Geochemistry of ultramafic rocks and hornblendite veins in the Fiskenæsset layered anorthosite complex, SW Greenland: Evidence for hydrous upper mantle in the Archean

Abstract

The Fiskenæsset Complex, SW Greenland, contains the world's best preserved Archean (∼2970Ma) layered anorthosite, leucogabbro, gabbro, and ultramafic association. The complex was emplaced into Archean oceanic crust distal from continental lithosphere and later intruded by tonalites, trondhjemites and granodiorites (TTG) constituting Archean continental crust. The complex and bordering TTG intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions.This study presents new whole-rock major and trace element, petrographic, and SEM (Scanning Electron Microscope)-BSE (backscatter electron) image data for a 45–60m-thick ultramafic sill and cross-cutting hornblendite veins. The ultramafic sill is composed of ∼75% olivine–pyroxene hornblendite, ∼15% pyroxene hornblendite, and ∼10% hornblende pyroxenite. Despite granulite facies metamorphism and multiple phases of deformation, the Fiskenæsset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and Alaskan-type ultramafic complexes. Hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism. Formation of orthopyroxene–magnetite symplectitic (vermicular) intergrowths, mainly at the expense of olivine, is attributed to chemical reactions between late stage, residual hydrous melts and olivine.Field, geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion. High MgO (16–31wt.%) contents suggest that the sill was emplaced as a crystal mush, rather than as melt only. Petrographic observations and BSE images indicate that olivine, orthopyroxene, and clinopyroxene were the major crystal phases, whereas hornblende represents the main interstitial liquid phase in the crystal mush. The sill exhibits three different REE patterns, corresponding to three cryptic layers. Although the origin of these REE patterns is not fully understood, they may reflect the intrusion of three batches of magma, containing different hornblende-forming melt/crystal ratios. Geochemical data indicate that the cross-cutting hornblendite veins likely originated from evolved, late-stage melts but were not related to the melts of the ultramafic sill.Large negative Nb anomalies (Nb/Nb*=0.04–0.66) suggest that the Fiskenæsset hornblendites, hornblende pyroxenites, and hornblendite veins were derived from a hydrous sub-arc mantle peridotite. Phanerozoic hornblendites are typically associated with supra-subduction zone ophiolites and magmatic arcs. Accordingly, it is suggested that water was recycled to the source of the Fiskenæsset hornblendites through subduction of altered oceanic crust. Recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites, but also played an important role in the formation of TTG-dominated Archean continental crust. Therefore, the field, petrographic and geochemical data presented in this study suggest that the origin of the Mesoarchean to Neoarchean terrane in the Fiskenæsset region, SW Greenland, is consistent with subduction zone geodynamic processes, rather than non-uniformitarian, density-driven, vertical, crustal overturn, and diapiric processes.