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Metabentonite geochemistry: magmatic cycles and graptolite extinctions at Dob's Linn, southern Scotland

Published online by Cambridge University Press:  03 November 2011

R. A. Batchelor  and
J. A. Weir
R. A. Batchelor
Affiliation:
R. A. Batchelor and J. A. Weir, Department of Geography and Geology, University of St Andrews, St Andrews, Fife KY16 9ST, Scotland, U.K.
J. A. Weir
Affiliation:
R. A. Batchelor and J. A. Weir, Department of Geography and Geology, University of St Andrews, St Andrews, Fife KY16 9ST, Scotland, U.K.
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Abstract

The Moffat Shale Group is a condensed, variable and partly pelagic sequence of mudrocks of Llandeilo—Llandovery age. The sequence has a five-fold lithological subdivision based mainly on the occurrence of grey mudstones within a succession otherwise dominated by fully euxinic black graptolitic mudrocks. Associated with the black mudrocks, especially in the Llandovery, are metabentonite beds which achieve a climax, both in thickness and in number, within the top quarter of the mudrock sequence. A geochemical and mineralogical study has confirmed a volcanic origin for the metabentonites. Major element data highlight a carbonate-dominated environment above the gregarius—convolutus Zones boundary. Phosphorus levels reach a peak at the same boundary, as well as at the Caradoc—Ashgill boundary where phosphorite horizons are known from Wales and Norway. Immobile trace elements have highlighted regular changes in source magma composition. Prolonged periods of crystal fractionation in magmas of intermediate composition gave rise, on eruption, to large volumes of silicic ash which had a deleterious effect on graptolite species and led to local extinctions. Regular fluctuations in ash composition from silicic to intermediate are ascribed to alternating fractionation and magma mixing cycles.

Keywords

Moffat Shale GroupmudrockspetrogenesisSouthern Uplandsvolcanic ash
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 79 , Issue 1 , 1988 , pp. 19 - 41
Copyright
Copyright © Royal Society of Edinburgh 1988

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