Microstructure characteristics of the ganet-bearing schist from Nam Co formation, Son La area, Song Ma suture zone, Northwestern Vietnam

Abstract

The garnet-bearing schists of the Nam Co formation have an identical mineral assemblage consisting of garnet, chlorte, albite, quartz and muscovite, together with accessory apatite, zircon, monazite, xenotime, and ilmenite. An aggregate of muscovite and chlorite defines the major foliations (Sn). Both albite and garnet occur as a porphyroblast, ranging in size 0.2÷1 mm and 0.5÷1.2 mm, respectively. Albite porphyroblasts commonly have the curved to sigmoidal inclusion trails defined by graphitic materials (Sn-1). Garnet porphyroblasts in the sample is generally characterized by paucity of inclusions and retrograde corona of bitotite and chlorite. Garnet also occursas an inclusion within albite porphyroblast. Porphyroblastic garnet showsthe compositional zonation typified by a bell-shaped spessartine profile balanced by increasing almandine from core to rim. Whereas, inclusion garnet is homogeneous compositions with rich in almandin and poor in spessatin, pyrop and grossula. All the above microstructures suggest two deformation and metamorphic stages (M1and M2)thatwere affected to politicrocksof the Nam Co formation, Song Ma suture zone