Early Miocene Post-collision Andesite in the Takab Area, NW Iran

Abstract

Abstract Miocene magmatic rocks are exposed as lava flows (OKV group), subvolcanic rocks (SRG group), and as lavas interbedded with shallow-basin sedimentary layers (TVN group) in the Takab area, NW Iran. Zircon U–Pb dating yields ages of 18 to 15 Ma. Whole-rock chemistry shows that most of the Early Miocene magmatic rocks are andesite with subordinate dacite. The magmatic rocks have low contents of MgO, Ni, Cr, Ti, Nb and Ta, and high concentrations of Li, large ion lithophile elements such as Rb, K and Ba, and light rare earth elements. The OKV and SRG groups have similar initial 87Sr/86Sr ratios (0·70557–0·70768) and εNd(t) values (+1·0 to +2·2). The TVN group show larger variations of 87Sr/86Sr(i) ratios from 0·70628 to 0·71033 and εNd(t) values from –3·8 to +1·6. This implies a greater role of involvement of supra-crustal domains in the evolution of the TVN group relative to the SRG and OKV groups. Early Miocene magmatic rocks in the Takab area are situated between the Sanandaj–Saqqez Cretaceous calc-alkaline andesite in the SW and the Late Eocene–Oligocene (35–27 Ma) ocean island basalt-like Mianeh–Hashtrood magmatic belt of possible back-arc affinity in the NE. In addition, Late Eocene (40–37 Ma) syn-collision granites in the Baneh–Marivan area along the Zagros suture zone along the west side of the Sanandaj–Saqqez Cretaceous calc-alkaline andesite body indicate that collision of the Arabian Plate and the NW Iran Block occurred in the Late Eocene. These observations support the idea that Early Miocene andesites (18–15 Ma) in the Takab area were generated after collision, which was also associated with doubling of the thickness of the continental crust in the Zagros suture zone, thinning of continental crust far from the Zagros suture zone, and development of shallow-basin sedimentary rocks in NW Iran. Partial melting of mafic calc-alkaline bodies at depth or highly metasomatized fossil mantle owing to thinning of continental crust and asthenospheric upwelling may represent possible sources for the Late Miocene andesite. We conclude that andesitic rocks, even with typical arc signatures, are not always generated in an active margin and that some were probably generated in a post-collision tectonic regime. Misinterpretation of the arc signature can result in erroneous assumptions as to the geodynamic regime, and in the particular case of NW Iran, the timing of collision of the Arabian and Iranian plates.