A complete suite of tonalite-trondhjemite-granodiorite (TTG) gneisses,together with associated monzogranitic gneisses are recognized in Yunzhong Mountain area,Shanxi,through detailed field geological mapping.Zircon LA-ICP-MS U-Pb dating demonstrates that the emplacement age of TTG gneisses is 2486 ± 9Ma,and of monzogranitic gneisses is 2448 ± 11Ma.The TTG gneisses show geochemical features similar to those of high-Al TTG assemblage.The trondhjemitic gneisses are characterized by high SiO2(69.82％ ～ 73.54％),Na2O (4.19％ ～ 5.16％) and CaO (1.45％ ～ 3.03％).They have Al2O3 contents and aluminum saturation index (A/CNK) ranging from 14.31％ to 15.74％ and from 1.00 to 1.03,respectively,and could be classified as Na-rich and weakly peraluminous series.The tonalitic gneisses have geochemical compositions comparable to those of the trondhjemitic gneisses but exhibit lower K2O and higher CaO.The granodioritic gneisses display relatively low SiO2 (61.23％ ～ 61.97％) and constitute a metaluminous to weakly peraluminous series,with Na2O/K2O ratios between 1.00 and 1.32,Al2O3 contents ranging from 15.03％ to 15.73％,aluminum saturation index (A/CNK) from 0.96 to 1.05.In contrast to the TTG gneisses,the studied monzogranitic gneisses are enriched in K2O relative to Na2O (K2O/Na2O =1.22 ～ 1.44) and depleted in Al2O3 (12.81％ ～ 14.27％).These rocks have low rare earth element (REE) contents (52.71 × 10-6 ～ 262.0 × 10-6) and are characterized by strong enrichment of LREE relative to HREE on the chondrite-normalized diagrams.The primitive mantle normalized diagrams of the TTG show sizeable peaks at K,Rb,Th and sizeable troughs at Nb,Ta,Sr,P,Ti.Simultaneously,the TTG gneisses have low Cr and Ni contents.These geochemical characteristics are analogous to those of arc magmatic rocks.In-situ zircon Lu-Hf isotopic analyses show that both granodioritic gneisses and monzogranitic gneisses have positive εHf(t) values.The two-stage Hf model ages of the granodioritic gneisses have a range of 2.5 ～ 2.6Ga closing to the emplacement age,which imply the TTG gneisses may be derived from a juvenile crust source.However,the εHf(t) values of the ～2.45Ga monzogranitic gneisses plot within the field of the 2.7 ～ 2.8Ga average continental crust reservoir,suggesting a significant contribution of preexisting crustal materials in their magma genesis.Our geochemical results,coupled with the available knowledge of regional geological setting,imply that the TTG gneisses and monzogranitic gneisses in Yunzhong Mountain area represent the igneous products in continental marginal arc by partial melting of flat oceanic slab and remelting of underplated ancient continental crust,respectively.The emplacement of the newly-recognized TTG gneisses indicate a major event of horizontal crustal accretion,and further suggest the cratonization had been finished in Yunzhong Mountain area in Late Neoarchean.%通过详细的野外地质工作,本文在山西云中山地区识别出一套岩石组合完整的TTG片麻岩岩系,与TTG片麻岩共生的还有后期发育的二长花岗片麻岩.LA-ICP-MS锆石U-Pb定年结果表明,花岗闪长质片麻岩的形成时间为2486±9Ma,二长花岗片麻岩的形成时间为2448±11Ma.地球化学特征显示研究区TTG片麻岩为高铝TTG.奥长花岗质片麻岩总体表现为高SiO2 (69.82％～73.54％)、富Na2O(4.19％ ～5.16％)、高CaO(1.45％ ～3.03％)的特征,Al2O3含量为14.31％～15.74％,铝饱和指数(A/CNK)为1.00～1.03,属富钠过铝质系列岩石;英云闪长质片麻岩的特征与奥长花岗片麻岩基本相似,但K2O含量更低,CaO含量更高;花岗闪长质片麻岩SiO2 (61.23％ ～ 61.97％)含量相对偏低,Na2O/K2O为1.00～1.32,Al2O3含量为15.03％ ～ 15.73％,铝饱和指数(A/CNK)为0.96 ～ 1.05,为准铝质或过铝质系列岩石.与上述TTG片麻岩不同的是,本区出露的二长花岗片麻岩表现为富钾(K2O/Na2O=1.22 ～ 1.44)、贫Al2O3(12.81％～14.27％)的特征.岩石稀土总量较低(ΣREE =52.71×10-6 ～262.0×10-6),轻稀土富集,重稀土亏损,轻、重稀土强烈分馏.微量元素具有富集K、Rb、Th等大离子亲石元素,亏损Nb、Ta、Sr、P、Ti等元素的特点,Cr、Ni等相容元素含量也较低,表现出岛弧岩浆的特征.花岗闪长片麻岩和二长花岗片麻岩εHf(t)值均为正值,前者成岩年龄与二阶段模式年龄较为接近,暗示其来源为2.5～2.6Ga的新生地壳,而后者成岩年龄与二阶段模式年龄则相差较多,其测点大都落在2.7～2.8Ga地壳演化线的区域内,表明二长花岗片麻岩来自于2.7 ～2.8Ga的古老地壳.根据上述地球化学特征,并结合区域地质特征,推测研究区高Al的TTG片麻岩是“平俯冲”状态下洋壳部分熔融的产物,而二长花岗片麻岩为底侵古老地壳部分熔融的产物,形成于大陆边缘弧的构造环境,代表云中山地区一次重要的以水平增生为主的地壳生长事件,同时也表明在新太古代末期,云中山地区已完成克拉通化,进入陆内演化阶段.
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