The Ti Record of Quartz in Anatectic Aluminous Granulites

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The distribution and concentration of Ti in quartz was assessed in five rutile-bearing anatectic aluminous granulites from the Grenville Province, Canada, each with previously constrained P–T conditions of metamorphism. Characterisation of quartz in these samples with the aid of cathodoluminescence (CL) mapping revealed two distinct types in each sample, resorbed quartz partly consumed by melting reactions during the prograde portion of metamorphism and quartz grown from crystallisation of partial melt during retrogression. In two of the samples, pseudomorphs after former melt in the form of quartz overgrowths on existing quartz were discovered using CL which are not visible by other methods. The P–T conditions recorded by each type based on Ti-in-quartz thermobarometry are in poor agreement with microstructures and P–T results inferred from phase equilibria modelling. The negative correlation between Ti incorporation into quartz and P suggests that prograde quartz, which is expected to record near-peak P–T, should be Ti-poor relative to retrograde quartz, which crystallised at conditions several kbar lower than peak P. The opposite trend was found; retrograde quartz is typically relatively Ti-poor and quartz overgrowths may be nearly Ti-free. Additionally, prograde quartz has variable Ti contents and fails to systematically record near peak P–T. Despite the presence of rutile in the samples, which is generally considered to ensure a Ti-saturated system, these results are strong evidence for Ti-undersaturation of quartz caused by disequilibrium between quartz and rutile. In addition, Zr-in-rutile thermometry generally gives lower temperatures than expected for these rocks, which can be explained by retrograde resetting of Zr in rutile and potentially by disequilibrium between rutile and zircon. Based on these results, the degree of Ti (or Zr) saturation achieved at any stage of metamorphism should be assessed with caution, as co-existence of quartz and rutile (or rutile and zircon) within a thin section or sample may not be a sufficient criterion for equilibrium.