CORDILLERAN-STYLE METAMORPHIC CORE COMPLEXES IN CENTRAL IRAN AND IMPLICATIONS FOR THE TETHYSIDES

A ~100 km long N-S belt of metamorphic core complexes is localized along the boundary between the Yazd and Tabas tectonic blocks of the central Iranian microcontinent, between the towns of Saghand and Posht-e-Badam. Amphibolite facies mylonitic gneisses are structurally overlain by east-tilted upper crustal stratified rocks including thick (>1 km), steeply dipping, nonmarine siliciclastic and volcanic strata. Near the detachment, the gneisses are generally overprinted by chlorite brecciation. Cross-cutting relationships along with U-Pb zircon and Ar/Ar age data indicate that migmatitization, mylonitic deformation, volcanism and sedimentation all occurred in the middle Eocene, between ~49 and 42 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 metasedimentary strata >10 km thick. A horizontal transect of (U-Th)/He apatite and zircon ages through the section shows little spatial variation within either phase. Zircon ages of 100-130 Ma and apatite ages of 15-22 Ma indicate pre-mid-Cretaceous tilting of the crustal section. These results define three events: (1) a Cretaceous period of upper crustal cooling of the western Tabas block, which appears to correspond to regional Jura-Cretaceous tectonism and erosion recorded by a strong angular unconformity below mid-Cretaceous strata throughout central Iran (e.g., Stocklin, 1968); (2) profound ~E-W middle Eocene crustal extension, plutonism and volcanism; and (3) early Miocene exhumation of both core complex and Tabas block assemblages to uppermost crustal levels. The discovery of these and other complexes located broadly within the mid-Tertiary magmatic arcs of Iran suggests continuity of Tertairy core-complex development in all major segments of the entire Alpine-Himalayan orogenic system.