Petrochemistry, geochronology and tectonic implication of the Chiang Khong-Lampang Tak volcanic belt, Northern Thailand

The Chiang Khong-Lampang-Tak volcanic belt (CLT) is a key element in the central part of the Sukhothai Fold Belt in northern Thailand, lying between the Chiang Mai (lnthanon) Suture and the Nan-Sra-Kaeo Suture. This study reports detailed petrochemical, geochronological and structural aspects of rocks constituting the CLT, and uses this information to elucidate the tectonic evolution of this geologically important part of mainland SE Asia. The CL T is constituted by a number of sub-belts, and these have been examined in two major study areas, namely, in northernmost Thailand along the Mekong River border region with Laos, and in further south in the Lampang area. Volcanic and dyke rocks in the CLT formed mainly between 220 and 230 Ma, and they display a striking range of compositions and affinities, from low-K rocks with transitional tholeiitic affinities, through medium-K calc-alkaline, to dominant high-K calc-alkaline compositions. Basaltic rocks dominate some sub-belts, whereas felsic rocks predominate in others. Although previous studies have unanimously suggested that these rocks formed in a continental margin volcanic arc, it is suggested here that these rocks formed in a post-collisional, extensional Basin and Range-type setting. It is proposed that crustal thickening due to the Late Middle Triassic collision between the Shan-Thai and Indochina terranes was followed by relaxation rifting and gravitational collapse of the new crustal collage, leading to the post-collisional magmatism of the CLT. Laser ablation ICP-MS U-Pb dating of zircons in this study has shown that most CLT igneous rocks crystallized in late Middle to early Late Triassic (-220-230 Ma). In many rocks, age histograms for zircons showed a peak at 220 Ma and another at 230 Ma. The cause of these bimodal zircon ages remains uncertain and there are no internal textural differences between the zircon grains in either group. Permian zircons have been found in some Late Triassic CLT igneous rocks and are inherited possibly from basement Permian granites associated with prior subduction. Detrital zircons m CL T sedimentary rocks indicate that Late Devonian to Early Carboniferous, Permian and Late Triassic rocks were exposed in the provenance areas of these sandstones. The Devonian to Early Carboniferous and Permian detrital zircons formed in the earlier, subduction-related magmatic events, whereas the Late Triassic zircons are locally derived from the CLT volcanics. Several structural transects of the CL T, in both the northern and southern study areas, showed that these rocks are dominated by a thin-skinned east-vergent fold and thrust structural style, characterized by upright close to open folds with locally overturned bedding. The steep regional cleavage is axial planar to regional folds and becomes more intense to the east and near thrusts. In some localities, the steep cleavage is overprinted by a shallow dipping cleavage, the latter is possibly related to shallow west-dipping thrusts associated with Late Triassic deformation. The thin-skinned thrust faulting involved mainly east-directed transport and was overprinted by local back thrusts. A west-dipping mylonite zone along the SW margin of the Chiang Khong Central Sub-belt is correlated with the regional deformation event that produced LIS tectonites of the Ban Huak area, southeast of the Chiang Khong area. Anomalous thrust fault orientations in the Ban Huak area represent lateral ramps. It is concluded that the structure of the CLT is similar to the thrust geometry described by previous workers from further east and south in the Sukhothai Fold Belt. The compressional event in the Latest Triassic, after the CLT extensional magmatism, and which disrupted the CLT into the present sub-belts, remains problematic. It possibly represents a final minor event at least 10 m.y. after the accretion of the Shan-Thai to Indochina terranes. Rocks showing the fold and thrust style regional structure of the CLT and the Sukhothai Fold Belt are overlain by Latest Triassic to Early Jurassic Khorat Group molasse and affected by normal and strike-slip faulting. This late structure is related to the extensional rift basin development in northern Thailand and possibly associated with the Eocene collision between the India and Asia. A new tectonic model for the geodynamic evolution of the northern Thailand region is proposed, based on the new data from this study and a comprehensive review of the extensive published literature. West-directed subduction during the Permian within the ocean basin (Palaeo-Tethys) between the Indochina and Shan-Thai terranes produced a typical West Pacific-type intra-oceanic arc-backarc basin system. Eventual closure of this segment of ocean led to collision of the arc system with the leading (western) edge of Indochina, perhaps in the Late Permian. Fallowing locking of the plate boundary by this collision event, continued convergence between the Indochina Terrane and the Shan-Thai Terrane forced a subduction polarity reversal, to east-directed subduction beneath the Indochina Terrane. Ongoing Latest Permian east-directed subduction led to construction of the Loei arc on the western edge of the Indochina Terrane, then to arrival of the leading edge of the Shan-Thai Terrane at the trench, and a Late Middle to Early Late Triassic continent-continent collision, which thoroughly reworked and restructured the fold belt formed in the earlier arc-continent collision. Although such an event is expected to generate east-dipping thrusts over much of the area affected by the collision, backthrusting of the older fold belt collage against the buttress of the Indochina Terrane may have produced the widespread west-dipping structures over much of the Sukhothai Fold Belt and Simao Terrane. Relaxation rifting and gravitational collapse of the new crustal collage in Late Triassic led to the post-collisional mainly calc-alkaline magmatism of the Chiang Khong-Lampang-Tak volcanic belt, and subsequent deposition of the Lampang Group. Cessation of extension and post-collisional magmatism may have been followed by Latest Triassic thrust faulting responsible for the present disposition of the sub-belt that constitute the CLT. This thrusting may have been due to further 'locking up' along the Indochina - Shan-Thai suture, but it remains problematic. Finally, the Khorat Group molasse was deposited over the collision zone.