The Itsaq Gneiss Complex of southern West Greenland; the world's most extensive record of early crustal evolution (3900-3600 Ma)

Abstract

The Itsaq Gneiss Complex of southern West Greenland contains the best-preserved occurrences of ⩾ 3600 Ma crust. Its known area is ∼ 3000 km 2 with almost continuous exposure in some places. SHRIMP UPb zircon geochronology shows that the gneiss complex had a complicated early history, having been added to, and modified in, several events starting at ∼ 3900 Ma. Supracrustal, mafic and ultramafic rocks comprise approximately 10% of the complex and range in age from ⩾ 3870 to ∼ 3600 Ma. A large portion of the Isua supracrustal belt and some other bodies contain sequences of LREE-enriched, mafic (locally pillow-structured) to felsic volcanic and volcaniclastic rocks (some deposited from turbidity currents) and abundant diverse chemical sediments. These sequences might have formed in an environment analogous to present-day volcanic arcs. Other (mostly ⩾ 3800 Ma?) units dominated by LREE-depleted, high Ti/Zr mafic rocks (as found in komatiites and komatiitic basalts free of crustal contamination) with layers of banded iron formation might have been derived from volcanic edifices formed as a result of plume activity. Only in the youngest supracrustal sequences (∼ 3600 Ma) are detrital sediments derived from mixed-provenance clastic sources an important component. Layered anorthosite, gabbro and peridotite units (some ⩾ 3800 Ma) derived from large deep crustal intrusions are also widespread. In addition there are massive dunites and harzburgites. In the rare cases where their original mineralogy and texture are preserved, these contain aluminous spinel, indicating equilibration in the lowermost crust or upper mantle. Approximately 90% of the complex consists of grey gneisses, the dominant protoliths having been tonalites and less abundant trondhjemites, quartz-diorites, diorites and granodiorites. The protoliths were intruded in several events starting at ∼ 3870 Ma. Like other suites of Archaean grey gneisses, they were formed by partial melting, probably in an arc environment, of buried (subducted?) amphibolite, leaving residual hornblende ± garnet. Granites form approximately 10% of the complex. The oldest are 3650 Ma leucogranites, which probably formed by deep crustal anatexis of predominantly tonalitic gneisses. There are also ∼ 3625 Ma augen granites and ferrogabbros, whose chemistry resembles that of some A-type, within-plate granite suites. The evolution of the Itsaq gneiss complex is marked by increasing compositional diversity with time. Pre-3650 Ma lithologies are predominantly mafic and ultramafic rocks, tonalites and diorites. The first recorded regional metamorphic event occurred at 3650 Ma, and was marked by localised partial melting and intrusion of leucogranites. This might record crustal thickening, brought about by collision of different blocks of tonalite-dominated crust. A further thermal event occurred just after 3600 Ma, as shown by PbPb titanite and feldspar ages and local intrusion of granites. Deposition of sediments at ∼ 3600 Ma which were derived from mixed-age sources suggests their derivation from an extensive block of “continental” crust.