A juvenile oceanic island arc origin for the Archean (ca. 2.97 Ga) Fiskenæsset anorthosite complex, southwestern Greenland: Evidence from oxygen isotopes

- The Fiskenæsset Complex has modern mantle-like O-isotope compositions (δO18=5.8‰).
- The δO18 values increase from olivine-rich ultramafic rocks (δO18=4.6‰) to plagioclase-rich anorthosites (6.7‰).
- Field relationships and O-isotope data suggest that the Fiskenæsset Complex formed in an oceanic setting.
- Mantle-like δO18 values are inconsistent with substantial recycling of continental crust.
- Field and geochemical data are consistent with Phanerozoic-like plate tectonic processes in the Mesoarchean.

The Archean (ca. 2.97 Ga) Fiskenæsset layered intrusion, southwestern Greenland, consists of an association of anorthosite, leucogabbro, gabbro, hornblendite, pyroxenite, peridotite and dunite. The intrusion is characterized by well-preserved igneous layering, cumulate texture and primary igneous minerals including olivine, pyroxene, plagioclase, hornblende and chromite. We use new whole-rock (n=36) and mineral (n=32) oxygen isotopic data for all major lithologic units from the best preserved stratigraphic section of the Fiskenæsset Complex at Majorqap qâva to revisit geodynamic and petrogenetic hypotheses proposed for the origin of Archean terranes. The Fiskenæsset Complex has modern mantle-like whole-rock O-isotope compositions (δO18=5.8±0.5‰). Average δO18 values increase from peridotite (δO18=5.0‰), through hornblendite (δO18=5.7‰), gabbro (δO18=5.8‰), pyroxene hornblendite (δO18=6.0‰) and leucogabbro (δO18=6.3‰), to anorthosite (δO18=6.3‰). These whole-rock isotopic compositions reflect the approximate modal abundances of olivine (average δO18=4.9‰), hornblende (average δO18=5.7‰), clinopyroxene (average δO18=6.4‰) and plagioclase (average δO18=6.4‰) in each rock type, as a consequence of mineral fractionation in the magma chamber(s). Field relationships and the absence of crustal contamination suggest that the Fiskenæsset Complex formed in an oceanic setting. Subduction zone-like whole-rock trace element signatures and mantle-like δO18 and initial εNd values are consistent with formation of these rocks in a juvenile oceanic island arc setting. Field and geochemical data from the Fiskenæsset region and adjacent terranes suggest that the origin of Archean crust in southwestern Greenland is consistent with Phanerozoic-like plate tectonic processes rather than density-driven sinking, delamination and diapiric processes requiring formation of greenstone belts and anorthosite complexes on pre-existing continental crust. Mantle-like δO18 values in the ca. 2.97 Ga Fiskenæsset Complex are inconsistent with substantial recycling of continental crust, indicating that the Fiskenæsset rocks originated distal from continental sources.