Zircon U–Pb geochronology, and elemental and Sr–Nd–Hf–O isotopic geochemistry of post-collisional rhyolite in the Chiang Khong area, NW Thailand and implications for the melting of juvenile crust
Volcanic rocks are widely exposed within the Chiang Khong–Lampang–Tak igneous zone in NW Thailand. A representative rhyolite sample from the Chiang Khong area yielded a zircon U–Pb age of 230.7 ± 1.1 Ma (n = 20, MSWD = 0.98). The Chiang Khong rhyolites are characterized by low TiO2 (0.29–0.62 wt%) and MgO (0.04–0.82 wt%) with A/CNK values of 0.95–1.06 (one outlier at 1.47), and can be classified as highly fractionated I-type rhyolites. They are enriched in LILEs and LREEs, and depleted in HFSEs. Two representative samples have 87Sr/86Sr (i) ratios of 0.70497 and 0.70527, and the εNd (t) values fall at +1.1 and +1.3, respectively. εHf (t) and δ18O in zircon are between +4.7 to +12.0 and 5.3 to 5.9 ‰, respectively. Our geochemical data suggest that the Chiang Khong rhyolites formed by partial melting of juvenile mafic lower crust in a post-collisional setting. Deep crustal anatexis was probably induced by upwelling asthenospheric mantle, shortly after slab detachment subsequent to closure of the Paleo-Tethys.