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

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Abstract

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. The Western 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 the Western 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.

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