Chemical and textural equilibration of garnet during amphibolite-facies metamorphism: The influence of coupled dissolution-reprecipitation

Metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase. Relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low temperature amphibolite-facies metamorphic reactions. Garnet is widely assumed to be a reactant in staurolite-isograd reactions, and the evidence for this has been assessed in the Late Proterozoic Dalradian pelitic schists of the Scottish Highlands. Three-D imaging of garnet porphyroblasts in staurolite-bearing schists reveal a good crystal shape and little evidence of marginal dissolution, however there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction. Staurolite forms directly adjacent to the garnet, and its nucleation is strongly associated with deformation of the muscovite-rich fabrics around the porphyroblasts. “Cloudy” fluid inclusion-rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet. Such cloudy garnet typically has a Mg-rich, Mn-poor composition and is interpreted to have formed during a coupled dissolution-reprecipitation process, triggered by a local influx of fluid. All garnet in the muscovite-bearing schists present in this area is potentially reactive, irrespective of the garnet composition, but very few of the schists contain staurolite. The staurolite-producing reaction appears to be substantially overstepped during the relatively high pressure Barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions. Fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history. The remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning. Such zoning in metamorphic garnet created during coupled dissolution-reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re-equilibration. Fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists. In addition the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history. However the partial re-equilibration of the porphyroblasts during coupled dissolution-reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions

The partial equilibration of garnet porphyroblasts in pelitic schists and its control on prograde metamorphism, Glen Roy, Scotland

Garnet porphyroblasts in sillimanite-bearing pelitic schists contain complex textural and compositional zoning, with considerable variation both within and between adjacent samples. The sillimanite-bearing schists locally occur in regional Barrovian garnet zone assemblages and are indicative of a persistent lack of equilibrium during prograde metamorphism. Garnet in these Dalradian rocks from the Scottish Highlands preserves evidence of a range of metamorphic responses including initial growth and patchy coupled dissolution- reprecipitation followed by partial dissolution. Individual porphyroblasts each have a unique and variable response to prograde metamorphism and garnet with mainly flat compositional profiles co-exists with those containing largely unmodified characteristic bell-shaped Mn-profiles. This highlights the need for caution in applying traditional interpretations of effective volume diffusion eliminating compositional variation. Cloudy garnet with abundant fluid inclusions is produced during incomplete modification of the initial porphyroblasts and these porous garnet are then particularly prone to partial replacement in sillimanite-producing reactions. The modification of garnet via a dissolution-reprecipitation process releases Ca into the effective whole rock composition, displacing the pressure-temperature positions of subsequent isograd reactions. This represents the first report of internal metasomatism controlling reaction pathways. The behaviour of garnet highlights the importance of kinetic factors, especially deformation and fluids, in controlling reaction progress and how the resulting variability influences subsequent prograde history. The lack of a consistent metamorphic response, within and between adjacent schists, suggests that on both local and regional scales these rocks have largely not equilibrated at peak metamorphic conditions

Late-magmatic mineral assemblages with siderite and zirconian pyroxene and amphibole in the anorogenic Mt Gibraltar microsyenite, New South Wales, Australia, and their petrological implications

The Mt Gibraltar intrusion near Mittagong and Bowral in New South Wales, Australia (lat. 34°27′54″S, long. 150°25′44″) is a small intrusive body of hypersolvus microsyenite emplaced into the Triassic Hawkesbury Sandstone of the Sydney Basin in Jurassic time, possibly related to extensional faulting. The rock itself consists of intermediate alkali feldspar with minor titanomagnetite and interstitial pyroxene ranging from nearly pure hedenbergite to ≈ Hd34Aeg65 in composition. It is crosscut by an irregular system of late-magmatic veins consisting of homogeneous alkali feldspar (≈ Ab50Or50), clinopyroxene evolving from sodic hedenbergite to zirconium-rich aegirine, arfvedsonite andsiderite. During postmagmatic evolution of the veins, microcrystalline or amorphous silica precipitated together with calcite filling miarolitic cavities. The late-magmaticmineralassemblage of the veins indicate crystallisation (at assumed 700 bar pressure) at T = 650–670 °C, log fO2 = − 22. This corresponds to conditions very close to the magnetite–wustite curve. Zirconium-bearing pyroxene has a restricted stability field in the system SiO2–ZrO2–FeO–FeO1.5–NaO0.5–HO0.5, at moderately elevated peralkalinity and intermediate silica activity. Under such conditions, pyroxeneand amphibole will act as effective sinks for Zr, preventing crystallisation of magmatic zircon or more exotic Zr silicates. The Mt Gibraltar microsyenite is therefore a rare example of an igneous rock in which zirconium is camouflaged in pyroxeneand amphibole rather than forming its own minerals