Contrasting Textural and Chemical Signatures of Chromitites in the Mesoarchaean Ulamertoq Peridotite Body, Southern West Greenland

Abstract

Peridotites occur as lensoid bodies within the Mesoarchaean orthogneiss in the Akia terrane of Southern West Greenland. The Ulamertoq peridotite body is the largest of these peridotites hosted within the regional orthogneiss. It consists mainly of olivine, orthopyroxene, and amphibole-rich ultramafic rocks exhibiting metamorphic textural and chemical features. Chromitite layers from different localities in Ulamertoq show contrasting characteristics. In one locality, zoned chromites are hosted in orthopyroxene-amphibole peridotites. Compositional zonation in chromites is evident with decreasing Cr and Fe content from core to rim, while Al and Mg increase. Homogeneous chromites from another locality are fairly uniform and Fe-rich. The mineral chemistry of the major and accessory phases shows metamorphic signatures. Inferred temperature conditions suggest that the zoned chromites, homogeneous chromites, and their hosts are equilibrated at different metamorphic conditions. In this paper, various mechanisms during the cumulus to subsolidus stages are explored in order to understand the origin of the two contrasting types of chromites.

Titanian Clinohumite-Bearing Peridotite from the Ulamertoq Ultramafic Body in the 3.0 Ga Akia Terrane of Southern West Greenland

“…Ilmenite is locally observed in olivine grains that are in contact with titanian clinohumite (Figure 4f). Dusty olivine is also commonly observed in other peridotites from the Ulamertoq body [40]. Orthopyroxene has a finer grain size (200 µm in average) compared to olivine and is uniformly distributed (Figure 3c).…”

The Mesoarchean Amikoq Layered Complex of SW Greenland: Part 1. Constraints on theP–Tevolution from igneous, metasomatic and metamorphic amphiboles

“…The aluminous garnet–cordierite–sillimanite–biotite–ilmenite–plagioclase–quartz–spinel mineral assemblage observed in the marginal sil–grt–crd gneiss, either represents an altered metavolcanoclastic protolith, or a metasomatic product. Highly aluminous gneisses have been reported from Kangerdluarssuk in the Maniitsoq region (Yakymchuk and Szilas, 2018), as marginal rocks of the nearby Ulamertoq ultramafic complex (Guotana et al , 2018), and from the Neoarchean Storø Supracrustal Belt (Szilas et al , 2016b). At Amikoq, the sil–grt–crd gneiss at the West Margin is not associated with ultramafic lithologies and it is separated from the bottom noritic sequence of the ALC by a thin quartz-rich metasomatised ‘amphibolite’ layer (Fig.…”

Metasomatic Reactions between Archean Dunite and Trondhjemite at the Seqi Olivine Mine in Greenland

“…Studies of the Ulamertoq ultramafic body (Figure 1) indicated that the peridotites of the Fiskefjord region, which Seqi is part of, potentially experienced serpentinization followed by complete dehydration [31,32]. This is supported by the occurrence of humite-group minerals, and the commonly observed magnetite inclusions in olivine.…”