Unravelling Garnet Aggregate Microstructures: Multiple Nucleation and Coalescence Versus Brittle Deformation

Abstract

Very particular garnet porphyroblast microstructures relating to a growth process that implies multiple nucleation plus grain-boundary energy minimisation driven rotation of individual garnet grains during coalescence to a single crystal (Spiess et al., 2001) have the capability to constrain the step limiting factors of reaction progress during metamorphism. Indeed, the degree of coalescence, and hence the type of microstructure that evolves by this process, relates to significantly different ratios of interface reaction rates vs. diffusion rates (Dobbs et al., 2003), and reasonably to differences in the metamorphic settings. The main importance of these coalescence microstructures might indeed be in the recognition of such interrelationships. However, similar garnet microstructures can also form by brittle deformation of a single porphyroblast, rather than by coalescence of an aggregate of individual grains. Because the implications from such deformation microstructures are totally different, there is need to unambiguously recognising their distinguishing features. We have analysed the microstructure of a garnet porphyroblast that has been deformed to an ellipsoidal aggregate of garnet grains during brittle deformation within a mylonite zone. We have used EBSD and OC to analyse the garnet substructures as well as the degree of crystallographic misorientation between adjacent grains, and we have used X-ray mapping plus quantitative EDS microchemical analysis to unravel any significant compositional zoning. The data we have obtained clearly show that coalescence microstructures are completely different from brittle deformation microstructures. Specifically, for the brittle deformed garnet porphyroblast we have observed that: 1) X-ray mapping does not reveal zoning patterns that support multiple nucleation and the setting up of growth rate differentials throughout the ellipsoidal porphyroblast domain; 2) OC images do not show substructures of microfaceted sub-domains that relate to impingement; 3) Uncorrelated EBSD data are statistically random, and the crystallographic orientation of adjacent grains do not show any tendency for a reduction in misorientation. In addition, our data also suggests that during brittle deformation fracturing of garnet to an aggregate of grains is controlled by the existing stress field, the distribution of quartz inclusions, and the crystallography of garnet. This latter aspect is also confirmed by the dispersion path of EBSD data plotted within pole figures. References: Spiess R., Peruzzo L., Prior D.J. and Wheeler J., 2001. Development of garnet porphyroblasts by multiple nucleation, coalescence and boundary driven rotations. Journal of Metamorphic Geology, 19, 269-290. Dobbs H. T., Peruzzo L., Seno F., Spiess R. and Prior D. J. 2003. Unravelling the Schneeberg garnet puzzle: a numerical model of multiple nucleation and coalescence. Contributions to Mineralogy and Petrology (published online first).