Archaean Gneiss Complex Research Articles

Hydro-mechanical effects of seismic events in crystalline rock barriers

Abstract. Under ideal conditions, owing to its extremely low matrix permeability, crystalline rock can constitute a suitable hydro-geological barrier. Mechanically, its high strength and stiffness provide advantages when constructing a repository and for long-term stability. However, crystalline rock usually occurs in a fractured form, which can drastically alter hydromechanical (HM) barrier functions due to increased permeability and decreased strength. Seismic events have the potential to alter these HM properties by activating faults, increasing their transmissibility, creating new fractures or altering network connectivity (De Rubeis et al., 2010). Therefore, it is of high importance to build computational models to allow assessment of the HM effects of seismic events in a Deep Geologic Repository (DGR) in crystalline rock, as illustrated in Fig. 1. For this purpose, we consider a DGR in Russia (Yeniseysky site) for high-level waste in crystalline rock (Proterozoic and Archaean gneiss complexes) that is located close to a potentially seismically active area (Jobmann, 2016). Here, we present a coupled HM simulation, using OpenGeoSys (Kolditz et al., 2012), of a large-scale, three-dimensional finite-element model of the Yeniseysky site to assess the consequences of seismically induced stress-field changes on the local stress field and the fluid flow. This research also provides an outlook of current model development geared towards a more detailed assessment of seismically induced hydro-mechanical processes in porous and fractured rocks.

Tectono-stratigraphic terranes in Archaean gneiss complexes as evidence for plate tectonics: The Nuuk region, southern West Greenland

Prior to 1970 grey gneiss complexes were interpreted as partially-melted sedimentary sequences. Once it was recognised from the Nuuk region that they comprised calc-alkaline igneous complexes, it was understood that such complexes world-wide were dominated by TTG (trondhjemite-tonalite-granodiorite) initially found to have juvenile Sr, Nd and, subsequently, Hf isotopic signatures. Between 1970 and 1985 the Nuuk region gneiss complex was interpreted by the non-uniformitarian ‘super-event’ model of crust formation which proposed occasional but extensive crust formation, with craton-wide correlation of granulite facies metamorphism and deformational phases. The igneous rocks formed in a late- Meso- to early Neoarchaean super-event engulfed crust formed in an Eoarchaean super-event. Mapping and reinterpretation at Færingehavn showed there are three TTG gneiss domains, each with different early accretionary, metamorphic and tectonic histories, separated by folded meta-mylonites. This established the key feature of the tectono-stratigraphic terrane model; that each terrane has an early intra-terrane history of crust formation, deformation and metamorphism, upon which is superimposed a later deformation and metamorphic history common to several terranes after they were juxtaposed. Remapping and >250 U-Pb zircon age determinations have refined the geological evolution of the entire Nuuk region, and has confirmed at least four main crust formation events and two collisional orogenies with associated transient high pressure metamorphism within clockwise P-T-t loops. Via independent corroborative studies the tectono-stratigraphic terrane model has been accepted for the Nuuk region and, through the discovery of similar relations across other gneiss complexes, its mode of evolution is found to be applicable to Archaean high-grade gneiss complexes worldwide. The TTG and mafic components that dominate each terrane have geochemistry interpreted to indicate subduction-related magmatism at convergent plate boundaries. Each terrane is thus dominated by juvenile additions to the crust. Intra-terrane sedimentary rocks show near unimodal age distributions in contrast to those near the boundaries which are more diverse and complex. The combined geochronological, metamorphic and structural evidence of convergence of these terranes leading to collisional orogeny, this indicates that plate tectonic processes operated throughout the Archaean.

Mapping lithology of the Sarfartoq carbonatite complex, southern West Greenland, using HyMap imaging spectrometer data

The Sarfartoq carbonatite complex occurs in the southern West Greenland in a transition zone between Archaean gneiss complex to the south and a Proterozoic mobile belt to the north. The Sarfartoq carbonatite complex consists of a core zone composed of dolomite carbonatite and minor søvite (calcite carbonatite) surrounded by a fenite zone and a marginal zone of gneisses frequently altered due to hydrothermal activity. High spatial and spectral resolution imaging spectrometer data recorded by the HyMap imaging system were used to map lithology of the Sarfartoq carbonatite complex. A careful analysis of the spectral reflectance properties of the carbonatite lithology preceded the HyMap data analysis stage. The spectral reflectance measurements showed that the various lithologic units including dolomite carbonatite, søvite, fenite and the marginal alteration zone have distinct spectral reflectance characteristics. The analysis of the HyMap data was based on an unsupervised clustering algorithm, the Self Organizing Maps (SOM), for the mapping of the main lithology and a hierarchical tree for the mapping of sparsely occurring søvite rocks. Spectral mixture analysis was applied to map fractional abundances and compare with the SOM results. The resulting lithological map shows the spatial distribution of dolomite carbonatite, søvite, fenite with abundant carbonatite dykes (representing the outer core of the carbonatite complex), fenite and hematized gneiss (marginal alteration zone). The results compare well with the field data collected for the assessment of the mapping accuracy and due to the spatially contiguous nature of the hyperspectral data could be used to better map the outcropping carbonatite lithology. The spectral reflectance measurements and the mapping results provide information of petrological importance for the carbonatite core zone.

The whole rock Sm–Nd ‘age’ for the 2825 Ma Ikkattoq gneisses (Greenland) is 800 Ma too young: Insights into Archaean TTG petrogenesis

The Ikkattoq gneisses of the Archaean gneiss complex in the Nuuk region, southern West Greenland, are the orthogneiss component within the amphibolite facies Tre Brødre terrane. They have mostly granodioritic compositions, with a small amount of quartz diorite. Sm–Nd isotopic data for a quartz diorite and five granodiorite Ikkattoq gneiss samples from within 5 km of the Ikkattoq (fjord) type locality yielded a regression with a slope equivalent to 2005 ± 52 Ma (MSWD = 0.72). Regardless of the low MSWD, this cannot be the true age of the Ikkattoq gneisses, because all Ikkattoq gneisses yield U–Pb zircon dates of c. 2825 Ma and they are cut by the undeformed 2560 Ma Qôrqut granite complex. This anomalously young regression ‘age’ resulted instead from mixing of different Nd components, indicating that the Ikkattoq gneisses are derived from mixed source materials. Taking the true age of the Ikkattoq gneisses as 2825 Ma from U–Pb zircon dating, the range of initial ε Nd in the Ikkattoq gneisses is − 7.1 to − 1.8. The negative initial ε Nd values mean that older, light rare earth enriched, sialic crust contributed to the igneous precursors of the Ikkattoq gneisses. This Nd evidence for contribution of older sialic crust is supported by positive ε Sr values for the Ikkattoq gneisses. With ε Nd values as low as − 7.1 this older crustal component has to be Eoarchaean. The presence of scarce quartz diorites (low SiO 2, high MgO) suggests that ultramafic rocks (upper mantle?), metasomatised by the passage of fluids or silicic melts, were another contributing source. The Ikkattoq gneisses are proposed as a complex suite incorporating material derived from melting of much older sialic crust and probably upper mantle. The intercalation of tectonostratigraphic terranes during collisional orogeny at c. 2720 Ma destroyed the architecture of this 2825 Ma magmatic system, and the Ikkattoq gneisses now form a slice tectonically isolated from their source region. In terms of trace element parameters, the Ikkattoq gneisses resemble Phanerozoic volcanic arc granites. Thus an Andean-style arc setting for the generation of the Ikkattoq gneiss precursors is possible. Other Archaean TTG suites of the Nuuk region are generally thought to represent predominantly juvenile additions to the crust. In the broadest sense they do, because isotopic work over the past 30 years has demonstrated that they do not represent wholesale recycling of considerably older crust. However in detail, within these broadly juvenile suites, a contribution from older crust can be detected. Thus, c. 3000 Ma type-Nûk gneisses from around Nuuk town show a spread in ε Nd values down to − 1.7. In this case, the likely older crustal component was 3230 Ma quartz diorite that occurs as enclaves in the c. 3000 Ma suite. Thus to a lesser or greater degree, some Meso- to Neoarchaean TTG suites in the Nuuk region display the same internal complexities and evidence for mixed sources as modern arc suites developed near the margins of older crust.

Palaeoproterozoic and Archaean gneiss complexes in northern Greenland: Palaeoproterozoic terrane assembly in the High Arctic

The Precambrian shield of northern Greenland has been investigated by SHRIMP U–Pb zircon dating of 14 orthogneisses and granitoids plus 5 metasediments, integrated with mapping by the Geological Survey of Denmark and Greenland and whole-rock Nd isotopic studies. The Inglefield Mobile Belt is a tract of Palaeoproterozoic sedimentation, plutonism, polyphase deformation and high-grade metamorphism that underlies Inglefield Land and northern Prudhoe Land. In the southern part of the belt at 78°30′N, the E–W-trending Sunrise Pynt Straight Belt is a high-grade, but structurally late, shear zone with contrasts in the geology on either side. South of the Sunrise Pynt Straight Belt, ca. 1980 Ma diorites and tonalites were emplaced into older orthogneisses and metasediments. Detrital zircons from two metaquartzites (deposited on Archaean basement?) yielded complex age spectra from ca. 3250 Ma to 2350 Ma, with 2600–2450 Ma grains dominant. In associated mica schist, low Th/U, 1923 ± 8 Ma zircons date high-grade metamorphism. The most southern orthogneiss investigated (77°45′N) is Neoarchaean (ca. 2600 Ma), in agreement with previously published isotopic data. North of the Sunrise Pynt Straight Belt to 79°10′N an amphibolite–granulite-facies complex with extensive pelitic to psammitic paragneisses are the oldest rocks recognised. Two psammitic paragneisses yielded unrounded zircons with a unimodal detrital age population centred on 2000–1980 Ma. Their source could be ca. 1980 Ma orthogneisses from south of the Sunrise Pynt Straight Belt, or from basement inliers in the North-East Greenland Caledonian fold belt. The metasediments were intruded by tonalites and diorites with dates of 1949 ± 13 Ma and 1943 ± 11 Ma, and then by granitoids (free of zircon inherited from older rocks) with ages of 1924 ± 29 Ma to 1915 ± 19 Ma. The metasediments show development of low Th/U zircon overgrowths at ca. 1920 Ma, coeval with the granitoids. Finally, other granites, some locally transformed into gneisses, have ages of 1783 ± 22 Ma to 1741 ± 15 Ma. Inherited zircons in the latter are up to 2650 Ma old. A granite dyke with a zircon age of 1783 Ma has an T DM age of 2981 Ma and a strongly negative ɛ Nd at 1.78 Ga, indicating derivation by melting or reworking of Archaean crust. Thus, by 1800 Ma this juvenile Palaeoproterozoic terrane had probably overridden crust with Archaean components. North of Inglefield Land, Precambrian crystalline rocks are obscured until Victoria Fjord (81°30′N). One reconnaissance orthogneiss sample from there contains ca. 3400 Ma oscillatory-zoned zircons, which probably date the rock, rather than being a xenocrystic component in a younger rock. Thus, from north to south there is an assemblage of Archaean, Palaeoproterozoic and Archaean to early Palaeoproterozoic gneiss terranes. The Inglefield Mobile Belt is dominated by juvenile Palaeoproterozoic arc crust trapped between two unrelated blocks of Archaean crust of contrasting age. The collision, and probably thrusting of a Palaeoproterozoic arc over a southern Archaean foreland, occurred at ca. 1920 Ma—dated by metamorphic zircon. The new isotopic results consolidate the regional mapping of Archaean and Palaeoproterozoic complexes across northern Baffin Bay that show continuity from Canada into Greenland, without displacement across the Nares Strait.

Chapter 5 Laxfordian Intrusive Rocks

Igneous intrusions of Palaeoproterozoic age occur within the outcrop of the Loch Maree Group and also within the Archaean gneiss complex, cutting the 'Scourie' dykes. Since they post-date either the dykes or the c. 2.0 Ga Loch Maree Group (LMG), they are considered to be Laxfordian in age. They are broadly granitoid in composition, and fall into three categories: (a) a suite of gneissose granodiorites and tonalites, including the Ard gneiss and Mill na Claise gneiss, which intrude the LMG; (b) thin granite sheets cutting both Archaean basement and 'Scourie' dykes; and (c) granitoid pegmatites. The Ard gneiss sensu stricto (Park 1964) is a gneissose granodioritic to tonalitic body (Fig. 5.1 A) whose outcrop extends from the type locality on the An Ard peninsula [802 751], where it is 600 m wide, southeastwards to Dubh Loch where it narrows to about 150 m. Similar gneisses, which are grouped with the Ard gneiss for convenience, occur in the core of the Mill na Claise fold (the Mill na Claise gneiss), and form a narrow belt along the SW side of the Mill na Claise amphibolite from Shieldaig Lodge to the SW end of Dubh Loch (the Cloiche gneiss) (see map). All these gneisses enclose numerous sheet-like bodies of amphibolite similar to, though coarser than, the amphibolites associated with the LMG metasediments. In the NW, on An Ard peninsula and NE of Lochan Dubh nan Cailleach, these amphibolites form distinct sheets up to 150 m wide. Further SE, at Druim Ruadh [822

A new fragment of the early earth crust: the Aasivik terrane of West Greenland

The Aasivik terrane is a ∼1500 km 2 complex of gneisses dominated by ∼3600 Ma components, which has been discovered in the Archaean craton of West Greenland, ∼20–50 km south of the Paleoproterozoic Nagssugtoqidian orogen. The Aasivik terrain comprises granulite facies tonalitic to granitic gneisses with bands of mafic granulite, which include disrupted mafic dykes. Four gneiss samples of the Aasivik terrain have given imprecise SHRIMP U–Pb zircon ages of 3550–3780 Ma with strong loss of radiogenic lead and new growth of zircon probably associated with a granulite facies metamorphic event(s) at ∼2800–2700 Ma. To the Southeast, the Aasivik terrane is in tectonic contact with a late Archaean complex of granitic and metapelitic gneisses with apparently randomly distributed mafic and ultramafic units, here named the Ukaleq gneiss complex. Two granitic samples from the Ukaleq gneiss complex have U–Pb zircon ages of 2817 ± 10 and 2820 ± 12 Ma and t zircon ε Nd values of 2.3–5.4. Given their composition and positive ε Nd values, they probably represent melts of only slightly older juvenile crust. A reconnaissance SHRIMP U–Pb study of a sample of metasedimentary rock from the Ukaleq gneiss complex found ∼2750–2900 Ma zircons of probable detrital origin and that two or more generations of 2700–2500 Ma metamorphic zircons are present. This gneiss complex is provisionally interpreted as a late Archaean accretionary wedge. A sample of banded granulite facies gneiss from a complex of banded gneisses south of the Aasivik terrain here named the Tasersiaq gneiss complex has yielded two zircon populations of 3212 ± 11 and 3127 ± 12 Ma. Contacts between the three gneiss complexes are mylonites which are locally cut by late-post-kinematic granite veins with SHRIMP U–Pb zircon ages of ∼2700 Ma. The isotopic character and the relationships between the lithologies from the different gneiss complexes suggest the assembly of unrelated rocks along shear zones between 2800 and 2700 Ma. The collage of Archaean gneiss complexes were intruded by A-type granites, here named the Umiatsiaasat granites, at ∼2700 Ma, later than the tectonic intercalation of the gneiss complexes.

Geochemistry and tectonic significance of peridotitic and metakomatiitic rocks from the Ussuit area, Nagssugtoqidian orogen, West Greenland

Lenses of sheared and metamorphosed ultramafic rocks are common within Palaeoproterozoic supracrustal rocks in the Ussuit area, central Nagssugtoqidian orogen, West Greenland. They occur in two settings: (1) along the tectonic contact between an Archaean gneiss complex and allochthonous Palaeoproterozoic arc rocks; and (2) at a structurally higher level, away from the contact with the Archaean basement. Ultramafic rocks in the first setting comprise strongly depleted peridotites, on the basis of petrographic and chemical evidence interpreted as mantle fragments, as well as hornblenditic rocks, which may represent original mafic gabbros of crustal origin. The presence of these rocks is evidence of crustal scale tectonic activity in the central part of the Nagssugtoqidian orogen. The possibility is considered that the rocks may be remnants of a disrupted ophiolite complex; strong deformation, however, prevents confirmation of this model by field evidence. Ultramafic rocks from the higher level consist mainly of amphiboles. Based on major and trace element data they are interpreted as original komatiitic lavas. Their presence suggests magmatic activity related to a mantle plume early in the history of the Nagssugtoqidian orogen.

Reassessment of the north-western border zone of the Palaeoproterozoic Ketilidian orogen, South Greenland

As part of ongoing research into the plate tectonic setting of the Palaeoproterozoic Ketilidian orogen led by the Geological Survey of Greenland and Denmark, four geologists from Denmark and the U.K. re-examined parts of the north-western border zone in July–August 1997. The field work was generously supported by the Danish Natural Science Research Council and the Carlsberg Foundation. One team studied the Proterozoic (Ketilidian) sedimentary and volcanic rocks and the regional structure, working from six inland camps along the variably deformed Archaean–Proterozoic unconformity between Midternæs and Qoornoq and on Arsuk Ø (Fig. 1). A second team investigated the plutonic and kinematic evolution of the Kobberminebugt area at the northwestern margin of the Julianehåb batholith (Fig. 1); the latter forms the central part of the Ketilidian orogen (Chadwick & Garde 1996). In addition, samples of volcanic and granitic rocks were collected for geochemical studies and dating of depositional and tectonic events. The first systematic study of the Ketilidian orogen took place in the 1960s and was largely concentrated in its western and southern parts (Allaart 1976). Essential new data from the central and eastern parts of the orogen were acquired during the Survey’s SUPRASYD project (1992–1996; e.g. Garde & Schønwandt 1994, 1995; Garde et al. 1997; Stendal et al. 1997), which was initiated with the aim of assessing the potential for mineral resources in supracrustal sequences (Nielsen et al. 1993). In the course of the SUPRASYD project a new plate-tectonic model for the entire orogen was also published (Chadwick & Garde 1996), in which the orogen is viewed as the result of oblique convergence between the Archaean craton of southern Greenland and a supposed oceanic plate located south of the present orogen, which was subducted towards the north. Chadwick & Garde (1996) also suggested a new division of the Ketilidian orogen into a ‘Border Zone’ adjacent to the Archaean craton, the ‘Julianehåb batholith’ (formerly the Julianehåb granite) in the central part of the orogen, and the ‘Psammite Zone’ and ‘Pelite Zone’ to the south-east, which largely consist of deformed and metamorphosed erosion products derived from the evolving batholith. The north-western border zone and the Ketilidian supracrustal sequences were mapped in the 1960s by Harry & Oen Ing Soen (1964), Watterson (1965), Bondesen (1970), Higgins (1970), Muller (1974), Berthelsen & Henriksen (1975) and Pulvertaft (1977). It was shown that an Archaean gneiss complex (part of the Archaean craton of southern West Greenland) and Palaeoproterozoic basic igneous rocks (the so-called ‘MD’ (metadolerite) dyke swarms and related intrusions), and the unconformably overlying Ketilidian supracrustal rocks are progressively affected by Ketilidian deformation and metamorphism towards the Julianehåb batholith in the south and south-east. Boundary relationships were reviewed by Henriksen (1969). Where the Ketilidian supracrustal rocks are best preserved at Midternæs and Grænseland, Bondesen (1970) and Higgins (1970) divided them into the Vallen Group, which largely consists of sedimentary rocks (Table 1), and the overlying Sortis Group, in which basic pillow lavas and related doleritic to gabbroic sills predominate. Supracrustal rocks of presumed Palaeoproterozoic age further south have previously been referred to as the Qipisarqo and Ilordleq Groups (Berthelsen & NoeNygaard 1965; Allaart et al. 1969). Based on data collected in the 1960s, the earliest plate-tectonic interpretation of the Ketilidian orogen included a prominent suture in Kobberminebugt (Windley 1991). However, other critical aspects of Windley’s model were not substantiated during the Survey’s studies in 1992–1996 (Chadwick et al. 1994; Chadwick & Garde 1996), and a re-evaluation of the north-west border zone was therefore a natural focus of subsequent investigations.

Early Archaean Isua supracrustal belt,West Greenland: pilot study of the Isua Multidisciplinary Research Project

The Isua belt of 3.8–3.7 Ga metavolcanic and metasedimentary rocks, is located 150 km north-east of Nuuk, within the Archaean gneiss complex of West Greenland. Most of this gneiss complex consists of late Archaean rocks with a minor component of early Archaean age, including the oldest known supracrustal rocks on Earth. The Isua belt contains the best preserved of the oldest supracrustal components and is therefore of vital importance in providing information on the oldest known terrestrial environments and a prospective locality in which to search for the earliest traces of life on Earth (Mojzsis et al. 1996). The Isua Multidisciplinary Research Project (IMRP) aims to coordinate a reinvestigation of the geology of the Isua belt and adjacent tonalitic gneisses with a broad-based, diversely skilled, international research team. IMRP is supported by the Danish Natural Science Research Council, the Commission for Scientific Research in Greenland and the Minerals Office of the Greenland Government (from 1998, the Bureau of Minerals and Petroleum). The project began in 1997 with a pilot study of the north-east sector of the Isua belt to test the feasibility of reinvestigating the early Archaean geology on the basis of new mapping. Five weeks of field work were carried out by a core of four geologists (P.W.U.A, C.M.F., S.M., J.S.M.) augmented by visits of shorter duration by G. Arrhenius, A. Hofmann, V.R. McGregor, S. Mojzsis, R.K. O’Nions and H.K. Schønwandt. This report outlines the geological background to the current study and the results of the 1997 pilot project.

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

We report new field and petrographic observations, and mineral-chemical data, on the amphibolite-facies Buksefjorden and granulite-facies Nordland anorthosites, which occur in different tectonostratigraphic terranes within the Archaean gneiss complex of SW Greenland. The Buksefjorden body [from the Akulleq (middle) terrane] is dominated by plagioclase and Ca-amphibole, but shows widespread effects of retrograde hydration (epidote, chlorite). Most plagioclase compositions are in the An60–82 range, with the majority of samples showing average core compositions ∼An76, whereas rims or recrystallized margins are ∼An65. Most grains in the An70–82 range display optically visible Huttenlocher intergrowths. Amphiboles at Buksefjorden are mainly magnesio-hornblende with X Mg ranging from 0.70 to 0.45. The Nordland anorthosite [from the Akia (northern) terrane] is also dominated by plagioclase and Ca-amphibole, but contains additional clinopyroxene (∼Ca47Mg38Fe15) as well as minor orthopyroxene (∼En68), spinel and corundum. Plagioclase at Nordland shows an equilibrated, equigranular texture, consistent with prolonged slow cooling from high temperatures. Despite this textural equilibration, plagioclase at Nordland shows a striking range of compositions from An28 to An97, most of which is found in single thin sections. A distinctive feature is the presence of discrete anorthite (+ spinel ± corundum) domains in some samples. Although a number of explanations may apply, we consider these domains to result from prograde mass transfer reactions involving Ca-amphibole and plagioclase. Amphibole compositions at Nordland show similar X Mg to those at Buksefjorden, but are more aluminous, alkalic, and titanian. This shift to more pargasitic compositions is consistent with the contrasts in metamorphic grade between the two anorthosite bodies. At Buksefjorden, there is no correlation between the amount of modal Ca-amphibole and plagioclase composition, which would be expected if amphibole was produced solely through metamorphism. Our results suggest, alternatively, that the primary igneous mineralogy of these rocks may have been plagioclase (∼An76) + hornblende + pyroxene + magnetite. The primary mineralogy at Nordland is less certain, but it is noteworthy that no rocks contain anorthite of unambiguous igneous origin, in contrast to some other occurrences of Archaean anorthosites.

Timing of late Archaean terrane assembly, crustal thickening and granite emplacement in the Nuuk region, southern West Greenland

A breakthrough in understanding the structural and metamorphic evolution of Archaean gneiss complexes occurred with the recognition that the Nuuk region of southern West Greenland comprised separate terranes assembled in the late Archaean. From northwest to southeast these are: the Akia (3220-2970 Ma), Akulleq (3870-3600 and 2820 Ma) and Tasiusarsuaq (2920-2860 Ma) terranes. The minimum time of assembly is recorded by the first event common to all component terranes. Using SHRIMP UPb zircon geochronology the oldest events common to all terranes (including emplacement of crustally derived granites, contemporaneous metamorphism and anatexis) have been dated at 2710–2725 Ma. In the Akulleq terrane areas where in situ diatexite formed, abundant granitoid sheets were intruded and common growth of metamorphic zircon (mostly low Th U ) in most lithologies occurred. In the Akia and Tasiusarsuaq terranes there was only intrusion of a lesser number of ∼ 2720 Ma granitoid sheets, because metamorphic zircons of that age have not been found. The ∼ 2720 Ma event is interpreted as marking, or shortly following, terrane assembly, when the diverse components of the Akulleq terrane were tectonically juxtaposed with the other two. This new documentation of metamorphic and associated igneous events within an accreting cratonic region is an illustration that the stabilisation of extensive areas of Archaean gneisses can be due to accretionary tectonics long after the individual components were first formed.

Tectonostratigraphic terranes within Archaean gneiss complexes: examples from Western Australia and southern West Greenland

New field work and isotopic data show that the Godthabsfjord region of West Greenland consists of a collage of tectonostratigraphic terranes, which evolved separately prior to tectonic juxtaposition in the late Archaean. In Western Australia the Narryer Gneiss Complex, which lies on the northwestern margin of the Yilgarn Craton, is, unlike the Godthabsfjord region, very poorly exposed (less than 1 % ). In consequence it is impossible to follow geological boundaries in this complex, and instead the complex has been studied by a very extensive use of within-grain zircon U-Pb geochronology on the ion microprobe SHRIMP. The zircon geochronology suggests that the Narryer Gneiss Complex also consists of several discrete terranes of early to mid Archaean gneisses. In both the Godthabsfjord region and the Narryer Gneiss Complex, late Archaean juxtaposition of terranes was accompanied by intrusion of crustally­derived granites, deformation, and amphibolite facies metamorphism. Thus some Archaean high grade gneiss complexes consist of terranes that underwent independent evolution until they were brought together at a later time. In this respect their anatomy resembles post-Archaean orogenic belts that formed as a consequence of plate tectonic processes.

The Archaean gneiss complex of northern Labrador A review of current results, ideas and problems

1. The early Archaean rocks in northern Labrador can be subdivided into the ea. 3.78 Ga Nulliak supracrus­tal association, the migmatitic Uivak I gneisses, the dominant phase of which was emplaced at ea. 3.73 Ga, and the Uivak II augen gneiss. Inherited low-U rounded inclusions within igneous zircons in the Uivak I gneisses have ages between 3.73 and 3.86 Ga and are more likely to have been derived from a pre-existing high-grade metamorphic gneiss complex than from the Nulliak association. In the early Archaean there were probably several rapid cycles of sedimentary deposition and volcanism followed by emplacement of major plutons. Mid Archaean gneisses are more abundant in northern Labrador than previously realised. The late Archaean metamorphic history of these gneisses is different from the history of the early Archaean gneisses. Whereas an important part of the mid Archaean suite was emplaced in granulite facies and retrogressed at the time of granitoid veining at ea. 2.99 Ga, the major part of the early Archaean rocks were reworked under granulite facies conditions in a sequence of closely spaced events between 2. 7 and 2.8 Ga. The two groups of gneisses had different metamorphic histories until ea. 2.7 Ga, but late and post-tectonic granites of 2.5- 2. 7 Ga age cut across both. It is suggested that the terrane model in southern West Greenland can be extended to Labrador and that tectonic intercalation of early and mid Archaean gneisses took place around 2.7 Ga. Correlation between the Maggo gneisses around Hopedale, mid Archaean gneisses in northernmost Labrador and gneisses from the Akia terrane in West Greenland is suggested. Like the Malene supracrustals in West Greenland the Upernavik supracrustals in Labrador are composite associations, the youngest of which are thought to have been deposited around 2. 7 Ga.

The Archaean gneiss complex in E. Hebei Province, North China: Geochemistry and evolution

Abstract The Caozhuang and Shuichang gneiss complexes from the western margin of the Qianan granite-gneiss dome, eastern Hebei, North China were investigated for their isotopic and trace element geochemistry. The oldest rocks so far recognized in China occur near Caozhuang in the south of the area. The Caozhuang gneiss complex consists of supracrustal rocks and multiphase orthogneisses. The supracrustals comprise a sequence of amphibolites, quartzites, felsic gneisses, marble and BIF, which are thought to be laid down in a shallow water environment. A ca. 3.5 Ga Sm-Nd isochron has been obtained from the amphibolites, and also from four felsic gneisses of the Caozhuang supracrustals. The orthogneisses from the Caozhuang complex appear to have been generated in three main cycles: at ca. 3.5, ca. 2.7 and ca. 2.5 Ga. The earliest Huangbaiyu gneiss was broken up and contaminated by the intrusions of granodiorites and late granitic gneisses. The granodiorites have yielded a ca. 2.7 Ga Rb-Sr isochron, the relatively high ISr[=0.7057±2 (2σ)] for the rocks suggesting that the granodiorites were formed by anatexis of older continental crust. Late granitic gneisses have a U-Pb zircon age of 2.5 Ga, and they are remarkably similar in appearance and chemical composition to the Huangbaiyu gneisses, suggesting that the gneiss complex as a whole is a mixture of gneiss types of widely different ages. The Shuichang gneiss complex contains supracrustal rocks, granitic rocks and charnockites. The supracrustal rocks include greywackes, semi-pelites and BIF, which have been metamorphosed in the granulite facies and occur within a polyphase complex of deformed leucogranites, microcline granites and charnockites. The age of the supracrustals is not known, except that they predate charnockites of 2.65 Ga. There is a close relationship between the production of the granites and the charnockites. It is suggested that differences in mineral and chemical composition between granites and charnockites could result from partial melting processes occurring in the Shuichang metasedimentary gneisses.

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.

Archaean gneiss complex of Eastern Hebei province, North China

Archaean gneiss complex of Eastern Hebei province, North China

Geochemistry of the Archaean Gneiss Complex and Associated Rocks of the Kanakapura Area, Karnataka, South India

Abstract The area of present investigation forms a part of the Archaean high-grade gneiss terrain of Karnataka. The rocks comprise both acid and basic lithologies and involve the late Archaean Closepet granite. Petrographically the acid rocks span the range, tonalite to granite, whilst the basic rocks are amphibolites and two-pyroxene granulites. The acid 'rocks have calc-alkaline affinities and resemble those from the transition zone to the east. Trace elements show them to be relatively undepleted with K/Rb ratios of >500. Simulated REE patterns of gneisses reveal that there are three broad groups: two of which are steep, but at different REE abundances, suggesting high pressure evolution; and one relatively flat group suggesting low pressure evolution. The basic rocks have a tholeiitic affinity, but are clearly separate, the granulite facies types being slightly more magnesian. The trace element chemistry of these basic rocks has been disturbed by migmatization. The REE patterns of Closepet granite are steep and may be related to the partitioning of accessory minerals such as allanite. An attempt is made to establish the relative position of the granulite facies metamorphism with respect to the regional fabric and retrogression.

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 > 3600 crustal Pb with younger material. Sm-Nd data points (Collerson & 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.