Cadomian tectonic evolution of Iran: records of an unusually hot and broad extensional convergent margin on the northern margin of Gondwana

• Published in: International geology review 2024-04, Vol.66 (7), p.1352-1372
• Main Authors: Sepidbar, Fatemeh, Homam, Seyed Masoud, Ghaemi, Farzin, Stern, Robert J., Jun, Hong, Karsli, Orhan, Gholami, Majid
• Format: Article
• Language: English
• Subjects: active margin; Cadomian; Cambrian; Continental crust; Convergence; Evolution; Geochemistry; Gneiss; Gondwana; Granite; Kariznou region; Lithosphere; Magma; NE Iran; Palaeozoic; Paleoceanography; Paleozoic; Passive margins; Plate margins; Proto-Tethys Ocean; Radiometric dating; Sedimentary rocks; Sediments; Subduction; Subduction (geology); Tectonics; Trace elements; Volcanic rocks; Zircon
• ISSN: 0020-6814; 1938-2839
• DOI: 10.1080/00206814.2023.2238219

The continental crust of Iran is dominated by abundant calc-alkaline and alkaline plutonic and volcanic rocks and by rifted basins filled with mostly terrigenous sedimentary rocks that formed at a Late Ediacaran to Cambrian extensional convergent plate margin along the northern margin of Gondwana. Here we present new zircon U-Pb age, geochemical, and isotopic data from plutonic (granite-granodiorite) and metamorphic (gneiss) rocks in the Kariznou region of NE Iran to provide insights into the nature of the Cadomian convergent margin of Iran. Geochemical data indicate calc-alkaline signatures, characterized by strong depletions in Nb, Ta, P, and Ti and arc-like trace element patterns. New zircon U-Pb ages show that calc-alkaline granitoids and granitic gneiss formed at ~564 to 537 Ma and 538 Ma, respectively. Bulk rock Sr-Nd isotopic data of calc-alkaline rocks have εNd(t) = -5.42 to −5.53 and −6.93 to −7.43 for granite and gneiss, respectively. The gneisses show stronger interaction with and/or re-melting of older continental crust than do granitic rocks. We interpret Kariznou magmatic rocks as forming in association with strong extension accompanied by crustal assimilation. Extension initiated ~570 Ma with the deposition of Late Ediacaran sediments, and magmatism began at 545-535 Ma, generating calc-alkaline magmas. The tectonomagmatic evolution of the Kariznou region encapsulates the prolonged transition of Cadomian Iran from a strongly extensional convergent margin, possibly as a result of oblique oceanic subduction and slab roll-back of the subducting Proto-Tethys oceanic lithosphere, culminating in the formation of an Early Palaeozoic passive margin on the northern side of Gondwana.