Late Paleocene adakitic granitoid from NW Iran and comparison with adakites in the NE Turkey: Adakitic melt generation in normal continental crust

Abstract

Late Paleocene adakitic granitoids are rare in Iran except in the northwest. We focused on the adakitic Saqqez-Takab pluton, which occupies an area of ~600 km2. New U-Pb zircon dating yield crystallization ages of 58–56 Ma (Thanetian). These granitoids comprises granite, quartz monzonite and monzogranite with granular to mylonitic textures. They have high SiO2 abundances (59.1–78.6 wt%), high ratios of Sr/Y (50–490) and La/Yb ratios (19–237), along with high Ba (779–2466 ppm) and light rare earth element contents (e.g., La = 11–70 ppm). Saqqez-Takab granitoids have compositions that are similar to high-silica adakites, including low Mg- number (Mg# < 0.5), TiO2 (<0.93 wt%), Cr (<38 ppm) and Ni (<55 ppm). K2O/Na2O ratios of these granitoids are high (>0.7), resembling K-rich adakitic granitoids. Initial ratios of 87Sr/86Sr and εNd(t) vary from 0.7044 to 0.7053 and −2.8 to +2.0 respectively. These isotopic signatures differ from those of typical subducting slab-derived adakites (O-type adakite) with high positive εNd(t) and from collision-related adakites (C-type adakite) with negative εNd(t). The geochemical and isotopic characteristics of Saqqez-Takab granitoids are most consistent with magma being produced by amphibole-dominated fractionation of hydrous melts of subcontinental lithospheric mantle accompanied by minor assimilation of lower mafic calc alkaline continental crust. Distribution of similar-ages adakites in the NW Iran and E Pontides of Turkey suggests that these melts formed similarly. Upwelling of hot asthenosphere due to Neotethys slab rollback led to partial melting of lower continental crust calc alkaline mafic rocks and/or amphibole fractionation from mafic magma produced K-rich adakitic rocks in eastern Turkey and NW Iran during the late Paleocene.