Nagssugtoqidian Mobile Belt Research Articles

Genesis of the Paleoproterozoic Ammassalik Intrusive Complex, south-east Greenland

The Paleoproterozoic Ammassalik Intrusive Complex of south-east Greenland is located at the margin between the Rae Craton to the north and the North Atlantic Craton to the south. The complex is part of the Nagssugtoqidian Mobile Belt emplaced at mid- to lower-crustal level and represents an eastward continuation of the Nagssugtoqidian Orogen of western Greenland. We present new whole-rock geochemical and isotopic data (whole-rock Rb-Sr, Sm-Nd, U-Pb, Lu-Hf and zircon U-Pb, Lu-Hf, O) acquired from the Ammassalik Intrusive Complex, with the aim of reassessing its petrogenesis and evolution. The intrusive rocks and surrounding host-rocks show typical calc-alkaline signatures as well as negative Ta, Nb and Ti anomalies. The new U-Pb age data indicate a protracted magmatic history over ca. 40 Ma, involving at least three successive magmatic pulses between ca. 1910 and 1870 Ma emplaced into a granulite facies biotite-garnet paragneiss with ca. ≤ 1990 Ma protolith ages. Whole-rock trace element and isotope geochemistry indicate that the magmas that formed the Ammassalik Intrusive Complex were derived from a mildly enriched (εNd0 = −0.8 to –3.8; εSr0 = +24 to +42) and deep, garnet-bearing (MREE/HREE fractionated patterns) mantle source, possibly the sub-continental lithospheric mantle. However, the observed decreasing zircon εHf values with time (εHf0 = +2.8 to −12.5) indicate increasing degrees of crustal contamination involving variably 18O-enriched crustal material (δ18O = 7–9‰) into the mafic magmas of the complex. This interpretation suggests that the mantle-derived magma do not have a deep, garnet-bearing source, but that the continental arc signature in the Ammassalik Intrusive Complex igneous rocks may have been inherited from the assimilation of partial melts of the garnet-bearing host-rocks. In addition, based on the new crystallisation ages broadly coeval with the Columbia supercontinent build up, marked by the formation of world-class Ni-Cu deposits globally, it is proposed that the Ammassalik Intrusive Complex and the south-east Greenland region were in direct continuity of the mineralised Kotalahti belt (Svecofennian Province, Finland). Together with the recent discovery of Ni-Cu mineralisation in the Ammassalik Intrusive Complex, our study further supports the high prospectivity of south-east Greenland for orthomagmatic Ni-Cu deposits.

New interpretations on palaeomagnetic data from the Nagssugtoqidian mobile belt in Greenland

The principles governing directions of magnetisation in a metamorphic terrain, which has experienced slow cooling during uplift, are reiterated. These principles are illustrated in areas affected by the Nagssugtoqidian metamorphism in West and East Greenland. Directions of magnetisation in the Kangâmiut dykes in the Nagssugtoqidian block and in the craton to the south are similar to each other. The directions in the dykes are close to those in the surrounding gneisses, implying magnetisation within a relatively short time. Their palaeomagnetic poles are close to those from Nagssugtoqidian localities elsewhere in West and East Greenland.Between-site and within-site directions in the dykes are asymmetrical, which is attributed to Apparent Polar Wandering. A statistical method of plotting Apparent Polar Wander Paths is presented. Such paths from dykes in the Archaean and Nagssugtoqidian regions are similar, from which a Composite path is drawn, thus establishing that the Apparent Polar Wander Path for Greenland lies between 0°N, 310°E, and 22°N, 260°E, in the Palaeoproterozoic at about 1750Ma, upon cooling after the Nagssugtoqidian metamorphism. Similar Apparent Polar Wander Paths have been drawn from other localities in West Greenland. Numerous Poles from East and West Greenland fall close to this path, and are thus of Nagssugtoqidian age. The mean Nagssugtoqidian pole from eight representative studies lies at 20.3°N, 278.3°E (dp/dm=4.8°/6.4°).

K‐feldspar sand‐grain provenance in the Triassic, west of Shetland: distinguishing first‐cycle and recycled sediment sources?

AbstractSandstone provenance studies can help constrain palaeogeographic reconstructions and ancient drainage system scales and pathways. However, these insights can be obscured by difficulties in geochemically distinguishing or adequately characterizing potential sourcelands, or by failure to identify sedimentary recycling. Triassic basins west of Shetland accumulated ∼2.5 km of sand‐rich sediment. The Middle‐Upper Triassic Foula Formation represents fluvial, aeolian and sabkha facies deposited in the northern interior of the Pangaean supercontinent. Published U‐Pb zircon geochronology and heavy mineral analysis suggest that these sandstones were derived from East Greenland. They contain significant fresh K‐feldspar which is likely to be first cycle and derived directly from its source. Pb isotopic analyses of individual K‐feldspar sand‐grains show a single, unradiogenic Pb population, consistent with the provenance indicated by U‐Pb zircon geochronology. Archaean and Palaeo‐Mesoproterozoic rocks—the Nagssugtoqidian Mobile Belt, the Lewisian Complex or equivalents—are the likely source, with terranes south of the Moine Thrust (Grampian, Caledonian and Variscan) ruled out by both the Pb and U‐Pb data. However, it is not possible to distinguish between rift flank sources to the east and west, as both areas have similar crustal affinity and/or share the same tectonic history. It is possible that the sediment was derived from the West Shetland Platform and not from Greenland. The comparison of provenance signals from robust and less stable mineral phases provides a means of recognizing sedimentary recycling. Robust zircon populations and less stable feldspar in Foula Formation sandstones concur in indicating the same source, suggesting that they are likely to be first cycle. The Triassic sand supply can be contrasted with that in Upper Carboniferous (Namurian) basins in the north of England where a significant zircon population has no corresponding K‐feldspar component. This zircon population is likely to have been recycled from Lower Palaeozoic greywackes from the Southern Uplands Belt or it's along strike extension. Copyright © 2009 John Wiley & Sons, Ltd.

Magnetic susceptibility ellipsoids in Nagssugtoqidian and Archaean rocks in South-East Greenland.

Measurements of magnetic susceptibility have been carried out on Precambrian rocks in south-east Greenland in the Nagssugtoqidian mobile belt from Ammassalik northwards to its boundary with the Archaean craton, and slightly beyond. Directions of maximum susceptibility are the best defined, and are as follows: Ammassalik: Declination = 3º, Inclination = 40º,α95 = 7º; Nagssugtoqidian/Archaean “boundary”: Declination = 311º, Inclination = 62º, α95 = 10º; area enclosing post-tectonic plutons: Declination = 194º, Inclination = 87º, α95 = 17º. The boundary is invisible to the directions of maximum susceptibility. A shear zone near the boundary has been studied in detail. The maximum directions of the samples are tightly grouped and lie in the plane of the zone, whilst the intermediate directions rotate about the maximum direction as the zone is approached, until they lie in its plane. Such rotation is widespread in the boundary area. A plate tectonic explanation for the maxima from the boundary and from Ammassalik is proposed as follows: the maximum direction from the boundary is attributed to subduction and collision of the Archaean plate arriving from the north-east, followed by a vertical component imprinted by the emplacement of the plutons. The maximum direction at Ammassalik is due to overriding Archaean crust coming from the north. Anisotropy of magnetic susceptibility is useful in detecting shear zones and rock fabric when these are not apparent in the field or hand specimen.

Two tectonically significant enclaves in the Nordre Strømfjord shear zone at Ataneq, central West Greenland

The Nordre Strømfjord shear zone is a 1.8 Ga zone of largescale, transcurrent and sinistral ductile shear (Sørensen et al. 2006) within the Nagssugtoqidian Mobile Belt (NMB) of central West Greenland. It has been hypothesised that the NMB is a suture between two Archaean continental masses (Kalsbeek et al. 1987). During field work in 2005 along the Nordre Strømfjord shear zone in the fjord Ataneq (Fig. 1), some unusual rock types were discovered that preserve evidence of magmatic and metamorphic processes not previously reported in the area. These observations include the first indication of high-pressure (HP) metamorphism in West Greenland and the first reported occurrence of a cumulate of giant orthopyroxene. The tectonic telescoping of these features together within the Nordre Strømfjord shear zone has important implications for reconstructing the Palaeoproterozoic history of this region, and provides evidence that processes typical of Phanerozoic continent–continent collision zones (e.g. the Caledonian and Alpine systems) operated at least as far back as 1.8 Ga ago.

Assimilation and high-pressure fractional crystallization (AFC) recorded by Paleo-proterozoic mafic dykes, Southeast Greenland

The northern margin of the Nagssugtoqidian mobile belt in Southeast Greenland exposes a suite of moderately fractionated Fe-rich tholeiitic dykes of Paleo-proterozoic age. The dykes were intruded during extension of the crust prior to the development of the Nagssugtoqidian mobile belt. Although the dykes recrystallized under amphibolite facies conditions during Proterozoic orogenesis, they suffered little deformation. Excluding two very evolved samples, the compositions range between: 7.8–4.6 wt.% MgO, 44.5–52.9 wt.% SiO 2 and 1.8–6.9 wt.% total alkalis. One group of mafic dykes shows distinct mantle-normalized trace element patterns with high abundance of low field strength elements and light rare earth elements and low abundances of high field strength elements. These characteristics are consistent with a process of fractional crystallization coupled with assimilation of the regional granulitic crust. Relatively high rates of assimilation to fractional crystallization (0.7) are required to generate the level of incompatible trace elements. This points to lower crustal conditions, and the assimilation is believed to have taken place at the base of the continental crust. Trace element variations indicate fractionation at high pressure involving clinopyroxene as the main extracted phase. We evaluate two fractionation models corresponding to a pressure of 0.9 and 1.5 GPa, respectively, and show that the trace element variations require polybaric fractionation at pressures from a maximum of 1.5 GPa to a minimum of 0.9 GPa.

Geochronology of Archaean and Proterozoic events in the Ammassalik area, South-East Greenland, and comparisons with the Lewisian of Scotland and the Nagssugtoqidian of West Greenland

The Ammassalik area in South-East Greenland forms a link between the Precambrian terrains of West Greenland and northwest Scotland. It consists mainly of Archaean orthogneisses reworked during a Proterozoic tectonothermal event. Proterozoic ortho- and paragneisses, major leuconorite-charnockite complexes, suites of metadolerite dykes and posttectonic granites also occur. SmNd, RbSr, PbPb and zircon UPb isotope data yield ages of ∼ 3.0–2.8 Ga for Archaean basement gneisses, 2.2-2.1 (?) Ga for Proterozoic gneisses, 1.9-1.8 Ga for the Proterozoic reworking, 1.89 Ga for the leuconorite-charnockite complexes, and 1.7-1.6 Ga for the late granites. Proterozoic reworking has disturbed most Archaean isotope systems, and contamination with Archaean material makes dating of Proterozoic rocks difficult. In several cases discordant dates are obtained from different isotope systems. The chronologies of the Lewisian complex of Scotland and the West Greenland Nagssugtoqidian mobile belt compare closely with that of the Ammassalik area, and support the correlation between these areas suggested by earlier studies. There are, however, also major differences between the Nagssugtoqidian and the mobile belt in the Ammassalik area, and precise correlation across the Inland Ice of Greenland is still ambiguous.

Geochemical mapping and geological reconnaissance in the Nagssugtoqidian mobile belt, West Greenland

The Early Proterozoic Nagssugtoqidian mobile belt is geologically poorly known despite its location in the more populated part of West Greenland (Fig. 1). A detailed geological map (1:100 000; Olesen, 1984) is available for the western part of the Nordre Strømfjord area, but the remaining areas have only been mapped at 1:500 000 scale with geological reconnaissance along the coast and aerial photograph interpretation of inland areas (Map sheet 3: Søndre Strømfjord - Nugssuaq; Escher, 1971).

SIMS analysis of REE in pyroxenes and amphiboles from the Proterozoic Ikasaulak intrusive complex (SE Greenland): implications for LREE enrichment processes during post-orogenic plutonism

Rare-earth and selected trace elements (Ti, Cr, V, Sr and Zr) have been analyzed by secondary ion mass spectrometry (SIMS) in pyroxenes and amphiboles from both the lower and upper zones of the post-orogenic Proterozoic Ikasaulak intrusive complex (Nagssugtoqidian mobile belt of southeast Greenland). The orthopyroxenes from the harzburgites and olivine-orthopyroxenites (lower zone) are LREE-depleted. In contrast. orthopyroxenes from gabbros (the most mafic members of the upper zone) display U-shaped chondrite-normalized patterns and REE contents which increase from core to rim. In the lower zone, clinopyroxene coexists with pargasitic amphibole, both exhibiting unusually high LREE, Sr and Zr contents. In the upper zone, clinopyroxene is less LREE-enriched and is replaced by late- or post-magmatic hornblende. The variations observed in both the major- and trace-element composition of the ferromagnesian phases indicate that the Ikasaulak intrusive complex did not result from a single magma injection. An early injection of magma was responsible for the lower zone and strongly interacted with the country rocks. Fractional crystallization of this magma was probably accompanied by metasomatism effected by highly LREE-enriched hydrous fluids (Ce abundances up to 1300 × chondrite). The most mafic members from the upper zone were derived from a late injection of magma. This magma probably mixed with the residual liquid of the earlier injection, thus retaining the geochemical signature of the contamination processes which had occurred in the lower zone. Mixing and assimilation processes led the pyroxenes to concentrate REE contents higher than those theoretically predicted on the basis of the usual distribution coefficients for basaltic systems. The late growth of hornblende is unrelated to the LREE-enrichment processes. The ion microprobe analyses of the main mafic rock types of the Ikasaulak intrusive complex indicate that: (i) the parental magmas (tholeiites probably marked by a slight LREE-enrichment) were contaminated during the early cumulus processes; (ii) the degree of LREE-enrichment and HFSE (high field strength elements) depletion in parental magmas, as deduced from the whole-rock chemistry, may be overestimated.

Mafic and ultramafic pods with eclogitic relics from the Proterozoic Nagssugtoqidian mobile belt of East Greenland

The cores of amphibolitic and ultramafic pods within the basement complex of the Nagssugtoqidian mobile belt of East Greenland preserve metastable relics of older high-pressure assemblages. The bulkrock geochemistry of amphibolites indicates that their protoliths derive from low-pressure crystal fractionation (mainly controlled by plagioclase, pyroxene and olivine) of tholeiitic melts of probable MORB affinity. Amphibolites from the cores of the pods are corona-textured and provide microtextural evidence of a polyphase retrogression subsequent to a high- T eclogitic event, producing omphacite-garnet assemblages. Symplectitic intergrowths of Ca-clinopyroxene and plagioclase are the result of the unmixing of the older omphacite, whilst garnet is partly replaced by a fine-grained pseudomorph of orthopyroxene, anorthitic plagioclase and magnetite. This suggests subsequent partial re-equilibration in the intermediate-pressure granulite facies, producing clinopyroxene-orthopyroxene-plagioclase-magnetite assemblages. The final event in the amphibolite facies was controlled by the influx of dominantly hydrous fluids. According to kinetic constraints, the amphibolites developed coronitic or granoblastic textures. The coronas consist of plagioclase and hornblende between remaining garnet and unmixed clinopyroxene. Re-crystallized granoblastic amphibolites do not retain relics; according to the bulk rock chemistry, plagioclase and hornblende coexist with either garnet or clinopyroxene. Adjacent ultramafic rocks conform with this metamorphic evolution, being retrogressed from the spinel ±amphibole lherzolite facies to the tremolite-chlorite peridotite facies. Estimated equilibrium conditions for the final amphibolite facies event are T = 600 + 70°C and P = 5.2 ± 1 kbar. Mafic and ultramafic pods from the basement complex of the Nagssugtoqidian mobile belt of East Greenland probably represent fragments of an oceanic crust which was subjected to a subduction event of Proterozoic age.

The metamorphic history of the Nagssugtoqidian mobile belt, southern East Greenland

The metamorphic history of the Nagssugtoqidian mobile belt, southern East Greenland

The Blokken gneisses: a re-appraisal

A group of quartzo-feldspathic gneisses – the Blokken gneisses – is described in the literature as a syn-tectonic igneous suite intruded into the central part of the Nagssugtoqidian mobile belt of South-East Greenland in early Proterozoic time. The gneisses are reported to be transgressive to early structures (Nag. 1) and to show only later (Nag. 2) tectonic fabrics. Field data are at variance with this interpretation. No grounds exist for maintaining the gneisses in question as a distinct unit separable from the regional Archaean gneisses. Consequently the term should be abandoned.

Geochemical characteristics of proterozoic post-orogenic magmatism in the Nagssugtoqidian Mobile Belt of southeast Greenland

The final event in the Nagssugtoqidian (NAG) Mobile Belt of East Greenland is represented by the post-orogenic intrusion of plutonic bodies into an Archaen-Proterozoic polymetamorphic basement. The petrogenesis of plutonic rocks and dykes, which make up the Ikasaulak Intrusive Complex, is interpreted on the basis of their major- and trace-element geochemistry. The investigated intrusive complex consists of a lower ultramafic-mafic sequence and an upper-intermediate felsic sequence. A compositional gap exists at the 60–70% SiO 2 range. Silica-rich rocks, corresponding to a minimum-melt composition, are characterized by strong LREE enrichment and highly fractionated REE patterns, consistent with an origin by partial melting of a garnet-rich source, probably triggered by the intrusion of mantle-derived melts. Most primitive mafic rocks show enrichment in LREE and large-ion lithophile elements and depletion in Nb and Ta. Although these data are suggestive of magmas generated at destructive plate margins, there is no geological reason for assuming a subduction-related origin of the melts. Thus trace-element compositions appear to be a feature of the mantle source, probably retaining the geochemical signature of the crust-generating processes occurring during the previous NAG ocean subduction (1920-1840 Ma). Petrogenetic modelling of the major- and trace-element compositions indicates that fractional crystallization can only partially account for the compositional differences up to 60% SiO 2 and other mechanisms, such as concurrent assimilation and fluid activity, are involved. The present data suggest that the final event in the mobile belt was the stabilization of the sub-continental mantle, which occurred at least 200–300 Ma after the subduction-related crust-generating processes.

Nagssugtoqidian mobile belt of West Greenland: a cryptic 1850 Ma suture between two Archaean continents—chemical and isotopic evidence

New chemical and isotopic data permit the recognition of a cryptic suture zone between two Archaean continental masses within the Nagssugtoqidian mobile belt of West Greenland. This discovery has important implications for Precambrian crustal evolution: suture zones may not always be identifiable from geological field observations, with the consequence that mobile belts in which undetected sutures exist may be mis-identified as ensialic, and thought to require special non-plate tectonic models to account for their development. The Nagssugtoqidian belt consists mainly of Archaean gneisses reworked during the Proterozoic, with metamorphic grade and degree of isotopic disturbance increasing towards the centre of the belt. At the centre of the belt the Nagssugtoqidian includes metasediments and calc-alkaline volcanic and plutonic rocks of Proterozoic age, almost always strongly deformed and metamorphosed. From isotopic evidence (Sr i ca. 0.703; model μ 1 values ca. 8.0; initial ε Nd ca. 0) it is clear that the Proterozoic igneous rocks do not include any significant contributions derived from the Archaean crust, and the chemistry of the rocks, together with the isotope data, suggests that they were formed at a destructive plate margin. The Proterozoic rocks are found in a narrow zone (up to 30 km wide) between the Archaean gneisses to the north and south of Nordre Strømfjord, and are interpreted as reflecting the existence of a suture between two Archaean continental blocks. Zircon U Pb data and other isotope evidence show that subduction started before ca. 1920 Ma ago, and lasted until ca. 1850 Ma when collision occurred, with consequent crustal thickening, high-grade metamorphism and local anatexis. Given the time-span for the operation of subduction, the existence of a wide Nagssugtoqidian ocean can be inferred, even for slow rates of plate motion. The Proterozoic and Archaean gneisses in the Nagssugtoqidian belt are very similar lithologically and chemically, and it has only been possible to distinguish between them using isotopic criteria. Suture zones of this kind are very difficult to detect, and may be present elsewhere within the reworked Archaean terrains of northern Greenland and Canada.

The East Greenland Nagssugtoqidian mobile belt compared with the Lewisian complex

Summary The rocks, structures and geological history of the Lewisian complex are similar to those of the Nagssugtoqidian mobile belt in East Greenland. Both regions comprise a heterogeneous, mainly tonalitic, gneiss complex which formed 2900–2700 Ma. Both gneiss complexes were intruded by basic dykes and intensely deformed and metamorphosed during two major episodes. The first episode, called Inverian in Scotland, is broadly equivalent to the Nag. 1 episode in Greenland dominated by major dextral strike-slip shear zones about 2600–2300 Ma. The second episode, the Laxfordian of Scotland, is broadly equivalent to the Nag. 2 episode of Greenland associated with the southward translation of sheets of rock along northward-dipping thrusts and shear zones about 1900–1700 Ma. Both gneiss complexes were raised to a high crustal level by 1600–1400 Ma and extensively eroded.

The boundary between Proterozoic and Archaean crustal blocks in central West Greenland and northern Labrador

Summary The contact between the Early Proterozoic Nagssugtoqidian Mobile Belt and the Archaean craton in West Greenland, and that between the Churchill Province and the Archaean (Nain) craton in Northern Labrador represent two cross-sections of an Early Proterozoic orogenic boundary. The Archaean cratons in both regions display the peripheral effects of Proterozoic orogenic events. The best indicators of Proterozoic effects on the Archaean cratons are the Late Archaean-Early Proterozoic dykes and the Early Proterozoic supracrustal rocks which unconformably overlie the gneisses and dykes. In Greenland the boundary zone evolution is characterized by Late Archaean (Nag. 1) transcurrent shearing, Early Proterozoic Kangâmiut dyke intrusion and subsequent (Nag. 2) ductile overthrusting. Ductile overthrusts are characteristic of the Nagssugtoqidian-Archaean boundary and attest to substantial shortening of the crust in this region. In Labrador the boundary region has been divided into several distinct zones. Although facets of its evolution are analogous to the boundary in Greenland there are several fundamental differences, the most important of which is that transcurrent movement appears to have been dominant over ductile overthrusting. These differences are provisionally related to contrasting tectonic settings of the two areas during Early Proterozoic continental collision, the Greenland segment being the frontal collision zone whilst the Labrador segment documents slightly oblique transcurrent motion.

The tectonic framework of the Precambrian shield of Greenland A review of new isotopic evidence

There is now reliable isotopic evidence that most of the crystalline basement of Greenland consists of Archaean (>2500 Ma) rocks: only in the Ketilidian mobile belt of southemmost Greenland no Archaean rocks have yet been found. Proterozoic orogenic activity was widespread in the Nagssugtoqidian and Rinkian mobile belts of central and northem West Greenland, and peaked at c. 1850 Ma. There is some evidence that the Nagssugtoqidian mobile belt may define the boundary between two once separate Archaean continental plates and it is possibie that comparable plate boundaries also exist elsewhere in Greenland.

Reconnaissance dating of Archaean rocks from South-East Greenland

A major project of geological investigations in South-East Greenland is planned for 1986 and 1987 with the aim of producing sheet 14 of the 1:500000 geological map series covering Greenland. The northern part of the map sheet is occupied by the Nagssugtoqidian mobile belt, and the southern part consists mainly of Archaean rocks. Because of difficulties of access, the Archaean part of the area is poorly known. Geological reconnaissance has been carried out by Bridgwater & Gormsen (1969), and, as a preparation for the 1986 and 1987 expeditions, by Escher & Nielsen (1982, 1983) and Nielsen & Escher (1985). This report presents reconnaissance Rb-Sr and Pb-Pb whole-rock age determinations from the Archaean part of the map sheet.

Exploration of the Sarfartôq carbonatite complex, southern West Greenland

Carbonatite was emplaced in two stages at c. 600 Ma in the border between the Archaean craton and the Nagssugtoqidian mobile belt. During and after emplacement hydrothermal activity gave rise to fenitisation and alteration with increased contents of Nb and REE adjacent to the igneous core.

Dykes and deformation in the Ikertoq zone of the Nagssugtoqidian at S0ndre Str0mfjord Airport, West Greenland - a discussion

Rb-Sr isotope evidence indicates that deformation in the border zone between the Archaean craton and the Nagssugtoqidian mobile belt in West Greenland took place both during the late Archaean (at ea. 2600 Ma) and during the Proterozoic Nagssugtoqidian orogeny (1850-1600 Ma). The structure (fabric) of the rocks is the combined effect of these two episodes of deformation.