ALONG-AXIS VARIATIONS IN GRANITE MINERALOGY: MAGMATIC CONDITIONS ACROSS THE ZAGROS OROGENIC BELT

The Zagros belt of Iran is part of the Alpine-Himalayan orogen that formed through the collision between Africa and Eurasia. We focus on the northeastern tectonic subdivision that represents the leading edge of the orogen, the NW-SE trending Urumieh-Dokhtar magmatic assemblage (UDMA), which comprises volcanic and pyroclastic sequences of Oligo-Miocene age. The oldest UDMA granites occur in the northwest and progressively younger granites outcrop to the southeast, consistent with tectonic models of oblique and diachronous collision. Petrographic and mineral chemistry analysis of representative thin sections from plutons spanning the UDMA reveal along-axis variations that record differences in silica and calcium contents presumably related to evolving thermal and chemical conditions as well as the modern level of exposure. Samples from northwestern UDMA contain potassic feldspar while those from the southeast contain biotite in addition to the minerals (plagioclase, quartz, calcic amphibole, clino- and orthopyroxene) present in all samples. We report new thermobarometric calculations that document these along-axis variations and relate them to the oblique collisional setting.