Abstract:

Triassic magmatism widely developed in the southern Lancangjiang zone, southwestern China. The study of their precise UPb geochronology and characteristics of rock associations shows that (1) small amount of EarlyTriassic (~248 Ma) volcanic rocks were preserved and cropped out in the area, which consist mainly of thin bed of andesitic rocks intercalated with minor basaltic andesites; that (2) the MiddleTriassic magmatism was characterized by extensive felsic igneous rocks with minor basalts interbedded in the middle of volcanic sections in part of areas, and the late MiddleTriassic rhyolites and main body of Lincang granitic batholith have the same extrusive/intrusive time with the age of ~230 Ma; that (3) the LateTriassic volcanic sequences are principally composed of basalts with minor interlayered rhyolites, exhibiting typical characteristics of bimodal volcanics. In addition, the features of elemental geochemistry of all samples suggest that (1) the EarlyTriassic andesitic rocks, characterized by characteristic natures of island arc volcanics, likely erupted in the subduction setting; that (2) the generation of LateTriassic rhyolites with an affinity of Alike granite and synchronously main body granites in the Lincang batholith displaying similar geochemical features, is closely related with the postcollision or latecollision setting; that (3) although the LateTriassic basic rocks partly show island arc volcanics such as highAl contents and NbTa depletion, some of them possessing highMg contents and mostly having traceelement ratios similar to intraplate basalt, which indicates that they erupted in the postcollision rift environment. Integrated with previous data in the study area, we conclude that the subduction and closing of main PaleoTethys Ocean (ChangningMenglian Ocean) in the Sanjiang area of southwestern China probably had completed until the late EarlyTriassic, and the succeeding continentcontinent/arc collision and postcollision rifting developed in the MiddleTriassic and LateTriassic, respectively, and finally the evolution of PaleoTethys finished in the end of the LateTriassic.

Key words:  Triassic magmatism; island arc; postcollision; southern Lancangjiang zone; PaleoTethys