Geochronological and geochemical constraints on the intermediate-acid volcanic rocks along the Chiang Khong–Lampang–Tak igneous zone in NW Thailand and their tectonic implications

• Main Authors: Qian X., Wang Y., Srithai B., Feng Q., Zhang Y., Zi J., He H.
• Format: Journal
• Published: 2017
• Online Access: https://www.scopus.com/inward/record.uri?partnerID=HzOxMe3b&scp=85016151928&origin=inward; http://cmuir.cmu.ac.th/jspui/handle/6653943832/40518
• Institution: Chiang Mai University

© 2017 International Association for Gondwana Research Volcanic rocks preserved in the Lampang–Den Chai area in NW Thailand are important components of the giant Paleotethyan igneous belt. Constraining their age and petrogenesis is critical for better understanding their temporal-spatial relationship with the Lancangjiang igneous zone and the Paleotethyan tectonic evolution in SE Asia. The volcanic suite is constituted by intermediate to acid rocks with zircon U–Pb ages of 240.4 ± 1.7 Ma and 240.6 ± 1.9 Ma for the representative andesitic and rhyolitic samples, respectively. Volcanic sequence is dominated by calc-alkaline andesites, dacites and rhyolites. The andesitic and dacitic samples are characterized by high Mg# (37–57) and TiO 2 (0.91–1.59 wt%), and can be classified as high-Mg series. They are enriched in LILEs and LREEs and depleted in HFSEs. Representative andesitic samples have 87 Sr/ 86 Sr (i) ratios of 0.70398–0.70567, ε Nd (t) values of +3.6–+3.9, zircon ε Hf (t) values of +2.8–+8.0 and δ 18 O values of 7.01–8.11‰, respectively. The rhyolitic samples are characterized by high Mg# (38–70) and low TiO 2 (0.25–0.61 wt%). They are enriched in LILEs and LREEs, along with 87 Sr/ 86 Sr (i) = 0.70468–0.70645, ε Nd (t) = +2.0–+4.3 and zircon ε Hf (t) = +5.7–+13.6. Geochemical signatures suggest that the andesitic and dacitic samples might originate from a newly modified mantle source by slab-derived fluids and recycled sediments, and rhyolitic samples were derived from juvenile mafic crust. It is proposed that the Middle Triassic high-Mg volcanic rocks in the Lampang–Den Chai area formed in response to slab roll-back during transition of tectonic regime from subduction to continental collision between the Sibumasu and Indochina blocks. These rocks constitute part of the Chiang Khong–Lampang–Tak igneous zone, and can northerly link with the Lancangjiang igneous zone and southerly extend to the Chanthaburi, Malaysia and Singapore areas.