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

• Published in: Geological journal (Chichester, England) England), 2021-06, Vol.56 (6), p.3396-3414
• Main Authors: Rahmani Javanmard, Somayeh, Verdecchia, Sebastián O., Yakymchuk, Chris, Broekmans, Maarten A. T. M.
• Format: Article
• Language: English
• Subjects: Andalusite; Batholiths; Boroujerd; Complexity; contact metamorphism; Cordierite; Hamedan; Iran; Jurassic; Magma; Metamorphism; metapelite; Mineral composition; Modelling; Orogeny; Phase equilibria; phase equilibrium; Sanandaj–Sirjan Zone; Schists; Sillimanite; Subduction
• ISSN: 0072-1050; 1099-1034
• DOI: 10.1002/gj.4135

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°C in andalusite schists and ~700°C in cordierite–sillimanite migmatites. The pressure does not exceed 3.5 kbar, implying a maximum depth of ~13 km. A hairpin clockwise P–T path in the Boroujerd region was inferred from mineral relations (e.g., replacement of andalusite1 by sillimanite in the migmatites in the heating stage, which was followed by a retrograde episode with growth of andalusite2) 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. 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. Microstructural relations and mineral compositions from andalusite schist and migmatite units in the Boroujerd region indicate a tectono‐thermal evolution represented by two metamorphic events: a low‐grade M1 and the main M2. Although M1 is linked to an older episode (ca. 180 Ma), M2 was coeval with magmatism in Boroujerd Intrusive Complex (ca. 169–171 Ma). Peak P–T conditions for M2 event are estimated at ~640–660°C and ~3 kbar in schist and ~700°C and 3.5 kbar in migmatites from phase equilibrium modelling and Ti‐in‐biotite thermometry. In the migmatites, andalusite (And1) is mantled by sillimanite, which suggests replacement during heating, which was followed by a retrograde episode (And2 + Bt + Ms) following a hairpin clockwise P–T evolution. This study demonstrates that metamorphic sequences of the north‐west Sanandaj–Sirjan Zone developed under similar metamorphic conditions in the Lower to Middle Ju