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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Earth Sci.</journal-id>
<journal-title>Frontiers in Earth Science</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Earth Sci.</abbrev-journal-title>
<issn pub-type="epub">2296-6463</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="publisher-id">1031842</article-id>
<article-id pub-id-type="doi">10.3389/feart.2022.1031842</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Earth Science</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Intra-oceanic subduction in the southeastern Paleo-Asian ocean: Evidence from early Permian gabbro of the Diyanmiao ophiolite in central Inner Mongolia, North China</article-title>
<alt-title alt-title-type="left-running-head">Zhang et al.</alt-title>
<alt-title alt-title-type="right-running-head">
<ext-link ext-link-type="uri" xlink:href="https://doi.org/10.3389/feart.2022.1031842">10.3389/feart.2022.1031842</ext-link>
</alt-title>
</title-group>
<contrib-group>
<contrib contrib-type="author">
<name>
<surname>Zhang</surname>
<given-names>Xiawei</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1889230/overview"/>
</contrib>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Li</surname>
<given-names>Yingjie</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
<xref ref-type="corresp" rid="c001">&#x2a;</xref>
<uri xlink:href="https://loop.frontiersin.org/people/2116539/overview"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Wang</surname>
<given-names>Genhou</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Wang</surname>
<given-names>Shuai</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Kong</surname>
<given-names>Xingrui</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Wang</surname>
<given-names>Xiaodong</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Liu</surname>
<given-names>Zhibin</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
</contrib>
</contrib-group>
<aff id="aff1">
<sup>1</sup>
<institution>School of the Earth Sciences and Resources</institution>, <institution>China University of Geosciences</institution>, <addr-line>Beijing</addr-line>, <country>China</country>
</aff>
<aff id="aff2">
<sup>2</sup>
<institution>Hebei Key Laboratory of Strategic Critical Mineral Resources</institution>, <institution>Heibei GEO University</institution>, <addr-line>Shijiazhuang</addr-line>, <country>China</country>
</aff>
<aff id="aff3">
<sup>3</sup>
<institution>College of Earth Sciences</institution>, <institution>Hebei GEO University</institution>, <addr-line>Shijiazhuang</addr-line>, <country>China</country>
</aff>
<aff id="aff4">
<sup>4</sup>
<institution>Hebei Province Collaborative Innovation Center for Strategic Critical Mineral Research</institution>, <institution>Hebei GEO University</institution>, <addr-line>Shijiazhuang</addr-line>, <country>China</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>
<bold>Edited by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/1685878/overview">Fan Yang</ext-link>, Lanzhou University, China</p>
</fn>
<fn fn-type="edited-by">
<p>
<bold>Reviewed by:</bold> <ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/2008841/overview">Yinhang Cheng</ext-link>, China Geological Survey, China</p>
<p>
<ext-link ext-link-type="uri" xlink:href="https://loop.frontiersin.org/people/2056481/overview">Leilei Dong</ext-link>, University of Science and Technology Beijing, China</p>
</fn>
<corresp id="c001">&#x2a;Correspondence: Yingjie Li, <email>liyingjie820@126.com</email>
</corresp>
<fn fn-type="other">
<p>This article was submitted to Structural Geology and Tectonics, a section of the journal Frontiers in Earth Science</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>04</day>
<month>01</month>
<year>2023</year>
</pub-date>
<pub-date pub-type="collection">
<year>2022</year>
</pub-date>
<volume>10</volume>
<elocation-id>1031842</elocation-id>
<history>
<date date-type="received">
<day>30</day>
<month>08</month>
<year>2022</year>
</date>
<date date-type="accepted">
<day>12</day>
<month>12</month>
<year>2022</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2023 Zhang, Li, Wang, Wang, Kong, Wang and Liu.</copyright-statement>
<copyright-year>2023</copyright-year>
<copyright-holder>Zhang, Li, Wang, Wang, Kong, Wang and Liu</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.</p>
</license>
</permissions>
<abstract>
<p>Investigation of intra-oceanic subduction can improve our understanding of plate tectonic processes and the history of continental growth. Evidence for intra-oceanic subduction in the Paleo-Asian Ocean has recently become an important focus of research, including the Diyanmiao ophiolite in central Inner Mongolia, North China. Here, we report a newly discovered occurrence of early Permian gabbro in the Diyanmiao ophiolite zone. The gabbro yields a weighted mean zircon U&#x2013;Pb age of 294.4 &#xb1; 2.2&#xa0;Ma. The gabbro samples are characterized by moderate SiO<sub>2</sub> (47.32&#x2013;50.51&#xa0;wt%), low TiO<sub>2</sub> (0.26&#x2013;0.54&#xa0;wt%) and K<sub>2</sub>O (0.04&#x2013;0.75&#xa0;wt%), and high Na<sub>2</sub>O (1.84&#x2013;4.52&#xa0;wt%) contents, high Na<sub>2</sub>O/K<sub>2</sub>O ratios (2.92&#x2013;58.29), and depleted chondrite-normalized light rare Earth element patterns that are similar to N-MORB. The gabbros show slightly lower contents of high-field-strength elements (e.g., Nb, Ta, and Ti) and slightly higher contents of large-ion lithophile elements (e.g., K, Rb, Ba, and U) relative to N-MORB. In addition, the gabbros show high &#x3b5;<sub>Nd</sub> (<italic>t</italic>) values (8.0&#x2013;9.8) that are similar to those of forearc basalt in the Diyanmiao ophiolite and N-MORB. Integrating these new data with available results for ophiolite and arc-magmatic rocks of central Inner Mongolia, we propose that the studied gabbro was formed during the initial stage of intra-oceanic subduction and that the Paleo-Asian Ocean was still in a subduction setting during the early Permian.</p>
</abstract>
<kwd-group>
<kwd>geochronology</kwd>
<kwd>gabbro</kwd>
<kwd>early permian</kwd>
<kwd>intra-oceanic subduction</kwd>
<kwd>Paleo-Asian Ocean</kwd>
</kwd-group>
</article-meta>
</front>
<body>
<sec id="s1">
<title>1 Introduction</title>
<p>Ophiolites represent fragments of ancient oceanic lithosphere in continental orogenic belts and record the magmatic evolution, metamorphic history, and structural processes of oceanic lithosphere, and also provide important information on the development and extinction of ancient oceanic basins (<xref ref-type="bibr" rid="B45">Nicolas, 1989</xref>). Ophiolites form in a variety of plate tectonic settings (e.g., <xref ref-type="bibr" rid="B45">Nicolas, 1989</xref>; <xref ref-type="bibr" rid="B14">Dilek and Robinson, 2003</xref>; <xref ref-type="bibr" rid="B13">Dilek and Furnes, 2009</xref>, <xref ref-type="bibr" rid="B11">2011</xref>, <xref ref-type="bibr" rid="B12">2014</xref>; <xref ref-type="bibr" rid="B30">Kusky, 2011</xref>; <xref ref-type="bibr" rid="B56">Santosh et al., 2016</xref>) and are eventually obducted onto continental margins or incorporated into accretionary prisms and orogens, thereby serving as marker zones of accretion and suturing between crustal blocks that were originally separated by oceanic basins (<xref ref-type="bibr" rid="B11">Dilek and Furnes, 2011</xref>). Consequently, the geology, geochemistry, and geochronology of ophiolites and spatially associated lithological units provide important insights into the evolution of major orogenic belts (<xref ref-type="bibr" rid="B58">Shervais et al., 2004</xref>).</p>
<p>The Central Asian Orogenic Belt (CAOB) was formed during a prolonged process involving subduction, accretion, and collision of the Paleo-Asian oceanic plate (<xref ref-type="bibr" rid="B15">Dobretsov et al., 1995</xref>). This belt was subsequently superimposed and transformed by the Mesozoic Mongolia&#x2013;Okhotsk and Pacific tectonic regimes. The CAOB is considered the world&#x2019;s largest Phanerozoic accretionary orogen and is located between the Siberian Craton to the north and the Tarim and North China cratons to the south (<xref ref-type="fig" rid="F1">Figure 1A</xref>). The belt contains intra-oceanic forearc materials, oceanic islands and seamounts, and ancient oceanic crust fragments as a result of the subduction of the Paleo-Asian oceanic plate (<xref ref-type="bibr" rid="B2">Badarch et al., 2002</xref>; <xref ref-type="bibr" rid="B71">Windley et al., 2007</xref>; <xref ref-type="bibr" rid="B75">Xiao et al., 2009</xref>, 2014; <xref ref-type="bibr" rid="B29">Kr&#xf6;ner et al., 2017</xref>; <xref ref-type="bibr" rid="B54">Safonova, 2017</xref>; <xref ref-type="bibr" rid="B83">Yang et al., 2017</xref>, <xref ref-type="bibr" rid="B82">2021</xref>; <xref ref-type="bibr" rid="B7">Cheng et al., 2019</xref>; <xref ref-type="bibr" rid="B41">Ma et al., 2021</xref>).</p>
<fig id="F1" position="float">
<label>FIGURE 1</label>
<caption>
<p>
<bold>(A)</bold> Schematic tectonic division of north China-Mongolia segment (modified from <xref ref-type="bibr" rid="B23">Jahn, 2004</xref>). <bold>(B)</bold> Sketch geological map of the late Paleozoic crust of southeastern Inner Mongolia (modified from <xref ref-type="bibr" rid="B74">Xiao et al., 2003</xref>). <bold>(C)</bold> Simplified geological map of the Diyanmiao area (modified after <xref ref-type="bibr" rid="B33">Li et al., 2020a</xref>).</p>
</caption>
<graphic xlink:href="feart-10-1031842-g001.tif"/>
</fig>
<p>The southeastern part of the CAOB is a key area for exploring the Paleozoic tectonic evolution of this orogenic belt, as it contains numerous ophiolite belts, including the Erenhot&#x2013;Hegenshan ophiolite belt (EHOB), Linxi belt, and Xar Moron River belt (XMRB), which are thought to mark the location of final closure of the Paleo-Asian Ocean (PAO) (<xref ref-type="bibr" rid="B66">Wang and Liu, 1986</xref>; <xref ref-type="bibr" rid="B64">Tang, 1990</xref>; <xref ref-type="bibr" rid="B3">BGMRIMAR, 1996</xref>; <xref ref-type="bibr" rid="B74">Xiao et al., 2003</xref>; <xref ref-type="bibr" rid="B60">Song et al., 2015</xref>; Yuri et al., 2020) (<xref ref-type="fig" rid="F1">Figures 1B, C</xref>). Although the EHOB is traditionally considered to have formed in a mid-ocean ridge basalts (MORB) (<xref ref-type="bibr" rid="B46">Nozaka and Liu, 2002</xref>), the formation of most Inner Mongolia ophiolites is ascribed to a supra-subduction zone (SSZ) environment on the basis of the geochemistry of their magmatic rocks (e.g., <xref ref-type="bibr" rid="B59">Shervais, 2001</xref>). Such ophiolites include the Neo-Tethyan ophiolite in SW Turkey (<xref ref-type="bibr" rid="B1">Aldanmaz et al., 2009</xref>) and the Troodos ophiolite in Cyprus (<xref ref-type="bibr" rid="B47">Osozawa et al., 2012</xref>).</p>
<p>Two models have been proposed for the late Paleozoic evolution of the PAO. Some studies have suggested that continuous subduction of the Paleo-Asian oceanic plate induced multiple accretion events on the Uliastai continental margin (UCM) and the northern margin of the Sino-Korean paleoplate during the Paleozoic. The collision of two paleoplates along the Solonker&#x2013;Linxi or Xar Moron River belts led to the final formation of the CAOB during the late Permian or Early Triassic (<xref ref-type="bibr" rid="B72">Windley, 1993</xref>; <xref ref-type="bibr" rid="B74">Xiao et al., 2003</xref>; <xref ref-type="bibr" rid="B9">Cheng et al., 2014</xref>, <xref ref-type="bibr" rid="B8">2015</xref>). Other studies have argued that the PAO closed during the early or middle Paleozoic along the EHB or the Xilin Hot&#x2013;Heihe suture zone and that the Carboniferous and Permian ophiolites in the area formed in a rift setting (<xref ref-type="bibr" rid="B76">Xu et al., 2013</xref>; <xref ref-type="bibr" rid="B79">Zhao et al., 2015</xref>). The main difference between the two models is whether the southeastern PAO was in a subduction or extensional tectonic setting during the late Paleozoic.</p>
<p>During recent work, we identified early Permian gabbro fragments in the Diyanmiao ophiolitic m&#xe9;lange of southeastern Inner Mongolia. These fragments represent northward subduction of the Paleo-Asian oceanic plate beneath the southern margin of the Siberian plate and formed after the early Carboniferous&#x2013;early Permian (<xref ref-type="bibr" rid="B34">Li et al., 2018</xref>) (<xref ref-type="fig" rid="F2">Figure 2</xref>). In this paper, we combine new data on the petrology, geochronology, and geochemistry of gabbro from the Diyanmiao ophiolite with results of previous studies in this area to establish the evolution of the PAO during the late Carboniferous and early Permian.</p>
<fig id="F2" position="float">
<label>FIGURE 2</label>
<caption>
<p>Simplified geological map of the Naolaiketu ophiolite in Diyanmiao (modified after <xref ref-type="bibr" rid="B33">Li et al., 2020a</xref>).</p>
</caption>
<graphic xlink:href="feart-10-1031842-g002.tif"/>
</fig>
</sec>
<sec id="s2">
<title>2 Geological setting</title>
<p>Five NEE&#x2013;SWW-trending tectonic zones have been identified in southeastern Inner Mongolia: the Southern orogenic belt, the Solonker suture zone, the Northern orogenic belt (NOB), the EHOB, and the UCM from south to north (<xref ref-type="fig" rid="F1">Figure 1B</xref>) (<xref ref-type="bibr" rid="B74">Xiao et al., 2003</xref>; <xref ref-type="bibr" rid="B25">Jian et al., 2012</xref>).</p>
<p>The UCM contains post-collisional basalts, andesites, and pyroclastic deposits of late Carboniferous&#x2013;Permian age. These volcanic&#x2013;sedimentary sequences overlie Cambrian to Devonian shallow-marine and continental fine clastic rocks (<xref ref-type="bibr" rid="B77">Zhang et al., 2011</xref>; <xref ref-type="bibr" rid="B32">Li et al., 2014</xref>; Anas et al., 2020). The EHOB lies to the south of the UCM and is characterized by numerous ophiolite slices, including the Hegenshan, Baiyinbulage, and Diyanmiao ophiolites from west to east (<xref ref-type="bibr" rid="B35">Li et al., 2015</xref>), which are enclosed in Carboniferous&#x2013;Permian flysch (<xref ref-type="bibr" rid="B66">Wang and Liu, 1986</xref>; <xref ref-type="bibr" rid="B37">Liang, 1991</xref>). The EHOB extends from Erenhot, close to the China&#x2013;Mongolia border, in the southwest to Hegenshan in the northeast. The EHOB is separated from the NOB to the south by the Solonker suture zone (<xref ref-type="fig" rid="F1">Figure 1B</xref>). The NOB comprises the Xilin Gol and Baolidao Arc complexes (<xref ref-type="bibr" rid="B74">Xiao et al., 2003</xref>). The Xilin Gol complex, which is thought to represent Precambrian basement (e.g., <xref ref-type="bibr" rid="B18">Ge et al., 2011</xref>; <xref ref-type="bibr" rid="B61">Sun et al., 2013</xref>), consists mainly of biotite and muscovite schists, quartz&#x2013;feldspar gneisses, and plagioclase amphibolites (<xref ref-type="bibr" rid="B74">Xiao et al., 2003</xref>; <xref ref-type="bibr" rid="B6">Chen et al., 2009</xref>). This complex is unconformably overlain by a cover of late Carboniferous and Permian marine strata (<xref ref-type="bibr" rid="B34">Li et al., 2018</xref>). The Baolidao Arc complex, which extends from Bayanbaolidao to Xilinhot, is composed of cumulate gabbros, gabbro diorites, quartz diorites, tonalites, and granites, and represents a magmatic arc formed by northward seafloor subduction (<xref ref-type="bibr" rid="B74">Xiao et al., 2003</xref>, <xref ref-type="bibr" rid="B75">2009</xref>; <xref ref-type="bibr" rid="B26">Jian et al., 2008</xref>).</p>
</sec>
<sec id="s3">
<title>3 Field and petrographic features</title>
<p>The Diyanmiao ophiolite is located in West Ujimqin, eastern Inner Mongolia (<xref ref-type="fig" rid="F1">Figure 1B</xref>), and is divided into the Naolaiketu ophiolite to the south and the Baiyinbulage ophiolite to the north (<xref ref-type="bibr" rid="B34">Li et al., 2018</xref>; <xref ref-type="bibr" rid="B33">2020a</xref>). The Naolaiketu ophiolite is exposed in the Naolaiketu&#x2013;Diyanmiao area as an ENE&#x2013;WSW-trending belt with a width of &#x223c;3&#xa0;km and length of &#x223c;28&#xa0;km. In the ophiolite belt, well-developed ductile deformation is shown by the development of folds, ENE&#x2013;WSW-trending stretching lineations, and mylonitic microstructures, as well as domains with strongly developed rhombic-meshed structure and weakly deformed domains. The lithological units comprise serpentinized harzburgite, layered gabbro, fine-grained isotropic gabbro, spilite, basalt, and quartz keratophyres, from bottom to top, and are overlain by chert. Of these, spillite and fine-grained isotropic gabbro are the most widely exposed. At the site of fault contact of the ophiolite and early Permian strata (Shoushangou and Dashizhai formations), well-developed brittle deformation structures are observed, including cleavages and cataclasite bands. Compressed and fragmented belts of ophiolite range in width from several meters to hundreds of meters.</p>
<p>For this study, samples of fine-grained isotropic gabbro (<xref ref-type="fig" rid="F2">Figure 2</xref>) were obtained for geochemical and geochronological analyses (<xref ref-type="fig" rid="F3">Figures 3A, B</xref>). The gabbro mainly consists of fine-grained, tabular plagioclase (40&#x2013;45&#xa0;vol%) and granular pyroxene (35&#x2013;40&#xa0;vol%), with minor magnetite and ilmenite (&#x223c;5&#xa0;vol%). The rock shows hypidiomorphic and granular textures under the microscope (<xref ref-type="fig" rid="F3">Figures 3C, D</xref>). The rocks have been strongly altered, including replacement of plagioclase by zoisite and pyroxene by uralite.</p>
<fig id="F3" position="float">
<label>FIGURE 3</label>
<caption>
<p>Macrofeatures and photomicrographs of the gabbro in Diyanmiao ophiolite. <bold>(A)</bold>-outcrop of gabbro; <bold>(B)</bold>-hand specimen of gabbro; <bold>(C,D)</bold>-photomicrographs of gabbro; Pl, plagioclase; Px, pyroxene; Chl, chlorite; Urt, uralite.</p>
</caption>
<graphic xlink:href="feart-10-1031842-g003.tif"/>
</fig>
</sec>
<sec id="s4">
<title>4 Analytical techniques</title>
<sec id="s4-1">
<title>4.1 <italic>In situ</italic> zircon U&#x2013;Pb dating</title>
<p>One sample of gabbro (TW0166) was selected for <italic>in situ</italic> zircon U&#x2013;Pb dating. Zircon grains were separated by crushing, sieving, and standard heavy-liquid techniques, followed by hand-picking under a binocular microscope at the Laboratory of Regional Geology and Mineral Resources Research Institute of Hebei Province, Langfang, China. The separated zircons were mounted in epoxy resin discs and polished to expose the grain interiors for analyses. Transmitted- and reflected-light photomicrographs, as well as cathodoluminescence (CL) images, were obtained to reveal zircon internal structure. CL images were acquired using a scanning electron microscope (JSM6510) equipped with a GATAN CL instrument at Beijing Gaonianlinghang Geo Analysis, Beijing, China.</p>
<p>Zircon U&#x2013;Pb dating was performed using laser-ablation&#x2013;multi-collector&#x2013;inductively coupled plasma&#x2013;mass spectrometry (LA&#x2013;MC&#x2013;ICP&#x2013;MS) at the Tianjin Institute of Geology and Mineral Resources, China. The instrumental conditions and analytical processes were similar to those described by <xref ref-type="bibr" rid="B39">Liu et al. (2008)</xref>. Operating conditions included a laser spot diameter of 32&#xa0;&#x3bc;m and a laser frequency of 7&#xa0;Hz. Contents of U and Pb were calibrated by using the standard zircon TEMORA (417&#xa0;Ma), while zircon GJ-1 (597&#xa0;Ma) was used as the external standard. The Excel-based software ICPMSDataCal program (<xref ref-type="bibr" rid="B39">Liu et al., 2008</xref>) was used to calculate U and Th contents and U&#x2013;Pb dating results, including calibrations, and the ISOPLOT 4.15 program (<xref ref-type="bibr" rid="B40">Ludwig, 2003</xref>) was used to plot results.</p>
</sec>
<sec id="s4-2">
<title>4.2 Whole-rock major and trace element analyses</title>
<p>We selected eight representative samples of the gabbro for bulk-rock major- and trace-element analyses at the Tianjin Geological Survey Center of China Geological Survey, China. Major elements were analyzed using X-ray fluorescence (XRF) spectrometry (Rigaku 3270E) with high-dispersion Echelle optics. Analyses of Chinese national standard rock samples GSR-1, GSR-2, and GSR-3 revealed that the analytical precision is generally better than &#xb1;2% for most major elements.</p>
<p>Bulk-rock trace-element analyses were performed using ICP&#x2013;MS at the Tianjin Geological Survey Center of China Geological Survey following the analytical procedure of <xref ref-type="bibr" rid="B39">Liu et al. (2008)</xref>. The analytical precision for most trace elements is better than 5%, as monitored by analyses of Chinese national standard samples GSR-1 and GSR-3. The analytical precision was better than &#xb1;5% for trace elements.</p>
</sec>
<sec id="s4-3">
<title>4.3 Sr and Nd isotopes</title>
<p>Sr and Nd isotope ratios were measured for 12 samples using a Thermo-Finnigan VG Sector 54 thermal ionization mass spectrometer at the Tianjin Geological Survey Center of China Geological Survey following the analytical procedures described by <xref ref-type="bibr" rid="B44">Miao et al. (2008)</xref>. Ratios of <sup>87</sup>Rb/<sup>86</sup>Sr and <sup>147</sup>Sm/<sup>144</sup>Nd were calculated using the Rb, Sr, Sm, and Nd contents determined by ICP&#x2013;MS. The mass fractionation corrections for isotopic ratios were measured using a value of 0.1194 for <sup>86</sup>Sr/<sup>88</sup>Sr and 0.7219 for <sup>146</sup>Nd/<sup>144</sup>Nd. The USGS rock standard BCR-2 was used to evaluate the separation and purification process of Rb, Sr, Sm, and Nd, yielding <sup>87</sup>Sr/<sup>86</sup>Sr &#x3d; 0.704963 &#xb1; 0.000003 and <sup>143</sup>Nd/<sup>144</sup>Nd &#x3d; 0.512354 &#xb1; 0.000004.</p>
</sec>
</sec>
<sec id="s5">
<title>5 Analytical results</title>
<sec id="s5-1">
<title>5.1 Zircon U&#x2013;Pb geochronology</title>
<p>Zircon CL images from the fine-grained isotropic gabbro sample TW0166 are shown in <xref ref-type="fig" rid="F4">Figure 4</xref>, and zircon U&#x2013;Pb isotope data are given in <xref ref-type="table" rid="T1">Table 1</xref>. Most of the zircons are clear, euhedral, and short or granular prismatic crystals with lengths of 50&#x2013;100&#xa0;&#x3bc;m and aspect ratios of 1:1 to 2:1. The grains show well-developed straight and wide oscillatory zoning in CL images, consistent with mafic volcanic or gabbroic rocks (<xref ref-type="bibr" rid="B60">Song et al., 2015</xref>).</p>
<fig id="F4" position="float">
<label>FIGURE 4</label>
<caption>
<p>
<bold>(A)</bold> Representative cathodoluminescence (CL) images of zircons from gabbro sample TW0166 <bold>(B)</bold> Zircon U-Pb concordia diagrams and weighed mean age for the gabbro sample TW0166.</p>
</caption>
<graphic xlink:href="feart-10-1031842-g004.tif"/>
</fig>
<table-wrap id="T1" position="float">
<label>TABLE 1</label>
<caption>
<p>LA-MC-ICP-MS zircon U-Pb isotopic analysis of the gabbro in Diyanmiao ophiolite.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th rowspan="2" align="center">Spots no.</th>
<th colspan="2" align="center">Element (ppm)</th>
<th rowspan="2" align="center">Th/U</th>
<th colspan="6" align="center">Isotopic ratios</th>
<th colspan="6" align="center">Apparent age (Ma)</th>
</tr>
<tr>
<th align="center">Th</th>
<th align="center">U</th>
<th align="center">
<sup>206</sup>Pb/<sup>238</sup>U</th>
<th align="center">1&#x3c3;</th>
<th align="center">
<sup>207</sup>Pb/<sup>235</sup>U</th>
<th align="center">1&#x3c3;</th>
<th align="center">
<sup>207</sup>Pb/<sup>206</sup>Pb</th>
<th align="center">1&#x3c3;</th>
<th align="center">
<sup>206</sup>Pb/<sup>238</sup>U</th>
<th align="center">1&#x3c3;</th>
<th align="center">
<sup>207</sup>Pb/<sup>235</sup>U</th>
<th align="center">1&#x3c3;</th>
<th align="center">
<sup>207</sup>Pb/<sup>206</sup>Pb</th>
<th align="center">1&#x3c3;</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td colspan="16" align="center">TW0166 Gabbro</td>
</tr>
<tr>
<td align="center">&#x2003;01</td>
<td align="center">354</td>
<td align="center">340</td>
<td align="center">1.04</td>
<td align="center">0.0475</td>
<td align="center">0.0005</td>
<td align="center">0.3425</td>
<td align="center">0.0093</td>
<td align="center">0.0523</td>
<td align="center">0.0014</td>
<td align="center">299</td>
<td align="center">3</td>
<td align="center">299</td>
<td align="center">8</td>
<td align="center">299</td>
<td align="center">61</td>
</tr>
<tr>
<td align="center">&#x2003;02</td>
<td align="center">98</td>
<td align="center">122</td>
<td align="center">0.80</td>
<td align="center">0.0471</td>
<td align="center">0.0005</td>
<td align="center">0.3439</td>
<td align="center">0.0210</td>
<td align="center">0.0530</td>
<td align="center">0.0032</td>
<td align="center">296</td>
<td align="center">3</td>
<td align="center">300</td>
<td align="center">18</td>
<td align="center">328</td>
<td align="center">139</td>
</tr>
<tr>
<td align="center">&#x2003;03</td>
<td align="center">131</td>
<td align="center">102</td>
<td align="center">1.28</td>
<td align="center">0.0461</td>
<td align="center">0.0005</td>
<td align="center">0.3434</td>
<td align="center">0.0258</td>
<td align="center">0.0540</td>
<td align="center">0.0041</td>
<td align="center">291</td>
<td align="center">3</td>
<td align="center">300</td>
<td align="center">22</td>
<td align="center">372</td>
<td align="center">171</td>
</tr>
<tr>
<td align="center">&#x2003;04</td>
<td align="center">16</td>
<td align="center">74</td>
<td align="center">0.22</td>
<td align="center">0.0475</td>
<td align="center">0.0005</td>
<td align="center">0.3465</td>
<td align="center">0.0442</td>
<td align="center">0.0529</td>
<td align="center">0.0069</td>
<td align="center">299</td>
<td align="center">3</td>
<td align="center">302</td>
<td align="center">38</td>
<td align="center">323</td>
<td align="center">294</td>
</tr>
<tr>
<td align="center">&#x2003;05</td>
<td align="center">65</td>
<td align="center">138</td>
<td align="center">0.47</td>
<td align="center">0.0476</td>
<td align="center">0.0005</td>
<td align="center">0.3435</td>
<td align="center">0.0190</td>
<td align="center">0.0523</td>
<td align="center">0.0029</td>
<td align="center">300</td>
<td align="center">3</td>
<td align="center">300</td>
<td align="center">17</td>
<td align="center">298</td>
<td align="center">125</td>
</tr>
<tr>
<td align="center">&#x2003;06</td>
<td align="center">83</td>
<td align="center">159</td>
<td align="center">0.52</td>
<td align="center">0.0468</td>
<td align="center">0.0005</td>
<td align="center">0.3455</td>
<td align="center">0.0201</td>
<td align="center">0.0535</td>
<td align="center">0.0031</td>
<td align="center">295</td>
<td align="center">3</td>
<td align="center">301</td>
<td align="center">18</td>
<td align="center">351</td>
<td align="center">129</td>
</tr>
<tr>
<td align="center">&#x2003;07</td>
<td align="center">308</td>
<td align="center">135</td>
<td align="center">2.28</td>
<td align="center">0.0459</td>
<td align="center">0.0005</td>
<td align="center">0.3500</td>
<td align="center">0.0285</td>
<td align="center">0.0553</td>
<td align="center">0.0045</td>
<td align="center">289</td>
<td align="center">3</td>
<td align="center">305</td>
<td align="center">25</td>
<td align="center">426</td>
<td align="center">183</td>
</tr>
<tr>
<td align="center">&#x2003;08</td>
<td align="center">156</td>
<td align="center">399</td>
<td align="center">0.39</td>
<td align="center">0.0463</td>
<td align="center">0.0005</td>
<td align="center">0.3461</td>
<td align="center">0.0077</td>
<td align="center">0.0542</td>
<td align="center">0.0012</td>
<td align="center">292</td>
<td align="center">3</td>
<td align="center">302</td>
<td align="center">7</td>
<td align="center">379</td>
<td align="center">49</td>
</tr>
<tr>
<td align="center">&#x2003;09</td>
<td align="center">22</td>
<td align="center">78</td>
<td align="center">0.28</td>
<td align="center">0.0465</td>
<td align="center">0.0006</td>
<td align="center">0.3411</td>
<td align="center">0.0391</td>
<td align="center">0.0532</td>
<td align="center">0.0062</td>
<td align="center">293</td>
<td align="center">3</td>
<td align="center">298</td>
<td align="center">34</td>
<td align="center">337</td>
<td align="center">262</td>
</tr>
<tr>
<td align="center">&#x2003;10</td>
<td align="center">50</td>
<td align="center">125</td>
<td align="center">0.40</td>
<td align="center">0.0476</td>
<td align="center">0.0005</td>
<td align="center">0.3435</td>
<td align="center">0.0220</td>
<td align="center">0.0523</td>
<td align="center">0.0033</td>
<td align="center">300</td>
<td align="center">3</td>
<td align="center">300</td>
<td align="center">19</td>
<td align="center">300</td>
<td align="center">145</td>
</tr>
<tr>
<td align="center">&#x2003;11</td>
<td align="center">36</td>
<td align="center">87</td>
<td align="center">0.41</td>
<td align="center">0.0466</td>
<td align="center">0.0005</td>
<td align="center">0.3400</td>
<td align="center">0.0279</td>
<td align="center">0.0529</td>
<td align="center">0.0044</td>
<td align="center">294</td>
<td align="center">3</td>
<td align="center">297</td>
<td align="center">24</td>
<td align="center">324</td>
<td align="center">188</td>
</tr>
<tr>
<td align="center">&#x2003;12</td>
<td align="center">14</td>
<td align="center">178</td>
<td align="center">0.08</td>
<td align="center">0.0469</td>
<td align="center">0.0005</td>
<td align="center">0.3385</td>
<td align="center">0.0174</td>
<td align="center">0.0523</td>
<td align="center">0.0027</td>
<td align="center">296</td>
<td align="center">3</td>
<td align="center">296</td>
<td align="center">15</td>
<td align="center">299</td>
<td align="center">116</td>
</tr>
<tr>
<td align="center">&#x2003;13</td>
<td align="center">215</td>
<td align="center">239</td>
<td align="center">0.90</td>
<td align="center">0.0455</td>
<td align="center">0.0005</td>
<td align="center">0.3479</td>
<td align="center">0.0314</td>
<td align="center">0.0554</td>
<td align="center">0.0049</td>
<td align="center">287</td>
<td align="center">3</td>
<td align="center">303</td>
<td align="center">27</td>
<td align="center">429</td>
<td align="center">198</td>
</tr>
<tr>
<td align="center">&#x2003;14</td>
<td align="center">281</td>
<td align="center">335</td>
<td align="center">0.84</td>
<td align="center">0.0462</td>
<td align="center">0.0005</td>
<td align="center">0.3490</td>
<td align="center">0.0125</td>
<td align="center">0.0548</td>
<td align="center">0.0019</td>
<td align="center">291</td>
<td align="center">3</td>
<td align="center">304</td>
<td align="center">11</td>
<td align="center">403</td>
<td align="center">80</td>
</tr>
<tr>
<td align="center">&#x2003;15</td>
<td align="center">21</td>
<td align="center">40</td>
<td align="center">0.52</td>
<td align="center">0.0473</td>
<td align="center">0.0007</td>
<td align="center">0.3407</td>
<td align="center">0.0629</td>
<td align="center">0.0522</td>
<td align="center">0.0097</td>
<td align="center">298</td>
<td align="center">4</td>
<td align="center">298</td>
<td align="center">55</td>
<td align="center">295</td>
<td align="center">422</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>Fifteen spots were analyzed on 15 zircon grains from sample TW0166. Contents of Th and U and Th/U ratios of the zircons are 14&#x2013;354&#xa0;ppm, 40&#x2013;399&#xa0;ppm, and 0.73&#x2013;4.67, respectively. All ages show concordance on a<sup>206</sup>Pb/<sup>238</sup>U&#x2013;<sup>207</sup>Pb/<sup>235</sup>U concordia diagram (<xref ref-type="fig" rid="F4">Figure 4</xref>). The analytical result yield a weighted mean <sup>206</sup>Pb/<sup>238</sup>U age of 294.4 &#xb1; 2.2&#xa0;Ma (MSWD &#x3d; 1.6, 2&#x3c3;), which is interpreted as the crystallization age of the gabbro.</p>
</sec>
<sec id="s5-2">
<title>5.2 Major and trace element compositions</title>
<p>The Diyanmiao gabbro samples have SiO<sub>2</sub> &#x3d; 46.94&#x2013;50.51&#xa0;wt% (mean &#x3d; 48.62&#xa0;wt%) and MgO &#x3d; 7.56&#x2013;10.02&#xa0;wt% (mean &#x3d; 8.88&#xa0;wt%), with Mg<sup>&#x23;</sup> values of 65.0&#x2013;75.6 (mean &#x3d; 70.3). The samples are characterized by high contents of Al<sub>2</sub>O<sub>3</sub> (14.26&#x2013;17.53&#xa0;wt%) and CaO (9.92&#x2013;14.61&#xa0;wt%), but low contents of Fe<sub>2</sub>O<sub>3</sub>
<sup>T</sup> (as FeO &#x2b; Fe<sub>2</sub>O<sub>3</sub>, 5.53&#x2013;8.31&#xa0;wt%), TiO<sub>2</sub> (0.26&#x2013;0.54&#xa0;wt%), MnO (0.12&#x2013;0.17&#xa0;wt%), and P<sub>2</sub>O<sub>5</sub> (0.01&#x2013;0.04&#xa0;wt%). The gabbro samples also have low contents of total rare Earth elements (REEs) (&#x2211;REEs) (10.76&#x2013;20.68&#xa0;ppm, mean &#x3d; 15.98&#xa0;ppm) and low ratios of light REEs to heavy REEs (LREE/HREE) (0.97&#x2013;1.31), (La/Yb)<sub>N</sub> (0.47&#x2013;0.89), (Gd/Yb)<sub>N</sub> (0.91&#x2013;1.07), (La/Sm)<sub>N</sub> (0.56&#x2013;0.99), and (Sm/Nd)<sub>N</sub> (1.12&#x2013;1.28). But positive Eu anomalies (Eu/Eu&#x2a; &#x3d; 1.12&#x2013;1.53), similar to the characteristics of N-MORB, including flat REE patterns (<xref ref-type="table" rid="T2">Table 2</xref>; <xref ref-type="fig" rid="F5">Figure 5A</xref>). In a primitive-mantle-normalized spider diagram (<xref ref-type="fig" rid="F5">Figure 5B</xref>), the gabbros show positive Rb, K, and Sr anomalies and negative Th, Ta, Zr, and Nb anomalies. Depletion in Nb relative to La and Th indicates the involvement of continental crust or arc materials during magmatic evolution (e.g., <xref ref-type="bibr" rid="B55">Santosh et al., 2017</xref>).</p>
<table-wrap id="T2" position="float">
<label>TABLE 2</label>
<caption>
<p>Major (wt%), trace element (&#xd7;l0<sup>&#x2212;6</sup>) analyzing results of the gabbro in Diyanmiao ophiolite and N-MORB.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="center">Rock type</th>
<th colspan="8" align="center">Gabbro</th>
<th align="center">N-MORB</th>
</tr>
<tr>
<th align="center">Sample NO.</th>
<th align="center">XT0160</th>
<th align="center">XT0162</th>
<th align="center">XT0163</th>
<th align="center">XT0164</th>
<th align="center">XT0166</th>
<th align="center">XT0167</th>
<th align="center">XT0168</th>
<th align="center">XT0169</th>
<th align="left"/>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="center">SiO<sub>2</sub>
</td>
<td align="center">47.75</td>
<td align="center">48.38</td>
<td align="center">49.10</td>
<td align="center">50.51</td>
<td align="center">46.94</td>
<td align="center">48.92</td>
<td align="center">50.07</td>
<td align="center">47.32</td>
<td align="center">49.30</td>
</tr>
<tr>
<td align="center">Al<sub>2</sub>O<sub>3</sub>
</td>
<td align="center">16.68</td>
<td align="center">16.56</td>
<td align="center">16.94</td>
<td align="center">17.53</td>
<td align="center">16.88</td>
<td align="center">15.26</td>
<td align="center">14.26</td>
<td align="center">15.11</td>
<td align="center">17.04</td>
</tr>
<tr>
<td align="center">Fe<sub>2</sub>O<sub>3</sub>
<sup>T</sup>
</td>
<td align="center">8.74</td>
<td align="center">8.00</td>
<td align="center">8.04</td>
<td align="center">7.27</td>
<td align="center">5.86</td>
<td align="center">6.99</td>
<td align="center">6.67</td>
<td align="center">7.23</td>
<td align="center">6.82</td>
</tr>
<tr>
<td align="center">CaO</td>
<td align="center">12.11</td>
<td align="center">11.03</td>
<td align="center">10.14</td>
<td align="center">9.92</td>
<td align="center">15.70</td>
<td align="center">13.47</td>
<td align="center">14.50</td>
<td align="center">14.61</td>
<td align="center">11.70</td>
</tr>
<tr>
<td align="center">MgO</td>
<td align="center">8.24</td>
<td align="center">8.04</td>
<td align="center">8.23</td>
<td align="center">7.56</td>
<td align="center">9.19</td>
<td align="center">10.02</td>
<td align="center">9.92</td>
<td align="center">9.86</td>
<td align="center">7.19</td>
</tr>
<tr>
<td align="center">K<sub>2</sub>O</td>
<td align="center">0.75</td>
<td align="center">0.26</td>
<td align="center">0.25</td>
<td align="center">0.27</td>
<td align="center">0.04</td>
<td align="center">0.05</td>
<td align="center">0.04</td>
<td align="center">0.09</td>
<td align="center">0.16</td>
</tr>
<tr>
<td align="center">Na<sub>2</sub>O</td>
<td align="center">2.19</td>
<td align="center">4.52</td>
<td align="center">3.78</td>
<td align="center">3.65</td>
<td align="center">2.39</td>
<td align="center">1.84</td>
<td align="center">1.84</td>
<td align="center">2.59</td>
<td align="center">2.73</td>
</tr>
<tr>
<td align="center">TiO<sub>2</sub>
</td>
<td align="center">0.54</td>
<td align="center">0.48</td>
<td align="center">0.51</td>
<td align="center">0.51</td>
<td align="center">0.26</td>
<td align="center">0.44</td>
<td align="center">0.35</td>
<td align="center">0.47</td>
<td align="center">1.49</td>
</tr>
<tr>
<td align="center">P<sub>2</sub>O<sub>5</sub>
</td>
<td align="center">0.04</td>
<td align="center">0.03</td>
<td align="center">0.03</td>
<td align="center">0.03</td>
<td align="center">0.02</td>
<td align="center">0.02</td>
<td align="center">0.01</td>
<td align="center">0.03</td>
<td align="center">0.16</td>
</tr>
<tr>
<td align="center">MnO</td>
<td align="center">0.15</td>
<td align="center">0.13</td>
<td align="center">0.14</td>
<td align="center">0.12</td>
<td align="center">0.12</td>
<td align="center">0.16</td>
<td align="center">0.17</td>
<td align="center">0.17</td>
<td align="center">0.17</td>
</tr>
<tr>
<td align="center">LOI</td>
<td align="center">2.75</td>
<td align="center">2.44</td>
<td align="center">2.78</td>
<td align="center">2.57</td>
<td align="center">2.58</td>
<td align="center">2.57</td>
<td align="center">2.12</td>
<td align="center">2.49</td>
<td align="left"/>
</tr>
<tr>
<td align="center">Na<sub>2</sub>O/K<sub>2</sub>O</td>
<td align="center">2.92</td>
<td align="center">17.38</td>
<td align="center">15.12</td>
<td align="center">13.52</td>
<td align="center">58.29</td>
<td align="center">40.00</td>
<td align="center">48.42</td>
<td align="center">30.47</td>
<td align="center">17.06</td>
</tr>
<tr>
<td align="center">Mg<sup>&#x23;</sup>
</td>
<td align="center">65.00</td>
<td align="center">66.48</td>
<td align="center">66.84</td>
<td align="center">67.2</td>
<td align="center">75.56</td>
<td align="center">73.84</td>
<td align="center">74.53</td>
<td align="center">72.88</td>
<td align="center">65.27</td>
</tr>
<tr>
<td align="center">Rb</td>
<td align="center">10</td>
<td align="center">8</td>
<td align="center">8</td>
<td align="center">8</td>
<td align="center">5</td>
<td align="center">5</td>
<td align="center">5</td>
<td align="center">5</td>
<td align="center">0.56</td>
</tr>
<tr>
<td align="center">Cr</td>
<td align="center">352</td>
<td align="center">296</td>
<td align="center">348</td>
<td align="center">318</td>
<td align="center">581</td>
<td align="center">769</td>
<td align="center">646</td>
<td align="center">612</td>
<td align="left"/>
</tr>
<tr>
<td align="center">Co</td>
<td align="center">37</td>
<td align="center">32.1</td>
<td align="center">36.3</td>
<td align="center">33</td>
<td align="center">32.1</td>
<td align="center">38.2</td>
<td align="center">35.8</td>
<td align="center">31.3</td>
<td align="left"/>
</tr>
<tr>
<td align="center">Ni</td>
<td align="center">115</td>
<td align="center">125</td>
<td align="center">148</td>
<td align="center">136</td>
<td align="center">107</td>
<td align="center">96.7</td>
<td align="center">103</td>
<td align="center">99.9</td>
<td align="left"/>
</tr>
<tr>
<td align="center">Sc</td>
<td align="center">34.8</td>
<td align="center">28</td>
<td align="center">36</td>
<td align="center">28.1</td>
<td align="center">37</td>
<td align="center">68</td>
<td align="center">43.6</td>
<td align="center">41.4</td>
<td align="left"/>
</tr>
<tr>
<td align="center">V</td>
<td align="center">193</td>
<td align="center">138</td>
<td align="center">157</td>
<td align="center">143</td>
<td align="center">154</td>
<td align="center">238</td>
<td align="center">214</td>
<td align="center">208</td>
<td align="left"/>
</tr>
<tr>
<td align="center">Zr</td>
<td align="center">20.1</td>
<td align="center">15.6</td>
<td align="center">14.8</td>
<td align="center">15.4</td>
<td align="center">4.23</td>
<td align="center">11.8</td>
<td align="center">7.48</td>
<td align="center">13.7</td>
<td align="center">74</td>
</tr>
<tr>
<td align="center">Hf</td>
<td align="center">0.75</td>
<td align="center">0.6</td>
<td align="center">0.58</td>
<td align="center">0.6</td>
<td align="center">0.21</td>
<td align="center">0.54</td>
<td align="center">0.3</td>
<td align="center">0.55</td>
<td align="center">2.05</td>
</tr>
</tbody>
</table>
<table>
<thead valign="top">
<tr>
<th align="center">Rock type</th>
<th colspan="8" align="center">Gabbro</th>
<th align="center">N-MORB</th>
</tr>
<tr>
<th align="center">Sample NO.</th>
<th align="center">XT0160</th>
<th align="center">XT0162</th>
<th align="center">XT0163</th>
<th align="center">XT0164</th>
<th align="center">XT0166</th>
<th align="center">XT0167</th>
<th align="center">XT0168</th>
<th align="center">XT0169</th>
<th align="left"/>
</tr>
</thead>
<tbody valign="top">
<tr>
<td align="center">Ta</td>
<td align="center">0.04</td>
<td align="center">0.04</td>
<td align="center">0.04</td>
<td align="center">0.04</td>
<td align="center">0.03</td>
<td align="center">0.03</td>
<td align="center">0.03</td>
<td align="center">0.03</td>
<td align="center">0.13</td>
</tr>
<tr>
<td align="center">Sr</td>
<td align="center">122</td>
<td align="center">138</td>
<td align="center">143</td>
<td align="center">152</td>
<td align="center">141</td>
<td align="center">117</td>
<td align="center">112</td>
<td align="center">127</td>
<td align="center">90</td>
</tr>
<tr>
<td align="center">Ba</td>
<td align="center">45.1</td>
<td align="center">68.2</td>
<td align="center">74.3</td>
<td align="center">94.9</td>
<td align="center">41.8</td>
<td align="center">58.4</td>
<td align="center">43.6</td>
<td align="center">43.7</td>
<td align="center">6.3</td>
</tr>
<tr>
<td align="center">Nb</td>
<td align="center">1.06</td>
<td align="center">0.82</td>
<td align="center">0.79</td>
<td align="center">0.96</td>
<td align="center">0.84</td>
<td align="center">1.06</td>
<td align="center">1.02</td>
<td align="center">0.86</td>
<td align="center">2.33</td>
</tr>
<tr>
<td align="center">Cs</td>
<td align="center">0.78</td>
<td align="center">0.7</td>
<td align="center">0.77</td>
<td align="center">0.78</td>
<td align="center">0.45</td>
<td align="center">0.54</td>
<td align="center">0.42</td>
<td align="center">0.72</td>
<td align="center">0.01</td>
</tr>
<tr>
<td align="center">Ga</td>
<td align="center">15.2</td>
<td align="center">13.1</td>
<td align="center">14.4</td>
<td align="center">14</td>
<td align="center">10.9</td>
<td align="center">14.2</td>
<td align="center">10.2</td>
<td align="center">12.4</td>
<td align="left"/>
</tr>
<tr>
<td align="center">Pb</td>
<td align="center">3.07</td>
<td align="center">3.07</td>
<td align="center">2.9</td>
<td align="center">2.77</td>
<td align="center">5.54</td>
<td align="center">3.29</td>
<td align="center">3.17</td>
<td align="center">3.31</td>
<td align="center">0.3</td>
</tr>
<tr>
<td align="center">Th</td>
<td align="center">0.05</td>
<td align="center">0.04</td>
<td align="center">0.04</td>
<td align="center">0.03</td>
<td align="center">0.03</td>
<td align="center">0.10</td>
<td align="center">0.14</td>
<td align="center">0.06</td>
<td align="center">0.12</td>
</tr>
<tr>
<td align="center">U</td>
<td align="center">0.05</td>
<td align="center">0.04</td>
<td align="center">0.05</td>
<td align="center">0.04</td>
<td align="center">0.05</td>
<td align="center">0.03</td>
<td align="center">0.03</td>
<td align="center">0.04</td>
<td align="center">0.05</td>
</tr>
<tr>
<td align="center">Y</td>
<td align="center">14.3</td>
<td align="center">10.6</td>
<td align="center">11.1</td>
<td align="center">10.6</td>
<td align="center">7.62</td>
<td align="center">13.1</td>
<td align="center">10.3</td>
<td align="center">10.9</td>
<td align="center">28</td>
</tr>
<tr>
<td align="center">La</td>
<td align="center">1.47</td>
<td align="center">1.32</td>
<td align="center">1.35</td>
<td align="center">1.27</td>
<td align="center">0.96</td>
<td align="center">1.08</td>
<td align="center">1.57</td>
<td align="center">1.30</td>
<td align="center">2.50</td>
</tr>
<tr>
<td align="center">Ce</td>
<td align="center">3.18</td>
<td align="center">2.55</td>
<td align="center">2.62</td>
<td align="center">2.41</td>
<td align="center">1.52</td>
<td align="center">2.23</td>
<td align="center">1.63</td>
<td align="center">2.47</td>
<td align="center">7.50</td>
</tr>
<tr>
<td align="center">Pr</td>
<td align="center">0.67</td>
<td align="center">0.55</td>
<td align="center">0.57</td>
<td align="center">0.52</td>
<td align="center">0.33</td>
<td align="center">0.48</td>
<td align="center">0.5</td>
<td align="center">0.47</td>
<td align="center">1.32</td>
</tr>
<tr>
<td align="center">Nd</td>
<td align="center">3.80</td>
<td align="center">3.07</td>
<td align="center">3.15</td>
<td align="center">2.91</td>
<td align="center">1.82</td>
<td align="center">2.89</td>
<td align="center">2.58</td>
<td align="center">2.73</td>
<td align="center">7.30</td>
</tr>
<tr>
<td align="center">Sm</td>
<td align="center">1.46</td>
<td align="center">1.15</td>
<td align="center">1.15</td>
<td align="center">1.07</td>
<td align="center">0.73</td>
<td align="center">1.21</td>
<td align="center">0.99</td>
<td align="center">1.07</td>
<td align="center">2.63</td>
</tr>
<tr>
<td align="center">Eu</td>
<td align="center">0.75</td>
<td align="center">0.54</td>
<td align="center">0.6</td>
<td align="center">0.57</td>
<td align="center">0.43</td>
<td align="center">0.56</td>
<td align="center">0.49</td>
<td align="center">0.49</td>
<td align="center">1.02</td>
</tr>
<tr>
<td align="center">Gd</td>
<td align="center">2.05</td>
<td align="center">1.54</td>
<td align="center">1.7</td>
<td align="center">1.6</td>
<td align="center">1.01</td>
<td align="center">1.89</td>
<td align="center">1.44</td>
<td align="center">1.54</td>
<td align="center">3.68</td>
</tr>
<tr>
<td align="center">Tb</td>
<td align="center">0.38</td>
<td align="center">0.3</td>
<td align="center">0.31</td>
<td align="center">0.3</td>
<td align="center">0.2</td>
<td align="center">0.36</td>
<td align="center">0.29</td>
<td align="center">0.3</td>
<td align="center">0.67</td>
</tr>
<tr>
<td align="center">Dy</td>
<td align="center">2.53</td>
<td align="center">1.96</td>
<td align="center">2.08</td>
<td align="center">1.93</td>
<td align="center">1.39</td>
<td align="center">2.42</td>
<td align="center">1.98</td>
<td align="center">2.03</td>
<td align="center">4.55</td>
</tr>
<tr>
<td align="center">Ho</td>
<td align="center">0.56</td>
<td align="center">0.42</td>
<td align="center">0.46</td>
<td align="center">0.43</td>
<td align="center">0.31</td>
<td align="center">0.53</td>
<td align="center">0.42</td>
<td align="center">0.44</td>
<td align="center">1.01</td>
</tr>
<tr>
<td align="center">Er</td>
<td align="center">1.62</td>
<td align="center">1.2</td>
<td align="center">1.31</td>
<td align="center">1.18</td>
<td align="center">0.90</td>
<td align="center">1.50</td>
<td align="center">1.20</td>
<td align="center">1.27</td>
<td align="center">2.97</td>
</tr>
<tr>
<td align="center">Tm</td>
<td align="center">0.25</td>
<td align="center">0.18</td>
<td align="center">0.20</td>
<td align="center">0.18</td>
<td align="center">0.14</td>
<td align="center">0.22</td>
<td align="center">0.18</td>
<td align="center">0.19</td>
<td align="center">0.46</td>
</tr>
<tr>
<td align="center">Yb</td>
<td align="center">1.69</td>
<td align="center">1.22</td>
<td align="center">1.30</td>
<td align="center">1.20</td>
<td align="center">0.89</td>
<td align="center">1.52</td>
<td align="center">1.18</td>
<td align="center">1.23</td>
<td align="center">3.05</td>
</tr>
<tr>
<td align="center">Lu</td>
<td align="center">0.27</td>
<td align="center">0.18</td>
<td align="center">0.20</td>
<td align="center">0.18</td>
<td align="center">0.13</td>
<td align="center">0.24</td>
<td align="center">0.18</td>
<td align="center">0.19</td>
<td align="center">0.46</td>
</tr>
<tr>
<td align="center">&#x2211;REE</td>
<td align="center">20.68</td>
<td align="center">16.18</td>
<td align="center">17</td>
<td align="center">15.75</td>
<td align="center">10.76</td>
<td align="center">17.13</td>
<td align="center">14.63</td>
<td align="center">15.72</td>
<td align="center">39.11</td>
</tr>
<tr>
<td align="center">LREE</td>
<td align="center">11.33</td>
<td align="center">9.18</td>
<td align="center">9.44</td>
<td align="center">8.75</td>
<td align="center">5.79</td>
<td align="center">8.45</td>
<td align="center">7.76</td>
<td align="center">8.53</td>
<td align="center">22.27</td>
</tr>
<tr>
<td align="center">HREE</td>
<td align="center">9.35</td>
<td align="center">7.00</td>
<td align="center">7.56</td>
<td align="center">7.00</td>
<td align="center">4.97</td>
<td align="center">8.68</td>
<td align="center">6.87</td>
<td align="center">7.19</td>
<td align="center">16.84</td>
</tr>
<tr>
<td align="center">LREE/HREE</td>
<td align="center">1.21</td>
<td align="center">1.31</td>
<td align="center">1.25</td>
<td align="center">1.25</td>
<td align="center">1.16</td>
<td align="center">0.97</td>
<td align="center">1.13</td>
<td align="center">1.19</td>
<td align="center">1.32</td>
</tr>
<tr>
<td align="center">(La/Yb)<sub>N</sub>
</td>
<td align="center">0.59</td>
<td align="center">0.73</td>
<td align="center">0.70</td>
<td align="center">0.71</td>
<td align="center">0.73</td>
<td align="center">0.48</td>
<td align="center">0.90</td>
<td align="center">0.71</td>
<td align="center">0.59</td>
</tr>
<tr>
<td align="center">&#x3b4;Eu</td>
<td align="center">1.33</td>
<td align="center">1.24</td>
<td align="center">1.31</td>
<td align="center">1.33</td>
<td align="center">1.53</td>
<td align="center">1.13</td>
<td align="center">1.25</td>
<td align="center">1.17</td>
<td align="center">1.00</td>
</tr>
</tbody>
</table>
</table-wrap>
<fig id="F5" position="float">
<label>FIGURE 5</label>
<caption>
<p>
<bold>(A)</bold> Chondrite-normalized REE (<xref ref-type="bibr" rid="B5">Boynton, 1984</xref>)and <bold>(B)</bold> trace element patterns (Sun and McDonough, 1989) for the gabbro and fore-arc basalt (FAB) in the Diyanmiao ophiolite. Data of Diyanmiao FAB is after (<xref ref-type="bibr" rid="B33">Li et al., 2020a</xref>). N-MORB values are after <xref ref-type="bibr" rid="B63">Sun and McDonough, 1989</xref>.</p>
</caption>
<graphic xlink:href="feart-10-1031842-g005.tif"/>
</fig>
</sec>
<sec id="s5-3">
<title>5.3 Sr and Nd isotopic compositions</title>
<p>The Diyanmiao gabbros show high <italic>&#x3b5;</italic>
<sub>Nd</sub> (<italic>t</italic> &#x3d; 294.4&#xa0;Ma) values (&#x2b;8.0 to &#x2b;9.8). The <italic>&#x3b5;</italic>
<sub>Nd</sub> (<italic>t</italic>) values are similar to FABs of the Diyanmiao ophiolite (<xref ref-type="table" rid="T3">Table 3</xref>; <xref ref-type="fig" rid="F6">Figure 6</xref>). In contrast, the gabbros have lower <sup>87</sup>Sr/<sup>86</sup>Sr values (0.702934&#x2013;0.703131) compared with the FABs (0.70466&#x2013;0.70517) (<xref ref-type="fig" rid="F6">Figure 6</xref>; <xref ref-type="table" rid="T3">Table 3</xref>), which might be due to alteration, given the high mobility <xref ref-type="fig" rid="F7">Figure 7</xref> of Rb and Sr.</p>
<table-wrap id="T3" position="float">
<label>TABLE 3</label>
<caption>
<p>Sr&#x2013;Nd isotopic compositions of the gabbro in Diyanmiao ophiolite.</p>
</caption>
<table>
<thead valign="top">
<tr>
<th align="center">Sample</th>
<th align="center">Age</th>
<th align="center">
<sup>87</sup>Rb/<sup>86</sup>Sr</th>
<th align="center">
<sup>87</sup>Sr/<sup>86</sup>Sr</th>
<th align="center">2&#x3b4;&#x2a;</th>
<th align="center">(<sup>87</sup>Sr/86Sr)i</th>
<th align="center">
<sup>147</sup>Sm/<sup>144</sup>Nd</th>
<th align="center">
<sup>143</sup>Nd/<sup>144</sup>Nd</th>
<th align="center">2&#x3b4;</th>
<th align="center">&#x3b5;<sub>Nd</sub> (<italic>t</italic>)</th>
</tr>
</thead>
<tbody valign="top">
<tr>
<td colspan="10" align="center">Gabbro</td>
</tr>
<tr>
<td align="center">&#x2003;XT0166</td>
<td align="center">294</td>
<td align="center">0.1026</td>
<td align="center">0.703131</td>
<td align="center">0.000007</td>
<td align="center">0.7028</td>
<td align="center">0.2425</td>
<td align="center">0.513092</td>
<td align="center">0.000001</td>
<td align="center">8.0</td>
</tr>
<tr>
<td align="center">&#x2003;XT0167</td>
<td align="center">293</td>
<td align="center">0.1236</td>
<td align="center">0.703052</td>
<td align="center">0.000009</td>
<td align="center">0.7027</td>
<td align="center">0.2532</td>
<td align="center">0.513206</td>
<td align="center">0.000001</td>
<td align="center">9.8</td>
</tr>
<tr>
<td align="center">&#x2003;XT0168</td>
<td align="center">295</td>
<td align="center">0.1291</td>
<td align="center">0.702934</td>
<td align="center">0.000007</td>
<td align="center">0.7025</td>
<td align="center">0.2320</td>
<td align="center">0.513128</td>
<td align="center">0.000001</td>
<td align="center">9.1</td>
</tr>
<tr>
<td align="center">&#x2003;XT0169</td>
<td align="center">295</td>
<td align="center">0.1139</td>
<td align="center">0.702975</td>
<td align="center">0.000008</td>
<td align="center">0.7026</td>
<td align="center">0.2370</td>
<td align="center">0.513174</td>
<td align="center">0.000001</td>
<td align="center">9.8</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn id="Tfn1">
<label>
<sup>87</sup>
</label>
<p>Note: (1) (<sup>87</sup>Sr/<sup>86</sup>Sr)<sub>i</sub> &#x3d;Sr/<sup>86</sup>Sr&#x2212;<sup>87</sup>Rb/<sup>86</sup>Sr &#xd7; (e<sup>&#x3bb;T</sup>&#x2212;1), where &#x3bb;<sub>Rb</sub> &#x3d;1.42 &#xd7; 10<sup>&#x2212;11</sup>year<sup>&#x2212;1</sup>. (2) &#x3b5;<sub>Nd</sub> (<italic>t</italic>) &#x3d; {[<sup>143</sup>Nd/<sup>144</sup>Nd&#x2212;<sup>147</sup>Sm/<sup>144</sup>Nd &#xd7; (e<sup>&#x3bb;T</sup>&#x2212;1)]/[<sup>143</sup>Nd/<sup>144</sup>Nd]<sub>CHUR(0)</sub>&#x2212;(<sup>147</sup>Sm/<sup>144</sup>Nd)<sub>CHUR(0)</sub> &#xd7; (e<sup>&#x3bb;T</sup>&#x2212;1)&#x2212;1} &#xd7; 10,000, where &#x3bb;<sub>Sm</sub>&#x3d;6.54 &#xd7; 10<sup>&#x2212;12</sup>year<sup>&#x2212;1</sup>; (<sup>143</sup>Nd/<sup>144</sup>Nd)<sub>CHUR(0)</sub>&#x3d;0.512638; (<sup>147</sup>Sm/<sup>144</sup>Nd)<sub>CHUR(0)</sub> &#x3d; 0.1967.</p>
</fn>
</table-wrap-foot>
</table-wrap>
<fig id="F6" position="float">
<label>FIGURE 6</label>
<caption>
<p>Isotopic plot of whole-rock <italic>&#x3b5;</italic>
<sub>Nd</sub> (<italic>t</italic>) vs. (<sup>87</sup>Sr/<sup>86</sup>Sr)<sub>i</sub> for the gabbro in the Diyanmiao ophiolite; Data sources: <xref ref-type="bibr" rid="B44">Miao et al. (2008)</xref> for the field of Hegenshan ophiolite; <xref ref-type="bibr" rid="B33">Li et al. (2020a)</xref> for Diyanmiao FAB.</p>
</caption>
<graphic xlink:href="feart-10-1031842-g006.tif"/>
</fig>
<fig id="F7" position="float">
<label>FIGURE 7</label>
<caption>
<p>
<bold>(A)</bold> Th-Hf-Ta diagram and <bold>(B)</bold> Th/Yb-Nb/Yb diagram for the gabbro and FAB in the Diyanmiao ophiolite. Data for FAB from <xref ref-type="bibr" rid="B33">Li et al., 2020a</xref>.</p>
</caption>
<graphic xlink:href="feart-10-1031842-g007.tif"/>
</fig>
</sec>
</sec>
<sec sec-type="discussion" id="s6">
<title>6 Discussion</title>
<sec id="s6-1">
<title>6.1 Geochronology of the gabbro in Diyanmiao ophiolite</title>
<p>Previous zircon U&#x2013;Pb analyses of gabbro from the Diyanmiao ophiolite have yielded ages of 346 &#xb1; 2, 357 &#xb1; 4, and 340 &#xb1; 14&#xa0;Ma (<xref ref-type="bibr" rid="B60">Song et al., 2015</xref>; <xref ref-type="bibr" rid="B34">Li et al., 2018</xref>), and Diyanmiao FAB has LA-ICP-MS zircon U-Pb age of 335.6 &#xb1; 2.6&#xa0;Ma (<xref ref-type="bibr" rid="B33">Li et al., 2020a</xref>). Previous studies have reported SHRIMP U&#x2013;Pb ages of 354 &#xb1; 7&#xa0;Ma and 333 &#xb1; 4&#xa0;Ma for microgabbro and plagiogranite from the Hegenshan ophiolite, respectively (<xref ref-type="bibr" rid="B25">Jian et al., 2012</xref>), 354 &#xb1; 5&#xa0;Ma and 353 &#xb1; 4&#xa0;Ma for gabbro and 345 &#xb1; 6&#xa0;Ma for plagiogranite from the Eastern Erenhot ophiolite (<xref ref-type="bibr" rid="B78">Zhang et al., 2015</xref>), and 343 &#xb1; 7&#xa0;Ma for plagiogranite from the Jiaoqier ophiolite (<xref ref-type="bibr" rid="B43">Miao et al., 2007</xref>).</p>
<p>The EHOB is considered to be a giant ophiolitic complex that developed during the Carboniferous (<xref ref-type="bibr" rid="B66">Wang and Liu, 1986</xref>; <xref ref-type="bibr" rid="B64">Tang, 1990</xref>; <xref ref-type="bibr" rid="B60">Song et al., 2015</xref>). <xref ref-type="bibr" rid="B34">Li et al. (2018)</xref> suggested that the Erenhot&#x2013;Hegenshan paleo-ocean underwent intra-oceanic subduction, resulting in the formation of incipient arc crust during the early Carboniferous. The structural location between the early Permian Shoushangou and Dashizhai formations indicates that the tectonic emplacement of the Diyanmiao ophiolite occurred after the early Permian.</p>
<p>The new LA&#x2013;MC&#x2013;ICP&#x2013;MS zircon U&#x2013;Pb age of 294.4 &#xb1; 2.2&#xa0;Ma obtained from the Diyanmiao gabbro in this study gives an early Permian age for the Diyanmiao ophiolite. The age distribution of magmatic rocks of the Diyanmiao ophiolitic m&#xe9;lange indicates that the m&#xe9;lange includes rocks corresponding to at least two magmatic events, which provides important evidence regarding the timing of closure of the PAO.</p>
</sec>
<sec id="s6-2">
<title>6.2 Petrogenesis of the gabbro in Diyanmiao ophiolite</title>
<p>The studied gabbro samples show high LOI values (2.12&#x2013;2.78&#xa0;wt%), indicating variable alteration (<xref ref-type="bibr" rid="B34">Li et al., 2018</xref>), and some of the major elements (e.g., Si, Ca, K, and Ba) and trace elements (Cs and Rb) may have been mobilized to variable extents during post-magmatic hydrothermal alteration or metamorphism (<xref ref-type="bibr" rid="B19">Gillis and Banerjee, 2000</xref>). In contrast, some transition elements (e.g., Cr, Ti, Ni, and V), Th, high-field-strength elements (HFSEs), and REEs (<xref ref-type="bibr" rid="B48">Pearce, 1983</xref>) are considered to be relatively immobile during hydrothermal seafloor alteration and low-grade metamorphism (<xref ref-type="bibr" rid="B49">Pearce, 2008</xref>). Consequently, the following discussion of petrogenesis is based mainly on the transition elements, HFSEs, and REEs.</p>
<p>Although there are differences in the major element contents between the Diyanmiao gabbros and FABs, the compositional signatures of REEs, HFSEs, and large-ion lithophile elements (LILEs) are similar (<xref ref-type="fig" rid="F5">Figure 5</xref>; <xref ref-type="bibr" rid="B33">Li et al., 2020a</xref>), suggesting a similar origin. The major-element compositions of the gabbros are similar to those of N-MORB, while the gabbros show a depleted REE pattern, indicating that their source more depleted than N-MORB source region. In comparison with N-MORB, the gabbros have variable enrichment in Rb, Ba, and K, and depletion in Nb, Ta, Zr, and Ti (<xref ref-type="fig" rid="F5">Figure 5B</xref>), consistent with the characteristics of island-arc tholeiite in a subduction zone, suggesting the involvement of subduction fluids.</p>
<p>In a Th&#x2013;Hf&#x2013;Ta diagram, five gabbro samples plot in the N-MORB field, and two plots on or near the boundary between N-MORB and tholeiitic basalt (<xref ref-type="fig" rid="F7">Figure 7A</xref>). In an Nb/Yb vs Th/Yb discrimination diagram, three gabbro samples plot within the weak subduction field, and four plots in the N-MORB field (<xref ref-type="fig" rid="F7">Figure 7B</xref>). These results reveal a subduction-related signature for the gabbros. The gabbros also display a depletion trend relative to the estimated bulk mantle composition, as well as a Th enrichment trend in the volcanic arc field (<xref ref-type="fig" rid="F7">Figure 7A</xref>), which is consistent with rock genetic models for the initial stages of the evolution of ensimatic marginal basins (<xref ref-type="bibr" rid="B50">Pearce et al., 1984</xref>). The Nb/La values of gabbro is 0.58&#x2013;0.98 (mean&#x3d;0.73), which is higher than the mean value of continental crust (Nb/La mean&#x3d;0.7), but the crystallization differentiation does not lead to a decrease in Nb/La (<xref ref-type="bibr" rid="B80">Zhao et al., 1997</xref>). Therefore, the original magma of the Diyanmiao gabbro likely evolved by fluid metasomatism in an island-arc setting during oceanic subduction, rather than by crystallization and differentiation processes.</p>
</sec>
<sec id="s6-3">
<title>6.3 Tectonic implications of the gabbro: Constraints of intra-oceanic subduction in the southern Paleo-Asian ocean</title>
<p>The tectonic evolution of the CAOB is not yet fully resolved. This orogenic belt is thought to have formed by continuous accretion <italic>via</italic> multistage trench retreat or the simultaneous operation of multiple subduction zones (e.g., <xref ref-type="bibr" rid="B75">Xiao et al., 2009</xref>; <xref ref-type="bibr" rid="B29">Kr&#xf6;ner et al., 2017</xref>). However, other studies have argued that the belt was formed by a series of orogenic processes involving collision, prolonged post-collisional stratification, and extension, with the PAO closing during the late Carboniferous&#x2013;early Permian (e.g., <xref ref-type="bibr" rid="B24">Jahn et al., 2009</xref>; <xref ref-type="bibr" rid="B21">Han et al., 2011</xref>).</p>
<p>Uncertainty remains concerning the tectonic evolution of the EHOB, including the formation of the Hegenshan ophiolite, despite the efforts of many studies (e.g., <xref ref-type="bibr" rid="B65">Tang and Yan, 1993</xref>; <xref ref-type="bibr" rid="B53">Robinson et al., 1999</xref>; <xref ref-type="bibr" rid="B74">Xiao et al., 2003</xref>, <xref ref-type="bibr" rid="B75">2009</xref>; <xref ref-type="bibr" rid="B26">Jian et al., 2008</xref>, <xref ref-type="bibr" rid="B27">2010</xref>; <xref ref-type="bibr" rid="B44">Miao et al., 2008</xref>; <xref ref-type="bibr" rid="B77">Zhang et al., 2011</xref>; <xref ref-type="bibr" rid="B17">Eizenh&#xf6;fer et al., 2014</xref>, <xref ref-type="bibr" rid="B16">2015</xref>; <xref ref-type="bibr" rid="B60">Song et al., 2015</xref>). Some studies have proposed that the Hegenshan ophiolite formed in a mid-ocean-ridge environment (e.g., <xref ref-type="bibr" rid="B46">Nozaka and Liu, 2002</xref>; <xref ref-type="bibr" rid="B60">Song et al., 2015</xref>). Other studies have considered that the Hegenshan ophiolite formed in an SSZ environment, such as a back-arc tectonic environment (<xref ref-type="bibr" rid="B44">Miao et al., 2008</xref>; <xref ref-type="bibr" rid="B68">Wang et al., 2008</xref>; <xref ref-type="bibr" rid="B17">Eizenh&#xf6;fer et al., 2014</xref>, <xref ref-type="bibr" rid="B16">2015</xref>; <xref ref-type="bibr" rid="B78">Zhang et al., 2015</xref>) or an island arc&#x2013;marginal basin system (<xref ref-type="bibr" rid="B53">Robinson et al., 1999</xref>).</p>
<p>The zircon U&#x2013;Pb age of 294.4 &#xb1; 2.2&#xa0;Ma obtained in this study for the Diyanmiao gabbro coincides with the formation age of late Carboniferous island-arc magmatic rocks in central Inner Mongolia that are found together with early Carboniferous Diyanmiao ophiolite (&#x223c;340&#xa0;Ma) (<xref ref-type="bibr" rid="B34">Li et al., 2018</xref>), forearc basalt (335.6&#xa0;Ma), and boninite (&#x223c;328&#xa0;Ma) (<xref ref-type="bibr" rid="B33">Li et al., 2020a</xref>), and late Carboniferous adakite (315&#xa0;Ma) (<xref ref-type="bibr" rid="B67">Wang et al., 2021</xref>). These results support the interpretation that the southern PAO had not closed by the early Permian but was undergoing intra-oceanic subduction. In other words, the late Carboniferous and early Permian ages record the initial stages of subduction of the Diyanmiao intra-oceanic arc within the southern PAO, with subsequent subduction of oceanic crust. On the basis of our field observations, geochemical data for the Diyanmiao ophiolite, and a comparison with the Izu&#x2013;Bonin&#x2013;Mariana initial subduction system (<xref ref-type="bibr" rid="B22">Ishizuka et al., 2014</xref>), we infer a model of subduction initiation in the Diyanmiao intra-oceanic arc during the early Carboniferous to early Permian. The initiation of early Carboniferous intra-oceanic subduction in the southeastern PAO began along a transform fault or fault zone (<xref ref-type="bibr" rid="B36">Li et al., 2020b</xref>) (<xref ref-type="fig" rid="F8">Figure 8A</xref>). After the initial sinking of the slab, asthenospheric upwelling and decompression melting (<xref ref-type="bibr" rid="B51">Reagan et al., 2017</xref>) led to forearc spreading and the formation of the Diyanmiao FABs (335.6&#xa0;Ma; <xref ref-type="fig" rid="F8">Figure 8B</xref>; <xref ref-type="bibr" rid="B33">Li et al., 2020a</xref>; <xref ref-type="bibr" rid="B36">2020b</xref>). As the subduction initiation process continued, leading to further decompression melting, boninites (328&#xa0;Ma) were generated at this stage (<xref ref-type="fig" rid="F8">Figure 8C</xref>). The newly discovered gabbro (294&#xa0;Ma) may represent the magma event of the subducted plate at a greater depth (<xref ref-type="fig" rid="F8">Figure 8D</xref>). The above information revealed that the initial subduction of intra-oceanic may have occurred in the eastern part of the Paleo-Asian Ocean from the early Carboniferous to the early Permian.</p>
<fig id="F8" position="float">
<label>FIGURE 8</label>
<caption>
<p>Evolution of the Early Carboniferous Diyanmiao intra-oceanic forearc system in the central Inner Mongolia, N China, as a typical example of subduction initiation of the southeastern PAO, based on the subduction infancy model (<xref ref-type="bibr" rid="B69">Whattam and Stern, 2011</xref>; <xref ref-type="bibr" rid="B51">Reagan et al., 2017</xref>). <bold>(A)</bold> Oceanic crust boundary preceding subduction initiation. <bold>(B)</bold> Early Carboniferous subduction initiation, leading to forearc spreading and formation of FABs. <bold>(C)</bold> As the subduction initiation process continued, boninites and transitional lavas were formed. <bold>(D)</bold> As the subduction process continued, the gabbro were formed. DMM is depleted MORB mantle.</p>
</caption>
<graphic xlink:href="feart-10-1031842-g008.tif"/>
</fig>
<p>The timing of the transformation of the PAO tectonic domain to the Paleo-Pacific tectonic domain has become an important Research Topic. Most studies have argued that the PAO tectonic system ended during the late Permian or Middle Triassic and continued locally through to the early Late Triassic (<xref ref-type="bibr" rid="B74">Xiao et al., 2003</xref>; <xref ref-type="bibr" rid="B31">Li, 2006</xref>; <xref ref-type="bibr" rid="B70">Wilde, 2015</xref>; <xref ref-type="bibr" rid="B20">Guo et al., 2016</xref>). <xref ref-type="bibr" rid="B42">Mao et al. (2020)</xref> proposed that late Permian to late Triassic igneous rocks on both sides of the Mongol&#x2013;Okhotsk suture belt record an active-continental-margin environment associated with bidirectional subduction of Mongol&#x2013;Okhotsk oceanic lithosphere. <xref ref-type="bibr" rid="B38">Liang et al. (2021)</xref> argued that subduction of the Paleo-Pacific Plate began in the early Permian. Continued subduction triggered back-arc basin spreading during the early Permian&#x2013;late Triassic (280&#x2013;232&#xa0;Ma). Closure of the basin resulted in the emplacement of accretionary materials in the Yuejinshan area during the late Triassic&#x2013;early Jurassic (232&#x2013;180&#xa0;Ma), which also corresponds to the time of formation of high-pressure metamorphic belts on the southern margin of the Khanka Massif (230&#x2013;220&#xa0;Ma) and western margin of the Jiamusi Massif (210&#x2013;180&#xa0;Ma) (<xref ref-type="bibr" rid="B81">Zhou et al., 2014</xref>; <xref ref-type="bibr" rid="B62">Sun et al., 2015</xref>; <xref ref-type="bibr" rid="B20">Guo, 2016</xref>; <xref ref-type="bibr" rid="B4">Bi et al., 2017</xref>).</p>
</sec>
</sec>
<sec sec-type="conclusion" id="s7">
<title>7 Conclusion</title>
<p>The geochemical characteristics of the Diyanmiao gabbro are similar to those of the Diyanmiao FABs. The gabbro is characterized by depletion in Nb, Ta, and Ti, similar to N-MORB, suggesting that the gabbro formed in a subduction setting in the Paleo-Asian Ocean. LA&#x2013;MC&#x2013;ICP&#x2013;MS U&#x2013;Pb dating of zircon grains from the Diyanmiao gabbro yielded an age of 294.4 &#xb1; 2.2&#xa0;Ma, indicating that the gabbro from the Diyanmiao ophiolite formed during the early Permian. The initial subduction of intra-oceanic may have occurred in the eastern part of the Paleo-Asian Ocean from the early Carboniferous to the early Permian. <xref ref-type="bibr" rid="B28">Karimov et al., 2020</xref>, <xref ref-type="bibr" rid="B57">Shcherbakov et al., 2020</xref>, <xref ref-type="bibr" rid="B73">Xiao and Santosh, 2014</xref>.</p>
</sec>
</body>
<back>
<sec sec-type="data-availability" id="s8">
<title>Data availability statement</title>
<p>The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.</p>
</sec>
<sec id="s9">
<title>Author contributions</title>
<p>XZ: Investigation, Formal analysis, Visualization, Conceptualization, Supervision, Project administration, Funding acquisition, Writing&#x2014;original draft, Writing&#x2014;review &#x26; editing. YL: Supervision, Project administration, Funding acquisition. GW : Supervision, Project administration. SW:Project administration, Funding acquisition. XK: Project administration, Funding acquisition. XW: Project administration. ZL: Formal analysis, Funding acquisition.</p>
</sec>
<sec id="s10">
<title>Funding</title>
<p>This research was funded by the National Natural Science Foundation of China, Grant/Award Numbers: 41972061, China Geological Survey, Grant/Award Numbers: 1212011120711 and the Science and technology innovation team program of Hebei GEO University (KJCXTD-2021-07).</p>
</sec>
<sec sec-type="COI-statement" id="s11">
<title>Conflict of interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec sec-type="disclaimer" id="s12">
<title>Publisher&#x2019;s note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
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