Not until recently at least three separated locations of originally thought to be older regional metamorphic rocks of Thailand have been extensively mapped and petrographically investigated in details by undergraduate students of the Geoscience Program, Mahidol University at Saiyok, Kanchanaburi province. These areas belong to the Western Shan-Thai, which from north to south, includes Uthong area, west of Suphan Buri; Tha Muang-Tha Maka area, east of Kanchanaburi and Pran Buri area, north of Prachuab Khiri Khan. Based on lithologic resemblances and grades of metamorphism, the rocks in these areas had been assigned range of ages from Inferred Precambrian, Cambrian-Ordovician and Ordovician. Careful characterization of rock associations, revealed nonexistence of paragneiss and migmatite in these areas, contradictory to those that usually occur in the true high grade regional metamorphic rocks of the inferred Precambrian age exposed elsewhere. Traceable to the outside lower grade regionally metamorphosed meta-argillaceous limestone and meta-argillite in two of the studied areas, the equilibrated higher grade calc-silicate hornfels and micaceous schistose hornfels inside the contact aureole signified by the presence of orthopyroxene among others, suggests the characteristic minerals of the contact metamorphism. More foliated contact metamorphic rocks which might have been misleaded mapped as Inferred Precambrian age such as in Lin Thin area and Khao Chong In see area, in Kanchanaburi and Khanom area, in Nakhorn Sri Thammarat are to be unraveled. The common occurrences of high temperature/low pressure regime in the inferred Precambrian high grade regional metamorphic rocks of western Shan-Thai terrane as earlier published indicates the Buchan or Abukuma type facies series.


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