Origin of the Zedang and Luobusa Ophiolites,Tibet

The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）.     

秦岭造山带松树沟元古宙蛇绿岩及其大地构造背景

秦岭造山带松树沟蛇绿岩大约是在９８３Ｍａ，构造就位于秦岭杂岩（Ｐｔ１）之上的异地构造岩片．由变质橄榄岩、堆积橄榄岩和火山岩系组成。变质橄榄岩以纯橄榄岩为主。堆晶岩分为单斜辉石型和斜方辉石型两类。火山岩由拉斑玄武岩和低铝安山岩组成，εＮｄ（Ｔ）为＋４．１～＋６．４，表现出Ｎ、Ｔ、Ｐ三种稀土分配型式和非正常洋脊玄武岩地球化学的特征。综合分析认为，这个古老蛇绿岩是在一种位于洋中脊之上的洋岛环境中形成的，代表了晋宁期华北板块与扬子板块之间古洋盆的残骸。     

内蒙古西乌珠穆沁旗迪彦庙蛇绿岩的识别

新识别出的内蒙古西乌珠穆沁旗迪彦庙蛇绿岩位于中朝古板块与西伯利亚古板块之间的兴蒙造山带中部。通过对迪彦庙蛇绿岩进行详细的野外地质调查和岩石学、岩石地球化学的研究,发现迪彦庙蛇绿岩由孬来可吐和白音布拉格两个蛇绿岩带组成,各带宽约3km,延伸约30km,蛇绿岩各单元出露齐全。岩性由下到上主要为蛇纹石化方辉橄榄岩、层状-块状辉长岩、斜长岩、细碧岩、枕状玄武岩、角斑岩-石英角斑岩及硅质岩。蛇纹石化方辉橄榄岩稀土配分模式具SSZ型蛇绿岩的地幔橄榄岩特征;枕状玄武岩具岛弧拉斑玄武岩(IAT)特征;硅质岩的Al2O3/(Al2O3+Fe2O3)比值显示大陆边缘沉积环境特征。     

北祁连山扎麻什地区东沟蛇绿岩LA-ICP-MS锆石U-Pb测年及其地球化学特征

东沟蛇绿岩位于北祁连造山带中东段的扎麻什一带,主要由辉橄岩、辉长岩和基性火山岩组成较为完整的蛇绿岩单元。对基性火山岩进行单颗粒锆石LA-ICP-MS U-Pb同位素测定,获得499.3Ma±6.2Ma年龄加权平均值,代表蛇绿岩的形成年龄,相当于晚寒武世。岩石地球化学研究表明,该蛇绿岩中的基性火山岩属于拉斑玄武岩系列,球粒陨石标准化稀土元素分配模式为近平坦型,(La/Yb)N在0.97～1.26之间;微量元素分配模式除个别大离子亲石元素(Ba、Rb、U、K)外基本为平坦型曲线,Nb、Ta、Zr、Hf无亏损,显示出洋中脊玄武岩(N-MORB)的地球化学特征;在Zr-Zr/Y和Ti/100-Zr-3Y等构造环境判别图中,所有样品数据点均落入MORB区域内,表明其形成于洋中脊环境。经区域对比,该蛇绿岩与玉石沟、川刺沟蛇绿岩等一起构成了大洋扩张脊型蛇绿岩带。     

绿岩套和蛇绿岩套的区分标志

综合绿岩、蛇绿岩的有关概念、地质环境、岩石学、地球化学特征,论述蛇绿岩和绿岩之间的异同。 目前由于太古宙和显生宙地质构造研究的进展,使人们对绿岩套和蛇绿岩套的特征及其相互之间的异同性、联系性和形成环境越来越感兴趣。同时,在不同地区的地质构造研究中,对上述两种岩套在概念上和成因上存在混淆。本文将讨论这一问题。     

北祁连东段柏木峡—门岗峡地区蛇绿岩的识别及其区域构造意义

为加强对北祁连早古生代多岛弧盆系蛇绿混杂岩的调查,选取柏木峡—门岗峡蛇绿岩开展岩相学、年代学和地球化学研究。柏木峡—门岗峡蛇绿岩位于青海省海东市互助县,构造上处于北祁连造山带中东段。由橄榄岩、辉长岩和基性火山岩组成较为完整的蛇绿岩单元。对辉长岩进行单颗粒锆石LA-ICP-MS U-Pb同位素测年,获得²⁰⁶Pb/²³⁸U加权平均年龄为(525.2±1.1) Ma(MSWD=0.06),代表了蛇绿岩的形成年龄,相当于早寒武世。岩石地球化学研究表明,该蛇绿岩中的基性火山岩属于拉斑系列,具有洋岛玄武岩的地球化学特征;玄武岩Th/Yb-Nb/Yb和TiO₂/Yb-Nb/Yb等构造环境判别图显示,该套蛇绿岩的形成环境与俯冲作用无关。结合详细的野外调查和区域对比,认为该蛇绿岩代表早古生代北祁连洋壳,与玉石沟—川刺沟等蛇绿岩共同构成了达坂山—玉石沟蛇绿岩带。     

新疆天山地区榆树沟-铜花山蛇绿岩特征和构造背景

榆树沟-铜花山蛇绿岩出露于南天山北缘,属哈萨克斯坦-伊犁板块和塔里木板块之间的缝合带。蛇绿岩已被构造肢解,主要由超镁铁岩、堆晶岩、熔岩类组成。超镁铁岩以方辉橄榄岩为主,显示典型的亏损地幔岩特征,橄榄石为富镁型,Fo为90;斜方辉石为顽火辉石,En为90,单斜辉石含量少;副矿物铬尖晶石属低铬型,Cr^#值(＝Cr/(Cr+Al)×100)16~28,Mg^#值(=Mg/(Mg+Fe)×100)63~75,反映了深海橄榄岩特征。归一化后榆树沟超镁铁岩MgO含量38.84%~44.53%,Al₂O₃为1.51%~3.63%,CaO为0.42%~5.77%,成分近于大洋二辉橄榄岩;铜花山的超镁铁岩叠加碳酸岩化,LREE强烈富集,可能经历了俯冲洋壳流体改造。熔岩类在榆树沟和铜花山均有较大规模产出,其中榆树沟玄武岩为主,铜花山安山岩和英安岩较多。熔岩多已遭受绿片岩相海底热液蚀变。榆树沟玄武岩的REE含量总体比铜花山的低,稀土配分模式均为轻稀土富集型;玄武岩的微量元素特征表明其源区可能遭受过流体作用影响。铜花山三个高镁火山岩化学成份具有SiO₂(32%~36%)含量很低、MgO(20.12%~28.50%)含量高、K₂O+Na₂O(0.06%~0.46%)含量小于2%的特征,综合分析可归为苦橄岩类。堆晶岩通常以构造小岩块产在超镁铁岩块中或基性熔岩中,以辉长岩为主,有少量橄榄辉石岩等,岩石蚀变、构造变形强烈。堆晶岩的成分接近玄武岩,存在LREE富集和略亏损平坦两种类型。岩石地球化学特征表明榆树沟和铜花山的蛇绿岩形成于MOR构造环境,但具有受俯冲带流体改造特征,并伴生有岛弧火山岩。     

中朝陆台北侧褶皱带(中段)蛇绿岩的地球化学特征

中朝陆台北侧褶皱带(中段)中,出露有两条蛇绿岩带:一条是温都尔庙加里东期蛇绿岩带(简称南带);一条是索伦山-贺根山华力西期蛇绿岩带(简称北带)。两条岩带具有不同的时空格局和明显的地球化学差异。通过对两条蛇绿岩带地球化学研究,讨论了蛇绿岩形成的古构造环境。南带蛇绿岩可能是在岛弧边缘附近海盆地扩张脊中形成的;北带蛇绿岩可能是在大洋中脊形成的,它标志着中朝板块和西伯利亚板块之间的碰撞带位置。     

新疆东昆仑阿其克库勒湖西缘地区蛇绿岩的确证

新疆东昆仑阿其克库勒湖西缘出露了一套超基性—基性岩。该蛇绿岩的野外地质、岩石学以及岩石化学特征表明它们是由变质橄榄岩和堆晶岩两大单元组成的蛇绿岩 ,是否存在蛇绿岩的其它单元值得进一步研究。同时 ,这些特征表明其与青海东昆仑清水泉蛇绿岩相似 ,指示它们可能为同一洋壳的残余。     

Study on the Tectonic Setting for the Ophiolites in Xigaze, Tibet

The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence sections are represented by Jiding, Dejixiang, Baigang, and Dazhuqu ophiolites and consist of three–four units. The complete ophiolite sequence in order from the bottom to top consists of mantle peridotite, cumulates, sheeted sill dike swarms, and basic lavas±radiolarian chert. These cumulates are absent in the remaining blocks of Dejixiang and Luqu. The age of radiolaria in the radiolarian chert is Late Jurassic–Cretaceous. The basalt and ultramafic rock of the ophiolite also are overlaid by Tertiary Liuqu conglomerate, which contains numerous pebble components of ophiolite, indicating that the Tethys Ocean began to close at the end of Cretaceous Period. The isotopic data of gabbro, diabase, and albite granite in the Xigaze ophiolite are approximately 126–139 Ma, which indicates that the ophiolite formed in the Early Cretaceous. The K–Ar age of amphibole in garnet amphibolite in the ophiolite mélange is 81 Ma, indicating that tectonic ophiolite emplacement occurred at the end of Late Cretaceous.