Geochemistry of metabasites from the North Shahrekord metamorphic complex, Sanandaj-Sirjan Zone: Geodynamic implications for the Pan-African basement in Iran

Abstract

Metabasic rocks (amphibolites, garnet-amphibolites and eclogites) of the North Shahrekord Metamorphic Complex (NSMC) of the Sanandaj-Sirjan structural Zone in southwest Iran are part of a ductile shear zone forming lenses or blocks associated with orthogneiss, paragneiss and metagranite. Geochemical characteristics (immobile trace elements and Sr-Nd isotope ratios) of these metabasites reveal similar tholeiitic protoliths. The rocks are slightly altered during metamorphism but no evidence for sea floor alteration was found. Rare earth elements (REE) display relatively flat patterns or a slight enrichment of light REEs. The (La/Yb)cn ratios range from 1.42 to 4.91, the Nb/La ratio from 0.50 to 1.07, and the Sm/Nd ratio from 0.24 to 0.31 resembling to a continental back-arc basin. The (87Sr/86Sr)t isotopic ratios display a large range between 0.7041 and 0.7090, while 143Nd/144Nd(t) varies from 0.5120 to 0.5122 and between εNd(t) 2.5 and 7.2. Immobile trace element patterns and Sr-Nd isotopic characteristics reveal mixed sources of depleted mantle and enriched mantle type II with a contribution from a supra-subduction zone and some effects of crustal contamination. Therefore, the data indicates formation of metabasites in a supra-subduction zone setting during late Neoproterozoic to early Cambrian times. The generation and emplacement of OIB-like magmas in a back-arc environment is likely the result of rollback of oceanic subducting slab. The formation of NSMC Ediacaran to Cambrian basement rocks is approximately contemporary with other arc and back-arc basin basalts reported from Alborz, Central Iran and Turkey. These developed as a result of Proto-Tethys subduction beneath the Gondwana supercontinent during Ediacaran to early Cambrian times.