Petrology and deformation of the Donchai group metamorphic rocks in Muang Pan and Chae Hom Districts, Lampang Province

Abstract

The Donchai Group metamorphic rocks in Northern Thailand are distribute in the Sukhothai terrane and the Inthanon Zone. They are inferred as the Silurian-Devonian rocks correlating with the Thong Pha Phum Group which contains tentaculites and graptolites fossils. The low-grade metamorphic rocks in the study area, Muang Pan and Chae Hom Districts, Lampang Province, can be subdivided into seven groups using their appearance characteristics. They are composed of metasandstone, layered-quartzite, phyllite, metagabbro, gneiss, quartz-schist, and slate. Their relict bedding or foliation lied in a nearly north-south direction and dipping to the east. The layered-quartzite shows S-C fabric on mesoscale and microscale on the western part of the study area, indicating a sinistral movement direction. The phyllite outcrops show S2 crenulation cleavages and andalusite porphyroblast overprinting on the S1 foliation. On the eastern side, the σ-type porphyroclasts in gneiss and quartz-schist outcrops indicated a dextral sense of shear pointing to the south on the outcrop scale. In contrast, their mantled porphyroclasts show a sinistral shear direction pointing to the south under a polarizing microscope. The geochemical analyses used the X-ray fluorescence (XRF) spectrometer for bulk composition, used the X-ray Diffractometer (XRD) and the Electron Probe Microanalyzer (EPMA) for mineral compositions. Geochemistry results, coupled with petrographic observation, were applied for the geothemobarometric estimation. The selected five samples: metapelitic quart-schists and spotted phyllite contain interesting and resembling mineral assemblages. Their equilibrium minerals include quartz, K-feldspar, plagioclase, biotite, muscovite, andalusite/staurolite, chlorite, rutile, magnetite, and cordierite. The cordierite minerals are pervasively altered under a microscope but assumed as stable before. The pseudo section revealed that the representative samples metamorphosed in the hornblende hornfels facies. From the petrographic study, the mineral assemblages of other samples are proper in the greenschist facies. Thus, the low-grade metamorphic rocks of the Donchai Group in the study area are metamorphosed ranging in the hornblende hornfels up to the greenschist facies condition. They experienced a temperature of 350-650 ºC, a pressure of 1-10 kbars, and under at depth above 30 km. The field evidence and microstructures exhibited at least three deformation and metamorphism events. The D1 deformation and the M1 metamorphism are assumed to occurring with the initial stage of the Paleo-Tethys closure in the Middle Triassic. The D2 deformation and the M2 metamorphism resulted from the intruding of granitic bodies in the Late Triassic to Early Jurassic considering P-T estimation result. This event might be related to the crustal thickening of the Indosinian orogeny’s late stage. The two fault sets appearing in the study area implied occurring later in the Cenozoic era during D3 deformation. The strike-slip faults occurred with the India-Eurasia collision due to the crustal thickening phase. The normal and thrust faults related to the basin extension in Mainland SE Asia. These two fault groups interpreted as developed during the Eocene to Miocene controlling by subduction pull and crustal thickening’s collapse stage of the Himalayan orogeny.