Abstract
A Precambrian fayalite granite outcropping at Lower King, near Albany, Western Australia, is interpreted as a high-Fe 2+/(Fe 2+ + Mg) analogue of charnockite. Calculation of the original titanomagnetite composition from analytical data on ilmenomagnetite ‘exsolution’ intergrowths suggests initial crystallisation of opaque oxides at about 940°C and 10 −12 bars f o 2 . This result indicates a magmatic origin for the rock. Other determinable points on the T-f o 2 cooling curve of the fayalite granite pluton include crystallisation of biotite at roughly 800–820°C and 10 −14.5 bars f o 2 , and final equilibration of opaque oxides below 550°C and 10 −23 bars f o 2 . Mineralogical data on nearby granulite facies country rocks suggest a regional total pressure of roughly 5 kb, and hence the depth of pluton emplacement was probably around 18–19 km. Thus the Lower King fayalite granite is believed to have crystallised from water-deficient, high-T melt or partial melt generated, possibly from metasedimentary rocks, deep in the crust under granulite facies conditions.
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