Uivak Gneisses Research Articles

Laser ablation split-stream analysis of the Sm-Nd and U-Pb isotope compositions of monazite, titanite, and apatite – Improvements, potential reference materials, and application to the Archean Saglek Block gneisses

Continued improvements in both ICPMS (inductively-coupled plasma mass spectrometry) and laser ablation technologies are fueling advancements in accessory mineral investigations and their related isotope systems of interest, and are now being applied to a wide range of geological applications. In this contribution we present an updated methodology for laser ablation split-stream (LASS) analysis of the light rare earth element (LREE) enriched minerals monazite, titanite, and apatite for simultaneous analysis of the U-Th-Pb age (or trace element content) and the Sm-Nd isotope system. The data were collected with the current generation of high-sensitivity ICPMS and laser systems (ThermoFinnigan NeptunePlus MC-ICPMS and Element XR SF-ICPMS- coupled to a RESOlution 193 nm ArF excimer laser system). The increased sensitivity of these ICPMS instruments allows for improved spatial resolution and the ability to target minerals which previously contained insufficient concentrations of elements of interest (e.g., Sm-Nd in apatite), making their analysis difficult, if not impossible, using less sensitive instruments. Furthermore, the higher sensitivity allows less aggressive ablation parameters that facilitate thin section sampling and reduces inter-element and isotopic fractionation.To assess and improve the accuracy, precision, and efficiency of the technique, three new potential LASS reference materials (RMs) are evaluated for dual U-Pb and Sm-Nd analysis (Tory Hill apatite, Tory Hill titanite, and Steenskralkamp monazite). The homogeneity of these materials was first assessed using reconnaissance laser ablation analyses, with final characterization of U-Pb age and Sm-Nd isotope composition using isotope-dilution thermal ionization mass spectrometry (ID-TIMS). The precision and accuracy of the LASS method is explored using secondary mineral reference materials of known age and Sm-Nd isotope composition. The utility of the technique is evaluated with a case study of monazite and apatite from the Eoarchean Uivak Gneiss complex of Labrador, Canada. Finally, we suggest that there is a wide variety of geological applications for this methodology, such as multi-mineral detrital provenance, crustal growth, and petrogenic studies.

Gneiss-forming events in the Saglek Block, Labrador; a reappraisal of the Uivak gneiss

The Archean gneiss complex of the Saglek Block of Labrador is a part of the North Atlantic Craton, and is correlated with southern West Greenland, both being metamorphosed during a ca. 2.7 Ga event. The main component of the complex is the Eoarchean Uivak orthogneiss, which includes lenses of the Nulliak supracrustal assemblage. Both lithologies are cut by the mafic Saglek metadykes. The Uivak gneisses have been divided into Uivak I grey gneiss and Uivak II augen gneiss. The former underwent ca. 3.6 Ga high-T metamorphism prior to the intrusion of the latter. However, the exact age, nature, and extent of Uivak II gneiss are poorly understood. We present geochemical and geochronological results for both these orthogneisses to help refine the various hypotheses that have been proposed concerning the nature of their protoliths. Magmatic ages of 3746 ± 5 and 3717 ± 6 Ma are consistent with previous estimates for the age of Uivak I gneiss. Uivak II augen gneiss from Maidmonts Island, where there is a clear intrusive relationship between the Uivak II and Uivak I gneissic protoliths, has an age of 3325 ± 3 Ma. This is similar to an homogeneous grey gneiss from St. John’s Harbour, with an age of 3318 ± 5 Ma. Grey gneiss from Big Island is distinctively younger (3219 ± 7 Ma), and equivalent to the ca. 3.24 Ga Lister gneiss. Our study shows that granitic gneisses classified as Uivak II were emplaced 200–300 million years after ca. 3.6 Ga metamorphism and deformation of the Uivak I gneiss. The igneous protolith of Uivak II gneiss pre-dates the Lister gneiss by about 100 Ma. The Uivak I and Lister gneisses are geochemically similar, and are both Tonalite–Trondhjemite–Granodiorite (TTG) gneisses, whereas the Uivak II gneiss is a granitoid partially derived from pre-existing crust. We propose abandoning the term ‘Uivak II gneiss’, and renaming ca. 3.3 Ga granitoids, after the type locality, as Maidmonts gneiss. This restricts the term ‘Uivak gneiss’ to Eoarchean TTG gneisses and removes the necessity for subdividing them into Uivak I and II.

Complexity of the early Archean Uivak Gneiss: Insights from Tigigakyuk Inlet, Saglek Block, Labrador, Canada and possible correlations with south West Greenland

The Saglek Block of Labrador comprises Eoarchean to Neoarchean lithologies, metamorphosed at high temperature at ca. 2.7 Ga. Here, we investigate the gneisses of Tigigakyuk Inlet, previously identified as the locality exposing the most ancient rocks in the Saglek Block. New geochronological and geochemical results reveal a multistage history. Precise magmatic emplacement ages of 3.75–3.71 Ga refine the age of the Uivak Gneiss. Zircon rims and neoblastic grains with low Th/U record metamorphism at ca. 3.6 and 2.8–2.7 Ga. Magmatism between these tectono-metamorphic events is recorded by the presence of meta-mafic dykes in the gneisses, gabbroic enclaves in ca. 2.7 Ga syn-tectonic granitoids, as well as by a ca. 3.56 Ga age for monzonitic gneiss in which metamorphic zircon is present as xenocrysts. Felsic (TTG) magmatism between ca. 3.75 Ga and 3.71 Ga, as well as metamorphism at both ca. 3.6 Ga and 2.8–2.7 Ga, is also recognised in the Itsaq Gneiss Complex of south West Greenland, and is restricted to the Færingehavn Terrane. Our new data enable a more rigorous correlation between these formerly conjugate parts of the North Atlantic Craton.

A prolonged granitoid formation in Saglek Block, Labrador: Zonal growth and crustal reworking of continental crust in the Eoarchean

The Archean continental crusts account for ca. 20% of the present volume, but the thermal history of the Earths' mantle suggests much more continental crusts were formed in the early Archean. Because the Archean continental crust underwent severe metamorphism, it is important to avoid influence by the later thermal events. We carried out a comprehensive geochronological work of Cathodoluminescence (CL) observation and U-Pb dating of zircons from orthogneisses and supracrustal rocks over the Saglek Block to obtain their protolith ages. The zircons were classified into three domains of core, mantle and rims, and the cores were further classified into three groups of inherited, altered and zoned cores based on the zonation on the CL images. We estimated the protolith ages from Pb-Pb ages of the zoned-cores of zircons with low U contents.We made a detailed sketch of a small outcrop in St. John's Harbour South (SJHS) area, and classified the orthogneisses and mafic enclaves into seven generations based on the geologic occurrence. The first and second generations comprise mafic rocks and lack magmatic zircons. We conducted CL imaging and U-Pb dating of zircons from the third, sixth and seventh generation of the orthogneisses to estimate the protolith ages at 3902 ± 25, 3892 ± 33 and 3897 ± 33 Ma for each, supporting the presence of the over 3.9 Ga Iqaluk Gneiss. The geological occurrence that the mafic rocks occur as enclaves within the 3.9 Ga Iqaluk Gneiss indicates that they are the oldest supracrustal rocks in the world. Our geochronological and geological studies show the Uivak Gneiss is quite varied in lithology and age from 3.6 to >3.9 Ga, and tentatively classified into six groups based on their ages. The oldest Uivak Gneiss components including the Iqaluk Gneiss are present around the SJHS area, and the orthogneisses become young as it is away. The lines of evidence of overprinting of younger granitoid on older granitoid in small outcrops and geological-map scale as well as presence of inherited zircons even in the oldest suite suggests that crustal reworking played an important role on erasing the ancient crusts.

Widespread tungsten isotope anomalies and W mobility in crustal and mantle rocks of the Eoarchean Saglek Block, northern Labrador, Canada: Implications for early Earth processes and W recycling

Well-resolved 182W isotope anomalies, relative to the present mantle, in Hadean–Archean terrestrial rocks have been interpreted to reflect the effects of variable late accretion and early mantle differentiation processes. To further explore these early Earth processes, we have carried out W concentration and isotopic measurements of Eoarchean ultramafic rocks, including lithospheric mantle rocks, meta-komatiites, a layered ultramafic body and associated crustal gneisses and amphibolites from the Uivak gneiss terrane of the Saglek Block, northern Labrador, Canada. These analyses are augmented by in situ W concentration measurements of individual phases in order to examine the major hosts of W in these rocks. Although the W budget in some rocks can be largely explained by a combination of their major phases, W in other rocks is hosted mainly in secondary grain-boundary assemblages, as well as in cryptic, unidentified W-bearing ‘nugget’ minerals. Whole rock W concentrations in the ultramafic rocks show unexpected enrichments relative, to elements with similar incompatibilities. By contrast, W concentrations are low in the Uivak gneisses. These data, along with the in situ W concentration data, suggest metamorphic transport/re-distribution of W from the regional felsic rocks, the Uivak gneiss precursors, to the spatially associated ultramafic rocks.All but one sample from the lithologically varied Eoarchean Saglek suite is characterized by generally uniform ∼+11 ppm enrichments in 182W relative to Earth's modern mantle. Modeling shows that the W isotopic enrichments in the ultramafic rocks were primarily inherited from the surrounding 182W-rich felsic precursor rocks, and that the W isotopic composition of the original ultramafic rocks cannot be determined. The observed W isotopic composition of mafic to ultramafic rocks in intimate contact with ancient crust should be viewed with caution in order to plate constraints on the early Hf–W isotopic evolution of the Earth's mantle with regard to late accretionary processes. Although 182W anomalies can be erased via mixing in the convective mantle, recycling of 182W-rich crustal rocks into the mantle can produce new mantle sources with anomalous W isotopic compositions that can be tapped at much later times and, hence, this process should be considered as a mechanism for the generation of 182W-rich rocks at any subsequent time in Earth history.

Occurrence and geochronology of the Eoarchean, ∼3.9 Ga, Iqaluk Gneiss in the Saglek Block, northern Labrador, Canada: Evidence for the oldest supracrustal rocks in the world

Understanding Earth’s early evolution requires decoding the cryptic geological record that is preserved in a few extremely rare terrains on Earth. The Saglek–Hebron Block in the North Atlantic Craton is one of the oldest terrains in the world. To better understand the Eoarchean history of the Saglek–Hebron Block, we undertook a comprehensive geological and geochronological investigation of this terrain. The Saglek–Hebron locality contains orthogneisses and supracrustal rocks, which are classified into two groups of the Eoarchean and Mesoarchean suites based on field associations defined by intrusion of the Mesoarchean mafic Saglek dykes. The Eoarchean suites contain the Nulliak supracrustal rocks and Uivak Gneisses. In this paper, we report cathodoluminescence (CL) observations and U–Pb ages of zircons from the Eoarchean orthogneisses to constrain the evolution of the Saglek–Hebron Block. We made detailed sketch maps (1:20) of critical outcrops to establish relative chronological relationships between lithologies. At least seven generations of mafic and felsic units were recognized in an outcrop of the St John’s Harbour South area. The first and second generations are mafic supracrustal rocks whereas the third to seventh generations are tonalitic gneisses, which are intruded into the supracrustal units. We study CL observation, geochemistry and U–Pb dating of zircons from the oldest and youngest tonalitic gneisses.The CL observation of internal structures of the zircons showed they typically comprise three domains of core, mantle and rim. The cores have clear oscillatory zoning, whereas the mantles lack oscillatory zoning and exhibit dark CL emission and structureless CL images. The obvious correlation between the ages and chemical compositions of U contents and Th/U ratios indicates that the cores with low U contents and high Th/U ratios preserve the protolith age. The cores with clear oscillatory zoning, low U contents and high Th/U ratios of zircons from the oldest suite plot on a concordia line with ages ranging from 3953 to 3797Ma. The oldest and average ages of the six oldest spots on the concordia line are 3953±54 and 3920±49Ma, respectively. On the other hand, the average age of the four old spots of zircons from a younger generation of orthogneiss is 3869±63Ma. The geological and geochronological relationships between the third and seventh generation orthogneisses are consistent with each other, confirming an Eoarchean age of ca. 3.9Ga for the oldest suite of the Uivak Gneisses. We name this oldest suite the Iqaluk Gneiss. As the protolith of the Iqaluk Gneiss was clearly intruded into the Nulliak supracrustal rocks, the Nulliak supracrustals must have a minimum age of ca. 3.9Ga, indicating that they are the oldest supracrustal rocks on Earth. But, further studies should be necessary to obtain more precise age of the Iqaluk Gneiss and Nulliak supracrustal rocks.

Geology of the Eoarchean, > 3.95 Ga, Nulliak supracrustal rocks in the Saglek Block, northern Labrador, Canada: The oldest geological evidence for plate tectonics

The Earth is a unique planet, which has been highly evolved, diversified and complicated through geologic time, and underwent many key events, including giant impact, magma ocean, core formation, large-scale mantle differentiation and late heavy bombardment, especially in its dawn. But, our knowledge of early Earth is limited due to the lack of the Hadean supracrustal rocks. The supracrustal rocks with the Eoarchean ages provide key evidence for the Earth's early evolution, but few supracrustal rocks have been comprehensively investigated. Therefore, we mapped in seven areas of the Saglek Block, northern Labrador, where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses. Early studies suggested that some of them have the Mesoarchean ages because of the lack of the Mesoarchean Saglek dyke, but we found the Saglek dykes in the areas to recognize the Eoarchean Nulliak supracrustal rocks and Uivak Gneiss in all the areas. Recent reassessment of U–Pb dating and cathodoluminescence observation of zircons from the oldest suites of the Uivak Gneiss showed that the Uivak Gneiss has the Eoarchean age, >3.95Ga, and forms the Iqaluk–Uivak Gneiss series. Because our geological survey clearly showed that the Iqaluk–Uivak Gneisses were intruded into the Nulliak supracrustal belts, the Nulliak supracrustal rocks are the oldest supracrustal rock in the world. The supracrustal belts consist of piles of fault-bounded blocks, which are composed of the ultramafic rocks, mafic rocks and sedimentary rocks in ascending order, similar to modern ocean plate stratigraphy (OPS). In addition, small-scale duplex structures are found over the areas. The presence of duplex structure and OPS indicates that the >3.95Ga Nulliak supracrustal belts originate from an accretionary complex. The presence of the accretionary complex, ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early Earth.

Early Archæan U/Pb fractionation and timing of late Archæan high-grade metamorphism in the Saglek–Hebron segment of the North Atlantic Craton

The Archæan gneiss complex of the Saglek–Hebron area in northern Labrador is dominated by polyphase ∼3.7 Ga old tonalitic-trondhjemitic-granodioritic (TTG) Uivak I Gneiss, that exhibit 3.8–3.9 Ga inheritance. These gneisses are interleaved with a heterogeneous group of supracrustal rocks known as the Nulliak supracrustal assemblage. Early Archæan ultramafic rocks occur either as tectonically-emplaced slivers of lithospheric mantle along structural contacts within the Uivak I Gneiss, or as layers of metakomatiite in Nulliak supracrustal units. The metakomatiites yield a Pb–Pb isochron age of 3845±160 Ma, reflecting a significant change from an initial Archæan mantle-like μ 1-value of 7.9 to a range of μ 2-values of 2.0–4.2. Members of the tectonically-emplaced lithospheric mantle suite have less well correlated 206 Pb/ 204 Pb and 207 Pb/ 204 Pb compositions. Both ultramafic suites were collected in an area identified as transitional between granulite and amphibolite facies. However, model calculations show that the data scatter exhibited by the lithospheric mantle cannot be accommodated by post-3.85 Ga U-depletion. Evidence is provided that the lithospheric mantle suite reflects variable degrees of mixing between a depleted mantle and a high-μ reservoir suggesting that the two ultramafic rock suites had different evolutionary histories. Timing of late Archæan reworking during medium- to high-grade metamorphism was determined by mineral whole-rock dating of a garnet pyroxenite from an area dominated by Kiyuktok Gneiss (reworked ∼3.75 Ga Uivak Gneiss). Sm–Nd mineral whole-rock data for this gneiss yielded an isochron equivalent to an age of 2570±17 Ma, significantly younger than its Pb–Pb isochron age of 2735±43 Ma. The negative ε Nd( t)=−7.9 reflects a significant crustal residence time, indicating that the Sm–Nd system of the whole rock has been reset. The internal Pb mineral isochron is interpreted to reflect the maximum age of peak metamorphism. The age discrepancy of ca. 165 Ma between the Sm–Nd and the Pb–Pb isochron ages suggests slow cooling at a minimum rate of 1.3°C Ma −1. This indicates that the Saglek–Hebron segment of the North Atlantic Craton experienced gradual uplift during late Archæan exhumation.

Petrological and whole rock isotopic characteristics of tectonically juxtaposed Archaean gneisses in the Okak area of the Nain Province, Labrador: relevance for terrane models

In a recent model (Schiøtte et al., 1990), the Archaean Nain Province of northern Labrador is described as a tectonic collage of terranes with different origins and metamorphic histories that was assembled in the late Archaean. 3200–3260 Ma granulite facies gneisses that are intercalated with the regional > 3500 Ma Uivak gneisses in the northern (Saglek) block of the Nain Province are interpreted as tectonic slivers of a terrane of mid-Archaean rocks represented by the Maggo gneisses of the Hopedale block 200 km farther south. This terrane is thought to correlate with the Akia terrane in southern West Greenland. In the Nain Province as opposed to West Greenland, the tectonic juxtaposition is thought to have taken place along subvertical planes. There is overlap in the range of chemical compositions exhibited by the early and mid-Archaean orthogneisses in the Okak area of the Saglek block, but on average the mid-Archaean gneisses are more mafic and less silica-rich. Their major element chemistry exhibits much simpler evolutionary trends. The REE chemistry is not related to the major element chemistry in any straightforward way in either early or mid-Archaean gneisses. REE patterns show some correlation with the local concentration of accessory REE-bearing minerals and are subject to metamorphic redistribution as late as 2560 Ma. In spite of this redistribution, late Archaean ϵNd-values can be used as a rough guide to the duration and complexity of the pre-juxtaposition history. ϵNd (2560 Ma) of individual samples ranges from −4.4 to −7.0 in the mid-Archaean gneisses as opposed to values < −7.7 in the early Archaean gneisses. The difference in metamorphic history also shows up in the whole rock Pb isotopic signature: the mid-Archaean granulite facies gneisses were juvenile intrusions, the source of which can be estimated from model μ1-values of ∼7.8–8.0. They crystallized in granulite facies sometimes between their emplacement at ∼3235 Ma and an event of granitoid veining at ∼2990 Ma. They do not exhibit the complexities or distinctly non-juvenile 207Pb204Pb ratios recorded from the early Archaean gneisses. The major volume of exposed early Archaean rocks in the Saglek block recrystallized under granulite facies metamorphism between 2780 and 2740 Ma. A suite of early Archaean granitic-granodioritic gneisses in granulite facies from the Okak area differs from previously investigated Uivak gneisses by showing evidence for larger pre-metamorphic variation in UPb (207Pb204Pb ratios that range from 14.02 to 16.30) and lower ThU. Secondary U loss during late Archaean metamorphism gave rise to a PbPb “palaeoisochron”. A late Archaean SmNd isochron with a strongly negative initial ϵNd has been obtained on this gneiss suite. The suite also exhibits other chemical peculiarities such as high Cr contents. A complex petrogenesis within a strongly differentiated early Archaean crust is suggested. There is no straightforward correlation between possible subterranes within terranes consisting of early Archaean rocks in the Nain Province and West Greenland.

U–Pb ages of single zircons within "Upernavik" metasedimentary rocks and regional implications for the tectonic evolution of the Archaean Nain Province, Labrador

Detrital zircons and their postdepositional overgrowths from three units of the "Upernavik" supracrustal association in the northern (Saglek) block of the Archaean Nain Province have been dated with the ion microprobe SHRIMP. In one unit, from the granulite-facies area in inner Saglek Fiord, the zircon population is dominated by early Archaean grains thought to be derived from the Uivak gneisses. Recrystallization and growth of new zircon within this metasediment took place during granulite-facies metamorphism at 2761 ± 12 Ma (2σ), which is also a younger limit on the age of deposition.In a second unit, from the amphibolite-facies area in outer Saglek Fiord, detrital zircons have predominantly mid- and late Archaean ages. The mid-Archaean zircons are comparable in age to the 3235 Ma Lister gneisses. The ages of the late Archaean detrital zircons (2800–2960 Ma) do not correspond to any known rock complex in the Saglek block, but plutonic rocks associated or correlative with the ca. 2840 Ma Kanairiktok Plutonic Suite of the southern (Hopedale) block are a possible source for many of the grains. Overgrowths were dated at 2690–2730 Ma in this sample.A third metasedimentary unit from the Okak Bay area, 100 km south of Saglek Fiord, also contains detrital zircons with ages comparable to that of the Lister gneisses (3235 Ma). The age of recrystallization and zircon overgrowths was dated at ca. 2560 Ma in this sample. A single grain dated at ca. 2780 Ma is considered most likely to be detrital, which would imply an age of deposition between ca. 2780 and 2560 Ma for this unit.The results show that although late Archaean depositional ages are possible for all three units, the "Upernavik" supracrustal association is composite and sediments in different units have widely different sources and metamorphic histories. These conclusions support a new model for the Nain Province according to which separate terranes were tectonically juxtaposed in the late Archaean. In this model, the age of plutonic and supracrustal rocks and their metamorphic histories prior to juxtaposition differ from one terrane to another.

U–Th–Pb ages of single zircons in Archaean supracrustals from Nain Province, Labrador, Canada

Ion-probe U–Pb dating of igneous zircons from an acid metavolcanite belonging to the Nulliak supracrustal association of the Saglek area, northern Labrador, suggests an original age of 3776 ± 8 Ma (2σ), interpreted as the deposition age of the volcanite. In accordance with the field model, this is slightly but significantly older than the previously obtained ion-probe U–Pb age of igneous zircons from the main phase of the Uivak gneisses, which carries inclusions of the supracrustal association. A secondary event of zircon growth recorded in the metavolcanite has a maximum age of 3065 ± 8 Ma (2σ).A metapelite belonging to the Weekes amphibolite supracrustal association of the Hopedale area south of Nain contains rounded, low-U zircons of 3258 ± 24 Ma (2σ) age. Occasional textural evidence suggests that some of these zircons are most likely detrital grains of igneous origin, in which case 3258 ± 24 Ma is a minimum age of the source from which the sediment was derived and a maximum age for the deposition of the sediment. However, the possibility that the age represents recrystallization of the sediment under high-grade conditions cannot be ruled out. A 2841 ± 5 Ma (2σ) age obtained on a younger generation of euhedrally zoned high-U zircons from the metapelite is within error of the published bulk zircon U–Pb age of the tonalitic component of the Kanairiktok plutonic suite that intrudes the older orthogneiss and supracrustal units in the area. These younger zircons are most likely to have grown during migmatization of the metasediment at the time of emplacement of the suite.The zircon data give no positive evidence to support correlation between the Nulliak supracrustals and the Weekes amphibolite association.

The early Archaean Nulliak (supracrustal) assemblage, northern Labrador

The Nulliak (supracrustal) assemblage, the remains of ca. 3800 Ma succession of volcanic and sedimentary rocks, was broken up by intrusion of the protoliths of the early Archaean Uivak orthogneisses and then deformed, metamorphosed, and variably metasomatised several times under upper amphibolite to granulite facies conditions in the Archaean. Amphibolites of "komatiitic basalt" and tholeiitic chemical affinity are the most important Nulliak assemblage lithologies. High Al2O3 metagabbroic rocks and anorthosites also occur. Interlayered with the amphibolites are marbles, calc-silicate rocks, and banded iron formation, interpreted as chemical sediments that were probably laid down in a shallow-water environment. Also found are felsic rocks probably derived by reworking of penecontemporaneous felsic volcanic rocks, and garnet- and sillimanite-bearing paragneisses derived from pelites. All these lithologies are randomly interlayered on a scale down to 1 m or less. The occurrence of 3850 – 3900 Ma cores for zircons in the surrounding polyphase Uivak gneisses suggests there may be an ancient sialic component in them, which could possibly represent basement upon which at least part of the Nulliak assemblage formed.

Ion probe U–Th–Pb zircon dating of polymetamorphic orthogneisses from northern Labrador, Canada

Rounded zircons included within igneous zircons from the Uivak I gneisses, northern Labrador, have ages of up to 3863 ± 12 Ma (2σ), showing the (former) existence of rocks that predate the Uivak gneisses by more than 100 Ma. The igneous zircons themselves have experienced varying amounts of early Pb loss, but age estimates based on maximum 207Pb/206Pb from three separate gneiss samples agree to within error at 3732 ± 6 Ma (2σ), which is taken as the age of emplacement of the igneous precursor to the dominant component of the Uivak gneisses. A new generation of zircons was formed during migmatization at ca. 3620 Ma. The volumetrically less important Lister gneiss was emplaced at 3235 ± 8 Ma (2σ). In the late Archaean, different parts of the gneiss complex were subjected to different grades of metamorphic overprinting. The zircon chronology of gneisses affected by granulite facies metamorphism suggests a rapid sequence of events, with simultaneous recrystallization of old zircons and growth of new U-poor zircons at 2766 ± 17 Ma (2σ), followed by partial melting at 2744 ± 4 Ma (2σ), and further recrystallization and growth of U-poor zircons afterwards. No late Archaean zircon growth has been recorded from orthogneisses unaffected by granulite fades metamorphism.

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.

Chemical and isotopic effects of late Archaean high-grade metamorphism and granite injection on early Archaean gneisses, Saglek-Hebron, northern Labrador

Summary Early Archaean (3700–3800 Ma) layered tonalitic to granodioritic Uivak gneisses form approximately 60–70% of the Archaean gneiss complex between Saglek and Hebron fiords, northern Labrador. Effects of late Archaean metamorphic and igneous activity can be recognized on the geochemistry and isotope chemistry of the Uivak gneisses: depletion in rocks affected by 2900 Ma granulite facies metamorphism, with a loss of K, Rb and redistribution of REE and a corresponding enrichment of LIL elements in the amphibolite facies area. LIL enrichment is at least in part by addition from intrusive late Archaean granitoid sheets. The depleted granulites (K/Rb 500–2000) show a scatter of data points on a Rb/Sr- 87 Sr/ 86 Sr diagram to the left of a 3560 Ma reference isochron (ISr 0.700) consistent with marked Rb loss and some homogenization of Sr. Amphibolite facies gneisses from a broad transitional zone into granulite facies have K/Rb values of 200–300 and plot on a c. 3560 Ma isochron with a high ISr (0.7038) consistent with a slight loss of Rb at 2900 Ma but without marked Sr homogenization. Amphibolite facies gneisses away from the transition zone show marked enrichment in LIL elements with unusually low K/Rb ratios (100–200). Rb/Sr data plot close to a 3560 Ma reference isochron, ɛSr 3560 values are strongly negative, interpreted as due to Rb addition at 2900 Ma rather than derivation from an unusually depleted mantle source. Pb-Pb isotope data from the depleted granulite show a cluster of unradiogenic data points. Samples from a monolith of gneisses in a transitional zone between amphibolite and granulite facies yields a 3808 Ma isochron (μ 1 8.5) suggesting there was locally little disturbance of the Pb/U system at 2900 Ma. Pb-Pb data from the Rb-enriched amphibolite facies area show a markedly higher proportion of radiogenic Pb compared to the granulite facies rocks which cannot be explained by U movement alone. Local galena mineralization suggests that Pb as well as U was mobile during the late Archaean and we interpret the results as mixtures of &gt; 3600 crustal Pb with younger material. Sm-Nd data points (Collerson &amp; McCulloch 1982) show a scatter about a 3665 Ma isochron with an average ɛNd 3665 of +2.48. Marked LIL + LREE enrichment is suggested and we interpret the scatter of ɛNd 3665 values as being due to late movement of LREE. The late Archaean granitoid sheets have a chemistry and petrology suggesting derivation largely by partial melting of earlier sial with modifications by fluid redistribution of alkalis. New minerals such as allanite and apatite grow in older rocks adjacent to young veins. Sr, Nd and Pb data all suggest mixing between early Archaean crust and a component with mantle-like isotopic character at c. 2900 Ma. The mixing process was complex and took place by ingestion of older rocks in younger granite, mixing of partial melts of different origin and fluid transport of components from younger intrusions into older gneisses.

Nd and Sr isotopic crustal contamination patterns in an Archaean meta-basic dyke from northern Labrador

Samples from the core to the margin of a 20 cm wide meta-dolerite dyke are sequentially enriched in K, Rb, Sr, and the light REE's. Rb-Sr and Sm-Nd compositional and isotopic profiles in the dyke are interpreted to be the result of selective contamination with components of country rock derivation, rather than the result of simple bulk mixing. 87Rb 86Sr ratios are higher at the edge of the dyke than at its centre, although they are somewhat irregular, due probably to the effects of subsequent alteration. This profile and one shown by unsupported 87Sr are both consistent with contamination of the dyke by a fluid phase derived by the breakdown of biotite. Common Sr shows a parallel, albeit weaker, contamination profile which is interpreted to reflect the contribution of a Sr-bearing phase such as plagioclase. 147Sm 144 Nd ratios and 144Nd concentrations increase and decrease respectively from the margin to the core of the dyke. In addition, the margin of the dyke is significantly less radiogenic than the interior. This contrasts with the relatively radiogenic character of an adjacent pegmatite vein. As this sample does not lie on an anticipated contamination profile between the Uivak gneisses and the dyke it is concluded that the REE contamination of the dyke occurred by the addition of a REE-enriched fluid phase which gained access to the dyke by flow along the dyke-pegmatite interface. If it is assumed that both the Rb-Sr and Sm-Nd contamination profiles are the result of diffusion limited processes, then the observations of scale made in this paper suggest that the rate of diffusion of Nd is an order-of-magnitude slower than that for Sr. In view of the scale and nature of these profiles, ages obtained from isotopic data for such mafic dykes must be interpreted with some care. Nevertheless, in spite of these limitations the ϵ Nd values for the least contaminated specimens provide a clear indication that the Saglek dykes were derived from a depleted mantle source with ϵ Nd ⩾ +2.

The Archean gneiss complex of northern Labrador. 2. Mineral ages, secondary isochrons, and diffusion of strontium during polymetamorphism of the Uivak gneisses

Strontium isotope systematics of minerals from the ca. 3600 Ma old Uivak gneisses and of compositionally layered Uivak I gneisses, which define secondary isochrons, are the result of Sr isotopic homogenization during thermal events ca. 2800, ca. 2500, and ca. 1800 Ma ago. The strontium isotope data provide information about the thermal history of the Archean gneiss complex in northern Labrador, the scale of Sr isotopic equilibration, and mechanisms of Sr diffusion during polymetamorphism.The mineral data indicate that biotite and amphibole were both partially open to Sr diffusion during the Hudsonian Orogeny ca. 1800 Ma ago. However, cooling rates and closure to diffusion of Sr in these phases varied throughout the terrane. Potassium feldspars were largely closed to diffusive loss of Sr during the 1800 and 2500 Ma events, and they record evidence of a ca. 2800–2900 Ma thermal event.Ages defined by secondary isochrons correlate with variations in the widths of the gneissic layers and correspond with geologically established events in the Nain and Churchill Provinces. The secondary isochrons developed as a result of local isotopic equilibration on different scales during these thermal events. The variable scales of equilibration reflect the effects of different Sr diffusion mechanisms. As a melt phase was not produced in the gneisses at the times indicated by the secondary isochrons, diffusion must have occurred under subsolidus conditions. Temperatures were therefore too low for extensive volume diffusion to have taken place. Exchange of Sr isotopes over distances greater than 5–10 cm must have involved grain boundary diffusion or some form of infiltration metasomatism.

Iron Formations in Archaean Granulite-Gneiss Belts With Special Reference to Southern India

Abstract In Archaean granulite-gneiss belts there are banded iron formations (BIF) less than 20m thick which are weakly banded magnetite quartzites occurring in a supracrustal sequence of quartzites, mica schists, marbles and metavolcanic amphibolites. This sequence forms stratigraphic markers within voluminous intrusive tonalitic to trondbjemitic gneisses. These rocks are highly folded and metamorphosed to high amphibolite or granulite facies at 7-12 kb pressure in the lower Archaean crust. Such iron formations are well represented in Tamil Nadu, Southern Karnataka, Kerala and Andhra Pradesh in South India; in Hebei Province in China; Kola Peninsula, Stanovoy Range, and Ukraine in USSR; Kambui schists of Sierra Leone; Nimba series of Liberia; Limpopo belt of southern Africa; Imataca complex of Venezuela; Uivak gneisses of Labrador; and Amitsoq gneisses of West Greenland. They are obviously different in type and environment from the Proterozoic Superior type of BIF; it is shown here how they are different from the Archaean Algoma type of BIF. They were deposited in a quartzite-pelite-carbonate association and they lack the silicate-carbopate-sulphide facies of the Algoma-type which, in contrast, was deposited with a greywacke-flyschconglomerate- shale association in subsiding basins (Greenstone belts). Because they are dissimilar to the Algoma-type of BIF in both their sedimentary facies and tectonic environment (the granulite-gneiss belts are not highly metamorphosed greenstone belts), they should be given a separate status and we tentatively suggest they be termed the Tamil Nadu type.

Reworking of sialic crust as represented in late Archean–age gneisses, northern Labrador

Regionally reworked sialic crust has been recognized in the form of ∼ 2,800-m.y.-old nebulitic rocks (Kiyuktok gneisses) with high initial 87Sr/86Sr ratios (∼ 0.7081) and Pb isotope ratios that suggest a history of the source back to at least 3,500 m.y. B.P. The field, geochemical, and isotopic data are consistent with these rocks having formed by in situ mobilization of granitic components in the ∼ 3,600-m.y.-old Uivak gneisses by the introduction of aqueous fluids some 2,800 m.y. ago. Contemporary but subordinate tonalitic-granodioritic orthogneisses derived from discrete intrusive bodies have contrasting Sr isotope compositions. Some display low initial 87Sr/86Sr ratios (∼ 0.7030) that suggest derivation from a source with mantlelike Sr-isotope composition. However, most of the late Archean–age intrusive rocks display higher initial 87Sr/86Sr ratios (∼ 0.7056, 0.7059). Recognition of these contrasting isotopic patterns suggests that processes of late Archean crust formation and stabilization in the North Atlantic craton were more complex than previously realized. Crustal reworking due to interaction of old crust with aqueous fluids must have been accompanied by the introduction of granitic melts derived by melting of ancient deep crustal rocks or contamination of juvenile granitic melts by ancient deep crustal material.