The Oligo-Miocene Ardestan quartz diorite to tonalite is part of widespread Cenozoic magmatism within the Urumieh-Dokhtar Magmatic Arc (UDMA) of Iran. The Ardestan pluton is mainly composed of varying proportions of plagioclase feldspar (normally zoned from bytownite to andesine), amphibole (magnesio-hornblende) and biotite. Biotite exhibits a range of Al values (~2-2.8 apfu) over very restricted Fe # ratios (0.42 to 0.56) which are characteristic of continental arc magmatic suites. High Ti 2 O contents of biotite (10) ratios suggesting a magmatic origin. T-fO 2 calculations of oxygen fugacity between-13.6 to-16.9 indicate oxidizing crystallization conditions between the Ni-NiO (NNO) and Fe 2 O 3-Fe 3 O 4 (HM) buffers. Tight linear trends of log (X F /X OH), log (X Cl /X OH) and log (X Cl /X OH) versus X Mg represent a narrow range of fH 2 O, fHF and fHCl, clearly indicating that physico-chemical conditions were essentially constant throughout the formation of magmatic biotite. The shape of crystal size distribution curves along with the medium Al and Mg contents in amphibole and biotite, respectively, are consistent with a history of magma mixing involving injections of mafic magma into the evolving felsic chamber. Calculated residence time for Ardestan plagioclase crystals of ~630 years support field evidence that these plutons were emplaced at shallow depths.