ANATOMY OF AN EARLY ARCHEAN GNEISS COMPLEX - 3900 TO 3600 MA CRUSTAL EVOLUTION IN SOUTHERN WEST GREENLAND

The Early Archean complex of southern West Greenland consists of polyp hase, tonalitic-trondhjemitic-granodioritic (TTG) and granitic Amitsoq gneisses with inclusions of volcanic and sedimentary rocks, gabbros, and ultramafic rocks. In this complex, rocks of similar appearance and composition were found to be of different ages by U-Pb zircon dating; the Amitsoq gneisses comprise 3870, 3820-3810, 3760, 3730, 3700, and 3625 Ma TTG and 3660-3650 and 3625 Ma granites, and their inclusions b elong to several supracrustal sequences with a similar spread of ages. These results show that the complex grew by episodic addition of new TTG and welding together of rocks of different ages. A possible plate- tectonics scenario is as follows: Melting of subducted mafic (oceanic) crust formed greater-than-or-equal-to 3700 Ma microcontinents consist ing of TTG suites with predominantly mafic inclusions. At 3650 Ma, col lision between microcontinents caused crustal thickening, high-grade m etamorphism, and emplacement of leucogranites. At 3625 Ma, subduction at the edge of the >3625 Ma continental mass created a new crustal add ition comprising both TTG and granite, while granites were emplaced in to the > 3625 Ma continental mass.