Geochemistry and tectonic significance of peridotitic and metakomatiitic rocks from the Ussuit area, Nagssugtoqidian orogen, West Greenland

Lenses of sheared and metamorphosed ultramafic rocks are common within Pala eoproterozoic supracrustal rocks in the Ussuit area, central Nagssugtoqidia n orogen, West Greenland. They occur in two settings: (I) along the tectoni c contact between an Archaean gneiss complex and allochthonous Palaeoproter ozoic are rocks; and (2) at a structurally higher level, away from the cont act with the Archaean basement. Ultramafic rocks in the first setting compr ise strongly depleted peridotites, on the basis of petrographic and chemica l evidence interpreted as mantle fragments, as well as hornblenditic rocks, which may represent original mafic gabbros of crustal origin. The presence of these rocks is evidence of crustal scale tectonic activity in the centr al part of the Nagssugtoqidian orogen. The possibility is considered that t he rocks may be remnants of a disrupted ophiolite complex; strong deformati on, however, prevents confirmation of this model by field evidence. Ultrama fic rocks from the higher level consist mainly of amphiboles. Based on majo r and trace element data they are interpreted as original komatiitic lavas. Their presence suggests magmatic activity related to a mantle plume early in the history of the Nagssugtoqidian orogen. (C) 1999 Elsevier Science B.V , All rights reserved.