Metamorphic Caledonides Research Articles

Chapter 62 The Port Askaig Formation, Dalradian Supergroup, Scotland

Abstract The Port Askaig Formation (Fm.) is a thick glaciogenic succession within the Dalradian Supergroup that consists of over 700 m of variably dolomitic diamictite, conglomerate, sandstone mudstone and minor dolomite, and is bounded by mixed siliciclastic–carbonate successions of the Islay (Lossit) and Bonahaven formations. These strata are exposed in the metamorphic Caledonides of Scotland, although excellent preservation of sedimentary structures can be found at several sites. An extensional setting for this succession has been proposed based on stratigraphic and structural arguments. The available chemostratigraphic data include the Chemical Index of Alteration, δ 13 C and Sr-isotope values. Palaeomagnetic analyses have been shown to be subject to post-depositional Caledonian overprinting. There is also continued debate over the regional palaeogeographical reconstructions of the Scottish promontory for this time period. The succession is chronologically poorly constrained with U–Pb analyses of stratigraphically much higher or lower deposits. The thick succession is thought to record glacially influenced marine sedimentation and reworking of unstable sediments in a tectonically active setting with evidence of ice-margin fluctuations. Alternative palaeoenvironmental interpretations that focus on glacial terrestrial processes and emphasize climatic influence instead of tectonic activity have also been proposed. The overlying carbonate is a lithologically diverse coastal complex and so does not fit the Neoproterozoic norm. Research has to date focused on the stratigraphic and sedimentological aspects of this succession, as well as some of the broader palaeogeographical and structural features of the Dalradian basin. Future efforts should focus on the chronological, structural and palaeogeographical constraints of this succession.

Late Caledonian extensional reactivation of a ductile thrust in NW Scotland

Evidence is given for ductile extension across a fault zone located on the western seaboard of the Northern Highlands of Scotland. The fault zone lies within the metamorphic Caledonides and extensional movement occurred during the waning stages of Caledonian orogenic activity. An earlier phase of westerly directed overthrusting took place on the same fault zone at higher metamorphic temperatures but after the peak of Caledonian metamorphism. The compression may be broadly coeval with early displacements on the Moine Thrust Zone which probably lies offshore some 10 km to the northwest. The extensional reactivation could relate to local crustal instability resulting directly from overthrusting; it may have been due to local gravitational collapse following overthickening of the crust. Alternatively the extensional movement may witness the onset of regional gravitational collapse of the orogen during the late Silurian to early Devonian. Reactivation accords with interpretations of the movement histories of similarly eastward dipping offshore faults recognized on seismic reflection profiles.

The uplift magnetisation record in the central scandinavian caledonides: results from basic igneous complexes

To identify the palaeomagnetic record acquired during slow post-orogenic uplift and cooling in the central part of the Scandinavian Caledonides we have investigated four separated synorogenic basic complexes in the Middle and Upper Allochthons. A shallow N-S axis of magnetisation is common to three of these bodies and therefore postdates all major tectonism. It is defined in the Møsjoen Gabbro ( D/I = 7°/3°, N = 23 samples, α 95 = 5.0° ), in the Sulitjelma Gabbro ( D/I=20°/−9°, N = 22 samples, α 95 = 6.6° )and the Råna norite ( D/I = 9°/−16°, N=19 samples, α 95 = 7.9° ). No vestige of this uplift remanence is recognised in the Lyngen Ophiolite, where a characteristic remanence of D/I = 65°/43° ( N = 19 samples, α 95 = 10.6° ) is recorded. The possibility that the N-S shallow axis of magnetisation is of mid-Carboniferous age, is considered and rejected. Instead it is attributed to the interval of post-tectonic cooling of the Caledonian nappe pile between synorogenic events dated ca.. 420 Ma and the uplift to surface levels defined by later Devonian events. It corresponds closely to an axis of magnetisation extensively represented within the metamorphic Caledonides and Scotland and believed to record a post-tectonic thermochemical event. Subordinate components record Mesozoic overprinting events and relict Caledonian magnetisations which have escaped replacement during this thermo-tectonic event. The latter collectively imply a rapid movement from high to low palaeolatitudes in Late Ordovician and Early Silurian times.

Caledonian ductile thrusting in a Precambrian metamorphic complex: The Moine of northwestern Scotland

Structural models drawn from external thrust-belts have been applied to a part of the metamorphic Caledonides in northwestern Scotland. Internal deformation of the amphibolite-facies Moine rocks was accomplished primarily by northwest-directed ductile thrusting (“sliding”), which was succeeded within a significant portion of the Moine by upright folding above a basal detachment. The Moine can be regarded as a stack of three major nappes, each of which has distinctive structural, stratigraphic, and metamorphic characteristics. Caledonian shortening within the Moine is of the order of 100 km, yet geothermometry and geobarometry confirm that all tectonic units had equilibrated at similar mid-crustal depths during a Precambrian, “Ardgourian” metamorphic event (∼1000 Ma). Kyanite-grade Caledonian metamorphism implies that ductile thrusting and steep refolding took place beneath a considerable overburden, although an intervening stage of uplift is attested to by the Caledonian reworking of low-pressure thermal aureoles. Final uplift of the complex took place during or after emplacement onto the foreland by the Moine thrust and related structures. Prior to Caledonian tectonics, the Moine was only a few kilometres thick but hundreds of kilometres in lateral extent, perhaps as a result of post-Ardgourian, pre-Caledonian crustal stretching. The pre-existing geometry of the interface between Moine cover and Lewisian basement exerted a major control on Caledonian structural development.

Late Caledonian sinistral displacements in Britain: Implications for a three‐plate collision model

In recent years, most geotectonic syntheses of the North Atlantic Caledonides have adopted a two plate configuration, with a roughly E‐W closure direction between Laurentia and Baltica producing the N‐S striking Scandinavian and East Greenland Caledonides and inducing dextral strike‐slip along the NE‐SW oriented British sector of the Iapetus suture. The North German‐Polish Caledonides are a third arm to the Appalachian‐N. Atlantic Caledonide orogen. Recently, Ziegler has proposed that the mid‐Palaeozoic deformation belts of S. Britain ‐ N. Germany‐Poland and also of central Europe were produced by the northward impact of microcontinental fragments onto the southern margin of the already sutured continental mass of Laurentia‐Baltica. Some ascribe the whole Appalachian orogen to a sequence of terrane accretion events in mid‐Ordovician through mid‐Devonian time. A N‐S collision direction would induce sinistral shear across the British sector of the Iapetus suture. In this paper, we outline the evidence for sinistral displacements within the British Caledonides and for their age. We conclude that the “non‐metamorphic Caledonides” were produced by the northerly accretion of a Cadomian terrane in late Silurian‐early Devonian time, and that this event was entirely separate from the earlier Laurentia‐Baltica collision which produced the post‐Grampian thrust related “metamorphic Caledonides” of Scotland.

The ending of the Caledonian orogeny in Scotland

The geological evolution of the metamorphic Caledonides during the 100-million-year period from the phase of crustal thickening initiated by the Grampian orogeny to the emplacement of late granites at c. 400 Ma is reviewed. Shortening and westward transport associated with the Grampian orogeny were diachronous; deformation began in the weak Dalradian basin and extended north-westward into the area of crystalline Moine rocks. An important change in tectonic regime, coinciding roughly with the closing of the Iapetus Ocean, was recorded at c. 425 Ma by the onset of sinistral transcurrent motions on NE–SW lines parallel to the Caledonian belt. Regional uplift of the thickened crustal welt, accompanied by deep erosion, led to the removal of 20–30 km of overburden from the Highlands, mainly in Ordovician and Silurian times. Known Lower Palaeozoic deposits of Scotland cannot account for the volume of sediment so produced and it is suggested that other basins were located in a major transform zone beneath the northern North Sea. The onset of late Caledonian magmatism coincided with the start of the post-collisional regime of strike-slip motion. The distribution of granites and appinites was influenced by transverse (NW–SE) dislocations in the lower crust and the emplacement of individual intrusions was often fault-controlled. The deep transverse lineaments acted as conduits for both sub-crustal and crustal melts over periods of up to 25 million years. It is suggested that mantle rocks metasomatized by fluids generated at an underlying subduction zone were the main source of the sub-crustal melts, and that magmatism was triggered from above by vertical and horizontal block movements. This paper is a contribution to IGCP Project 27.

Major southward thrusting of the Dalradian rocks of Connemara, western Ireland

The Connemara schists are of Dalradian (late Precambrian–Cambrian) age and since the work of Kilburn et al.1 it has been apparentthat they occupy an anomalous geographical position south-southeast of the strike continuation of the Dalradian rocks of Scotland and Ireland, and south of the inferred position of the Highland Boundary Fault (Fig. 1). Also, the Connemara schists are unusual in having reached a higher metamorphic grade and having had a different metamorphic history from most other parts of the Dalradian. The model which we propose here for the Connemara Dalradian, that of an allochthonous thrust sheet, offers an explanation for these anomalies and highlights hitherto unsuspected similarities between the tectonic histories of the south-east margins of the metamorphic Caledonides in the British Isles and Scandinavia.

Scale and timing of movements on the Great Glen fault, Scotland

The Great Glen fault, for which a strike-slip displacement of about 2,000 km is proposed from paleomagnetic data by Van der Voo and Scotese, transects the metamorphic Caledonides of Scotland. Comparisons of pre-Caledonian basement complexes, folded cover successions, and late Caledonian intrusive suites occurring in the Northern and Grampian Highlands on either side of the fault reveal a close stage-by-stage correspondence in the geologic histories of these terrains. It is argued that these correspondences are incompatible with displacements of more than about 200 km on the Great Glen fault.

The Glen Kyllachy Granite and its bearing on the nature of the Caledonian Orogeny in Scotland

The Tomatin (Findhorn) Granite in the NW Grampian Highlands has been separated into two distinct complexes: a late-tectonic Glen Kyllachy Granite (tectonically foliated) and a post-tectonic Findhorn Granite (flow foliated). For the Glen Kyllachy complex, Rb-Sr analyses of muscovites from the granite and from an associated suite of cross-foliated pegmatites yield an emplacement age of 443 +5 −15 Ma. Whole-rock Rb-Sr data support field and textural evidence that the pegmatite and granite emplacement was late-tectonic to the last (F3) phase of Caledonian folding. Initial granite 87 Sr/ 86 Sr of 0.7176 supports field and geochemical evidence of derivation from upper crustal metasediments first metamorphosed during the Grenville event. For the Findhorn Granite, concordant Rb-Sr and K-Ar mineral data establish an age of 413± 5 Ma, and an initial 87 Sr/ 86 Sr of c. 0.706 indicates a lower crustal and/or mantle source. This age and isotopic contrast between these granites is characteristic of the whole Grampian region, in which a plutonic hiatus between c . 440 and 415 Ma coincides with the peak of sedimentary accretion in the Southern Uplands, and may be explained in terms of the lack of hydrous materials passing down the associated subduction zone. Structural, metamorphic and radiometric evidence suggests (a) that the late-F3 Glen Kyllachy pegmatites are comparable with the 442 ± 7 Ma old, syn-F3 pegmatites in the N Highlands—both pegmatite suites are situated in the axial zone of the metamorphic Caledonides displaced by the Great Glen Fault; and (b) that the Caledonian, c . 454 Ma old ‘peak’ tectonic-metamorphic event (D2) previously documented in the N Highlands coincides with a Caradocian basin closure along the Highland Boundary Fault. For Silurian events in the N Highlands we propose a separate subduction history; in this region, the peak of calc-alkaline plutonism was accompanied by continuous folding and westward thrusting at the same time as a plutonic hiatus in the Grampian Highlands. Current evidence suggests that the c . 160 km or greater sinistral movements on the Great Glen Fault were not initiated until late Silurian–early Devonian time.

Temporal Changes in Heat-Flow Distribution Associated with Metamorphism in the SW Scottish Highlands and the Lepontine Alps

In mountain belts with low- or medium-pressure metamorphism the distribution of metamorphic zones is influenced by the shape of the heat source and the anisotropism of heat transfer. Both isograds and chrontours indicate that higher heat flow is associated with regional belts of steep schistosity. In metamorphic Caledonides of the south-west Scottish Highlands a laterally extensive heat source was focussed by the belt of steep schistosity at the root zone of the Tay Nappe to form a thermal ridge. On cooling, the isotherms retreated NE along the steep belt towards the center of the heat source. In the Swiss Alps the Lepontine phase of metamorphism is related to a small, slightly elongate heat source. Its lateral effects parallel to the root zone of the Pennine nappes were exaggerated by structural anisotropy. As the belt cooled, the focus of the heat-source migrated westwards, parallel to the root zone at about 3 mm/yr.

Deformation in the Caledonides of England, Ireland and Scotland

Summary The past few years have witnessed a remarkable change in our view of the British Caledonides resulting from radiometric work, application of plate tectonic models as well as basic field work. Overshadowing all else has been the acceptance of a later Proterozoic (probably Grenville) orogeny in the Moine rocks occurring between the Moine thrust and the Great Glen fault. The northern Moines are clearly polymetamorphic in character, but several matters are still controversial, for example the separation of Proterozoic and Caledonian fabrics and the areal extent of the older orogeny. This northern block was subjected to intense and repeated tectonism and, above all, large thrust translation during the Caledonian orogeny and the rocks are now juxtaposed with the older Proterozoic and Archaean gneisses of the Caledonian foreland. In Ireland small areas of Grenville basement are in slide contact with the younger Moine and Dalradian rocks. South of the Great Glen fault above a postulated Grenville basement, are late Proterozoic sediments (Central Highland Granulites and Dalradian) which were orogenically deformed in lower Ordovician times. In this area the essence of the structural pattern established some years ago stands unquestioned for the most part, but significant modifications have been made, notably the recognition of a slice of ‘Cadomian’ basement in the Dalradian pile of NE Scotland, and possible pre-Caledonian basement in W Ireland. In Ireland the structural pattern in the Scottish Highlands may be recognised in the north, but becomes considerably modified in the west where the strike becomes E-W. The picture of the Scottish Highlands as a metamorphic Alpine-type orogen, dating from the early Ordovician, and undergoing uplift during Ordovician and later times, is complemented by the recent recognition of the Southern Uplands zone (later, non-metamorphic Caledonides) as being an accretionary prism, on the Aleutian model, in which imbricate thrusts developed in series towards the south above the subducting oceanic crust of Iapetus. The Iapetus suture lies between the Southern Uplands and northern England and passes through central Ireland. The pattern of Ordovician vulcanicity in northern England, Wales and SE Ireland has been attributed to southerly directed subduction of the Iapetus crust beneath the opposing continental margin. Final closure of Iapetus in Silurian and Devonian times, seems to have been marked by relatively mild tectonic activity in the ‘metamorphic Caledonides’, but by considerable compression across the slate belts of the Lake District, Wales and SE Ireland with polyphase structural evolution.

Metamorphic rocks in the SW Ox Mountains Inlier, Ireland; their structural compartmentation and place in the Caledonian orogen

Three tectono-sedimentary units having Caledonian tectono-thermal histories are recognized within the metamorphic rocks of the SW Ox Mountains Inlier. The different structural levels represented by each of these units were juxtaposed along tectonic contacts late in the individual tectono-thermal histories. The highest structural unit is the virtually unmetamorphosed, probably Southern Highland (Upper) Dalradian, Ardvarney Formation which can be related on a regional scale to the Donegal 'Tay Nappe' structure. The Raheen Barr succession probably ranges from the Islay to Easdale subgroups of the Middle Dalradian and can be correlated with the metamorphic rocks of the Nephin Beg succession in NW Co. Mayo. The Ox Mountains succession represents the deepest structural level seen and consists of basic volcanic rock, graphite schist, marble quartzite, psammite, and semi-pelite. It forms the envelope to the syn- to late kinematic 500 ± 18 Ma Ox Mountains Granodiorite and stratigraphically it can best be compared with the Appin and Argyll groups of the Dalradian of Connemara. The differing tectono-thermal histories of the different structural levels in this part of the metamorphic Caledonides of western Ireland possibly reflect the diachronous nature of the deformation and metamorphism in the orogen. Interaction during juxtaposition must have involved movement related to more than one direction of tectonic transport.

Dating events in the Metamorphic Caledonides: impressions of the symposium held in Edinburgh, September 1971

Synopsis The symposium comprised seventeen contributions on the following subjects: palaeontological evidence for the age of the Dalradian, the Ordovician of western Ireland and other formations relevant to the dating of events (2 papers); the sequence of tectonothermal events in parts of the Moine Thrust Belt and the Moine Nappe (4 papers); deformation rates (1 paper); isotope ages and their relation to events in the Moinian and Dalradian areas (4 papers); early events in the Scandinavian Caledonides and their isotope dating (3 papers); and wider geotectonic aspects, including a plate tectonics synthesis (1 paper), a comparison of Newfoundland and Ireland (1 paper), and new palaeomagnetic evidence for the location of the main continental masses in the Palaeozoic (1 paper). The symposium failed to settle the question of the significance of the Precambrian isotope ages obtained from various parts of the Moine outcrop. Several new Precambrian ages were put forward to augment those already published, but at the same time further geological evidence was adduced to show that the earliest tectonic structures in the Moinian were post-Arenig. Evidence was presented to show that Dalradian sedimentation in Scotland and Ireland north of the Highland Boundary Fault lasted from perhaps 700 m.y. ago (top of the Lower Dalradian = 660 m.y.) through the Cambrian into the Lower Ordovician (Arenig or Llanvirn). The main folding and metamorphism of the Dalradian north of the Highland Boundary Fault occurred around 500–505 m.y. ago. In western Ireland south of the Highland Boundary Fault, Dalradian sedimentation ceased before the Arenig and the main polyphase folding and metamorphism occurred about 520 m.y. ago. The younger D 3 dedformation phase in the Scottish Dalradian may be about 460 m.y. old and possibly equivalent to the pre-Silurian deformation of the Ordovician of western Ireland. It now seems probable that early Caledonian folding in the Scandinavian Caledonides is confined to the North Cape and Bergen regions and may be about 530 m.y. old. Elsewhere in Scandinavia the main Caledonian folding is of Upper Silurian date; K-Ar ages around 435 m.y. are reported in the Trondheim area. The Varanger Tillite is 660 m.y. old. Palaeomagnetic evidence was put forward which demonstrated the existence of a vast Hercynian ocean which closed around the beginning of the Carboniferous, but there is no evidence for a Caledonian ocean between the presently exposed forelands in Britain that could have been wider than 2000 km.

Conditions in the Metamorphic Caledonides During the Period of Late Orogenic Cooling

Conditions in the Metamorphic Caledonides During the Period of Late Orogenic Cooling

Conditions in the Metamorphic Caledonides during the Period of Late-orogenic Cooling

AbstractThe paper deals with regional variations in the contact–effects of Caledonian granites and in the characters of certain associated minor intrusions in the metamorphic Caledonides. It is suggested that these variations are related to the thermal structure of the fold–belt at the time of emplacement.