Abstract
AbstractThe response of garnet and zircon to prograde amphibolite-facies metamorphism in late Proterozoic mica schists from the Scottish Highlands has been investigated. Spatial analysis of zircon populations using scanning electron microscopy was undertaken in Dalradian Schists that have undergone a sequence of prograde garnet growth and localised breakdown reactions involving coupled dissolution–reprecipitation. Fluid availability and matrix permeability strongly control this metamorphic response and different generations of garnet contain radically different populations of metamorphic micro-zircon and associated changes in the detrital zircon population. Micro-zircon abundance increases during garnet growth, whereas that of detrital zircon decreases. The mineralogy of the matrix influences zircon abundance in porphyroblast phases, where garnet overgrows a micaceous matrix zircon-rich garnet forms and where it overgrows a quartzofeldspathic matrix the result is zircon-poor garnet. Following garnet growth, micro-zircon abundance decreases at each stage of the prograde reaction history, with sillimanite-zone schists containing the lowest abundance, suggesting micro-zircons are texturally less stable at staurolite- and sillimanite-grade metamorphism. Micro-zircons are distributed evenly across host minerals in the matrix, with the exception of retrograde chlorite where micro-zircons are absent due to fluids removing Zr before new zircon can precipitate. There is an overall decrease in the mode of zircon at each stage of the reaction history, indicating that zircon is a highly reactive phase during amphibolite-facies metamorphism and is very sensitive to individual prograde and retrograde reactions.
Highlights
Dissolution-reprecipitation occurs when a fluid comes into contact with a mineral with which it is undersaturated, resulting in the dissolution of the less stable parent phase and reprecipitation of a more stable daughter phase (Putnis & Putnis, 2007; Putnis, 2009; Ruiz-Agudo et al, 2014; Altree-Williams et al, 2015; Konrad-Schmolke et al, 2018)
Garnet is characterised by sluggish volume diffusion at amphibolite facies and may record changing P-T-X during growth and post-growth modification (e.g. Jiang & Lasaga, 1990; Spear, 1991; Raimondo et al, 2017)
Three polished sections were selected for analysis due to the varying proportions of staurolite and cloudy garnet
Summary
Dissolution-reprecipitation occurs when a fluid comes into contact with a mineral with which it is undersaturated, resulting in the dissolution of the less stable parent phase and reprecipitation of a more stable daughter phase (Putnis & Putnis, 2007; Putnis, 2009; Ruiz-Agudo et al, 2014; Altree-Williams et al, 2015; Konrad-Schmolke et al, 2018). Garnet in these schists has been studied previously (Dempster et al, 2017; 2019), and the garnet characteristics defined in these studies has been used as a context for understanding the reaction history. Primary clear garnet is inclusion-poor and represents essentially unmodified porphyroblasts, whereas cloudy garnet locally develops, containing abundant fluid inclusions has been texturally and chemically modified by CDR (Martin et al, 2011; Dempster et al., 2017). Garnet is one of the most studied minerals in metamorphic petrology (Hollister, 1966; Yardley, 1977; Dempster, 1985; Hames & Menard, 1993; Carlson, 2006; Baxter et al, 2017), making it the perfect canvas to understand the behaviour of zircon during metamorphism
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