Caledonian Fold Belt Research Articles

Caledonian and late Caledonian Europe: a working hypothesis involving two contrasted compressional/extensional scenarios

The tectonomagmatic and metamorphic structuration of the European Caledonian realm suggests that two mutually perpendicular compressional/extensional scenarios developed during the Ordovician-Devonian time-span. As a result of the mid Ordovician Grampian compressional scheme (Caledonian s.s.), a major extensional province developed further east from the Caledonian foldbelt in continental Europe. This scenario ended by early/mid Devonian, with the complete locking of North America, Baltica and Gondwana into a Pangaea supercontinent, thus triggering a contrasted tectonic environment which might be termed <date Caledonian», characterized by compression and plutonism in western continental Europe, and extensional detachment tectonics and volcanism in the northern part of the Caledonian foldbelt (Scotland, southwestern Norway and eastern Greenland). The overall evolution of this realm can be summarized in terms of inversion tectonics (s.l.) processes, with periods of crustal thinning followed by thickenning and vice versa

Structural age of the Connemara gabbros, western Ireland

A ‘D2’ age of intrusion for the Connemara gabbros is rejected. The gabbros were intruded 490 Ma ago, probably just prior to the D3 deformation and, together with younger members of the Metagabbro and Gneiss complex, were responsible for a widespread sillimanite-grade contact metamorphism which overprinted the regional ‘D2’ gamet-staurolite ± kyanite metamorphism and the major D3 folds. Comparison with the NE Scotland gabbros is thereby strengthened, supporting the idea that they were both part of a coeval magmatic arc developed at the margin of the Caledonian fold belt.

Regional geology and 1:500 000 mapping in North-East Greenland

A three-year field mapping programme was initiated in 1988 aiming at regional geological studies and geological mapping in North-East Greenland between latitudes 75° and 78°N. This region encompasses relatively little known parts of the Caledonian fold belt and the overlying post-Caledonian sequences, which lie north of the better known regions of central East Greenland (Henriksen, 1989). Major aims of the programme include compilation a 1:500 000 geological map, and an understanding of the general geology of the region.

The crystalline rocks of western and southern Dove Bugt, North-East Greenland

The land areas around Dove Bugt are dominated by quartzo-feldspathic orthogneisses with a complex history of emplacement and deformation. The oldest rocks recognised are supracrustal units, mainly marbles and rusty-weathering metasedimentary rocks. Locally gabbro-anorthosite units are associated with the supracrustal rocks. These rock units are cut by different phases of the gneisses, of which the most abundant are grey banded orthogneisses. Two different varieties of pink orthogneiss are also recognised. Different types of tabular amphibolitic bodies, relicts of dykes or sills, cut most rock types. The area lies within the East Greenland Caledonian fold belt, but available isotopic data suggest that the crystalline rocks include Archaean and early Proterozoic suites which have undergone uncertain degrees of Caledonian reworking. It is not clear at present how much of the deformation history of at least four periods of deformation and associated mylonitisation is Caledonian, and how much pre-Caledonian.

High pressure metamorphic rocks and their tectonic environment in northeastern China

This paper deals with the high-pressure metamorphic rocks mainly of the Ondor Sum Group in Inner Mongolia and the Heilongjiang Group in the Jiamusi massif. A high pressure/low temperature metamorphic mineral assemblage of glaucophane schist facies and greenschist facies characterized by lawsonite, jadeitic pyroxene, glaucophane and pumpellyite has been recognized in the Ondor Sum Group. In northeastern China the high pressure metamorphic rocks are mainly distributed in the ancient intercontinental area, north of the Sino-Korean platform. A part of them is distributed on the outer margin of the Caledonian fold belt along the north edge of the Sino-Korean platform and it was dated at 445.6 ± 15 Ma by 39Ar/ 40Ar isochron method on glaucophane schist and at 509 ± 40 Ma by RbSr isochron method on whole rock of meta-basalt. These isotopic data indicate an early Paleozoic age of the subduction on the north edge of the Sino-Korean platform. The other part is the Heilongjiang Group of Jiamusi massif on the outer margin of the Xingkaiian fold belt. The Heilongjiang Group was intruded by a granite of 460 Ma (KAr). The lithological feature of the high pressure metamorphic rocks of the Heilongjiang Group is possibly correlated to that in the Altay-Sayan district (540–900 Ma). These facts suggest a probable Sinian-early Cambrian subduction.

Origin of infracrustal (I-type) granite magmas

ABSTRACTI-type granites are produced by partial melting of older igneous rocks that are metaluminous and hence have not undergone any significant amount of chemical weathering. In the Lachlan Fold Belt of southeastern Australia and the Caledonian Fold Belt of Britain and Ireland there was a major magmatic event close to 400 Ma ago involving a massive introduction of heat into the crust. In both areas, that Caledonian-age event produced large volumes of I-type granite and related volcanic rocks. Granites of these two areas are not identical in character but they do show many similarities and are markedly different from many of the granites found in Mesozoic and younger fold belts. These younger, dominantly tonalitic, granites have compositions similar to those of the more felsic volcanic rocks forming at the present time above subduction zones. The Palaeozoic granites show little evidence of such a direct relationship to subduction. Within both the Caledonian and Lachlan belts there are some granites with a composition close to the younger tonalites. A particularly interesting case is that of the Tuross Head Tonalite of the Lachlan Fold Belt, which can be shown to have formed from slightly older source rocks by a process that we refer to as remagmatisation which has caused no significant change in composition. Since remagmatisation has reproduced the former source composition in the younger rocks, the wrong inference would result from the use of that composition to deduce the tectonic conditions at the time of formation of the tonalite. Granites, particularly the more mafic ones, will generally have compositions reflecting the compositions of their source rocks, and attempts to use granite compositions to reconstruct the tectonic environment at the time of formation of the granite may be looking instead at an older event. This is probably also the case for some andesites formed at continental margins.Several arguments can be presented in favour of a general model for the production of I-type granite sources by underplating the crust, so that the source rocks are infracrustal. Such sources may contain a component of subducted sediments with the consequence that some of the compositional characteristics of sedimentary rocks may be present in I-type source rocks and in the granites derived from them. The small bodies of mafic granite and gabbro associated with island arc volcanism have an origin that can be related to the partial melting of subducted oceanic crust or of mantle material overlying such slabs and can be referred to as M-type. These rocks have compositions indistinguishable from those of the related volcanic rocks, except for a small component of cumulative material. The tonalitic I-type granites characteristic of the Cordillera are probably derived from such M-type rocks of basaltic to andesitic composition, which had been underplated beneath the crust. Some of the more mafic tonalites of the Caledonian-age fold belts may also have had a similar origin. More commonly, however, the plutonic rocks of the older belts are granodioritic and these probably represent the products of partial melting of older tonalitic I-type source rocks in the deep crust, these having compositions and origins analogous to the tonalites of the Cordillera. In this way, multiple episodes of partial melting, accompanied by fractionation of the magmas, can produce quite felsic rocks from original source rocks in the mantle or mantle wedge. These are essential processes in the evolution of the crust, since the first stages in this process produce new crust and the later magmatic events redistribute this material vertically without the addition of significant amounts of new crust.

Archaean U-Pb zircon ages from the Scoresby Sund region, East Greenland

U-Pb analyses on zircons are reported for two rock colleetions from infracrustal complexes within the East Greenland Caledonian fold belt in the Scoresby Sund region. Calculated minimum ages range from 2800 to 2965 Ma, and are the oldest ages obtained in the region using this method.

Rb-Sr and K-Ar Caledonian ages from the Upper Eleonore Bay Group and Cambrian metasediments of the East Greenland Caledonian fold belt

Fine sediment fractions &lt; 2 u extracted from various formations of Upper Proterozoic and Cambrian age in Canning Land and Segelsällskapet Fjord in East Greenland are anchimetamorphic to strongly diagenetic. Rb-Sr and K-Ar isotopic analyses indicate that the metamorphic and diagenetic changes correspond to the main Caledonian event at c. 450 Ma while some of the data are consistent with ayounger event at nearly 400 Ma, possibly connected to late Caledonian tectonics. The study also shows the peculiar features of the isotopic diagrams often observed in such conditions: parallel isotopic lines, discordant behaviour of the Rb-Sr and K-Ar chronometers, and the necessity of collecting sets of samples from small homogeneous areas.

Syn-orogenic alkaline magmatism and its relationship to the Moine Thrust Zone and the thermal state of the Lithosphere in NW Scotland

The alkaline complexes of NW Scotland lie in a zone sub-parallel to the Moine thrust and represent the NW edge of the otherwise overwhelmingly calc-alkaline late-orogenic magmatism of the Caledonian fold-belt. The emplacement of intrusives in Assynt overlapped the thrust movements and new U-Pb zircon ages for the Loch Ailsh intrusion (439 ± 4 Ma) provide are liable bracket, when combined with existing age data for the Loch Borralan intrusion (430 ± 4 Ma), for the main movements on the Ben More thrust plane. A detailed history of the main tectonic and igneous events in the Moine Thrust Zone is tabulated. Zircons from the undeformed Cnoc-nan-Cuilean syenite, emplaced in Moine rocks near Loch Loyal, give an age of 426 ± 9 Ma, in contrast to the deformed Glen Dessary syenite for which earlier work gave 456 ± 5 Ma. The linear distribution of the alkaline complexes to the west of the calc-alkaline plutons, and the ultra-potassic character of the Loch Borralan complex (the most westerly), are consistent with, but do not prove, generation related to a W-dipping subduction zone. The persistence of the magmatism, over 30 Ma, in a belt being shortened by thrusting, suggests a source in the mantle lithosphere. The absence of magmatism further to the NW and the production of calc-alkaline magmas to the SE of this zone, is compatible with a model in which the thermal state of the lithosphere exercises the most important control on the nature of Late Caledonian magmatism.

Gravity measurements in Jameson Land and neighbouring parts of East Greenland

In the summer of 1982 a regional gravity survey of Jameson Land and adjacent East Greenland areas was carried out by the Danish Geodetic Institute. The purpose of this survey was primarily to contribute to the ongoing efforts to evaluate the hydrocarbon potential of the area. Together with gravity data from supplementary surveys in 1983 and 1984, a total of 379 new gravity stations has been established in the central East Greenland area. The gravity station spacing ranges from 5 to 30 km, most dense in southern and central Jameson Land, where a small area around J. P. Koch Fjeld was covered with a station spacing down to 1 km to investigate local structural disturbances in the area.&#x0D; In the paper results of the surveys are given, including details of the processing of the raw gravimeter readings, barometric elevations and terrain correction computations. Additionally the results of earlier, unpublished surveys in the Scoresby Sund and Mesters Vig regions are presented. Based on available onshore and offshore gravity data, Bouguer anomaly maps are outlined for Jameson Land and for the central East Greenland region 69°N to 73°N, 30°W to 19°W.&#x0D; The gravity data of the region show very large anomalies, with Bouguer anomalies varying from 90 mgal on the continental shelf to -180 mgal near the edge of the Inland Ice at the centre of the Caledonian fold belt. This variation is consistent with general isostatic principles, and probably primarily indicates changes in crustal thickness. Over the Jameson Land post-Caledonian sedimentary basin a smooth anomaly picture is found, with an E-W trending high crossing the central part of the area, and SSW-NNE trending lows and highs in the southern part, probably closely related to overall basin structure and basement geology. In the local area around J. P. Koch Fjeld a small positive anomaly seems to be associated with the structural disturbances, thus probably indicating shallow, relatively high-density rocks within the sedimentary sequence.

Palaeomagnetic re-examination of the basal Caithness Old Red Sandstone; aspects of local and regional tectonics

A palaeomagnetic re-examination of the basal strata of the Caithness Old Red Sandstone has given results that are fully compatible with previous palaeomagnetic findings in this region. After structural correction the dominant remanence component has D = 205°, I = +3°, α 95 = 6.4° ( N = 27). The existence of this shallow inclined magnetization in the Middle Devonian strata of Caithness invalidates the model, proposed by Van der Voo and Scotese (1981), involving a ca. 2000 km sinistral offset along the Great Glen Fault in the Carboniferous. However, the available data are in favour of a few hundred kilometres sinistral movement along this fracture zone. However, the possibility of there having been a much larger transcurrent shift between Europe and North America in late/post-Devonian times, accumulated along various fracture zones within the Caledonian fold belt, is discussed. On the basis of an inferred overprinted magnetization, it is tentatively concluded that the tectonic deformation of the Old Red Sandstone of Caithness has a mid-Jurassic or younger age.

The thrust and shear zones of the Moine thrust zone and the NW Scottish Caledonides

The Caledonian thrust belts of NW Scotland exhibit many features typical of foreland fold-and-thrust belts. The major structures propagated westwards towards the foreland, the thin carapace of Cambro-Ordovician sediments being accreted in imbricate and duplex zones beneath the major thrust sheets. On a broad scale, the rules of thrust geometry are obeyed but many local anomalies are found, particularly in the Assynt region. In Assynt, several large fault-bounded gravity-spreading structures, surge zones, have been identified. Extensional faults have been traced into strike-slip and then contractional faults. There are zones of extensional and contractional imbricate faults and extensional and contractional ductile flow. Similar structures have been found along the length of the Moine thrust belt, suggesting that gravity spreading was an important mechanism during the later stages of movement. Within the main Caledonian fold belt there are several major tectonic slides, which are recognized by a local increase in intensity of deformation. The majority are NW-verging and were originally gently dipping. In the Highlands SE of the Great Glen there are early major SE-verging recumbent folds associated with considerable crustal thickening reflected in high pressure (~12kbar) metamorphism. Many of the major slides may have formed by spreading away from this thickened crust. The Caledonian deformation and metamorphism occurred later in the Moines and Moine thrust zone than in the Dalradian and Grampian Moines to the SE of the Great Glen. This may reflect different levels of deformation; alternatively, it may argue for a large strike-slip displacement on the Great Glen fault, juxtaposing crustal blocks which had experienced different ages of Caledonian deformation and metamorphism. The late movement on the Moine thrust involved spreading from an area of thickened Caledonian crust formed either by the stair-step trajectory of the basal thrust to the Moines and, or, by thickening due to intrusion of large masses of late Caledonian granites

Scheelite mineralization in Central East Greenland

Numerous scheelite-mineralized areas have been located in the Caledonian fold belt of Central East Greenland (70° – 74 1/2° N). The fold belt comprises Archaean and Proterozoic metamorphic complexes flanked by Late Proterozoic and Lower Palaeozoic sediments, and intruded by Upper Proterozoic and Caledonian granitic rocks. Scheelite mineralization occurs in: marble-skarns in Upper Proterozoic metasediments often spatially associated with Caledonian or older granitic intrusives; skarns and quartz veins in Late Proterozoic sediments (Lower EBG quartzite) up to 7 km from outcropping Caledonian granites; veins in fault zones cutting late Proterozoic sediments (Upper EBG limestones). In this paper, the three types are described with emphasis on structural setting, mineralogy and trace-element chemistry. It is concluded that most scheelite mineralization is associated with Caledonian igneous activity, either at or near granite contacts or as mineralization distal to granites at depth. Of the various types described, the scheelite veins in Late Proterozoic limestone are considered to be of the best economic potential.

The relationship between Caledonian nappe tectonics and Silurian turbidite deposition in North Greenland

The relationship between the northern part of the Caledonian fold belt of East Greenland and the North Greenland fold belt is explored within the context of Silurian sedimentation and tectonics. A model is developed relating the Caledonian nappes of eastern North Greenland to major tectonic and sedimentological features of the Silurian of the remainder of North Greenland. The abrupt change from a thin succession of predominantly slowly deposited muds to a thick succession of rapidly deposited sandstone turbidites at the Ordovician/Silurian boundary, in the deep-water basin N of the carbonate platform in North Greenland, is directly related to uplift and westerly directed nappe emplacement in the eastern Caledonides. It is suggested that uplift to the E of North Greenland, which commenced in the late Ordovician or early Silurian, provided the main source of the Silurian turbidites of North Greenland. This is corroborated by the dominant westerly transport direction of the Silurian turbidites. Nappe emplacement during the latest Llandovery (Silurian), and foundering of the whole of the carbonate platform of eastern North Greenland E of the Victoria Fjord Arch (approximately 60 000 km 2 ), is marked by the replacement of shallow water carbonates by basin turbidites. These events were directly related to isostatic readjustments as the nappes advanced onto the eastern fringe of the platform.

Deep structure of the Scottish Caledonides revealed by the MOIST reflection profile

The Moine and Outer Isles Seismic Traverse (MOIST) is a deep crustal reflection profile shot at sea off the north coast of Scotland in 1981. Spectacular reflections are observed from the Moho and from thrust zones within the Caledonian fold belt and foreland. Deep reflection profiling of the continental crust of the United States under the direction of the COCORP group, has amply demonstrated the value of this technique when applied to suitable geological problems (see, for example, refs 1,2). A similar group, the British Institutes Reflection Profiling Syndicate (BIRPS), has now been set up in the UK to organize such projects. Funds are provided by the Natural Environment Research Council (NERC) through the core group based at Cambridge University. However, a preliminary experiment, of which MOIST is the result, was organized through the Institute of Geological Sciences (IGS).

Non-existence of the Carolinidian orogeny in the Prinsesse Caroline – Mathilde Alper of Kronprins Christian Land, eastern North Greenland

In 1961 John Haller proclaimed the presence of an older, pre-Caledonian, orogeny in the northern part of the Caledonian fold belt of East Greenland. Haller called it the 'Carolinidian orogeny' after the Prinsesse Caroline-Mathilde Alper in Kronprins Christian Land in easternmost North Greenland, where the pre-Caledonian structures were regarded to be well preserved (Haller, 1970, p. 48). The Carolinidian fold belt was considered to extend from the northern part of East Greenland over most of North-East and North Greenland to northern Ellesmere Island in arctic Canada (Haller, 1970, fig. 18).

Isotopic evidence for a Precambrian metamorphic event within the Charcot Land window, East Greenland Caledonian fold belt

Rb-Sr, U-Pb and K-Ar analyses on rocks and minerals from a tectonic window below a Caledonian thrust sheet in the westernmost part of the Scoresby Sund region (70°-72°N) give evidence for a Precambrian age of formation. The Charco't Land supracrustal sequence rests on a basement that is probably of Archaean development and older than at least 2100 m.y. The major regional metamorphism of the supracrustal rocks is probably not much older than the intrusion of two post-kinematic bodies, i.e. about 1840 m.y. Low-grade metamorphism in a tillite and low-grade retrogressive overprinting of the supra­crustal rocks are related to Caledonian orogenesis.

Tectonic slides in the Caledonides

Summary Despite the continuing use of the term ‘tectonic slide’ in the Caledonian fold belt of Britain and Ireland for the past fifty years, the concept has found few advocates elsewhere. To many geologists outside the British Isles it remains a subject of some confusion. A review of the literature suggests that slides are a general term for faults which form in close association with syn-metamorphic regional deformation. They are often related to major folds although this is not a diagnostic feature. Neither is any movement sense implied in the term slide; thus, they may be thrusts, lags, oblique slip etc. Classically, these structures lie along and subparallel to the boundaries of major lithological units but transgress the stratigraphy on a larger scale. Structural analogies with the thrusts of the lower grade or non-metamorphic deformed cover sequences can often be seen in slides. However, they are more commonly and possibly fundamentally the result of differing responses to deformation of contrasting, adjacent lithologies in areas of higher strain and metamorphism.

Pre-permian depositional environments around the brabant massif in Belgium, The Netherlands and Germany

Pre-Permian sedimentation in northwestern Europe has been controlled by the structural evolution of this area. Cambro-Silurian deposition has been influenced by partly synsedimentary movements (among others Ordovician-Silurian uplift south of the Brabant/Condroz zone, such as the Stavelot-Venn Massif). Presence, respectively absence of important late Caledonian deformation has subdivided northwestern Europe into three major sedimentary environments during the Devono-Carboniferous: the Caledonian fold belt and the Cornwall-Rhenish Basin which are separated by the Belgo-Dutch platform. Subsequently, the Hercynian or Variscan orogenies have gradually reduced the sedimentary area and produced the overall withdrawal of the marine environment. Eventually, large-scale overthrusts - such as the Dinant Nappe - masked parts of the original sedimentary basins.

A resurgent cauldron in the early Paleozoic of Wales, U.K.

Abstract The paper presents a controversial interpretation of a mid-Ordovician volcano-sedimentary complex. It deals with the cyclic interdependence of intrusive, volcanic, and sedimentary processes, due to the development of a nearshore resurgent cauldron in the Caledonian fold belt of North Wales. Deformed volcanotectonic features include a resurgent dome and apical graben, surrounded by a moat and peripheral crescentic ring-fault, constituting a caldera 20 km in diameter. The resurgent Snowdon caldera developed through three cycles of ash-flow volcanism resulting from the continuous supply of magma into a shallow magma chamber emplaced beneath the floor of a marine basin. Each ash-flow cycle was preceded by the emergence, above sea level, of a geotumour that subsequently collapsed following eruption and evacuation of the magma chamber. Localized unconformities at the base of individual ash-flow cycles are compared with caldera margin and associated collapse features. Deeper-seated effects of caldera collapse are expressed as gaps in the Ordovician sequence due to normal faulting along the structural boundary of the caldera. Major ash-flow fissure vents were located at points of maximum unloading of the magma chamber by distention faults in its roof. Explosive mechanisms were triggered by rapid pressure release due to tectonic erosion. The presence of a resurgent cauldron implies that the Ordovician succession of North Wales is more complete than recorded in the literature, and that Caledonian structures were largely predetermined by Caradocian volcano-tectonics.