Crystal size distribution (CSD) and textural evolution of accessory apatite, titanite and allanite during four stages of metamorphism: An example from the Moine Supergroup, Scotland

Abstract

Crystal size distributions (CSD) and shapes of accessory apatite, titanite and allanite are investigated in three texturally distinct garnet zones (Z1-Z3) and in the matrix (Z4) of a garnet-epidote- biotite gneiss from the Moine Supergroup. Additionally, textural relationships and interactions between the accessory minerals and surrounding rock-forming minerals are considered, results of numerical CSD modelling are presented, and geochronological and geological consequences of the inferred CSD evolutions are discussed. Textures and CSDs indicate that the accessory minerals were in, or near, a stage of nucleation and initial growth immediately prior to garnet Z1 over-growth, and formed within less than 20 000 years, either by a size-independent or size-dependent growth mechanism. Subsequently, the CSDs were modified by different growth mechanisms, as supported by several parameters including CSDs, grain numbers, grain sizes, specific volumes and others. The apatite CSD evolution from Z1 to Z4 is consistent with open-system LPE (Law of Proportionate Effects) growth accompanied and followed by supply controlled random ripening, whereas transformation of the original titanite CSD is more consistent with Ostwald ripening, temporarily accompanied by positive or negative McCabe growth. The allanite CSDs also point to Ostwald ripening between Z3 and Z4. The textural observations indicate that the growth evolution of the accessory phases was influenced by mineral reactions with surrounding rock-forming minerals, as well as by deformation and matrix coarsening, in a manner similar to that found in more simple ceramic systems. The observed textures require a successive temperature rise throughout the tectono-metamorphic evolution of the investigated rock, in agreement with existing P-T data. Fast nucleation and initial growth of the accessory minerals during Z1 was perhaps initiated by contact metamorphism, whereas subsequent growth and annealing (Z2-Z4) result from regional metamorphic events. © Oxford University Press 2004; all rights reserved.