High-pressure – Low-temperature metamorphic rocks of Iran and their geodynamic significance: A review

High-pressure-low-temperature (HP-LT) rocks such as eclogites and blueschists preserve valuable information about the history and geodynamics of subduction zones. HP-LT rocks exposed in Iran formed during subduction of Paleo-Tethys and Neo-Tethys oceanic lithosphere beneath the Iran microplate and were subsequently exhumed from the Permian to the Early Eocene. The Shanderman and Anarak complexes are associated with Paleo-Tethys, and the Shahrekord, Hajiabad-Esfandaghe, Sistan, Makran, and Sabzevar complexes are related to Neo-Tethys. The assemblage garnet + omphacite + amphibole (Na-, Na-Ca) + white mica + albite ± zoisite formed at the peak metamorphism stage of the eclogites, and amphibole (Na-, Na-Ca) + albite ± white mica ± zoisite ± epidote ± lawsonite formed in blueschists. Retrograde metamorphism replaced peak assemblages with epidote-amphibolite and greenschist facies mineral assemblage (Ca-amphibole + Na-Ca plagioclase + epidote + chlorite). For the various complexes, geothermobarometry calculations and phase diagram modeling show peak P-T conditions of 23 kbar - 600 °C (Shanderman), 14 kbar - 560 °C (Anarak), 25 kbar - 670 °C (Shahrekord), 17 kbar - 530 °C (Hajiabad-Esfandaghe), 24 kbar - 650 °C (Sistan), 15 kbar - 560 °C (Makran), and 17 kbar - 570 °C (Sabzevar). Their retrograde P-T conditions are ∼6 kbar - 470 °C, 7 kbar - 500 °C, 6 kbar - 530 °C, 10 kbar - 450 °C, 8 kbar - 500 °C, 7 kbar − 500 °C and 6 kbar - 490 °C, respectively. The obtained P-T conditions represent a geothermal gradient of 12–16 °C/km for the Paleo-Tethys HP-LT complexes and 7–11 °C/km for the Neo-Tethys. Field and petrographic studies reveal that subduction metamorphism occurred mostly along clockwise P-T paths, except for the Hajiabad-Esfandaghe, Makarn and a subunit of the Anarak complexes that endured counter-clockwise paths. The differences may show a greater subduction angle of Neo-Tethys oceanic lithosphere than Paleo-Tethys, resulting in a cold geothermal gradient and development of continental back-arc basins during the Mesozoic.