Chemical and Thermal Evolution of Archaean Sialic Crust, Southern West Greenland

Abstract

The Archaean rocks of West Greenland are predominantly banded quartzofeldspathic gneisses enclosing thin migmatized sheets of older metabasic and metasedimentary lithologies. Isotopic dating (Moorbath & Pankhurst, 1976) indicates that much of the Archaean crust presently exposed in southern West Greenland was generated from predominantly upper mantle sources 3000-2800 m.y. ago. Parts of this crust crystallized under prograde granulite facies conditions 2950-2750 m.y. ago. The granulite gneisses were severely depleted in some of the lithophile and heat producing elements, Si, Na, Sr, Rb, U and Th, during the granulite metamorphism. These elements appear to have been transferred to higher crustal levels by the migration of a dispersed vapour phase. The pressure and temperature recorded by amphibolite and granulite facies assemblages have been estimated using thermometers and barometers calibrated against the results of phase equilibrium experiments. These estimates (800 °C, 10.5 kb and 630 °C, 7.3 kb for granulite and amphibolite facies respectively) indicate that the Archaean continental crust in southern West Greenland was at least 30–40 km thick 2800 m.y. ago. Water vapour pressures in the granulites were extremely low, 0.3 to 0.1 Ptotal. The thermal evolution of the Archaean crust during the period 3000-2700 m.y. was controlled by the emplacement of large volumes of acid-intermediate melts into a relatively thin metabasic crust. The thermal perturbations generated by this convective transfer of heat from the upper mantle to the crust relaxed during the period immediately following crustal accretion. Prograde granulite facies assemblages could have developed under strongly dehydrating conditions and progressively falling temperatures, or during a phase of rising temperature in the lower crust.