Pyrometamorphic breakdown of cordierite-muscovite intergrowths

摘要

Backscattered electron imaging of cordierite-muscovite intergrowths near the margin of a schist xenolith in trachytic tephra of the Wehr Volcano, East Eifel area, Germany, show that they have undergone disequilibrium melting to form spinel + biotite + sillimanite/?mullite + peraluminous melt. Textures indicate that in the initial stage of reaction, spinel, biotite and sillimanite/?mullite formed with melt produced in cordierite and spinel + ?mullite with melt after muscovite, providing evidence of two different reaction pathways. Fe-Mg distributions between biotite/spinel, cordierite/spinel, cordierite/ biotite suggest a melting temperature of ~770°C, implying overstepping of various calculated and experimentally determined mineral breakdown curves of between 30 and 170°C during short-term heating before the xenolith was quenched on eruption.