Reaction textures in Al-rich metabasite; implication for metamorphic evolution of the eastern border of the Middle Austroalpine basement units

Al-rich metabasite from the Speik Complex in the Eastern Alps contains coexisting kyanite, staurolite, garnet and calcic amphibole. Compositional zoning and inclusions trails in minerals suggest a prograde metamorphic history to high-pressure amphibolite facies conditions. The formation of kyanite and staurolite was controlled by bulk rock composition, mainly by high Al content in some metabasites. Textural relations and thermodynamic calculations indicate isothermal decompression from 1.1–1.2 GPa/580 °C to 0.5 GPa/580 °C that resulted in formation of Ca-rich plagioclase and new amphibole replacing garnet and locally corundum substituting for clinozoisite and kyanite. The replacement of amphibole by biotite and chlorite and plagioclase by margarite, and the formation of prehnite and pumpellyite were due to isobaric cooling to very low-grade conditions.