TEXTURAL ANALYSIS AND EPMA GEOCHRONOLOGY OF MONAZITE IN METAPELITES FROM THE THERMAL AUREOLE OF THE BALLACHULISH IGNEOUS COMPLEX, SCOTLAND

Monazite with a variety of textures and forms is documented in metapelites in the thermal aureole of the Ballachulish Igneous Complex. Grains were characterized by BSE imaging and element mapping, and based on textural complexity, analyzed by EMPA.

Monazite from Leven Schist show little variability in the size and shape along the temperature gradient. Compositional mapping show multiple textures that include patchy zoning, curvilinear domain boundaries, and core-mantle-rim textures. Samples from the lower temperature samples in the Appin Phyllite are small, discrete and rounded in form. With increasing temperature some grains become slightly larger, are found in clusters, and a bimodal size distribution develops. In the Creran Succession, at increasing temperatures, the occurrence of monazite clusters decreases and the few grains that occur display dissolution textures on grain edges.

Comparison of textures to experimental work products, suggests that the decrease in grain size and dissolution textures in the Creran Succession is compatible with the presence of a fluid with hard anionic ligands present (e.g. S, F). Meanwhile the textures in the Appin Phyllite and Leven Schist are similar to those predicted by dissolution-reprecipitation reactions in the presence of more alkaline fluids. This is compatible with models that proposed metamorphic fluids in the Creran Succession were produced by devolitization reactions, while those in the Appin Phyllite and Leven Schist were mixtures of metamorphic and igneous derived fluids.

EPMA trace element analysis of four samples from the Levin Schist led to the identification of four age populations: 467 ± 4 Ma, 450 ± 3 Ma, 431 ± 3 Ma, and 414 ± 2 Ma. By comparison with the literature data the 467 ± 4 Ma population overlaps with the proposed age of the Grampian event, while the 450 Ma dates lie within the proposed age range of the Moine event. The Ballachulish Igneous Complex and its associated thermal metamorphism are dated between 421-427 Ma. The 431 and 414 Ma populations are most probably correlated with emplacement of the Ballachulish Complex, and subsequent hydrothermal circulation and metasomatism and during cooling, which is compatible with textures observed in monazite.