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[1] Burial histories of subduction zone rocks are often difficult to accurately constrain, owing to a lack of robust mineral geobarometers applicable to high pressure mineral assemblages. Knowledge of the depthhistories of subduction is, however, required for our understanding of global geochemical cycles, subduction-related seismicity, and the evolution of destructive tectonic boundaries. The high spatial resolution of quartz inclusion geobarometry can be used to determine pressure evolution during metamorphic growth of individual garnet crystals. Quartz inclusions in garnet from Sifnos, Greece, preserve such a record of the pressure of garnet growth, allowing detailed reconstruction of the metamorphic evolution of these rocks. Pressure-dependent Raman spectra of quartz inclusions were combined with elastic modeling to infer the conditions at which they were trapped during garnet growth. All measured inclusions suggest that garnet growth occurred between 19 and 20.5 kbars, with little evidence for significant pressure variation during the garnet growth interval, which is interpreted to record $100 C of heating. Coupled with thermometry and geochronology, these results show that early, cold burial was followed by a phase of rapid heating, which immediately preceded exhumation. Garnet growth occurred primarily during this heating phase.Components: 6,458 words, 5 figures, 1 table.

resorbed crystal rims). In general, such elements would be those that tend to be strongly partitioned into garnet over other matrix phases including Mn, Y, Lu and other HREE (e.g., Kohn and Spear 2000; Kelly et al. 2011; Gatewood et al. 2015). In some cases, secondary minerals may form at a garnet resorption surface driven by the sudden and proximal influx of those particular elements. Xenotime, for example, has been observed decorating the resorbed surfaces of garnets where localized flux of Y (from the resorbed garnet) promotes growth of xenotime (e.g., Gatewood et al. 2015). In this case, there is a useful textural link between resorbed garnet surface and neocrystallized accessory phase xenotime. Textural evidence for polymetamorphism. We use the term 'polymetamorphic garnet' to describe crystals that grew during more than one tectonic 'event' separated by a significant hiatus that can be resolved texturally, chemically, or temporally with existing methods. Several examples of polymetamorphic garnet have been recognized in the Alpine-Himalayan system that grew during multiple orogenic cycles, separated by millions to hundreds of millions of years. Argles et al. (1999) dated fractions of garnet crystals, identifying that only crystal rims record Tertiary metamorphism, with crystal cores hundreds of millions of years older. Gaidies et al. (2006), Herwartz et al. (2011), Robyr et al. (2014) and (Lanari et al. 2017) recognized, garnet zonation textures in which Alpine overgrowths are vividly separated by prominent microstructural and chemical discontinuities from older Variscan cores. Manzotti and Ballevre (2013) recognized chemically heterogeneous detrital garnet cores-derived from multiple sources-that were overgrown by a homogeneous Alpine garnet rim. All of these examples serve to illustrate some of the textural and chemical means by which polymetamorphic garnet may be recognized. Of course, only by adding direct geochronology to each of these growth generations may we establish the absolute chronology and length of the hiatus between phases of garnet growth. Porphyroblast nucleation and growth models. Metamorphic porphyroblastic minerals comprise vivid records of progressive nucleation and growth kinetics. Treated individually, each porphyroblast may record aspects of the rock's overall history colored by local chemicaltextural features. Taken as a whole, porphyroblast crystal size distributions, their relative spatial disposition, and chemical zonation reveal information about the nucleation and growth process rock wide (e.g.