Caledonian Fold Belt Research Articles

P‐Wave Tomography for 3‐D Radial and Azimuthal Anisotropy Beneath Greenland and Surrounding Regions

AbstractWe present the first 3‐D images of P‐wave radial anisotropy (RAN) and azimuthal anisotropy (AAN) down to 750‐km depth beneath Greenland and surrounding regions. Three P‐wave velocity models (isotropic, RAN, and AAN) are determined by applying a regional tomographic method to simultaneously invert P wave arrival times of 1,309 local events and P wave relative traveltime residuals of 7,202 teleseismic events, which were recorded mainly by the latest GLISN network. A high‐velocity body located beneath northeast Greenland (NEG) exhibits a strong negative RAN and a strong AAN with N‐S to NE‐SW oriented fast‐velocity directions (FVDs). The FVDs are generally consistent with the direction of the fold axis of the Caledonian fold belt, which outcrops in NEG. Beneath the Iceland, Jan Mayen, and Svalbard hotspots, a strong positive RAN and a negligible or weak AAN are revealed, which may reflect effects of upwelling mantle plumes. Among the three hotspots, a weak AAN with a constant FVD is only revealed beneath Iceland, which may reflect the existence of background mantle flow. The RAN and AAN features beneath the Labrador Sea, Davis Strait, and Baffin Bay suggest the following scenario on breakup between Greenland and Canada: the breakup was initiated at the Labrador Sea due to local mantle upwelling, but the northward propagation of the breakup was blocked by a strong high‐velocity anomaly beneath Davis Strait; the opening of Baffin Bay might be caused passively by far‐field plate forces.

A healed strike-slip plate boundary in North Greenland indicated through associated pull-apart basins

Abstract In the North Atlantic, Laurentia–Eurasia break-up commenced in the Late Carboniferous, largely following the structural grain of the Caledonian Fold Belt. However, in the Arctic region, a 45° offset in the plate boundary between North Greenland and Svalbard was determined by a number of pre-Caledonian fundamental faults in North Greenland. As a result, this segment of the plate boundary experienced significant episodes of combined transtension and transpression, in part controlled by the movement of a temporarily independent Greenland Plate. Late Permian–Mesozoic deposits in the North Greenland Wandel Sea Basin record the plate-boundary history along this offset, in our view in a series of at least 20, variously disturbed, pull-apart basins, most of which can be assigned to four major episodes of pull-apart basin formation. The direction of the pre-existing fundamental faults, in combination with the regional variation in rock properties of both the basin floor and basin fill, explains the marked differences in tectonic style recorded along the plate boundary.

Texture and tectonic attribute of Cenozoic basin basement in the northern South China Sea

Based on the drilling data, the geological characteristics of the coast in South China, and the interpretation of the long seismic profiles covering the Pearl River Mouth Basin and southeastern Hainan Basin, the basin basement in the northern South China Sea is divided into four structural layers, namely, Pre-Sinian crystalline basement, Sinian-lower Paleozoic, upper Paleozoic, and Mesozoic structural layers. This paper discusses the distribution range and law and reveals the tectonic attribute of each structural layer. The Pre-Sinian crystalline basement is distributed in the northern South China Sea, which is linked to the Pre-Sinian crystalline basement of the Cathaysian Block and together they constitute a larger-scale continental block—the Cathaysian-northern South China Sea continental block. The Sinian-lower Paleozoic structural layer is distributed in the northern South China Sea, which is the natural extension of the Caledonian fold belt in South China to the sea area. The sediments are derived from southern East China Sea-Taiwan, Zhongsha-Xisha islands and Yunkai ancient uplifts, and some small basement uplifts. The Caledonian fold belt in the northern South China Sea is linked with that in South China and they constitute the wider fold belt. The upper Paleozoic structural layer is unevenly distributed in the northern South China. In the basement of Beibu Gulf Basin and southwestern Taiwan Basin, the structural layer is composed of the stable epicontinental sea deposit. The distribution areas in the Pearl River Mouth Basin and the southeastern Hainan Basin belong to ancient uplifts in the late Paleozoic, lacking the upper Paleozoic structural layers. The stratigraphic distribution and sedimentary environment in Middle-Late Jurassic to Cretaceous are characteristic of differentiation in the east and the west. The marine, paralic deposit is well developed in the basin basement of southwestern Taiwan but the volcanic activity is not obvious. The marine and paralic facies deposit is distributed in the eastern Pearl River Mouth Basin basement and the volcanic activity is stronger. The continental facies volcano-sediment in the Early Cretaceous is distributed in the basement of the western Pearl River Mouth Basin and Southeastern Hainan Basin. The Upper Cretaceous red continental facies clastic rocks are distributed in the Beibu Gulf Basin and Yinggehai Basin. The NE direction granitic volcanic-intrusive complex, volcano-sedimentary basin, fold and fault in Mesozoic basement have the similar temporal and spatial distribution, geological feature, and tectonic attribute with the coastal land in South China, and they belong to the same magma-deposition-tectonic system, which demonstrates that the late Mesozoic structural layer was formed in the background of active continental margin. Based on the analysis of basement structure and the study on tectonic attribute, the paleogeographic map of the basin basement in different periods in the northern South China Sea is compiled.

Advances in research of Asian geology—A summary of 1:5M International Geological Map of Asia project

The International Geological Map of Asia at a 1:5,000,000 scale (IGMA5000) is the first digital Asian geological map under the standard of the Commission for the Geological Map of the World (CGMW). Major advances that have been achieved in compiling the map are manifested in the following understandings.(1)Large amounts of Mesozoic volcanic rocks occurring in the eastern Asian coastal area are mainly Cretaceous instead of Upper Jurassic–Lower Cretaceous. Most of the Carboniferous–Permian volcanic rocks in Central Asia seem not to be arc volcanics, but the product of an extensional stage. The basal boundary of the Meso-Neoproterozoic Jixian section in China is not dated at 1.8Ga as defined previously, but less than 1.68Ga.(2)The most significant Neoarchean tectono-thermal events in the Sino-Korean craton and the Indian craton took place at 2.5Ga rather than at 2.7Ga. The basement of the Yangtze craton was finally formed at 0.75–0.8Ga, which is 0.2–0.3Ga later than the Greenville orogenic cycle. Geologically, South China is identified to be an Early Paleozoic Caledonian foldbelt. The Qinling belt, where no oceanic basin was developed in Triassic times, is not an Indosinian collisional orogen, but a continental crust subduction one. When Pangea was formed, Indo-Australian Gondwana had been joined to Paleo-Asia and between them there was no oceanic basin, i.e. no Paleo-Tethys which continued from Paleozoic to Mesozoic. A huge Indosinian orogenic belt existed on the southern margin of Paleo-Asia to the north of the Zagros–Himalayas.(3)Asia is a composite continent consisting of three major cratons—the Siberian, Indian and Arabian and three huge orogenic belts with a number of minor cratons and numerous microcontinents included. The main body of the Asian continent took its shape during the Mesozoic. The orogenic belts belong respectively to three global tectonic domains: the Paleo-Asian, Tethyan and Pacific. The small cratons, such as Sino-Korea, Yangtze, Tarim, and Sibumasu are thought to be affiliated to the tectonic transform zone between Gondwana and Siberia. They had been situated on the northern margin of Gondwana before the disappearance of the Paleo-Asian Ocean, and were lying on the southern margin of Paleo-Asia after the closing of the Paleo-Asian Ocean and then the opening of the Tethys. The fact that ophiolites in Asia appear to get progressively younger in age from north to south throws light on the Phanerozoic evolutionary process of the dispersion of Gondwana and the accretion of Asia accompanied by a southward migration of its orogenic belts.

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.

Gravity-inferred crustal thickness of Greenland

Greenland's crust contains clues for understanding the dynamics of the Earth's second largest ice-sheet, as well as the Iceland hotspot over the past 100 Ma. However, our knowledge of the sub-ice crust is very limited due to the paucity of outcrops and seismic observations, particularly in central Greenland. In order to augment these sparse constraints, we modeled terrain and airborne free-air gravity data which provide estimates for Moho and related crustal thickness. These estimates were obtained from the inversion of isostatically corrected terrain effects where the computed gravity effects of the terrain's rock, water, and ice components were subtracted from the terrain-correlated free-air anomalies. The terrain-correlated free-air components were estimated from the correlation spectrum between the free-air anomalies and the terrain's gravity effects which are assumed to reflect isostatic disturbances of the crustal terrain. The results indicated an average crustal thickness of about 37 km for Greenland, with greater values by 5–10 km, in the southern Greenland Archean blocks and Caledonian Foldbelt, and comparably lower values in northern Greenland. The thinner crustal thickness estimates in the north are strongly correlated with the faster basal melt regions of the ice sheet and glaciers. The crustal thickness map provides constraints for an improved interpretation of the Iceland hotspot track in northern Greenland at about 100–70 Ma.

Morphotectonic evolution of two depressions at the southern border of the Baikal rift system

Detailed study of two dry depressions in the Baikal rift system: the E–W Khoito Ghol-Tunka-Bistraya depression and the SW–NE Gusinoje–Ivolga depression, aims to provide a better understanding of tectonic control on the intershoulder relief evolution after the rift opened. Both depressions are grabens and both feature a suite of 10–20 km-wide basins alternating with more or less massive highs. Field and laboratory analysis shows that this pattern is of recent tectonic origin and that local breaking-up and subsidence followed the general sinking which originally formed the grabens. The subsidence belts seem to have gradually shifted north and northeastwards. Geomorphological analysis reveals that in both depressions the highs are remnants of a former pediment which was broken up. The young basins display numerous relevant hydrographic anomalies of the secondary channels and a general water-logging. They also suggest that the subsidence belts have gradually shifted north-and northeastwards. In the Gusinoje–Ivolga depression evidence was found of a Plio-Pleistocene river course, parallel to the Selenga river, which was later dismantled by the breaking-up. This depression, parallel to the Baikal rift and belonging to the Mesozoic system of grabens in the Caledonian fold belt, seems to have been included into the general system of rifts during the Pliocene tectonic phase. As for the main hydrographic axes, the Selenga river was set on a Palaeogene-age planation surface before the first tectonic phase and kept its original course. The Irkut river flowed in the Khoito Gol–Tunka–Bistraya depression after the first tectonic phase and was not affected by the later breaking-up. In contrast, the secondary drainage network is largely discordant. Despite their different geotectonic contexts, the two depressions show a similar development of relief pattern ,which poses the question of the style of rift dynamics after the main Pliocene tectonic phase.

The crustal structure of central East Greenland-I: From the Caledonian orogen to the Tertiary igneous province

SUMMARY We present a regional crustal model of the East Greenland Fjord Region between 69 ◦ N and 74 ◦ Nw hich spans the Caledonian fold belt and the adjoining Devonian and Mesozoic basins. The model is a compilation of existing seismic models that were partly reinterpreted and newly derived results from different modelling approaches. Remodelling of 33 stations on three deep seismic lines in the southern area yielded consistent P-wave velocity models for the entire Fjord Region. Seismic velocities, between 5.5 km s −1 near the surface and 6.9 km s −1 in the lower crust are typical for regions of Palaeozoic age. Moho depths up to 48 km in the seismic models suggest the existence of a crustal root beneath the Caledonian orogen. Shear wave modelling of 51 stations on six refraction seismic profiles resulted in S-wave velocities between 3.2 km s −1 in the upper crust, 4.0 km s −1 at the crust‐mantle boundary and 4.2 km s −1 in the partial magmatic underplating of the lower crust in the northern area. Calculation of Poisson’s ratio portrays a fairly homogeneous crust with only slight variations in Poisson’s ratio of 0.26‐0.30 for the uppermost crystalline crust and 0.22‐0.24 for the middle crust. These values cannot be linked to lithological variations because they are either small-scale or span several tectonic provinces. Finite-difference modelling and amplitude analysis confirm models without magmatic underplating in the lower crust of the southern Hall Bredning, Scoresby Sund area.

NE Atlantic break-up: a re-examination of the Iceland mantle plume model and the Atlantic–Arctic linkage

Final break-up of Pangaea – opening of the NE Atlantic (NEA) and the Arctic Eurasia Basin – was associated with significant magmatism (in the NEA) and is commonly ascribed to thermal effects from a proto-Iceland plume. The plume is often assumed to be fixed with respect to the Earth’s core and to have governed NEA break-up. It is argued here that the Iceland anomaly, past and present, cannot represent a fixed plume, nor be rooted at the core–mantle boundary and that the Greenland–Faroes Ridge is inconsistent with a classic time-transgressive hotspot track. It is shown that the Iceland anomaly has probably been located at the constructive plate boundary (Mid-Atlantic Ridge and antecedents) since its inception. While recent studies allow for some ‘wandering’ of hotspots relative to the core and mantle, it is considered unlikely that such drift of a mantle plume would precisely match lithospheric drift in order to achieve constant centering on the spreading ridge. The alternative view is, therefore, supported – that the anomaly is an upper mantle response to plate break-up. The two pulses of NEA magmatism are related to separate phases of North Atlantic break-up. Early Paleocene magmatism ( c . 62–58 Ma) was governed by a short-lived attempt at seeking a new rift path, intermediate in time and space between the Labrador Sea–Baffin Bay and the NEA–Eurasia Basin rifts. The voluminous Early Eocene magmatism ( c . 56–53 Ma) along the NEA margins was related to final break-up of Pangaea, exploiting the collapsed Caledonian fold belt. The interpretations here are at odds with Iceland representing a classic Morgan-type plume and it is suggested that the magmatism in the NEA and the Iceland anomaly represent a ‘top-down’ effect of plate tectonics.

Serpentinitic Mélanges Associated with HP and UHP Rocks in Central Asia

Serpentinitic mélanges containing eclogites, blueschists, and jadeitic rocks have been studied within the following foldbelts: Borus (West Sayan), Chagan-Uzun (Gorny Altai), Chara (East Kazakhstan), Kokchetav (North Kazakhstan), and Maksyutov (Southern Urals). The West Sayan (Borus belt) and Gorny Altai (Chagan-Uzun belt) are Caledonian foldbelts formed after collisions of paleo-seamounts with primitive island arcs, which resulted in the exhumation of eclogites and jadeitic rocks. The Chara belt of Hercynian age was formed during the Late Carboniferous-Permian collision of the Siberian and Kazakhstan continents and represents a strike-slip zone comprising serpentinitic mélanges and HP units of different ages. In Caledonian time, the mélanges represent an accretionary wedge at the margin of the Kazakhstan continent. The Kokchetav and Maksyutov foldbelts of, respectively, Caledonian and Hercynian ages resulted from collision of microcontinents with island arcs and formation of sheeted complexes incorporating large lenses of UHP-HP rocks and less serpentinitic mélanges. Two types of serpentinitic mélange occur in all suture zones formed as a result of collision of island arcs with seamounts or other island arcs. Type I mélange contains HP rocks in antigorite or olivine-talc-antigorite schists associated with high-temperature mantle peridotites, which can be regarded as part of the hanging wall of a paleosubduction zone. Type II mélange is a part of an obducted oceanic crust and contains blocks and inclusions of LP and LT metamorphic rocks associated with island arc and/or seamount terranes. The suture zones formed by continent/island arc or microcontinent collision (the Kokchetav megamélange and the lower unit of the Maksyutov complex) comprise no large sheets of serpentinitic mélange, but only subordinate lenses of mélanges with HP rocks (the Maksyutov lower unit) or small and sporadic ultramafic lenses inside a tectonic mélange with a metasedimentary matrix.

Confirmation and significance of Caledonian metallogenic epoch for gold deposits in South China

Caledonian gold deposits are widely distributed in South China. They are developed in both South China Caledonian fold belt and adjacent Proterozoic Jiangnan uplift. The host rocks are Proterozoic metamorphosed microclastic rocks in the Jiangnan uplift and Proterozoic and Cambrian strata, as well as Chengjiang and Caledonian igneous bodies in the South China fold belt. The distinct differences between the Caledonian and the most developed Yenshanian gold deposits in South China are reflected in age and host-rock type, relations to Yenshanian magmatic activities, element association, mineral assemblage and gold deposit type. The studies have proven that the Caledonian epoch is a principal metallogenic period of gold deposits in South China. This conclusion is of very important enlightening significance in exploration of Caledonian gold deposits in South China as well as in other Caledonian fold belts and adjacent uplifts in China.

U–Pb geochronology of Neoproterozoic and Caledonian tectonothermal events in the East Greenland Caledonides

The East Greenland Caledonides occupy a crucial position in plate-tectonic reconstructions of the Late Mesoproterozoic to Early Neoproterozoic Grenville–Sveconorwegian belt. We present new field and isotopic data from the northern Stauning Alper which indicate that the 1050–930 Ma history of the area was characterized by deposition of extensive clastic sequences. Sources of detritus were dominated by rocks of Mesoproterozoic age, with only limited contributions from Archaean sources, suggesting deposition at a distance from the present Caledonian foreland. A Neoproterozoic granite (938±13 Ma) provides evidence for thermal perturbation at a time of extensional collapse and uplift recorded in NW Scotland, the Grenville Belt of Canada, Labrador and the Sveconorwegian of SW Sweden and southern Norway. Widespread anatexis in the northern Stauning Alper at c. 430–425 Ma resulted from both collisional melting and decompression melting on uplift contemporaneous with the early part of orogenic collapse of the Caledonian Fold Belt. Caledonian deformation was focused at the zone of most extensive granite emplacement. Isotopic evidence suggests that Caledonian granites, previously thought to be entirely post-kinematic, actually predate late Caledonian extension.

Radiolarian fauna found from Tieshidas Group in East Kunlun

The Tieshidas Group in Qimantag, a branch of the East Kunlun Mountains, was classically considered the Caledonian basement, and classified into Middle-Upper Ordovician, Upper Ordovician or simply Lower Paleozoic. The radiolarian fauna was found, for the first time, from a chert block in the Tieshidas Group at Yaziquandaban (Pass) north to Ayakum Lake. They areAstroentactinia?mirousi Gourmelon,As. cfbiaciculata Nazarov,Archocyrtium cf.diductum Deflandre,Ar. aff.diductum Deflandre,Deflandrellium? sp.,Entactinia vulgaris Won,En. cf.additiva Foreman,En.? sp.,Entactinosphaera palimbola Foreman,Pylentonema? sp.,Triaenosphaera sp. andTr.? sp. The geological age of this radiolarian fauna is Early Carboniferous. Evidently, the classical interpretation about the age of Tieshidas Group needs to be checked and modified. Except the Ordovician proved with the formerly discovered fossils, it also includes, at least, Lower Carboniferous rocks. There was still an oceanic basin at Qimantag during Early Carboniferous (and even a long geological time after then). Qimantag is not a Caledonian fold belt, and the Tieshidas Group is not the Caledonian basement.

Post-collisional extension of the East Greenland Caledonides: a geophysical perspective

Crustal extension during and following continental collision is well documented in the Arctic Caledonian fold belt. However, models for the post-collisional extension of the Caledonides are mainly based on geoscientific data from Scandinavia. For a more complete understanding of the evolution of the Caledonides, knowledge of the crustal structure of East Greenland is vital. Seismic and gravity studies have revealed a pronounced Moho topography and a west-dipping lower crustal reflector beneath the fjord region of East Greenland. These deep crustal structures are related to Late Caledonian extensional structures at the surface. The observations can be satisfactorily explained by applying simple shear or eduction models proposed for upper crustal extension in Scandinavia to the complementing lower crustal structures in East Greenland. However, exhumation of the Caledonian Northeast Greenland eclogite province cannot be accomplished by these models. Instead, a synthesis of geoscientific data has shown marked differences in the crustal structure of East Greenland north and south of about 76°N, indicating a different crustal evolution of the northern and southern parts of the East Greenland Caledonides.

Conclusion of the 1:500 000 mapping project in the Caledonian fold belt in North-East Greenland

The second and last field season of the mapping project in the southern part of North-East Greenland (72°–75°N) was carried out in 1998 with full accomplishment of all planned goals. The general overview mapping of the East Greenland Caledonian fold belt has now been completed, in total covering a stretch of more than 1300 km between latitudes 70°N and c. 81°30´N (Fig. 1). The Survey’s systematic regional 1:500 000 mapping programme in the East Greenland Caledonides started with mapping in the Scoresby Sund region (70°–72°N) in 1968, and in the course of 13 field seasons spread over 30 years has involved more than 50 geologists for one or more field seasons each; several Survey geologists have participated in all 13 field seasons. The mapping of the Caledonian fold belt and the adjacent pre- and post-Caledonian rocks will be presented on five 1:500 000 map sheets of which three are already published and a fourth is under compilation. The scientific results are documented in numerous publications both in international journals and in the Survey’s own Bulletins and Reports. Review articles on various parts of the East Greenland Caledonian fold belt include those of Haller (1971), Henriksen & Higgins (1976), Higgins & Phillips (1979), Henriksen (1985, 1986), Hurst et al. (1985), Peel (1985), Peel & Sønderholm (1991), Higgins (1994) and Jepsen et al. (1994). Preliminary results of the 1997 and 1998 field work have been published in Survey reports (Higgins & Frederiksen 1998, 1999), and are summarised by Henriksen (1998) and in this article.

Geochronology of granitic and supracrustal rocks from the northern part of the East Greenland Caledonides: ion microprobe U–Pb zircon ages

Granitoid rocks from different settings within the northern part of the East Greenland Caledonian fold belt have yielded U–Pb zircon dates between 2000 and 1730 Ma, confirming the Palaeoproterozoic origin of the crystalline basement. Widespread sandstone sequences occur both in the Caledonian fold belt and in the foreland to the west; all of these have been assigned to the Independence Fjord Group, and attributed a Mesoproterozoic age on geological maps. However, metarhyolitic rocks associated with the sandstones in the Caledonian fold belt have yielded an age of 1740 ± 6 Ma, significantly older than anticipated. Zircon ages for a sandstone sample in the same area suggest deposition after the end of Palaeoproterozoic orogenic events, but in part prior to emplacement of the rhyolitic rocks at 1740 Ma; sandstone from another locality may have been deposited before emplacement of the latest Proterozoic granite sheets. Field relations suggest that some granitic veins and sheets might be Caledonian in age, but, with one possible exception, all those analysed proved to be Proterozoic. The apparent absence of Caledonian granites in the northern part of the East Greenland Caledonides, despite regional high-grade metamorphism, may be related to the lack of major occurrences of pelitic supracrustal rocks within the crystalline basement complexes.

Re-Os isotopic dating of molybdenites in the Xiaoliugou W (Mo) deposit in the northern Qilian mountains and its geological significance

The Xiaoliugou is a large-scale W (Mo) deposit newly explored in the Northern Qilian Mountains Caledonian fold belt, Northwestern China. It is a vein and skarn type deposit genetically associated with the Xiaoliugou Caledonian granodiorite stock. Re-Os isotopic dating for the molybdenite in the deposit yields an isochron age of 462 ± 13 Ma (2σ) and model ages of 436 to 496 Ma. These results suggest that W-Mo mineralization occurred before the collision of plates, and mineralized substances mainly originated from the crust.

An F-rich, Sn-bearing volcanic-intrusive complex in Yanbei, South China

The igneous complex that hosts the Yanbei porphyry tin deposit is situated along the eastern margin of the southeast China Caledonian fold belt. It intrudes a Late Jurassic granitic batholith and was unconformably covered by Late Cretaceous red beds. The complex consists mainly of volcanic (ignimbrites and minor rhyolitic lavas, tuffs, and subvolcanic rhyolite porphyry) and intrusive rocks (topaz-bearing granite porphyry and granite). These rocks are generally peraluminous with F- and VI Al-rich biotite as the main mafic mineral, and F-rich and OH-poor topaz as an accessory mineral in the granite porphyry and granite, and as an essential mineral in the topazite. Topaz in the granites is usually interstitial and crystallized during the late magmatic stage. The volcanic rocks and granites are rich in Si, K, F (SiO 2 > 75 wt %, Na 2 O/K 2 O = 0.03-0.72, F = 0.06-6.48 wt %) and incompatible trace elements (Rb, Zr, Nb, W, Sn ) and poor in Mg, Ti, Ca, P and compatible trace elements (Sr, Eu ). The chondrite-normalized rare earth element (REE) patterns for the granites generally have a distinct V shape, with strong Eu depletion (Eu/Eu (super [whitesunwithrays]) = 0.04-0.20). The main rocks of the complex have low epsilon Nd (T) values (-9.3 to -8.9 for volcanic rocks, -4.0 to -4.8 for granites), and high delta 18 O values of quartz from dacite (9.5ppm) and granites (9.6-10.1ppm). The difference in epsilon Nd (T) between the volcanic rocks and granites indicates that they are not comagmatic. The source of the volcanic rocks probably contains a greater metasedimentary component than that of the granites.The Yanbei porphyry tin deposit is located along the contact zone between topaz-bearing granite porphyry and rhyolite porphyry, and consists principally of one orebody which makes up 99 percent of the total reserves. Of the metallic minerals (5% by volume), cassiterite, pyrite, and chalcopyrite are dominant, sphalerite, wolframite, and galena are less important, and siderite, molybdenite, and stannite are minor. Cassiterite accounts for 96 percent of the total Sn reserves and stannite is the subordinate Sn-bearing mineral that occurs as rims around chalcopyrite. Cassiterite occurs mostly in disseminated form and as veinlets. The wall-rock alteration is intense and can be roughly grouped into three zones spatially arranged around the granite porphyry, namely: topaz-quartz, chloritic, and sericitic zones. Most of the orebody occurs in the topaz-quartz and chloritic zones. Hydrothermal alteration caused a decrease in K and Na and an increase in Al, Fe, Ca, Sn, W, and Cu. The delta 18 O values of quartz and cassiterite from the mineralized zones vary from 6.0 to 11 per mil, and the calculated delta 18 O (sub H 2 O) values of the ore-forming fluid range from 4.2 to 7.8 per mil. The altered rocks have elevated delta 18 O (sub H 2 O) values (14.5-9.7ppm) relative to fresh rocks. The pyrite from the ore has a limited range of delta 34 S values (-0.3-+1.5ppm). These data suggest a magmatic source for the ore-forming hydrothermal fluid.

Aeromagnetic study of the continental crust of northeast Greenland

A reconnaissance‐type aeromagnetic survey of northeast Greenland was undertaken by the Alfred Wegener Institute for Polar and Marine Research between 1993 and 1996. These data were compiled in a regional magnetic anomaly map at 3700 m altitude, covering the Caledonian orogen of east Greenland and parts of the Precambrian shield of north Greenland. The survey area was divided into regions with distinct magnetic anomalies. The interpretation of the magnetic anomalies is based on comparison with exposed geological structures and is locally supported by in situ measurements of magnetic susceptibilities. Middle Proterozoic magmatic rocks determine the magnetic character of the Precambrian shield of north Greenland. Rifting and downfaulting of the Precambrian basement at its northern and eastern margin are reflected by gradients in the magnetic field. The Caledonian fold belt of east Greenland could be subdivided into a northern only weakly magnetic part, where pervasive Caledonian reworking probably demagnetized the crust, and a southern part, where isolated positive anomalies occur. Magnetite survived Caledonian metamorphism in these areas. The sensitivity of magnetite to Caledonian metamorphism was recognized in several places in the Caledonides of east Greenland. This observation might be useful in determining the much debated degree of Caledonian reworking of the crystalline complexes of east Greenland.

Crustal contamination in Palaeogene East Greenland flood basalts: plumbing system evolution during continental rifting

Palaeogene central East Greenland flood basalts have Sr, Nd and Pb isotope characteristics consistent with crustal contamination of Icelandic-type basalts. In the Lower Basalts of central East Greenland, amphibolite facies gneiss is the dominant contaminant at low stratigraphic levels, whereas contamination with granulite facies gneiss prevails in the upper part of the lava succession. The contamination probably occurred by partial melting of the crust, and can be modelled by assimilation–fractionation–crystallisation processes. Similar contaminants have been detected in lavas from ODP Leg 152 off the SE Greenland coast, although in a reverse sequence. Contamination in Lower Basalts and ODP Leg 152 lavas, which erupted through an Archaean/Proterozoic substrate, differs from lavas in NE Greenland, which overly the Caledonian fold–belt and large Mesozoic sediment deposits. In the NE Greenland lavas, the dominant contaminant appears to be a dacitic component formed by partial melting of Mesozoic sediments. Changes in magma supply rates from the mantle may be the determining factor for the sequence of contaminants and may also control the depth of ponding in crustal chambers. The compositional variation of the least contaminated East Greenland basalts suggests that melts were produced from mantle sources similar to present-day Iceland and North Atlantic MORB at the time of continental breakup but distinction between mantle sources is very complicated. REE systematics indicate a maximum in initial depth of mantle melting at Kangerlussuaq during formation of the East Greenland flood basalts, consistent with a plume being centered in that region.