On the early archaean isua iron-formation,West Greenland

In the pre-3.7 Ga old Isua supracrustal belt, West Greenland, a banded iron-formation occurs. The iron-formation can be subdivided into different facies according to composition and mineralogy, and these facies resemble the facies subdivision of younger Archaean and Precambrian iron-formations. The geochemistry of the Isua iron-formation indicates that the secular variation in the contents of phosphorus, calcium and aluminium as well as the Na/K ratios of Precambrian iron-formations can be extended into the Early Archaean. A remarkable feature of the Isua iron-formation is the high chalcopyrite/iron-sulphide ratio. Field relationships and geochemical evidence indicate that the iron-formation is mainly of submarine-exhalative origin from brines of basaltic pedigree.     

Chemical assembly of Archean anorthosites from amphibolite- and granulite-facies terranes, SW Greenland

We report whole-rock, major- and trace-element compositions (obtained by XRF and INA methods) for the amphibolite-facies Buksefjorden and granulite-facies Nordland anorthosites, SW Greenland. In a previous petrologic study on the same sample suite, we documented differences in texture, mineralogy, and mineral compositions between these two anorthosite bodies. Chemical analyses confirm differences in composition between the two bodies, but these differences cannot be explained by variations in metamorphic conditions, and point towards differences in the nature of their protoliths. Analyzed Nordland samples are anorthosites and leucogabbros with 88-98% normative plagioclase, whereas those from Buksefjorden include anorthosite, leucogabbro, and gabbro with ~55-95% normative plagioclase. Two or more compositional groupings can be recognized at each site, which correspond to differences in color and mineralogy of the hand samples. Samples from Buksefjorden are mainly quartz-normative, whereas those from Nordland are olivine (- nepheline) normative. Other differences include higher Ni/Co ratio and REE contents in the granulite-facies anorthosites from Nordland. REE pattern shapes are similar, however, being moderately fractionated at ~0.5-102 chondrites with positive Eu-anomalies. Calculated equilibrium melt patterns are similar for both anorthosites, being relatively flat at ~50-1502 chondrites, suggesting unfractionated (but evolved) parental magmas. Olivine must have been present in the protoliths of the Nordland rocks compared with Buksefjorden. Otherwise, the protoliths contained plagioclase with variable An-content (~An₆₂-An₉₂) and a mafic component with variable Fe/Mg (mg ~0.3-0.8). This mafic component was either hornblende or a combination of ortho- and clinopyroxene in fixed proportions, plus a small amount of magnetite. Mixing calculations demonstrate that some Buksefjorden anorthosites contain two varieties of plagioclase: a calcic type that may correspond to cumulus crystals, and a sodic-type that may correspond to a trapped-melt component. On plots of normative whole-rock An versus mg, compositions of the Buksefjorden and Nordland anorthosites form crude negative arrays that differ from the generally positive trends of mafic layered intrusions (Kiglapait, Skaergaard) and from the generally flats trends of plagioclase-rich cumulate rocks (St. Urbain and Stillwater anorthosites). This difference provides further evidence for the distinctive nature of Archean calcic-anorthosite complexes compared with other types of mafic intrusions. Moreover, this distribution of data points is consistent with the assembly of the protolith of the SW Greenland anorthosites mainly as mixtures of plagioclase and hornblende. Finally, the field for the Buksefjorden and Nordland anorthosites overlaps only slightly with that for the Fiskenaesset Complex, thus extending the known range of compositions for Archean anorthosites in West Greenland.     

Comparative petrology of Archaean anorthosites in amphibolite and granulite facies terranes, SW Greenland

We report new field and petrographic observations, and mineral-chemical data, on the amphibolite-facies Buksefjorden and granulite-facies Nordland anorthosites, which occur in different tectonostratigraphic terranes within the Archaean gneiss complex of SW Greenland. The Buksefjorden body [from the Akulleq (middle) terrane] is dominated by plagioclase and Ca-amphibole, but shows widespread effects of retrograde hydration (epidote, chlorite). Most plagioclase compositions are in the An_60–82 range, with the majority of samples showing average core compositions ∼An₇₆, whereas rims or recrystallized margins are ∼An₆₅. Most grains in the An_70–82 range display optically visible Huttenlocher intergrowths. Amphiboles at Buksefjorden are mainly magnesio-hornblende with X _Mg ranging from 0.70 to 0.45. The Nordland anorthosite [from the Akia (northern) terrane] is also dominated by plagioclase and Ca-amphibole, but contains additional clinopyroxene (∼Ca₄₇Mg₃₈Fe₁₅) as well as minor orthopyroxene (∼En₆₈), spinel and corundum. Plagioclase at Nordland shows an equilibrated, equigranular texture, consistent with prolonged slow cooling from high temperatures. Despite this textural equilibration, plagioclase at Nordland shows a striking range of compositions from An₂₈ to An₉₇, most of which is found in single thin sections. A distinctive feature is the presence of discrete anorthite (+ spinel ± corundum) domains in some samples. Although a number of explanations may apply, we consider these domains to result from prograde mass transfer reactions involving Ca-amphibole and plagioclase. Amphibole compositions at Nordland show similar X _Mg to those at Buksefjorden, but are more aluminous, alkalic, and titanian. This shift to more pargasitic compositions is consistent with the contrasts in metamorphic grade between the two anorthosite bodies. At Buksefjorden, there is no correlation between the amount of modal Ca-amphibole and plagioclase composition, which would be expected if amphibole was produced solely through metamorphism. Our results suggest, alternatively, that the primary igneous mineralogy of these rocks may have been plagioclase (∼An₇₆) + hornblende + pyroxene + magnetite. The primary mineralogy at Nordland is less certain, but it is noteworthy that no rocks contain anorthite of unambiguous igneous origin, in contrast to some other occurrences of Archaean anorthosites. Received: 17 January 1996 / Accepted: 12 March 1997     

Stratiform tourmalinites in the Archaean tungsten province of West Greenland

Summary In the Archaean Malene supracrustal rocks of West Greenland different types of stratiform tourmalinites have been found. The present article describes schistose anthophyllite-rich tourmalinites hosted in anthophyllite-cordierite schists. It is shown that the boron is of submarine exhalative origin and was absorbed from seaweater by clay minerals. Tourmaline was formed at an early stage of metamorphism up to staurolite grade. At higher metamorphic grades staurolite became unstable and porphyroblasts of cordierite and tourmaline were formed. The boron is suggested to be from the same brines which supplied tungsten for the extensive stratabound scheelite occurrences found in banded amphibolites and in some tourmalinites in the Malene supracrustal belt.

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Stratiforme Turmalinite in der archaischen Wolfram-Provinz von West-Grönland

Zusammenfassung In den archaischen suprakrustalen Gesteinen Westgrönlands kommen verschiedene schichtgebundene Turmalinite vor. Schiefrige anthophyllitreiche Turmalinite, die in Anthophyllit-Cordierit-Schiefern auftreten, sind der Gegenstand dieser Untersuchung. Das Bor stammt aus submarinen Exhalationen und wurde an Tonmineralen absorbiert. Turmalin wurde bereits in einem frühen Stadium der Metamorphose bis hin zur Staurolith-Fazies gebildet. Mit steigender Metamorphose bildeten sich Cordierit- und Turmalin-Porphyroblasten auf Kosten von Staurolith. Es wird angenommen, daß das Wolfram in den weitverbreiteten schichtgebundenen Scheelitvorkommen der suprakrustalen Gesteine des Malene-Gürtels ebenso wie das Bor aus submarinen Exhalationen stammt.

     

A major shear zone within the Nagssugtoqidian of West Greenland

A regional lineament of highly parallelized planar and linear structures can be followed over a distance of 150 km from the coast to the Inland Ice at Nordre Strømfjord in central West Greenland. This shear zone is situated within the Nagssugtoqidian mobile belt and transects an area of intricate interference structures, from which it has been formed by high shear strain. By using data for the orientation of planar structures outside and within the zone, the shear strain is calculated to be approximately 6. This value applies to the shear zone close to the coast where its width is approximately 15 km. The zone is cut by the granulite/amphibolite facies boundary, and towards lower metamorphic grade the width of the zone decreases. At the same time the shear strain increases, so that the offset across the zone could be constant irrespective of width. At the coast, where the zone has been mapped, the fanning of planar structures shows the zone to be wedge-shaped, thinning upwards.It is therefore suggested that the zone represents a deep-seated ductile part of a major, transcurrent fault with a sinistral displacement of at least 100 km.     

Geochemistry of Archean metasedimentary rocks from West Greenland

Archean metasedimentary rocks occur as components of the Isua supracrustals, Akilia association and Malene supracrustals of southern West Greenland. Primary structures in these rocks have been destroyed by metamorphism and deformation. Their chemistry and mineralogy is consistent with a sedimentary origin, but other possible parents (e.g. acid volcanics, altered pyroclastic rocks) cannot be excluded for some of them. There is little difference in the composition of metasedimentary rocks from the early Archean Isua supracrustals and probable correlative Akilia association. Both have a wide range in rare earth element (REE) patterns with $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$ ranging from 0.61?5.8. The REE pattern of one Akilia sample, with low $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$, compares favourably with that of associated tholeiites and it is likely that such samples were derived almost exclusively from basaltic sources. Other samples with very steep REE patterns are similar to felsic volcanic boulders found in a conglomeratic unit in the Isua supracrustals. Samples with intermediate REE patterns are best explained by mixing of basaltic and felsic end members. Metasedimentary rocks from the Malene supracrustals can be divided into low silica (≤55% SiO₂) and high silica (>77% SiO₂) varieties. These rocks also show much variation in $\text{La}{}_{\mathrm{<mtext>N</mtext>}}\text{Yb}{}_{\mathrm{<mtext>N</mtext>}}$ (0.46?14.0) and their origin is explained by derivation from a mixture of mafic volcanics and felsic igneous rocks. The wide range in trace element characteristics of these metasedimentary rocks argues for inefficient mixing of the various source lithologies during sedimentation. Accordingly, these data do not rigorously test models of early Archean crustal composition and evolution. The systematic variability in trace element geochemistry provides evidence for the bimodal nature of the early Archean crust.     

Glacial landscape evolution in the Uummannaq region,West Greenland

The Uummannaq region is a mosaic of glacial landsystems, consistent with hypothesized landscape distribution resulting from variations in subglacial thermal regime. The region is dominated by selective linear erosion that has spatially and altitudinally partitioned the landscape. Low altitude areas are dominated by glacial scour and higher elevations are dominated by plateaux or mountain valley and cirque glaciers. The appearance and nature of each landscape type varies locally with altitude and latitude, as a function of bedrock geology and average glacial conditions. Selective linear erosion has been a primary control on landscape distribution throughout Uummannaq, leading to plateau formation and the growth of a coalescent fjord system in the Uummannaq region. This has allowed the development of the Uummannaq ice stream's (UIS) onset zone during glacial periods. Fjord development has been enhanced by a downstream change in geology to less‐resistant lithologies, increasing erosional efficiency and allowing a single glacial channel to develop, encouraging glacier convergence and the initiation of ice streaming. The landscape has been affected by several periods of regional uplift from 33 Ma to present, and has been subject to subsequent fluvial and glacial erosion. Uplift has removed surfaces from the impact of widespread warm‐based glaciation, leaving them as relict landsurfaces. The result of this is a regional altitude‐dependent continuum of glacial modification, with extreme differences in erosion between high and low elevation surfaces. This study indicates that processes of long‐term uplift, glacial erosion/protection and spatial variability in erosion intensity have produced a highly partitioned landscape.     

Quaternary marine stratigraphy and geochronology in central West Greenland

A new stratigraphic framework is proposed for the Quaternary of a portion of central West Greenland, based primarily on faunal and geochronologic studies of shallow shelf deposits. Twenty-four occurrences of pre-Holocene deposits were already known in West Greenland and data from 18 new sites are presented, together with new information on some of the previously described localities. Four pre-Holocene marine events are described. The interglacial Ivnaarssuit and Nordre Laksebugt marine events are considered to be Middle Pleistocene in age. The interstadial Laksebugt marine event is considered to be late Middle Pleistocene, whereas the Svartenhuk marine event is correlated with the last interglacial. For the last glacial period an extensive ice shelf is proposed west of Disko. The oldest postglacial deposits are dated at 10,470 ± 130 ¹⁴C-years BP.     

Systematic Sr-isotopic variation in alkaline rocks from West Greenland

Mesozoic lamprophyre and carbonatite dykes from southern West Greenland, which are believed to have evolved from a single mantle-derived magma by crystal fractionation and liquid immiscibility, have initial Sr-isotopic ratios which are negatively correlated with the increasing contents of Sr, Nb and Zr through the suite. The isotopic variation is suggested to have resulted from contamination with radiogenic crustal Sr after the formation of the various rock types of the suite.     

Stratabound scheelite in altered Archaean komatiites,West Greenland

Extensive scheelite-bearing calc-silicate zones with elevated contents of chromium, nickel, copper, zinc, yttrium and barium are found in Archaean supracrustal rocks of the Nuuk area, West Greenland. The calc-silicate zones are largely stratabound and occur in mafic and ultramafic metavolcanic rocks. The calc-silicates are interpreted as amphibolite facies metamorphosed alteration zones in komatiitic intrusive and extrusive rocks as well as in tholeiitic pillow lavas. The alteration which antedates the amphibolite facies metamorphism and the main deformation of the supracrustal rocks, presumably took place at the sea-floor or immediately below the sea-floor. The brines which caused the alteration were enriched in elements such as tungsten, boron, sulphur, iron, copper and zinc. The boron was precipitated as thin tourmalinites and the iron in massive to semi-massive sulphide bodies. The scheelite in the calc-silicates is mainly found as disseminated grains, as stringers and as veinlets. Rare veins up to 15 cm wide with massive scheelite are encountered. Channel sampling revealed grades up to 0.35% W over a width of 2.5 m. Highly anomalous bromine contents up to 418 ppm and equally anomalous chlorine/bromine ratios down to 3 have been found in scheelite-bearing calc-silicates. Somewhat similar anomalous bromine contents and Cl/Br ratios have been reported from serpentinites in the Outokumpu ore body, Finland. At Mount Isa, northern Australia and in the Antimony line in the Murchinson greenstone belt, southern Africa anomalous Cl/Br ratios have been observed. The possible role of bromine in the ore-forming processes is as yet poorly understood.     

Earliest Holocene deglaciation of the central Uummannaq Fjord system,West Greenland

Uummannaq Fjord, West Greenland, held the Uummannaq Ice Stream system that drained an estimated ~6% of the Greenland Ice Sheet (GrIS) during the Last Glacial Maximum. Published ages for the final deglaciation in Uummannaq Fjord vary from as early as c. 9.8 ka to as late as c. 5.3 ka. Assessing this variability requires additional chronological controls to improve the deglaciation history of central West Greenland. Here, we combine ¹⁴C dating of lake sediment cores with cosmogenic ¹⁰Be exposure dating at sites adjacent to the present GrIS margin in the central‐inland sector of the Uummannaq Fjord system. We find that ice retreated to or within the present GrIS margin at 10.8±0.2 ka (n = 6). Although this ‘final deglaciation’ to or within the present GrIS margin across the Uummannaq Fjord system varies from c. 10.8 to 5.3 ka, all chronologies indicate collapse from the continental shelf to the inner fjords at c. 11.0 ka, which occurred at a net retreat rate of 300–1100 m a⁻¹. The Uummannaq Fjord system deglaciated c. 1000 years earlier than the major fjord system to the south, Disko Bugt. However, similarly rapid retreat rates of the two palaeo‐ice stream systems suggest that their collapse may have been aided by high calving rates. The asynchronous deglaciation of the GrIS throughout the Uummannaq Fjord system probably relates to the influence of varying fjord geometry on marine glacier behaviour.     

Palaeoecology of two lake basins from Disko,West Greenland

The macrofossils, lithology and chronology of two lake basins on Disko in central West Greenland have been investigated. Both lakes were isolated from the sea in the mid-Holocene as a result of relative land uplift. A marine-brackish-limnic sequence was recovered from the Qivittut Lake. The marine fauna is unusually diverse, whereas the brackish water fauna consists only of few species. Immediately after isolation the lake passed through a short-lived eutrophic phase, after which stable, mesotrophic conditions were established. From the second lake basin, the Fortunebay Lake, only limnic sediments were recovered. This lake record demonstrates that rather stable conditions have persisted since mid-Holocene times. Both lakes show anomalously high accumulation rates.     

A continuous record of Holocene eolian activity in West Greenland

Eolian landforms are widespread alongside proglacial valley-sandurs in West Greenland and comprise low-relief sand sheets, climbing dunes, and upland loess. Sedimentary facies mainly reflect distance to outwash-source zones and the influence of vegetation cover. The sediments show stratification types typical for poorly to moderately vegetated sand-sheets, alternately laminated silt/peat sequences, and unstratified loess. Twenty-five accelerator mass spectrometry ¹⁴C dates provide the basis for the chronostratigraphy of the inland eolian deposits. ¹⁴C dates from interstratified sand-sheets suggest that the bulk of eolian sands were deposited prior to 3400 cal yr B.P. and after 550 cal yr B.P. This two-phase formation for the inland dunes most likely reflects local changes in proglacial floodplain development and meltwater rerouting associated with a significant recession of the Greenland ice sheet during the mid Holocene climate optimum. Subsequent floodplain regeneration and renewed sand-sheet formation after 550 cal yr B.P. followed when the ice margin readvanced to its present position. In contrast, atmospheric deposition of regionally derived silt in upland peat mires has been continuous since at least 4750 cal yr B.P. Silt influx data demonstrate a strongly episodic history of the intensity of eolian activity over the past five millennia, which tentatively reflects alternating periods of (winter) aridity associated with the variable incursion of maritime air masses over the interior ice-free areas of West Greenland.     

Rare earth elements in the early Archaean Isua iron-formation, West Greenland

The rare earth element (REE) patterns in the 3.8 Ga-old Isua iron-formation are generally flat, resembling those of some primitive basalts. Samples with positive, negative or no europium anomaly were found. It is shown that diagenesis and metamorphism did not significantly change the REE patterns. The presence or absence of europium anomalies in iron-formations cannot be used as an indicator of the presence or absence of oxygen in the atmosphere during the Archaean and Precambrian. The REE contents cannot be used to distinguish Algoma-type from Superior-type iron-formations. There appears to be a striking similarity between the Archaean submarine exhalations and modern submarine hydrothermal systems. It is considered likely that Archaean and early Precambrian seawater had a chondritic REE pattern with a slight enrichment of light REE.     

Strata bound scheelite in the Archean Malene supracrustal belt,West Greenland

In the Archean of Western Greenland a tungsten province ca. 300 km long and up to 120 km wide has been discovered with extensive banded amphibolites containing up to 2% W and 0.16 ppm gold. The tungsten occurs as scheelite which is associated with tourmalinites and strata-bound tourmaline-rich layers in amphibolites of presumed tuffaceous origin and with an iron-formation containing high amounts of tungsten, zinc, copper, lead, molybdenum, and tin. The scheelite is shown to be strata-bound and of submarine exhalative origin.     

The young Earth and the story of the early Archaean rocks of West Greenland

Broadly the science of geology has passed through a number of distinct phases. In the early days attention was focussed on establishing a stratigraphic framework, concentrating on fossils and lithologies—the days of mapping and systematizing of sedimentary successions and the uncovering of the succession of life. Later, in the early twentieth century, geologists became much more interested in igneous rocks. By the 1960s attention turned to the ocean basins, culminating in the acceptance of the paradigm of plate tectonics. At the end of the 1960s, one area of geology that remained relatively little understood was the huge span of time represented by the Precambrian, about 80 per cent of Earth's history. By the 1960s this was changing. Radiometric dating was beginning to show the relative ages of such terranes and new methods of mapping were beginning to be used.     

Calcite-dolomite thermometry: Negative evidence from the Marmorilik Formation,West Greenland

Calcite grains from 19 composite dolomite-calcite marble samples from the Marmorilik Formation, West Greenland, have been analysed for Ca, Mg, and Fe with the microprobe in order to estimate metamorphic temperatures using the calcite-dolomite solvus geothermometer. The results show that there are large variations within single samples, and, insofar as any thermal gradient can be detected, it is the opposite of that indicated by mineral parageneses. Current practice in the application of the method is therefore questioned.     

Conditions of Archean granulite metamorphism in the Godthab-Fiskenaesset region, southern West Greenland

The Nordlandet peninsula (Akia terrane) and the Tasiusarsuaq terrane in southern West Greenland were metamorphosed to granulite facies at 3.0 and 2.8 Ga respectively. Temperatures of metamorphism are estimated using magnetite + ilmenite, garnet + orthopyroxene, garnet + clinopyroxene and garnet + biotite thermometers. Barometry has been carried out in the two terranes using eight different garnet barometers. A uniform set of activity models for all minerals, including the garnet activity model of Newton et al. (1986), is applied to each barometer in order to permit comparison. Pressure estimates using the different barometers are generally quite consistent (±1.5 kbar). Use of the Newton et al. (1986) garnet activity data results in pressures similar to those obtained using other garnet activity models.
Peak metamorphic conditions on the Nordlandet peninsula are estimated to have been 800 ± 50°C, 7.9 ± 1.0 kbar. Values of log f _O2 are estimated to have been 1.1 to 2.0 above the quartz + magnetite + fayalite buffer from assemblages of magnetite + ilmenite and quartz + magnetite + ferrosilite. Peak metamorphic conditions in the Tasiusarsuaq terrane are estimated to have been 780 ± 50°C, 8.9 ± 1.0 kbar. Estimates of f _H2O and f _CO2 using biotite, amphibile, grossular + anorthite and grossular + scapolite equilibria are low in both terranes. These results suggest that granulite metamorphism was fluid absent in both terranes, and that the metamorphism in the Akia terrane and possibly also in the Tasiusarsuaq terrane was initiated by the injection of large volumes of magma into the lower crust.     

Terminal environment, topographic control and fluctuations of West Greenland glaciers

Glaciers are commonly regarded as sensitive indicators of climatic change, but iceberg calving can partially decouple glacier oscillations from climatic forcing. Recent fluctuations (1942–85) of 72 West Greenland outlet glaciers were studied using aerial photographs, and nine of them examined in the field. Despite similar climatic forcing, variable glacier behaviour is apparent. Eighty-four percent of the land terminating glaciers have been retreating or stable during the period, whereas more than half the tidewater and lake terminating glaciers have been advancing. The calving glaciers exhibit much greater variability in frontal behaviour. Patterns of change suggest that the land-terminating glaciers are controlled dominantly by variations in summer temperature, but that calving dynamics have caused the tidewater and lake-calving glaciers to respond to climatic change in non-linear ways. Dynamic and response contrasts are apparent between freshwater and tidewater glaciers. Trough geometry is of great significance in controlling the nature and magnitude of frontal change of calving glaciers; in particular, topographic 'pinning points' represent potentially stable locations within the fjord at which stillstands almost invariably occur, irrespective of climatic change or regional trends.