Geology and thermochronology of Tertiary Cordilleran-style metamorphic core complexes in the Saghand region of central Iran

Abstract

An ~100-km-long north-south belt of metamorphic core complexes is localized along the boundary between the Yazd and Tabas tectonic blocks of the central Iranian micro-continent, between the towns of Saghand and Posht-e-Badam. Amphibolite facies mylonitic gneisses are structurally overlain by east-tilted supracrustal rocks including thick (>1 km), steeply dipping, nonmarine siliciclastic and volcanic strata. Near the detachment (the Neybaz-Chatak fault), the gneisses are generally overprinted by chlorite brecciation. Crosscutting relationships along with U-Pb zircon and ^(40)Ar/^(39)Ar age data indicate that migmatization, mylonitic deformation, volcanism, and sedimentation all occurred in the middle Eocene, between ca. 49 and 41 Ma. The westernmost portion of the Tabas block immediately east of the complexes is an east-tilted crustal section of Neoproterozoic–Cambrian crystalline rocks and metasedi-mentary strata >10 km thick. The ^(40)Ar/^(39)Ar biotite ages of 150–160 Ma from structurally deep parts of the section contrast with ages of 218–295 Ma from shallower parts, and suggest Late Jurassic tilting of the crustal section. These results define three events: (1) a Late Jurassic period of upper crustal cooling of the western Tabas block that corresponds to regional Jurassic–Cretaceous tectonism and erosion recorded by a strong angular unconformity below mid-Cretaceous strata throughout central Iran; (2) profound, approximately east-west middle Eocene crustal extension, plutonism, and volcanism (ca. 44–40 Ma); and (3) ~2–3 km of early Miocene (ca. 20 Ma) erosional exhumation of both core complex and Tabas block assemblages at uppermost crustal levels, resulting from significant north-south shortening. The discovery of these and other complexes within the mid-Tertiary magmatic arcs of Iran demonstrates that Cordilleran-style core complexes are an important tectonic element in all major segments of the Alpine-Himalayan orogenic system.