Metamorphic stages in mountain belts during a Wilson cycle: A case study in the central Sanandaj–Sirjan zone (Zagros Mountains, Iran)

Polymetamorphic units are important constituents of continent–continent collisional orogens, and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision. This study reports the metamorphic conditions and evolution of the Dorud–Azna metamorphic units in the central part of the Sanandaj–Sirjan zone (SSZ), Iran. Here, new geothermobarometry results are integrated with ⁴⁰Ar/³⁹Ar mineral and Th–U–Pb monazite and thorite ages to provide new insight of polyphase metamorphism in the two different basement units of the SSZ, the lower Galeh-Doz orthogneiss and higher Amphibolite-Metagabbro units. In the Amphibolite-Metagabbro unit, staurolite micaschist underwent a prograde P–T evolution from 640 ± 20 °C/6.2 ± 0.8 kbar in garnet cores (M1) to 680 ± 20 °C/7.2 ± 1.0 kbar in garnet rims (M2). Three Th–U–Pb monazite ages of 306 ± 5 Ma, 322 ± 28 Ma and 336 ± 39 Ma from the garnet-micaschists testify the Carboniferous age of M1 metamorphism. In the same unit, the metagabbro records P–T conditions of 4.0 ± 0.8 kbar and 580 ± 50 °C in the (magmatic) amphibole core (Late Carboniferous intrusion) to 7.5 ± 0.7 kbar and 700 ± 20 °C in the amphibole rim indicating a prograde P–T path during subsequent burial (M1). New ⁴⁰Ar/³⁹Ar dating of white mica from the staurolite micaschist yielded a staircase pattern ranging from 36 ± 12 Ma to 170 ± 2 Ma. This implies polymetamorphism with a minimum Late Jurassic cooling age through the Ar retention temperature of ca. 425 ± 25 °C after M2 metamorphism and a Paleogene low-grade metamorphic overprint (M3), while ⁴⁰Ar/³⁹Ar white mica dating of garnet micaschist yielded a plateau age of 137.84 ± 0.65 Ma. We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and is likely between 180 and 200 Ma. Furthermore, it is overprinted at about 36 Ma under retrogressive low-grade M3 metamorphism (at temperatures of ~350–240 °C) during final shortening and exhumation. In the underlying Galeh-Doz unit, the Panafrican granitic orthogneiss intruded at P–T conditions of 3.2 ± 4 kbar and 700 ± 20 °C, then it was metamorphosed and deformed at 600 ± 50 °C and 2.0 ± 0.8 kbar (metamorphic stage M1) prior to Late Carboniferous intrusion of mafic dikes. ⁴⁰Ar/³⁹Ar dating of amphibole from the Galeh-Doz orthogneiss gave plateau-like steps between 260 and 270 Ma, representing the age of cooling through ca. 500 °C after the M1 metamorphic event. Interestingly, the results of this study demonstrate polyphase metamorphic histories in both the Galeh-Doz orthogneiss and Amphibolite-Metagabbro units at different P–T conditions and final thick-skinned Paleogene emplacement of these units over the underlying low-grade metamorphic June Complex. Our findings suggest that both units are affected by high-T/low-P Late Carboniferous orogenic metamorphism along with the bimodal magmatism, as result of rifting. We propose that the Early Jurassic amphibolite-grade M2 metamorphism of the SSZ is correlated with the initial subduction of the Neotethyan Ocean. Eventually, the investigated units reflect various stages of a Wilson cycle, from rifting to initiation of the subduction in final plate collision.