Highly refractory Archaean peridotite cumulates: Petrology and geochemistry of the Seqi Ultramafic Complex, SW Greenland

Abstract

This paper investigates the petrogenesis of the Seqi Ultramafic Complex, which covers a total area of approximately 0.5 km2. The ultramafic rocks are hosted by tonalitic orthogneiss of the ca. 3000 Ma Akia terrane with crosscutting granitoid sheets providing an absolute minimum age of 2978 ± 8 Ma for the Seqi Ultramafic Complex. The Seqi rocks represent a broad range of olivine-dominated plutonic rocks with varying modal amounts of chromite, orthopyroxene and amphibole, i.e. various types of dunite (s.s.), peridotite (s.l.), as well as chromitite. The Seqi Ultramafic Complex is characterised primarily by refractory dunite, with highly forsteritic olivine with core compositions having Mg# ranging from about 91 to 93. The overall high modal contents, as well as the specific compositions, of chromite rule out that these rocks represent a fragment of Earth’s mantle. The occurrence of stratiform chromitite bands in peridotite, thin chromite layers in dunite and poikilitic orthopyroxene in peridotite instead supports the interpretation that the Seqi Ultramafic Complex represents the remnant of a fragmented layered complex or a magma conduit, which was subsequently broken up and entrained during the formation of the regional continental crust. Integrating all of the characteristics of the Seqi Ultramafic Complex points to formation of these highly refractory peridotites from an extremely magnesian (Mg# ~ 80), near-anhydrous magma, as olivine-dominated cumulates with high modal contents of chromite. It is noted that the Seqi cumulates were derived from a mantle source by extreme degrees of partial melting (>40%). This mantle source could potentially represent the precursor for the sub-continental lithospheric mantle (SCLM) in this region, which has previously been shown to be ultra-depleted. The Seqi Ultramafic Complex, as well as similar peridotite bodies in the Fiskefjord region, may thus constitute the earliest cumulates that formed during the large-scale melting event(s), which resulted in the ultra depleted cratonic keel under the North Atlantic Craton. Hence, a better understanding of such Archaean ultramafic complexes may provide constraints on the geodynamic setting of Earth’s first continents and the corresponding SCLM

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

“…These lithologies have gradational contacts. The peridotite body also has irregular sheets and pods of pink granite, which also cuts the surrounding gneiss [21,22]. The lithologies in Ulamertoq are comparable to those found in the Seqi ultramafic complex (pyroxene-amphibole peridotites or amphibole harzburgites), and tend to occur randomly [22].…”

“…Exposures of these peridotites can be found in the Miaggoq, Seqi, Ulamertoq, and Amikoq ultramafic bodies, which are also associated with noritic rocks [1] ( Figure 1B). [22]); (C) Geologic map of Ulamertoq peridotite body showing the sampling locations of chromitites with zoned chromites (filled circle) and homogeneous chromites (filled square) (adapted from Garde [21]).…”

“…The immediately surrounding orthogneiss host also includes smaller pods of amphibolite, as well as peridotite ( Figure 1C). The Ulamertoq peridotite body is dominated by olivine-rich plutonic rocks containing orthopyroxene [22]); (C) Geologic map of Ulamertoq peridotite body showing the sampling locations of chromitites with zoned chromites (filled circle) and homogeneous chromites (filled square) (adapted from Garde [21]).…”

“…The peridotite body also has irregular sheets and pods of pink granite, which also cuts the surrounding gneiss [21,22]. The lithologies in Ulamertoq are comparable to those found in the Seqi ultramafic complex (pyroxene-amphibole peridotites or amphibole harzburgites), and tend to occur randomly [22]. The classification used by Szilas et al [22] to refer to olivine-rich rocks with varying amounts of orthopyroxene and amphibole is also adopted.…”

“…The lithologies in Ulamertoq are comparable to those found in the Seqi ultramafic complex (pyroxene-amphibole peridotites or amphibole harzburgites), and tend to occur randomly [22]. The classification used by Szilas et al [22] to refer to olivine-rich rocks with varying amounts of orthopyroxene and amphibole is also adopted. The ultramafic host and envelope will be simply referred to as peridotites hereafter.…”