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Early Palaeozoic metamorphic history of the Midland Valley, Southern Uplands–Longford-Down massif and the Lake District, British Isles

Published online by Cambridge University Press:  03 November 2011

G. J. H. Oliver
Affiliation:
Department of Geology, University of St Andrews, St Andrews KY16 9ST, Scotland.
J. L. Smellie
Affiliation:
British Antarctic Survey, Madingley Road, Cambridge CB3 0ET, England.
L. J. Thomas
Affiliation:
Department of Geology, University of St Andrews, St Andrews KY16 9ST, Scotland.
D. M. Casey
Affiliation:
British Petroleum Development Limited, Fairburn Industrial Estate, Dyce, Aberdeen AB2 0PD, Scotland.
A. E. S. Kemp
Affiliation:
Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland.
L. J. Evans
Affiliation:
Department of Geology, University of St Andrews, St Andrews KY16 9ST, Scotland.
J. R. Baldwin
Affiliation:
Department of Geology, University of St Andrews, St Andrews KY16 9ST, Scotland.
B.C. Hepworth
Affiliation:
Robertson Research Laboratories, Tyn-y-Coed, Llandudno LL30 ISA, Wales.

Abstract

A model for the early Palaeozoic metamorphic history of the Midland Valley and adjacent areas to the S in Scotland, England and Ireland is based on the results of new field mapping, thin section petrography, electron probe microanalysis, X-ray diffractometry, conodont and palynomorph colouration and graptolite reflectance measurement.

The oldest metamorphic rocks of the Midland Valley of Scotland, excluding xenoliths in post-Silurian lavas, are possibly the blueschist occurrences in the melange unit of the Ballantrae complex. These may be tectonised remnants of (?)pre-Arenig ocean-floor subducted during closure of the Iapetus Ocean. In the early Ordovician, the melange terrane was dynamothermally metamorphosed during obduction of newly-formed ocean crust. The obduction process piled up a thick sequence of various ocean-floor types such that burial metamorphism in parts reached pumpellyite-actinolite facies; elsewhere prehnite-pumpellyite and zeolite facies was attained.

Whilst the Midland Valley acted as an inter- or fore-arc basin during the Late Ordovician and Silurian and experienced burial metamorphism, an accretionary prism was formed to the S. Accretion, tectonic burial and metamorphism of ocean-floor and trench sediment was continuous in the Southern Uplands and the Longford-Down massif of Ireland through Late Ordovician to Late Silurian times. Rocks at the present-day surface vary from zeolite facies to prehnitepumpellyite facies. Silurian trench-slope basin sediments can be recognised in part by their lower grade of burial metamorphism. Greenschist facies rocks of the prism probably lie close to the surface.

The Lake District island-arc terrane of Northern England has an early Ordovician history of burial metamorphism up to prehnite-pumpellyite facies. The Late Ordovician and Silurian metamorphic history is one of sedimentary burial complicated by tectonism and intrusion of granite plutons to a relatively high level. The Iapetus suture is marked by a weak contrast in metamorphic grade.

Type
Southern Uplands, granites and synthesis
Copyright
Copyright © Royal Society of Edinburgh 1984

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