OLIGOCENE MAFIC-ALKALINE MAGMATISM IN NORTH AND NORTHWEST OF IRAN: EVIDENCE FOR THE SEPARATION OF THE ALBORZ FROM THE URUMIEH-DOKHTAR MAGAMTIC ARC

Paleogene arc volcanics and intrusives represent the most voluminous episode of magmatism in Iran. Igneous rocks of this period are mainly found in two belts (excluding the east Iranian Eocene volcanic zone): the NW-trending Urumieh-Dokhtar zone of central Iran and a less well defined belt that stretches from the Azarbaijan Province into the Alborz mountains. Identical lithologic sequences of the same age in both belts suggest that the two originally formed a single continental margin magmatic arc that was subsequently rifted apart. Eocene calcalkaline, volatile-rich, magmatism formed felsic to intermediate rocks that are common in the arc, as are widespread, thick, marine and continental pyroclastic deposits. In contrast, Oligocene magmatism was characterized by drier magmas of mafic-alkaline compositions that formed numerous igneous bodies in the Azarbaijan-Alborz magmatic belt. The plutonic systems exhibit differentiation from olivine gabbros to alkali monzonites and nepheline syenites. Distinctive analcime basanite flow rocks with similar geochemistry and age are found in both Azarbaijan-Alborz and Urumieh-Dokhtar zones and are related to this tectonomagmatic episode. Despite high alkalinity and common silica undersaturation, the Oligocene rocks are characterized by high LIL/HFS elemental ratios, indicating subduction-related origins.

Oligocene mafic-alkaline magmatism occured during intra-arc spreading that formed widespread sedimentary basins between the Alborz and central Iran, in which the fluvial Lower Red, marine Qom, and fluvial Upper Red formations were deposited. This extensional event probably resulted from slab roll-back due to slowed spreading in the Indian Ocean after India collided with Eurasia.