Hercynite-quartz granulites: phase relations, and implications for crustal processes

Abstract

The assemblage Zn-poor hercynite + quartz, occurring with cordierite, garnet and/or sillimanite, is diagnostic of high-temperature granulite-facies metamorphism at mid-crustal depths. It has been recorded from a handful of terrains worldwide, in which peak temperatures probably exceeded 800°C, and pressures were typically in the range 4-7 kbar, lower than that required for the association sapphirine + quartz. Uncertainty in the absolute P-T conditions of hercynite-quartz rocks results from unresolved inconsistencies in experimental studies of cordierite equilibria, and from the difficulty of reconstructing the peak compositions of minerals, particularly of oxides, which generally show exsolution and corona textures. The coronas record information about the retrograde P-T path, while in some cases larger-scale mineral associations, such as hercynite-quartz aggregates replacing cordierite porphyroblasts, reveal part of the prograde history. Most hercynite-quartz terrains lack evidence for an earlier high-pressure history, and cooled isobarically from a very hot, transient geotherm. The most generally applicable models involve a combination of magmatic advection and lithospheric thinning. -from Author