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Petrology and geochemistry of post-obduction dykes of the Ballantrae complex, SW Scotland

Published online by Cambridge University Press:  03 November 2011

F. V. Holub ,
H. Klápová ,
B. J. Bluck  and
D. R. Bowes
F. V. Holub
Affiliation:
Department of Petrology, Charles University, Albertov 6, 128 43 Prague 2, Czechoslovakia.
H. Klápová
Affiliation:
Department of Petrology, Charles University, Albertov 6, 128 43 Prague 2, Czechoslovakia.
B. J. Bluck
Affiliation:
Department of Geology, University of Glasgow, Glasgow G12 8QQ, Scotland.
D. R. Bowes
Affiliation:
Department of Geology, University of Glasgow, Glasgow G12 8QQ, Scotland.
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Abstract

Doleritic intrusions known to post-date the obduction of the Ballantrae complex during Arenig times record changing magma provenance during the cooling and serpentinisation of obducted peridotite. There are two groups of dolerites with different petrographical and geochemical characteristics.

The earlier emplaced group, which is subordinate, is characterised by amphibole formed under low metamorphic facies conditions and the virtual absence of Fe–Ti oxides. Chemically these rocks are of primitive tholeiitic character and are similar to modern island-arc basalts. They were derived from a strongly depleted mantle source region. The source region of the later emplaced group, which is predominant, was much less depleted. Clinopyroxene and plagioclase, much of it albitised, are the dominant minerals. Fe–Ti oxides also are common and there are pronounced chilled margins and well-preserved ophitic textures. Although showing mildly alkaline tendencies, these rocks originated from a tholeiitic parental magma of “within-plate” type.

Both groups show evidence of rodingitisation and associated alteration related to serpentinisation of the peridotite. During the metasomatic activity, only some elements were mobile, while Al2O3, total Fe, MgO, TiO2, Cr, Ni, Nb, Y and Zr remained almost constant. Assessment of original magma type and geotectonic environment, and demonstration that both groups of doleritic rocks show the products of fractionation, has relied heavily on data for the apparently immobile elements and on petrographical study which identified those rocks least affected by alteration.

Keywords

albitisationamphiboleanalysisArenigbasaltdiscriminant functiondoleritefractionationgeotectonic environmentimmobile elementisland-arc basaltmetasomatismmicroprobepyroxenerodingiteserpentinitetholeiite
Type
Midland Valley
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. 211 - 223
Copyright
Copyright © Royal Society of Edinburgh 1984

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