Zircon U-Pb-Hf and whole-rock Sr-Nd isotopic insights on the Silurian syenite-monzonite shoshonitic magmatism of Central Iran

The Palaeozoic magmatic history of Central Iran is related to the subduction of the Proto-Tethys Ocean and the amalgamation of Gondwana continental fragments. This study presents whole-rock geochemistry, Sr-Nd isotope and zircon U-Pb-Hf ages of shoshonitic plutonic rocks (e.g., monzonite-syenite) from the north Posht-e-Badam (NPB) district with a view to constrain their petrogenetic and tectono-magmatic evolution during the Palaeozoic. The geochemical data show moderate enrichment in large-ion lithophile elements and Ti, large positive Pb anomalies, and depletion or moderate enrichment in high-field-strength elements (e.g., Nb and Ta), typical of continental arcs. Zircon LA-ICP-MS U-Pb ages are in the range of 444-428 Ma. The Sr-Nd-Hf isotope data, for example, (Sr-87/Sr-86)i = 0.7073 to 0.7089, epsilon Nd-(t) = -1.4 to -0.2, and epsilon Hf-(t) = -3.6 to +7.7, indicate derivation of the magma from a mantle source. The numerical modelling shows a contribution of less than 10% crustal assimilation and similar to 3% sediment into the melt-derived mantle. We propose that metasomatic enrichment occurred during the subduction of Proto-Tethyan Ocean between the Iranian microcontinents, leading to the formation of a shoshonitic melt. We infer that the recorded ages coincide with a continental rift episode followed by a rear arc tectonic region that spread along the northern active margin of Gondwana. Further, the change in magma affinity from calc-alkaline (similar to 474-512 Ma) to shoshonitic (444-428 Ma) in Central Iran is probably related to the gradual steepening of the slab dip angle and trench-ward migration of the front arc into the region above metasomatically enriched rear arc mantle.