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A mantle-derived dolomite silicocarbonatite from the southwest coast of Ireland

Published online by Cambridge University Press:  05 July 2018

A. E. Brady  and
K. R. Moore
A. E. Brady*
Affiliation:
Department of Earth and Ocean Sciences, School of Natural Sciences, National University of Ireland Galway, Galway, Ireland Mkango Resources Ltd, Suite 1400, 700 2nd Street SW, Calgary, Alberta T2P 4V5, Canada
K. R. Moore
Affiliation:
Camborne School of Mines, College of Engineering, Mathematics and Physical Sciences, University of Exeter, Cornwall Campus, Penryn, Cornwall TR10 9EZ, UK
*
*E-mail: aoife@mkango.ca
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Abstract

The magma source and evolution of a zoned breccia pipe on the southern Beara Peninsula in southwest Ireland are investigated using the geochemistry of the host mineral assemblages. The clast-poor inner zone of the pipe has a magnesium-rich silicocarbonatite whole-rock composition (14.30 wt.% MgO; 31.80 wt.% SiO2). The silicocarbonatite has retained an ultimate mantle source 13C isotopic composition after metamorphism, consistent with the presence of mantle debris. The silicocarbonatite is Cr-, Ni- and Co-rich (847 ppm, 611 ppm and 60 ppm, respectively) but REE depleted compared with volcanic dolomite carbonatites worldwide. The mineral assemblage consists of Sr-rich (0.55 wt.% SrO) ferroan dolomite, magnesite and pseudomorphs of chlorite after phlogopite, consistent with derivation from a carbonated and hydrated mantle. However, chrome spinel crystals (≤4 40.14 wt.% Cr2O3) are compositionally indistinguishable from unmetasomatized spinel macrocrysts in kimberlites. The silicocarbonatite is inferred to represent a magma produced by partial melting of metasomatized mantle at physical conditions between those in which primary dolomite carbonatite and ultramafic magmas of high-pressure origin form. The primary silicocarbonatite magma ascended and sampled mantle material in a manner similar to kimberlite, and subsequently lost volatile components due to release of metasomatic fluids and later metamorphism.

Keywords

Cahermorecarbon isotopecarbonatitechloritedolomiteIrelandmantlemagnesiteoxygen isotopesilicocarbonatitespinel
Type
Research Article
Information
Mineralogical Magazine , Volume 76 , Issue 2 , April 2012 , pp. 357 - 376
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2012

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