The Early Miocene Marshenan intrusion, in the central part of the Urumieh-Dokhtar magmatic arc (UDMA), central Iran, includes granodiorite-granite and diorite; it is a classic example of a Cu-Au barren intrusive system. Zircon U–Pb dating indicates that the diorite display an age of 20.32 ±0.36 Ma, coeval with granodiorite rocks (20.5±0.8 Ma). These rocks are composed of feldspar, quartz, amphibole, biotite, titanite, and magnetite. In the granodiorites, the plagioclase composition ranges from oligoclase to bytownite, the amphiboles are magnesio-hornblende and biotites is Mg rich, related to calc-alkaline orogenic suites in the region. Plagioclase phenocrysts exhibits oscillatory zoning and marked changes in the abundance of elements, such as Ba, Sr, and Fe, suggesting magma mixing/mingling may have had a role in generating these parental magmas. The average calculated P–T conditions of the granodiorite and diorite rocks are about 730 °C and 2.1 kbar and 733 °C and 1.7 kbar, respectively, corresponding to near solidus conditions equal to emplacement depths of ~ 7 to 8 km. Magmatic H2O contents and ƒO2 calculated from crystallized amphiboles indicate that the Marshenan granodiorite had initial magmatic H2O contents ~ 5 wt. % and relatively high ƒO2 (ΔNNO; ave. 1.3) and the diorite had initial magmatic H2O contents ~ 4.7 wt. % and relatively high ƒO2 (ΔNNO; ave. 1.5), both consistent with the presence of phases, such as hornblende, biotite, magnetite, and titanite. In comparison with other intrusions in the UDMA, the Marshenan intrusion formed via the same physico-chemical mechanisms like other barren intrusions, whereas the fertile intrusions exhibit slightly higher temperatures, pressures, ƒO2, and H2O values than the barren intrusions. These compiled data suggest that, in spite of the high magmatic H2O and ƒO2 contents, the Marshenan and other barren intrusions in the UDMA will not produce porphyry Cu mineralization, unlike the giant Kerman deposit, probably due to magma source, magmatic evolution processes, timing of volatile exsolution, pre-existing crustal-scale fractures, coeval volcanism, and extended duration of volatile saturated crystallization to subsolidus conditions.
