METAMORPHIC P–T CONDITIONS AND RETROGRADE PATH OF THE HP BARROVIAN METAMORPHIC ZONES NEAR CAIRN LEUCHAN, SCOTLAND

Since Barrow (1893), the Barrovian metamorphic zones in the Caledonides of Scotland have become the world type locality for the medium-P/T metamorphic facies series. However, the peak P–T conditions of the Barrovian zone are open to question, as well as the much-quoted progressive P–T curve (e.g., Fettes et al., 1985; Baker, 1986; Phillips et al., 1999; Baxter et al., 2002; Vorhies and Ague, 2011).

We have examined the metamorphic P–T conditions and associated relations of the Barrovian zones near Glen Muick by focusing on the petrology and thermodynamics of rocks at Cairn Leuchan, where garnet-rich lenses or pods (garnets 50–95 wt.%) are included in garnet amphibolite in the highest grade zone (sillimanite–K-feldspar; Baker and Droop, 1983). The representative mineral assemblages in the garnet-rich lenses (garnetite) and garnet-amphibolite are Grt + Qtz + Cpx + Pl + Amp ± Ep and Grt + Amp + Qtz + Pl ± Cpx ± Ep, respectively. The chemical compositions of constituent minerals are the same in both the garnetites and garnet amphibolite. The metamorphic P–T conditions of these rocks were estimated by thermodynamic calculations using THERMOCALC ver. 3.3.3 (Holland and Powell, 1998) and PERPREX ver. 6.6.6 (Connolly, 2005). These results show that the investigated rocks underwent high-pressure granulite facies metamorphism at P= ca. 13–14 kbar and T = ca. 770–800 °C. The pressure condition is higher than in previous works. Integration of our new results with published data suggests that the highest grade Barrovian zone was subducted to eclogite facies depths, much deeper than previously considered. The retrograde P–T path indicates isothermal decompression or temperature decrease with decompression. The position of these highest P–­T rocks with others in Glen Muick in the central Barrovian zone bordered symmetrically by lower P–T zones is consistent with exhumation and retrogression of an extruded wedge that gave rise to the main Grampian terrane of the Caledonide orogen.