Mixed Caledonian appinite magmas: implications for lamprophyre fractionation and high Ba-Sr granite genesis |
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Authors: | M B Fowler P J Henney |
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Institution: | (1) Division of Geology and Cartography, Oxford Brookes University, Headington, Oxford OX3 0BP, U.K., GB;(2) British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K., GB |
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Abstract: | Ach'Uaine Hybrid appinites represent a rare example of lamprophyric magmas that were demonstrably exactly contemporaneous
with felsic differentiates, preserved within a suite of minor, hypabyssal intrusions emplaced at the end of the Caledonian
orogeny in northern Scotland. Numerous small stocks, bosses and dykes show outcrop-scale relationships characteristic of mingling
between lamprophyric and syenitic magmas, and are commonly cut by sharp-sided granite veins. The mafic rocks are characterised
by Ni and Cr abundances and MgO sufficiently high to signal derivation from a mantle source within which radiogenic 87Sr/86Sr and nonradiogenic 143Nd/144Nd ratios require significant time-integrated incompatible element enrichment. This is manifest in high Ba, Sr and light REE
abundances and incompatible element ratios in the derived magmas directly comparable with those of high Ba-Sr granitoids and
related rocks. Quantitative major element, trace element, radiogenic and stable isotope modelling is consistent with early
fractionation of clinopyroxene and biotite, accompanied by minor crustal assimilation, having driven the evolving lamprophyric
magma to cogenetic syenite. Subsequent derivation of granite required a major change to feldspar-dominated crystal fractionation
with continued, still minor contamination. The elemental and isotopic characteristics of the granitic terminus are so similar
to high Ba-Sr granitoids both locally and worldwide, that these too may have had large mantle components and represent significant
juvenile additions to the crust.
Received: 26 September 1995 / Accepted: 5 June 1996 |
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