Neoproterozoic–Early Cambrian tectono-magmatic evolution of the Central Iranian terrane, northern margin of Gondwana: Constraints from detrital zircon U–Pb and Hf–O isotope studies

Abstract

U–Pb dating and oxygen and Lu–Hf isotope analyses are applied to ~400 detrital zircon grains from the Neoproterozoic–Cambrian Kahar, Bayandor and Zaigun sandstones. The results reveal the evolutionary history of the Central Iranian continental crust in the northern margin of Gondwana during the Neoproterozoic–Cambrian. The U–Pb dating produces major peaks of crystallization ages at 0.5–0.7Ga and minor peaks around the Tonian, Paleoproterozoic and Neoarchean. The zircon population in the Zaigun sandstone is dominated by long-transported grains and exhibits slightly different zircon distribution patterns than those from the older Kahar and Bayandor units. The zircon population ages and Hf isotopes of the Zaigun sample are very similar to the Neoproterozoic–Early Palaeozoic siliciclastic units in the Arabian Nubian shield (ANS) and Turkey, which suggests the late to post–Pan-African unroofing of the Afro–Arabia realm as the main process for detritus accumulation in Central Iran during the early Palaeozoic. A significant proportion of the Tonian-aged zircons (~64%) in the Kahar and Bayandor samples show positive εHf(t) values, whereas those with late Cryogenian–Ediacaran ages have high δ18O and variable εHf(t) values (~−30‰ to +17‰), suggesting that the crustal evolution of provenance of the Tonian-aged zircons commenced in an island arc setting and continued in an active continental margin. All the samples contain pre-Neoproterozoic zircons that are ca 1.9–2.3Ga or 2.5–3.2Ga, which are much older than the known Neoproterozoic igneous rocks in Iran and are more consistent with pre-Neoproterozoic igneous-metamorphic rocks in the eastern ANS and northern Africa. These ages support the eastern sector of the Afro–Arabia margin as a provenance for the detrital zircons in the oldest sedimentary sequences of Iran during the late Neoproterozoic–Cambrian. The Hf model ages of zircons with mantle-like δ18O values suggest that a significant amount of continental crust in the provenance of the detrital zircons was generated at around 1.0–2.0 and 3.0–3.5Ga, likely by mantle-derived mafic magmas, and subsequently reworked during crustal differentiation into younger, more felsic crust with varying crustal residence times.