Early Paleozoic mafic intraplate magmatism in the Binaloud zone, NE Iran: Implications for the long-lived mantle plume activity in the northern margin of Gondwana

Documentation of continental rifting processes such as mantle plume activity is important for understanding the nature and evolution of tectonic plates. In this study, we report detailed petrological, geochronological, geochemical, Sr-Nd isotope, and mineral chemical data for basalt, and coarse-and fine-grained gabbro from the Binaloud zone of the Alborz, which is the easternmost extension of the Avalonian-Cadomian Orogenic Belts and the western parts of Indo-Australia Orogenic Belt, central-east Asia. Uranium-lead zircon dating of fine-and coarse-grained gabbro indicates that they have been formed at 461.8 +/- 8.2 and 435.0 +/- 4.5 Ma, respectively. The basalt and gabbro have large variations in elemental and isotopic compositions, with 44.0-50.0 wt% SiO2, 124-411 ppm Sr, and epsilon Nd(t) of +0.1 to +3.3. All the rocks show OIB-like or transitional OIB-/E-MORB-like geochemical characteristics, without noticeable Nb-Ti depletion, diagnostic of an intraplate affinity. Whole-rock geochemical and isotopic compositions combined with mineral compositions suggest that both basalt and gabbro have been generated by a plume/asthenospheric mantle (OIB-type) source mixed with enriched sub-continental lithospheric mantle components. Partial melting of such a source in the transitional spinel-garnet stability field was followed by different degrees of fractional crystallization of olivine, clinopyroxene, and plagioclase. Our study demonstrates that roll-back of the subducting Tornquist (Eastern Ipateous ocean) oceanic lithosphere has been followed by plume activity, continental rifting and Paleo-Tethys opening during the Silurian period.