Geochemistry of high grade gneisses, with examples from West Greenland and British Columbia.

The ancient continental massifs of the world are typically composed of granulite grade met metamorphic rocks of calc-alkaline affinity. A model, based on experimental work, is derived to explain the processes by which the igneous precursors of the deep crustal gneisses were generated. It is suggested that they are the results of melting of a mafic garnet - clinopyroxene rock, possibly with small amounts of amphibole and plagioclase. Their composition was subsequently modified by fractionation of amphibole and plagioclase under upper mantle/lower crustal conditions. Granulite grade gneisses are commonly found to be depleted in trace elements such as Th. U, and Rb when compared with rocks of equivalent major element composition that have not suffered granulite grade metamorphism. Possible models for the depletion of these elements are considered, and a model is derived that suggests that dehydration ration melting, involving the breakdown of biotite and amphibole. Best explains both the depleted and anhydrous nature of typical lower crustal granulites. Water activity (a(H20)) plays a key role in the determining the equilibrium mineral assemblage of rocks under lower crustal conditions. Gneisses from West Greenland and British Columbia are studied in detail, in order to constrain the variability of a(H20). Analysis of the mineral assemblages reveals that differences in a(H20) exist on a local scale under both amphibolite and granulite grade conditions. suggesting that a pervasive fluid phase of constant composition did not accompany metamorphism. It is suggested that the variability of a (H20) is due to the control exerted by the mineral phases.