K–Ar ages and petrology of the late Miocene pumices from the Maragheh Formation, northwest Iran

Pumice beds of the Maragheh Formation, which is distributed on the plains at the foot of the late Cenozoic Sahand volcano, NW Iran, consist of the Mordaq tuff bed, lower pumice beds (A and B), middle pumice bed and upper pumice bed, in ascending order. New hornblende and plagioclase K–Ar ages of the pumices from these are: Mordaq tuff bed: 8.14 ± 0.27 Ma, lower pumice beds (A): 7.54 ± 0.22 Ma, lower pumice beds (B): 6.95 ± 0.28 Ma, middle pumice bed: 7.87 ± 0.29 Ma, upper pumice bed: 6.96 ± 0.31 Ma. The middle pumice bed is a flood deposit, and the chemical compositions of glass, hornblende and biotite in pumices overlap those from the lower pumice beds (A). These features indicate that the pumices of the middle pumice bed were derived from a similar source to the lower pumice beds (A). Chemical compositions of pumices from the Maragheh Formation fall in the dacite field of the total alkali-silica (TAS) classification diagram and slightly differ from those of the Plio–Pleistocene rocks from the Sahand volcano. Chemical compositions of late Miocene Maragheh Formation pumices show adakitic features with high Sr contents (698–545 ppm), and low Y (12–8 ppm). It is inferred that the source materials of the adakitic dacite magma were amphibole eclogite or garnet amphibolite originating from the Neo-Thetys plate subducted beneath the Eurasia plate.