The peridotite plugs of Rum: Crystal settling and fabric development in magma conduits

Holness, M. B. and Sides, R. E. and Prior, David J. and Cheadle, Michael J. and Upton, Brian G.J. (2012) The peridotite plugs of Rum: Crystal settling and fabric development in magma conduits. Lithos, 134–13. pp. 23-40. DOI

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The Rum Igneous Complex contains numerous circular to elongate exposures of peridotite cumulates within the layered series and the country rock. The extent of thermal effects on quartzo-feldspathic country rock surrounding the Mam Tuath and West Sgaorishal plugs demonstrates that these are pipe-like bodies, inclined at various angles to the vertical, that remained at magmatic temperatures for periods of up to several months. Tabular olivine grains within the plugs preserve a weak sub-horizontal fabric that is stronger in the plug centres than at their margins. The olivine mode increases from ~ 50 vol.% at the plug margins to ~ 85 vol.% at the centre. The spatial arrangement of the olivine grains is consistent with their being clustered, with the increase in mode towards the plug centres achieved by a combination of overgrowth and mechanical re-arrangement. Comparison of fabrics with those of the Ard Mheall peridotites of the Rum Western Layered Intrusion is consistent with porosity reduction in the Ard Mheall cumulates also having been primarily achieved by overgrowth and mechanical re-arrangement: compaction via intra-crystalline deformation did not occur. We suggest that the Rum ultramafic plugs are deep-level cross-sections presenting the ultimate ‘snap-shots’ of volcanic conduits through which olivine-rich magmas flowed for substantial periods, probably giving rise to fire-fountains of the kind associated with Hawaiian eruptions. The exposed parts of the plugs are now miniature cumulate bodies that developed in situ from the settling of olivine phenocrysts once flow had ceased within the conduit.

Item Type: Article
Uncontrolled Keywords: 2012AREP; IA63;
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title: Lithos
Volume: 134–13
Page Range: pp. 23-40
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 07 Mar 2012 16:07
Last Modified: 23 Jul 2013 10:03

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