A petrogenetic grid for pelites in the Ballachulish and other Scottish thermal aureoles

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Abstract
The Ballachulish ‘Granite’ is a composite Devonian intrusive complex surrounded by a distinctive thermal aureole developed in regionally deformed and metamorphosed Dalradian sediments on the W coast of Scotland. The most abundant rocks in the aureole are pelites, which show a progression of assemblages from quartz–muscovite–chlorite up to a variety of high-grade assemblages involving combinations of cordierite, corundum, spinel and, rarely, hypersthene and garnet. Metamorphic zones have been mapped around the granite, which are defined by the following reactions going upgrade: Mu + Chl + Q = Cd + Bi + V { Mu + Bi + Q = Cd + Kf + V or     Mu + Cd = Q + Bi + As + V Mu + Q = As + Kf + V { Q + Bi + As = Cd + Kf + V or     Mu + Cd = Bi + As + Kf + V Mu = Cor + Kf + V Bi + As = Cor + Kf + Cd + V The restricted occurrences of assemblages involving spinel, hypersthene and garnet do not allow higher grade zones to be mapped. Variations in the reaction sequence as a consequence of bulk compositional factors, in particular the development of quartz-bearing versus quartz-absent assemblages, are described. Details of the mineral assemblages from Ballachulish are combined with high-grade assemblage data from the Belhelvie, Lochnagar and Comrie aureoles to construct a comprehensive schematic petrogenetic grid. The grid involves the minerals quartz, chlorite, muscovite, biotite, cordierite, alumino-silicate, K-feldspar, corundum, spinel, hypersthene and garnet, whose assemblage relationships are modelled in the system K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O (KFMASH).