Mesoarchaean peridotite-norite cumulates of SW Greenland - The Miaggoq ultramafic complex (original) (raw)
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Mineralogical Magazine, 2021
Whole-rock major-and trace-element data are presented on a sample collection from the >3 Ga Amikoq Layered Complex (ALC), and hosting amphibolites within the Mesoarchean Akia terrane, SW Greenland. The lithologies range from leuconorite to melanorite/feldspathic orthopyroxenite, orthopyroxenite to harzburgite through to dunite, and tholeiitic basaltic-picritic mafic host rocks. The Amikoq Layered Complex samples are primitive (Mg#: 65-89) with elevated Ni and Cr contents. However, the absence of troctolitic lithologies and the presence of two orthopyroxene compositional trends, suggests that the successions might not be comagmatic. On the basis of trace-element cumulate models, relatively low Ni contents and minor negative Sr-Eu anomalies in some high-Ti ultramafic rocks, it is not possible to exclude a petrogenesis related to a melt similar to that of the mafic host-rocks. Ultramafic samples with U-shaped trace-element distribution patterns are petrogenetically related to the noritic sequences, either through cumulus mineral accumulation or melt-rock reactions. Assimilation-fractional-crystallisation modelling of melanorites nevertheless require the parental melt to have been contaminated/mixed with a component of island-arc-like tholeiite affinity. A boninite-like parental melt might have been derived from the subcontinental lithospheric mantle of the Akia terrane, or alternatively via assimilation of an ultramafic parental melt with island-arc-like tholeiite. Given the complex geological evolution and high-grade metamorphic overprint of the Amikoq Layered Complex, we are unable to differentiate between the two models.
Precambrian Research, 2021
The northern part of the North Atlantic Craton (NAC) in southern West Greenland comprises a large tract of exposed Meso-Neoarchaean continental crust, divided into the ca 3300-2900 Ma Akia and ca 2900-2500 Ma Tuno terranes. We combine aeromagnetic, stream sediment geochemical, new litho-chemical and zircon geochronological data with previously published data to re-evaluate the crustal architecture and evolution of the Akia terrane and its boundary towards the Tuno terrane. The previously recognised, but overlooked, Alanngua complex, situated between the Akia and Tuno terranes is bounded by aeromagnetic lineaments interpreted as Neoarchaean shear zones and has a distinct spectrum of Neoarchaean magmatic and metamorphic zircon ages that are rare in the Akia terrane. The Alanngua complex comprises components derived from both the Akia and Tuno terranes and is interpreted as a tectonic melange created during the Neoarchaean assembly of the NAC. Within the Akia terrane, the chemistry of orthogneiss samples indicate that a large percentage is too mafic to classify as TTG s.s., implying that not only partial melting of mafic crust, but also some yet unaddressed mantle involvement is necessary in their formation. Previous models for the generation of the ca. 3015-2990 Ma quartz-dioritic Finnefjeld and Taserssuaq complexes conflict with their geochemical variation. We argue that the complexes may be genetically linked to buried gabbro-dioritic rocks indicated by their strong aeromagnetic response. Formed at same time are carbonatite, high-Mg gabbro and tonalite-trondhjemite, and we propose that this wide spectrum of rocks was formed by lithospheric and crustal melting in response to asthenospheric up-welling possibly in an extensional setting. Periods of extensive magmatism in the Akia terrane were previously recognised at ca. 3220-3180 Ma and 3070-2970 Ma. We now subdivide the latter period into three episodes: juvenile basaltic-andesitic volcanism at 3070-3050 Ma; tonalitic and dioritic plutonism at 3050-3020 Ma, and gabbroic-dioritic plus tonalitic-trondhjemitic plutonism at 3020-2985 Ma. This last episode was immediately followed by crustal reworking during collision at 2980-2950 Ma.
Earth and Planetary Science Letters, 2024
Several ultramafic enclaves found within the Eoarchean Itsaq Gneiss Complex (IGC) in southern West Greenland have previously been interpreted as representing mantle relicts. However, as Archean ultramafic rocks are frequently overprinted by metamorphism and late-stage metasomatism, it can be difficult to distinguish between peridotites representing mantle residues or primitive crustal cumulates. Therefore, detailed evaluation of individual Eoarchean peridotite occurrences is required to better constrain their origin. Here, we present new petrological observations and geochemical data for the >3.8 Ga Narssaq Ultramafic Body (NUB) in the IGC of SW Greenland. The NUB ultramafic rocks have high FeOt and Cr contents with flat chondrite-normalized trace element patterns and positive Cr-Mg correlations, which distinguish them from mantle rocks. In addition, the variations in olivine and spinel compositions are consistent with a fractional crystallization process. The above geochemical features suggest that the ultramafic rocks of NUB cannot be explained as mantle residues. Instead, their major element compositions are related to the spatially associated tholeiitic amphibolites, indicating that these ultramafic rocks likely represent crustal cumulates derived from high-Mg magmas. This interpretation is supported by thermodynamic modeling using MELTS, which shows that the compositions of the ultramafic rocks in the NUB can be reproduced via fractional crystallization of local high-Mg tholeiitic basalts, involving the accumulation of olivine, spinel, and possibly clinopyroxene, in conjunction with trapped melt. The NUB ultramafic rocks can thus form as a natural consequence of fractional crystallization of regular highly magnesian tholeiitic magmas, making a cumulate origin the simplest explanation for Eoarchean peridotites in general. This also implies that the formation of the NUB did not involve refertilization of subduction-like components as previously assumed, and thus did not require subduction to have taken place in the Eoarchean.
Precambrian …, 2011
Layered anorthosite complexes are typical components of Archean crustal domains. However, the geodynamic settings in which they were emplaced are still discussed as geological relationships are often ambiguous. Here we report major, trace element and high-precision high-field-strength-element (HFSE) data for the recently discovered well preserved Naajat Kuuat Anorthosite Complex from the inner Ameralik fjord region, southern West Greenland. The dataset is complemented by the first combined Hf–Nd isotope analyses for Archean layered anorthosite complexes and U–Pb zircon geochronology. The data contribute to the small database on Archean layered anorthosite complexes and are used to unravel the origin of these complexes and the tectonic regime involved. Fractional crystallisation of olivine, pyroxene, plagioclase and possibly amphibole controls major and trace element variations in the layered intrusion. There are two groups of amphibolites: (1) a group with primitive mantle normalized trace element patterns are similar to those of MORB-like basalts and (2) typical island-arc tholeiites (IAT), apparently indicating an island-arc setting. Lu–Hf regression lines yield an age of 2985 ± 59 Ma (MSWD 4) within the error of the Sm–Nd regression age of 2929 ± 110 Ma (MSWD 17). The initial ɛHf(2985) for the Naajat Kuuat rocks range from +1.6 to +5.8 and the initial ɛNd(2985) range from +0.4 to +3.9, either indicating variably depleted mantle sources or variable degrees of crustal contamination. In contrast to most mafic assemblages, ratios of Nb/Ta are highly variable (7.85 to 18.6), reflecting fractionation and accumulation of amphibole, ilmenite and pyroxene. The MORB-like parental liquids have the highest Nb/Ta of ca. 18, consistent with a mantle source overprinted by melt-like components from subducting oceanic crust with high Nb/Ta. The accumulation of plagioclase forming the anorthosites and the primary fractionation of amphibole as well as the occurrence of high-Al basalts within the Naajat Kuuat complex argue for hydrous parental liquids in support of an island-arc related setting. Zircon U–Pb geochronology from the anorthosite and adjacent tonalites reveal major tonalite intrusion into the complex at ca. 2802 Ma and a second regional event at ca. 2710 Ma, in accord with crustal heating due to micro-continent amalgamation and crustal thickening. Altogether, the geochemical data can be interpreted with a geodynamic model, where anorthosite-complex associated rocks intrude into tectonically thickened island-arc crust. Crustal thickening is possibly triggered by island-arc accretion, leading to the emplacement of TTG bodies that further thickened the crustal pile. Further collision and amalgamation with other proto-crustal assemblages might have led to enhanced crustal magmatism and granulite facies metamorphism.
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.
Mineralogical Magazine, 2020
The metamorphic history of the Mesoarchean Amikoq Layered Complex within the Akia terrane of SW Greenland was characterised by electron microprobe mineral data and detailed petrography on 12 representative samples, integrated with zircon U–Pb geochronology and petrology. The complex intruded into a >3004 Ma supracrustal association now consisting of granoblastic metabasites with subordinate quartz-rich gneiss. Supracrustal host rocks contain a relict high-temperature assemblage of orthopyroxene–clinopyroxene (± pigeonite exsolution lamellae, exsolved at ~975–1010°C), which is interpreted to pre-date the Amikoq intrusion. Cumulate to granoblastic-textured rocks of the main Amikoq Layered Complex range modally from leuconorite to melanorite, orthopyroxenite to harzburgite/dunite and rare hornblende melagabbro. Observed mineralogy of main complex noritic lithologies is essentially relict igneous with orthopyroxene–biotite and hornblende–plagioclase thermometers yielding temperatures...
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.
Different geodynamic models exist for the growth and differentiation of Archean continental crust, ranging from horizontal tectonics with subduction zones to vertical tectonics with foundering of greenstone sequences. U-Pb zircon geochronology, field relationships, and pressure-temperature constraints from granulite-facies metabasite of the Akia Terrane of the North Atlantic Craton in West Greenland show that this terrane grew through two major magmatic growth episodes: an earlier one at c. 3.2 Ga, and a later one at c. 3.05-2.97 Ga. Phase equilibrium modelling for assemblages related to the latter indicates temperatures of > 800°C at < 0.9 GPa, consistent with a high apparent geothermal gradient and implies thin crust. Granulite-facies metamorphism and partial melting occurred in the absence of pervasive ductile deformation as indicated by nebulitic, undeformed pyroxene-bearing leucosome in metabasite gneiss. Trace element modelling suggests that c. 3.0 Ga tonalite at the current exposure level in the Akia Terrane was generated at pressures of > 0.8 GPa in the stability field of garnet. U-Pb zircon geochronology and existing Hf isotope data are also consistent with a model involving protracted Mesoarchean magmatic growth with limited mantle addition during a prolonged period of high temperatures in a relatively stagnant tectonic regime prior to Neoarchean compressional tectonism in the Akia Terrane.