Pb isotope geochemistry of the late Miocene–Pliocene volcanic rocks from Todeshk, the central part of the Urumieh–Dokhtar magmatic arc, Iran: Evidence of an enriched mantle source

Abstract

The late Miocene–Pliocene volcanic rocks from Todeshk, south-east of Isfahan, are located in the middle of the Urumieh–Dokhtar magmatic belt. The belt is considered the subduction-related magmatic arc. The late Miocene–Pliocene calc-alkaline volcanic rocks are mainly andesite and dacite. The rocks have been formed during the post-collisional stage of the Zagros orogen. Geochemical data show the enrichment of light rare-earth elements and large ion lithophile elements such as Cs, Rb, K, Pb, Ba and Th as well as the depletion of elements with high field strength such as Nb, Ta and Ti. The Pb–Sr–Nd isotopic ratios of the studied rocks are characterised by 206Pb / 204Pb = 18.41 –18.72; 207Pb / 204Pb = 15.64 –15.67; 208Pb / 204Pb = 38.49 –38.83; 207Pb / 206Pb = 0.8372 –0.8496; 208Pb / 206Pb = 2.0743 –2.0905; 87Sr / 86Sr = 0.7051 –0.7068 and 143Nd / 144Nd = 0.5125. The rocks have Δ 7 / 4 Pb = 15.44 –15.82 and Δ 8 / 4 Pb = 57.26 –60.44. Based on petrological studies and the whole rock Pb, Sr and Nd isotopes data, the late Miocene–Pliocene calc-alkaline volcanic rocks have been generated from the partial melting of the subduction-related metasomatised mantle. Additionally, the slab-derived melts and fluids were recycled into the mantle source. The data demonstrate that terrigenous sediments accompanied by a subducted slab play an important role in the formation of the enriched mantle as the source of volcanic rocks.