Phase equilibria modeling of metapelitic hornfelses using pseudosections; implications for contact metamorphism

The Dorbeh contact aureole, located south of Urmia, NW Iran, comprises hornfelses, calc-silicate rocks, and calcite dolomite marbles. The metapelites consist of garnet + cordierite + sillimanite + biotite + chlorite + ilmenite ± andalusite ± chloritoid ± spinel ± quartz. Two significant mineral parageneses are recognized in the rocks. One of them is a low-grade paragenesis, which includes chlorite and chloritoid, without alumino-silicates or cordierite (P1). The second one is chloritoid- and chlorite-free and andalusite/sillimanite- and cordierite-bearing (P2). The most important feature of the Dorbeh contact aureole is the presence of characteristic reaction textures in the pelitic hornfelses, formed due to their especially low silica whole rock chemistry, along with the scarcity of staurolite, which is predicted by models. According to the pressure–temperature (P-T), pressure–chemical composition (P-X), and temperature–chemical composition (T-X) pseudosections on several metapelitic samples, including both spinel present (quartz absent), and quartz present (spinel absent) samples, a pressure of ~3.2 kbar and temperature of ~700 °C are estimated for the observed peak paragenetic equilibria including garnet + cordierite + sillimanite ± spinel ± quartz. In addition, low-grade paragenesis was formed in low-grade zones at an estimated pressure of ~2.5 kbar and temperature of ~450 °C. Residual chlorite and chloritoid at the high temperature conditions, in addition to the lack of staurolite, are signatures of a high degree of overstepping during the prograde metamorphism. The rapid variations in metamorphic conditions caused incomplete reactions and the unstable presence of such low temperature phases in the peak conditions.