Abstract
Charnockitic rocks that occur in many Proterozoic high-grade metamorphic terranes have crystallized (or recrystallized) under high-temperature and low water pressure conditions: they usually contain two pyroxenes and mesoperthite as well as orthoclase, plagioclase, quartz and traces of hornblende and biotite. Estimates of physical conditions based on subsolidus reactions independant of water pressure show that crystallization occurred in the range 800 to 900°C at pressures varying from 7 kbar in the Grenville province to 10 kbar in Africa and in Norway. These physical conditions correspond to a depth of charnockite genesis of about 25–35 km and a gradient of 25 to 30°C per km. Charnockite terranes might therefore correspond to regions of crustal thickening as do those which are associated with continental collision processes. If such is the case, their actual distribution might help in understanding Proterozoic plate motion.
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