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A re-evaluation of the blueschist facies

Part 2. The role of the mineral in metamorphism

Published online by Cambridge University Press:  01 May 2009

R. M. Wood
R. M. Wood
Affiliation:
Department of Mineralogy and Petrology, Downing Place, Cambridge
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Summary

Within blueschist facies rocks it is common to find that mineral assemblages represent a series of attempts to arrive at changing states of equilibrium. To interpret such rocks it is necessary to study how the mineral constrains and determines chemical interaction. The gradual process of chemical exchange between minerals, during changes in conditions, is interrupted by the nucleation of a new mineral species. Mineral reactions are instigated by the nucleation of one new mineral species, though when a reaction is seen at an equilibrated completion the reaction path cannot be traced. In low temperature metamorphism the preservation of partial mineral reactions is very widespread. The growth, adaptation and resorption of a mineral are considered in order to demonstrate: (1) how and why a mineral should zone during growth: (2) the competitive reaction paths that may allow a mineral to change composition, even across and internal solvus; and, (3) through the consideration of a few simple parameters, the prediction of a variety of resorption textures. It is proposed that through an understanding of the way in which the mineral can store information on the chemical history of the rock it is possible to obtain a metamorphic analysis without recourse to the conventional phase equilibrium metamorphic model - a frequent misapplication in the blueschist facies.

Type
Articles
Information
Geological Magazine , Volume 116 , Issue 3 , May 1979 , pp. 191 - 201
Copyright
Copyright © Cambridge University Press 1979

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