A BILLION-YEAR (3800-2800 MA) HISTORY OF INTRA-OCEANIC VOLCANISM, EO- TO NEOARCHEAN GREENSTONE BELTS, SW GREENLAND

Eo- to Neoarchean greenstone belts in SW Greenland are composed mainly of an association of mafic to ultramafic volcanic rocks, gabbros, peridotites, and minor chemical and siliciclastic sedimentary rocks. These greenstone belts were deformed and metamorphosed at amphibolite facies, and occur within multiply-deformed orthogneisses. The Eoarchean (3800-3700 Ma) Isua greenstone belt is composed mainly of tholeiitic pillow basalts, boninitic and picritic pillows and flows. The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt also contains tholeiitic basalts, picrites and boninites, and is intruded by peridotites, gabbros and diorites. The Neoarchean (2950 Ma) Fiskenaesset amphibolites are spatially associated with layered anorthosites, leucogabbros and peridotites, and display a tholeiitic composition. The ca. 2860 Ma Bjornesund-Ikkatup Nuna greenstone belt is composed of mafic to felsic volcanic rocks including tuffs and ignimbrites. The Isua and Ivisaartoq boninites have depleted to enriched LREE patterns, negatively fractionated HREE patterns, and large negative Nb anomalies. In contrast, tholeiitic basalts and picrites in the Isua and Ivisaartoq belts have flat to moderately enriched LREE patterns and flat to slightly depleted HREE patterns, and share the negative Nb anomalies of the associated boninites. On the Th/Yb versus Nb/Yb diagram, all these volcanic rocks plot within the field of intra-oceanic subduction-derived magmas, as for most Phanerozoic ophiolites. On the log-transformed immobile trace element ratio diagrams (e.g. La/Th, Sm/Th, Yb/Th, and Nb/Th), they plot in the IAB field. On the IAB-MORB-CFB-OIB diagram, they display a trend projecting from MORB to IAB field. This trend reflects the enrichment of the Archean depleted upper mantle by melts and/or fluids released from subducted oceanic crust and/or sediments in a convergent margin following the initiation of intra-oceanic subduction. Collectively, the trace element characteristics of mafic to ultramafic volcanic rocks in the Isua, Ivisaartoq, Fiskenaesset, and Bjornesund-Ikkatup supracrustal assemblages suggest that intra-oceanic subduction processes played an important role in the growth of Archean continental crust in SW Greenland.