Mineral Equilibria Modeling of the Granulite–Eclogite Transition: Effects of Whole-Rock Composition on Metamorphic Facies Type-Assemblages


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Abstract

P–T–X(composition) pseudosections constructed for natural 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. Increasing ferric iron content results in a marked contraction of garnet–plagioclase assemblages, and an expansion of orthopyroxene, kyanite, quartz and ilmenite stability across the compositional range of monzodioritic–gabbroic protoliths, and the expansion of plagioclase stability up-pressure in gabbroic–peridotgabbroic compositions. Omphacite granulite defines a transitional stage between garnet granulite and eclogite in monzodioritic to 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 modeling of intermediate and mafic equilibria involving more than a few per cent partial melt. Omphacite-bearing granulite mineral equilibria in dioritic protoliths are far less sensitive to changes in whole-rock oxidation state than gabbroic protoliths; a doubling of whole-rock oxygen content displaces the modeled granulite–eclogite transition in gabbroic assemblages by 0·4 GPa up-pressure. Results of low-H2O, high-O equilibria modeling best validate 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.

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