Bulk rock composition: A key to identifying invisible prograde reactions in zoned garnet

Callum J. Hetherington, Ronan Le Bayon

Research output: Contribution to journalArticle

9 Scopus citations

Abstract

Chemical zonation of prograde metamorphic minerals is commonly attributed to net transfer reactions, intracrystalline diffusion or the growth of compositionally differing zones of the same phase during consecutive geological events. Evidence for such events or processes is commonly recorded in mineral relics and/or inclusions, or through the interpretation of geochronological data. Complex chemical zonation has been recorded in garnet grains from garnet-biotite-muscovite gneiss from the Berisal Complex, Simplon Region, Switzerland. None of the features (e.g. relic inclusions) used to interpret such zoning via petrography are observed or applicable; therefore, the reactions occuring on the prograde P-T path and its shape and trajectory cannot be defined. In the absence of features necessary to permit a classical investigation of equilibrium-disequilibrium phases and their role in the growth of the garnets, an approach based on the bulk rock composition of the rock has been investigated. For a given bulk rock chemistry, equilibrium phase diagrams are constructed over a specified P-T range using DOMINO software. Important minerals are identified and their isopleths calculated to model the variation in composition of each phase in response to variations in pressure and temperature. The modelled chemical changes are compared with the observed chemical zonations and the Alpine metamorphic history of the gneiss and its garnet grains is reconstructed. The modelling of the prograde mineral reactions using a rock's bulk rock composition, combined with observed chemical zonations in metamorphic minerals, is a valuable method for constraining the prograde P-T path when standard methods are inapplicable.

Original languageEnglish
Pages (from-to)57-67
Number of pages11
JournalSchweizerische Mineralogische und Petrographische Mitteilungen
Volume85
Issue number1
StatePublished - 2005

Keywords

  • Berisal Complex
  • Bulk rock composition
  • P-T-X conditions
  • Prograde metamorphic reactions
  • Zoned garnet

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