Eoarchean ultramafic rocks represent crustal cumulates: A case study of the Narssaq ultramafic body, southern West Greenland (original) (raw)
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Contributions to Mineralogy and Petrology, 2019
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 perido-tites 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 ultra-mafic 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 Eoar-chean 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.
Mesoarchaean peridotite-norite cumulates of SW Greenland - The Miaggoq ultramafic complex
Lithos, 2023
Several studies focused on the ultramafic bodies of the Archaean continental crust in southern Greenland in order to gain information on early Earth petrogenetic, metamorphic and metasomatic processes. This research provides the first petrological dataset of the Miaggoq Ultramafic Complex (~1 km 2) in the Akia terrane, with a minimum age of 2997 ± 15 Ma. It comprises ultramafic (dunite, peridotite) and mafic (orthopyroxenite, norite) rocks along with chromitites and provides a window into Mesoarchaean mantle compositions. Field observations, such as chromitite bands, mineral layering, and orthopyroxenite oikocrysts in peridotites coupled with chemical analysis displaying high abundance of chromites in the dunitic rocks and high forsterite contents (Mg# 91 to 92.5) of the olivines, all point to a layered cumulate origin for the Miaggoq body. Pseudosection calculations along with geothermometry estimations reveal peak metamorphic conditions of 850-1100 • C at pressures of 0.7-1.25 GPa under anhydrous conditions followed by a possible metamorphic overprint at 650-800 • C and 0.7 GPa with relatively dry melting (0.025-0.125 wt% H 2 O). MELTS fractional crystallization coupled with cumulate modelling approximated the compositional trends with conditions on ~3 kbar with 1 wt% H 2 O. This research concludes that the Miaggoq body represents a layered cumulate complex that was derived by large degrees of partial melting of the mantle with possible assimilation (synonymous with contamination) of basalts in the crust. Overall, this study provides complementary data for the Mesoarchaean cumulate bodies of the Akia Terrane and their petrological processes.
Geoscience Frontiers, 2018
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 re-fractory 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.
Geochimica et Cosmochimica Acta, 2023
The extensive exposure of the Archean continental crust in southern West Greenland makes it an important window into the tectonic evolution of early Earth. Here, we provide a comprehensive geochemical data set for tholeiitic amphibolites (meta-basalts), calc-alkaline leucoamphibolites (meta-andesites), and ultramafic rocks (meta-cumulates) for the Mesoarchean Bjørneøen Supracrustal Belt, Nuuk region, SW Greenland. This data helps constrain the geodynamic setting in which these rocks formed. The volcanic rocks display two distinct geochemical trends in terms of their La/Sm ratios. Nonetheless, both tholeiitic amphibolites and leucoamphibolites have negative Nb-Ta-Ti anomalies and thus geochemical features associated with apparent island arc or crust contamination processes. Uranium-lead zircon dating of a leucoamphibolite yields an age of 3077 ± 6 Ma, which is older than regional orthogneisses. A series of models for both major element variation (thermodynamics-based) and trace element variation (partition coefficient-based) implies that fractional crystallization of tholeiitic basalt can effectively produce the observed ultramafic rocks. Such ultramafic cumulates had low degrees of crystallinity, reflecting open system magmatic process at shallow depths likely representing magma conduits in a volcanic pile. The geochemical features of the andesites are distinct from the basalts and our modeling excludes a connection via fractional crystallization or crustal assimilation of the two suites. Instead, the andesites formed via high degrees of mixing between basaltic and felsic endmember magmas, requiring elevated temperatures in the mid-to lower-crust. The introduction of felsic components could be derived from partial melting of mafic lower crust, for example by mafic underplating or via some other process that achieves such anatexis, or alternatively by the addition of rhyolitic melt from extensive fractional crystallization. Mixing and homogenization of basaltic and felsic endmembers to produce andesites may occur in modern-style subduction environments, although this could also be feasible in other geodynamic settings in a hotter early Earth.
Geoscience Frontiers, 2022
Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Belt (ISB), hosted in the Itsaq Gneiss Complex of southwestern Greenland. These bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic record. Ultramafic rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral assemblages. Ti-rich humite-group minerals such as titanian chondrodite (Ti-Chn) and titanian clinohumite (Ti-Chu) often occur as accessory phases in the metamorphosed ultramafic rocks. The Ti-rich humite minerals are associated with metamorphic olivine. The host olivine is highly forsteritic (Fo96-98) with variable MnO and NiO contents. The concentrations of the rare-earth elements (REE) and high-field strength elements (HFSE) of the metamorphic olivine are higher than typical mantle olivine. The textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial melting. The close association of olivine, antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between ∼1.3–2.6 GPa within the antigorite stability field (<700 °C). The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels.
We present bulk-rock geochemical and U-Pb zircon age constraints on a ∼580 m thick sequence of Mesoarchean metavolcanic rocks from SW Greenland. The rocks were arguably deformed into a tight synform and meta-morphosed under amphibolite facies conditions, where relict volcanic structures testify to their igneous origins. The sequence includes picrites, tholeiitic basalts and calc-alkaline andesitic to dacitic schists, interbedded with syn-volcanic mafic and felsic feeder intrusions. Amongst late aplitic intrusions, four conform to a minimum U-Pb age for the entire succession of 2929 ± 5 Ma, which is slightly older than a protolith intrusion age of 2902 ± 4 Ma for the regional grey orthogneisses. The metavolcanic section describes an overall inward increase in SiO 2 and Al 2 O 3 , coupled with a decrease in MgO, total iron (FeO T) and CaO, across a major discontinuity that separates: (1) A tholeiitic suite with high FeO T , low Al 2 O 3 /TiO 2 coupled with flat REE-patterns, interpreted as mantle-derived signatures; and (2) a more evolved calc-alkaline suite with relatively low FeO T , high Al 2 O 3 /TiO 2 and steep REE-patterns, yet relatively high Mg, Ni and Cr, which resemble adakites (or high-Mg andesites). A systematic greater negative Nb-anomaly (Nb/Nb *) and LREE-enrichment (La/Sm) N up through the metavolcanic section is consistent with an increasing subduction zone signature, and thereby a likely arc setting for all rocks. The overall field relationships argue strongly for the deposition of (i) enriched tholeiites (including basal picrites that could be either lavas or cumulates), (ii) more depleted tholeiites that also intruded the enriched tholeiites, before (iii) an abrupt transition into distinctly different calc-alkaline and adakite-like volcanics that evolved from andesites to dacites. Thus, the Nigerlikasik section records tholeiitic magmatism, succeeded by calc-alkaline adakitic magmatism during the evolution of an Archean island arc. In our single arc model, we discuss the possibilities of the early tholeiitic suite being formed through progressively more hydrous melting of a juvenile mantle wedge and subsequent low-P fractionation of plagioclase-bearing cumulates; whereas the adakitic suite more likely formed through high-P hornblende + garnet fractionation of an enigmatic parent that was possibly sourced from a crustally contaminated mantle.