Fiskenaesset Complex Research Articles

Formation, origin and geographic typing of corundum (ruby and pink sapphire) from the Fiskenæsset complex, Greenland

Metamorphic petrology observations on rubies found in-situ in their host-rock are combined with geochemical measurements and optical microscopy observations on the same rubies, with the aim of connecting the ruby-forming metamorphic reaction to a unique fingerprint for these minerals. The Fiskenæsset complex in Greenland is used as an area of this case study. Isochemical pressure-temperature sections were calculated based on electron microprobe and whole-rock geochemistry analyses, and compared to field observations. Rubies formed from reaction between olivine/serpentine and anorthite, triggered by the intrusion of a 2.71 Ga pegmatite. Al is sourced from the anorthite reacting to calcic amphibole, silica from the pegmatite reacts with olivine/serpentine to anthophyllite, Cr3+ is mobile in the pegmatitic fluid, giving colour to the rubies. The ruby-forming reaction occurs at about 640 °C and 7 kbar. In order to establish the unique fingerprint for this ruby-bearing ultramafic complex, laser-ablation inductively-coupled-plasma mass-spectrometry trace-element measurements, oxygen isotope compositions, optical microscopy and scanning electron microscopy were applied. Due to the setting in an ultramafic rock-anorthosite-leucogabbro complex, the fingerprint of the rubies from the Fiskenæsset complex is rather unique. Compared to rubies from other localities, Fiskenæsset complex rubies contain high Cr, intermediate Fe, and low V, Ga, and Ti concentrations, low oxygen isotope values (1.6–4.2‰) and a rarely-observed combination of optical growth features and mineral inclusions like anthophyllite+biotite. Results for other Greenland localities are presented and discussed as well. Even though these are derived from ultramafic rock settings too, they record different trace-element ratios and oxygen isotope values, resulting from variations in the Archaean ruby-forming reaction.

Mantle and crustal sources of Archean anorthosite: a combined in situ isotopic study of Pb–Pb in plagioclase and Lu–Hf in zircon

Isotopic analyses of ancient mantle-derived magmatic rocks are used to trace the geochemical evolution of the Earth’s mantle, but it is often difficult to determine their primary, initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration. We present in situ analyses by LA-MC-ICPMS for the Pb isotopic compositions of igneous plagioclase (An75–89) megacrysts and the Hf isotopic compositions of BSE-imaged domains of zircon grains from two mantle-derived anorthosite complexes from south West Greenland, Fiskenaesset and Nunataarsuk, which represent two of the best-preserved Archean anorthosites in the world. In situ LA-ICPMS U–Pb geochronology of the zircon grains suggests that the minimum crystallization age of the Fiskenaesset complex is 2,936 ± 13 Ma (2σ, MSWD = 1.5) and the Nunataarsuk complex is 2,914 ± 6.9 Ma (2σ, MSWD = 2.0). Initial Hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 eHf units. In terms of Pb isotopic compositions of plagioclase, both anorthosite complexes share a depleted mantle end member yet their Pb isotopic compositions diverge in opposite directions from this point: Fiskenaesset toward a high-μ, more radiogenic Pb, crustal composition and Nunataarsuk toward low-μ, less radiogenic Pb, crustal composition. By using Hf isotopes in zircon in conjunction with Pb isotopes in plagioclase, we are able to constrain both the timing of mantle extraction of the crustal end member and its composition. At Fiskenaesset, the depleted mantle melt interacted with an Eoarchean (~3,700 Ma) mafic crust with a maximum 176Lu/177Hf ~0.028. At Nunataarsuk, the depleted mantle melt interacted with a Hadean (~4,200 Ma) mafic crust with a maximum 176Lu/177Hf ~0.0315. Evidence from both anorthosite complexes provides support for the long-term survival of ancient mafic crusts that, although unidentified at the surface to date, could still be present within the Fiskenaesset and Nunataarsuk regions.

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 ~0.5–10× chondrites with positive Eu-anomalies. Calculated equilibrium melt patterns are similar for both anorthosites, being relatively flat at ~50–150× 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.

Deformation of chromite: S.E.M. investigations

The Scannning Electron Microscope operated in the back-scattered electron mode has been applied in a microstructural analysis of chromites. Samples from the Oman, Vourinos (Greece) and Tiebaghi (New Caledonia) ophiolites, displaying a variety of mantle flow and emplacement structures, have been examined and compared with undeformed chromite from the Bushveld intrusion and deformed chromite from the high-grade metamorphic Fiskenaesset complex. Channelling contrast and electron channelling patterns provide information on grain boundaries, subgrains, inclusions and cracks, as well as defects in the chromite lattice. These microstructures suggest that dislocation mechanisms are active in chromite deforming under mantle conditions. Recovery is quick whereas nucleation is slow and, accordingly, recrystallization of chromite occurs by migration of grain boundaries, driven either by strain energy or by interfacial energy, increasing the grain size during deformation. Dislocation mechanisms, however, only provide a secondary mode of deformation compared to brittle fracturing and passive rotation of chromite grains and fragments in the weaker (silicate) matrix. Diffusion-accommodated flow may also be significant.

Radiogenic neon in an Archaean anorthosite

The isotopic composition of Ne trapped in an Archaean anorthosite (age > 2.9 Ga) from the Fiskenaesset Complex in west Greenland is highly anomalous in comparison to the corresponding present-day atmospheric value. The most extreme values measured are 20 Ne 22 Ne = 3.9 ± 0.4 and 21 Ne 22 Ne = 0.48 ± 0.05 . The results were initially obtained using a traditional Nier ion source but later confirmed with a Baur-Signer ion source and a liquid helium cryogenic technique for separating Ne from the much greater quantities of He released during pyrolysis. From a comparison of the measured 21Ne excess/ 4He radiogenic with the calculated production ratio of these isotopes, the anomalies can be understood in terms of the Wetherill reactions, 18O(α,n) 21Ne and 19F( α,n) 22Na( β +) 22Ne. The generation of 21Ne and 22Ne from these reactions must have occurred in the first 1.7 Ga of the Earth's history as the concentration of U and Th in the anorthosite is orders of magnitude too low for the in situ production of the observed quantities. Finally, the 20 Ne 22 Ne , 21 Ne 22 Ne and 21Ne exc/ 4He ratios, trapped more than 2.9 Ga ago, agree well with the corresponding values in continental natural gases emanating today.

Comment on “Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex, southern West Greenland: Nature of the parent magma” by B. L. Weaver, J. Tarney and B. Windley

Evidence that REE have been metamorphically redistributed between plagioclase feldspar and mafic minerals, as suggested by Weaver et al. (1981) is not forthcoming from the six Fiskenaesset rocks used in the study. REE patterns of the separated feldspars and of two rocks containing small modal amounts of mafic minerals are consistent with a light REE-enriched magma during the formation of the Fiskenaesset Complex.

Magnesian högbomite in a sapphirine-bearing rock from the Fiskenaesset region, W. Greenland

AbstractHögbomite occurs in a 20 × 80 cm gedrite-rich lens between meta-volcanic amphibolite and anorthosite at the top of the Fiskenaesset Complex in the Angnertussoq area 13 km SE of Fiskenaesset. Between coarse prismatic gedrites there are dusters with chlorite, spinel, sapphirine, rutile, högbomite and corundum. Högbomite forms 200 µm grains often associated with rutile, surrounded by spinel and sapphirine but never in contact with gedrite. Usually sapphirine forms rims on spinel and contains relict gedrite. Corundum is intergrown with spinel and sapphirine. Reactions suggested by the textures are: 1. Gedrite + spinel + rutile → sapphirine + högbomite + chlorite. 2. Sapphirine → chlorite + corundum + spinel. 3. Högbomite → rutile + corundum + spinel. In some areas without rutile or högbomite the reaction gedrite + spinel + ? → sapphirine + chlorite took place.Mg/(Mg + Fe + Mn) ratios are in the following order: sapphirine (0.95–0.92) > chlorite (0.94) > hornblende (∼ 0.91) > anthophyllite (∼ 0.86) > gedrite (∼ 0.86) > spinel (0.77–0.74) > högbomite (0.75–0.71). Högbomite usually has an anhydrous total of c. 98 %; on the basis of 31 oxygens and with total Fe as FeO one representative analysis has Mg 4.59, Fe 1.73, Zn 0.02, Ni 0.05, Al 14.78, Cr 0.02, Ti 1.2 atoms per formula unit.

Geochemistry and petrogenesis of Archaean metavolcanic amphibolites from Fiskenaesset, S. W. Greenland

The meta-volcanic amphibolites closely associated with the Fiskenaesset anorthosite complex can be subdivided on the basis of trace element patterns or mineral chemistry; by far the most abundant type has light rare-earth element (LREE) depleted REE patterns and displays a wide range in trace element abundances. Chemically comparable amphibolites can be recognised throughout the ca. 2800 M.yr. West Greenland terrain. The geochemistry of the basaltic amphibolites is dominantly controlled by fractional crystallisation processes, although variable degrees of partial melting may also be important. The required crystal extract (plagioclase dominated) in the proposed fractionation scheme is very similar to the primary mineralogy of cumulates of the Fiskenaesset complex and trace element models support a genetic relationship between the anorthosite complex and enclosing host amphibolites. The application of trace element discrimination to assign tectonic environment in the Archaean is arguable. However, details of the trace element chemistry (especially chondritic La/Ta ratios) are taken to suggest, out of a range of likely tectonic environments, an ocean floor, rather than island arc, affinity for the Fiskenaesset amphibolites. The large ion lithophile (LIL) elements display erratic distribution patterns, but are generally enriched relative to the REE. This appears not to be related to high-grade metamorphism but may be a relict feature of seafloor alteration. The association of the cumulate sequence with meta-volcanic amphibolites and metasediments probably represents an ocean floor assemblage emplaced into the lower crust during crustal accretion.

Comparison of platinum, palladium, and rhodium distributions in some layered intrusions with special reference to the late differentiates (upper zone) of the Bushveld Complex, South Africa

The Stillwater, Fiskenaesset, and Bushveld Complexes are all composed of layered ultramafic and mafic rocks in which the cumulus phases generally appear in a regular order, which have similar petrologic and chemical characteristics, which are all Precambrian in age, and which contain platinum-group elements that vary widely in their abundances and relative proportions. Two of the complexes, the Stillwater and Bushveld, contain platinum-group element enriched horizons not yet found in the Fiskenaesset. The ratio of Pt/(Pt + Pd) in rocks from the Stillwater tends to increase from the basal zone to the middle of the ultramafic zone, decreases to the beginning of the banded zone, and then increases again through the banded zone; many fluctuations, however, are superimposed upon this generalized trend. The trends of Pt/(Pt + Pd) ratios appear to show a positive correlation with the Mg/(Mg + Fe (super +2) ) ratio in olivine. In the Fiskenaesset Complex, the Pt/(Pt + Pd) ratio increases rapidly from 0.3 to 0.7 from the lower gabbro and ultramafic units through the leucogabbro units, then increases slowly through the anorthosite and upper gabbro unit. Superimposed on this generalized trend are increasing and decreasing patterns within each unit.Analyses of platinum, palladium, and rhodium for 69 rocks from the main and upper zones of the eastern Bushveld from the Roossenekal area show generally low concentrations of less than 1 ppb Pt; 0.1 ppb Pd; and 0.1 ppb Rh, but larger amounts occur in some magnetite layers and sulfide-bearing zones. Within the main zone up to the pyroxenite marker, from the pyroxenite marker to below lower magnetite layer 2, and in the upper zone, the Pt/(Pt + Pd) ratio decreases with stratigraphic position, although with minor fluctuations in these intervals. The concentration levels of the platinum-group metals appear to be directly related to the amount of sulfide material present and the proportion of intercumulus material. Variations in Pt/(Pt + Pd) ratios in the Bushveld and Fiskenaesset Complexes are presumably related to changes in major mineral compositions as they are in the Stillwater Complex.

Platinum, palladium, and rhodium in the Fiskenaesset Complex, southwestern Greenland

The Archean Fiskenaesset complex contains low concentrations of Pt, Pd, and Rh in comparison with the Stillwater Complex, Montana, but variations in these concentrations with stratigraphic height show trends related to the results of igneous differentiation, even though most of the rocks have been metamorphosed and the complex has been fragmented by granitoid intrusions and repeatedly deformed. Pt, Pd, and Rh are concentrated in ultramafic rocks and chromitites, but relatively high concentrations also occur locally in anorthosite and leucogabbro which are rich in disseminated sulfides. The concentrations of Pt and Pd are inversely related: the Pt:Pd + Pt ratio tends to increase upward. The Rh: Pd + Pt + Rh shows less variation but tends to increase slightly in the upper part of the intrusion. A new fire-assay atomic absorption method was developed to analyze the low concentrations of these metals present in the Fiskenaesset complex, and this method is described.

Plagioclase twin laws and fabrics in three specimens of anorthosite

Twinning patterns and petrofabrics of plagioclases are examined in three specimens of anorthosite from the Bushveld Complex, the Quebec Massif, and the Fiskenaesset Complex. Their plagioclases have petrographical characteristics exhibiting their different petrogeneses. In an anorthosite from the Bushveld Complex, plagioclase grains are twinned after the albite-Carlsbad, pericline, albite and Carlsbad laws. Frequency percentage of the albite-Carlsbad and Carlsbad laws reaches 43% Plagioclase grains in the adcumulate layers are developed with their composition plane (010) subparallel to the cumulate plane, whereas those in the heteradcumulate layers are developed with their composition plane (010) subperpendicular to the cumulate plane. In an equigranular anorthosite from the Quebec Massif, plagioclase grains are polysynthetically twinned after the albite and pericline laws with rare examples of the albite-Carlsbad and Carlsbad laws. Frequency percentage of the latter two laws is only 1% together. Some regularities are recognized in the petrofabrics of c-axis and (010) plane. In a calcic anorthosite from the Fiskenaesset Complex, plagioclase grains are polysynthetically twinned, exclusively according to the pericline law or a combination of pericline and albite laws. The pericline law is predominant and reaches 64% and this twinning pattern cleaarly differs from that of the former two anorthosites.

Pipes of mafic pegmatite in the stratiform Fiskenaesset anorthosite complex, southwest Greenland

Pipes of hornblende gabbro and ultramafite cut across cumulate layering in the lower part of the Fiskenaesset complex. The pipes form irregular networks in which all vertical portions are massive and pegmatitic, whereas some horizontal portions are layered. Two kinds of layering occur: mineral-graded layers formed by precipitation of plagioclase and hornblende, and comb layers formed by the upward growth of tabular hornblende crystals. Alternations of these layers show that magma with plagioclase phenocrysts was flushed through the pipes in pulses, perhaps associated with erratic propagation of cracks and magmatic drilling.

Rare earth element abundances in rocks and minerals from the Fiskenaesset Complex, West Greenland

Rare earth element (REE) abundances determined by activation analysis in rocks, plagioclase and mafic separates from the Fiskenaesset Complex are presented together with data on major and trace elements in the minerals. The REE data for the rocks and plagioclases are distinct from those of many other anorthositic complexes and the abundances are some of the lowest recorded for plagioclase from terrestrial anorthosites. The bulk and trace element compositions of the Fiskenaesset plagioclases show a number of similarities to those of lunar plagioclases. The plagioclases show a positive Eu anomaly of about 10 and a depletion in the heavy REE relative to the light ones. The mafic separates are enriched in the heavy REE relative to the light ones, and show no Eu anomaly except in one sample with a positive anomaly not attributable to plagioclase contamination. It is estimated, from experimental partition coefficient data, that the REE pattern in the magma at an early stage of fractionation was La (17×) to Lu (0.7× chondrites) with a possible positive Eu anomaly. This highly fractionated REE pattern may be attributed to partial melting of a garnet-bearing source.

Petrochemistry and mineralogy of early Precambrian anorthositic rocks of the Limpopo belt, southern Africa

Two sections of the anorthosite ‘complex’ were examined at Messina, South Africa and at Pikwe, Botswana. Thirty XRF whole-rock analyses of samples in stratigraphic order show that alkalies at Messina increase upwards from leuco-gabbros to anorthosites, but no such correlation was found at Pikwe. Electron probe analyses of plagioclases in 33 samples indicate extensive normal zoning and variation (An 31-An 80) especially at Pikwe. The highest An-contents tend to vary monotonically with stratigraphic position, whereas the lowest values are erratic. Twenty-two amphibole analyses indicate that SiO 2, TiO 2 and MgO/FeO increase, decrease and increase, respectively, with height at Messina, but 22 analyses from Pikwe show no such trends. Their compositions are similar to those from the Fiskenaesset complex. Aluminous chromites are Fe-rich, also like the Fiskenaesset ones. The Limpopo anorthositic rocks belong to a layered igneous body that was recrystallized by regional metamorphism and subjected to erratic metasomatism. The original Limpopo and Fiskenaesset bodies strongly resemble gabbroic and peridotitic relics in the Peninsular Ranges Batholith in S. California.

Proceedings

17 January 1973 Geological Information Group Meeting Geological documentation problems and geosystems 31 January 1973 Ordinary General Meeting Igneous Studies, held at the Linnean Society Lecture Room. 1. Demonstration on part of the Fiskenaesset complex, west Greenland. B. F. Windley, University of Leicester. The Fiskenaesset layered intrusion is over 2900 m.y. old, has a strike length of at least 500 km, is bordered by meta-volcanic amphibolites and occurs as a conformable layer up to 2 km thick in high grade gneisses. It has been folded several times and recrystallized by granulite and amphibolite facies metamorphisms. The intrusion has a well-preserved igneous stratigraphy passing upwards ideally through dunites, peridotites, pyroxenites, leuco-gabbros, minor chromiferous ultramafics, gabbros, leuco-gabbros, and anorthosites containing chromitites (up to 20 m thick). Mineral layering, cumulate textures, and grading are common. 70 whole rock XRF and 128 electron probe mineral analyses from one traverse demonstrate that the main differentiation trend of the body is intact, and that some cryptic chemical variations are preserved in olivine, orthopyroxene, hornblende, plagioclase, magnetite and chromite. The original high alumina basalt magma formed under a high water pressure which was responsible for the early crystallization of magnetite, primary igneous amphibole, a high calcium content in plagioclase (An 96–80 ) and a thick sequence of late anorthositic fractionates. The igneous differentiation pattern of this layered intrusion has been only partly reorganized by subsequent, largely isochemical, metamorphic re-equilibration. Field work for this study was carried out with the Geological Survey of Greenland. Co-authors of the petrochemistry are R.K.Herd,

On the discussion of the origin of the Fiskenaesset complex

On the discussion of the origin of the Fiskenaesset complex

Pre-metamorphic folded chromite deposits of stratiform type in the early Precambrian of West Greenland

The low-grade chromite deposits at Fiskenaesset, West Greenland, occur in 0,5–20 m wide horizons in anorthosites which represent the main component of an original layered intrusion, the Fiskenaesset complex. An exposed length of 125 km of this has been mapped, forming conformable stratigraphic horizons in the early Precambrian gneiss basement. The pre-metamorphic chromite deposits have been affected by at least two major phases of folding and metamorphism, but magmatic features are still recognizable. Mineralogical data on the chromite are given together with the petrography of the chromitite. The primary magmatic and secondary metamorphic features of the Fiskenaesset deposits are described and discussed in relation with similar chromite deposits of stratiform type elsewhere in the world.