Charnockite
“The orthopyroxene-bearing granite”
Charnockite is a distinctive high-grade metamorphic rock with a granitic composition but containing orthopyroxene (hypersthene) instead of the biotite and hornblende typical of normal granite. This dry, anhydrous mineral assemblage indicates formation at extreme temperatures deep in the lower crust, often above 800°C.
orthopyroxene (hypersthene), quartz, feldspar (perthite), garnet, clinopyroxene, biotite (minor)
Chennai (India — type locality), Sri Lanka, Madagascar, Adirondacks (USA), Enderby Land (Antarctica), Limpopo Belt (Southern Africa)
6-7
Charnockite is named after Job Charnock, the founder of Calcutta (Kolkata) — his tombstone in the city's St. John's Church is made of charnockite, and it was from this tombstone that the rock was first formally described.
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What does the presence of orthopyroxene instead of biotite and hornblende in charnockite indicate about the conditions of formation?
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