When deformed together in nature, calcite rocks invariably appear weaker and more ductile than quartz rocks. This can be reconciled with experimental flow data only by taking into account grain-size-sensitive (GSS) flow in calcite rocks, which is predicted to dominate even at grain sizes on the order of 1mm at middle metamorphic grades. Using new experimental data that demonstrate the transition between intracrystalline plastic and GSS flow of quartz rocks, we predict that unnaturally small grain sizes at temperatures of 700°C or higher are required for GSS flow of quartz in nature. Thus natural flow of quartzite is expected to occur by intracrystalline plastic processes, even after recrystallization to a fine grain size.

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