The highest grade pelitic and semi-pelitic rocks of the Ballachulish contact aureole show structural and textural features indicative of partial melting. Three locations were selected for detailed petrographic study which show the development of boudinage and breccia structures, discrete leucosomes, and small-scale network features. One more extensively melted area (the Chaotic Zone) showed boudinage and breccia structures developed on a much larger scale. Rocks from the Chaotic Zone were divided into several groups: hornfels fragments, laminated metasediment, granulose metasediment, granitic material, and penetrative vein systems. The development of the above features is related to melting processes. Discrete leucosomes and small-scale network features in the less extensively melted rocks represent accumulations of melt which could not be contained along the grain boundaries in the host rock. In the Chaotic Zone large bodies of rock have melted to such a degree that the critical melt fraction was exceeded, and the rock behaved as a ductile body. The hydraulic fracturing responsible for formation of boudinage and breccia structures operates on a larger scale in this area due to the larger degrees of melting involved. An important control on the distribution of melt, and melt-related processes, is the original compositional layering of the rocks concerned. Throughout the melting history, movements of the melt phase are thought to have been minor and the orientation of original layering is often preserved, even after large degrees of melting. Quantitative study of the angular relationships between mineral grains showed that the approach to textural equilibrium in these rocks is poor. In rocks which contain abundant evidence of melting, non-equilibrium dihedral angles (in the range 50-60<SUP>o</SUP>) are commonly preserved which coincide with experimentally-derived wetting angles for granitic-type silicate liquids. In rocks where melting is thought to have been at a minimum, the approach to textural equilibrium is better. Dihedral angles measured are in the range 90<SUP>o</SUP> to 110<SUP>o</SUP>, which corresponds to expected values for equilibrium solid/solid angles between the phases present. A δ18O study of leucocratic segregations shows that in general, they have the same δ18O composition as the immediately adjacent matrix material, consistent with a local origin. Extensively melted rocks are coincident with large-scale oxygen isotope homogenisation. This indicates enhanced communication between rock layers and thus supports petrographic evidence suggesting the presence of an interconnected melt phase over large distance. Consideration of isotopic equilibration between the mixed grains in these rocks shows that late-stage fluid activity was at a minimum. A comparison of traditional methods of testing for equilibrium, and application of Giletti's model for slow cooling in a closed system showed that Giletti's model provided a closer approach to the observed δ18O compositions.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:653934 |
| Date | January 1990 |
| Creators | Linklater, Claire Marie |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/12429 |
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