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The petrology of the Lugar Sill, SW Scotland

Published online by Cambridge University Press:  03 November 2011

C. M. B. Henderson  and
F. G. F. Gibb
C. M. B. Henderson
Affiliation:
Department of Geology, The University, Manchester M13 9PL, England.
F. G. F. Gibb
Affiliation:
Department of Geology, The University, Sheffield S1 3JD, England.
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Abstract

A 49 m complete section through the 288 Ma Lugar Sill obtained from drill cores can be subdivided into nine units. The uppermost four units are teschenitic and are mirror images of the bottom four. A 35 m thick central unit consists of theralite passing down into kaersutite theralite and then picrite. Marginal chilling 'of the units indicates multiple intrusion from the outside inwards. Olivine in the central unit (Fo88–90) encloses Cr-rich spinels and increases in amount inwards to over 50%. Clinopyroxene, kaersutite and biotite show symmetrically increasing Fe/Mg from the centre of the sill outwards. Most major and trace elements vary symmetrically throughout the sill with those in the central unit reflecting mainly olivine distribution but incompatible elements exhibit upward enrichment. Remarkably, the most-evolved rocks in the sill are at its margins. The sill was formed by multiple injections of successively less-evolved teschenitic magmas followed by a larger pulse of theralitic liquid carrying abundant olivine phenocrysts. The amount of olivine in this final pulse increased during emplacement. Subsequent in-situ differentiation in the central unit, with upward enrichment in residual liquid and volatiles, gave rise to lugarites. The various magmas were produced in a lower-level magma chamber by differentiation of a mantle-derived, alkali-rich picritic magma.

Keywords

Alkalinedifferentiated silllugaritemultiple intrusionPermo-Carboniferouspicriteteschenitetheralite
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 77 , Issue 4 , 1987 , pp. 325 - 347
Copyright
Copyright © Royal Society of Edinburgh 1987

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