MIDDLE JURASSIC FLARE-UP AND CRETACEOUS MAGMATIC LULL IN THE CENTRAL SANANDAJ-SIRJAN ARC, IRAN: AN ANALOGY WITH THE SOUTHWESTERN UNITED STATES

Subduction of Neotethys oceanic lithosphere underneath the Iranian part of the Cimmerian continent probably began in the Triassic, but the magmatic arc did not fully develop until the Jurassic. The numerous granitoid bodies that invade lower Jurassic schists in the Sanandaj-Sirjan zone are mostly I-type, with trace element signatures indicative of a normal continental arc setting. These plutons have been assigned ages from Jurassic to Cretaceous. However, new U-Pb zircon dates from granitoids exposed in the Hamadan-Esfahan segment of the Sanandaj-Sirjan arc cluster in a narrow time interval from 180 to 165 Ma, suggesting a magmatic pulse analogous to the flare-up event in the Sierra Nevada batholith. While magmatism paused during the Cretaceous in this segment, it continued to the northwest and southeast, implying: 1) incidence of slab segmentation, 2) switch to shallow slab subduction in the central segment and 3) persistence of normal subduction in the adjacent segments. This new arrangement initiated Cretaceous to Paleocene, Laramide-style flat slab subduction in a median sector that covered parts of Iran from the Hamedan-Esfahan segment in the south to the Alborz in the north. Cretaceous magmatism in this middle sector is absent in the Sanadaj-Sirjan and Urumieh-Dokhtar zones and only occurs farther north in the Alborz. In contrast, outside this area Cretaceous granitoids are present within the Sanandaj-Sirjan zone (e.g., Qorveh area) and in the Urumieh-Dokhtar zone (e.g., Shirkuh of Yazd).