塔里木板块东北缘I型花岗岩年代学与地球化学研究

塔里木板块广泛发育新元古代岩浆岩,它们对研究塔里木新元古代构造演化及塔里木对Rodinia超大陆汇聚和裂解的响应具有重要意义。本文报道了塔里木板块东北缘I型花岗岩的U-Pb年代学、原位Hf同位素以及全岩地球化学研究结果。该花岗岩具有较高的SiO₂含量（平均值为71.61%）,低的镁铁质含量（FeO^T和MgO平均值为1.52%和0.72%）;Na₂O/K₂O比值介于0.51~3.76,平均值为1.12;铝饱和度（A/CNK）介于0.69~1.16,属于碱性-钙碱性过渡型花岗岩。花岗岩表现为高场强元素（HFSE,如：Nb、Ta、Ti、Th等）的亏损以及大离子亲石元素（LILE,如：Rb、Sr、Ba、U等）的相对富集。样品Nb/Ta、Zr/Hf平均值分别为10.93、36.86,说明该花岗岩形成时有流体的参与。受斜长石的影响,花岗岩表现出明显的Eu正异常,Eu/Eu^*平均值1.39。该花岗岩²⁰⁶Pb/²³⁸U加权平均年龄为748.8±6.1Ma,其原位Hf同位素ε_Hf（t）值范围在-16.5~-9.7之间,对应两阶段模式年龄t_DM2介于2.30~2.74Ga,指示该期花岗岩可能为古元古代地壳的部分熔融,可能形成于古洋壳向南俯冲塔里木板块产生的大陆弧后环境。结合之前在塔北缘的研究工作,提出了塔里木板块北缘新元古代弧后伸展体系,并且认为该体系主要受控于Rodinia超大陆的裂解。     

塔里木盆地北缘二叠纪巴立克立克组凝灰岩年代学、地球化学及构造意义

塔里木盆地北缘广泛分布的二叠纪火山岩,对于理解天山造山带晚古生代构造演化具有重要研究意义。本文以哈拉峻地区巴立克立克组凝灰岩为研究对象,开展了地球化学、年代学和Lu-Hf同位素研究,结果显示凝灰岩富硅(SiO₂=74.59％~76.18％)、富碱(Na₂O+K₂O=7.48％~7.79％)、低铝(Al₂O₃=13.80%~14.29%),属于钾玄质过铝质流纹岩类;具轻稀土元素富集、重稀土元素相对亏损的特征,具Eu弱正异常,相对富集Rb、Th等大离子亲石元素(LILE),亏损P、Nb、Ta、Ti等高场强元素(HFSE),Nb/Ta值平均为16.60,Sr/Y值平均为2.42,Mg^#值平均为34.71;LA-ICP-MS锆石U-Pb年代学分析结果显示巴立克立克组凝灰岩的结晶年龄为280.47±2.58 Ma,εHf(t)值为-0.73~2.12,二阶段模式年龄(t_DM2)为1 318~1 145 Ma。结合塔里木盆地北缘早二叠世的构造演化、岩浆活动及沉积分布等信息,指出巴立克立克组沉积于南天山洋壳向塔里木板块俯冲的板内裂谷盆地中,具非造山板内岩浆岩特征,可能为地幔柱作用背景下,岩石圈地幔底侵导致地壳物质部分熔融的产物,继承了壳源和幔源的双重源区特征。     

柴北缘东段奥陶纪埃达克岩-富Nb玄武岩:对大陆深俯冲之前大洋俯冲及地壳增生的启示

在柴北缘东段识别出早古生代埃达克岩-富Nb玄武岩的火山岩组合。埃达克岩富Na₂O、贫K₂O,K₂O/Na₂O比值介于0.14~0.43之间;高Sr（614×10^-6~1043×10^-6）,但亏损Y（3.26×10^-6~14.1×10^-6）和Yb（0.33×10^-6~1.46×10^-6）,具有高的Sr/Y比值（44~282）;富集Sr、Ba等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素及Cr、Ni、Co、V等相容元素。富Nb玄武岩富Na₂O、贫K₂O、高TiO₂,其Nb含量较高,介于16.9×10^-6~17.9×10^-6之间,具有高的Nb/Ta、Nb/U、（Nb/La）_N比值,同时富集高场强元素。埃达克岩锆石U-Pb定年得到453±4Ma和457±4Ma的结晶年龄。锆石ε_Hf（t）范围较大,介于3.40~13.23之间,对应的二阶段模式年龄t_DM2介于1059~566Ma之间,显示以新生物质为主的特征。综合研究表明柴北缘东段埃达克岩可能为岛弧环境下俯冲的南祁连大洋板片部分熔融的产物。板片来源的埃达克质熔体交代或与上覆地幔楔橄榄岩反应,导致被交代的地幔橄榄岩部分熔融而形成富Nb玄武质岩浆。柴北缘东段埃达克岩-富Nb玄武岩火山岩组合的厘定表明南祁连洋可能直到~455Ma之前并未完全闭合,同时表明俯冲大洋板片的部分熔融可能是柴北缘早古生代地壳增生的一种重要方式。     

新疆东天山石炭纪火山岩及其构造意义

东天山石炭纪火山岩北至吐哈盆地南缘,向南受控于阿奇克库都克断裂,对恢复古生代期间古亚洲洋的构造演化具有重要的意义。通过岩石学、同位素年代学及地球化学研究发现:(1)东天山石炭纪火山岩是一套钙碱质玄武岩-安山岩-流纹岩建造,玄武岩包括N-MORB和IAT型两种类型,指示东天山地区在石炭纪存在一个弧间盆地,其北部边界可能为东准噶尔的卡拉麦里蛇绿岩带;(2)东天山吐哈盆地南缘底坎儿组中流纹岩LA-ICP-MS锆石U-Pb 年龄为320 ±1.2Ma(MSWD=1.3),代表了底坎儿组火山岩主体岩浆结晶时代;(3)对比东天山及邻区石炭纪火山岩地球化学,发现在SiO₂含量相近的情况下,由北向南东天山石炭纪火山岩中玄武岩的K₂O、K₂O+Na₂O、K₂O/Na₂O、LILE(Rb、Ba、Sr等)和HFSE(Nb、Ta、Th、Zr等)呈规律性增加,具有明显的成分分带,与环太平洋中-新生代岛弧和活动大陆边缘产出的火山岩的成分极性变化一致。认为东天山石炭纪火山岩成分分带可能是古亚洲洋在石炭纪向东南准噶尔-吐鲁番-哈密陆块下斜向俯冲的结果。     

"三江"北段昌都陆块晚三叠世钾质-超钾质火山岩成因及地质意义

昌都微陆块内部发育一套晚三叠世粗安岩.本文基于系统的野外踏勘,对具有代表性的火山岩进行岩石学、地球化学等研究.结果显示该区火山岩的SiO₂含量为51.36%~58.04%,全碱含量(Na₂O+K₂O)为5.03%~7.84%,根据岩石K₂O,MgO及K₂O/Na₂O比值判断,属钾质-超钾质火山岩,具高K、高Al、低Ti的特征.本区岩石强烈富集大离子亲石元素(LILE)和轻稀土元素(LREE),Nb、Ta、Ti的负异常明显,整体特征类似于弧火山岩.其稀土总量较高(270×10^-6~960×10^-6),轻重稀土分馏明显,重稀土分布形态较平缓.岩石具有较高的初始锶同位素比值(⁸⁷Sr/⁸⁶Sr)_i=0.7150~0.7176和较低的初始钕同位素比值(¹⁴³Nd/¹⁴⁴Nd)_i=0.51180~0.51184,ε_Nd(t)=-10.9~-10.2,二段Nd模式年龄为t_2DM=1.82~1.88Ga.本区岩石的地球化学特征显示其源区是由俯冲板片所产生的流体/熔体交代的岩石圈地幔,源区矿物组成分析结果显示,其源区以尖晶石二辉橄榄岩为主,含少量石榴石二辉橄榄岩及金云母.本区钾质-超钾质岩石虽然与西藏南部的钾质-超钾质岩石有相似的地球化学特征,但却产于碰撞后的构造背景之下.结合其所具有的弧火山岩特征及时代构造背景判断其为一种"滞后型"弧火山岩,其形成过程可以概括为三个阶段.     

贵州水城二叠纪钠质粗面玄武岩的地球化学特征及其源区

贵州水城二叠纪玄武岩位于峨眉山大火成岩省东部。该玄武岩全岩SiO₂的含量为44.5%~50.04%,TiO₂的含量为2.38%~2.74%,MgO的含量为5.74%~7.96%, Mg^#值较低为0.40~0.49,Na₂O含量高,为4.81%~7.19%,并且Na₂O/K₂O>4,属于钠质粗面玄武岩即夏威夷岩。具有ΣREE富集的右倾型稀土元素分布模式,稀土和微量元素特征和Pb同位素特征显示洋岛玄武岩OIB的地球化学特征,(⁸⁷Sr/⁸⁶Sr)_i=0.70482~0.70503,ε_Nd(t)和(²⁰⁶Pb/²⁰⁴Pb)_<em>t变化范围较窄:1.3~1.8和17.21~17.62。与贵州威宁黑石头和织金二叠纪玄武岩比较,水城玄武岩富碱,TiO₂含量低,Na₂O、MgO和Al₂O₃含量高,造成峨眉山大火成岩省东部贵州境内三个地方玄武岩不同性质的主要原因是由于地幔源区不同,分离结晶程度和地壳混染程度的不同,水城玄武岩来源于交代富集地幔,是峨眉山地幔柱上升至石榴石稳定区发生部分熔融,地幔柱的部分熔融体和富含挥发分的大陆岩石圈地幔混合,在上升到地表过程中受到轻微的地壳混染所形成。     

江西冷水坑矿床含矿花岗斑岩的Sr-Nd及锆石Hf-O同位素研究

江西冷水坑铅锌矿床是武夷成矿带北段的典型矿床,矿区内含矿花岗斑岩锆石U-Pb年龄为（162.0±2.0）Ma,岩浆分异程度较高,具有高SiO₂（69.46%~75.52%）、高K₂O（K₂O/Na₂O>3.22）、强过铝质（ASI=1.20~1.96）的特征。岩体发育较明显的Eu负异常,富集强不相容元素Rb、Th、U,而亏损高场强元素Ba、Sr、Nb、Ta、P、Ti。全岩同位素测试结果显示,冷水坑含矿花岗斑岩具有极高的⁸⁷Sr/⁸⁶Sr初始比值（0.74005~0.76518）与低的ε_Nd值（-11.48~-10.78）,锆石Hf-O同位素测定值变化范围较小,¹⁷⁶Hf/¹⁷⁷Hf介于0.282317~0.282460,具有负的ε_Hf（t）值（-12.39~-7.62）和相对较高的δ¹⁸O值（7.23‰~8.81‰）,指示岩浆主要来自于成熟地壳,可能起源于北武夷地区变质基底,源岩为基底变质岩,这可能与古太平洋板块俯冲形成的挤压背景下的地壳熔融有关。     

阿尔金索尔库里地区元古代流纹岩锆石SHRIMP U-Pb定年及其地质意义

阿尔金索尔库里北山彩虹沟塔什达坂群卓阿布拉克组流纹岩锆石SHRIMP U-Pb 年龄为920±20Ma (MSWD=1.50),代表流纹岩的喷出时间。岩石的SiO₂介于68.33%~70.60%,K₂O>Na₂O,K₂O/Na₂O平均值6.42,属弱过铝质(ACNK平均为1.25)钙碱性系列(σ平均为2.76),为一套典型的壳源流纹岩岩石系列。稀土元素含量高(平均为211.3×10^-6),轻稀土富集(L/H平均为6.57),Eu亏损中等(δEu平均为0.30)。微量元素以富集Rb、Th、K、Zr,而相对亏损Ba、Sr、Eu、Ti、Nb、Ta为特征。岩石微量及稀土元素具有典型的板内火山岩特征,而岩石显著的低Sr特征(25.0×10^-6~65.3×10^-6,平均值为36.8×10^-6))表明其并非源自加厚的地壳,而是起源于斜长石稳定的正常地壳。阿尔金流纹岩是由进入上地壳高位岩浆房的拉斑玄武岩浆的底侵作用直接使上地壳部分熔融,形成酸性火山岩的原始岩浆。该岩浆体系沿区域断裂构造体系上升,并经历了较强的结晶分异和演化,最终形成阿尔金索尔库里北山流纹岩系列。     

木其滩岩组形成时代、地球化学特征——对北秦岭构造演化的制约

出露于北秦岭造山带中的木其滩岩组主要由斜长角闪岩组成,其原岩为玄武岩-亚碱性玄武岩。对木其滩岩组斜长角闪岩中的锆石进行LA-ICP-MS U-Pb年代学分析,获得同位素年龄762.5±4.6Ma,为成岩年龄。地球化学上,这些斜长角闪岩显示低钛 (TiO₂=0.57%~2.16%),低K₂O/Na₂O比值(0.06~0.55)和中等-高 Mg^#(45.3~68.7)特征,为低钛拉斑玄武岩。在球粒陨石标准化图解中,表现出轻重稀土分异,无Eu异常的特征,类似于E-MORB。这些斜长角闪岩以Rb、Ba、Th和La富集,低(⁸⁷Sr/⁸⁶Sr)_i 比 (0.7038~0.7040) 和高的ε_Nd(t)值(+4.1~+6.9)为特征,表明其来源于地幔,成分上相似于FOZO。(Th/Nb)_N, Nb/La 和 Ba/La的比值特征表明它们经历了不同程度的地壳混染。所有这些特征,结合区域地质资料阐明木其滩岩组斜长角闪岩代表的是大约762.5±4.6Ma商丹洋盆初始裂解时的洋壳残片。     

江达-维西火山岩浆弧中段德钦岩体年代学、地球化学及岩石成因

本文对江达-维西火山岩浆弧德钦岩体的寄主岩石——花岗闪长岩及其镁铁质微粒包体(MME)——闪长岩进行了详细研究。二者LA-ICPMS锆石U-Pb年龄分别为254.6±1.8Ma和253.5±1.6Ma,二者形成时代一致。地球化学研究结果表明,花岗闪长岩富K₂O和Na₂O,且K₂O>Na₂O,富Al₂O₃,A/CNK平均为0.96;闪长岩富K₂O和Na₂O,但K₂O2O,富Al₂O₃和MgO,A/CNK平均为0.72;花岗闪长岩的稀土总量低于闪长岩,二者轻重稀土分馏明显,配分曲线为右倾的轻稀土富集型;二者均富集大离子亲石元素而亏损高场强元素Nb、Ta等;二者均具有相对较高的Mg^#(58.8~65.8),并具有相对较高的相容元素Cr、Ni含量(花岗闪长岩平均值分别为115×10^-6和31.6×10^-6,闪长岩平均值分别为398×10^-6和98.2×10^-6;花岗闪长岩和闪长岩¹⁷⁶Hf/¹⁷⁷Hf的平均值分别为0.282383和0.282287,二者ε_Hf(t)平均值分别为-8.3和-11.8,反映了二者属于I型花岗岩,可能为岩浆混合作用的产物。地球化学特征及Hf同位素组成一致显示岩浆来源于地壳的部分熔融,伴有不同比例的地幔物质加入,形成于弧陆碰撞-后碰撞的构造背景,暗示金沙江结合带在~255Ma已经进入了弧陆碰撞-后碰撞的地质时期。     

U–Pb zircon geochronology and geochemistry of Neoproterozoic volcanic rocks in the Tarim Block of northwest China: implications for the breakup of Rodinia supercontinent and Neoproterozoic glaciations

As the lowest volcanics-bearing unit of the Neoproterozoic succession, the Beiyixi Formation is the key to understanding the early response to the breakup of the Roninia supercontinent in the Tarim Block. The SHRIMP analyses of zircons from the volcanic rocks at the bottom of the Beiyixi Formation yield a weighted mean ²⁰⁶Pb/²³⁸U age of 755 ± 15 Ma. This is interpreted as the eruption age of the Beiyixi volcanic rocks. The Beiyixi volcanic rocks consist of bimodal basalt and dacite-rhyolite with a SiO₂ gap between 55% and 65%. The mafic rocks display negative ɛ_Nd (755 Ma) values (−9.9 to −10.8), moderate enrichment in LILE and variable depletion in Nb, Ta and P, resembling those of the tholeiitic basalts in continental rift. Geochemical and Nd isotopic characteristics suggest that the mafic rocks were derived from partial melting of an enriched lithospheric mantle reservoir. The felsic rocks show negative ɛ_Nd (755 Ma) values (−7.9 to −9.2), negative Nb, Ta, P and Ti anomalies, very high La_N/Yb_N (62–92) ratios and LILE abundances, and may be generated by melting of eclogites or garnet amphibolites in the lower crust, as a result of basalt emplacement into continental crust during continental rifting. The age of 755 ± 15 Ma indicates that the Beiyixi glaciation took place later than 755 Ma and it could be correlated with the Chang’an glaciation in the Yangtze Block and the Sturtian–Rapitan glaciation in other Rodinia Blocks. The geochemical characteristics of the Beiyixi volcanic rocks resemble those of the rift-related magmatism in other Rodinia Blocks, suggesting that the Beiyixi volcanism was a part of global magmatism during the breakup of Rodinia supercontinent. The age and geochemical features of the Beiyixi volcanic rocks also reveal that the mantle plume activity spread to the northwestern margin of the Rodinia supercontinent and probably resulted in the breakup between Australia and Tarim Blocks.     

Review Section

ABSTRACT

The petrology, geochronology, and geochemistry of the early Permian volcanic rocks from Houtoumiao area, south Xiwuqi County in central Inner Mongolia of China, are studied to elucidate the early Permian tectonic setting of the region. The volcanic rocks, which are interbedded with sandstone, feature both mafic and felsic compositions and show a bimodal nature. Zircon U–Pb dating reveals that the volcanic rocks formed at 274–278 Ma, similar to the ages of bimodal magmatism in neighbouring areas. The mafic rocks are composed of tholeiitic basalt, basaltic andesite, basaltic trachyandesite, and trachyandesite. They are rich in Th, U, and LILEs, depleted in HFSEs Nb, Ta, and Ti, and have positive ε_Nd(t) values (+3.6 to +7.9). Geochemical analyses indicate that the mafic rocks originated from metasomatized lithospheric mantle. The felsic volcanic rocks are mainly rhyolite, with minor trachyte and dacite. They have different evolutionary tendencies of major elements, chondrite-normalized REE patterns, and isotopic compositions from the mafic volcanic rocks, which preclude formation by fractional crystallization of mafic melts. The ε_Nd(t) values of the felsic rocks are similar to those of the Carboniferous Baolidao arc rocks in the region. It is suggested that Permian felsic melts originated from the partial melting of Carboniferous juvenile arc-related rocks. By comparison with typical Cenozoic bimodal volcanism associated with several tectonic settings, including rift, post-collisional setting, back-arc basin, and the Basin and Range, USA, the bimodal volcanic rocks in central Inner Mongolia display similar petrological and geochemical characteristics to the rocks from back-arc basin and the Basin and Range, USA. Based on the analysis of regional geological data, it is inferred that the early Permian bimodal volcanic rocks in the study area formed on an extensional continental margin of the Siberian palaeoplate after late Carboniferous subduction–accretion.     

北山南带音凹峡地区酸性火山岩年代学、地球化学研究:二叠纪裂谷岩浆作用的新证据

北山造山带位于中亚造山带南缘中段地区,是中亚造山带、塔里木克拉通和华北克拉通的构造结合部。年代学资料表明北山南带晚古生代存在一期重要的岩浆活动,所形成的岩石类型包括了镁铁-超镁铁质杂岩、花岗岩类以及酸性火山岩。音凹峡地区位于北山南带,该地区广泛分布着厚度巨大的二叠系。这套地层主要是由酸性火山岩及火山碎屑岩组成,流纹岩的锆石LA-ICP-MS U-Pb年龄为273±1 Ma,为早二叠世。针对音凹峡地区酸性火山岩的地球化学研究发现,其具有高SiO_2、K_2O+Na_2O、Al_2O_3含量、低Fe_2O_3~T、Mg O、P_2O_5含量;富集轻稀土元素,重稀土元素无分异,具有明显的Eu负异常;微量元素方面,酸性火山岩富集Cs、Rb、Th、U、Zr和Hf等元素,而相对亏损Ba、Nb、Ta、Sr、P、Ti等元素。酸性火山岩的锆石ε_(Hf)(t)值为-6.0~3.9,具有较老的Hf同位素模式年龄,t_(DM2)=1046~1669 Ma。音凹峡酸性火山岩的地球化学证据表明这套火山岩系可能由中-新元古代壳源岩石部分熔融形成,并与幔源岩浆进行了不同程度的混合,之后发生了分离结晶作用。综合音凹峡地区同时代镁铁质岩石、花岗岩类的研究成果,以及该地区二叠系沉积建造及火山岩特征分析可知,北山南带在早二叠世应处于大陆裂谷的构造背景。     

拉萨地块西部亚热地区林子宗群火山岩地球化学特征及其构造意义

广泛分布于拉萨地块南部的林子宗群火山岩被普遍认为是新特提斯洋北向俯冲消减结束并由同碰撞向后碰撞构造背景转化过程中的岩浆作用产物,记录了从新特提斯洋俯冲结束到印度-欧亚大陆碰撞-后碰撞的丰富信息。本文对采集于拉萨地块西部亚热地区的林子宗群火山岩样品进行了年代学和岩石地球化学研究,结果显示:林子宗群火山岩为一套钾玄质系列的流纹岩,该岩石具有高硅(w(SiO_2)=73.90%～74.03%)、高钾(w(K_2O)=6.15%～6.28%)、高钾钠比(w(K_2O)/w(Na_2O)=2.14～2.65),富集K、Rb、Th、U、Hf等元素,亏损Nb、Ta、P、Ti、Sr等元素,具有明显的Eu负异常;林子宗群火山岩样品LA-ICP-MS锆石U-Pb年龄为50.1 Ma±0.3 Ma—51.3Ma±0.4Ma之间;结合前人研究,笔者认为亚热地区林子宗群火山岩为形成于印度-欧亚大陆同碰撞背景下的大陆地壳部分熔融的产物。     

塔里木二叠纪基性-中性-酸性岩浆岩的年代学及其地质意义

塔里木西部柯坪地区出露的早二叠纪玄武岩与盆地内其他地区广泛分布的玄武岩共同构成一个面积约0.25Mkm²的溢流玄武岩省。巴楚地区出露的早二叠纪辉绿岩、正长岩、石英正长岩与石英正长斑岩等多种岩浆岩构成了连续的基性-中性-酸性岩浆序列。野外露头观察表明,巴楚麻扎尔塔格地区的基性-中性-酸性岩浆岩为近同时侵位。柯坪塔格开派兹雷克组顶部层位玄武岩的锆石SHRIMP U-Pb年龄为279.0±4.5Ma, 该年龄值可限定该区二叠纪溢流玄武岩喷发的最晚时限。巴楚麻扎尔塔格石英正长斑岩脉的锆石SHRIMP U-Pb年龄为273.0±3.7Ma, 与前人测定的巴楚辉绿岩脉和正长岩的锆石年龄在误差范围内一致,表明巴楚地区的基性-中性-酸性岩浆岩为近同时侵位,时间在275Ma左右。综合已发表的塔里木二叠纪岩浆岩高精度年代数据,本文提出：整个塔里木大火成岩省岩浆作用的持续时间约为20Myrs; 291~287Ma喷发的巨量溢流玄武岩可能构成塔里木大火成岩省的主体,是地幔柱头部熔融的直接产物; 283~272Ma侵位的巴楚、一间房和塔北地区的其它类型岩浆岩体积较小,为塔里木大火成岩省晚期岩浆作用的产物。     

The Tarim picrite–basalt–rhyolite suite, a Permian flood basalt from northwest China with contrasting rhyolites produced by fractional crystallization and anatexis

We report major and trace element composition, Sr–Nd isotopic and seismological data for a picrite–basalt–rhyolite suite from the northern Tarim uplift (NTU), northwest China. The samples were recovered from 13 boreholes at depths between 5,166 and 6,333 m. The picritic samples have high MgO (14.5–16.8 wt%, volatiles included) enriched in incompatible element and have high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd isotopic ratios (εNd (t) = −5.3; Sr_i = 0.707), resembling the Karoo high-Ti picrites. All the basaltic samples are enriched in TiO₂ (2.1–3.2 wt%, volatiles free), have high FeOt abundances (11.27–15.75 wt%, volatiles free), are enriched in incompatible elements and have high Sr and low Nd isotopic ratios (Sr_i = 0.7049–0.7065; εNd (t) = −4.1 to −0.4). High Nb/La ratios (0.91–1.34) of basalts attest that they are mantle-derived magma with negligible crustal contamination. The rhyolite samples can be subdivided into two coeval groups with overlapping U–Pb zircon ages between 291 ± 4 and 272 ± 2 Ma. Group 1 rhyolites are enriched in Nb and Ta, have similar Nb/La, Nb/U, and Sr–Nd isotopic compositions to the associated basalts, implying that they are formed by fractional crystallization of the basalts. Group 2 rhyolites are depleted in Nb and Ta, have low Nb/La ratios, and have very high Sr and low Nd isotopic ratios, implying that crustal materials have been extensively, if not exclusively, involved in their source. The picrite–basalt–rhyolite suite from the NTU, together with Permian volcanic rocks from elsewhere Tarim basin, constitute a Large Igneous Province (LIP) that is characterized by large areal extent, rapid eruption, OIB-type chemical composition, and eruption of high temperature picritic magma. The Early Permian magmatism, which covered an area >300,000 km², is therefore named the Tarim Flood Basalt.     

羌北-昌都地块那益雄组火山岩锆石U-Pb年代学及地球化学特征

本文对青藏高原羌北-昌都地块阿布日阿加措地区的晚二叠世那益雄组火山岩进行了年代学和地球化学研究。该火山岩主要由玄武岩、安山玄武岩、安山岩、流纹岩、凝灰岩组成,具有碱性玄武岩到酸性熔岩的特征。锆石U-Pb年代学研究表明该火山岩的形成时代为251. 1±4. 8～249. 6±1. 3 Ma之间。地球化学分析结果表明,该地区的流纹岩具有高的Si O2(74. 85%～77. 55%)和Na2O+K2O(5. 40%～6. 61%)含量,较低的MgO、K2O和Ca O含量,Al2O3含量低且稳定,里特曼指数平均为1. 15,小于3. 3。安山岩Si O2含量55. 13%～56. 28%,Na2O+K2O含量4. 13%～6. 15%,里特曼指数平均为2. 20,小于3. 3,属于钙碱性安山岩。碱性玄武岩Si O2含量51. 49%,Na2O+K2O含量6. 34%,里特曼指数为4. 73,属于碱性系列。稀土元素配分曲线为富集LREE的右倾型。另外,富集大离子亲石元素(LILE) Th、U,亏损高场强元素(HFSE) Nb、Ta等特征,均说明羌北-昌都地块阿布日阿加措地区的火山岩形成于陆缘岛弧环境。     

Petrology and geochemistry of Early Permian volcanic rocks in South Tian Shan,NW China: implications for the tectonic evolution and Phanerozoic continental growth

Situated in the southwest of the Central Asian Orogenic Belt (CAOB), the South Tian Shan (STS) Block is a key area for understanding the final accretion of the CAOB. A suite of volcanic rocks interbedded with continental sediments from the Xiaotikanlike Formation lies along the southwestern edge of the Tian Shan orogen. Laser-ablation-inductively coupled plasma-mass spectrometer U–Pb dating provided a crystallization age of 295.0 ± 2.8 Ma (MSWD = 1.3), suggesting an Early Permian magmatic event. The volcanic rocks show a variable composition, with dominant rhyolites and dacites, subordinate basaltic andesites and few basalts. The felsic rocks are enriched in K and exhibit remarkably negative anomalies in Ba, Sr, Eu, P and Ti. These anomalies associated with their high negative ε _Nd(t) values and old Nd model ages suggest that they are most likely sourced from ancient lower crustal rocks. The mafic rocks are characterized by high Mg#, Cr, Ni contents, negative Nb, Ta anomalies and pronounced enrichment in light rare earth elements as well as mild enrichment in large-ion lithophile elements. The mafic rocks are thus inferred to derive from enriched subcontinental lithospheric mantle. The petrographic and geochemical characteristics of the Xiaotikanlike Formation volcanic rocks indicate that they were generated under a post-collisional regime. Therefore, the final collision between the Tarim Craton and the Kazakhstan–Yili terrane took place before Early Permian, most probably at Late Carboniferous. Differing from other tectonic units of the CAOB, the recycling of ancient lithospheric crust played a significant role in the continental growth of the STS Block.     

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K₂O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K₂O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO₂-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.     

北山石炭纪-二叠纪火山岩成因及构造背景

北山古生代火山岩尤其是石炭纪-二叠纪火山岩的形成环境及成因备受学者关注且长期以来存在争议。本文收集了近年来发表的关于北山石炭纪-二叠纪火山岩研究的地球化学数据,岩石地球化学特征显示北山石炭纪玄武岩主要为安山玄武岩,属拉斑系列,二叠纪火山岩主要为安山玄武岩和亚碱性玄武岩,落入拉斑系列及过渡区;石炭纪玄武岩和二叠纪玄武岩均具有LREE富集的球粒陨石标准化稀土元素配分模式,轻重稀土元素分馏程度均较低。在微量元素蛛网图上,石炭纪-二叠纪遭受地壳混染的玄武岩呈现出明显的Nb-Ta亏损和微弱的Ti亏损特征,而未遭受地壳混染作用的绝大多数石炭纪-二叠纪玄武岩主要呈现出与OIB相似的"隆起"状不相容元素标准化配分模式。岩石成因分析认为,石炭纪-二叠纪玄武质岩浆可能主要来源于地幔柱,部分石炭纪-二叠纪玄武岩在形成演化过程中遭受了明显的大陆地壳混染作用,导致其出现十分相似于岛弧或活动大陆边缘的地球化学特征。结合区域构造演化分析及构造环境判别,认为石炭纪-二叠纪玄武岩均形成于大陆板内环境。