Formation of spinel-cordierite-plagioclase symplectites replacing andalusite in metapelitic migmatites of the Alvand aureole, Iran

Abstract

AbstractSpinel–cordierite–plagioclase symplectites partially replacing andalusite occur in the metapelitic migmatite rocks of the Alvand aureole within the Sanandaj–Sirjan metamorphic belt, Hamadan, Iran. The presence of melt shows that corona development occurred under partial melting conditions. Spinel is predicted to grow with cordierite at around 700°C. Exhaustion of the available SiO2 and/or separation of sillimanite/andalusite from SiO2-rich matrix domains by cordierite resulted in the formation of localized low-silica activity domains and thus triggered the growth of spinel in the rim of andalusite, the reaction Sil/And + Bt = Crd + Spl + Kfs + melt, as the most common reaction for the development of coronas in the metapelitic of Alvand aureole. The breakdown of garnet to plagioclase + sillimanite, dehydration melting and the formation of spinel–plagioclase symplectite could occur during heating or decompression; these textures are limited to the contact aureole in the studied area, so heating is perhaps the more likely explanation for formation of the symplectites in the metapelitic rocks of the Alvand aureole. The P–T diagram, inferred paths and zoning profiles of garnet do not account for the decompression history of the terrane.