The Petrology And Litho-geochemistry Characteristics Of The Volcanic Rocks In Mts.mangkangshan Area And Its Significance,East Tibet

The volcanic rocks in the Mts.Mangkangshan area are exposed to the Changdu block in the east of Tibet,which is the northern part of the Sanjiang tectonic belt.The paper discusses the mineral assemblage of rocks and its structure and texture by studying the petrological characteristics of volcanic rocks in Mts.Mangkangshan area;The geochemical characteristics of rocks are discussed through the abnormal distribution of chemical constituents and trace rare earth elements;The formation time of rock in the Mts.Mangkangshan area was determined by the zircon U-Pb dating.Finally,the combination feature of volcanic rocks is summed up,the volcanic rock facies are determined,the volcanic structures and texture are speculated,and the tectonic setting of the volcanic rocks at that time is conjectured in the Mts.Mangkangshan area.In this paper,the age of volcanic rocks by derived date from the LA-ICP-MS zircon U-Pb isotopic dating of volcanic rocks in the Mts.Mangkangshan area is obtained.The age of the Lawula formation is 32.1Ma;The age of the Malasongduo formation is239.5-248.8Ma;The age of the Xiayacun formation is 257.8Ma.In addition to Lawula formation,other volcanic rocks were classified in the same time as the previous ones.The study of petrology of volcanic rocks in the Mts.Mangkangshan area show that The Lawula formation is weak volcanic eruption facies and overflow facies;the lower part of the Malasongduo formation volcanic rock is explosive facies with strong explosive characteristics,and the upper part is the overflow facies dominated by spillway,and The lower part of the Xiayacun formation volcanic rocks is an eruption phase dominated by volcanic clastic rocks,and the middle part is a spillway with lava mainly in the middle,and the upper part is a spillway and a weak explosive phase with the lava and breccia at the upper part.The study of geochemical characteristics of volcanic rocks in the Mts.Mangkangshan area show that the rock types of volcanic rocks in the Lawula formation are potassium basaltic series;It is a unique rich K series for orogenic belts;Combined with the characteristics of rock assemblage,it is speculated that the erupting environment is terrestrial orogenic environment.The rock series of the Malasongduo formation volcanic rocks are high potassium calc alkaline series;In trace elements,LILE showed enrichment and HFSE showed a deficit;The contents of light rare earth elements in rare earth elements are obviously greater than those of heavy rare earth elements,and heavy rare earth fractionation is obvious;It shows that it is formed in the volcanic arc of extruding environment.Most of the Xiayacun formation volcanic rocks in the K₂O-SiO₂ diagram are in the calcium alkaline series and the high potassium calcium alkaline series;Combined with volcanic facies and its trace and rare earth characteristics,it is speculated that it is the product of island arc or continental margin environment.The late Permian,Changdu-Mangkang block is affected by the jinshajiang ocean basin subduction,ocean basin closed.until the Triassic,Jinshajiang ocean basin closed completely.In the process to produce the Xiayacun formation,Malasongduo formation volcanic rocks;To Cenozoic period,the influence of the Eurasian plate and Indian plate,Changdu-Mangkang block had undergone an intracontinental orogeny in the tectonic environment of extrusion and strike-slip.