Dismembered Fragments of Meso- to Neoarchean Supra-subduction Zone Oceanic Crust: Field and Geochemical Evidence From 2800-3075 Ma Greenstone Belts in the Nuuk Region, SW Greenland

The Ivisaartoq (ca. 3075 Ma) and Storø(ca. 2800-2840 Ma) greenstone belts are the largest Meso- to Neoarchean belts in the Nuuk region, SW Greenland. Several unnamed greenstone belts (2972-3070 Ma) in the Ujarassuit Nunaat area appear to be continuation of the Ivisaartoq belt. These greenstone belts have undergone polyphase deformation and metamorphism at amphibolite facies conditions. They are dominated by metabasalts, with minor intercalations of intermediate metavolcanic rocks. They also include gabbros, picrites, cumulate rocks with relict igneous clinopyroxene, and serpentinite lenses with relict olivine. Primary magmatic features including pillow flows, inter-pillow breccias, and cumulate layering are best preserved in the Ivisaartoq belt. Metasedimentary rocks occur as thin layers (0.5-1.0 m) of aluminous paragneisses and mica schists concordant to the regional foliation. However, the Storøand Ivisaartoq belts include thick (> 200 m) units of metasediments. These supracrustal belts occur within different exotic terranes separated by high-grade mylonites. In the Ujarassuit Nunaat area, rare thrust-fold structures bring into contact the supracrustal rocks and the underlying TTG-gneisses. The metabasalts display flat to slightly fractionated REE patterns (La/Smcn = 0.9-1.8; Gd/Ybcn = 1.0-1.3) whereas the intermediate rocks possess more fractionated patterns (La/Smcn = 1.8-3.2; Gd/Ybcn = 1.9-3.1). Both types of metavolcanic rocks posses consistent negative Nb anomalies (ca. 0.2- 0.8). Major and trace elements discriminate two groups of metasediments. Mafic metasediments are geochemically similar to the associated basaltic amphibolites. Intermediate to felsic metasediments are abundant despite the dominant basaltic composition of these belts. They display enriched LREE (La/Smcn = 1.1-7.0) and near-flat HREE (Gd/Ybcn = 0.8-2.7) patterns, pronounced negative Nb anomalies, and variable concentrations of transition metals (Ni = 50-330 ppm). The geochemical characteristics of the basaltic rocks suggest partial melting of a shallow mantle source slightly modified by subduction components. The occurrence of picritic flows and rare felsic to intermediate volcanic rocks also indicate a subduction zone environment. Intermediate to felsic metasedimentary rocks are interpreted to represent volcanogenic sediments sourced from nearby arc edifices that have been eroded and are not preserved in the geological record. All these field and geochemical characteristics indicate that the studied greenstones represent relicts of Archean oceanic crust formed in a suprasubduction zone setting.