Dating of Oligocene granitoids in the Khak-Sorkh area, Central Urumieh-Dokhtar arc, Iran, and a genetic linkage with the associated skarn iron deposit

Abstract

Calc-silicate assemblages and skarn-type iron ores are associated with granite and granodiorite bodies intruding Triassic-Jurassic sedimentary strata in the Khak-Sorkh area in the central Urumieh-Dokhtar magmatic arc (UDMA), Iran. The intrusive bodies are metaluminous to slightly peraluminous, high-K calc-alkaline, and of I-type affinity. The skarn-type iron ore at Khak-Sorkh occurs in several ore bodies of variable sizes consisting of magnetite, subordinate hematite, and minor sulfides. The prograde assemblage of diopsidic pyroxene, andraditic garnet, and olivine is locally preserved, but the common intimate association of the iron ore with serpentine and lesser phlogopite suggests that ore formation occurred during retrograde hydrous reactions. Zircon dated using the isotope dilution TIMS U-Pb technique yields overlapping ages of 30.05 ± 0.06 Ma and 29.94 ± 0.09 Ma for two granite samples, consistent with the age of 29.51 ± 0.13 Ma for coexisting monazite, constraining the time of magma emplacement. A similar age of 29.53 ± 0.11 Ma was obtained for zircon grains from a highly epidotized granite offshoot enclosed in dolomitic limestones. Zircon from granodiorite provides a maximum age of 31.23 ± 0.16 Ma, with coexisting xenotime and titanite yielding 29.49 ± 0.10 M 29.67 ± 0.42 Ma, respectively. The 207Pb/204Pb ratios for epidote from both endoskarn and exoskarn (15.584–15.669) are identical implying a common fluid source. They are distinguished from those for calcite from the host limestone (15.774–15.779), the latter consistent with upper crustal sources. The new results confirm that significant arc-type plutonism occurred in the central UDMA during the Oligocene, consistent with the widespread I-type plutons in southern and northern sections of the UDMA.