Origin and tectonic implications of an Early Paleozoic (460–440 Ma) subduction-accretion shear zone in the northwestern Yunkai Domain, South China Songfeng Liu a , Songbai Peng a, ⁎, Timothy Kusky a,b,c , Ali Polat a,d , Qingsen Han a a School of Earth Sciences, Center for Global Tectonics, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China b Three Gorges Research Center for Geo-hazards, China University of Geosciences, Wuhan 430074, China c Department of Geological Engineering, Middle East Technical University, Ankara, Turkey d Department of Earth and Environmental Sciences, University of Windsor, Ontario, Canada abstract article info Article history: Received 8 June 2018 20 September 2018 Accepted 7 October 2018 Available online 09 October 2018 The geological evolution of the Early Paleozoic Wuyi-Yunkai orogen in South China is a scientific question of a long-standing debate. We document the presence of a NE-NEE-striking Early Paleozoic subduction-accretion shear zone, a possible mélange belt, in the northwestern Yunkai Domain. The northwestern Yunkai shear zone consists predominantly of tectonically juxtaposed fragments of Early Paleozoic flysch, arc volcanic rocks, and a forearc ophiolite. The Yunkai shear zone displays typical mélange structures in several locations; however, these structures are not continuous throughout the shear zone. The shear zone provides evidence for greenschist to amphibolite facies metamorphism and intense deformation resulting from Early Paleozoic to Early Mesozoic tectonic events. The flysch fragments are characterized by northwestward younging, northwestward thrusting, and northwestward migration of deformation and metamorphism. The arc volcanic rocks consist of basaltic- andesite, andesite, and dacite with a mainly sanukitic composition and also include Nb-enriched basalts. They are characterized by enrichment of LREEs, LILEs, Pb and depletion of HFSEs, suggesting a continental forearc or a continental arc tectonic setting. The ophiolitic fragments consist of MORB-like basalt and dolerite/gabbro. They have slightly depleted to flat LREE patterns and are characterized by enrichment of LILEs and Pb and deple- tion of HFSEs, indicating a continental forearc setting. Zircon U-Pb analyses yield 460–443 Ma and 455–437 Ma ages for the sanukitic volcanic rocks and ophiolitic fragments, respectively, suggesting that they formed in the Late Ordovician to Early Silurian. Both the sanukitic volcanic rocks and ophiolitic fragments possess negative to positive zircon ε Hf (t) values (-11.0 to +2.3), indicating that they may have been generated by partial melting of an old subarc mantle wedge source metasomatized by slab-derived fluids and/or melts. Recognition of the Early Paleozoic subduction-related magmatism and subduction-accretion structures in the northwestern Yunkai Domain has important implications for the tectonic history of the Wuyi-Yunkai orogen, South China. Collectively, combined with previous studies, we propose that the Huanan oceanic lithosphere began to subduct southeast- ward beneath the Yunkai terrane (arc) as early as 460 Ma, and the subduction continued between 460 and 440 Ma. © 2018 Elsevier B.V. All rights reserved. Keywords: Northwestern Yunkai Domain Arc volcanic rocks Forearc ophiolites Early Paleozoic tectonic mélange Subduction-accretion orogeny Yangtze/Cathaysia suture 1. Introduction Early Paleozoic orogenesis along the northern Gondwana margin mainly involved subduction-accretion and collisional processes, re- vealed by Early Paleozoic ophiolite belts and high-pressure (HP) or ultra-high-pressure (UHP) metamorphic rocks, such as in the North Qinling, North Tongbai, North Qilian, Altyn and Qaidam orogens (Xiao et al., 2009; Burrett et al., 2014; Yan et al., 2015; Dong and Santosh, 2016; Li et al., 2017 and reference therein). During the Early Paleozoic (480–400 Ma) era, a series of continental /microcontinental blocks, in- cluding the Yangtze, Cathaysia, Tarim, Qaidam, Alxa, North Qinling, Qilian, Oulongbuluke, South Qiangtang, Lhasa, Lanping-Simao, and In- dochina in the SE Asia, accreted to the northern Gondwana margin, resulting in the closure of the Proto-Tethys Ocean (Li et al., 2017, and reference therein). However, some geologists proposed that the Early Paleozoic Wuyi-Yunkai orogen in South China is a special intraplate orogen, which is not only different from the Early Paleozoic subduction-collision framework in SE Asia but also characterized by un- usual intraplate orogenic processes due to intrusion of a large volume of granitoids (Charvet, 2013; Chen et al., 2012a; Shu, 2012; Xia et al., 2014; Xu et al., 2016b). Therefore, it is especially important to better Lithos 322 (2018) 104–128 ⁎ Corresponding author. E-mail address: psongbai@aliyun.com (S. Peng). https://doi.org/10.1016/j.lithos.2018.10.006 0024-4937/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos