PHASE EQUILIBRIA MODELLING OF BARROVIAN METAMORPHISM FROM THE TYPE LOCALITY, SCOTLAND

The metamorphic evolution of the metamorphosed pelitic and semi–pelitic rocks from the classic Barrovian section in Glen Esk, Scotland, is re–examined via phase equilibria modelling in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O model system using the software THERMOCALC v.3.37 (Powell & Holland, 1988), the thermodynamic dataset of Holland & Powell (2011) and entirely new thermodynamic models for metapelitic minerals (White et al., 2014). This study is the first application of these models to rocks. The constructed P–T pseudosections show that peak metamorphic conditions for biotite-, garnet-, staurolite-, and kyanite-grade units lie along the same linear piezothermal array, whereas peak sillimanite-grade conditions occur at lower P. Microstructural evidence in conjunction with published age data constrains sillimanite-grade metamorphism to have occurred after higher-Pkyanite-grade conditions. This is interpreted to record a sillimanite-grade contact metamorphic overprint, attributed to post-peak (but syn-orogenic) magmatism, on a regional-scale background metamorphic sequence that formed in response to arc-continental collision and crustal loading.

Further, the staurolite and sillimanite isograds are only weakly affected by bulk rock variations, whereas the garnet isograd is more strongly affected. The formation of garnet is strongly dependent on the bulk rock composition, especially the MnO content with the most MnO-rich rocks typically having garnet stability down to lower T. For the lowest MnO content (0.07 mol%) the garnet isograd is stable at 5 kbar and 575 °C and for the highest MnO content (0.2 mol%) the garnet isograd is stable at 5 kbar and 500 °C. Further, the staurolite isograd is more related to a low variance reaction, which is basically grt + chl = bi + st, and these low variance fields are less strongly affected by bulk rock composition.