title: Controls on zircon dissolution and crystallization during metamorphism
Creators: name: McElhinney, Tara R.
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: garnet, zircon, metamorphism, scottish highlands, Leven Schists, Appin Phyllites, polymetamorphosed, microzircon, dissolution-reprecipitation, coupled dissolution-reprecipitation, cloudy garnet.

description: The response of zircon to prograde metamorphic reactions has been investigated in late Proterozoic metapelites from the Scottish Highlands. Samples from rocks undergoing greenschist and amphibolite facies metamorphism record changes in the zircon populations associated with specific stages in the reaction history. Appin Phyllites from Onich record changing zircon distribution and morphology across three distinct events, an early regional event is recorded in the matrix, a later contact event produces a population of zircon within biotite porphyroblasts, and finally partial retrogression of biotite produces chlorite. Leven Schists from Glen Roy record a progressive sequence of metamorphism from garnet zone to staurolite zone to sillimanite zone. Dissolution-reprecipitation occurs at each stage of the reaction history in the Leven Schists and produces modified garnet which can be recognised based on the resulting change to the texture and chemistry of product garnet. Within the Leven Schists, two distinct stages of modification can be recognised; the formation of cloudy garnet during staurolite growth and the formation of secondary clear garnet during sillimanite growth. During staurolite formation, dissolution-reprecipitation partially replaces garnet with large, irregular quartz inclusions and small &lt;1 μm rounded fluid inclusions. Additionally, the original concentric growth zoning is modified producing localised low pyrope zones of cloudy garnet and releasing Ca into the rock. The products of dissolution-reprecipitation are more reactive than clear garnet owing to the high defect density within cloudy garnet. The release of Ca from dissolution-reprecipitation also moves the sillimanite isograd to lower P-T space. As a result, the higher the proportion of cloudy garnet formed during staurolite formation, the more likely the rocks will react to form sillimanite. Sillimanite formation results in the formation of reequilibrated secondary clear garnet, where fluid inclusions are eradicated and a low Ca, high Mg product garnet forms. Garnet therefore records three identifiable stages in its temporal evolution; (1) clear, unmodified garnet, (2) cloudy garnet formed via dissolution-reprecipitation during staurolite formation, and (3) secondary clear garnet, reequilibrated during sillimanite formation. Analysis of zircon within the matrix and biotite porphyroblasts in the Appin Phyllites, and within garnet in the Leven Schists enable an understanding of how zircon populations change through metamorphism. Within both the Appin Phyllites and Leven Schists zircon is present in two distinct populations; (a) inherited detrital zircon, and (b) authigenic microzircon. Microzircon are fine &lt;1 μm2, euhedral zircon that display no internal structure or zonation. Microzircon appear to be reactive in the presence of fluids and thus have the ability to trace fluid-mediated metamorphic processes. By analysing zircon populations in garnet in the Leven Schists, the stages in temporal evolution of garnet can be utilised to understand temporal changes to the morphology and distribution of zircon. Within clear garnet, zircon are heterogeneously distributed, the primary influence on the abundance of zircon is the mineralogy of the matrix. Micaceous matrix, and the garnet that overgrows it, contains more microzircon than quartz-rich matrix. Following dissolution-reprecipitation, detrital zircon and particularly microzircon decrease in abundance. Additionally, microzircon become finer and are more frequently hosted within fluid and mineral inclusions, suggesting these populations formed during dissolution-reprecipitation. Within sillimanite-zone schists the secondary clear garnet contains the lowest number of microzircon. The Appin Phyllites record a much higher abundance of microzircon in later formed contact porphyroblast phases than within the earlier formed regional matrix. Both the Leven Schists and Appin Phyllites record an absence of microzircon within chlorite-retrogressed domains suggesting that effective microzircon dissolution occurs during chloritization but there is no new zircon crystallization. This is the first comprehensive study of prograde zircon evolution, recording consistent changes to the distribution and morphology of zircon from lower greenschist Appin Phyllites to mid-amphibolite facies Leven Schists. These changes present the opportunity to trace metamorphic processes in complex polymetamorphic rocks where dating presently proves near impossible.