Does Neoproterozoic-Early Paleozoic (570–530 Ma) basement of Iran belong to the Cadomian Orogeny?

Abstract

Late Neoproterozoic–Early Paleozoic basement in Iran mainly comprises granites and mafic rocks locally affected by high-grade metamorphism. In this contribution, we collate whole rock chemical data for more than 600 samples; Sr–Nd isotope data for 200 samples; and zircon U-Pb ages of > 2500 grains. On the basis of zircon U-Pb ages, Iranian basement spans 570–530 Ma. The mafic rocks are tholeiitic and the felsic (granitic) rocks show affinities for predominantly I-and S-type granite with minor A-type granite. Sr-Nd isotope ratios show two discrete sources for the basement rocks. The mafic rocks yield younger Nd model ages (TDM < 1.0 Ga) and lack old zircon grains (mainly less than 800 Ma) confirming a juvenile mantle source. Conversely, the felsic rocks have older TDM ages (2.5 to 1.6 Ga) similar to inherited zircon 238U-206Pb ages (2.5 Ga to 650 Ma), which reflects recycling of old (Late Archean-Early Proterozoic) continental crust in the generation of felsic melts in the Late Neoproterozoic. Based in part on a recent study of Late Neoproterozoic Gondwana evolution, long-lived Proto-Tethys subduction beneath northern Gondwana (Hun terranes) may have caused: (1) Proto-Tethys slab avalanche in the lower mantle; (2) subsequent triggering of mantle plume emplacement; and (3) the calving off of a ribbon of Hun terranes from northern Gondwana. That Iran was far from the Cadomian orogeny is supported by the fact that ‘Cadomian’ terranes of Iran show evidence of having formed in an extensional tectonic regime as opposed to that of a subduction zone as for Cadomian basement in Europe. Hence, the common-use of Cadomian basement to explain Ediacaran–Cambrian magmatism in Iran is a misinterpretation. This misconception stems largely from an erroneous correlation of Iran with North America and Europe, as opposed to correlation with Central and Eastern Asia such as microcontinents in the Tibetan Plateau.