Thermal history of excavation of Archaean gneisses from the base of the continental crust

Abstract

General considerations of the mineral assemblages likely to be developed at the crust-mantle boundary, the manner in which such rocks are brought to the surface and the criteria for identifying them are followed by a specific model of events in the Scourie gneiss complex. The Scourie gneisses equilibrated their strontium isotopes to upper mantle ratios 2.8 Ga (1 Ga = 10 9 years) ago at the base of a c. 50 km thick crust. Conditions of 15 kbar, 1250°C with low water fugacity, imply thick crust (c. 50 kin) very high crustal geothermal gradient (c. 25°C/km over 50 km) and probable residual character of the entire complex due to loss of granitic fractions. Recovery history was marked by (i) rapid reduction in pressure to c. 11 kbar at high temperature (consistent with near adiabatic uplift) followed by (ii) rapid fall in temperature to c. 450°C at pressures of 11–7 kbar (consistent with rapid cooling of the outer parts of the earth in the late Archaean) prior to dyke intrusions at 2.2–2.4 Ga. Subsequent thermal history until surface exposure 10 Ga ago is poorly marked by mineralogical events. Closure of different mineral systems to chemical change at several stages allows the thermal history of excavation to be unravelled.