A comprehensive re-look into Jurassic-Cretaceous Neotethyan active margin in Iran: Evidence of deep mantle flow into subduction magma factory and formation of transitional basalts

Subduction zones display the most diverse magma compositions from various calc-alkaline differentiates to alkaline mafic associations, but mechanisms generating Nb-Enriched Basalts (NEB) and High-Nb Basalts (HNB) in such settings have received less attention from the researchers. Occurrences of NEB and HNB magmas in the Mesozoic Neotethyan active margin in Iran, known as the Sanandaj-Sirjan Zone, provide an excellent natural lab for examining this interesting petrologic puzzle. The Middle-Jurassic typical calc-alkaline arc plutons exhibit a NW-SE subduction parallel trend. In the northwestern sector of the zone, the Late Jurassic-Early Cretaceous magmatic rocks (JCMR) define a chain that transects the Middle-Jurassic arc trend. The two cross-cutting magmatic chains are chemically and genetically different. The JCMR include three distinct compositional groups: (i) tholeiites that are depleted in high-field-strength elements (HFSE) and show subduction-related geochemical fingerprints, (ii) NEB with higher abundances of Nb and other HFSE, and (iii) HNB with the highest abundances of Nb and other HFSE. The latter two groups do not display typical arc signatures and are transitional between arc and plume-type magmas. These three groups of rocks show linear major and trace element, and Sr-Nd isotopic variation trends extending from Mid-Ocean Ridge Basalts-like tholeiites to Oceanic Island Basalts-like HNB. Hence, a two-component mixing process is suggested for explaining the compositional variation of the JCMR. We propose that a vertical window resulted from slab tear created the geodynamic environment for juxtaposition of magmas originated at different mantle depths and formation of the transitional basalts by magma mixing.