THE ITSAQ GNEISS COMPLEX OF SOUTHERN WEST GREENLAND - THE WORLDS MOSTEXTENSIVE RECORD OF EARLY CRUSTAL EVOLUTION (3900-3600-MA)

The Itsaq Gneiss Complex of southern West Greenland contains the best- preserved occurrences of greater than or equal to 3600 Ma crust. Its k nown area is similar to 3000 km(2) with almost continuous exposure in some places. SHRIMP U-Pb zircon geochronology shows that the gneiss co mplex had a complicated early history, having been added to, and modif ied in, several events starting at similar to 3900 Ma. Supracrustal, m afic and ultramafic rocks comprise approximately 10% of the complex an d range in age from greater than or equal to 3870 to similar to 3600 M a. 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 turb idity currents) and abundant diverse chemical sediments. These sequenc es might have formed in an environment analogous to present-day volcan ic arcs. Other (mostly greater than or equal to 3800 Ma?) units domina ted by LREE-depleted, high Ti/Zr mafic rocks (as found in komatiites a nd komatiitic basalts free of crustal contamination) with layers of ba nded iron formation might have been derived from volcanic edifices for med as a result of plume activity. Only in the youngest supracrustal s equences (similar to 3600 Ma) are detrital sediments derived from mixe d-provenance elastic sources an important component. Layered anorthosi te, gabbro and peridotite units (some greater than or equal to 3800 Ma ) derived from large deep crustal intrusions are also widespread. In a ddition there are massive dunites and harzburgites. In the rare cases where their original mineralogy and texture are preserved, these conta in aluminous spinel, indicating equilibration in the lowermost crust o r upper mantle. Approximately 90% of the complex consists of grey gnei sses, the dominant protoliths having been tonalites and less abundant trondhjemites, quartz-diorites, diorites and granodiorites. The protol iths were intruded in several events starting at similar to 3870 Ma. L ike other suites of Archaean grey gneisses, they were formed by partia l melting, probably in an are environment, of buried (subducted?) amph ibolite, leaving residual hornblende +/- garnet. Granites form approxi mately 10% of the complex. The oldest are 3650 Ma leucogranites, which probably formed by deep crustal anatexis of predominantly tonalitic g neisses. There are also similar to 3625 Ma augen granites and ferrogab bros, whose chemistry resembles that of some A-type, within-plate gran ite suites. The evolution of the Itsaq gneiss complex is marked by inc reasing 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 wa s marked by localised partial melting and intrusion of leucogranites. This might record crustal thickening, brought about hy collision of di fferent blocks of tonalite-dominated crust. A further thermal event oc curred just after 3600 Ma, as shown by Pb-Pb titanite and feldspar age s and local intrusion of granites. Deposition of sediments at similar to 3600 Ma which LE ere derived from mixed-age sources suggests their derivation from an extensive block of ''continental'' crust.