Petrochemistry and metamorphism of Lansang metamorphic suites, Changwat Tak

Abstract

Western metamorphic belt in Thailand elongated within north-south direction has been grouped into the Chiang Mai-Lincang belt. As part of this metamorphic belt, the Lansang National Park located in the west of Changwat Tak is an affected area from northwest-southeast striking Mae Ping fault. Along the fault zone, rocks have been strongly deformed yielding steep foliation and lineation. Based on petrographic study, these rocks are mainly characterized by gneisses showing very fine- to coarse-grained textures which consist of quartz, plagioclase, K-feldspar, biotite and minor amounts of chlorite and amphibole. Calc-silicate and impure marble are partly interlayered in these gneisses; they are composed of calcite, quartz, feldspar, amphibole and garnet. Moreover, granitic gneiss, intruded into the host gneisses and calc-silicate, appears to have similar composition of gneiss. Based on geothermobarometry, these rocks should have undertaken high-grade metamorphism belonging to upper greenschist to lower amphibolite facies with P-T estimates of 560-580°C and 5.7-7.5 Kbar for coarse-gained gneiss, 545-560°C and 0.5-0.8 Kbar for fine-grained gneiss, 440-500°C and 3.0-5.0 Kbar for very fine-grained gneiss and 480-510°C and 6.0-6.8 Kbar for impure marble. According to geochemistry, these rocks appear to have several evolution stages. Granitic and sedimentary protoliths, initial rocks of gneisses and calc-silicate, may have been situated in the area prior to regional metamorphism. Subsequently, dykes and veins crossed cut into these gneissic and calc-silicate rocks before later metamorphism appears to have continued and reached the ductile-brittle stage along with uplifting onto the surface.