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Relationships Between Late Caledonian Lamprophyric, Syenitic, and Granitic Magmas in a Differentiated Dyke, Southern Scotland

Published online by Cambridge University Press:  05 July 2018

R. Macdonald ,
N. M. S. Rock ,
C. C. Rundle  and
O. J. Russell
R. Macdonald
Affiliation:
Department of Environmental Science, University of Lancaster, Lancaster LA1 4YQ
N. M. S. Rock
Affiliation:
Department of Geology, University of Western Australia, Nedlands 6009, Western Australia
C. C. Rundle
Affiliation:
British Geological Survey, 64 Gray's Inn Road, London WC1X 8NG
O. J. Russell
Affiliation:
Department of Environmental Science, University of Lancaster, Lancaster LA1 4YQ
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Abstract

The 2 km × 5 m Newmains Dyke in the Scottish Southern Uplands consists mainly of primitive (mg number > 70), at least partly mantle-derived lamprophyre (vogesite spessartite, SiO2 50–60%), with hornblende-pyroxene-rich cumulates (SiO2 < 50%) and small amounts of quartz syenitic to granite residua (60–73% SiO2). The lamprophyres and cumulates yield an Rb-Sr whole-rock isochron age of 395±9 Ma (MSWD 2.0), and (87Sr/86Sr)i 0.70514±5—identical values to the nearby Criffell granitic pluton. After some 65% hornblende + clinopyroxene fractionation of parent lamprophyre magma, a volatile phase separated and metasomatically enriched the country rock greywackes in K, Ba, and Rb. The remaining quartz syenitic liquids became contaminated by 87Sr from these greywackes, which have a mean (87Sr/86Sr)395 value of 0.70789±114. Further fractionation of the contaminated syenitic liquids gave the granitic rocks, which have (87Sr/86Sr)395 values of 0.7056–64. The dyke appears to demonstrate in situ some of the hidden processes previously invoked to explain the evolution of the Criffell pluton itself.

Keywords

lamprophyresyenitegraniteCaledonianNewmains DykeScotland
Type
Petrology
Information
Mineralogical Magazine , Volume 50 , Issue 358 , December 1986 , pp. 547 - 557
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1986

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