High magmatic flux during Alpine-Himalayan collision: Constraints from the Kal-e-Kafi complex, central Iran

Voluminous Eocene gabbros to granites of the Kal-e-Kafi backarc composite intrusion were emplaced prior to the Alpine-Himalayan collision in the central Iranian backarc, but the reasons for precollisional high arc and backarc magmatic productivity (60-53 Ma) are unclear. Diagnostic geochemical signatures are high K2O-Sr contents and successively depleted middle rare earth element (REE) patterns, reflecting a highly metasomatized source and an increasing role for amphibole and garnet (0%-10%) in the relatively younger granites. Release of concealed K-Sr-rich fluids from oceanic fractures and faults during buckling and bulging of a precollisional choking oceanic slab, and melting of phlogopite-bearing lithosphere with subsequent interaction of the melt with lower crustal garnet-amphibolite of a similar to 40-km-thick crust, can explain the Kal-e-Kafi geochemical and isotopic signatures. Gravimetric data indicating a similar to 39 km present-day backarc crustal thickness are consistent with geochemical results but also imply little if any collisional crustal thickening of the central Iranian Plateau. High Eocene arc-backarc melt flux prior to collision in this region reflects vigorous thermal convection, which may in fact be diagnostic of collisional magmatism, explaining the presence of post-collisional shoshonitic melts in this and other collisional orogenic settings.