minerals Article Identification and Geological Significance of Early Jurassic Adakitic Volcanic Rocks in Xintaimen Area, Western Liaoning Zhi-Wei Song 1, Chang-Qing Zheng 1,2,*, Chen-Yue Liang 1,2 , Bo Lin 1, Xue-Chun Xu 1, Quan-Bo Wen 1, Ying-Li Zhao 1, Cheng-Gang Cao 3 and Zhi-Xin Wang 4 1 College of Earth Sciences, Jilin University, Changchun 130061, China; [email protected] (Z.-W.S.); [email protected] (C.-Y.L.); [email protected] (B.L.); [email protected] (X.-C.X.); [email protected] (Q.-B.W.); [email protected] (Y.-L.Z.) 2 Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Land and Resources, Jilin University, Changchun 130061, China 3 Qinghai Bureau of Environmental Geology Exploration, Xining 810008, China; [email protected] 4 Shandong No.3 Exploration Institute of Geology and Mineral Resources, Yantai 264000, China; [email protected] * Correspondence: [email protected]; Tel.: +86-13894881089 Abstract: The Western Liaoning area, where a large number of Jurassic-Cretaceous volcanic rocks are exposed, is one of the typical areas for studying the Mesozoic Paleo-Pacific and Mongolia-Okhotsk subduction process, and lithospheric destruction of North China Craton. The identification and investigation of Early Jurassic adakitic volcanic rocks in the Xintaimen area of Western Liaoning is of particular significance for exploring the volcanic magma source and its composition evolution, tracking the crust-mantle interaction, and revealing the craton destruction and the subduction of Citation: Song, Z.-W.; Zheng, C.-Q.; oceanic plates. Detailed petrography, zircon U–Pb dating, geochemistry, and zircon Hf isotope Liang, C.-Y.; Lin, B.; Xu, X.-C.; Wen, studies indicate that the Early Jurassic intermediate-acidic volcanic rocks are mainly composed Q.-B.; Zhao, Y.-L.; Cao, C.-G.; Wang, of trachydacites and a few rhyolites with the formation ages of 178.6–181.9 Ma. Geochemical Z.-X. Identification and Geological characteristics show that they have a high content of SiO2, MgO, Al2O3, and total-alkali, typical of Significance of Early Jurassic Adakitic the high-K calc-alkaline series. They also show enrichment of light rare earth elements (LREEs) and Volcanic Rocks in Xintaimen Area, large ion lithophile elements (LILEs), depletion of heavy rare earth elements (HREEs) and high field Western Liaoning. Minerals 2021, 11, strength elements (HFSEs), and have a high content of Sr and low content of Y and Yb, suggesting 331. https://doi.org/10.3390/ " min11030331 that they were derived from the partial melting of the lower crust. The Hf(t) values of dated zircons and two-stage model ages (TDM2) vary from −11.6 to −7.4 and from 1692 to 1958 Ma, respectively. Academic Editor: Tamara Bayanova During the Early Jurassic, the study area was under long-range tectonic effects with the closure of the Mongolia-Okhotsk Ocean and the subduction of the Paleo-Pacific plate, which caused the Received: 4 February 2021 basaltic magma to invade the lower crust of the North China Craton. The mantle-derived magma Accepted: 18 March 2021 was separated and crystallized while heating the Proterozoic lower crust, and part of the thickened Published: 23 March 2021 crust melted to form these intermediate-acidic adakitic volcanic rocks. Publisher’s Note: MDPI stays neutral Keywords: adakite; zircon U-Pb age; North China Craton; Western Liaoning; Paleo-Pacific plate with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction Adakite has attracted much attention due to its unique magma source, geodynamic significance, and close relationship with metal deposits. The term “adakite” originally Copyright: © 2021 by the authors. refers to a set of intermediate-acidic igneous rocks formed by slab melting in the subduc- Licensee MDPI, Basel, Switzerland. tion environment, with geochemical characteristics of MgO ≤ 3 wt.%, SiO2 ≥ 56 wt.%, This article is an open access article Al2O3 ≥ 15 wt.%, La/Yb ≥ 20, Sr/Y ≥ 40, and/or low Y ≤ 18 ppm [1–4]. However, distributed under the terms and some intermediate-acidic igneous rocks with the geochemical characteristics of adakite are conditions of the Creative Commons not necessarily developed in an island arc environment or by the melting of subducted Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ slabs [5–11]. These rocks are collectively referred to as adakitic rocks. In this study, a set of 4.0/). intermediate-acidic adakitic volcanic rocks with high-K calc-alkaline series affinity were Minerals 2021, 11, 331. https://doi.org/10.3390/min11030331 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 23 wt.%, Al2O3 ≥ 15 wt.%, La/Yb ≥ 20, Sr/Y ≥ 40, and/or low Y ≤ 18 ppm [1–4]. However, some intermediate-acidic igneous rocks with the geochemical characteristics of adakite are not necessarily developed in an island arc environment or by the melting of subducted slabs Minerals 2021, 11, 331 2 of 22 [5–11]. These rocks are collectively referred to as adakitic rocks. In this study, a set of intermediate-acidic adakitic volcanic rocks with high-K calc-alkaline series affinity were identified in the Western Liaoning area, which is located on the northern margin of the North China Cratonidentified (NCC; in Figure the Western 1). Liaoning area, which is located on the northern margin of the North China Craton (NCC; Figure1). As one of the oldest cratons in the world, the NCC has a long history of ~3.8 billion As one of the oldest cratons in the world, the NCC has a long history of ~3.8 billion years, which preservesyears, whicha relatively preserves complete a relatively tectonic complete evolution tectonic that evolutioncontains metamorphic that contains metamor- basement from phicArchean basement to Paleoproterozoic from Archean to Paleoproterozoic and Phanerozoic and Phanerozoiccaprock development caprock development (Figure 1) [12–19](Figure. The1 )[Western12–19]. Liaoning The Western is Liaoninglocated in is locatedthe northern in the northern margin margin of the ofNCC the NCC and and the eastern thepart eastern of the part Yanshan of the Yanshantectonic tectonic belt which belt whichdeveloped developed a large a large number number of of Meso- Mesozoic volcaniczoic-sedimentary volcanic-sedimentary basins basinswith lots with lotsof lacustrine of lacustrine paleontological paleontological fossils.fossils. Therefore, Therefore, WesternWestern Liaoning Liaoning is one is one of ofthe the classic classic areas areas to studystudy thethe Mesozoic Mesozoic tectonic-magma tectonic- and magma and biologicalbiological evolution evolution in inthe the NCC NCC (Figures (Figures1 1 and and2)[ 2)6 ,[186,,1820,–2026–].26]. Figure 1.FigureSimplified 1. Simplified map showing map the showing tectonic the subdivisions tectonic ofsubdivisions the North China of the Craton North and China the distribution Craton and of thethe basement rocks (modifieddistribution after of Zhao the et basement al. [13]; NCC rocks for (modified North China after Craton, Zhao SCC et al. for [ South13]; NCC China for Craton). North China Craton, SCC for South China Craton). Since the Mesozoic, the Western Liaoning region was affected by the superimposition Since the Mesozoic,of the NE the trending Western Paleo-Pacific Liaoning plate region tectonic was domain,affected Mongolia-Okhotskby the superimposition tectonic domain of the NE trendingand Paleo the nearly-Pacific EW plat trendinge tectonic Central domain, Asian Mongolia Orogenic-Okhotsk Belt (CAOB). tectonic In complexdomain tectonic regimes, the wide development of Mesozoic volcanic rocks began in the Early Jurassic and the nearly EW trending Central Asian Orogenic Belt (CAOB). In complex tectonic and ended in the late Early Cretaceous. Geochronology of the Mesozoic volcanic events regimes, the widein development the Western Liaoning of Mesozoic region volcanic have been rocks studied began in detail in the by Early predecessors, Jurassic and which can be ended in the latedivided Early Cretaceo into fourus. phases: Geochronology Early Jurassic of (Nandaling the Mesozoic or Xinglonggou volcanic events Period), in Latethe Jurassic Western Liaoning(Tiaojishan region have or Lanqibeen studied Period), in Early detail Cretaceous by predecessors, (Zhangjiakou which or can Yixian be Period),divided and Late into four phases:Cretaceous Early Jurassic (Daxingzhuang (Nandaling Period) or [6 ,24Xinglonggou,27–37]. However, Period), most ofLate the geochronologicalJurassic (Tiaojishan or Lanqiand biologicalPeriod), Early studies Cretaceous focused on (Zhangjiakou the Late Jurassic-Early or Yixian Cretaceous Period), and magmatic Late events, Cretaceous (Daxingzhuangand most scholars Period) believed [6,24,27 that–37 the]. However, Late Jurassic most magmatic of the geochronological rocks may be the result of and biological studiesthe Paleo-Pacific focused on subduction the Late Jurassic [6,20,21-Early] or the Cretaceous combined influencemagmatic of events, the subduction and and closure of the Mongolia-Okhotsk Ocean [38–40]. The widely distributed Early Cretaceous most scholars believed that the Late Jurassic magmatic rocks may be the result of the Paleo- magmatic rocks were formed under a strong extensional background, which was related to Pacific subductionthe [6,20,21] subduction or the of the combined Paleo-Pacific influence plate [ 20of, 21the,25 subduction,26,40,41]. However, and closure the distribution of the and Mongolia-Okhotsktectonic Ocean setting [38–4 of0]. the The Early widely Jurassic