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No AccessGeological Note

Low-Pressure Origin of High-Silica Rhyolites and Granites

High-silica rhyolites and granites (>75 wt% SiO2, anhydrous basis) are common features of the crust as part of both the volcanic and the plutonic records. While low crystallization pressure (<250 MPa) is typically inferred, it has been suggested that they form via polybaric evolution, with initial crystallization at relatively high pressures (>500 MPa). We use glass compositions derived from the EarthChem portal, selected natural examples from the literature, and rhyolite-MELTS calculations to show that the phase relations in the quartz-albite-orthoclase ternary dictate the silica content of silicic melts. In particular, we show that silica content of melts increases with decreasing pressure as a result of the displacement of the quartz-sanidine cotectic toward the Qz apex with decreasing pressure. It follows from our analysis that (1) the crust is expected to be stratified in terms of the silica content of residing melts; (2) high-silica glasses form at low pressure, requiring shallow-level crystallization, and preclude a polybaric evolution for many systems (e.g., Bishop Tuff); (3) the existence of high-silica pumice requires fractionation (or melting) at low pressure, showing that high-silica rhyolites are intrinsic to the shallow crust; and (4) low-pressure cumulates (or melting residues) must exist in the shallow crust, weighing in favor of the cumulatic nature of many granitoids found in plutons.