MESOZOIC-CENOZOIC MAGMATIC ARC SYSTEM OF IRAN

Tertiary calc-alkaline volcanic rocks in the Urumieh-Dokhtar arc are a prominent manifestation of subduction of Neotethyan oceanic crust beneath Iran. This phase of volcanism is, however, only a small part of a long and seemingly continuous history of Neotethys-related Iranian arc magmatism. Late Triassic and Jurassic calc-alkaline plutons in the Sanandaj-Sirjan zone of SW Iran are the earliest evidence of subduction zone magmatism and comprise remnants of the original Mesozoic continental arc. A Cretaceous reorganization of the arc system is suggested by the observation that, with the exception of some volcanic rocks in the northwesternmost part of the Sanandaj-Sirjan zone, most Cretaceous magmatism occurred in northern Iran. Subsequent Tertiary volcanism was distributed over a large area (encompassing the Urumieh-Dokhtar belt, the Alborz Mtns., large parts of eastern Iran, and many relatively small locations in central Iran) but was generally bracketed spatially by Cretaceous igneous rocks to the north and Triassic-Jurassic plutons to the south. We suggest that a single subduction zone, along what is now the Main Zagros reverse fault, was responsible for all of these elements of the Late Triassic to Miocene magmatic arc system of Iran. This interpretation implies that the record of Neotethyan arc magmatism in Iran does not encompass only the relatively narrow belt of Tertiary volcanics in the Urumieh-Dokhtar arc, but is actually spread over all of Iran and spans nearly 200 My. Changes in slab trajectory may be responsible for both the spatial distribution of arc magmas and temporal variations in magmatic productivity.