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

Abstract

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, (87Sr/86Sr)i = 0.7073 to 0.7089, εNd(t) = −1.4 to −0.2, and εHf(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 ~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 (~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.