Bulletin of the Geological Society of Malaysia, Volume 12, Aug. 1980, pp. 55 – 74
Department of Geology, Faculty of Science, Chulalongkorn University, Thailand
Abstract: Petrographic description is given of marble, quartzite, calcsilicate quartzofeldspathite and marble, metapelite (including schist and the Hua Hin Gneiss) and other gneissic rocks, namely the Pranburi Gneiss and the Hup Kapong Gneiss, which occur in the Pranburi-Hua Hin Metamorphic Complex.
Chemical analysis of various specimens of the Hua Hin, the Pranburi, and the Hup Kapong Gneisses shows that the bulk compositions of these rocks are fairly uniform. The ratio of Al/Na + K +½Ca indicates that the Pranburi Gneiss is wollastonite norm, whereas the Hua Hin and Hup Kapong Gneisses appear to be corundum norm. Moreover, major element ratios and field observations suggest that the Hua Hin and Hup Kapong Gneisses may have a sedimentary origin, whereas the Pranburi Gneiss shows igneous-like characteristics.
On the basis of mineral assemblages of metapelites, the P-T conditions of the Pranburi-Hua Hin Complex have been estimated to be at approximately 2.25-3.65 kbar and 610-680°C. However, the attempt to use mineral assemblages of calcsilicate rocks to determine the physical condition of the metamorphism is greatly affected by the degree of uncertainty in PH2O/PCO2 ratio.