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+Vor   Mu+Cd=Q+Bi+As+V
 
Mu+Q=As+Kf+V
 
{Q+Bi+As=Cd+Kf+Vor   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 K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH).

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