Disequilibrium in the Ross of Mull Contact Metamorphic Aureole, Scotland: a Consequence of Polymetamorphism

Abstract

The Ross of Mull pluton consists of granites and granodiorites and intrudes sediments previously metamorphosed at amphibolite facies. The high grade and coarse grain size of the protolith is responsible for a high degree of disequilibrium in many parts of the aureole and for some unusual textures. A band of metapelite contained coarse garnet, biotite and kyanite prior to intrusion, and developed a sequence of textures towards the pluton. In Zone I, garnet is rimmed by cordierite and new biotite. In Zone II, coarse kyanite grains are partly replaced by andalusite, indicating incomplete reaction. Coronas of cordierite þ muscovite around kyanite are due to reaction with biotite. In the highergrade parts of this zone there is complete replacement of kyanite and/or andalusite by muscovite and cordierite. Cordierite chemistry indicates that in Zone II the stable AFM assemblage (not attained) would have been cordierite þ biotite þ muscovite, without andalusite. The observed andalusite is therefore metastable. Garnet is unstable in Zone II, with regional garnets breaking down to cordierite, new biotite and plagioclase. In Zone III this breakdown is well advanced, and this zone marks the appearance of fibrolite and K-feldspar in the groundmass as a result of muscovite breakdown. Zone IV shows garnet with cordierite, biotite, sillimanite, K-feldspar and quartz. Some garnets are armoured by cordierite and are inferred to be relics. Others are euhedral with Mn-rich cores. For these, the reaction biotite þ sillimaniteþ quartz! garnetþ cordieriteþ K-feldsparþ melt is inferred. Using a petrogenetic grid based on the work of Pattison and Harte, pressure is estimated at 3 2 kbar, and temperature at the Zone II---III boundary at 650 C and in Zone IV as at least 750 C.