Isua supracrustal belt (Greenland)-A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology

The mafic-ultramafic units of the similar to 3.8 Ga Isua supracrustal belt (15B) in Greenland occur in a two-armed arcuate zone (eastern and western arms) and are grouped into two major tectonostratigraphic units based on their lithological and geochemical characteristics: (1) Undifferentiated amphibolites (UA), and (2) Garbenschiefer amphibolites (GA). The UA contains all major lithological units of a typical Penrose-type complete ophiolite sequence. The GA is composed dominantly of volcaniclastic and volcanic rocks, commonly found in immature island arcs. The available geochemical data from UA and GA show distinct differences between the two units. Compared with the geochemical evolution of some of the well known Phanerozoic ophiolites, the pillow lavas and associated dikes of the UA show a compositional range that is similar to typical MORB-type Ligurian ophiolites in the Western Alps-Apennines and those displayed by LIP-type Caribbean ophiolites. The GA is characterized by island arc tholeiite (IAT) to boninite-like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone (SSZ) ophiolites in the Mediterranean region. Our proposed geodynamic model for the ISB suggests that the UA was built by primary to differentiated, mantle-generated melts during seafloor spreading, little to moderately affected by subduction processes, and that the IAT to boninitic-like rocks of the GA formed at a later stage by melting from a strongly subduction-affected, depleted and hydrated mantle. Our interpretation of the ISB is that the UA and GA represent early and late stages, respectively, in the formation of a SSZ ophiolite. This implies that Phanerozoic-type plate tectonic processes, such as seafloor spreading and subduction, were operating by 3.8 Ga in the Palaeoarchean. (C) 2009 Elsevier B.V. All rights reserved.