| 1. |
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| 2. |
- Keulen, Nynke, et al.
(författare)
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Meso- and Neoarchaean geological history of the Bjornesund and Ravns Storo Supracrustal Belts, southern West Greenland: Settings for gold enrichment and corundum formation
- 2014
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 254, s. 36-58
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Tidskriftsartikel (refereegranskat)abstract
- To understand the Mesa- and Neoarchaean geological history of the Bjornesund region, southern West Greenland (ca. 62-63 degrees N), we have undertaken an integrated study combining field observations with U-Pb zircon geochronology by LA-ICP-MS, metamorphic petrology, and microstructural analyses. The Bjornesund region links the southern part of the Fiskenaesset anorthosite complex to the well-preserved metavolcanic rocks of the Ravn Story Supracrustal Belt and the Bjornesund Supracrustal Belt, so the development of this region is of key interest to unravelling the Archaean geology of the entire region. In this study, we couple the observed phases of gold-enrichment and corundum formation to the geological history. The leucogabbroic rocks of the Bjornesund region, which are part of the Fiskenaesset complex, intruded at ca. 2.95 Ga into the tholeiitic and calc-alkaline metavolcanic rocks of the two studied belts. Following isoclinal folding, the rocks were intruded by diorite (2.92 Ga) and granodiorite (2.91 and 2.88 Ga), which are now texturally orthogneisses. The area was affected by three major phases of folding; F1 (undated), F2 at 2.85-2.83 Ga, and F3 at 2.71-2.70 Ga. The former deformation event is associated with the gold-enrichment in alteration zones in shear folds in metavolcanic amphibolite near the contact with the orthogneiss. Intrusion of granitic sheets during the F3 deformation triggered the formation of corundum, which is locally of gem quality, at the contact between leucogabbroic and ultramafic rocks. Peak metamorphic conditions were calculated as 580-630 degrees C and 6-4 kbar at 2.85-2.83 Ga. Later retrogression in the southern part of the area at ca. 500-450 degrees C and 4.5-3 kbar, and extensional fault reactivation overprinted some of the older structures. (C) 2014 Elsevier B.V. All rights reserved.
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| 3. |
- Möller, Charlotte, et al.
(författare)
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An Iapetus origin for a layered eclogite complex in the northern Western Gneiss Region, Scandinavian Caledonides
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Ingår i: Journal of Metamorphic Geology. - 0263-4929.
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Tidskriftsartikel (refereegranskat)abstract
- The Western Gneiss Region (WGR) is a Precambrian basement domain in the Scandinavian Caledonides and one of the world's largest high- and ultrahigh-pressure terranes. The south–central WGR underwent regional eclogite facies metamorphism 415–400 Ma ago when Baltica subducted beneath Laurentia, during the Scandian orogeny. Eclogites in the WGR group into two traditional types: (1) Precambrian mafic intrusions metamorphosed in situ during Scandian continental subduction and (2) eclogites, garnet peridotites and garnet pyroxenites within ultramafic complexes derived from the subcontinental mantle beneath Laurentia. We document, using field relations, petrography, whole-rock geochemistry and secondary ion mass spectrometry (SIMS) zircon geochronology, a hitherto unrecognized third type of eclogite in the WGR that places new constraints on its tectonic architecture: an eclogitized fragment of oceanic crust from the Iapetus Ocean. The Kråkfjord eclogite complex is a km2-sized body with an interior consisting of kyanite eclogite (meta-troctolite) and subordinate layers and lenses of garnet peridotite, garnet websterite and kyanite–garnet leucotonalite. This interior is capped by Fe–Ti-rich eclogite, which locally contains subordinate pockets of migmatitic aluminous gneiss. The elemental abundances and isotopic compositions of the Fe–Ti-rich eclogites resemble those of mid-ocean ridge basalt (MORB). In contrast, the interior kyanite eclogites, peridotites and pyroxenites have compositions similar to the gabbroic cumulates in the lower oceanic crust of slow-spreading ridges. U–Pb SIMS dating of igneous zircon cores from a leucotonalite pod in the interior of the Kråkfjord complex yields Cambro-Ordovician igneous ages of 500–440 Ma, with the ~500 Ma age interpreted as the isotopically undisturbed age. This age matches those of Iapetan oceanic rocks exposed elsewhere in the mountain belt. Metamorphic zircon from an Fe–Ti-rich eclogite in the carapace of the Kråkfjord complex dates the eclogite facies metamorphism at 421.9 ± 2.2 Ma, synchronous with the continental collision. Zircon from a leucosome in Fe–Ti-rich retro-eclogite indicates an age of 408.5 ± 2 Ma for the crystallization of partial melt following the decompression. Detrital zircon core ages from a pocket of aluminous migmatitic gneiss in the carapace indicate derivation of sediment from the Baltic crust. Collectively, the data show that the eclogite complex (1) originated at an Iapetus spreading centre near the continent Baltica, (2) subducted to eclogite conditions during Scandian continental collision and (3) was tectonically intercalated with the Precambrian Baltica basement of the WGR.
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| 4. |
- Scherstén, Anders, et al.
(författare)
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Re-Os and U-Pb constraints on gold mineralisation events in the Meso- to Neoarchaean Storo greenstone belt, Storo, southern West Greenland
- 2012
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 200, s. 149-162
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Tidskriftsartikel (refereegranskat)abstract
- The Storo greenstone belt in Godthabsfjord, southern West Greenland consists of juxtaposed rock units of different age and origin, and hosts gold mineralisation that is associated with arsenopyrite along a contact between lithological units and along the axial plane of a large fold core. The age and origin of the gold is debatable, but in this paper, we present new arsenopyrite Re-Os and zircon U-Pb data to constrain the age of the Storo gold deposit. A 2.71 +/- 0.05 Ga arsenopyrite isochron and 2.707 +/- 0.008 Ga highly radiogenic arsenopyrite from a mineralisation along a rock contact, together with a 2.64 +/- 0.02 Ga arsenopyrite isochron along the axial plane of the fold core indicate a two-stage mineralisation process. While the 2.707 +/- 0.008 Ga highly radiogenic arsenopyrite provides firm support for an early mineralisation event, a mixing origin cannot yet be excluded for the 2.71 +/- 0.05 Ga isochron. The 2.64 +/- 0.02 Ga isochron is in perfect agreement with recent U-Pb zircon data (Nutman et al., 2007. Precambrian Research 159, 19-32) and these data are best explained by orogenic mineralisation during amphibolite facies metamorphism along structural weak planes. The initial Os-187/Os-188 value of 0.56 +/- 0.16 for the 2.64 +/- 0.02 Ga isochron indicates a crustal source for the metals, whereas the initial Os-187/Os-188 = -0.1 +/- 0.6 for the 2.71 +/- 0.05 Ga isochron remains unconstrained. Re-Os data are best explained by relatively short crustal residence times of less than 0.1 Ga, wherein the Os, and associated metals, were extracted from the mantle at a time younger than 2.8 Ga, and in which the 2.64 +/- 0.02 Ga stage formed by mobilisation of an earlier mineralisation. Detrital zircon constraints imply volcanism and sediment deposition for parts of the belt at <= 2.84 Ga and that these units were tectonically juxtaposed to similar to 3.05 Ga rocks within the belt. The units that were deposited <= 2.84 Ga record metamorphic zircon U-Pb ages of similar to 2.63 Ga, but evidence for 2.72 Ga metamorphism, which is ubiquitous in adjacent terranes, is lacking. (C) 2012 Elsevier B.V. All rights reserved.
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| 5. |
- Szilas, Kristoffer, et al.
(författare)
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Archaean andesite petrogenesis : insights from the Grædefjord Supracrustal Belt, southern West Greenland
- 2013
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 236, s. 1-15
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Tidskriftsartikel (refereegranskat)abstract
- We present new whole-rock major, trace and platinum-group element data, as well as Sm–Nd and Lu–Hfisotope data for meta-volcanic rocks from the Mesoarchaean Grædefjord Supracrustal Belt (GSB), locatedwithin the Tasiusarsuaq terrane, southern West Greenland. We also present new in situ zircon U–Pbisotope data (by LA-ICP-MS) for associated felsic rocks. This region has experienced amphibolite to lowergranulite facies metamorphism, causing re-equilibration of most mineral phases (including zircon).An intrusive tonalite sheet with a zircon U–Pb age of 2888 ±6.8 Ma, yields a minimum age for the GSB.The Sm–Nd and Lu–Hf isotope data do not provide meaningful isochron ages, but the isotope compositionsof the mafic rocks are consistent with the ca. 2970 Ma regional volcanic event, which is documented in pre-vious studies of the Tasiusarsuaq terrane. The major and trace element data suggest a significant crustalcontribution in the petrogenesis of andesitic volcanic rocks in the GSB. The trace element variation of theseandesitic leucoamphibolites cannot be explained by bulk assimilation–fractional-crystallisation (AFC)processes involving local basement. Rather, the observed patterns require binary mixing between basalticand felsic end-member magmas with between 50% and 80% contributions from the latter (depending onthe assumed felsic composition). Hf-isotope constraints point to contamination with pre-existing conti-nental crust with an age of ca. 3250 Ma. Basement gneisses of this age were previously described at twolocalities in the Tasiusarsuaq terrane, which supports the mixing hypothesis. Thus the felsic end-memberlikely represents melts derived from the local basement.Ultramafic rocks (18.35–22.80 wt.% MgO) in GSB have platinum-group element (PGE) patterns that aresimilar to magmas derived from high-degree melting of mantle, but they have relatively enriched traceelement patterns. We propose that the ultramafic rocks represent arc-related picrites or alternativelywere derived by melting of metasomatised sub-continental lithospheric mantle.Overall these new geochemical data from the Mesoarchaean Grædefjord Supracrustal Belt and thepetrogenetic mixing model in particular, are similar to observations from modern continental subductionzone environments, which also require large degrees of mixing with felsic basement melts. Therefore,we propose that the metavolcanic rocks formed in a modern-style subduction zone geodynamic setting,which due to the hotter Archaean mantle conditions allowed for substantial amounts of partial meltingand magma mixing, rather than assimilating pre-existing continental crust.
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| 6. |
- Szilas, Kristoffer, et al.
(författare)
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Complex calc-alkaline volcanism recorded in Mesoarchaean supracrustal belts north of Frederikshab Isblink, southern West Greenland: Implications for subduction zone processes in the early Earth
- 2012
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 208, s. 90-123
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Tidskriftsartikel (refereegranskat)abstract
- We present new geochemical data for three Mesoarchaean supracrustal belts (Ravns Storo, Bjornesund and Perserajoorsuaq) situated north of Frederikshab Isblink in southern West Greenland for which we propose the collective name 'The Ikkattup Nunaa Supracrustal Association'. They comprise mainly amphibolites of tholeiitic basalt composition and leucoamphibolites of calc-alkaline andesite composition. Both lithological units are cut by aplite sheets of tonalite-trondhjemite-granodiorite (TTG) composition with U-Pb zircon ages of c. 2900 Ma. Lu-Hf and Sm-Nd isochrons based on whole rock amphibolite and leucoamphibolite samples yield ages of 2990 +/- 41 Ma and 3020 +/- 78 Ma, respectively, which are within error of the age of the Fiskenaesset Complex situated 5-25 km to the north. Leucoamphibolites from the three supracrustal belts show apparent geochemical mixing trends between tholeiitic amphibolites and TTG gneisses, as the end-members. By assimilation-fractional-crystallisation (AFC) modelling we can show that one group of leucoamphibolites can indeed be explained by contamination of the parental melts by a TTG-like end-member and another group of high P2O5, La and Nb leucoamphibolites can be explained by contamination involving a hypothetical low-silica adakite end-member. However, the leucoamphibolites are juvenile with epsilon Nd-(2970 Ma) from +2.1 to +3.5 and epsilon Hf-(2970 (Ma)) of +3.5 to +4.3. Thus, the mafic source of the felsic contaminant melts must have been derived from a depleted mantle source more or less at the same time (<60 Ma) as the volcanism took place. Our preferred interpretation of the geochemical and isotope data is that the protoliths of the supracrustal rocks formed in an island arc setting, where early tholeiitic volcanism gave way to calc-alkaline volcanism in a maturing arc. The apparent AFC trends are thus explained by in situ partial melting of basaltic arc crust to form juvenile TTG- and adakite-melts that mixed with mafic magmas or contaminated their mantle source to produce the calc-alkaline leucoamphibolite protolith. This model has important implications for the general interpretation of other Archaean supracrustal belts, because AFC and geochemical mixing trends towards a TTG end-member are not uniquely diagnostic of crustal contamination, but may rather reflect processes operating at lower crustal or source levels in arcs, such as melting-assimilation-storage-homogenisation (MASH) or slab-melt (low-silica adakite) metasomatism of the mantle source, respectively. (C) 2012 Elsevier B.V. All rights reserved.
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| 7. |
- Szilas, Kristoffer, et al.
(författare)
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Highly refractory Archaean peridotite cumulates : Petrology and geochemistry of the Seqi Ultramafic Complex, SW Greenland
- 2018
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Ingår i: Geoscience Frontiers. - : Elsevier BV. - 1674-9871. ; 9:3, s. 689-714
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Tidskriftsartikel (refereegranskat)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.
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| 8. |
- Szilas, Kristoffer, et al.
(författare)
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Origin of Mesoarchaean arc-related rocks with boninite/komatiite affinities from southern West Greenland
- 2012
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937. ; 144, s. 24-39
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Tidskriftsartikel (refereegranskat)abstract
- We report whole-rock elemental and Sm-Nd isotope geochemical data from mafic-ultramafic supracrustal rocks from the Nunatak 1390 area in southern West Greenland. Additionally, we report the metamorphic temperature history for these rocks as derived from tourmaline thermometry on a tourmalinite inlier, as well as in situ U-Pb, Hf and O isotopic data from zircons extracted from tonalite-trondhjemite-granodiorite (TTG) gneisses that intruded the mafic-ultramafic sequence. The supracrustal rocks from the Nunatak 1390 area have a minimum age of c. 2900 Ma defined by U-Pb zircon ages of cross-cutting aplite sheets of TTG composition. The supracrustal sequence comprises mafic rocks with pillow structures and ultramafic rocks with no evidence of their protolith. They all have amphibolite-facies mineral assemblages and a peak metamorphic temperature of approximately 550 degrees C. The mafic sequence has relatively flat trace element patterns (La-N/Sm-N of 0.70-2.4) and mostly negative Nb-anomalies (Nb/Nb* of 0.30-1.0) and resembles modern island arc tholeiites. The mafic sequence can be divided into a high- and low-Ti group, where the former group has lower MgO, and significantly higher contents of incompatible elements such as TiO2, P2O5, Zr, Nb and Th. The ultramafic rocks have major and trace element compositions similar to Ti-enriched/Karasjok-type komatiites described in the literature. However, there are no textural indications that the ultramafic rocks from Nunatak 1390 are komatiites sensu stricto. The low-Ti group of the mafic sequence appears to have been derived from a N-MORB source, whereas the high-Ti group and the ultramafic rocks appear to have been derived from a mantle source that is more enriched than the N-MORB source. However, there is no difference in the initial epsilon Nd of the mafic and ultramafic rocks. Additionally, assimilation-fractional-crystallisation (AFC) modelling is consistent with this enrichment being caused by introduction of juvenile low-silica adalcite (slab-melt) into the mantle source region. Accordingly, we propose that the mafic and ultramafic rocks were derived from a similar type of mantle source, but that the ultramafic rocks were derived from a previously depleted mantle source that was refertilised by slab melts in a subduction zone setting. The high MgO contents of the ultramafic rocks could thus reflect a second stage of partial melting of a refractory mantle in a process similar to that which is suggested for the formation of modern boninites. We propose that the mafic-ultramafic sequence represents an island arc that evolved initially as a juvenile complex (c. 3000 Ma). However, inherited zircon grains in aplites and Hf isotope data recorded by the second intrusive TTG phase (c. 2850-2870 Ma), show that mixing with older pre-existing crust occurred during this event. Because the regional crust is dominated by TTGs of this younger age, our data suggests that it likely formed by accretion and melting of arcs of different ages and/or contamination of juvenile arcs by pre-existing continental crust rather than entirely by juvenile arc differentiation or melting. Our data thus supports melting of thickened mafic crust in an accretionary setting, rather than direct slab melting, as a mechanism for Archaean crust formation. (C) 2012 Elsevier B.V. All rights reserved.
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| 9. |
- Szilas, Kristoffer, et al.
(författare)
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Remnants of arc-related Mesoarchaean oceanic crust in the Tartoq Group of SW Greenland
- 2013
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Ingår i: Gondwana Research. - : Elsevier BV. - 1342-937X. ; 23:2, s. 436-451
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Tidskriftsartikel (refereegranskat)abstract
- The Tartoq Group, located in SW Greenland, consists of supracrustal rocks of mainly tholeiitic basaltic composition, including pillow lavas, sills/dykes and gabbros, as well as ultramafic rocks. Metamorphic grade ranges from greenschist fades to granulite facies. The Tartoq Group crops out as a series of blocks and slivers that are imbricated with originally intrusive Mesoarchaean TTG orthogneisses. The supracrustal rocks form part of a SE vergent fold and thrust belt consistent with the imbrication of TTG gneisses and supracrustal rocks along a convergent margin. LA-ICP-MS U-Pb zircon dating of an intrusive TTG sheet yields a minimum age of 2986 +/- 4 Ma for the Tartoq Group. This age is consistent with MC-ICP-MS Lu-Hf and Sm-Nd isotopic whole-rock data for mafic samples from different blocks of the Tartoq Group, which yield errorchron ages of 3189 +/- 65 Ma and 3068 +/- 220 Ma, respectively. The mafic supracrustal rocks of the Tartoq Group have chondrite-normalized REE patterns with La-CN/Sm-CN of 0.67-1.96 and rather flat primitive mantle-normalized multi-element patterns, except for scattered LILE contents, and generally negative Nb-anomalies with Nb/Nb* of 0.26-1.31. Th/Yb varies between 0.06 and 0.47 and Nb/Yb between 0.45 and 4.4 indicative of an arc affinity when compared to rocks from modern settings. The similar geochemistry of the different lithological units, together with their coeval formation, as evident from trace element geochemical trends, supports a co-magmatic origin for the rock assemblage and their formation as imbricated relics of oceanic crust Accordingly, we propose that the Tartoq Group represents remnants of Mesoarchaean oceanic crust, which formed in a suprasubduction zone geodynamic environment (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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