Magmatic Evolution In Weixi Area,Yunnan Province

In this study,the magmatic rock of Jicha complex in Weixi area is taken as the research object,and the detailed field geological investigation,indoor mineralogy and petrology research are carried out.By means of LA-ICP-MS,zircon Hf isotopic testing,whole rock major and trace element analysis,the restriction of arc volcanic rocks on the subduction and reduction of Lancang ocean crust in Weixi area is discussed.Meanwhile,the time-space evolution framework of volcanic rocks in Weixi area is established,the change of tectonic environment of volcanic rocks is discussed,and the restriction significance of volcanic rocks on arc land collision is revealed.The following achievements have been achieved:?1?The discovery of the Neoproterozoic?703Ma?sample reveals that there are Neoproterozoic basement in the west of Weixi area.The tectonic environment is determined to be continental plate by geochemical study.The U-Pb age?2119Ma-2156Ma?of the inherited zircon is similar to the basement of the Yangtze ancient land,suggesting that the western margin of the Yangtze ancient land is to the west of the middle section of Lancangjiang suture zone.?2?Based on the analysis of zircon CL image and zircon U-Pb dataing,it is concluded that the average age?297Ma?of a group of enclaves in the metamorphic peridotite sample can be used as the formation age of rocks.The contact relationship between rock mass and surrounding rock structure and geochemical mapping results show that Jicha rock mass is more likely to be part of ophiolite than Alaskan rock mass,it is supported that the expansion of the Jinshajiang and Lancangjiang basins was in Late Carboniferous.?3?The characteristics of sedimentary Intercalations in the volcanic sequence in Jicha area are observed in the field.The geochemical study and zircon dating of mylonitized rhyolite samples confirm that the acidic volcanic rocks in this area belong to Jiangda Weixi arc volcanic rocks,and the whole arc volcanic rock belt is in the similar sedimentary environment of fore arc or in arc basin.Compared with the previous Hf isotopic study of the Jiangda Weixi arc volcanic rocks,the positive value of?_Hf?t?of the arc volcanic rocks in Weixi area is relatively small.This study supports the preliminary cessation of subduction in Weixi area was at about 228Ma.?4?A late Jurassic gabbro intrusion event was recorded on the west side of the Lancangjiang suture zone.The geochemical study is consistent with the stratigraphic record.In the late Jurassic,the basic rocks were formed in the shallow marine oceanic crust environment on the west edge of the Lancangjiang suture zone.?5?Four magmatic events occurred in the Weixi area of the middle section of the Lancangjiang suture zone were combed,and the key time nodes of land and sea changes from Neoproterozoic to Jurassic in Weixi area were recorded,which provided materials for future generations to determine the edge and nature of the Lancangjiang suture zone.