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Geochronological evidence for phased volcanic activity in Fife and Caithness necks, Scotland

Published online by Cambridge University Press:  03 November 2011

R. M. Macintyre ,
R. A. Cliff  and
N. A. Chapman
R. M. Macintyre
Affiliation:
Isotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU, Scotland
R. A. Cliff
Affiliation:
Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, England
N. A. Chapman
Affiliation:
Environmental Protection Unit, Institute of Geological Sciences, Building 151, Harwell Laboratory, Didcot, Oxon OX11 ORA, England
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Abstract

In an attempt to establish a chronology for volcanic neck emplacement and so elucidate petrogenesis, isotopic studies have been carried out on various cumulate inclusions, blocks and megacrysts which occur chiefly in association with tuffs infilling several Scottish vents. K-Ar ages of 13 samples of low-pressure cumulate minerals (biotite, hornblende and pyroxene) from necks in East Fife indicate crystallisation at shallow depth at 314 Ma. U-Pb analyses of zircons are concordant at 318 Ma suggesting they are also members of this suite and their formation is penecontemporaneous with the Namurian volcanic activity which is welldocumented stratigraphically. By 295 Ma crystallisation of anorthoclase megacrysts had been completed, perhaps from the fractionated residuum. An eruption from considerable depth (within the stability field of garnet precipitation) then broke through to the surface bearing high-pressure megacrysts. This penetrated and disrupted the early cumulates carrying them to the surface and producing the diverse vent assemblages. K-Ar dating of basanites suggest that the Duncansby Ness neck in Caithness was emplaced around 270 Ma in the early Permian. For two Fife necks the balance of evidence favours an age of 290 Ma (Stephanian) for this final explosive activity associated with vent formation.

Keywords

agesanorthoclasebiotitecarboniferouscumulatedateshornblendeinclusionK-ArnodulePermianpyroxenetuffU-Pbventxenocryst
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 72 , Issue 1 , 1981 , pp. 1 - 7
Copyright
Copyright © Royal Society of Edinburgh 1981

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