Neoproterozoic magmatic flare-up along the N. Margin of Gondwana: The Taknar complex, NE Iran

Abstract

Magmatic “flare-up” is common in the formation of continental arc roots. The best-studied examples of such flare-ups are known from Cretaceous and younger continental arcs, but more ancient examples are preserved in Late Ediacaran-Cambrian or Cadomian arcs that formed along the northern margin of Gondwana. In this paper, we report new trace-element, isotopic and geochronological data on Cadomian magmatic rocks from the Taknar complex, NE Iran, and use this information to better understand episodes of flare-up, crustal thickening and magmatic periodicity in the Cadomian arcs of Iran and Anatolia. Igneous rocks in the Taknar complex include gabbros, diorites, and granitoids, which grade upward into a sequence of metamorphosed volcano-sedimentary rocks with interlayered rhyolites. Granodioritic dikes crosscut the Taknar gabbros and diorites. Gabbros are the oldest units and have zircon U-Pb ages of ca 556 Ma. Granites are younger and have U-Pb zircon ages of ca 552-547 Ma. Rhyolites are coeval with the granites, with U-Pb zircon ages of ~551 Ma. Granodioritic dikes show two U-Pb zircon ages; ca 531 and 548 Ma. Geochemically, the Taknar igneous rocks have calc-alkaline signature typical of continental arcs. Whole-rock Nd and zircon O-Hf isotopic data from Taknar igneous rocks show that these rocks were generated via mixing of juvenile magmas with older continental-crust components at an active continental margin. Compiled geochronological and geochemical data from Iran and Anatolia allow identification of a long magmatic flare-up along northern Gondwana. The compiled U-Pb results from both magmatic and detrital zircons indicate a flare-up started ~572 Ma and ended ~ 528 Ma. The Cadomian flare-up was linked to strong crustal extension above a S-dipping subduction zone beneath northern Gondwana. The Iran-Anatolian Cadomian arc represents a site of crustal differentiation and stratification and involved older (Archean?) continental lower-middle crust, which has yet to be identified in situ, to form the continental nuclei of Anatolia and Iran. The Cadomian crust of Anatolia and Iran formed a single block “Cimmeria”, that rifted away from northern Gondwana and was accreted to southern Eurasia in late Paleozoic time.