Zircon U–Pb–Hf isotopes and whole rock geochemistry of magmatic rocks from the Posht-e-Badam Block: A key to tectonomagmatic evolution of Central Iran

Abstract

The Posht-e-Badam Block in Central Iran likely formed part of the Neoproterozoic Arabian-Nubian Shield (ANS). However, its Phanerozoic history is not well constrained. Zircon UPb ages, Hf isotopic compositions and whole-rock geochemistry of igneous rocks from the Bafq district were determined to constrain their source and tectonomagmatic evolution. Two magmatic cycles are identified; early Paleozoic events associated with the Cadomian Orogeny resulting from Proto-Tethys subduction beneath the northern margin of Gondwana, and Cenozoic volcanism related to the Alpine-Himalayan Orogeny and closure of Neo-Tethys. The main plutonic and volcanic rocks record early Cambrian ages. The Zarigan, Narigan and Chahcholeh granitoids, volcanic rocks of the lower Cambrian volcano-sedimentary unit (CVSU) and the Zarigan gabbro have remarkably similar ages of 536 to 528 Ma. The Zarigan and Chahcholeh granitoids and volcanic rocks of the CVSU have positive εHf(t) values of 1.4 to 9.3, with Hf model ages (Hf-TDMc) of 715–1034 Ma. They are mostly A2-types, although some samples show affinity to A1- and I-types, suggesting an asthenospheric mantle source, modified by various amounts of crustal assimilation. The Narigan granite records εHf(t) values of −3.8 - 3.1, with Hf-TDMc of 970 to 1255 Ma and displays I-type characteristics, formed through mixing of mantle-derived melts with predominantly Neoproterozoic crust. All rocks were generated in a post-collisional setting. The Esfordy syenite and mafic dikes are younger and essentially coeval, with weighted mean ages of 457 ± 5 Ma and 451–448 Ma, respectively. The Esfordy syenite has A1-type characteristics, indicating possible derivation in an intraplate rift environment and was likely plume-related. The mafic dikes have OIB-like characteristics. Mafic rocks related to closure of Neo-Tethys include the ca. 23 Ma Bahabad diorite that records εHf(t) values of −16.8 to 10.1 (Hf-TDMc = 241–283 Ma) and an OIB-like signature, indicating assimilation of crustal material by asthenospheric mantle-derived melt.