Geochemistry and geochronology of Late Triassic volcanic rocks in the Chiang Khong region, northern Thailand

摘要

The Chiang Khong segment of the Chiang Khong–Lampang–Tak Volcanic Belt is composed of three broadly meridional sub-belts of mafic to felsic volcanic, volcaniclastic,and associated intrusive rocks. Associated sedimentary rocks are largely non-marine red beds and conglomerates. Three representative Chiang Khong lavas have Late Triassic (223–220 Ma) laser ablation inductively coupled mass-spectroscopy U–Pb zircon ages.Felsic-dominated sequences in the Chiang Khong Western and Central Sub-belts are high-K calc–alkaline rocks that range from basaltic to dominant felsic lavas with rare mafic dykes. TheWestern Sub-belt lavas have slightly lower high field strength element contents at all fractionation levels than equivalent rocks from the Central Sub-belt. In contrast, the Eastern Sub-belt is dominated by mafic lavas and dykes with compositions transitional between E-mid-oceanic ridge basalt and back-arc basin basalts. The Eastern Sub-belt rocks have higher FeO* and TiO2 and less light rare earth element enrichment than basalts in the high-K sequences. Basaltic and doleritic dykes in theWestern and Central sub-belts match the composition of the Eastern Sub-belt lavas and dykes. A recent geochemical study of the Chiang Khong rocks concluded that they were erupted in a continental margin volcanic arc setting. However, based on the dominance of felsic lavas and the mainly non-marine associated sediments, we propose an alternative origin, in a post-collisional extensional setting. A major late Middle to early Late Triassic collisional orogenic event is well documented in northern Thailand and Yunnan. We believe that the paucity of radiometric dates for arc-like lavas in the Chiang Khong–Lampang–Tak Volcanic Belt that precede this orogenic event, coupled with the geochemistry of the Chiang Khong rocks,and strong compositional analogies with other post-collisional magmatic suites, are features that are more typical of volcanic belts formed in a rapidly evolving post-collisional, basin-and range-type extensional setting.