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Metamorphic development of cordierite-amphibole rocks and mica schists in the vicinity of the Outokumpu ore deposit, Finland

Published online by Cambridge University Press:  03 November 2011

P. J. Treloar ,
T. J. Koistinen  and
D. R. Bowes
P. J. Treloar
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, England.
T. J. Koistinen
Affiliation:
Exploration Department, Outokumpu Oy, 83500 Outokumpu, Finland.
D. R. Bowes
Affiliation:
Department of Geology, University of Glasgow, Glasgow G 12 8QQ, Scotland.
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Abstract

Analysed compositions of coexisting phases from cordierite-amphibole-bearing rocks (cordierite-orthoamphibole-cummingtonite-garnet-staurolite-spinel-opaque ores) and mica schists (garnet-staurolite-biotite-muscovite-plagioclase-andalusite-quartz) have been used in conjunction with experimentally determined equilibria to estimate pressures and temperatures of metamorphism for the rocks of the Outokumpu district. The data do not determine a single point in P-T space, but bracket an area such that T ≈ 600 ± 50°C and P ≈ 3·5 ± 1 kb. These conditions represent the maximum temperature attained by the rocks, and the associated sulphide ore bodies, during the Svecokarelian orogeny and are consistent with a regional palaeogeotherm of c. 50°C/km.

Assessment of the composition of the coexisting phases in the cordierite-amphibole-bearing rocks, particularly the Fe-Mg-bearing phases, indicates that it is unlikely that either iron or magnesium was mobile during metamorphism and that the equilibrium assemblage represents essentially isochemical metamorphism of a rock composition that predates the metamorphism. A similar genesis is considered likely for the cordierite-anthophyllite rocks of Orijärvi in southern Finland which forms the basis of the classic statement of the magnesium-metasomatism hypothesis.

Keywords

anthophyllitebiotitecoexisting mineralscummingtonitegarnetgedriteisochemicalmagnesium-metasomatismmineral chemistryP-T conditionsreactionstaurolitestockworkSvecokarelian orogeny.
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 72 , Issue 3 , 1981 , pp. 201 - 215
Copyright
Copyright © Royal Society of Edinburgh 1981

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