Petrochemistry, U-Pb (zircon) age, and palaeotectonic setting of the Lampang volcanic belt, northern Thailand

Two NE-trending belts of mainly subaerial dacitic to rhyolitic flows and tuffs occur in the area between the towns of Lampang and Denchai in northern Thailand. In the western belt (Doi Ton), the rocks have been pervasively altered to quartz keratophyre; rocks in the eastern belt (Doi Luang) are generally less altered. Mobile chemical components such as Na2O and K2O show wide variation, particularly in samples from the Doi Ton bell. However, low Zr/TiO2 ratios and low Nb and Ta contents support an origin at a convergent plate margin. A positive epsilon Nd value of +4.9 for rhyolite from the Doi Luang belt supports derivation from a primitive crustal source. A rhyolite sample from the Doi Luang belt yielded a U-Pb zircon age of 240 +/- 1 Ma (early Mid-Triassic). The Doi Ton and Doi Luang belts are part of the Lampang volcanic bell, which can be traced to the north into the Lincang-Jinghong volcanic belt in southern China. Comparison with published petrological data from the Lincang-Jinghong bell shows strong similarity, including the widespread development of keratophyric mineralogy and chemistry. The Lampang-Lincang-Jinghong belt formed at an early Mid-Triassic convergent plate margin, and is similar in age and tectonic setting to the more mafic Phetchabun volcanic belt on the east side of the Nan River suture zone, These data constrain the timing of final amalgamation between the Indochina and Shan-Thai terranes to Mid-Triassic or younger.