Chiang Mai Journal of Science

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Meta- clastic sedimentary rocks and calc-silicate rocks from Khao Phai, immediate west of  Ban Choeng Chai, Wang Pong, Phetchabun were studied for their lithology and petrography, and thus lead to pressure-temperature estimates of  metamorphism. The meta- clastic sedimentary rocks show a hornfelsic texture, with mineral constituents of quartz, plagioclase, K-feldspar, sillimanite, andalusite, cordierite, biotite, muscovite, magnetite and/or monazite/zircon in variable proportions.  They are either non-porphyroclastic or porphyroclastic.  The calc-silicate rocks have non-porphyroblastic and porphyroblastic textures, and are made up of quartz, plagioclase, K-feldspar, clinopyroxene, garnet, phlogopite, epidote, vesuvianite, carbonates, magnetite, hematite/iron hydroxide and/or Fe sulfides.  The pressure and temperature of  metamorphism estimated from major informative metamorphic minerals in some rock samples, i.e. quartz and sillimanite/andalusite for meta-clastic sedimentary rocks, and quartz and clinopyroxene for calc-silicate rocks, are in ranges of 2-3 kbar and 546-607^oC, respectively.  The obtained pressure-temperature condition is in agreement with amphiolite facies metamorphism, inferred from overall metamorphic mineral assemblages.  The heat for this metamorphic event is very likely to have been released from Triassic(?) microdiorite/microgabbro or Triassic(?) granitic rocks in the vicinity.

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