Relict chromian spinels in Tulu Dimtu serpentinites and listvenite,Western Ethiopia: implications for the timing of listvenite formation

Serpentinites (massive and schistose) and listvenite occur as tectonic sheets and lenses within a calcareous metasedimentary mélange of the Tulu Dimtu, western Ethiopia. The massive serpentinite contains high-magnesian metamorphic olivine (forsterite [fo] ~96 mol%) and rare relict primary mantle olivine (Fo_90–93). Both massive and schistose serpentinites contain zoned chromian spinel; the cores with the ferritchromite rims preserve a pristine Cr/(Cr+Al) atomic ratio (Cr# = 0.79–0.87), suggesting a highly depleted residual mantle peridotite, likely formed in a suprasubduction zone setting. Listvenite associated with serpentinites of smaller ultramafic lenses also contain relict chromian spinel having identical Cr# to those observed in serpentinites. However, the relict chromian spinel in listvenite has significantly higher Mg/(Mg+Fe²⁺) atomic ratios. This suggests that a nearly complete metasomatic replacement of ultramafic rocks by magnesite, talc, and quartz to prevent Mg–Fe²⁺ redistribution between relict chromian spinel and the host, that is, listvenite formation, took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Considering together with the regional geological context, low-temperature CO₂-rich hydrothermal fluids would have infiltrated into ultramafic rocks from host calcareous sedimentary rocks at a shallow level of accretionary prism before a continental collision to form the East African Orogen (EAO).     

兴蒙造山带及华北板块北缘钼矿化——进展、规律、问题与成因初探

兴蒙造山带及其南侧受古亚洲洋南向俯冲所影响的华北板块北缘内各有1条显著的中生代斑岩钼成矿带,并在东西两侧首尾相连。文章综述了伸展环境下的斑岩钼矿床的研究进展,对兴蒙造山带及华北板块北缘内这2条钼成矿带的成矿背景、分布规律、矿床共生组合特点、成矿岩浆的属性、巨量金属和水的来源以及斑岩铜、钼矿化的异同等进行了总结,并从成矿岩浆源区塑造过程的角度初步探讨了巨型钼成矿带的形成特点。这2条钼成矿带在地质特征和区域矿床组合上非常相似,具有相似的启动时间和峰值时间,与区域内的斑岩铜矿化在时代上具有不共生的特点,矿化特征也与美国科罗拉多地区产出的高F型斑岩钼化类似,二者构成了统一的整体。元素地球化学对比研究显示,南、北2条钼成矿带的成矿岩浆与古生代斑岩铜成矿岩浆以及中生代的碱性岩浆均具有相似的特征,为脱水熔融的产物,并与古老陆下岩石圈地幔包体、新生代软流圈地幔玄武岩具有显著的差别。研究认为,南钼矿带的岩浆源区是亏损金属和水的古老岩浆源区在古生代洋片俯冲过程脱水交代改造后的产物,北钼矿带的岩浆源区是古生代洋片俯冲增生形成的富水源区。成(含)矿岩石Sr同位素研究显示,南、北2条钼成矿带成矿源区均启动于Rb/Sr比值较低的源区,受到上部高Rb/Sr比值地壳的混染;Nd同位素特征的对比研究显示,二者初始Nd值差别极大,但是Sm/Nd比值非常相似,显示放射性成因Nd的积累在三叠纪以后是一致的,也说明初始Nd值的差别是成矿源区塑造前所形成和继承的,同时也说明初始Nd值的差异可能掩盖了Mo成矿岩浆形成的真正原因。通过与世界范围内其他典型钼成矿带的对比研究,认为南、北2条钼成矿带成矿的岩浆源区位于陆下岩石圈地幔,古生代期间古亚洲洋向南、北两侧的俯冲在其形成过程中具有重要作用,主要体现在塑造富集型源区、水化造山带和增厚岩石圈等几个方面。俯冲改造、加厚并富集了水和大离子亲石元素的陆下岩石圈获得了地球化学上的不稳定性,在伸展构造环境(可能有多期伸展)驱动下,脱水熔融以达到稳定的趋势,在这个过程中,其化学成分将逐渐与古老陆下岩石圈地幔的化学成分趋于一致。因此,水化的陆下岩石圈地幔在伸展过程中的低程度批式脱水部分熔融,形成的富含金属和水的高分异型岩浆构成了成矿岩浆,并在岩石圈的不同尺度经过多阶段结晶分异-同化混染后,就位成为近矿岩浆房。陆下岩石圈脱水熔融的结束也意味着巨型热液钼矿化作用的结束,并决定了俯冲后巨型热液成矿带总的生命周期,这也与兴蒙造山带及华北板块北缘钼矿化(甚至其他热液型矿化)在早白垩世(约130 Ma)趋于减弱并熄灭的现象一致,也使得新生代的碱性岩浆岩不具有显著的脱水熔融特征。综上,笔者认为兴蒙造山带及华北板块北缘的斑岩钼矿化为一个统一的整体,属于古亚洲洋俯冲作用水化的源区在后期强烈伸展环境下部分熔融的产物,是古亚洲洋俯冲成矿作用的延续和发展,也是古生代塑造的富集型源区在中生代伸展构造驱动下的复合成矿作用。     

冀西北怀安地体高级变质表壳岩的锆石年代学研究

位于华北克拉通中部造山带中北段的怀安地体与内蒙孔兹岩带相接,出露高压麻粒岩和退变榴辉岩等多种高级变质岩,是洞悉华北克拉通古元古代构造演化历史的重要窗口。研究区变质表壳岩包括夕线石榴长英质片麻岩、石榴长英质粒状岩石以及紫苏黑云二长片麻岩。阴极发光图像特征显示研究区样品的锆石主要包括碎屑锆石和变质锆石,其中碎屑锆石具有岩浆结晶环带,而变质锆石为单颗粒或围绕着继承性碎屑锆石边部生长,内部结构均匀,Th/U比值较低。锆石LAICP-MS U-Pb定年结果与前人研究结果综合表明该区变质表壳岩石的碎屑锆石的207Pb/206Pb年龄主要集中在~2040Ma,其原岩形成时代与孔兹岩带变泥质岩石相近,均为~2.0Ga。变质锆石记录其变质时代为1957~1804Ma,结合前人对怀安地区变泥质岩和变基性岩变质作用和年代学研究结果,推测得出1.95~1.92Ga代表了峰期(高压)麻粒岩相变质时代,1.90~1.85Ga代表峰后减压阶段变质时代,而1.85~1.80Ga代表退变质晚期的时代。怀安地区变质岩石可能卷入了阴山陆块、鄂尔多斯陆块以及东部陆块间的先后碰撞造山过程,并持续较长时间(1.95~1.80Ga),最终拼贴为统一的整体。     

南秦岭铧厂沟金矿床铬云母~(40)Ar/~(39)Ar年代学及其大地构造意义

南秦岭铧厂沟金矿床位于勉略缝合带以南的逆冲推覆带内,矿体呈透镜体或脉状产于新元古界蚀变细碧岩及泥盆系灰岩中,受控于近EW向叠瓦状逆冲断层及韧脆性剪切带。为了准确厘定其成矿时代,对矿区蚀变细碧岩型矿石和含矿石英脉中的铬云母进行了年龄测定。2件铬云母样品的~(40)Ar/~(39)Ar坪年龄分别为209.4±2.3 Ma和211.5±2.5 Ma,相应的等时线年龄211.4±3.6 Ma和215.3±3.9 Ma,与坪年龄在误差范围内一致。因此,铧厂沟金矿床的成矿年龄为212~209 Ma。结合铧厂沟金矿床的大地构造位置、矿床地质特征及成因类型,推测铧厂沟金矿床形成于扬子板块与秦岭微板块的碰撞过程,其成矿年龄代表了两板块发生碰撞的下限。     

U-Pb dating of zircon from the Central Zone of the East Kunlun Orogen and its implications for tectonic evolution

LA-ICP-MS and SHRIMP U-Pb dating of zircons from orthogneisses and amphibolite from the Central Zone of the Kunlun Orogen is reported in this paper. One orthogneiss sample has metamorphic zircons yielding weighted average 206Pb/238U age of 517.0 5.0/-6.0 Ma, and the other orthogneiss sample con- tains zircons with inherited magmatic cores giving three population 207Pb/206Pb ages of 955 Ma, 895 Ma and 657 Ma for the magmatic protolith, and metamorphic recrystallized rims with peak 206Pb/238U ages of 559 12/?17 Ma and 516 ± 13 Ma. The amphibolite yielded three populations of weighted average 206Pb/238U age of 482.0 10/?8.0 Ma, 516.2 ± 5.8 Ma and 549 ± 10 Ma for the metamorphic zircons. These dating results recorded the tectonothermal events that occurred in the early Paleozoic and the Pre- cambrian time. The records of the Cambrian magmatic-metamorphic event in the Qinling Orogen, the Altyn Tagh belt, north margin of the Qaidam Block and the Kunlun Orogen suggest that continental assembly probably occurred in the early evolutionary history of the Proto-Tethys.     

关于造山带地层学的若干思考——昌宁—孟连带实例剖析

造山带中地层的展布具有明显不同于稳定克拉通地区的特点,充分了解这些特点并采取相应的对策,是正确解决造山带疑难地层问题的关键所在。通过对滇西昌宁—孟连带若干实例的剖析,探讨解决其中长期存在争议的地层问题的途径     

哀牢山造山带构造演化

哀牢山造山带造山活动始于晚二叠世的扬子陆块被动边缘的裂谷环境,经过印支及喜马拉雅期两次造山作用,形成了四个构造世代的叠加构造变形。这些不同时期的构造共生组合,既表现出陆内造山带垂向上不同构造层次构造环境的流变学特征,又反映了造山物质从表部下冲到地壳深处,又折返地表的复杂经历。清晰地刻划出哀牢山造山带的所经历的陆内俯冲、主期碰撞造山及后期卷入喜马拉雅造山活动的发展过程。     

河南鱼池岭钼矿床辉钼矿铼-锇同位素年龄及地质意义

河南嵩县鱼池岭钼矿床是东秦岭钼矿带新发现的大型斑岩钼矿床,产于被前人认为白垩纪的合峪花岗岩基内部。鱼池岭矿床5件辉钼矿样品的铼锇同位素模式年龄介于(134.3±2.0)~(141.8±1.6)Ma 之间,铼锇等时线年龄为144.3±5.2Ma（1σ,MSWD=3.7）。据此,结合前人获得的年龄资料,确定合峪复式花岗岩基的岩浆侵入序列为：(1) 144Ma之前的似斑状黑云母二长花岗岩,(2) 144Ma左右的黑云母二长花岗斑岩,(3) 136~127Ma左右的巨斑状黑云母二长花岗斑岩,(4) <127Ma的(石英)正长斑岩脉或岩株;合峪复式花岗岩基形成于晚侏罗世—早白垩世,而非前人认为的仅限于白垩纪;成岩成矿构造背景为碰撞加厚的造山带地壳和岩石圈的伸展减薄或减压增温体制,岩浆侵入和成矿作用与地壳快速隆升事件相伴随。     

甘肃阳山金矿碳氢氧同位素与成矿流体来源

甘肃文县阳山金矿的探明黄金储量已达308t,平均品位4.74g/t,是我国地质勘查储量最大的金矿床。该矿床产于西秦岭造山带,是一个同碰撞形成的类卡林型金矿床,矿体受EW向韧脆性剪切带控制,赋矿围岩为泥盆系碳质千枚岩-板岩-碳酸盐-硅质岩和侵入其中的花岗斑岩脉。热液石英的流体包裹体之δ~(13)C_(CO_2)(PDB)值为-2.5‰～-5.6‰,δ~(87)C_(CH_4(PDB))值为-23.1‰～-32.6‰,说明成矿流体来源于碳酸盐地层或相似岩石建造的变质或/和改造脱水作用。该结论尚被流体氢氧同位素研究结果所证明,成矿早阶段和主阶段流体的δ~(18)O_W值介于9.5‰～15.3‰,δD_W值为-86‰～-73‰。两件矿化晚阶段流体的δ~(18)O_W氧分别为2.7‰和6.8‰,表明有强烈的大气降水热液混入。总体而言,成矿流体系统从早到晚、从深到浅,由变质热液演变为大气降水热液。特别补充说明,上述研究结果和认识与我们根据流体包裹体和矿床地质研究得出的结论相吻合。     

Re-Os and U-Pb Geochronology of the Erlihe Pb-Zn Deposit,Qinling Orogenic Belt,Central China,and Constraints on Its Deposit Genesis

The Erlihe Pb-Zn deposit is an important mine of the Pb-Zn metallogenic zone in the South Qinling Orogen.It has been considered a sedimentary exhalative deposit in previous investigations because the ore body occurs concordantly at the transitional location of an upright fold.Re and Os isotopic analyses for paragenetic pyrites with sphalerite and galena from the ore body have been used to determine the timing of mineralization and to trace the source of metallogenic materials.The Re-Os isotopic data of f...