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The mineral chemistry and petrology of Tertiary pitchstones from Scotland

Published online by Cambridge University Press:  03 November 2011

R. J. Preston ,
M. J. Hole ,
J. Still  and
H. Patton
H. Patton
Affiliation:
Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, AB24 3UE, U.K.j.preston@abdn.ac.uk Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, AB24 3UE, U.K.
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Abstract

Sub-silicic to silicic pitchstones are widespread throughout the British Tertiary Igneous Province (BTIP), with examples being found at all the major igneous centres. Both highly porphyritic and almost completely aphyric varieties occur, and take the form of sills, dykes and lava flows. Here we present previously unreported mineral chemistry data on phenocryst and microcrystallite populations from a number of pitchstones from throughout the BTIP. Phenocryst assemblages are completely anhydrous, comprising mixtures of plagioclase, sanidine, fayalite, orthopyroxene, pigeonite, ferroaugite, ferrohedenbergite and quartz. Microcrystallite assemblages are also diverse, consisting of sanidine, ferrohedenbergite, fayalite and, occasionally, almost pure end-member ferrosilite, as well as hydrous phases such as ferrohornblende and biotite. Textural and mineral chemistry observations support interpretations derived from whole-rock and residual glass major element analyses, together with whole-rock trace element and the available Sr-Nd-Pb isotope data, that the Tertiary pitchstones of Scotland are either the products of intimate mixing between a range of basaltic magmas with hydrous crustal melts, or were formed by the crustal contamination of basaltic magmas.

Keywords

basaltBTIPcrustal meltscrystallitesdisequilibrium phenocrystsglassmagma mixing
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 89 , Issue 2 , 1998 , pp. 95 - 111
Copyright
Copyright © Royal Society of Edinburgh 1998

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