Permian-Triassic magmatism in response to Palaeotethys subduction and pre-Late Triassic arrival of northeast Gondwana-derived continental fragments at the southern Eurasian margin: Detrital zircon evidence from Triassic sandstones of Central Iran

摘要

The closure of Palaeotethys that led to the collision of the Cimmerian blocks with the southern Eurasian margin causing the Eo-Cimmerian orogeny during the Early Mesozoic is still controversially discussed. The Triassic Nakhlak Group in Central Iran is a key sedimentary succession for better understanding the closure of Palaeotethys and the Eo-Cimmerian orogeny in the Middle East. The Nakhlak Group is composed of the Alam (Olenekian to Middle Anisian), Bacioroq (?Upper Anisian to Middle Ladinian) and Ashin (Upper Ladinian to ? Carnian) formations, which consist mainly of volcaniclastic sandstones, mixed siliciclastic conglomerates, and marine carbonates. Here we present for the first time detrital zircon U-Pb ages from the Nakhlak Group to unravel its provenance and constrain its palaeotectonic position within the Palaeotethyan realm. Most detrital zircons from the Nakhlak Group are euhedral and subhedral with Permian-Triassic ages (ca. 280-240 Ma) suggesting sediment supply from Permian-Triassic magmatic rocks of the Silk Road Arc. Minor zircon populations show pre-Permian Palaeozoic ages, with age peaks at ca. 320 Ma and 480 Ma, which are probably derived from the basement on which the magmatic arc developed. Neoproterozoic-latest Mesoproterozoic (ca. 550-1100 Ma) and Palaeoproterozoic (ca. 1800-2200 Ma) zircon grains are anhedral (rounded). The latter are prominent in the upper Bayoroci Formation (Middle Ladinian) suggesting recycling of older sedimentary rocks. Sandstone petrography points toward an additional metamorphic provenance for this formation. This shortlived provenance change can be explained by tectonic uplift in the source area that led to erosion of metamorphosed rocks with a northeast Gondwanan affinity. It clearly indicates that northeast Gondwana-derived continental fragments likely belonging to the Cimmerian blocks already arrived at the southern Eurasian margin in pre-Late Triassic time. Current palaeotectonic models of the closure of Palaeotethys and the Eo-Cimmerian orogeny in the Middle East during the Triassic may need to be revised. (C) 2020 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.