Early (3700 Ma) Archaean rocks of the Isua supracrustal belt and adjacent gneisses

Abstract

A coherent stratigraphy is recognised in the highly deformed, amphibolite facies early Archaean Isua supracrustal belt. The supracrustal belt consists of layered rocks (in which sedimentary structures are locally preserved), ultramafic rocks and units of garbenschiefer (a massive Mg-Al rich, leucoamphibolite). The layered supracrustal rocks form two sequences, which are separated from each other tectonically. When folding is taken into account, these sequences are now less than 200 m thick. Sequence A forms most of the belt. In it there is a transition upwards from predominantly layered amphibolites with banded iron formation horizons to calc-silicate rocks, carbonates and layered felsic metasediments. Sequence B is restricted to the western edge of the eastern part of the supracrustal belt. It changes upwards from predominantly layered felsic metasediments to ferromagnesian mica schists. The supracrustal belt is regarded as a thin fragment from a thicker, more extensive volcanosedimentary pile. The early Archaean gneisses adjacent to the supracrustal belt consist of early multiphase tonalites which were first intruded by mafic dioritic dykes and then by granitic sheets. The granitic sheets were originaIly horizontal to gently inciined and form up to 40 per cent of the gneiss complex. Interdigitation of supracrustal rocks and gneisses in the Isukasia area is due to both the style of intrusion ofthe gneisses and to tectonic intercalation. Archaean basic dykes that cut the supracrustal belt and adjacent gneisses are ofseveral generations. Within and south of the supracrustal belt they are generally strongly deformed and have been recrystallised under amphibolite facies conditions; but in the north of the area they are generally better preserved. The dykes cut across several generations of structures in the supracrustal belt and the adjacent gneisses.