PETROGRAPHY AND DEFORMATION OF DONCHAI GROUP METAMORPHIC ROCKS IN MUANG PAN AND CHAE HOM AREA

Abstract

The Donchai Group are low-grade metamorphic rocks which are distributed in Northern Thailand. Their protoliths are pelitic rocks, quartzo-feldsparthic rocks, and mafic/ultramafic rocks. The Donchai Group in the study area experienced at least two metamorphism events. They show the spotted porphyroblasts that are the evidence of contact metamorphism overprinted on the early regional metamorphism. The andalusite porphyroblast aggregate sized 2–10 cm while the cordierite relicts sized 1–5 mm in diameters. The first event (D1) created their foliation that mostly dipping to the east while the second (D2) created the crenulation cleavages. In microscales, the microstructures show a dextral sense of shear. The mineral assemblages of metapelites are biotite, muscovite, quartz, andalusite, sillimanite, cordierite, garnet, and tourmaline. The metapsammites contain quartz, k-feldspar, plagioclase, biotite, muscovite, chlorite, apatite, zircon and titanite. The metabasites compose of plagioclase, quartz, amphibole, orthopyroxene, biotite, chlorite, and opaque minerals. From the petrographic study, the Donchai Group metamorphic rocks are metamorphosed under the greenschist facies.