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

Dunning, F. W.

doi:10.1144/sjg08030179

Cited by 18 publications

(4 citation statements)

References 25 publications

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“…Thus, Flinn & Pringle (1976) regard an Rb-Sr whole-rock isochron age of 530-25 Ma for the Colla Firth migmatite of Shetland as in complete agreement with ages of migmatisation from Connemara (520+--30 Ma, Dunning 1972), Norway (530-+35 Ma, Pringle & Sturt 1969) and consistent with that for the Duchray Hill migmatite reported here.…”

Section: Discussionsupporting

confidence: 83%

Migmatisation, deformation and porphyroblast growth in the Dalradian of Tayside, Scotland

Bradbury¹

1979

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In Tayside, migmatisation of kyanite zone Argyll and Southern Highland Dalradian metasediments occurred between the regional D2 and D3 episodes in the Grampian orogeny. The U-Pb zircon age for the emplacement of the 'Older' Ben Vuirich Granite (514 + 6/-7 Ma), which intervenes between D2 and D3, provides an approximate age for migmatisation. This is significantly younger than recent isotopic ages of migmatites from the W Scottish Highlands with which the Tayside migmatites were previously correlated. In Tayside, migmatisation largely post dated the development of the Tay nappe but pre-dated the late-stage modification of the Tummel steep belt and the Loch Tay inversion. Textural evidence indicates that migmatisation was broadly coeval with the climactic Barrovian metamorphism, the age of which is discussed.Migmatites commonly occur in the higher grade cores of metamorphic belts. In Scotland, Barrow (1893) assumed a causal relationship between the 'Older' Granites, which include migmatites (Harry 1958), and the surrounding metamorphic zones defined by index minerals. More recently this view has been disputed (e.g. Johnson 1963, p.133) and the relationship of early granites to migmatisation has remained problematical.Modern work in the NE Highlands has focused attention on the relative age of migmatitic features and the growth of porphyroblasts. On textural grounds, most workers concur with Chinner's (1966) proposal of a sillimanite-migmatite overprint on the earlier climactic pattern of zones characterised in their highest grade by kyanite (Barrovian) and andalusite (Buchan). This overprint has been related to the emplacement of the Newer Basic rocks (Ashworth 1972(Ashworth , 1975). Thus, the emplacement age of the Newer Basic rocks, possibly as young as 480 Ma (Pankhurst & Pidgeon 1976), would provide an approximate age for the sillimanite-migmatite event and a minimum age for climactic Dalradian metamorphism in NE Scotland.This paper considers the time relationship between deformation events and migmatisation within the kyanite zone outside the sillimaniteoverprint area.

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Section: Discussionsupporting

confidence: 83%

Migmatisation, deformation and porphyroblast growth in the Dalradian of Tayside, Scotland

Bradbury¹

1979

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“…5 ) it is clear that pre-Permian palaeomagnetic pole positions do not all fall on a single common polar path. On the contrary, Briden (1971) (also quoted by Dunning (1972) and illustrated in Smith et al (1973)) first showed that three distinct regions with distinct polar paths are evident, namely Gondwanaland, Siberia and Euramerica (all of Europe and North America north of the Hercynian-Alleghenian orogenic belt). Each of these regions may be regarded to a first approximation as a Palaeozoic crustal plate, although the Euramerican region is evidently more complicated, two large plates and at least one sub-plate may possibly be distinguished within it.…”

Section: Introductionmentioning

confidence: 99%

Palaeomagnetic Studies in the British Caledonides?VI Regional and Global Implications

1973

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Summary Palaeozoic palaeomagnetic data from the British Isles are reviewed and are found to be consistent with erratic polar shift from the vicinity of 10° N, 180° E in the Ordovician to 0°, 145° E in the Siluro‐Devonian and to 25° N, 160° E in the early Carboniferous. The Cambrian pole is less well established near 25° N, 170° E. The rates and times of shifts are discussed. World‐wide palaeomagnetic data suggest the existence of three groups of continents in the Early and Middle Palaeozoic—the Gondwanan, Euramerican and Siberian groups—separated by large oceans which closed in the Late Palaeozoic along the lines of the Hercynides and Uralides. The data from Gondwana and Siberia are internally consistent and each may have been a single plate to which only peripheral slices were added subsequently. Data from Euramerica are less coherent. The most probable explanation seems to be that this group of continents was loosely‐knit and consisted of the North American and the Baltic/Russian plates (which joined in the Silurian on the site of the Scandinavian Caledonides) and the British Isles sub‐plate which joined in the Devonian (on previously unrecognized sutures lying in the present North Atlantic and North Sea respectively). This interpretation takes all the data at face value and assumes that time‐coverage is adequate in all parts of the region. Alternatively, if time‐coverage was not adequate, Britain might have been part of the North American plate, but further work is required to discriminate between these interpretations. Palaeomagnetic evidence from the British Isles also indicates that little or no closure has taken place across the British Caledonides since Early Ordovician time. If so the British Isles sub‐plate may have been rotated in azimuth as well as being transported laterally before joining the Euramerican plate. Examples of rotation and remagnetization on a more local scale are cited, and are ascribed to tectonic and thermal effects of orogeny respectively. The Euramerica and Gondwana groups were already close together by early Carboniferous times, which is distinctly earlier than the Hercynian‐Alleghenian orogenic belt which may mark their juncture. It may be generally true that large scale crustal drift, involving consumption of a large area of oceanic crust between two continents, significantly predates the orogenic belt which forms when the two continents have met. If this is so, large scale closure across the British Caledonides might have occurred in Precambrian or Cambrian time, but palaeomagnetic data are not yet adequate to evaluate this.

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“…As Skevington (1971, p. 286) points out, the possibility that the specimen is incorrectly localized should be investigated in view of the Arenig to Llanvirn or Llandeilo age suggested for the Macduff Group by Downie et al (1971) on the basis of microfossils. Similarly the isotopic age data summarized by Dunning (1972) suggest an age of at least 501 ± 17 m.y. for the Newer Gabbros which metamorphose the Macduff Group in the south of its outcrop.…”

Section: H U G H Miller's Graptolitesmentioning

confidence: 69%

Hugh Miller's Graptolites

Trewin¹

1973

SJG

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

Cited by 18 publications

References 25 publications

Migmatisation, deformation and porphyroblast growth in the Dalradian of Tayside, Scotland

Migmatisation, deformation and porphyroblast growth in the Dalradian of Tayside, Scotland

Palaeomagnetic Studies in the British Caledonides?VI Regional and Global Implications

Hugh Miller's Graptolites

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