Petrogenesis And Tectonic Significance Of Triassic Volcanic Rocks From The Pantiange Area,Western Yunnan

The Sanjiang orogenic belt,located in the southeastern margin of the Qinghai–Tibet Tibet Plateau,belongs to the eastern part of the Tethys tectonic domain,which records the Tethyan tectonic evolution process completely and is an ideal region to study the Paleo-Tethys evolution.During the period of late Paleozoic to early Mesozoic,strong tectonic orogenic movement occurred in the Sanjiang area,which resulted in large-scale magma intrusion and eruption and formed a number of arc magmatic belts.However,the complicated evolution history of the multi-stage ocean basins blurred the detailed and specific evolution process in the Sanjiang area,and it made the explanation of the specific tectonic origin of some magmatic arc still controversial.The Jiangda-Weixi magmatic belt,located in the eastern margin of the North Qiangtang block,was formed during the late Paleozoic to early Mesozoic.The cause of the belt is still controversial.Some scholars believe that the arc magmatic belt was formed by the westward subduction of the Jinsha River Ocean and subsequent continental collision while the other researchers believe that the arc magmatic belt is formed by the eastward subduction of the main ocean basin of Paleo-Tethys along the ancient Tethys suture zone of the Longmucuo-Shuanghu-Lancangjiang-ChangningMenglian.The Jiangda-Wixi magmatic belt in the Pantiange area is composed of the Pantiange Formation and Cuiyibi Formation of the late Permian-early Middle Triassic.In this paper,the two sets of rock assemblages of Pantiange area in the northwestern of Weixi are selected as the research objects.Based on the geological mapping of 1/50,000,the rock assemblages and their developmental characteristics are found out through detailed field geological survey and laboratory study.On the strength of the comprehensive study of petrology,geochronology and geochemistry,the genesis and tectonic significance of rocks are discussed and a reliable constraint for the tectonic attribution of the Jiangda-Weixi magmatic belt is provided.Field measured section in the Pantiange area shows that the lithology of the Pantiange Formation is rhyolite with tuff,ignimbrite,argillaceous rock and a small amount of carbonate rock and the upper Cuibiyi Formation is made up by volcanic rock with clastic rock and a small amount of limestone.The volcanic rocks from the bottom to the top are characterized by a transition from intermediate(the Member 1)to intermediate acid(the Member 2).Two samples of volcanic rocks from the Cuibiyi Formation were collected and carried out zircon U-Pb dating analysis at 245~242 Ma,which belongs to the Middle Triassic.The petrogeochemical results show that the andesitic rocks of the Cuibiyi Formation belong to high-potassium calc-alkaline/shoshonitic series and has the characteristics of LREE enrichment and heavy REE depletion in the REE distribution diagram and the large ion lithophile elements enrichment,and the high field strength elements depletion in the trace element spider diagram.It is considered that the andesitic rocks of the Cuibiyi Formation were formed by the combination of crystallization differentiation and assimilation of mantle-derived basaltic magma.Based on the regional magmatic data,it is suggested that the Pantiange Formation and Cuibiyi Formation were formed in the continental margin arc environment related to the eastward subduction of the Longmucuo-Shuanghu-Lan cangjiang River Ocean.