A Discussion on the evolution of the Precambrian crust - The Archaean craton of the North Atlantic region

Abstract

The North Atlantic Archaean craton, exposed in parts of Greenland, Labrador and northwest Scotland, is a high-grade gneiss terrain which contrasts with Archaean granite-greenstone-belt terrains such as those of southern Africa. The tonalitic to granitic banded or agmatitic gneisses which occupy most of the craton are considered to be derived largely from granitic bodies emplaced within the crust. Early granitic gneisses of this type in Godthabsfiord are at least 3800 Ma in age and it is suggested that a granitic basement of similar age extended over much of the craton. Most of this early basement was reworked and interleaved with metamorphosed supracrustal rocks, with layered anorthositic complexes and with abundant tonalitic gneisses derived from younger intrusions. Identifiable metavolcanics and metasediments, forming narrow belts in the gneisses, occupy less than 20% of the craton; they include highly-metamorphosed basic, ultrabasic and intermediate-acid volcanic rocks with associated intrusions and predominantly chemical sedimentary rocks. Clastic sediments are preserved in the lower part of the Isua supracrustal belt where they are overlain by banded ironstones and metavolcanics. All these rocks suffered profound deformation and metamorphism which destroyed their primary relationships and culminated in the development of fold interference patterns without linear grain and in granulite or amphibolite-facies metamorphism ending at about 2800 Ma. Tectonic and metamorphic episodes over the next thousand million years were more localized and served to differentiate the Archaean craton from border-zones of early Proterozoic mobility.