Petrological and Geochemical Characteristics,Ar-Ar Geochronology Study and Their Tectonic Significance of Triassic Volcanic Rocks in Southern Lancang River Zone
Triassic volcanic rocks are widespread in southern LancangRiver 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. Geochemical characteristics indicate that Triassic volcanic rocks from southern LancangRiver 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. With the dating results, the main collision stage of LancangRiver 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, LancangRiverOcean closed.