Metamorphic mineral assemblages of gneisses along Doi-Inthanon Highway, Northern Thailand

The mineral assemblages of inferred Precambrian rocks of Chiang Mai -Tak gneiss belt along the Chomthong- Doi lnthanon Highway, were studied petrographically. These rocks are divided into mica-K-feldspar-sillimanite gneiss and biotite gneiss with intercalating marble and calc­ silicate. These two units of gneiss are separated by biotite-granite intrusions. The assemblages IJf the mica - K-feldspar - sillimanite gneiss is characterized by the coexistence of biotite, muscovite, K-feldspar, quartz and sillimanite. Fibrolitic sillimanite which is present in small amounts suggests the highest temperature conditions of the almandine - amphibolite facies. Textural evidence indicates generations of muscovite. The foliated small muscovite coexists with biotite appears to be formed during the earlier stage. of metamorphism. Retrograde muscovite is found as non-foliated coarse flakes containing relict fibrolitic sillimanite, suggesting the reaction: sillimanite+ K-feldspar + H2 0 ~ muscovite + quartz, as the last metamorphic episode. Water needed for this reaction may have been derived from pegmatite or aplite veins w.h,ich are widely distributed in the gneiss. ·' ·.