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Cryptic variation in the Rhum layered intrusion

Published online by Cambridge University Press:  05 July 2018

A. C. Dunham  and
W. J. Wadsworth
A. C. Dunham
Affiliation:
Department of Geology, University of Manchester
W. J. Wadsworth
Affiliation:
Department of Geology, University of Manchester
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Summary

Electron-microprobe analyses of cumulus olivine, chromite, pyroxene, and plagioclase from layered peridotites and allivalites of the Eastern and Western Layered Series of Rhum demonstrate the presence of cryptic variation. Olivine varies from Fo88-78 within individual units, and there are corresponding changes in the Mg/(Mg+Fe2+) ratios in the pyroxenes and chromites. Plagioclase changes are not so dramatic, but the An-content broadly follows the Mg/(Mg+Fe2+) ratio in the other minerals. The most Fe-(and Na-) rich phases do not occur at the top of lithological units, but some way below. The composition trend above them is reversed. The data are interpreted as the result of periodic infilling of a magma chamber, the new magma mixing with the remains of the previous pulse. Each pulse was followed by a period when fractional crystallization produced the layered rocks. New data on Ni in the olivines suggests that the ratio of the volume of initial magma to volume of layered rocks was about four to one, the initial magma being allied to the high-calcium low-alkali tholeiitic basalts of Skye.

Type
Research Article
Information
Mineralogical Magazine , Volume 42 , Issue 323 , September 1978 , pp. 347 - 356
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1978

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