The early Archaean to Proterozoic history of the Isukasia area, southern West Greenland

Abstract

The c. 3800 Ma Isua supracrustal belt and associated smaller bodies of supracrustal rocks are intruded by >3600 Ma orthogneisses. A coherent stratigraphic sequence is recognised consisting of interlayered metabasic rocks, metasediments derived from volcanic rocks, chemical sediments, and metabasic and ultramafic intrusions. Despite repeated deformation and high-grade metamorphism sedimentary structures are locally preserved. The depositional environment was probably an immersed volcanic region remote from areas of significantly older crust. Conglomeratic structures in a metachert and banded iron formation unit suggest shoaling and shallow water conditions. Felsic sediments locally preserve evidence of deposition from turbidite flows. The Isua supracrustal rocks are regarded as thin fragments of a thicker, more extensive sequence. The orthogneisses that intrude the supracrustal rocks consist of 3750-3700 Ma multiphase tonalites (the grey gneisses) which were first intruded by the basic Inaluk dykes, then by abundant shallow-dipping swarms of c. 3600 Ma granite sheets (the white gneisses) and finally by c. 3400 Ma pegmatitic gneiss sheets. These early Archaean rocks were metamorphosed under amphibolite facies conditions and repeatedly deformed prior to intrusion of the Tarssartôq basic dykes in the mid Archaean. In the late Archaean (3100-2500 Ma) there was polyphase metamorphism up to amphibolite facies grade and two or more stages of deformation and local intrusion of granitic gneiss sheets and pegmatites. However, despite general strong deformation there is a large augen of low deformation preserved within the arc of the Isua supracrustal belt. During the Proterozoic there was intrusion of basic dykes, major faulting with associated recrystallisation under uppermost greenschist to lowermost amphibolite facies conditions, followed by heating and intrusion of acid dykes at c. 1600 Ma. No profitable mineralisations have been located.