Implications for the origins of Eoarchean ultramafic rocks of the North Atlantic Craton: a study of the Tussaap Ultramafic complex, Itsaq Gneiss complex, southern West Greenland

Abstract

Ultramafic rocks found within the ~ 3.81 Ga Itsaq Gneiss Complex (IGC) have some mantle-like geochemical characteristics that have led to them being used to directly constrain the nature of the Eoarchean mantle. The discrimination of mantle peridotites that are the residues of partial melting, from cumulate peridotites generated by crystal accumulation from mantle-derived magmas can be difficult in ancient, altered ultramafic rocks whose field relations have been obscured by multiple tectonic episodes. Hence it is important to scrutinize significant individual occurrences of Eoarchean ultramafic rocks in some detail prior to using them to constrain the nature of Earth's early mantle. Here we present mineral chemistry, whole rock major-, trace-, and platinum-group-element abundances, and Re-Os isotope compositions of a previously unstudied large ultramafic enclave in the IGC-the Tussaap Ultramafic Complex (TUC)-with the aim of documenting its origin. High FeO contents of up to 15.5 wt% and correlations between MgO and Os provide strong evidence that the TUC evolved through fractional crystallization rather than partial melt extraction. In addition, co-variations of major elements in the TUC lithologies can be modeled via fractional crystallization of picritic basalts using MELTS. Later alteration and metasomatism of these ultramafic rocks has largely overprinted primary mineral chemistry and resulted in a redistribution of light rare earth elements, rendering these tools ineffective for ascertaining the origin of the TUC or quantifying some of the petrogenetic processes that formed the body. In addition, it is clear that many geochemical features used to identify residual mantle peridotites can also be produced by cumulate or alteration processes, such as some variations in olivine and chromite chemistry, whole rock Al/Si vs Mg/Si systematics, and trace and platinum group element patterns. Finally, combined discrimination diagrams for high field strength elements and moderately high 187 Os/ 188 Os ratios suggest the parental melt of the TUC partially assimilated basaltic crust prior to precipitating the TUC cumulates. As such, these rocks represent a variably obscured record of Eoarchean crystal fractionation from mantle-derived melts. Despite not being prima facie mantle rocks, it is possible that such early formed ultramafic cumulates in nascent continents found their way into the later-stabilized roots of Archean cratons, helping to explain the high compositional variability of cratonic peridotites. Keywords Eoarchean • Itsaq gneiss complex • Ultramafic cumulates • Platinum group elements • Re-Os isotopes Communicated by Timothy L. Grove.

Earth's Earliest Phaneritic Ultramafic Rocks: Mantle Slices or Crustal Cumulates?

“…This feature is commonly interpreted to represent melt depletion in mantle (e.g., Wang et al., 2013), which was used to explain the formation of “group 1” peridotites from the enclaves of meta‐tonalites, south of the Isua supracrustal belt (which have [Pt/Ir] PM of ∼0.2 to ∼0.5; Van de Löcht et al., 2018). However, similarly fractionated HSE patterns could be obtained by cumulates via fractional crystallization of olivine and HSE‐rich alloys (e.g., Coggon et al., 2015; McIntyre et al., 2019). Therefore, the relative depletion in Pt and Pd versus Ir and Ru may be consistent with cumulate origins and not exclusive to depleted mantle rocks (Figure 7 and Figure S7 in Supporting Information ).…”

“…In comparison, fresh or fluid-altered arc and abyssal peridotites either show significantly lower Th concentrations (generally <0.01 ppm) and/or more fractionated heavy REE as evident by lower (Gd/Yb) PM and Gd/Yb ratios (Figure 6). Pilbara rocks and some Eo-or Mesoarchean cumulates from southwestern Greenland (McIntyre et al, 2019;Szilas et al, 2014Szilas et al, , 2018 show relative depletion in Pt and Pd versus Ir and Ru, as evident by (Pt/Ir) PM ratios of ∼0.3-0.6 (Pilbara) and ∼0.02-5 (complied cumulates) (Figure 7). This feature is commonly interpreted to represent melt depletion in mantle (e.g., Wang et al, 2013), which was used to explain the formation of "group 1" peridotites from the enclaves of meta-tonalites, south of the Isua supracrustal belt (which have [Pt/Ir] PM of ∼0.2 to ∼0.5; Van de Löcht et al, 2018).…”

The Mesoarchean Amikoq Layered Complex of SW Greenland: Part 2. Geochemical evidence for high-Mg noritic plutonism through crustal assimilation

“…This is especially acute for Archean cratons such as the North Atlantic Craton in SW Greenland where the dominating tonalite-trondhjemite-granodiorite (TTG) orthogneisses (Garde, 1997;Nutman et al, 2015) dominate the rock record. There is important geological information hidden in the older subordinate ultramafic-mafic layered complexes, both in terms of geodynamic setting (Myers, 1985;Szilas et al, 2015b, Huang et al, 2012McIntyre et al, 2019) and economic aspects (Szilas et al, 2018;Waterton et al, 2020).…”

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

“…Important clues in unravelling early Earth geodynamics reside in the plagioclase-rich ultramafic–mafic layered complexes and associated supracrustal sequences found in Archean cratons such as the North Atlantic Craton (NAC) of southern Greenland (Szilas et al , 2015; McIntyre et al , 2019). For example, the Fiskenæsset anorthosite complex has been interpreted as arc-related cumulates (e.g.…”