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

R. Macdonald1, N. M. S. Rock2, C. C. Rundle3 and O. J. Russell1
1 Department of Environmental Science, University of Lancaster, Lancaster LA1 4YQ
2 Department of Geology, University of Western Australia, Nedlands 6009, Western Australia
3 British Geological Survey, 64 Gray's Inn Road, London WC1X 8NG

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: lamprophyre • syenite • granite • Caledonian • Newmains Dyke • Scotland

Mineralogical Magazine; December 1986 v. 50; no. 358; p. 547-557; DOI: 10.1180/minmag.1986.050.358.01
© 1986, The Mineralogical Society
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