THE ISUA SUPRACRUSTAL BELT (GREENLAND) - A VESTIGE OF A 3.8 GA SUPRASUBDUCTION ZONE OPHIOLITE, AND ITS SIGNIFICANCE FOR ARCHEAN GEOLOGY

The origin of mafic-ultramafic rocks in Archean greenstone belts has been a subject of debate, although some studies have shown the possible occurrence of incomplete ophiolites. The apparent lack of an idealized Penrose-type ophiolite, in particular sheeted dike complexes, in the Archean greenstone belts has led many workers to conclude that typical oceanic crust and hence seafloor spreading and plate tectonic processes did not occur in the earliest stages of Earth's history. We found within the amphibolites of the ca. 3.8 Ga Isua supracrustal belt (ISB) in SW Greenland a sheeted dike complex associated with pillow lavas, gabbros and ultramafic rocks, collectively forming an ophiolite – the Isua Ophiolite Complex (IOC). The geochemistry of the dikes and pillows in the IOC shows MORB to IAT affinities and their oxygen isotopes suggest an overprinted hydrothermal metamorphism typical of oceanic crust formed in seafloor spreading centers. Another major component (ca. 50%) of the ISB, a “garbenschiefer” unit (GU), comprises a volcano-sedimentary unit with gabbroic sill intrusions, and its metabasaltic rocks are geochemically similar to boninites. We have compared the reconstructed stratigraphy and geochemistry of the Archean IOC+GU to those of Phanerozoic SSZ ophiolites (i.e., Jurassic Mirdita ophiolite in Albania), whose crustal units display a progressive evolution from MORB-like to IAT to boninitic (extremely refractory) compositions and a Penrose-type pseudostratigraphy. These Phanerozoic SSZ ophiolites formed within rifted protoarc-forearc settings in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions. Slab rollback, mantle flow in the arc wedge corner and subduction-induced mantle metasomatism were important processes during their magmatic evolution. The similarities in the geochemical signatures, internal structure and stratigraphy, and igneous evolutionary patterns suggest that the IOC+GU collectively form an arc-forearc generated ophiolite, analogous to its Phanerozoic SSZ counterparts. The existence of an Archean SSZ ophiolite with a seafloor spreading origin strongly suggests that plate tectonics cycle driven by lithospheric subduction and surface cooling should have operated by 3.8 Ga.