Quaternary high-Nb basalts: existence of young oceanic crust under the Sanandaj-Sirjan Zone, NW Iran

摘要

Quaternary basaltic volcanoes are distributed in the northern part of the Sanandaj-Sirjan Zone (N-SSZ). Those in the Ghorveh area of the N-SSZ are characterized by low SiO_2, high alkalis, and LILE + LREE enrichment. They also have high Mg numbers (Mg# = 65-70) and high contents of Cr (>300 ppm), Ni (> 177 ppm), and TiO_2 (> 1.5 wt.%), suggesting that they crystallized directly from primary magma. The basalts are classified as high-Nb basalts (HNB), with Nb concentrations greater than 20 ppm. Their ~(87)Sr/~(86)Sr values range from 0.7049 to 0.7053 and their ε~0_(Nd) values lie between -0.2 and 1.1. The small negative values of ε~0_(Nd) indicate involvement of continental material in the evolution of the source magma in the area. Based on these new chemical and isotopic data and their relationship to the Plio-Quaternary volcanic adakites in northern Ghorveh, we propose that the partial fusion of metasomatized mantle associated with adakitic magma was responsible for generation of the HNB rocks following late Miocene collision of the Arabian and Iranian plates. Rollback of Neotethyan oceanic spreading and mantle plume activity caused a thinning of the northern SSZ lithosphere;;furthermore, the S wave tomography model beneath the N-SSZ supports this hypothesized lithospheric thinning. The HNB rocks have close spatial proximity and temporal association with adakites, which were formed by the subduction of young (<25 Ma) oceanic crust. Our discussion clarifies the role of the oceanic slab in the post-collision generation of the HNB basalts in this area. Our data confirm the relationship of the HNB rocks to the subduction zone instead of to the oceanic island basalt (OIB) type magma in extensional zones.