Mineral Equilibria Modelling of the Granulite-Eclogite Transition: Effects of Whole-Rock Composition on Metamorphic Facies Type-Assemblages
Abstract
Pressure-Temperature-X(composition) pseudosections constructed for monzodioritic to peridotgabbroic rock compositions using Thermocalc in the model system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (NCKFMASHTO) illustrate the dependence of granulite and eclogite assemblages on whole-rock composition at mid-crustal to upper-mantle conditions. Results of low-H2O equilibria modelling can be validated in the context of natural assemblages from Breaksea Sound, Fiordland, New Zealand, where co-facial monzodioritic granulite and peridotgabbroic eclogite formed at P≈1.8 GPa and T≈850°C. Increasing ferric iron content results in a marked contraction of garnet-plagioclase assemblages, and an expansion of orthopyroxene, kyanite, quartz and ilmenite stability across compositional range in monzodioritic protoliths, and the expansion of plagioclase stability up pressure in gabbroic compositions. Silicate liquid compositions calculated for a monzodioritic protolith using the haplogranite melt model do not accurately reflect trends in comparable experimental data, and refinement is needed for its application to the modelling of intermediate and mafic equilibria involving more than a few percent partial melt. Omphacite-bearing granulite mineral equilibria in dioritic protoliths are far less sensitive to changes in whole rock oxygen content than gabbroic protoliths; a doubling of whole rock oxyen content displaces the modelled granulite-eclogite transition in gaabroic assemblages by 0.4 GPa up-pressure.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T23B2398C
- Keywords:
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- 1012 GEOCHEMISTRY / Reactions and phase equilibria;
- 3611 MINERALOGY AND PETROLOGY / Thermodynamics;
- 3660 MINERALOGY AND PETROLOGY / Metamorphic petrology;
- 8159 TECTONOPHYSICS / Rheology: crust and lithosphere