Evidence of Mid-ocean ridge and shallow subduction forearc magmatism in the Nagaland-Manipur ophiolites,northeast India: constraints from mineralogy and geochemistry of gabbros and associated mafic dykes     

《Chemie der Erde / Geochemistry》2016,76(4):605-620

We discuss here the mineralogical and geochemical characteristics of mafic intrusive rocks from the Nagaland-Manipur Ophiolites (NMO) of Indo-Myanmar Orogenic Belt, northeast India to define their mantle source and tectonic environment. Mafic intrusive sequence in the NMO is characterized by hornblende-free (type-I) and hornblende-bearing (type-II) rocks. The type-I is further categorized as mafic dykes (type-Ia) of tholeiitic N-MORB composition, having TiO₂ (0.72–1.93 wt.%) and flat REE patterns (La_N/Yb_N = 0.76–1.51) and as massive gabbros (type-Ib) that show alkaline E-MORB affinity, having moderate to high Ti content (TiO₂ = 1.18 to 1.45 wt.%) with strong LREE-HREE fractionations (La_N/Yb_N = 4.54–7.47). Such geochemical enrichment from N-MORB to E-MORB composition indicates mixing of melts derived from a depleted mantle and a fertile mantle/plume source at the spreading center. On the other hand, type-II mafic intrusives are hornblende bearing gabbros of SSZ-type tholeiitic composition with low Ti content (TiO₂ = 0.54 wt.%–0.86 wt.%) and depleted LREE pattern with respect to HREE (La_N/Yb_N = 0.37–0.49). They also have high Ba/Zr (1.13–2.82), Ba/Nb (45.56–151.66) and Ba/Th (84.58–744.19) and U/Th ratios (0.37–0.67) relative to the primitive mantle, which strongly represents the melt composition generated by partial melting of depleted lithospheric mantle wedge contaminated by hydrous fluids derived from subducting oceanic lithosphere in a forearc setting. Their subduction related origin is also supported by presence of calcium-rich plagioclase (An_16.6–32.3). Geothermometry calculation shows that the hornblende bearing (type-II) mafic rocks crystallized at temperature in range of 565°–625 °C ± 50 (at 10 kbar). Based on these available mineralogical and geochemical evidences, we conclude that mid ocean ridge (MOR) type mafic intrusive rocks from the NMO represent the section of older oceanic crust which was generated during the divergent process of the Indian plate from the Australian plate during Cretaceous period. Conversely, the hornblende-bearing gabbros (type-II) represent the younger oceanic crust which was formed at the forearc region by partial melting of the depleted mantle wedge slightly modified by the hydrous fluids released from the subducting oceanic slab during the initial stage of subduction of Indian plate beneath the Myanmar plate.  相似文献   

云开地区新元古代蛇绿岩的地球化学证据及其构造意义   总被引：10，自引：0，他引：10  

彭松柏  金振民  付建明  何龙清  蔡明海  刘云华 《地质学报》2006,80(6):814-825

粤西云开隆起区西北缘信宜贵子、茶山一带浅变质云开群底部层状、似层状变基性、超基性岩的研究表明,变基性岩以变玄武岩、变辉长岩、斜长角闪岩为主,变超基性岩则主要为蛇纹石化橄榄岩、橄辉岩和角闪辉石岩,它们是遭受后期构造变形变质作用强烈改造的蛇绿混杂岩。变基性岩的岩石地球化学特征可分为略有差异的两类:茶山的TiO2=0．71％-2．16％(平均1．38％),(La／Yb)N=0．67-1．91,轻稀土略亏损-平坦,La／Nb、Ce／ Zr、Zr／Nb、Zr／Y、Ti／Y平均值分别为1．19、0．31、15．75、2．08和434．98,Nb／Th平均值为4．0,εNd(t)=1．4-1．8;贵子的TiO2=1．55％-1．89％(平均1．74％),(La／Yb)N=2．13-2．80,轻稀土略富集,La／Nb、Ce／Zr、Zr／Nb、Zr／Y、 Ti／Y平均值分别为0．84、0．22、13．04、4．15和351．93,Nb／Th平均值为5．8,εNd(t)=2．4-5．0,均显示俯冲带上 (SSZ)和洋中脊(E-MORB)构造环境的玄武岩,并来自富集地幔源区。变超基性岩蛇纹石化橄榄岩的稀土元素配分具典型的U形特征,显示为具地幔交代作用的LREE富集地幔残余。茶山和贵子变基性岩的Sm-Nd、Rb-Sr全岩同位素等时线年龄分别为824±77 Ma和667±43 Ma、663±1 7 Ma。这为华南存在新元古代-早古生代的洋盆提供了重要证据,而且在洋盆初始阶段有地幔柱或OIB物质组分的加入。  相似文献   

Discovery of two ophiolite complexes of different ages in the Khoy area (NW Iran)     

Morteza Khalatbari-Jafari  Thierry Juteau  Hervé Bellon  Hashem Emami 《Comptes Rendus Geoscience》2003,335(12):917-929

New field and laboratory studies on the ophiolite of Khoy (northwestern corner of Iran) lead to the discovery that there are not one, but two ophiolitic complexes in the Khoy area: (1) an old, poly-metamorphic ophiolite, whose oldest metamorphic amphiboles yielded a Lower Jurassic apparent ⁴⁰K–⁴⁰Ar age, and whose primary magmatic age should logically be pre-Jurassic (Upper-Triassic?); (2) a younger non metamorphic ophiolite of well dated Upper Cretaceous age. To cite this article: M. Khalatbari-Jafari et al., C. R. Geoscience 335 (2003).  相似文献   

On the initiation of subduction zones   总被引：1，自引：0，他引：1  

Sierd Cloetingh  Rinus Wortel  N. J. Vlaar 《Pure and Applied Geophysics》1989,129(1-2):7-25

Analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere. Numerical model studies show that if after a short evolution of a passive margin (time span a few tens of million years) subduction has not yet started, continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin.Although much geological evidence is available in supporting the key role small ocean basins play in orogeny and ophiolite emplacement, evolutionary frameworks of the Wilson cycle usually are cast in terms of opening and closing of wide ocean basins. We propose a more limited role for large oceans in the Wilson cycle concept. In general, initiation of subduction at passive margins requires the action of external plate-tectonic forces, which will be most effective for young passive margins prestressed by thick sedimentary loads. It is not clear how major subduction zones (such as those presently ringing the Pacific Basin) form but it is unlikely they form merely by aging of oceanic lithosphere. Conditions likely to exist in very young oceanic regions are quite favorable for the development of subduction zones, which might explain the lack of preservation of back-arc basins and marginal seas.Plate reorganizations probably occur predominantly by the formation of new spreading ridges, because stress relaxation in the lithosphere takes place much more efficiently through this process than through the formation of new subduction zones.  相似文献   

蛇绿岩研究的进展   总被引：12，自引：0，他引：12  

张旗 《地学前缘》1994,(Z1)

近几年蛇绿岩的研究已取得显著的进展，主要反映在下述几个方面：１．对于蛇绿岩的概念已有了新的认识，蛇绿岩代表古代的大洋岩石圈碎片，但并非正常的大洋岩石圈；２．关于蛇绿岩的多样性已经提出了许多不同的见解，最近有关特提斯与环太平洋蛇绿岩的对比研究使许多作者得出结论，它们具有不同的特征；３．蛇绿岩源区的研究表明，蛇绿岩并非唯一地来自亏损的软流圈地幔，不同地幔端元之间的混合以及来自陆壳物质的混染作用也是常见的现象；４．早元古代蛇绿岩的发现。预期９０年代，在蛇绿岩的多样性、地幔岩部分熔融、岩浆来源、蛇绿岩的侵位机制以及蛇绿岩与高压变质作用等方面将取得长足的进展．  相似文献   

Tectonics of the ophiolite belt from Naga Hills and Andaman Islands,India   总被引：1，自引：0，他引：1  

S. K. Acharyya  K. K. Ray  S. Sengupta 《Journal of Earth System Science》1990,99(2):187-199

The ophiolitic rocks of Naga Hills-Andaman belt occur as rootless slices, gently dipping over the Paleogene flyschoid sediments, the presence of blue-schists in ophiolite melange indicates an involvement of the subduction process. Subduction was initiated prior to mid-Eocene as proved by the contemporaneous lower age limit of ophiolite-derived cover sediment as against the accreted ophiolites and olistostromal trench sediment. During the late Oligocene terminal collision between the Indian and Sino-Burmese blocks, basement slivers from the Sino-Burmese block, accreted ophiolites and trench sediments from the subduction zone were thrust westward as nappe and emplaced over the down-going Indian plate. The geometry of the ophiolites and the presence of a narrow negative gravity anomaly flanking their map extent, run counter to the conventional view that the Naga-Andaman belt marks the location of the suture. The root-zone of the ophiolite nappe representing the suture is marked by a partially-exposed eastern ophiolite belt of the same age and gravity-high zone, passing through central Burma-Sumatra-Java. The ophiolites of the Andaman and Naga Hills are also conventionally linked with the subduction activity, west of Andaman islands. This activity began only in late Miocene, much later than onland emplacement of the ophiolites; it further developed west of the suture in its southern part. Post-collisional northward movement of the Indian plate subparallel to the suture, also developed leaky dextral transcurrent faults close to the suture and caused Neogene-Quatemary volcanism in central Burma and elsewhere.  相似文献   

西藏改则县多龙SSZ型蛇绿岩的锆石U-Pb年龄、岩石地球化学及Sr-Nd同位素特征:班公湖-怒江洋晚二叠世洋内俯冲的证据   总被引：1，自引：1，他引：0  

韦少港  宋扬  唐菊兴  刘治博  王勤  高轲  李壮  李发桥 《岩石学报》2019,35(2):505-522

西藏改则县多龙矿集区分布着大量的SSZ型蛇绿岩残片,是班公湖-怒江蛇绿岩带的重要组成部分,本文对代表性辉绿(玢)岩进行了锆石U-Pb定年、岩石地球化学及Sr-Nd同位素分析,获得辉绿玢岩加权平均年龄为252.1±1.5Ma(MSWD=0.09,n=33),表明多龙SSZ型蛇绿岩形成于晚二叠世。岩石烧失量普遍较高(1.57%～5.23%)的特征,表明样品普遍遭受蚀变;而且具有富硅、高铝、低MgO及低Mg~#值(36.8～63.1)的特征,说明其经历过一定程度的分离结晶作用。多龙辉绿岩和辉绿玢岩富集大离子亲石元素(LILE:Rb、Cs、K、Sr、P、Pb),亏损高场强元素(HFSE:Ta、Nb、Ti、Zr、Hf);稀土元素总量为23.27×10~(-6)～58.95×10~(-6),δEu为0.83～1.35,(La/Yb)_N为0.66～1.30,具有与岛弧拉斑玄武岩类似的稀土元素和总体一致的微量元素特征。岩石(~(87)Sr/~(86)Sr)_i值为0.707345～0.708748,(~(143)Nd/~(144)Nd)_i值为0.512669～0.512705,t_(DM2)值为305～456Ma,ε_(Nd)(t)值为+6.9～+8.8,表明其岩浆源区为亏损地幔源区,其地球化学成分具有洋内弧的特征,暗示其可能是洋内俯冲作用的产物。综合分析认为,班-怒洋壳于晚二叠世时(ca.252Ma)在板块汇聚边缘发生了类似西太平洋Izu-Bonin-Mariana的洋内俯冲作用,形成兼具MOR蛇绿岩亲缘性和俯冲带SSZ蛇绿岩地球化学印记的双重属性的多龙辉绿岩及辉绿玢岩的母岩浆;母岩浆经过结晶分异、上升侵位,最终于洋内俯冲带之上的弧前盆地环境下形成多龙辉绿岩和辉绿玢岩。  相似文献   

The Cansiwang Melange of Southeast Bohol (Central Philippines): Origin and tectonic implications     

Joel V. De Jesus  Graciano P. Yumul  JR    Decibel V. Faustino 《Island Arc》2000,9(4):565-574

Abstract The Cansiwang Melange underlies the Southeast Bohol Ophiolite Complex (SEBOC) and is composed mainly of sheared ophiolite-derived blocks such as harzburgites, microgabbros, basalts and cherts in a pervasive serpentinite matrix. Available field, as well as geophysical evidence show that this melange unit is not diapiric, nor does it have a sedimentary origin considering that it lacks slump and flow structures. A tectonic origin for the Cansiwang Melange is favored in view of the numerous thrust faults, which cut across the exposures, as well as the tectonic contacts that the melange has with the overlying and underlying formations. The presence of the Cansiwang Melange in between the SEBOC and the Alicia Schist provides evidence that the amphibolite of the Alicia Schist do not correspond to the metamorphic sole of SEBOC. Similar to what is recognized in the Josephine Ophiolite, this suggests a 'cold' emplacement of the ophiolite over the Alicia Schist. The Cansiwang Melange represents an accretionary prism product which marks the location of an ancient subduction zone in what is now Central Philippines.  相似文献   

Hf–Nd isotope constraints on the origin of Dehshir Ophiolite,Central Iran     

HADI SHAFAII MOGHADAM  ROBERT J. STERN  JUN‐ICHI KIMURA  YUKA HIRAHARA  RYOKO SENDA  TAKASHI MIYAZAKI 《Island Arc》2012,21(3):202-214

The peri‐Arabian ophiolite belt, from Cyprus in the west, eastward through Northwest Syria, Southeast Turkey, Northeast Iraq, Southwest Iran, and into Oman, marks a 3000 km‐long convergent margin that formed during a Late Cretaceous (ca 100 Ma) episode of subduction initiation on the north side of Neotethys. The Zagros ophiolites of Iran are part of this belt and are divided into Outer (OB) and Inner (IB) Ophiolitic Belts. We here report the first Nd–Hf isotopic study of this ophiolite belt, focusing on the Dehshir ophiolite (a part of IB). Our results confirm the Indian mid‐oceanic ridge basalt (MORB) mantle domain origin for the Dehshir mafic and felsic igneous rocks. All lavas have similar Hf isotopic compositions, but felsic dikes have significantly less‐radiogenic Nd isotopic compositions compared to mafic lavas. Elevated Th/Nb and Th/Yb in felsic samples accompany nonradiogenic Nd, suggesting the involvement of sediments or continental crust.  相似文献   

中国蛇绿岩型铬铁矿的研究进展及思考   总被引：1，自引：0，他引：1       下载免费PDF全文

胡振兴  牛耀龄  刘益  张国瑞  孙文礼  马玉鑫 《高校地质学报》2014,20(1):9-27

中国广泛分布有不同时期的蛇绿岩。相对于全球一些大型蛇绿岩铬铁矿床(如 Kempirsai,Bulquiza,Guleman 等), 我国同期蛇绿岩中赋存的铬铁矿床规模都较小(如萨尔托海,东巧,罗布莎等)。近年来的研究认识到绝大多数成矿的蛇绿 岩都形成于俯冲带上覆岩石圈。熔 - 岩反应是目前用来解释豆荚状铬铁矿成因的流行假说,但仍然不能解释铬元素的有效 富集过程——即铬铁矿矿床的形成过程。高铬含量是岛弧玄武岩原始岩浆的特征,但与成矿要求的铬含量相差甚远;岛弧 原始玄武岩浆结晶出富铬尖晶石,有利于成矿,但这个岩浆演化和相平衡过程仍然难以造就具有工业意义的矿体、矿床。 那么存在形成富铬熔体的机制吗?又如何运移这些富铬熔体并集中结晶成矿?这些仍然是豆荚状铬铁矿成因的关键问题, 有待于进一步思考研究。  相似文献