Disequilibrium thermal breakdown of staurolite: a natural example

Abstract

A partially melted pelitic xenolith, Wehr volcano, East Eifel, Germany, contains staurolite porphyroblasts pseudomorphed by a hercynite-rich assemblage that includes ferrogedrite, sillimanite-mullite(ss), quartz and siliceous, peraluminous glass, which can be schematically represented by the reaction, staurolite -> hercynite + gedrite + Al-silicate + quartz + melt. Minerals and glass of the pseudomorph are a disequilibrium breakdown assemblage that represents a time-temperature-transformation of staurolite during short-term heating and cooling. Microstructural evidence indicates the hercynite formed first together with melt, followed by ferrogedrite, and crystallization of Al-silicate and quartz from the melt. The absence of cordierite, the presence of ferrogedrite (a possible metastable intermediate phase to formation of almandine) in the staurolite breakdown assemblage and evidence for melting of muscovite + quartz and plagioclase (An(20)) + quartz in the xenolith indicates minimum pressure conditions of 0.35 GPa at ca. 665-700 degrees C with probable overstepping of staurolite stability by 40-95 degrees C. Such conditions were kinetically favourable for the formation of the metastable breakdown assemblage that was preserved by quenching on eruption.