A review of the geodynamic significance of hornblende-bearing ultramafic rocks in the Mesoarchean Fiskenæsset complex, SW Greenland

Abstract

The Fiskenæsset Complex, SW Greenland, is characterized by layered anorthosite, leucogabbro, gabbro, and ultramafic rock association. Ultramafic rocks consist mainly of hornblendite, hornblende peridotite, hornblende pyroxenite, and dunite. Despite upper amphibolite to granulite facies metamorphism, poly-phase deformation and multiple granitoid intrusions, primary igneous layers and mineral assemblages have been well preserved. Petrographic studies, including SEM-BSE imaging, reveal the presence of igneous hornblende occurring as an interstitial mineral to olivine, clinopyroxene, orthopyroxene, plagioclase, and chromite, as well as inclusions in these minerals, consistent with a hydrous mantle source. Large negative Nb-anomalies in whole-rock samples and hornblende grains suggest that the magmas of the Fiskenæsset Complex originated from a hydrous subarc mantle peridotite. Water was recycled to the source of the Fiskenæsset rocks through subduction of hydrated oceanic crust. Phanerozoic hornblende-bearing mafic and ultramafic rocks are typically associated with supra-subduction zone ophiolites and magmatic arcs. Recycling of water to the upper mantle via subduction of oceanic crust not only resulted in the generation of hornblende-rich rocks, but also played an important role in the formation of TTG-dominated Archean continental crust.