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 degrees C (Shanderman), 14 kbar -560 degrees C (Anarak), 25 kbar -670 degrees C (Shahrekord), 17 kbar -530 degrees C (Hajiabad-Esfandaghe), 24 kbar -650 degrees C (Sistan), 15 kbar -560 degrees C (Makran), and 17 kbar -570 degrees C (Sabzevar). Their retrograde P-T conditions are similar to 6 kbar -470 degrees C, 7 kbar -500 degrees C, 6 kbar -530 degrees C, 10 kbar -450 degrees C, 8 kbar -500 degrees C, 7 kbar -500 degrees C and 6 kbar -490 degrees C, respectively. The obtained P-T conditions represent a geothermal gradient of 12-16 degrees C/km for the Paleo-Tethys HP-LT complexes and 7-11 degrees 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.