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Nature and significance of beerbachites in the Ballantrae ophiolite, SW Scotland

Published online by Cambridge University Press:  03 November 2011

E. Jelínek ,
J. Souček ,
B. J. Bluck ,
D. R. Bowes  and
P. J. Treloar
E. Jelínek
Affiliation:
E. Jelínek and J. Soucek. Faculty of Science, Charles University, Albertov 6. 128 43 Prague 2., Czechoslovakia.
J. Souček
Affiliation:
E. Jelínek and J. Soucek. Faculty of Science, Charles University, Albertov 6. 128 43 Prague 2., Czechoslovakia.
B. J. Bluck
Affiliation:
B. J. Bluck and D. R. Bowes, Department of Geology, University of Glasgow, Glasgow G12 8QQ, Scotland.
D. R. Bowes
Affiliation:
B. J. Bluck and D. R. Bowes, Department of Geology, University of Glasgow, Glasgow G12 8QQ, Scotland.
P. J. Treloar
Affiliation:
P. J. Treloar, Department of Mineralogy and Petrology, University of Cambridge, Downing Place, Cambridge CB2 3EW, England.
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Abstract

Metamorphosed abyssal ocean-floor tholeiitiic rocks, little affected by crustal contamination and probably formed at a spreading ridge, occur as a dyke complex in banded gabbros with oceanic affinities and as small tectonic lenses in serpentinised peridotite. Igneous textures and mineral assemblages have been partly or completely replaced in both dykes and gabbros. Metamorphic development of clinopyroxene and very Ti-rich hornblende at 900-1000°C was followed by a low temperature actinolite-chlorite assemblage and then by pectolite-bearing veins. Geochemical variations resulting from magmatic and metasomatic processes have been distinguished and plots of major and trace elements used to establish tectonic environment. The very high temperature metamorphic mineral growth associated with beerbachite formation indicates that the dyke rocks were in a high heat-flow environment for a considerable time, presumably in the vicinity of a spreading ridge. Obduction of a very hot slab of oceanic lithosphere accounts for the superimposed mineral assemblages.

Spatially associated pillow lavas have been affected by crustal contamination and are not comagmatic with the dykes. There are petrochemical characters indicating that some of the lavas are ocean-floor basalts. For other lavas, features suggesting emplacement in a continental or an island-arc environment could also be consistent with development in a marginal basin. However, there is so much variation in composition of pillows from margin to centre, from small to large and from metasomatic activity, that their compositional fields cannot be used for discrimination of basaltic type with the same confidence as for the dykes. Accordingly, it is suggested that the petrochemistry of sheeted dyke complexes, rather than of pillow lavas, be used for this purpose in the study of ophiolites.

Keywords

comagmatiticgabbrosmetasomatismocean-floorpetrochemical plotpillow lavaserpentinitesheeted dyke complexspreading ridgetholeiite.
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 71 , Issue 3 , 1980 , pp. 159 - 179
Copyright
Copyright © Royal Society of Edinburgh 1980

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