Petrological, Geochemical, Geochronological Characteristics And Their Tectonic Significance Of Triassic Volcanic Rocks In Southern Lancangjiang Zone

The volcanic rocks in southern Lancangjiang zone play an important role inthe development of Sanjiang Tethyan tectonic belt. It is essential to clarify theirformation ages, geochemical characteristics and other issues in revealing thecomplex history of Paleo‐Tethys tectonic evolution and the dynamic mechanism. Inthis paper, Triassic volcanic rocks in southern Lancangjiang zone were studied basedon petrological, geochemical and Ar‐Ar isotopical characteristics.Middle‐Late Triassic volcanic sequences exposed in full in Yunxian area in thenorth of southern Lancangjiang zone, can be divided into 3 formations as MiddleTriassic Manghuai Formation (T2m), Late Triassic Xiaodingxi Formation (T3x) and LateTriassic Manghuihe Formation (T3mh) from the bottom up. Acid volcanics mainlyerupted in Manghuai Formation, formed a rhyolite and pyroclastic rock assemblage;In Xiaodingxi Formation developed basic‐intermediate lavas mixed with pyroclasticrocks; Rhyolite pyroclastic rocks and basalt coexist in Manghuihe Formation,showed"bimodal"volcanic features. The SiO2 of Triassic volcanic rocks from Yunxianrange 46.05%‐54.50% and 67.85%‐77.34%, lack of intermediate ones. The contentof Al2O3 is high, ranging 11.60%‐18.26%, averagely 14.79%. The TiO2 is relatively lowfrom 0.17% to 2.23%, averagely 0.87%. The K2O/Na2O ratio range 1.41‐2.21. TheREE content is between 80.20×10‐6 and 302.11×10‐6, with (La/Yb)N ratios of5.23‐22.00 (average 11.46). The chondrite‐normalized REE patterns show obviousright slanting, moderately enriched LREE and depleted HREE. Negative Euanomalies are also shown withδEu of 0.43‐1.08, averagely 0.71. The volcanic rocksgenerally enrich in Pb and Large Ion Lithophile Elements Rb, where Pb is stronglyenriched, and depleted in Sr and High Field Strength Elements Nb,Ta. Geochemicalcharacteristics and tectonic setting discrimination diagrams indicate that Triassicvolcanic rocks from southern Lancangjiang zone have dual characteristics of arcvolcanic rocks and continental intraplate volcanic rocks, suggesting that theirformation environment is transition‐type continental margin orogenic belts. Ar‐Ardating is carried on quartz andesite collected from Jinghong in the south ofsouthern Lancangjiang zone and yields the plateau age of 236.7±2.2 Ma, whichbelong to Middle Triassic. With the dating results, the main collision stage ofLancang River is inferred in Early Triassic. Middle Triassic and early Late Triassicare respectively the stress relaxation stage after collision and continued subductionstage of oceanic basin. Until the end of Late Triassic, subduction ended, LancangRiver Ocean closed.