Amitsoq Gneisses Research Articles

Initial Pb of the Amı̂tsoq gneiss revisited: implications for the timing of early Archaean crustal evolution in West Greenland — Reply

Abstract The rapid change in 207 Pb / 206 Pb ratio during early Earth history provides a powerful constraint for temporal resolution of Archaean crustal evolution. New and published data for Amitsoq gneiss feldspars and whole rocks define the Earth's least radiogenic silicate Pb known until now. The most important feature of the data is that independent of the exact slope of the regression in a common Pb diagram, a very well-defined intercept with plausible terrestrial Pb evolution lines is obtained between 3.63 and 3.66 Ga, identical to a combined Pb/Pb regression age of 3654±73 Ma (MSWD=17.6). We conclude that the magmatic precursors of the Amitsoq orthogneisses separated from a depleted mantle-like source at ca. 3.65 Ga. This contrasts sharply with claims based on ion-probe U–Pb zircon dating that the Amitsoq gneisses had a prolonged, episodic, magmatic evolution extending from ca. 3.87 to 3.60 Ga. Pb isotopes offer no evidence that any Amitsoq gneiss analysed until now began its evolution in the crust as long ago as 3.85 Ga. We propose that zircons significantly older than 3.65 Ga were inherited from chemically evolved rocks as old as ca. 3.85 Ga. Geochemical arguments are presented to support this proposal. New feldspar and whole rock Pb isotope data are also reported for sheets of quartz-dioritic gneiss on Akilia Island that yield the oldest zircon U–Pb age clusters in the range 3.84–3.87 Ga. These sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose C-isotope ratios are regarded as biogenic in origin. As above, Pb-isotopes in the sheets are consistent with a magmatic age of 3.65 Ga, which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers. Additional Pb-isotope data, as well as limited published Sm–Nd and U–Pb zircon data on the Akilia enclaves themselves, suggest an age between ca. 3.65 and 3.70 Ga for their original deposition, some 150–200 Ma younger than proposed previously. Thus the question of overlap of earliest life with a bolide impact scenario terminating ≥3.80 Ga (as on the moon) becomes irrelevant.

Extreme Nd-isotope heterogeneity in the early Archaean—fact or fiction? Case histories from northern Canada and West Greenland—Reply

SmNd data on rock suites from early Archaean provinces in northern Canada and West Greenland clearly demonstrate that tectonothermal (i.e. igneous, metamorphic, tectonic) processes which affected the rocks long after their formation produced open-system behaviour leading to effective resetting of the SmNd system accompanied by complete, or near-complete, Nd-isotope homogenisation. This means that extreme caution is required in interpreting highly variable initial ϵNd values of ancient rocks in terms of long-standing regional mantle heterogeneity or of complex mantle-crust interaction processes. In particular, calculated initial ϵNd values based on high-precision zircon UPb dates may be of little or no significance in terms of geochemical evolution of early mantle and crust source regions. A striking example is provided by the Acasta gneisses of northern Canada, with published SHRIMP UPb zircon dates in the range 3.6–4.0 Ga and apparent, initial ϵNd values in the range −4.8 to +3.6 (Bowring and Housh, 1995). A combination of 34 published and new SmNd whole-rock analyses for a wide range of rock types yields a regression (errorchron) age of 3371 ± 59 Ma (MSWD = 9.2), with initial ϵNd = −5.6 ± 0.7. Whilst the very negative initial ϵND provides strong, independent support for the extreme age of the Acasta gneiss protolith, resetting of the SmNd system at ∼ 3.4 Ga renders calculation of initial ϵNd based on the zircon UPb dates geologically meaningless. Analogous considerations for early Archaean Akilia enclaves and host Amitsoq gneisses of West Greenland suggest that their published range of initial ϵNd values of ∼ −4.5 to +4.5 at ∼ 3.73–3.87 Ga (Bennett et al., 1993) may be unrealistically wide and, therefore, inappropriate for modelling upper-mantle heterogeneity. In an attempt to determine a realistic initial ϵNd value, we have regressed 58 published and new SmNd data for two major rock units of the Isua supracrustal belt (felsites and mica-schists), regarded as having a short-term crustal history combined with minimal SmNd disturbance. The SmNd regression yields an age of 3776 ± 52 Ma (MSWD = 8.2), with initial ϵNd = +2.0 ± 0.6. This value is much closer to conventional depleted-mantle models (e.g., DePaolo et al., 1991) than to the value of ∼ +4.5 proposed by Bennett et al. (1993) for this age range. Our observations suggest that there may have been no major change in processes between early Archaean and more recent styles of depleted-mantle evolution.

Plumbing the Depths: Pb Isotope Constraints on the Origin of the 3.65 Ga Amîtsoq Gneiss

Archaean crust largely consists of tonalite-trondhjemite-granodiorite (TTG) gneiss provinces with associated greenstone belts. Understanding their genesis is a fundamental task in deciphering crustal evolution through time. The oldest well-exposed major TTG-greenstone belt terrain is the Itsaq gneiss complex of southern West Greenland. This complex consists of the TTG-type Amhsoq gneiss with its highly metamorphosed metasedimentary and mafic inclusions (the Akilia association), and the Isua greenstone belt. The isotope-geochemical signatures of these units, and the conclusions drawn from them, are highly debated issues with far-reaching implications for crustal and mantle evolution. The main controversy centres around the significance of ionprobe U-Pb zircon dates. Zircons from these rocks typically show a wide range (c. 3.60 to 3.87 Ga) of near-concordant U-Pb dates. Initial Nd isotope signatures of individual samples, when corrected for ~47Sm decay by assuming that the oldest U-Pb zircon dates represent the age of rock formation, span a wide range between +5 and 5 epsilon units (Bennett et al., 1993). These Nd isotope data have been interpreted to reflect both extreme mantle isotope heterogeneity and extreme LREE-depletion of early Archaean mantle, implying the existence and persistence of a voluminous crust during early Archaean times (McCulloch and Bennett et al., 1994). Such an interpretation of the Nd isotope data of the Itsaq gneiss complex would require a substantial (irreversible?) change in processes of crustal evolution between midand early Archaean times. Here we challenge the above scenario based on new, previously unpublished and published Pb isotope data for the Amitsoq gneiss. The ancient Amksoq gneiss is characterised by extremely low I.t (= 238U/2~ values and contains the earth's least radiogenic silicate Pb. In addition to previously published and unpublished whole-rock and feldspar Pb isotope data of Am]tsoq gneiss samples, we have specifically included samples in this study which contain zircons with some ion-probe U-Pb dates extending back to >3.8 Ga. Plotting all 84 samples (whole rocks, feldspars, leachates) in a common Pb diagram reveals two major features. Firstly, in spite of the wide area over which the samples were collected, a tight regression line (MSWD = 17.6) corresponding to an age of 3654_+73 Ma is obtained. The second, far more important, feature is that independent of the exact slope of the regression, a very well-defined intercept with plausible terrestrial Pb-isotope evolution curves is obtained. Thus, relative to the depleted mantle evolution curve of Kramers and Tolstikhin (1997), intercept ages of a family of regression lines calculated with sub-sets of the data range only between 3.64 and 3.68 Ga. The scatter around the Pb/Pb regression line is most probably the result of minor initial Pb-isotopic heterogeneity and/or some post-formational metamorphic disturbance. Whilst the slope of the Pb/Pb regression line could, in theory, be spurious if not all samples were genetically related, the really important intercept constraint from the least radiogenic Pb data cannot be discounted by arguing that unrelated rocks have been studied. The only straightforward interpretation of the Amhsoq Pb isotope data is that the magmatic precursors of the Amltsoq gneisses were formed over a short period of time (< 100 Ma), most likely c. 3650 Ma ago. The Pb-isotopic constraints are, and have always been, in obvious conflict with the extended (3.87 to 3.6 Ga) crustal growth history proposed by Nutman et al. (1996) and with the Nd isotope interpretation of Bennett et al. (1993). These authors summarily dismissed the Pb-isotope evidence by stating that these might reflect post-igneous disturbance. However, the only scenario under which the original Pb isotope ratios of 3.87 to 3.6 Ga old rocks could evolve into the observed Pb-isotope array would involve three stages. Firstly, a geological event must be proposed at 3.65 Ga capable of completely

High ammonium contents in the 3800 Ma Isua supracrustal rocks, central West Greenland

Ammonium contents of whole-rock and biotite samples from the metasedimentary sequence of the 3800 Ma Isua supracrustal belt, central Greenland, have been determined and compared with the ammonium contents of volcanic, sedimentary, metamorphic, and granitic rocks from the Late Archaean-Early Proterozoic. A sample of the Amitsoq gneiss has an NH 4 + content (whole-rock: 9 ppm, biotite: 29 ppm) that is similar to many younger igneous rocks. Ammonium contents of the supracrustal rocks vary with chemistry and mineralogy of the rock and most likely were inherited from the original sediments. A series of carbonate-bearing and basic plagioclase-bearing biotite-rich schists from sequence A have varying NH 4 + contents (whole-rock: 6–24 ppm) that increase with an increasing chemical /clastic component and decreasing volcanogenic component. Biotites in these rocks, together with biotites in mica schists of the same sequence, have high NH 4 + contents which vary over a small range of 54–95 ppm, suggesting that the NH 4 + in these rocks was contained mostly in authigenic clays of the original sediments. All these data imply that clay minerals were the major sink of NH 4 + or other nitrogen compounds on the Earth's surface at 3800 Ma. Two samples of graphite-bearing garnet-staurolite-mica schist from sequence B have remarkably high NH 4 + contents (whole-rock: 68 and 86 ppm, biotite: 234 and 253 ppm), which are as high as, or even higher than, those of common Late Archaean-Early Proterozoic argillites. It appears almost certain that the original, thick, clay-rich bottom sediments contained a large amount of NH 4 + or other nitrogen compounds. The most likely major source of nitrogen in these NH 4 +-enriched, graphite-bearing mica schists might be organic nitrogen.

Mineralogical and petrochemical characteristics and genesis of Laoniugou gneiss in Jiapigou gold mine, Jilin Province

Detailed mineralogical and petrochemical studies show that the Laoniugou gneiss of the Jiapigou gold mine is composed mainly of plagioclase gneiss and irregular to lentiform plagioclase amphibolite melanic enclaves. The major element contents show an obvious bimodal and trondhjemitic series evolutional trend. This situation is significantly different from that encountered in bimodal calc-alkalic volcanic rocks in the rift-type Archaean greenstone belt. The contents of Rb, Sr and Ba are 7–21 ppm, 153–363 ppm and 201–1451 ppm respectively, close to those of common Archaean grey gneisses. AU the samples of plagioclase gneisses show positive Eu anomalies (even up to 4.6). The protoliths of the plagioclase gneiss are high-Al2O3 trondhjemitic series rocks, belonging to typical TTG of Archaean high-grade metamorphic terrain. The gneiss is quite similar to the B-type Amitsoq gneiss of W. Greenland. The authors believe that the plagioclase amphibolite enclaves are the relics of ancient oceanic crust while the plagioclase gneiss is the TTG ancient intrusive rock resulting from partial melting of the oceanic crust.

UPb single-zircon age for the Tinissaq gneiss of southern West Greenland: A controversy resolved

Ion probe UPb dating of single zircons from a layered gneiss from Tinissaq, southern West Greenland, resolves a long standing controversy about the age of the oldest recognizable orthogneiss unit within the partly retrogressed granulite-facies area south of Ameralik. In spite of whole-rock isotopic evidence for late Archaean ages for these gneisses, some workers have used field criteria to correlate them with the early Archaean (> 3.5 Ga) Amitsoq gneisses of the Godthåbsfjord area farther north and explained the isotopic characteristics in terms of various secondary element migrations. Ages obtained on well-preserved igneous zircons from the Tinissaq gneiss suggest a 2922 ± 7 Ma (2σ) emplacement age for the igneous precursor to the gneiss, ruling out any correlation with the Amitsoq gneisses. A younger generation of igneous zircons may have formed at 2852±9 Ma (2σ), interpreted as the time of a migmatization event. All igneous zircons recrystallized to varying extent as massive U-poor zircons during granulite-facies metamorphism at 2819±23 Ma (2σ). Younger granitoid gneisses that break up the 2922-Ma gneiss were most probably emplaced approximately contemporaneously with this recrystallization event. There is little direct geological correlation across the amphibolite-granulite facies boundary between Tinissaq and Nuuk (Godthåb), in agreement with recent suggestions that the boundary is a major tectonic break.

Growth kinetics and step density in reflection high-energy electron diffraction during molecular-beam epitaxy: Shaun Clarke and Dimitri D Vvedensky, J appl Phys, 63, 1988, 2272–2283

Field studies in the vicinity of Saglek Bay, Labrador, demonstrated that it was possible to subdivide the Archaean gneiss complex into distinct lithologic units and erect a geologic chronology similar to that recognized in Godthaabsfjord, West Greenland. The Uivak gneisses are the oldest quartzo-feldspathic suite in the area and are distinguished from a younger gneissic suite in the field, the undifferentiated gneisses, by the presence of porphyritic basic dykes (Saglek dykes) within the Uivak gneisses. The Uivak gneisses range in composition from tonalites to granodiorites, with the two chemically distinct suites recognized: a grey granodioritic suite and an iron-rich plutonic igneous suite which locally intrudes or grades into a grey gneiss which strongly resembles the grey Uivak gneiss. Rb-Sr isotopic studies indicate an age of 3622 ± 72 m.y. (2σ) and initial Sr isotopic composition of 0.7014 ± 0.0008 (2σ) for the Uivak gneiss suite, i.e. grey gneiss plus iron-rich suite (λRb = 1.39 × 10−11 yr−1). The grey Uivak gneiss suite, treated independently, defines a Rb-Sr isochron with an age of 3610 ± 144 m.y. (2σ) and initial Sr isotopic composition of 0.7015 ± 0.0014 (2σ) which is indistinguishable from the age and initial ratio of the total Uivak gneiss suite, grey gneisses plus iron-rich suite. The undifferentiated gneisses define a Rb-Sr isochron with an age of 3121 ± 160 m.y. (2σ), and initial Sr isotopic composition of 0.7064 ± 0.0012 (2σ). The isotopic data support field observations suggesting the undifferentiated gneisses were derived by local remobilization of the grey Uivak gneisses. The Uivak gneisses resemble the Amitsoq gneisses of Godthaabsfjord both chemically and isotopically. The interpretation of the initial Sr isotopic composition of the Uivak gneisses is interpreted as the time of regional homogenization rather than the initial ratio of the plutonic igneous parents of the Uivak gneisses as suggested for the Amitsoq gneisses. Although the undifferentiated gneisses are contemporaneous with the Nuk gneisses of West Greenland, they do not form a well-defined calc-alkaline suite and may not be associated with major crustal thickening in the Labrador Archaean.

GaP/Si heteroepitaxy by complex ion beam sputtering: F Ishizuka et al, J appl Phys, 63, 1988, 2091–2093

Field studies in the vicinity of Saglek Bay, Labrador, demonstrated that it was possible to subdivide the Archaean gneiss complex into distinct lithologic units and erect a geologic chronology similar to that recognized in Godthaabsfjord, West Greenland. The Uivak gneisses are the oldest quartzo-feldspathic suite in the area and are distinguished from a younger gneissic suite in the field, the undifferentiated gneisses, by the presence of porphyritic basic dykes (Saglek dykes) within the Uivak gneisses. The Uivak gneisses range in composition from tonalites to granodiorites, with the two chemically distinct suites recognized: a grey granodioritic suite and an iron-rich plutonic igneous suite which locally intrudes or grades into a grey gneiss which strongly resembles the grey Uivak gneiss. Rb-Sr isotopic studies indicate an age of 3622 ± 72 m.y. (2σ) and initial Sr isotopic composition of 0.7014 ± 0.0008 (2σ) for the Uivak gneiss suite, i.e. grey gneiss plus iron-rich suite (λRb = 1.39 × 10−11 yr−1). The grey Uivak gneiss suite, treated independently, defines a Rb-Sr isochron with an age of 3610 ± 144 m.y. (2σ) and initial Sr isotopic composition of 0.7015 ± 0.0014 (2σ) which is indistinguishable from the age and initial ratio of the total Uivak gneiss suite, grey gneisses plus iron-rich suite. The undifferentiated gneisses define a Rb-Sr isochron with an age of 3121 ± 160 m.y. (2σ), and initial Sr isotopic composition of 0.7064 ± 0.0012 (2σ). The isotopic data support field observations suggesting the undifferentiated gneisses were derived by local remobilization of the grey Uivak gneisses. The Uivak gneisses resemble the Amitsoq gneisses of Godthaabsfjord both chemically and isotopically. The interpretation of the initial Sr isotopic composition of the Uivak gneisses is interpreted as the time of regional homogenization rather than the initial ratio of the plutonic igneous parents of the Uivak gneisses as suggested for the Amitsoq gneisses. Although the undifferentiated gneisses are contemporaneous with the Nuk gneisses of West Greenland, they do not form a well-defined calc-alkaline suite and may not be associated with major crustal thickening in the Labrador Archaean.

Evolution and emplacement of Archaean terranes in the Kapisigdlit area, southern West Greenland

In the Godthåbsfjord region, early Archaean Arnitsoq gneisses are found in association with the middle Archaean Nûk gneisses (McGregor, 1973). Early models of crustal evolution in the Godthåbsfjord region suggested that the Amitsoq gneisses were reworked in a major crustal accretion-differentiation event, which was marked by the injection of the voluminous Nûk gneisses (e.g. Bridgwater et al., 1974; McGregor, 1979; Moor- - bath et al., 1986). This event ,has been interpreted as having culminated in a granulite-amphibolite facies metamorphic event at c. 2800 Ma, which outlasted all significant ductile deformation (e.g. Wells, 1979; Coe, 1980).

Late-Archaean tectonics in the Færingehavn–Tre Brødre area, south of Buksefjorden, southern West Greenland

The Færingehavn–Tre Brødre area consists of three distinct terranes tectonically jux-taposed by a previously unrecognized event. Contacts between the terranes are mylonitic shear zones truncating lithological units in adjacent terranes. The terrenes are: (1) Færingehavn terrane, largely composed of early-Archaean Amîtsoq gneiss cut by younger Archaean granitic gneiss defined here as the Sätut gneiss; (2) Tre Brødre terrane comprising mid-Archaean Malene supracrustal rocks, anorthosite complex and polyphase Nûk gneisses; (3) Tasiusarsuaq terrane largely comprising mid-Archaean Nûk gneisses affected by c. 2800 Ma granulite facies metamorphism. The Tasiusarsuaq terrane is structurally above both the Færingehavn and Tre Brødre terranes which at c. 2800 Ma experienced a lower grade of metamorphism. Juxtaposition of the terranes took place between 2800 and 2500 Ma and involved thrusting and crustal thickening. Subsequent re-equilibration involved folding, steeply inclined shear belts, intrusion of synkinematic granitoids under amphibolite facies conditions and retrogression of granulite facies assemblages. This thrusting post-dates the 2800 Ma granulite facies metamorphism. It is younger and distinct from the thrusting postulated to explain the intercalation of the early-Archaean Amîtsoq gneisses and the mid-Archaean Malene supracrustal rocks, associated with intrusion of the Nûk gneiss precursors described from Godthåbsfjord. The tectonic breaks provide, for the first time, marker horizons which can be used to assess the amount and type of late Archaean deformation in the southern Godthåbsfjord region.

Geochronology and isotopic variation of the early Archaean Amitsoq gneisses of the Isukasia area, southern West Greenland

Relatively unaltered samples of the Amitsoq gneiss complex in the Isukasia area were identified and analysed despite the effects of metamorphism and local Proterozoic hydrothermal alteration and contamination. The parents of the three lithologies that make up the Amitsoq gneisses of the area were emplaced over about 320 Ma. Taking a weighted average of dates from U-Pb on zircons, Rb-Sr on WR, Sm-Nd on WR and Pb-Pb on WR, the most probable ages of intrusion are: grey gneisses at 3690 ± 50 Ma, white gneisses at 3590 ± 40 Ma and pegmatitic gneisses at 3370 ± 60 Ma. The Rb-Sr isochron initial 87Sr 86Sr ratios for the grey, white and pegmatitic gneisses are 0.7006 ± 1, 0.7016 ± 1 and 0.8248 ± 3, respectively. The corresponding average ϵ Nd ( T) values are +1.0, +0.7 and −0.2. Since the REE chemistry of the rocks that were the direct parents of the grey gneisses is not known, the +1.0 average ϵ Nd ( T) of the grey gneisses need not imply generation from a depleted mantle. The Sr initial ratio and ϵ Nd ( T) values for the white and pegmatitic gneisses are compatible with consecutive derivation at two discrete intervals by partial melting of crust mainly comprised of rocks similar to the grey gneisses. The progressive evolution of ϵ Nd with T for the Amitsoq gneisses in the early Archaean shows that values of ϵ Nd ( T) near zero in granitoids do not necessarily indicate derivation from the “undepleted” mantle (CHUR).

Alteration and metamorphism of Amitsoq gneisses from the Isukasia area, West Greenland: Recommendations for isotope studies of the early crust

Early Archaean gneisses (3400–3700 Ma) in the Isukasia area of West Greenland have been subjected to a series of local and regional metamorphic processes. Metasomatic alteration accompanied the intrusion of the early Archaean white gneisses (~3600 Ma) into the grey gneisses (~3700 Ma), and resulted in local formation of altered rocks. Pb-Pb isotope results on whole rock and feldspar from the gneisses reveal a major rearrangement of Pb isotopes at about 2600 Ma together with some local third stage Pb changes at a later time. This 2600 Ma event is also shown by Rb-Sr data for phengites in the ~3400 Ma Pegmatitic gneiss sheets and can be correlated with metamorphism accompanying the intrusion of a late Archaean pegmatite (~2600 Ma) swarm. Local hydrothermal alteration by fluids emanating from Proterozoic faults and fractures is petrographically recognised in and near major faults and lithological boundaries. This fault-controlled alteration, though only locally significant, may correlate with low grade thermal metamorphism reflected in a 1600 Ma Rb-Sr mineral isochron on biotites from both altered and unaltered gneisses. Oxygen isotope results show that this alteration probably was associated with influx of a low-δ 18O fluid, perhaps meteoric water. Selection of the least altered Amitsoq gneisses can be made on the basis of petrographic and field criteria, and furnishes the best material to study original geochemical and isotopic systematics in these early Archaean rocks.

A new geochronometer using lanthanum-138

We have applied a new geochronometer, based on the decay of 138La to 138Ba, to a sample of Amitsoq gneiss from West Greenland. Using a value for the electron-capture decay constant, λEC, of 4.44 × 10−12 yr−1, the resulting age (2,408 ± 24 Myr) is in good agreement with that from Sm–Nd systematics (2,410 ± 54 Myr). Application of this geological clock is limited to REE (rare earth element)-rich minerals, but its merit is that, for Archaean rocks, the age can be evaluated from one specimen on the assumption that the initial Ba isotope ratios are effectively constant.

Early Archaean Am�tsoq tonalites and granites of the Isukasia area, southern West Greenland: development of the oldest-known sial

Amphibolite facies early Archaean Amitsoq gneisses envelop and intrude the c. 3,800 Ma Isua supracrustal belt, Isukasia area, southern West Greenland. Most of these gneisses are strongly deformed, but in a c. 75 km2 augen of lower deformation, the Amitsoq gneisses are seen to comprise predominantly 3,750–3,700 Ma tonalitic grey gneisses that were intruded first by thin bodies of mafic to dioritic composition, known collectively as the Inaluk dykes, and then by c. 3,600 Ma white gneisses and finally by sporadic c. 3,400 Ma pegmatitic gneiss sheets. The grey gneisses could have formed by partial melting of crust consisting predominantly of basic rocks. The Inaluk dykes are interpreted as strongly fractionated basic melts of mantle origin, contaminated by crustal material. The white gneisses consist mostly of medium grained granite and occur as lenses and anastomosing sheets throughout their host of grey gneisses with subordinate inclusions of supracrustal rocks. The white gneisses have chemistry compatible with formation by partial melting at depth of a source dominated by grey gneisses. The igneous chemistry, including REE abundances, of the grey gneisses and white gneisses has been modified to varying degrees by metasomatism and assimilation reactions during the crystallisation of the white gneisses and also during subsequent tectonometamorphic events. The white gneisses are evidence for considerable reworking by anatexis of sialic crust in the early Archaean, 150 to 100 Ma after its formation. The white gneisses and the pegmatitic gneisses show that granitic rocks s.s. were important in the earliest Archaean, and are further evidence of the diversity of the oldest-known sial.

Malene amphibolites and related anorthosites from Akugdlerssuaq, inner Godthåbsfjord, with komatiite affinities

Mapping by helicopter reconnaissance in 1976 (Allaart et al. 1977) and subsequent more detailed mapping in 1981 (Nutman, 1982) showed that there are several units of anorthosite with associated amphibolite on Akugdlerssuaq, inner Godthåbsfjord. The anorthosite and amphibolite units are interspersed with units ofstrongly deformed Amitsoq gneisses (identified by the presence of basic dyke remains and by enclaves of banded iron formation) sheeted by younger gneisses. Most of the contacts between anorthosite and amphibolite units and the Amitsoq gneisses are clearly tectonic, whilst the remaining contacts could also be tectonic or are modified primary contacts (Nutman, 1982). The younger gneisses are of late Archaean age (Bridgwater &amp; Nutman, unpublished Rb-Sr isotopic data).

Mg-Fe-Mn distribution in amphiboles, pyroxenes, and garnets and implications for conditions of metamorphism of high-grade early Archaean iron-formation, southern West Greenland

AbstractRemnants of early Archaean (Akilia) iron-formation occur as enclaves within the c. 3700 Ma Amitsoq gneisses in the eastern Godthåb-Sukkertoppen region of southern West Greenland. Amphibole- and pyroxene-bearing assemblages are preserved respectively in these two areas. The amphibole-facies rocks comprise one, two, and three-amphibole-bearing assemblages, coexisting with magnetite and quartz (and locally garnet). Experimentally determined phase equilibria suggest that the amphibole-bearing types have not exceeded c. 700 °C, while distribution coefficients in the two-pyroxene ironformation indicate a maximum temperature of around 625 °C.Mg-Fe distribution relationships between the phases are:andwhere the numbers are generalized distribution coefficients and in phase a). Mn distribution is:and The distribution coefficients are variable both from one rock to another and internally from one set of grain pairs to another. The development of the observed variable silicate phase assemblages is dependent upon a complex interaction of not only metamorphic grade (P and T) and element availability, but more importantly aH2o and fo2. In particular, the possibility of reactions between observed assemblages containing cummingtonite + actinolite, magnesian actinotite + magnetite and ferrohypersthene+ferrosalite suggest lesser or greater degrees of dehydration under oxidizing or reducing conditions. For example:and

Contrasting styles of tectonism and magmatism in the late Archaean crustal evolution of the northeastern part of the Ivisârtoq region, inner Godthåbsfjord, southern West Greenland

Varieties of late Archaean gneisses recognized on the basis of field, petrographic and chemical data include multiphase tonalitic—granitic gneisses with abundant enclaves of amphibolite, granodioritic gneisses largely devoid of enclaves, leucocratic schlieric gneisses with pods of amphibolite and hornblendite and grey granitic gneisses with locally abundant enclaves of amphibolite. The bulk of the gneisses are correlated with the Nûk gneisses (c. 3000–2900 Ma) of outer Godthåbsfjord.Most of the gneisses were emplaced during a period of horizontal tectonic interleaving of thick slices of Amitsoq gneisses (c. 3700 Ma) and younger Malene supracrustal rocks (mainly amphibolites older than c. 2900 Ma). Interleaving took place on tectonic slides during regional, horizontal, heterogeneous simple shear movements. Flattening strains modified Ameralik amphibolite dykes in the Amitsoq gneisses and gave rise to S fabrics (banding and schistosity) in the Malene rocks, the S fabrics facilitating emplacement of voluminous Nûk gneisses mainly as broadly concordant sheets. Potash feldspar blastesis and localized partial melting occurred in some of the earlier phases of Nûk gneisses and possibly the Amitsoq gneisses, but the timing of these processes in relation to this period of emplacement is uncertain.High temperatures appear to have been sustained into the second period of deformation when large, mainly upright domes and synforms, some cuspate, were superimposed on the broadly horizontal pile of Amitsoq gneisses, Malene rocks and Nûk gneisses. Tectonic fabrics are dominated by mineral and ribbing lineations coaxial with folds of this second period. Magmatic emplacement of sheets of granitic gneisses and pegmatites continued, many of the sheets being injected coplanar with the second phase parasitic folds. Pegmatites and schlieric gneisses were generated by syntectonic and younger feldspar blastesis and partial melting. The close of late Archaean magmatism is marked by the post-tectonic Qôrqut granite and pegmatite complex (c. 2500 Ma).The horizontal tectonic movements, emplacement of gneisses, generation of domes and synforms and syntectonic injection of younger granitic gneisses appear to have taken place in the interval between c. 3000–2600 Ma ago. Similar relations between late Archaean tectonism and magmatism have been recognized elsewhere in the Archaean block of Greenland, but syntectonic injection of granitic gneisses and transformation of older gneisses to schlieric types appear to be more widespread in this part of the Archaean block in Ivisârtoq.

Stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt, southern west greenland

The highly deformed c. 3800 Ma Isua supracrustal belt is a fragment of a more extensive Early Archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic Amftsoq gneisses in the period 3800-3600 Ma. The supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 Ma, in the Late Archaean and locally at c. 1800 Ma. Layered sequences of rock of sedimentary and probable volcanic origin form over 50% of the belt. Bodies of high MgAl basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of Amitsoq gneisses. The layered rocks which are < 1 km thick are divided into two sequences, that are in faulted contact with each other. The way-up of these sequences has been determined from facing-directions of locally-preserved graded layering in felsic metasediments at several localities. The overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments. Clastic sedimentation took place against a background of chemical sedimentation, shown by interlayers of banded iron formation, metachert and calc-silicate rocks throughout the sequences. The felsic rocks locally preserve graded bedding and possible conglomerate structures, indicating deposition from turbidite flows and possibly as debris flows. Nodules in the felsic rocks contain structures interpreted as fiammé. There is an irregular enrichment in K 2O/Na 2O in many of the felsic rocks at constant SiO 2 and Al 2O 3 content, interpreted as owing to alteration of original andesitic to dacitic volcanic rocks. Banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking, possibly under shallow water conditions. Lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source.

The iron-rich suite from the Am�tsoq gneisses of southern West Greenland: early Archaean plutonic rocks of mixed crustal and mantle origin

A distinctive group of augen gneisses and ferrodiorites (termed the iron-rich suite) is a component of the early Archaean Amitsoq gneisses of southern West Greenland. The iron-rich suite outcrops south of the mouth of Ameralik fjord in an area that underwent granulite facies metamorphism in the early Archaean. The iron-rich suite forms approximately 30% of the Amitsoq gneiss of this area and occurs as sheets and lenses up to 500 m thick. The rest of the Amitsoq gneisses are predominantly tonalitic-granodioritic, banded grey gneisses. Despite intense deformation and polymetamorphism, there is local field evidence that the iron-rich suite was intruded into the grey gneisses after they had been affected by tectonism and metamorphism. The banded grey gneisses are interpreted as 3,700 to 3,800 Ma old; U-Pb zircon ages from the iron-rich suite give concordia intercepts at circa 3,600 Ma.

The Malene metasedimentary rocks on Rypeø, and their relationship to Amîtsoq gneisses

Two groups of Malene metasedimentary rocks on Rypeø, a small island south of Godthåb, have been studied in detail in order to evaluate their origin and relationship to older, adjacent Amîtsoq gneiss. Field observation shows that the metasediments range from massive to finely layered, and include an unusual pod-rock ('pseudoconglomerate'), which probably represents a series of deformed sedimentary layers (or lenses) of contrasting competence. Petrographic study indicates that the metasediments are quartz-rich, contain abundant plagioclase and biotite, and small but variable amounts ofsillimanite, muscovite, microcline and garnet. Rare earth elements (REE) in samples of four metasedimentary lithologies are similar to some published analyses of 'tonalitic' Amîtsoq gneiss. These observations suggest that the protolith of the Malene metasediments on Rypeø was dominated by sandstone type lithologies which evolved in a near-shore tidal to fiuviatile environment. These sediments were probably derived by weathering and erosion of Amitsoq gneiss and deposited unconformably on such a gneissic basement. Petrological study indicates that the Rypeø metasediments were metamorphosed to the beginning of Muse + Qtz breakdown (Musc-Sill-Kfsp transition zone), and locally underwent minor amounts of melting. The proposed original unconformable contact relationships, coupled with high-grade polymetamorphism - not only of the supracrustal rocks but also of Amitsoq gneiss - indicates substantial vertical crustal movement, perhaps as much as 75 km since the time of formation of Amitsoq gneiss at - 3750 Ma.