and relative depletion of HREE.The crystal tuff is relatively deficient in Rb?Sr and other large ion lithophile elements,and relatively enriched in Th?U and other high field strength elements.Diorite is relatively rich in Rb?K and other large ion lithophile elements;while Nb?Ta?Hf?Ti and other high field strength elements are relatively deficient.The zircon U-Pb age of the granodiorite in mori Village is 255.2±1.8Ma,and the age is late Permian.The geochemical characteristics of the rocks show that the geochemical characteristics of crystalline tuff are similar to those of seamount basalt,and they have the characteristics of oceanic island basalt.The combination of crystalline tuff and marble has the characteristics of oceanic island seamount combination,Therefore,the crystalline tuff is formed in the oceanic environment.The two types of diorite in the Kagong area are characterized by crust mantle mixing and formed in a post collisional environment.According to the comprehensive data,it is believed that the presence of the North Lancangjiang ocean is indicated by the crystalline tuff of the Kagong formation,while the diorite indicate the process from the closing of the North Lancangjiang ocean at the end of the Permian to the collisional subduction orogeny of the Triassic,which provides new data for the north extension of the Lancangjiang structural belt and its formation and evolution.