Geochronology and geochemistry of rhyolites from Hormuz Island, southern Iran: A new record of Cadomian arc magmatism in the Hormuz Formation
Hormuz Island, a salt-gypsum dome in the Persian Gulf in southern Iran, is a complex halotectonic melange comprising evaporites, carbonates, volcanic and volcaniclastic rocks, as well as low-grade metamorphic and sedimentary rocks. Based on trace element (including rare earth elements REE) compositions of whole rocks and zircon, Hormuz rhyolites are inferred to have formed from subduction-related magmas generated in an active continental margin setting. Ion microprobe analyses of zircon crystals yielded concordant U–Pb ages with weighted mean ^(206)Pb/^(238)U age of 558 ± 7 Ma (juvenile zircons in contrast to those from previous magmatic episodes or xenocrysts) along with younger and older discordant ages which likely represent Pb loss and the presence of xenocrystic domains, respectively. Trace element ratios and in particular REE patterns of juvenile zircon from Hormuz rhyolites indicate crystallization from continental crustal source rocks typical for subduction environments. The concordant ^(206)Pb/^(238)U zircon age agrees with ages obtained from most other structural zones of Iran which indicate regional consolidation of igneous basement during the Neoproterozoic to Early Cambrian. Furthermore, Hormuz rhyolite ages and compositions correlate with counterparts that co-evolved along the northern margin of Gondwana, and are now preserved along the southern coast of the Persian Gulf. Hormuz rhyolites erupted synchronously with the deposition of carbonates and evaporites, suggesting that volcanism occupied an extensional backarc or retroarc setting. Such depositional environments predominated in the northern Gondwana continental margin where convergent (Proto-Tethyan) and extensional (Najd) tectonic regimes coexisted.
© 2015 Elsevier B.V. Received 21 December 2014; Accepted 26 August 2015; Available online 12 September 2015. The first author would like to greatly appreciate Dr. Abdolrahim Houshmandzadeh, Dr. Thomas Johannes Degen and Sabine Walther for all their helpful suggestions. Editorial suggestions by Prof. Sun- Lin Chung are appreciated. We are very grateful to Dr. Hadi Shafaii Moghadam and an anonymous reviewer for their constructive reviews of the manuscript, and Prof. Christoph Gauert who provided us the XRF analysis. Support from the Department of Earth, Planetary, and Space Sciences of UCLA, the Martin-Luther-University Halle-Wittenberg Germany, the University of the Free State of South Africa, the Ministry of Sciences Research and Technology of Iran, and the University of Kharazmi is acknowledged. The ion-microprobe facility at UCLA is partly supported by a grant from the Instrumentation and Facilities Program, Division of Earth Sciences, National Science Foundation.