P-T-t path of the Boroujerd Complex, north-west Sanandaj-Sirjan Zone, western Iran: Insights from phase equilibrium modelling and thermobarometry

Variations in the pressures and temperatures of metamorphism reflect the complexity of collisional orogenic systems. In the north-west Sanandaj-Sirjan Zone of western Iran, migmatites in the Boroujerd area record earlier dynamothermal metamorphism followed by the development of a contact metamorphic aureole induced by the Jurassic Boroujerd Batholith. Integrated micro-textural observations, mineral compositions, and the results of phase equilibrium modelling indicate that peak metamorphic temperatures reached 640-660 degrees C in andalusite schists and similar to 700 degrees C in cordierite-sillimanite migmatites. The pressure does not exceed 3.5 kbar, implying a maximum depth of similar to 13 km. A hairpin clockwise P-T path in the Boroujerd region was inferred from mineral relations (e.g., replacement of andalusite(1) by sillimanite in the migmatites in the heating stage, which was followed by a retrograde episode with growth of andalusite(2)) and P-T pseudosections. Metamorphism of the Boroujerd area was coeval with voluminous intermediate-mafic magmatism, which provided the heat necessary for partial melting. This study demonstrates that metamorphic sequences of the north-west Sanandaj-Sirjan Zone developed under similar metamorphic conditions in the Lower to Middle Jurassic and record an extensional back-arc setting related to the subduction of Neo-Tethys.