0000000000073731
AUTHOR
Roger Powell
On the interpretation of retrograde reaction textures in granulite facies rocks
Retrograde reaction textures, such as partial pseudomorphs, coronae and symplectic coronae are a relatively common feature of granulite facies rocks and represent evidence of change of mineral modes or assemblages. Thus, such reaction textures represent one of the more potentially useful indicators of retrograde P-T evolutions. However, the interpretation of such textures requires the simultaneous consideration of the formation of the texture itself in terms of the spatial relationships in a rock, and of the changes in P-T conditions that drove the development of the reaction texture. Calculated phase diagrams provide a robust framework in which to consider reaction textures, especially via…
Using calculated chemical potential relationships to account for replacement of kyanite by symplectite in high pressure granulites
Calculated mineral equilibria are used to account for the formation of sapphirine-plagioclase, spinel-plagioclase and corundum-plagioclase symplectites replacing kyanite in quartz-plagioclase-garnet-kyanite granulite facies gneisses from the Southern Domain of the Athabasca granulite terrane, a segment of the Snowbird tectonic zone in northern Saskatchewan, Canada. Metamorphic conditions of >14 kbar and 800 °C are established for the high pressure, garnet-kyanite assemblage using constraints from P-T pseudosections and Zr-in-rutile thermometry. Replacement of kyanite by symplectites reflects the reaction of kyanite with the matrix following near-isothermal decompression to <10 kbar. The che…
Quantitative phase petrology of cordierite-orthoamphibole gneisses and related rocks
Cordierite-orthoamphibole gneisses and rocks of similar composition commonly contain low-variance mineral assemblages that can provide useful information about the metamorphic evolution of a terrane. New calculated petrogenetic grids and pseudosections are presented in the FeO-MgO-Al2O3-SiO2-H2O (FMASH), Na2O-CaO-K2O-FeO-MgO-Al2 O3- SiO2-H2O (NCKFMASH) and Na2O-CaO-K2O-FeO-MgO-Al2O3- SiO2-H2O-TiO2-Fe2 O3 (NCKFMASHTO) chemical systems to investigate quantitatively the phase relations in these rocks. Although the bulk compositions of cordierite-orthoamphibole gneisses are close to FMASH, calculations in this system do not adequately account for the observed range of mineral assemblages. Calcu…
High-grade metamorphism and partial melting of basic and intermediate rocks
Rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust. However, the former lack of appropriate thermodynamic models for silicate melt, amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite- and granulite facies metamorphism. In this work, we present phase diagrams calculated in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) compositional system for a range of natural basic and interm…
Activity-composition relations for the calculation of partial melting equilibria in metabasic rocks
A set of thermodynamic models is presented that, for the first time, allows partial melting equilibria to be calculated for metabasic rocks. The models consist of new activity–composition relations combined with end-member thermodynamic properties from the Holland & Powell dataset, version 6. They allow for forward modelling in the system Na (Formula presented.) O–CaO–K (Formula presented.) O–FeO–MgO–Al (Formula presented.) O (Formula presented.) –SiO (Formula presented.) –H (Formula presented.) O–TiO (Formula presented.) –Fe (Formula presented.) O (Formula presented.). In particular, new activity–composition relations are presented for silicate melt of broadly trondhjemitic–tonalitic compo…
Garnet and spinel lherzolite assemblages in MgO-Al2O3-SiO2 and CaO-MgO-Al2O3-SiO2: thermodynamic models and an experimental conflict
The recent publication of an updated thermodynamic dataset for petrological calculations provides an opportunity to illustrate the relationship between experimental data and the dataset, in the context of a new set of activity–composition models for several key minerals. These models represent orthopyroxene, clinopyroxene and garnet in the system CaO–MgO–Al2O3–SiO2 (CMAS), and are valid up to 50 kbar and at least 1800 °C; they are the first high-temperature models for these phases to be developed for the Holland & Powell dataset. The models are calibrated with reference to phase-relation data in the subsystems CaO–MgO–SiO2 (CMS) and MgO–Al2O3–SiO2 (MAS), and will themselves form the basis o…
Using calculated chemical potential relationships to account for coronas around kyanite: an example from the Bohemian Massif
Corona textures around kyanite, involving for example zoned plagioclase separating kyanite from the matrix, reflect the instability of kyanite with the matrix on changing P-T conditions, commonly related to decompression. The chemical potential gradients set up between the kyanite and the matrix as a consequence of slow Al diffusion drive corona development, with the zoning of the plagioclase reflecting the gradients. Calculated mineral equilibria are used to account for corona textures involving plagioclase ± garnet around kyanite, and replacement of kyanite by plagioclase + spinel symplectite, in quartz + plagioclase + K-feldspar + garnet + kyanite granulite facies gneiss from the Blanský…
Partial melting of metagreywacke: a calculated mineral equilibria study
Greywacke occurs in most regionally metamorphosed orogenic terranes, with depositional ages from Archean to recent. It is commonly the dominant siliciclastic rock type, many times more abundant than pelite. Using calculated pseudosections in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system, the partial melting of metagreywacke is investigated using several natural protolith compositions that reflect the main observed compositional variations. At conditions appropriate for regional metamorphism at mid-crustal depths (6–8 kbar), high-T subsolidus assemblages are dominated by quartz, plagioclase and biotite with minor garnet, orthoamphibole, sillimanite, muscovite and/or K-feldspar (±Fe–T…
On equilibrium in non-hydrostatic metamorphic systems
Metamorphic geology has accumulated a huge body of observation on mineral assemblages that reveal strong patterns in occurrence, summarised for example in the idea of metamorphic facies. On the realisation that such patterns needed a simple explanation, there has been considerable a posteriori success from adopting the idea that equilibrium thermodynamics can be used on mineral assemblages to make sense of the patterns in terms of, for example, the pressure and temperature of formation of mineral assemblages. In doing so, a particularly simple implicit assumption is made, that mineral assemblages operate essentially hydrostatically. Structural geologists have studied the same rocks for diff…
Retrograde melt-residue interaction and the formation of near-anhydrous leucosomes in migmatites
Considering physical segregation of melt from its residue, the chemical potentials of the components (oxides) are the same in both when segregation occurs. Then, as P–T conditions change, gradients in chemical potential are established between the melt-rich domains and residue permitting diffusional interaction to occur. In particular, on cooling, the chemical potential of H2O becomes higher in the melt segregation than in the residue, particularly when biotite becomes stable in the residue assemblage. Diffusion of water from the melt to the residue promotes crystallization of anhydrous products from the melt and hydrous products in the residue. This diffusive process, when coupled with mel…
The effect of Mn on mineral stability in metapelites revisited: new a -x relations for manganese-bearing minerals
The a-x relations recently presented in White et al. (, Journal of Metamorphic Geology, 32, 261-286) are extended to include MnO. This provides a set of internally consistent a-x relations for metapelitic rocks in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (MnNCKFMASHTO) system. The mixing parameters for the Mn-bearing minerals were estimated using the micro-ϕ{symbol} approach of Powell et al. (, Journal of Metamorphic Geology, 32, 245-260). Then the Mn-end-member thermodynamic properties were calibrated using a database of co-existing minerals involving literature data from rocks and from experiments on natural materials. Mn-end-members were calibrated for orthopyroxene, cordierit…
Phase equilibria modelling of kyanite-bearing anatectic paragneisses from the central Grenville Province
Kyanite-bearing paragneisses from the Manicouagan Imbricate Zone and its footwall (high-P belt of the central Grenville Province) preserve evidence of partial melting with development of metamorphic textures involving biotite-garnet ± kyanite ± plagioclase ± K-feldspar-quartz. Garnet in these rocks displays a variety of zoning patterns with respect to Ca. Pseudosection modelling in the Na O-CaO-K O-FeO-MgO-Al O - SiO -H O-TiO -O (NCKFMASHTO) system using measured bulk rock compositions accounts for the textural evolution of two aluminous and two sub-aluminous samples from the presumed thermal peak to conditions at which retained melt solidified. The prograde features are best explained by p…
On parameterizing thermodynamic descriptions of minerals for petrological calculations
A new regularization approach, termed micro-ϕ, is outlined for parameterizing activity–composition (a–x) relations and other aspects of the thermodynamic descriptions of minerals for petrological calculations. In the context of the symmetric formalism, a formulation of a–x relations that is easily generalizable to multi-component minerals, parameterization with micro-ϕ extends from where there are good data available to constrain, for example, interaction energies, to where there are little or no data. This involves decomposing the interaction energies, which are macroscopic between end-members, into their microscopic components involving interactions between elements on sites. Micro-ϕ invo…
New mineral activity-composition relations for thermodynamic calculations in metapelitic systems
New activity–composition (a–x) relations for minerals commonly occurring in metapelites are presented for use with the internally consistent thermodynamic dataset of Holland & Powell (2011, Journal of Metamorphic Geology, 29, 333–383). The a–x relations include a broader consideration of Fe2O3 in minerals, changes to the formalism of several phases and order–disorder in all ferromagnesian minerals where Fe–Mg mixing occurs on multiple sites. The a–x relations for chlorite, biotite, garnet, chloritoid, staurolite, cordierite, orthopyroxene, muscovite, paragonite and margarite have been substantially reparameterized using the approach outlined in the companion paper in this issue. For the fir…