Deformation history and U–Pb zircon geochronology of the high grade metamorphic rocks within the Klaeng fault zone,eastern Thailand

In eastern Thailand the Klaeng fault zone includes a high-grade metamorphic rock assemblage, named Nong Yai Gneiss, which extends about 30 km in a NW–SE direction along the fault zone. The rocks of this brittle-fault strand consist of amphibolite to granulite grade gneissic rocks. Structural analysis indicates that the rocks in this area experienced three distinct episodes of deformation (D₁–D₃). The first (D₁) formed large-scale NW–SE-trending isoclinal folds (F₁) that were reworked by small-scale tight to open folds (F₂) during the second deformation (D₂). D₁ and D₂ resulted from NE–SW shortening during the Triassic Indosinian orogeny before being cross-cut by leucogranites. D₁ and D₂ fabrics were then reworked by D₃ sinistral shearing, including shear planes (S₃) and mineral stretching lineations (L₃). LA–MC–ICP–MS U–Pb zircon dating suggested that the leucogranite intrusion and the magmatic crystallization took place at 78.6 ± 0.7 Ma followed by a second crystallization at 67 ± 1 to 72.1 ± 0.6 Ma. Both crystallizations occurred in the Late Cretaceous and, it is suggested, were tectonically influenced by SE Asian region effects of the West Burma and Shan-Thai/Sibumasu collision or development of an Andean-type margin. The sinistral ductile movement of D₃ was coeval with the peak metamorphism that occurred in the Eocene during the early phases of the India–Asia collision.     

泰国东南部Rayong省Klaeng地区的中-晚泥盆世放射虫

Tectonic subdivision of mainland Thailand has been discussed by means of Paleozoic and Mesozoic stratigraphy, micropaleontology represented by fo-raminiferal and radiolarian biostratigraphy and paleo-biogeography, tectonic settings of granitoids, and ul-tramafic rocks that are distinctive of suture lines. Re-cently, Ueno (2002) and Ueno and Charoentitilat (2011) proposed a new tectonic scheme of mainland Thailand and divided it into three geotectonic units: the Sibu-masu Block, Sukhothai Zone, and Indochina Block from west to east, which are separated by the Mae Yuan Fault, the Chiang Rai Tectonic Line, and the Nan–Uttaradit Suture, respectively. In Northern Thai-land, moreover, the peculiar Inthanon Zone is recog-nized in the eastern part of the Sibumasu Block, where Paleo-Tethyan oceanic rocks are widely distributed as tectonic slices. These geotectonic subdivisions in Northern Thailand are relatively clear. However, the tectonic subdivisions of Central and Southeast Thai-land have not been well documented and are still con-troversial because there is short of information about basement rocks in those areas. In this circumstance, we have examined Paleozoic to Mesozoic stratigraphy, lithology, and radiolarian ages of siliceous rocks dis-tributed in Southeast Thailand to clarify the geotec-tonic belongings of this area and the southern exten-sions of geotectonic units consisting of Northern Thailand. In the course of this research, we could ob-tain Middle to Late Devonian radiolarians from well-bedded siliceous rocks distributed in southwest of Klaeng, Rayong Province. In this study, we report lithology and age of the radiolaria-bearing rocks and discuss their stratigraphy and geological correlation nearby the Klaeng region.