NEW AND CONVENTIONAL APPROACHES: BARROVIAN METAMORPHISM FROM THE TYPE LOCALITY, SCOTLAND

In this study, new and conventional petrological methods are used and compared to determine P–T paths, including peak metamorphic conditions, for rocks from the classic Barrovian section in Glen Esk, Scotland. Pseudosections and composition isopleths were calculated in the MnNCKFMASHTO system using the Theriak-Domino program (de Capitani & Petrakakis, 2010) and the Holland and Powell (1998) database. The constructed pseudosections for garnet-, staurolite-, and sillimanite-grade rocks show that the P–T conditions of the garnet core composition isopleths lie close to the equilibrium garnet-in reaction. Geothermometry (Ferry & Spear, 1978) gives a temperature range for the peak metamorphic conditions that is consistent with the observed mineral assemblage and data from Vorhies & Ague (2011) for the garnet and staurolite zones whereas the geobarometry gives values that are approx. 1kbar lower than those from Vorhies & Ague (2011). Geothermobarometry on rocks from the sillimanite zone are consistent with data from both Vorhies & Argue (2011) and Dempster (1985).

QuiG (quartz-in-garnet) barometry was used to determine the pressure of quartz entrapment, and hence of garnet growth, in rocks from the three zones. All measured entrapment pressures were higher by 1–4kbar than pressures inferred from either the equilibrium pseudosections or thermobarometry. For example for the garnet zone, the measured entrapment pressure is around 6.5–7.5kbar compared to 3–4kbar from thermobarometry and equilibrium pseudosections. Furthermore, the entrapment pressures for garnet formation based on intersecting isopleths from the garnet core composition, indicating significant overstepping of the garnet-in reaction. Most importantly, QuiG barometry from the sillimanite zone sample is consistent with the occurrence of early kyanite and trace rutile in the sample, but inconsistent with the P–T path inferred from thermobarometry or the equilibrium pseudosection. The P–T history inferred from QuiG barometry and textural analysis is also inconsistent with a T–t history whereby the peak metamorphic temperatures in the entire terrane were caused by a thermal overprint from nearby plutons, although it is entirely consistent with late sillimanite having formed in response to a thermal pulse from these plutons.