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

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