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1.
为加深对西藏泽当地区新特提斯洋演化的认识,对西藏泽当蛇绿混杂岩带内的共国日二长花岗岩进行了岩石学、岩石地球化学、同位素及年代学等研究,研究显示:共国日二长花岗岩岩体规模小、岩性稳定,LA-ICP-MS锆石U-Pb年龄为(90.40±0.68)Ma,属晚白垩世;岩石地球化学具高硅、富铝、富钾、低钛和准铝质钙碱性花岗岩特征;轻稀土富集、重稀土亏损,具明显的负Eu异常,微量元素表现为相对富集Rb、K、Ba、Th、U等大离子亲石元素,显著亏损Nb、P、Ti等高场强元素;(87Sr/86Sr)i=0.705 708~0.706 284,(143Nd/144Nd)i=0.512 689~0.512 716,εNd(t)=2.00~2.51。以上特征表明,位于泽当蛇绿混杂岩带内的共国日二长花岗岩属于I型花岗岩,由正常岛弧岩浆形成,应为晚白垩世新特提斯洋向北俯冲形成的岛弧环境下俯冲带上部地壳部分熔融的产物,其不属于泽当蛇绿岩的组成部分,表明在90 Ma前泽当洋内弧已经消失。  相似文献   
2.
理塘混杂岩位于甘孜-理塘蛇绿混杂岩带中段新龙县-理塘县一带,其内部保存有完整的混杂岩系,包括蛇绿岩残片、洋岛残块、洋内弧残块、复理石建造、裂谷残片、高压变质岩等,是恢复和反演甘孜-理塘洋盆演化的理想地区。在总结前人研究的基础上,结合笔者近年来的研究成果,详细阐述了理塘混杂岩的物质组成、构造环境及形成时代,进一步约束了甘孜-理塘洋盆的时空、性质以及演化历程。LA-ICP-MS锆石U-Pb测年结果表明,甘孜-理塘混杂岩带内蛇绿岩年龄为(346±17)Ma、(286.2±5.1)Ma、(219.5±2.2)Ma、(216.1±2.3)Ma,洋岛年龄为(271±10)Ma、(245.1±1.5)Ma、(211.8±1.8)Ma,在侏罗纪瑞环山组粉砂岩夹层中测得碎屑锆石最新年龄为(196±3)Ma,结合大量的古生物化石鉴定结果,分析认为理塘混杂岩最早的年龄记录可追溯至中泥盆世,最晚可延至早白垩世,是甘孜-理塘洋盆中泥盆世-早白垩世连续演化的记录。综合以上研究成果,笔者还大致建立了甘孜-理塘洋盆晚古生代-中生代的演化过程模式。  相似文献   
3.
全球多地蛇绿岩型地幔橄榄岩和铬铁矿中发现微粒金刚石,并在中国西藏南部和俄罗斯乌拉尔北部的蛇绿岩铬铁矿中发现原位产出的金刚石,认为是地球上金刚石的一种新的产出类型,不同于金伯利岩型金刚石和超高压变质型金刚石。它们与呈斯石英假象的柯石英、高压相的铬铁矿和青松矿等高压矿物以及碳硅石和单质矿物等强还原矿物伴生,指示蛇绿岩中的这些矿物组合形成于深度150~300 km或者更深的地幔。金刚石具有很轻的C同位素组成(δ13C-18‰~-28‰),并出现多种含Mn矿物和壳源成分包裹体。研究认为它们曾是早期深俯冲的地壳物质,达到>300 km深部地幔或地幔过渡带后,经历了熔融并产生新的流体,后者在上升过程中结晶成新的超高压、强还原矿物组合,通过地幔对流或地幔柱作用被带回到浅部地幔,由此建立了一个俯冲物质深地幔再循环的新模式。蛇绿岩型地幔橄榄岩和铬铁矿中发现金刚石等深部矿物,质疑了蛇绿岩铬铁矿形成于浅部地幔的已有认识,引发了一系列新的科学问题,提出了新的研究方向。   相似文献   
4.
Within the Variscan Orogen, Early Devonian and Late Devonian high‐P belts separated by mid‐Devonian ophiolites can be interpreted as having formed in a single subduction zone. Early Devonian convergence nucleated a Laurussia‐dipping subduction zone from an inherited lithospheric neck (peri‐Gondwanan Cambrian back‐arc). Slab‐retreat induced upper plate extension, mantle incursion and lower plate thermal softening, favouring slab‐detachment within the lower plate and diapiric exhumation of deep‐seated rocks through the overlying mantle up to relaminate the upper plate. Upper plate extension produced mid‐Devonian suprasubduction ocean floor spreading (Devonian ophiolites), while further convergence resulted in plate coupling and intraoceanic ophiolite imbrication. Accretion of the remaining Cambrian ocean heralded Late Devonian subduction of inner sections of Gondwana across the same subduction zone and the underthrusting of mainland Gondwana (culmination of NW Iberian allochthonous pile). Oblique convergence favoured lateral plate sliding, and explained the different lateral positions along Gondwana of terranes separated by Palaeozoic ophiolites.  相似文献   
5.
丁青蛇绿岩位于班公湖-怒江缝合带东段,是该缝合带出露面积最大的蛇绿岩。为查明岩体成因,在丁青东岩体中实施了一口165.19m的钻孔。除最顶部有约0.5m厚的第四系残坡积物外,其余均为地幔橄榄岩。结合显微镜鉴定将岩心划分出17个岩性单元层,岩性主要以方辉橄榄岩为主,夹少量纯橄岩和含铬铁矿纯橄岩。地幔橄榄岩中橄榄石的Fo变化于88.79~93.73,铬尖晶石的Cr#变化于44.33~81.66,揭示丁青地幔橄榄岩可能经历过约20%~40%的中高度部分熔融作用;全岩地球化学分析表明其具有富镁(MgO=45.98%~49.45%)、贫铝(Al2O3=0.19%~1.37%)和贫钙(CaO=0.28%~0.70%)的特点,属于熔融程度较高的地幔残余物质。岩石具有明显不同于阿尔卑斯蛇绿岩的轻稀土元素富集特征,指示区内地幔橄榄岩先经历了较强程度的部分熔融,后经历了俯冲消减过程中的流体交代。利用地幔橄榄岩中的铬尖晶石成分计算母熔体Al2O3含量对应的FeO/MgO值,与不同构造环境原始岩浆成分相比较,发现丁青地幔橄榄岩母熔体大多处于玻安岩中。纯橄岩氧逸度估算FMQ=-3.05~-0.71,方辉橄榄岩氧逸度FMQ=-3.89~+1.47,显示丁青地幔橄榄岩有俯冲作用的参与。通过丁青钻孔岩心的研究,提出丁青东岩体可能形成于俯冲带之上的弧前环境这一观点。  相似文献   
6.
Zircon U–Pb dating of two samples of metagabbro from the Riwanchaka ophiolite yielded early Carboniferous ages of 354.4 ± 2.3 Ma and 356.7 ± 1.9 Ma. Their positive zircon εHf(t) values (+7.9 to +9.9) indicate that these rocks were derived from a relatively depleted mantle. The metagabbros can be considered as two types: R1 and R2. Both types are tholeiitic, with depletion of high-field-strength elements (HFSE) and enrichment of large-ion lithophile elements (LILE) similar to those of typical back-arc basin basalts (BABB), such as Mariana BABB and East Scotia Ridge BABB. Geochemical and isotopic characteristics indicate that the R1 metagabbro originated from a back-arc basin spreading ridge with addition of slab-derived fluids, whereas the R2 metagabbro was derived from a back-arc basin mantle source, with involvement of melts and fluids from subducted ocean crust. The Riwanchaka ophiolite exhibits both mid-ocean ridge basalts- and arc-like geochemical affinities, consistent with coeval ophiolites from central Qiangtang. Observations indicate that the Qiangtang ophiolites developed during the Late Devonian–early Carboniferous (D3–C1) in a back-arc spreading ridge above an intra-oceanic subduction zone. Based on our data and previous studies, we propose that an oceanic back-arc basin system existed in the Longmuco–Shuanghu–Lancang Palaeo-Tethys Ocean during the D3–C1 period.  相似文献   
7.
内蒙古西乌旗新发现的米斯庙蛇绿岩,位于中亚造山带东段迪彦庙SSZ型蛇绿岩带的西延米斯庙一带。大比例尺填图结果显示,米斯庙蛇绿岩呈NEE向展布,宽约5km,长约20km,蛇绿岩各单元出露较齐全,岩性主要为蛇纹石化方辉橄榄岩、暗色均质辉长岩、浅色均质辉长岩、斜长岩、枕状前弧玄武岩(fore-arc basalt,FAB)、枕状过渡熔岩和泥质硅质岩,局部见斜长花岗岩侵入于玄武岩之中,其岩性组合和岩石序列可与代表洋内初始俯冲的迪彦庙前弧蛇绿岩相对比。该米斯庙蛇绿岩岩石序列中辉长岩锆石U-Pb年龄为336.5±3Ma,与东部迪彦庙蛇绿岩的辉长岩同期。米斯庙FAB主要发育无斑结构和少斑结构,基质为球颗结构,枕状构造;枕状过渡熔岩,发育斑状结构,比FAB具有更多的斜长石斑晶。米斯庙FAB地球化学成分与大洋中脊形成的强亏损玄武岩相似,但与N-MORB相比,高场强元素(HFSE)Nb和Ta等含量略低于N-MORB,与弧拉斑玄武岩相似;与米斯庙FAB相比,米斯庙枕状过渡熔岩的地球化学特征是更加亏损Ti、HREE和HFSE,指示大洋板片的渐进性俯冲,类似于IBM前弧的演化。米斯庙蛇绿岩与迪彦庙前弧蛇绿岩均发育类似于马里亚纳前弧的FAB序列,两者延伸长度可达40km,表明在古亚洲洋东段早石炭世沿迪彦庙-米斯庙一带发育洋内初始俯冲,记录了早石炭世古亚洲洋东部洋内初始俯冲事件。  相似文献   
8.
贺根山蛇绿岩带是中亚造山带东部出露面积最大的晚古生代蛇绿岩带,然而研究者对它是大洋岩石圈的残余还是陆内岩石圈地幔伸展的产物仍存在较大争议,主要归因于一直缺少橄榄岩Re-Os同位素的有效制约。基于此,本文对贺根山蛇绿岩中方辉橄榄岩进行了系统的岩石学、地球化学和Re-Os同位素研究。方辉橄榄岩主要由橄榄石、斜方辉石、粗粒的铬尖晶石和少量单斜辉石组成。贺根山方辉橄榄岩具有难熔地球化学特征,显示出高的MgO(45.1%~48.1%)含量,低的CaO(0.09%~0.26%)和Al 2O 3(0.58%~0.90%)含量及Tb/Yb比值,低于原始上地幔的^(187)Os/^(188)Os(0.1236~0.1263)同位素组成;橄榄石具有较高的Fo值(91.0~91.7)和NiO含量(0.26%~0.48%),而且单斜辉石具有低的重稀土含量,暗示了贺根山方辉橄榄岩是尖晶石相陆下岩石圈地幔高程度(20%~30%)部分熔融的难熔残余。另外,方辉橄榄岩相对富集LREE和大离子亲石元素,亏损高场强元素(例如Nb、Ta),且单斜辉石具有高Ti/Eu值和低的La/Yb,揭示了地幔源区受到硅酸盐熔体交代作用的改造。另一方面,方辉橄榄岩显示出相对年轻的Re亏损模式年龄,t RD为454~825Ma,表明贺根山地区岩石圈地幔形成的时代应该在0.8Ga之前,而后经历过多期熔体抽离和交代,与区域上大规模地壳增生时代吻合,可能是罗迪尼亚超大陆裂解的响应。结合蛇绿岩中大量深源矿物(金刚石等)和捕获锆石的存在、南北两侧一致的地质记录,本文认为贺根山方辉橄榄岩可能是陆内伸展环境下深部地幔物质上涌减压熔融的产物。  相似文献   
9.
The Bulqiza ultramafic massif, which is part of the eastern Mirdita ophiolite of northern Albania, is world renowned for its high-Cr chromitite deposits. High-Cr chromitites hosted in the mantle section are the crystallized products of boninitic melts in a supra-subduction zone (SSZ). However, economically important high-Al chromitites are also present in massive dunite of the mantle-crust transition zone (MTZ). Chromian-spinel in the high-Al chromitites and dunites of the MTZ have much lower Cr# values (100Cr/(Cr+Al)) (47.7–55.1 and 46.5–51.7, respectively) than those in the high-Cr chromitites (78.2–80.4), harzburgites (72.6–77.9) and mantle dunites (79.4–84.3). The chemical differences in these two types of chromitites are reflected in the behaviors of their platinum-group elements (PGE). The high-Cr chromitites are rich in IPGE relative to PPGE with 0.10–0.45 PPGE/IPGE ratios, whereas the high-Al chromitites have relatively higher PPGE/IPGE ratios between 1.20 and 7.80. The calculated melts in equilibrium with the high-Cr chromitites are boninitic-like, and those associated with the high-Al chromitites are MORB-like but with hydrous, oxidized and TiO2-poor features. We propose that the coexistence of both types of chromitites in the Bulqiza ultramafic massif may indicates a change in magma composition from MORB-like to boninitic-like in a proto-forearc setting during subduction initiation.  相似文献   
10.
In the Veria-Naousa ophiolitic complex (north Greece), rodingite appears mainly in the form of cross cutting dykes within serpentinised peridotites. It is distinguished into three types, based upon the provenance of its protoliths, textural characteristics, mineralogical assemblages and geochemical affinities. Type I rodigites were derived from boninitic diabasic protoliths and their mineralogical assemblage include garnet + clinopyroxene + chlorite. Type II rodingites were formed at the expense of gabbroic precursors, comprising clinopyroxene + garnet + vesuvianite ± quartz, whereas Type III rodingites replaced diabasic tholeiitic protoliths comprising of garnets + vesuvianite + clinopyroxene + chlorite. Rodingitisation resulted in desilification, decrease of alkalies, Al, Fe, Mg and increase in Ca contents. In Type I rodingites the MREE (middle rare earth elements) and HREE (heavy rare earth elements) were slightly reduced. Type II rodingites experienced LREE (light rare earth elements) depletions, whereas MREE and HREE remained fairly stable. Restricted mobility of REE in Type III rodingites is assigned to shallow-level rodingitisation under decreasing pH.Rodingitisation occured in two distinct stages at fore-arc settings. The first stage occured under mildly oxidising conditions and enhanced CO2/H2O ratios. This stage affected the protoliths of all rodingite types. The second rodingitisation stage occured under more oxidising conditions and lower CO2/H2O ratios, which corresponds to the exhumation stage of the serpentinite-rodingite formations. Types II and III rodingites were subjected to further rodingitisation under the increasing influence of slab-derived hydrous phases at shallower depths, leading to the formation of late-stage andradite and vesuvianite. All stages of rodingitisation are estimated to have occurred under relatively moderate temperatures and pressure (~300 to 450 °C; ~2–6 kbar respectively).  相似文献   
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