新疆萨尔托海蛇绿岩中高铝型铬铁矿成因

发布时间：2018-01-07 02:17

本文关键词：新疆萨尔托海蛇绿岩中高铝型铬铁矿成因　出处：《中国地质科学院》2015年博士论文　论文类型：学位论文

【摘要】：近年在我国西藏和俄罗斯极地乌拉尔等地的蛇绿岩铬铁矿中新发现了金刚石等深部矿物群,使得对此前铬铁矿的浅部成因理论提出了质疑。铬铁矿中为什么会产有金刚石等深部地幔矿物?它们是如何形成的?这些异常矿物在全球不同类型和构造背景的蛇绿岩铬铁矿和地幔岩中是否普遍存在?其科学意义何在?新发现使得蛇绿岩型铬铁矿的成因,尤其金刚石等深部矿物的成因和深部地幔作用,成为板块构造面临的一个极具挑战性的新的前沿性科学问题。本文选择了新疆达拉布特蛇绿岩和其中的萨尔托海铬铁矿为研究目标。萨尔托海铬铁矿为高铝型,产在晚古生代的蛇绿岩中,已发现金刚石的西藏罗布莎和俄罗斯极乌拉尔铬铁矿均为高铬型,后两者的蛇绿岩形成时代分别为中生代和早古生代。说明萨尔托海与西藏和俄罗斯的铬铁矿不仅矿石类型不同,并且分属不同的造山带。因此,探讨其中是否含有金刚石等深部矿物及成因,具有重要科学意义。本文开展了达拉布特蛇绿岩和萨尔托海铬铁矿野外地质调查,室内综合研究和铬铁矿人工重砂选矿。研究内容包括蛇绿岩和铬铁矿的主、微量和铂族元素,地幔橄榄岩的矿物学和单矿物微量元素,铬铁矿中矿物包裹体分析,铬铁矿人工重砂单矿物分选和选出矿物的成分、结构和包裹体分析,以及基性岩类的锆石U-Pb定年和Hf同位素。主要取得如下进展： (1)达拉布特蛇绿岩形成于弧后扩张脊并受俯冲流体/熔体影响。地幔橄榄岩以方辉橄榄岩为主,含少量纯橄岩和二辉橄榄岩。方辉橄榄岩中尖晶石Mg#在54-62之间,Cr#在39-48之间,橄榄石Fo值在90-93之间。纯橄岩和方辉橄榄岩∑REE都低于球粒陨石,且具有LREE富集的U型稀土元素配分模式。方辉橄榄岩中单斜辉石∑REE介于深海和SSZ地幔橄榄岩中单斜辉石的∑REE之间,高场强元素Ti vs Yb和Ti vs Dy图解都显示单斜辉石可能是地幔橄榄岩含水部分熔融的残余。根据方辉橄榄岩中共存的橄榄石-尖晶石平衡计算,获得方辉橄榄岩的氧逸度在FMQ-0.14至FMQ+0.96之间,具MOR到SSZ过渡的特征,或弧后盆地至岛弧过渡的特征。地幔橄榄岩的单斜辉石、斜方辉石、橄榄石和尖晶石的主量元素成分也具有MOR到SSZ过渡的特征。尖晶石Cr#vs Mg#以及尖晶石Cr#与共存橄榄石Fo值图解显示,方辉橄榄岩为原始地幔20%-25%部分熔融的残余。蛇绿岩中基性岩类具有N-MORB和E-MORB的特征,表现出高场强元素Nb、Ta、Zr、Hf、Ti亏损,而大离子亲石元素U、K、Pb、Sr等富集的特点,反映俯冲带物质成分加入,尤其,基性岩锆石的Hf同位素特征表明有壳源物质的加入。 (2)通过蛇绿岩中基性岩的锆石U-Pb定年,限制了洋盆演化至少持续到早石炭世。本研究获得达拉布特蛇绿岩带中木哈塔依蛇绿岩的玄武岩锆石U-Pb年龄在(393.9±3.0)Ma之间,为达拉布特蛇绿岩形成的年龄。萨尔托海辉长岩脉锆石U-Pb年龄为(359.4±1.0)Ma,达拉布特辉长岩脉锆石U-Pb年龄为(342.0±2.7)Ma,阿克巴斯套辉长岩脉的锆石U-Pb年龄为(357.6±2.4)Ma,阿克巴斯套辉石岩脉的锆石U-Pb年龄为(350.6士4.0)。这些岩脉形成于大洋扩张的过程中由软流圈地幔部分熔融侵入于地幔橄榄岩中形成,表明大洋扩张持续到早石炭世。 (3)通过重砂选矿首次在847Kg萨尔托海高铝铬铁矿中发现了超高压,强还原性和壳源的20余种矿物。其中包括金刚石、单质铬、自然铁和单质硅等自然元素类；碳化物碳硅石；铁镍、铁镍铬合金等金属合金；方铁矿、金红石、赤铁矿、磁铁矿、钛铁矿、独居石、石英和铬尖晶石等氧化物类；方铅矿、闪锌矿、针镍矿、赫硫镍矿和毒砂等硫化物类；镁橄榄石、顽火辉石、透辉石、蛇纹石、锆石和长石等硅酸岩类。这些矿物与俄罗斯极地乌拉尔以及西藏罗布莎铬铁矿可以对比,表明了萨尔托海高铝铬铁矿可能和高铬铬铁矿一样,存在深部地幔成矿阶段。 (4)在铬铁矿中发现了大量的含水和无水的硅酸盐原生矿物包体。包括透辉石,顽火辉石,镁橄榄石,钠-金云母,Na-Cr韭闪石,钠长石等。铬铁矿选矿发现的壳源矿物金红石原岩为变基性岩,锆石中也存在大量的壳源含水矿物包体。这些矿物的出现暗示了铬铁矿在浅部含水上地幔的成矿过程。计算所得铬铁矿成矿的母岩浆A12O3、TiO2和FeO/MgO与MORB相似,表明成矿与含水的MORB熔体有关。 (5)提出了萨尔托海深部预富集和浅部改造成矿的成因模式。俯冲板片进入地幔过渡带,发生熔融,形成富Si、Cr、C、Fe、Ni等元素的强还原性流体,并在过渡带形成金刚石、碳硅石、单质硅、单质铬、单质铁、方铁矿、Fe-Ni合金等矿物。这些矿物在在扩张脊下方向上运移过程中被包裹于过渡带顶部或上部(14GPa)的铬铁矿中。铬铁矿在此阶段初步富集,携带这些超高压和强还原性矿物进入浅部的大洋岩石圈地幔。板片断裂使软流圈地幔上涌并同化俯冲板片,携带俯冲带壳源锆石和金红石等物质,进入弧后扩张脊中的岩浆房中,形成含水的MORB型岩浆并与地幔橄榄岩发生大规模的岩石／熔体反应,改造之前初步富集的铬铁矿,形成萨尔托海高铝型铬铁矿。铬铁矿形成时岩浆为MORB型贫硫和贫PGE岩浆,使得铬铁矿中几乎没有原生的铂族矿物和贱金属硫化物存在,而在岩浆演化后期,由于硫逸度不断增加、温度降低,形成赫硫镍矿、针镍矿等贱金属硫化物和硫钌矿等铂族矿物赋存于铬铁矿的裂隙间或粒间基质中。
[Abstract]:In ophiolitic chromitites in China Tibet and Russia, Ural and other places in the new polar diamond found deep mineral group, so this theory causes shallow before chromite questioned. In chromite why diamond producing deep mantle minerals? How are they formed? Whether these abnormalities the mineral is widespread in the global different types and tectonic background of ophiolite and chromite in mantle rocks? What are its scientific significance? The new discovery that causes ophiolitic chromite, especially the deep causes of diamond minerals and deep mantle, as plate tectonics faces a challenging new the frontier of science.
This paper chose Xinjiang DARBUT ophiolite and the Salto sea of chromite as the research target. Salto sea chromite is high aluminum, produced in the late Paleozoic ophiolite in Tibet, has found the diamond LUOBUSHA CHROMITE DEPOSIT IN Ural and Russia are extremely high chromium, the ophiolite formed respectively. The Mesozoic and early Paleozoic. The Salto sea and Tibet and Russia not only chromite ore types are different, and different orogenic belt. Therefore, to investigate whether it contains mineral genesis and deep diamond, has important scientific significance.
This paper carried out the DARBUT ophiolite and Salto sea chromite ore field geological survey, indoor comprehensive study and chromite artificial placer beneficiation. Research contents include ophiolite and chromite, and trace platinum group elements, mineralogy of mantle peridotite minerals and trace elements, analysis of mineral inclusions in chromite, chromium ore artificial placer mineral separation and select the single mineral composition, structure analysis and inclusions, and zircon U-Pb dating of basic rocks and Hf isotope. The main results are as follows:
(1) DARBUT ophiolite was formed in the back arc spreading ridge subduction and fluid / melt. The mantle peridotite, harzburgite, dunite and containing a small amount of two peridotite. Harzburgite in spinel Mg# at 54-62, Cr# at 39-48, Fo value in the 90-93 of olivine. Dunite and harzburgite are lower than the sigma REE chondrites and U with rare earth element enrichment LREE distribution patterns between REE. Sigma Sigma REE between clinopyroxene and SSZ deep mantle peridotite Clinopyroxenes in harzburgite, vs Yb and high field strength elements Ti Ti vs Dy graphic display of clinopyroxene may be residual partial melting of mantle peridotite water. According to the calculation of the coexistence of harzburgite in olivine - spinel harzburgite, obtain the oxygen fugacity between FMQ-0.14 to FMQ+0.96, with characteristics of MOR to SSZ transition, or to the island arc back arc basin The characteristics of the transition. The mantle peridotite clinopyroxene, orthopyroxene, major element compositions of olivine and spinel has the characteristics of MOR to SSZ transition. The spinel Cr#vs Mg# and spinel Cr# with coexisting olivine Fo graphic display, harzburgite residual 20%-25% primitive mantle partial melting. The characteristics of basic rocks of ophiolite with N-MORB and E-MORB, showed high field strength elements Nb, Ta, Zr, Hf, Ti loss, and large ion lithophile elements U, K, Pb, Sr etc. characteristics, especially the subduction zone components added, Hf isotopic characteristics of mafic zircon showed that adding crustal material.
(2) the zircon U-Pb dating of mafic ophiolite, limited oceanic basin evolution continued until at least the Early Carboniferous. This research obtains the DARBUT ophiolite belt in mohurtay ophiolite basalts in zircon U-Pb age (393.9 + 3) Ma, as the dalabut ophiolite formed Salto sea gabbro age. Pulse zircon U-Pb age (359.4 + 1) Ma, U-Pb zircon age of gabbro vein Da Bute (342 + 2.7) Ma, U-Pb zircon age akbastau gabbro vein for (357.6 + 2.4) Ma, akbastau pyroxenite veins zircon U-Pb age (350.6 + 4). These dikes formed in the mantle peridotite formed by partial melting of the asthenosphere mantle intrusion in ocean expansion, that the ocean continues to Early Carboniferous.
(3) the heavy dressing for the first time in 847Kg Salto haigao aluminum chromite found in ultra high pressure, more than 20 kinds of minerals with strong reducibility and crust source. Including diamond, elemental chromium, natural iron and silicon carbide carbon and other natural elements; silica; Fe Ni, Fe Ni Cr alloy and other metal alloys; Party Jin Hongshi, iron ore, hematite, magnetite, ilmenite, monazite, quartz and chrome oxide; galena, sphalerite, millerite, HEAZLEWOODITE and sulfides such as arsenopyrite; magnesium olivine, enstatite, diopside, serpentine, zircon and feldspar silicate rocks. These minerals can be compared with Russia the Ural and Tibet LUOBUSHA chromite, showed Tuohaigao chromite and sal aluminum high chromium chromium ore, there are deep mantle metallogenic stage.
(4) found a large number of hydrous and anhydrous silicate minerals of inclusions in chromite. Including diopside, enstatite, olivine, sodium phlogopite, pargasite Na-Cr, sodium feldspar. Found chromium ore dressing shell source mineral rutile protolith is metabasites, crust derived hydrous minerals a large number of inclusions in zircon. There are also these minerals suggest that the ore-forming process of chromite in the shallow water of the upper mantle. Magma A12O3 calculated chromite mineralization, similar to TiO2 and FeO/MgO and MORB, showed that the MORB melt and water mineralization related.
(5) proposed the Salto sea deep preconcentration and shallow mineralization genesis model. Subduction into the mantle transition zone, melting, forming the rich Si, Cr, C, Fe, Ni and other elements of the strong reduction of fluid, and in the transition zone of the formation of diamond, carbon silica, silicon, single organic chromium, iron and wustite, Fe-Ni alloy and other minerals. These minerals are wrapped in the transition zone at the top or on the upper part of the spreading ridge under the direction of migration in the process (14GPa) in chromite chromite. At this stage the preliminary enrichment, carrying these ultra high pressure and strong reduction of mineral into the shallow oceanic lithosphere the fracture plate. The mantle upwelling of asthenospheric mantle and assimilation of subducted slab subduction zone, carrying crustal zircon and Jin Hongshi etc, the ridge expansion into the magma arc, the formation of MORB type magma water and rock / melt reaction and large-scale transformation of mantle peridotite. Before the preliminary enrichment of chromite, formed Salto haigao aluminium chromite chromite magma formation. MORB and PGE in low sulphur depleted magma, the platinum mineral and base metal sulfides in chromite almost no native existence, and in the late magmatic evolution, because the sulfur fugacity is increasing, the decrease of temperature, the formation of Hector polydymite, millerite base metals such as ruthenium sulfide and sulfur ore platinum mineral occurrence in the chromite fissure or intergranular matrix.

【学位授予单位】：中国地质科学院
【学位级别】：博士
【学位授予年份】：2015
【分类号】：P618.3

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