Geology of the Outer Hebrides; memoir for 1:100 000 (solid edition) geological sheets, Lewis and Harris, Uist and Barra (Scotland)

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Authors:Fettes, D. J.; Mendum, J. R.; Smith, D. I.; Watson, J. V.
Author Affiliations:Primary:
British Geological Survey, Edinburgh, United Kingdom
Other:
Imperial College, United Kingdom
Source:197p. Availability: Br. Geol. Surv., Keyworth, United Kingdom. ISBN: 0-11-884478-4
Publication Date:1992
Note:In English. 250 refs. Color geologic map 1:250,000; Structural maps 1:100,000
Summary:The Outer Hebrides form a 210 km long archipelago on the NW seaboard of Scotland. The islands have never been systematically mapped by the Geological Survey but the latter have now collaborated with University research groups to complete the geological cover. The islands form the largest area of Archaean crust exposed in Britain, being composed mainly of Archaean and Lower Proterozoic quartzofeldspathic gneisses, metasediments, banded basic and ultrabasic intrusions and granites. Following Scourian deformation of the Archaean assemblage at 2800-2600 m.y., and intrusion of late-Scourian intermediate and granitic bodies, the gneisses were intruded by a widespread suite of tholeiitic dykes at approx 2400-2200 m.y., correlated with the Scourie dykes of the mainland of NW Scotland. In the southern part of South Harris a suite of gabbro, anorthosite, norite and diorite intrusions, also dated at approx 2200 m.y., form spectacular geology and scenery (although the more northerly part of South Harris was graphically described by Heddle as the most barren part of the British Isles). The South Harris igneous complex was intruded into metasediments which display granulite and upper amphibolite facies metamorphic assemblages. The Laxfordian deformation and metamorphism at approx 1700 m.y. resulted in widespread reworking of the gneisses and basic dykes. The Outer Hebrides thrust zone runs down the eastern edge of the archipelago and was initiated as a thrust belt late in the Laxfordian, being subsequently reactivated in an extensional mode in Caledonian times; it has produced extensive mylonitization, thrusting and the formation of bands of pseudotachylite-veined cataclasite <= 40 m thick. Late-Laxfordian granites, mainly a series of sheets, veins and pegmatites, intrude the gneiss complex, notably in the Uig Hills of Lewis and in South Harris. Permo-Carboniferous and Tertiary dykes are found throughout the Outer Hebrides, with Permo- Carboniferous quartz dolerites and camptonite-monchiquites mainly in Barra and South Uist, while the Tertiary basaltic dykes are more common in Harris and central Lewis. The memoir includes many chemical analyses of gneisses, metasediments, norites, gabbros, anorthosites and ultrabasic rocks but also reports numerous EPMA results for garnets, pyroxenes, amphiboles, biotites, plagioclases and scapolites. [R.A.H.]
Subjects:Areal geology; Geologic maps; Maps; Structural maps; Europe; Great Britain; Harris; Hebrides; Outer Hebrides; Scotland; United Kingdom; Western Europe; Barra; Lewis; North Uist; South Uist; Color geologic map
Abstract Numbers:93M/3421
Record ID:1993031842
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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