Kurunegala amphibolite-to-granulite transition: mapping carbonic fluid infiltration fronts?
What field petrology markers identify patches of incipient charnockite formation in amphibolite-grade biotite-hornblende gneisses around Kurunegala?
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3 Answers from travellers
The transition zone between the amphibolite-facies Wanni Complex and the granulite-facies Highland Complex around Kurunegala exhibits classic "incipient charnockitization." In quarry faces, light-grey hornblende-biotite gneiss displays localized irregular dark greenish-brown patches and vein networks of charnockite crossing foliation planes. Petrographic analysis reveals that carbon dioxide-rich (CO2) fluids infiltrated along ductile shear micro-fractures, lowering water activity (aH2O) and breaking down hydrous biotite and hornblende into dry orthopyroxene (hypersthene), quartz, and K-feldspar in-situ at 750°C.
The greenish discoloration is caused by microscopic sub-micron fluid inclusions and fine exsolution lamellae within feldspar and quartz grains.
Commercial stone quarries blast these charnockite masses extensively to produce high-strength railway ballast and marine breakwater armor stone.
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