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Xenoliths and their implications for the deep geology of the Midland Valley of Scotland and adjacent region

Published online by Cambridge University Press:  03 November 2011

Brian G. J. Upton ,
Peder Aspen  and
Robert H. Hunter
Brian G. J. Upton
Affiliation:
Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland.
Peder Aspen
Affiliation:
Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland.
Robert H. Hunter
Affiliation:
Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland.
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Abstract

Late Palaeozoic alkalic basalts in and around the Midland Valley of Scotland contain a wide variety of ‘plutonic’ xenoliths. Pyroxene-rich ultramark xenoliths (wehrlites, clinopyroxenites and garnet pyroxenites) may be representative of younger components within a dominantly peridotitic upper mantle represented by ubiquitous magnesian peridotite xenoliths. Glimmerites and other biotite-rich ultramafic xenoliths are probable samples of metasomatised upper mantle facies.

Xenoliths composed mainly of plagioclase, clinopyroxene ± orthopyroxene ± magnetite are widespread. These pyroxene granulites may typify the lower crustal layers. Garnet granulites are rare; such rocks may formerly have been important with loss of garnet occurring through retrograde metamorphism. Anorthositic xenoliths are relatively common. The lower crust may consist largely of anhydrous rocks, of gabbroic to anorthositic composition, ccurring as stratiform bodies of metacumulates.

Other xenoliths of igneous origin include tonalitic and trondhjemitic gneisses. Although these may play some role in the lower crust, they may be more abundant in the mid-crustal domains underlying the deformed upper Precambrian and lower Palaeozoic supracrustal strata. Xenoliths of quartzofeldspathic, granulitic gneisses containing garnet ± sillimanite ± rutile are also of widespread occurrence; many of these are of metasedimentary provenance and are regarded as being derived from the mid-crustal layers beneath the Southern Highlands, Midland Valley and Southern Uplands and their Irish counterparts.

Keywords

anorthositegranulitelherzolitelower crustmetasomatismpyroxenitespinelupper mantlewehrlite
Type
Regional framework
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 75 , Issue 2: Deep Geology of the Midland Valley of Scotland and Adjacent Regions. Bicentenary Symposium , 1984 , pp. 65 - 70
Copyright
Copyright © Royal Society of Edinburgh 1984

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