Origins of Large Peridotite Bodies within Mesoarchean Orthogneisses in SW Greenland

Abstract:

Peridotite bodies with sizes up to 500 x 1000 m are found within Mesoarchaean orthogneisses in the Fiskefjord region of SW Greenland. Here we present field observations, bulk-rock major, trace, and platinum-group element data, and mineral chemistry for some these peridotites. The largest of these (known as Seqi), has previously been mined for industrial grade olivine. Seqi is mainly dunitic, consisting of highly fosteritic olivine with a median Mg# of 92.6. It also hosts extensive layers of chromitite (up to 30 cm wide), which are always associated with interstitial orthopyroxene. The other peridotite bodies are generally thinner (<100 m thick, but up to 2000 m long) and are associated with coarse norite and orthopyroxenite with obvious cumulate textures. Amphibolite of tholeiitic basaltic composition is intercalated with these thinner peridotite bodies and suggests that they represent metamorphosed layered igneous complexes. Based on field observations, the latter predate the Mesoarchaean regional TTG-type orthogneisses dated at ca. 3000 Ma. We interpret the peridotites of the Fiskefjord region to have formed as ultramafic cumulates derived from Archean high-Mg magmas. The Seqi dunites contain hydrous minerals such as amphibole and phlogopite, and have U-shaped bulk-rock trace element patterns with elevated Th. This may support the currently accepted subduction zone model for the formation of the Archean crust in this region.