ACCESSORY MINERAL TEXTURE DEVELOPMENT AS A FUNCTION OF LITHOLOGY: A CASE STUDY FROM THE BALLACHULISH IGNEOUS COMPLEX METAMORPHIC AUREOLE

Understanding the behavior and stability of accessory minerals in response to metamorphic processes is an important and rapidly developing field of petrology. Interest is driven, in part by developments in micro-analytical techniques, and in part by the increasing integration of P-T-t-X data from individual sub-domains in complexly zoned grains to reconstruct complex multi-stage metamorphic histories. The measurement of compositional and textural information is now well established. Less well constrained are the physical parameters that control the compositional evolution of common accessory phases such as zircon and monazite, and in particular, the role, nature and composition of metamorphic fluids. The Ballachulish Igneous complex of the Scottish Highlands is surrounded by steeply-dipping metasediments which strike sub-perpendicular to the northeast and southwest pluton margins. The contact relationships result in a metamorphic profile where mineral behavior may be studied as a function of rock and fluid composition along T profiles. The textural evolution of detrital zircon, monazite and allanite in metacarbonate, calcsilicate, quartzite and metapelite from the contact aureole are described. Textures in metacarbonates include the preservation of oscillatory zoning and evidence of grain boundary dissolution. In quartzite, sub-rounded zircon grains have metamorphic overgrowths, while a LREE mineral (possibly allanite) has a relict/dendtritic texture indicating dissolution. The absence of new mineral growth suggests transportation of elements away from the point of mineral dissolution. In mica-schist and slate samples, LREE-silicate minerals show extensive evidence of dissolution with no apparent evidence of new mineral growth. Zircon displays evidence of new metamorphic overgrowth on preserved cores. When compared to previous studies and experimental data, it is proposed that lithologies where high concentrations of "strong" acid anion species may be present, dissolution of accessory minerals and transportation of REEs is favored. In lithologies with more oxidizing conditions and evidence feldspar recrystallization, dissolution, new mineral growth and/or overgrowth textures in accessory minerals are more common.