Reasons of Andalusite Overstepping in the Sillimanite and Spinel- Cordierite Zones within Migmatites of the Alvand Aureole, Hamadan


Department of Geology, Shahid Chamran University of Ahvaz


Migmatites rocks comprise a substantial part of the high-grade part of the Alvand aureole in the Hamadan, western Iran. Metapelitic migmatits consist of mesosomes with biotite+sillimanite+garnet+cordierite+spinel± orthopyroxene mineral assemblage and leucosomes with garnet+plagioclase+k-feldspar and quartz. Spinel-cordierite-plagioclase symplectites partially replacing andalusite occur in the mesosome of migmatite rocks. The presence of melt microstructure shows that corona development occurred under partial melting conditions. Spinel is predicted to grow with cordierite at around 700 to 750 °C, indicating that andalusite has been stable in this temperature in the Hamadan migmatite. Field evidence, petrographic observations, melt reactions and P-T estimation suggests that andalusite stability has reached up to spinel-cordierite zone due to rapid heating rate. Intrusion of the Alvand complex has been developed high grad metamorphism and migmatite rocks in the contact metamorphic aureole. Therefore, heating rate in the contact aureole is high (geothermal gradiant is 50° C/km) so, ΔS of Sill-And polymorphic reaction is reduced and andalusite overstepping has increased, which in the mesosome of Hamadan migmatite rocks is 200±50 ° C.


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