Mantle wedge involvement in the petrogenesis of Archaean grey gneisses in West Greenland

The Archaean crust in West Greenland is dominated by grey orthogneiss complexes formed in periods of crustal accretion at around 3.8, 3.6, 3.2, 3.0-2.9 and 2.8-2.7 Ga. The majority of the gneisses have tonalite-trondhjemite-granodiorite (TTG) compositions, while subordinate quartz-dioritic and dioritic gneisses have calc-alkaline compositions. The major and trace element chemistry of gneiss samples has been compiled from three large regions representing different terranes and ages in southern and central West Greenland, the Godthabsfjord, Fiskefjord and Disko Bugt regions. The TTG gneisses are typical for their kind and show little variation, except marked Sr enrichment in the Fiskefjord area and slight Cr enrichment in a unit within the Disko Bugt region. Thus, while most of the crust has probably formed from magmas derived by slab melting, local involvement of mantle-derived components is suggested. Most of the diorites have geochemical signatures compatible with mantle-derived parental magmas, i.e., elevated Mg, Cr and flat chondrite-normalised REE patterns. A group of quartz-diorite and diorite samples from the Fiskefjord region exhibits marked enrichment in Sr, Ba, P, K and REE, combined with steep REE patterns. A similar but much more pronounced enrichment in the same elements characterises Palaeoproterozoic subduction-related monzodiorites within the Nagssugtoqidian orogen, as well as carbonatites and carbonatitic lamprophyres within the same part of West Greenland. We argue that the parental magmas of the enriched diorites are derived by partial melting from regions within the mantle that have been metasomatised by carbonatite-related material, e.g., in the form of carbonate-apatite-phlogopite veins. Alternatively, ascending slab melts may have reacted with carbonatite-metasomatised mantle. Carbonatitic carbonates have high Sr and Ba, and carbonatitic apatite has high P2O5 and very steep REE spectra. Adding such a component to a peridotite-derived magma produces geochemical features similar to those of sanukitoids, except that high phosphorus is not described as typical of sanukitoids. We observe that the enriched diorites from Greenland are sanukitoid-like, although they are not sanukitoids by the original definition, and their genesis requires a twist to the current models for sanukitoid petrogenesis. (C) 2004 Elsevier B.V. All rights reserved.