Petrological and geochemical characteristics,Ar-Ar geochronology study and their tectonic significance of Triassic volcanic rocks in southern Lancangjiang zone

Triassic volcanic rocks are widespread in southern Lancangjiang zone of Sanjiang area,Middle-Late Triassic volcanic sequences exposed in full in Yunxian area in the north of southern Lancangjiang zone,can be divided into three formations as Middle Triassic Manghuai Formation(T2m),Late Triassic Xiaodingxi Formation(T3x) and Late Triassic Manghuihe Formation(T3mh) from the bottom to up,Acid volcanics mainly erupted in Manghuai Formation,formed a rhyolite and pyroclastic rock assemblage,Xiaodingxi Formation developed basic-intermediate lavas mixed with pyroclastic rocks,Rhyolite pyroclastic rocks and basalt coexist in Manghuihe Formation,showed "bimodal" volcanic features,Geochemical characteristics indicate that Triassic volcanic rocks from southern Lancangjiang zone have dual characteristics of arc volcanic rocks and continental intraplate volcanic rocks,suggesting that their formation environment is transition-type continental margin orogenic belts,Ar-Ar dating is carried on quartz andesite collected from Jinghong in the south of southern Lancangjiang zone and yields the plateau age of 236.7±2.2Ma,which belong to Middle Triassic,With the dating results,the main collision stage of Lancang River is inferred in Early Triassic,Middle Triassic and early Late Triassic are respectively the stress relaxation stage after collision and continued subduction stage of oceanic basin,Until the end of Late Triassic,subduction ended,Lancang River Ocean closed.