Crustal thickening in an extensional regime: application to the mid-Norwegian Vøring margin

The structure of the mid-Norwegian volcanic Vøring margin at the onset of the Maastrichtian–Paleocene extension phase reflects the cumulative effect of earlier consecutive rifting events. Lateral structural differences present on the margin at that time are a consequence of migration of the location of maximum extension in time between Norway and Greenland. The most important imprints (Moho depth, thermal structure) of these events on the lithosphere are incorporated in a numerical simulation of the final extension phase. We focus on a possible mechanism of formation of the Vøring Marginal High and address the relationship between spatial and temporal evolution of crustal thinning and thickening, uplift of the surface and strength of the lithosphere.It is found that the Vøring Basin formed the strongest part of the margin which explains why the Maastrichtian–Paleocene rift axis was not located here but instead jumped westward with respect to the earlier rift axes locations. The modeling study predicts that local crustal thickening during extension can be expected when large lateral thermal variations are present in the lithosphere at the onset of extension. Negative buoyancy induced by lateral temperature differences increases downwelling adjacent to the rifting zone; convergence of material at the particular part of the margin is mainly taken up by the lower crust. The model shows that during the final phase of extension, the crust in the Vøring Marginal High area was thickened and the surface uplifted. It is likely that this dynamic process and the effects of magmatic intrusions both acted in concert to form the Marginal High.     

Strike-slip faults parallel to crustal spreading axes: data from Iceland and the Afar Depression

Slickenside studies in regions of crustal spreading such as Iceland and the Afar Depression, East Africa, reveal that a significant number of faults parallel and close to rift axes are strike-slip rather than normal. Therefore, the pattern of brittle deformation in these regions does not conform to the classic two-dimensional schemes of oceanic tectonics and pre-oceanic rifting. Dip-slip and strike-slip faulting presumably alternated along or in the vicinity of spreading axes, indicate a varying stress field and a combination of transverse and longitudinal movements. In Iceland, strike-slip faults parallel to rifts are observed both west and east of the rift system as well as in a median area between overlapping rifts; the mechanisms proposed for their origin include accommodation of oblique convergence or divergence of crustal sections due to variations of spreading directions along axis and the interaction of overlapping rifts. In the Afar Depression this kind of fault is recorded west of the rift of Asal and can be imputed to reflect an interaction among rifts in the vicinity of the Afar triple junction. Rift-parallel strike-slip faults cannot however be assumed to be a feature of all crustal spreading axes due to the peculiarity of the examined regions: both of them are hot-spot areas and the Afar Depression lies at a triple junction.     

An analytical model of continental rift fault patterns

Two proposed mechanisms of rift initiation are crustal uplift alone and a combination of crustal uplift and regional horizontal extension. A three-dimensional, thick-plate, elastic analysis has been used to model the crustal stress state and the fault patterns associated with these mechanisms. Small ratios of uplift width to crustal thickness (<10) necessitate the thick-plate approach.For the crustal uplift model, the surface fault pattern is characterized by normal faults trending parallel to the major uplift axis at the uplift center and radial normal faults toward the ends of the major uplift axis. Zones of compressional structures (e.g., strikeslip and thrust faults) may develop at the periphery of the uplift. Superposition of regional horizontal tension with the stresses produced by crustal uplift eliminates the compressive stresses at the uplift periphery producing normal faults parallel to the major uplift axis at the uplift center and normal faults perpendicular to the major uplift axis at the uplift periphery.A comparison of these predicted fault patterns with the faults of the Rhine graben suggests that the combination of crustal uplift and regional horizontal extension contributed to the formation of that rift system. The stresses produced by crustal uplift promoted the formation of the central graben and the fan-shaped troughs toward the ends of the major uplift axes, while superposed regional horizontal tension eliminated the large compressive stresses at the uplift periphery promoting the normal faulting and dike intrusions observed on the Rhine graben flanks.     

Evolution of oceanic crust on the Kolbeinsey Ridge, north of Iceland, over the past 22 Myr

The evolution of oceanic crust on the Kolbeinsey Ridge, north of Iceland, is discussed on the basis of a crustal transect obtained by seismic experiment from the Kolbeinsey Ridge to the Jan Mayen Basin. The crustal model indicates a relatively uniform structure; no significant lateral velocity variations are observed, especially in the lower crust. The uniform velocity structure suggests that the postulated extinct axis does not exist over the oceanic crust formed at the Kolbeinsey Ridge, but supports a model of continuous spreading along the ridge after oceanic spreading started west of the Jan Mayen Basin. The oceanic crust formed at Kolbeinsey Ridge is 1–2.5 km thicker than normal oceanic crust due to hotter-than-normal mantle from the Iceland Mantle Plume. The observed generally uniform thickness throughout the transect might also indicate that the temperatures of the astheno-spheric mantle ascending along the Kolbeinsey Ridge have not changed significantly since the age of magnetic anomaly 6B.     

Petrogenesis of late stage magmatism at Hold with Hope, East Greenland

The Myggbukta caldera complex and a swarm of basic dykes constitute the latest Tertiary magmatism in the Hold with Hope region, East Greenland. The Sr and Nd isotope ratios of these rocks show coherent variations which extend to high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd values and require a contribution from continental lithosphere. Broad correlations with major element differentiation indices suggest that the continental component was incorporated during magmatic differentiation thereby favouring a crustal contamination process. Trace element concentrations are strongly correlated with isotopic compositions but display ranges for many incompatible elements which extend beyond likely crustal contaminant compositions. This is readily modelled by AFC processes in which the dominant cause of trace element enrichment is the concentration effect of fractional crystallisation rather than the composition of the contaminant. The simplest such models still require unrealistically high degrees of fractional crystallisation to explain the ten-fold enrichment of some trace elements. This can be overcome if the primary magmas entering the crust already had highly variable trace element compositions. Such variability is readily achieved if melts from different parts of the melting column escape without thorough homogenization. An AFC model which incorporates variability in parental magma composition is then able to simulate the range of compositions observed at Hold with Hope. This carries the implication that the variations observed are more readily attributed to changes in uncontaminated parental magma than to variations in the composition or amount of contaminant. Received: 5 March 1998 / Accepted: 16 June 1998     

Metallogeny of South Greenland: A review of geological evolution,mineral occurrences and geochemical exploration data

South Greenland has been the site of historic mining of cryolite, copper, graphite and gold, hosts mineral deposits with gold, uranium, zinc, niobium, tantalum, zirconium, hafnium, REE, iron, titanium, vanadium, fluorite and graphite, and has additional potential for lithium, beryllium, phosphorus, gallium and thorium. Data from stream sediment geochemical surveys document that South Greenland is enriched in a range of these elements relative to the rest of Greenland and to estimates of the upper crust composition. Distribution patterns for individual elements within south Greenland exhibit enriched regions that are spatially related to lithological units, crustal structure and known mineralisation.The Northern Domain of South Greenland includes the southernmost part of the orthogneiss-dominated North Atlantic craton. Orogenic gold mineralisation is hosted by quartz veins and hydrothermally altered rocks associated with shear zones intersecting the Mesoarchaean Tartoq Group of mafic metavolcanic rocks. Geochemical exploration indicates that additional potential for gold mineralisation exists within Palaeoproterozoic supracrustal rocks overlying the Archaean basement.Rocks formed during the Palaeoproterozoic Ketilidian orogeny occupy a major part of South Greenland and has been divided into two domains. The Central Domain is underlain by the Julianehåb igneous complex forming a 100 km wide ENE–WSW zone centrally across South Greenland. Intrusive and extrusive, mostly felsic magmatic rocks were emplaced in two main stages (1850–1830 and 1800–1780 Ma) in a continental arc setting. Positive anomalies in aeromagnetic data indicate that mafic plutons are common in the late igneous complex. Intra-arc mafic metavolcanic rocks contain syngenetic stratabound copper sulphide and epigenetic shear zone-hosted copper–silver–gold mineralisation at Kobberminebugt and Kangerluluk, whereas metasedimentary and metapyroclastic rocks contain stratabound uraninite mineralisation. Orthomagmatic iron–titanium–vanadium mineralisation is hosted by a gabbro. A potential for porphyry-type mineralisation related to the late intrusive stages of the Julianehåb igneous complex is suggested by showings with copper, molybdenum and gold together with stream sediment anomalies for these elements. Vein-type uranium mineralisation occurs in fault zones in the Julianehåb igneous complex related to Mesoproterozoic rifting.The Southern Domain contains an assemblage of Palaeoproterozoic metasedimentary and metavolcanic rocks that underwent moderate to strong deformation, peak HT–LP metamorphism and partial melting with subsequent retrograde exhumation at 1790–1765 Ma. The supracrustal rocks contain syngenetic Au, As, Sb, U, and Zn mineralisation in volcanic or graphite- and sulphide-rich sedimentary environments; graphite was mined historically at two sites. Many stream sediment gold anomalies are located in a NE-trending belt along the boundary between the early Julianehåb complex and the supracrustal rocks to the south. They reflect a number of auriferous quartz vein occurrences, including the Nalunaq gold deposit, hosted in a system of shear zones and probably generated as orogenic gold during Ketilidian accretion. The 1755–1730 Ma, A-type Ilua plutonic suite is the latest magmatic event in the Ketilidian orogen.The 1300–1140 Ma Gardar period involved continental rifting, sedimentation and alkaline magmatism. Numerous dykes and 10 ring-shaped intrusion complexes were formed across South Greenland. An orthomagmatic iron–titanium–vanadium deposit is hosted by troctolitic gabbro. Residual magmas and fluids resulting from extreme magmatic differentiation, possibly combined with assimilation of older crust, created mineral deposits including cryolite that was mined at Ivigtut, large low-grade deposits of uranium–rare earth elements–zinc at Kvanefjeld and tantalum–niobium–rare earth element–zirconium at Kringlerne, in the Ilímaussaq complex, as well as tantalum–niobium–rare earth elements at Motzfeldt Sø in the Igaliko complex.The South Greenland crustal evolution records effects of mantle processes, such as lithospheric extension, subduction and underplating, which resulted in recurrent magma emplacement in tectonically active environments. As such, the geology of South Greenland reflects events and circumstances that are favourable to the generation and preservation of hydrothermal ore-forming fluid systems during the Ketilidian orogeny as well as to the development of extreme rock compositions within the Gardar alkaline igneous province.     

Crustal structure of the East African Rift zone

A review of seismological data on the crustal structure of the East African Rift zone is presented. The only refraction line is that along the Gregory Rift, which indicates a 7.5 km/sec refractor which is presumed to be the Moho. The bulk of data is provided by surface-wave dispersion studies. Some preliminary measurements of crustal and sub-Moho velocities using the University of Durham array at Kaptagat in Kenya are included.

There is now a growing body of evidence that the crust is generally of shield type over the whole rift zone. The exception is along the axis of the Gregory Rift, where a low-velocity Moho and some crustal modification is apparent. This is presumably the result of magma intrusions and suggests some crustal separation along this section of the rift. Sub-Moho velocities are probably normal outside the rifts themselves, though anomalously low upper-mantle velocities are to be associated with rifting. There is firm evidence for thinning of the lithosphere along the eastern branch of the rift. A cross-section of the Gregory Rift which is consistent with the current data is presented.     

Geological aspects of deformation in continental shear zones

A method of kinematic analysis of structures, microstructuresand mineral preferred orientations, initially devised in the study of peridotites, has been applied to crustal rocks bearing evidence of large strains produced in metamorphic environments. Three tectonic lineaments (Angers-Lanvaux, Montagne Noire and Maydan) were selected. They illustrate a general situation arising in continental crusts when they are deformed by ductile transcurrent fault systems.The Angers-Lanvaux structure is bilaterally symmetric; its dominant feature is the horizontal stretching lineation which is parallel to the fold axes. The foliation and slaty cleavage in the most surficial formations wrap around the axis of the whole structure. The folds in the slates away from the axis also exhibit axes parallel to the general trend, but no stretching lineations. These folds are attributed to crustal shortening in a direction normal to the ductile fault. In the Montagne Noire recumbent folds are thrusted away from the axis of the structure over at least 25 km. The metamorphism is also centered on the structure and symmetrically reduced away from it. The core of the structure is occupied by a strongly lineated orthogneiss, cut by a late intrusive granite. The Maydan axial zone displays clear evidence of partial melting at various scales within the deformed gneisses: (1) in gashes perpendicular to the stretching lineation which in these anatectic formations tends to plunge at more than 45°; (2) in bands of deformed pegmatites; and (3) possibly in granites which on the one hand intrude the surrounding formations and on the other converge with increasing deformation on the fault zone. The quartz preferred orientations and microstructures in quartzite layers from Angers indicate that the plastic flow plane and direction lie, respectively, close to the foliation and lineation, the slight departure is ascribed to a flow with a rotational shear component.All this suggests a general model for the origin of such ductile zones. The horizontal relative displacement of crustal blocks along a ductile band is responsible for its overall steeply dipping foliation and horizontal lineations. Viscous heating progressively tends to concentrate the plastic flow along its axis. It is also responsible for the development of metamorphism and of anatexis at depth; the partially melted rocks tend to rise, building at shallower depth the arched structure in the axis of the ductile zone, with a continuing flowage parallel to this axis probably now in the solid state; they can also intrude the surrounding terrain as undeformed batholiths. The folds parallel to the stretching lineation in the axial zone are explained by the fact that, due to the escape of anatectic melts, the formations at depth flow in a narrowing channel. The upwelling of the axial structure induces a compression with folding in the surrounding sedimentary formations and gravity nappe sliding away from the axis.     

中国东部铜矿床类型、成矿环境、成矿集中区与成矿系统

通过对我国东部２２个大型以上及４０余个中型铜矿床的综合调研,并与国外对比得出如下结论：（１）中国中东部陆壳是在几个古陆块基础上增生起来的,铜矿成矿作用在空间上向板块边缘推移,与块体边缘地球化学急变带有密切联系,在时间上越来越新;（２）大型铜矿可分６大成矿期,以中生代最重要,次为晚古生代,中（新）元古代,（３）我国东部地壳运动频繁而又比较强烈,就形成大型铜矿床的环境而言,内生成较多的矿床类型比外生成     

Shear wave splitting beneath South Kamchatka during the 3-year period associated with the 1997 Kronotsky Earthquake

Shear wave splitting measurements in South Kamchatka during the 3-year period (1996–1998) in which the Kronotsky Earthquake (M=7.7, December 5, 1997) occurred are used to determine anisotropic parameters of the subduction zone and shear wave splitting variations with time. The local small seismic events recorded at the Petropavlovskaya IRIS station (PET) were analyzed. The dominant azimuths of the fast shear wave polarizations for the 3-year period are defined within N95±15°E, which are consistent with the general Pacific Plate motion direction. Modeling of fast shear wave polarizations shows that HTI model with the symmetry axis oriented along N15°E±10° fit well the observed data for events the focal depths of which are less than 80 km. For the greater depths, the orthorhombic symmetry of medium is not excluded. The anisotropy coefficient increases generally with depth from 1–2% in the crust to 4–7.5% in the subducting plate. Variations in time delays show a general increase up to 10–15 ms/km during 1996–1997 before the large crustal earthquake series (M≈5.5–7) in the Avacha Bay and before the Kronotsky Earthquake. Analysis of fast S-wave azimuths of mantle events reveals a temporal cyclic variation. The most regular variations are observed for fast azimuths of deep events with a period of about 172 days over the 3-year period. The fast polarizations of crustal events behave comparatively stable. It is assumed that the major instabilities in stress state are localized in the descending slab and influenced the upper mantle and comparatively stable crust.     

Geodetic reference systems for crustal motion studies

The behaviour of geodetic reference systems as a function of time cannot be ignored in determinations of position for secular geodynamic modelling. While non-specialist users are likely to appreciate the uncertainty surrounding the concept of a “fixed” point in regional surveys for crustal motion, its extension to global surveys based on techniques in dynamic geodesy is not widely understood. Such solutions are related to the natural coordinate system defined by the instantaneous geocentre (earth's centre of mass) and the instantaneous rotation axis whose location in earth space cannot be expected to be time invariant.The effect of such movements of the natural system of reference on the coordinates of points at the earth's surface is computed for a plausible model of the former. The resulting changes in solutions for three-dimensional position from dynamic considerations are discussed. It is shown that for the model adopted, the earth-space variations of the natural reference system with time produce coordinate changes with magnitudes not dissimilar to those resulting from reasonable variations in the earth's figure, though with different wavelengths, when computed from three-dimensional considerations alone.The resulting displacements, along with an accepted model for plate motions, are used to study changes in the shape of the level surfaces of the earth's gravity field with time. It is shown that plausible models for the deformation of the earth's figure produce significant changes in the geopotential and the shape of the geoid. However, the transfer of mass implied in extreme models of plate motion produce no significant changes in the datum level surface for crustal motion studies over periods as long as 10² yr.     

Ca isotope fingerprints of early crust-mantle evolution

The utility of ⁴⁰Ca/⁴⁴Ca as a tracer of pre-existing crustal contributions in early Archaean cratons has been explored to identify traces of Hadean crust and to assess the style of continental growth. The relatively short half-life of ⁴⁰K (∼1.3 Gy) means that its decay to ⁴⁰Ca occurs dominantly during early Earth History. If Archaean crust had a significant component derived from a more ancient protolith, as anticipated by “steady state” crustal evolution models, this should be clearly reflected in radiogenic ⁴⁰Ca/⁴⁴Ca ratios (or positive initial ε_Ca) in different Archaean cratons. A high precision thermal ionisation technique has been used to analyse the ⁴⁰Ca/⁴⁴Ca ratios of plagioclase separates and associated whole rocks in ∼3.6 Ga (early Archaean) samples from Zimbabwe and West Greenland. Three out of four tonalite, trondhjemite, granodiorite (TTG) suite samples from Zimbabwe display initial ⁴⁰Ca/⁴⁴Ca ratios indistinguishable from our measured modern MORB value (i.e., ε_Ca(3.6) ∼ 0). Greenland samples, however, are very diverse ranging from ε_Ca(3.7) = 0.1 in mafic pillow lavas and felsic sheets from the Isua supracrustal belt, up to very radiogenic signatures (ε_Ca(3.7) = 2.9) in both mafic rocks of the Akilia association and felsic TTG from the coastal Amîtsoq gneisses.At face value, these results imply the Zimbabwe crust is juvenile whereas most Greenland samples include an earlier crustal component. Yet the west Greenland craton, as with many Archaean localities, has experienced a complex geological history and the interpretation of age-corrected initial isotope values requires great care. Both felsic and mafic samples from Greenland display ε_Ca(3.7) so radiogenic that they are not readily explained by crustal growth scenarios. The presence of such radiogenic ⁴⁰Ca/⁴⁴Ca found in low K/Ca plagioclases requires Ca isotope exchange between plagioclase and whole rock during later metamorphic event(s). In addition the unexpectedly radiogenic Ca isotope ratios in some mafic samples reflect anomalous K/Ca ratios as a result of intense K-metasomatism ∼3.6 Ga. Thus Ca isotope measurements are not a robust tracer of crustal growth in the presence of intense tectono-metamorphic processes. Coupled with other isotope data, however, the degree of overprint can be estimated and the ⁴⁰Ca/⁴⁴Ca ratio of a little disturbed sample hints at a small contribution of Hadean protocrust in the coastal part of the Godthåbsfjord area (Southwest Greenland). In the majority of Zimbabwe TTG samples, unradiogenic initial Ca isotope ratios point to very little prior crustal history and minor subsequent disturbance. We thus infer that the modest initial ε_Nd ∼0.8 of the Zimbabwean samples is representative of the depleted mantle at ∼3.6 Ga. Furthermore, Ca isotope systematics provide little support for a “steady state” model of crustal growth.     

Deep crustal carbonates as CO2 fluid sources: Evidence from metasomatic reaction zones

Metasomatic reaction zones which developed at marble-pelitic schist contacts in a granulite facies terrane in West Greenland contain a consistent sequence of five mineralogical zones. Outward from the carbonates the zones are characterized by the assemblages grossular-diopside-meionite (I), meionite-anorthite-diopside (II), anorthite-diopside-edenitic hornblende (III), anorthite-enstatite (IV), plagioclase-almandine-sillimanite (V). Sphene is superceded by ilmenite between zones (II) and (III); quartz is present in all zones except zone I. Scapolite, plagioclase, clinopyroxene and mica exhibit a small degree of compositional variation which correlates with distance from the carbonate. These small compositional variations are superimposed on a strong CaO chemical potential gradient. Compositional features, zone distributions and CaO activity calculations demonstrate that the zones developed in response to CaO diffusion along a chemical potential gradient of 2 kcal/m. The CaO source appears to be carbonate rocks which release calcium as decarbonation reactions proceed. The maximum volume of CO₂ released in this process, and that released during discontinuous reactions in the marbles, will contribute a total volume of CO₂ approximately equivalent to the volume of carbonate in the rock. Calculations demonstrate that a terrane consisting of as little as 8% carbonate will release sufficient CO₂ to result in complete dehydration of an amphibolite terrane, at deep crustal conditions. Dehydration through CO₂ release will be accomplished either through rapid burial, which would prevent both equilibration of mineral assemblages and CO₂ release at intermediate crustal levels, or through diffusion-driven metasomatic reactions which would lead to CO₂ release primarily at the high temperatures of deep crustal environments. The latter process would be the dominant CO₂ source at deep crustal levels if carbonate rocks occur predominately as relatively thin layers.     

Precambrian–Palaeozoic geology of Smith Sound, Canada and Greenland: key constraint to palaeogeographic reconstructions of northern Laurentia and the North Atlantic region

Nares Strait separating Greenland and northernmost Canada is floored by continental crust. Most palaeogeographic reconstructions of Laurentia and the North Atlantic region model the seaway as the site of massive sinistral strike–slip and/or compression/transpression, subduction and collision, the supposed manifestations of the hypothetical Wegener Fault. However, these reconstructions fail to take into account the bedrock geology that represents within-plate evolution. Both sides of Smith Sound, the southernmost part of Nares Strait, expose the same early Proterozoic to early Palaeozoic assemblages that are unaffected by seaway-related tectonism or thermal activity. Smith Sound is an intact crustal block or `linchpin' demonstrating that there was no independent Greenland plate. North-west Greenland was not a leading plate margin neither was Nares Strait the site of the plate boundary between Greenland and North America. The Wegener Fault does not exist. The Smith Sound linchpin constitutes a key constraint that must be respected in any palaeogeographic reconstruction of the region.     

Lateral variation in structural style of mountain building: controls of rheological and rift inheritance

Many mountain belts exhibit significant along‐strike variation in structural style with changes in the width of the orogen, the geometry and kinematics of the crustal‐scale thrust system, and the degree of partitioning between pro‐ and retro‐wedge deformations. Although the main factors controlling first‐order structural style are understood, the cause of these lateral variations remains to be resolved. Here we focus on the Pyrenees, characterized by significant lateral variation in structural style with a thrust system involving more and thinner thrust sheets in the eastern section than in the western part. Similarly, the prior Mesozoic rifting event was characterized by significant lateral variation in structure. We integrate available geological and geophysical data with forward lithospheric scale numerical models. We show that lateral variation in crustal strength attributed to inherited Variscan crustal composition accentuated during Mesozoic rifting explains the variation in structural style observed during Pyrenean mountain building.     

Gravity anomalies over the Central Indian Ridge between 3<Superscript>∘</Superscript>S and 11<Superscript>∘</Superscript>S,Indian Ocean: Segmentation and crustal structure

High-resolution shipboard geophysical investigations along the Indian Ocean ridge system are sparse especially over the Carlsberg and Central Indian ridges. In the present study, the shipboard gravity and multibeam bathymetry data acquired over a 750 km long section of the Central Indian Ridge between 3 ^°S and 11 ^°S have been analysed to understand the crustal structure and the ridge segmentation pattern. The mantle Bouguer anomalies (MBA) and the residual mantle Bouguer anomalies (RMBA) computed in the study area have shown significant variations along the ridge segments that are separated by transform and non-transform discontinuities. The MBA lows observed over the linear ridge segments bounded by well-defined transform faults are attributed to the thickening of the crust at the middle portions of the ridge segments. The estimates of crustal thickness from the RMBA shows an average of 5.2 km thick crust in the axial part of the ridge segments. The MBA and relative RMBA highs along the two non-transform discontinuities suggests a thinner crust of up to 4.0 km. The most significant MBA and RMBA highs were observed over the Vema transform fault suggesting thin crust of 4 km in the deepest part of the transform fault where bathymetry is more than 6000 m. The identified megamullion structures have relative MBA highs suggesting thinner crust. Besides MBA lows along the ridge axis, significant off-axis MBA lows have been noticed, suggesting off-axis mantle upwelling zones indicative of thickening of the crust. The rift valley morphology varies from the typical V-shaped valley to the shallow valley floor with undulations on the inner valley floor. Segments with shallow rift valley floor have depicted well-defined circular MBA lows with persistent RMBA low, suggesting modulation of the valley floor morphology due to the variations in crustal thickness and the mantle temperature. These are supported by thicker crust and weaker lithospheric mantle.     

Metallogeny of Greenland

Greenland is the largest island on Earth, with 80% of its area covered by a thick ice sheet. The coastal areas are underlain by variable rocks ranging from Eoarchean to the most recent ages. Greenland has a mineral exploration tradition since its colonization in the 18th century, and mining of cryolite started in 1854. Since the 1960s, the country is explored systematically for various commodities, which however resulted only in limited mining activity in only a few successful mines. Most exploration has been based on prospecting followed by exploration around the exposed mineralization.The geology from North-West Greenland along the coast to the south and along the eastern coast north to Kangerlussuaq Fjord is dominated by deeply eroded Archean and Paleoproterozoic rocks. The metallogeny is largely controlled by mid crustal processes and the preservation potential of mineralization in the deeper crust. Significant mineralization is found in orthomagmatic Ni-PGE-Au sulfide and Cr- or Fe-Ti-V oxide systems and hypozonal orogenic gold systems in major shear zones. Interestingly, the ultramafic units of the orthomagmatic systems locally host gemstone-quality corundum mineralization. Graphite mineralization occurs in amphibolite-granulite facies metasedimentary units and in shear zones in Paleoproterozoic orogens. Mesozonal orogenic gold and iron ore in banded iron formation are restricted to localized lower metamorphic grade areas along the west coast. Larger units of preserved Paleoproterozoic metasedimentary and metavolcanic rocks are restricted to South, central East and central West Greenland, where base metal mineralization formed the significant Zn-Pb Black Angel deposit.Widespread sedimentation and localized mafic magmatism started in the late Paleoproterozoic in various continental to shallow marine basins and lasted with interruptions until the start of the Caledonian Orogeny. These late Paleoproterozoic to early Paleozoic sedimentary rocks are variably deformed and metamorphosed by subsequent orogeny and mainly preserved in northern and eastern Greenland. They host stratiform sedimentary base metal mineralization of only limited known extent, except the sedimentary exhalative Zn-Pb Citronen deposit in central North Greenland. The Caledonian and subsequently the Ellesmerian orogens affected the eastern and northern Laurentian margin, respectively. Mineral systems of only limited known extent related to this orogeny are Mississippi Valley Type Zn-Pb in the Ellesmerian foreland, mesozonal orogenic gold in Caledonian shear zones and magmatic-hydrothermal W-Sb ± Au ± Cu systems in and adjacent to Caledonian granites. Renewed and almost continuous sedimentation occurred from the Devonian until Paleogene in eastern Greenlandic basins. The sedimentary units host stratiform sedimentary base metal mineralization of only small known magnitude. The Paleogene in eastern and central West Greenland is characterized by widespread mafic-ultramafic magmatism, forming flood basalt and a series of intrusions in East Greenland. Nickel-sulfide mineralization is locally hosted by the mafic-ultramafic rocks in central West Greenland, whereas eastern Greenlandic mafic and felsic intrusions host significant orthomagmatic PGE-Au mineralization in Skaergaard, and magmatic hydrothermal Mo-Au-Ag mineralization in Malmbjerg and Flammefjeld.Western and southern Greenland was a relative stable shield from Paleoproterozoic times and is intruded by localized Meso- and Neoproterozoic alkaline and carbonatite suites, which form part of a larger Mesoproterozoic rift only in South Greenland. These intrusions host locally significant REE-Nb-Ta-U-Zn-Be in Kvanefjeld, Kringlerne and Motzfeldt deposits of South Greenland and the southern West Greenlandic Sarfartoq deposit. Diamond mineralization is spatially associated with the alkaline and carbonatite intrusions in southern West Greenland.The long and complex geological evolution recorded in Greenland appears to be in contrast with only few examples of successful mineral exploration and mining. Numerous mineral deposits are developed in neighboring Arctic countries, making the remote Arctic setting an unlikely single argument for the situation. Geological knowledge is still relatively basic for many parts of Greenland and modern geophysical and geochemical data is often only available at a regional scale, which makes knowledge- and mineral system-driven exploration difficult and costly. The review of the Greenlandic metallogeny in this paper, however, clearly shows the enormous potential for finding ores in a wide variety of settings.     

论岛弧的拱结构

基于海沟－岛弧－弧后盆地体系起伏变化的地壳厚度和岛弧平面上独特的延展方式，从结构力学上看，岛弧实质上是横跨于汇聚板块边界上的二铰拱，弧体及两端支点的地质构造特点都说明了这一点。板块俯冲边界上作用力的方式和方向相当于作用在二铰拱上的竖向均布载荷，因而岛弧选择的合理拱轴为抛物线。为了保持汇聚板块边界的平衡，岛弧呈抛物线形向外扩展，引起弧后盆地向岛弧一侧的扩张，这与大洋中脊的扩张有本质的不同。     

Magma plumbing systems in large igneous provinces: Inferences from cyclical variations in Palaeogene East Greenland basalts

We present compositional data on a 1,250-m-thick sequence of sparsely porphyritic lavas that comprise the Geikie Plateau Formation, part of the ~55-Ma break-up-related flood basalts in East Greenland. Major element compositions are relatively restricted (6.3–7.6 wt% MgO; 2.2–2.4 wt% TiO₂), with two excursions to more evolved compositions (2.4–3.4 wt% TiO₂) that are similar to the inferred parental magma of the nearby Skaergaard Intrusion. Major and trace element calculations show that fractional crystallisation is the principal control on magma compositions, and the cyclical sequential variations imply regular magma chamber replenishment events. Isotopic data indicate minor crustal assimilation, but with different contaminants for the main group (amphibolitic gneiss) and evolved cycles (granulitic gneiss). Rifting episodes may have allowed more primitive magmas to ascend to shallow crustal levels and subsequently fractionate to more evolved compositions in a separate chamber, which was perhaps similar to the source of the Skaergaard Intrusion.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial responsibility: I. Parsons     

The nature and genesis of marginal Cu–PGE–Au sulphide mineralisation in Paleogene Macrodykes of the Kangerlussuaq region, East Greenland

The Kangerlussuaq region of East Greenland hosts a variety of early Tertiary extrusive and intrusive igneous rocks related to continental break up and the passage of the ancestral Iceland plume. These intrusive bodies include a number of gabbroic macrodykes, two of which—the Miki Fjord Macrodyke, and the newly discovered Togeda Macrodyke—contain Cu–PGE–Au sulphide mineralisation along their margins. Sulphides occur as disseminated interstitial blebs and rounded globules of chalcopyrite and pyrrhotite with some Fe–Ti oxides and platinum-group minerals, comprising largely Pd bismuthides and tellurides. The globules are interpreted to have formed from fractionation of trapped droplets of an immiscible Cu- and Pd-rich sulphide melt and show geopetal indicators. Sulphur isotopes imply a local crustal source of S in these from pyritic sediments of the Kangerlussuaq Basin. Thus, generation of these sulphide occurrences was controlled by local country rock type. Low Ni/Cu and Pt/Pd ratios, also present in the Platinova reefs in the Skaergaard Intrusion, indicate that early fractionation of olivine may have depleted the magma of Ni and suggest the likely presence of a large magma chamber at depth. Xenoliths of Ni-rich olivine cumulates in the Miki Fjord Macrodyke may have been sourced from such a body. The location of thus far unidentified conduit or feeder zones to the macrodykes beneath the present day surface may represent potential targets for more massive sulphide orebodies.