西昆仑库地蛇绿岩中超镁铁岩及富Al型铬铁岩成因研究

发布时间：2018-04-28 22:27

本文选题：豆荚状铬铁矿 + 铂族元素　；参考：《吉林大学》2017年博士论文

【摘要】：I库地-其曼于特蛇绿岩带是青藏高原最北缘的一条缝合带,多数学者认为是原特提斯洋在早古生代闭合留下的遗迹,其中奥依塔格、柯岗、库地、其曼于特等地发育蛇绿岩。在这些蛇绿岩中,库地蛇绿岩保存较好,主要由不孜完沟(Buziwan Valley)处的镁铁-超镁铁岩以及一些克沟(Yixieke Valley)处的火山岩、火山碎屑岩组成。镁铁-超镁铁岩主要由大洋壳及地幔残片组成,包括方辉橄榄岩、纯橄岩、辉石岩及辉长岩。最近几年,在超镁铁岩中发现了数条铬铁矿体。矿体呈豆荚状、层状、似层状,分布于方辉橄榄岩和纯橄岩岩体中,矿体与围岩呈整合或不整合接触。火山岩及火山碎屑岩包括块状及枕状玄武岩、玻安岩、凝灰岩、安山质角砾岩和集块岩。前人对库地蛇绿岩的研究主要集中在火山岩方面,对镁铁-超镁铁岩的研究较弱,尤其是近些年在超镁铁岩中发现了铬铁矿体,更突显了超镁铁岩的重要性。为此,本文选择代表性的纯橄岩、方辉橄榄岩及铬铁岩作为研究对象,对它们进行了详细的岩相学、地球化学特征分析,探讨了岩石的成因、形成环境及形成时的氧化还原状态。对铬铁岩还进行了详细的矿物包裹体研究,探讨了矿物包裹体的成因及铬铁岩母岩浆性质。通过研究,我们发现纯橄岩主要有两种:一种分布在方辉橄榄岩层位的上部,呈层状、似层状,与上部的堆晶辉长岩距离较近,与方辉橄榄岩呈突变接触,分布广泛(I型);另一种分布在方辉橄榄岩中,呈透镜状、团块状分布,与方辉橄榄岩呈突变或渐变接触,分布较为局限(II型)。I型纯橄岩全岩具有较高的TiO2、MnO、Pt族铂族元素(PPGE:Pt,Pd)、PPGE/IPGE比值(IPGE:Os,Ir,Ru),较低的Mg#(0.88-0.90)及较低的CaO/Al2O3值(0.12-0.47)。其中橄榄石的Fo值、NiO较低,MnO含量较高,铬尖晶石Cr#较低,TiO2较高,为堆晶成因。II型纯橄岩全岩亏损易熔组分(如Al2O3、CaO、PPGE),富集难熔组分(如MgO、IPGE),其铂族元素配分模式为负斜率。此外,II型纯橄岩中,橄榄石具有较高的Fo值、NiO,较低的MnO;铬尖晶石具有较高的Cr#、较低的TiO2,为熔融残余成因。方辉橄榄岩中橄榄石具有较高的Fo值,斜方辉石和单斜辉石具有较高的Mg#值,铬尖晶石具有中等偏高的Cr#值。全岩具有较高IPGE,较低的PPGE,在铂族元素配分图解中,呈平的或负的斜率,与残余成因的方辉橄榄岩类似。方辉橄榄岩样品中,部分铬尖晶石颗粒较粗并包含橄榄石、辉石等硅酸盐矿物颗粒,且具有较高的TiO2含量(可达0.22 wt.%),是熔体与橄榄岩反应的结果,这与全岩微量元素地球化学特征一致。因此,方辉橄榄岩为亏损的地幔岩,后期受到一定的熔体交代作用。铬铁岩有三种:块状铬铁岩、浸染状铬铁岩及脉状铬铁岩。块状铬铁岩主要由铬尖晶石、橄榄石和单斜辉石组成,铬尖晶石呈自形晶,TiO2含量较高(0.3 wt.%),全岩富集PPGE,亏损IPGE,在铂族元素配分图解中呈正斜率,与堆晶成因的铬铁岩类似。浸染状铬铁岩和脉状铬铁岩主要由铬尖晶石和橄榄石组成。铬尖晶石形态不规则,沿橄榄石颗粒边界彼此连接,TiO2含量较低(0.3 wt.%),全岩富集IPGE,亏损PPGE,铂族元素配分模式呈负斜率,为饱含铬尖晶石的熔体和纯橄岩反应而来。铬铁岩中的铬尖晶石含有很多矿物包裹体,包括无水硅酸盐矿物(如橄榄石、单斜辉石)、含水硅酸盐矿物(如角闪石、金云母)、氧化物(如铬尖晶石)、碳酸盐矿物(如方解石)、磷酸盐矿物(磷灰石)、贱金属硫化物(如镍黄铁矿、黄铜矿、方黄铜矿)等。其中,橄榄石、单斜辉石等硅酸盐矿物呈单颗粒形成存在,它们形成于铬尖晶石结晶前,在岩浆阶段被铬尖晶石包裹;角闪石、金云母等矿物,主要和其他矿物共生在一起,是在铬尖晶石重结晶作用过程中,由熔体和早期结晶的矿物经过复杂的反应而来。硫化物则是在含硫化物的流体从岩浆的分离过程中形成的。铬铁岩母岩浆成分与拉斑玄武质熔体类似,且铬尖晶石中含有很多含水硅酸盐矿物包裹体,母岩浆应为含水的拉斑玄武质熔体。样品的氧逸度值(FMQ+0.61-FMQ+2.27)及地幔橄榄岩的矿物化学成分均反映了弧下和弧前的环境。由于库地火山岩具有俯冲开始阶段火山岩的特征,因此我们认为库地蛇绿岩中的超镁铁岩及铬铁岩也形成于板片的俯冲开始阶段,它们最初形成于弧下(火山前缘)环境,之后受到地幔角流(the mantle flows)的影响,被拖拽至弧前盆地就位。
[Abstract]:The I Kudi - its manuuyu ophiolite belt is a suture zone on the most northern margin of the Qinghai Tibet Plateau. Most scholars believe that the original Tethys ocean was left behind in the early Palaeozoic, including O Bea Tagg, kolgang and Kudi, which are specially developed by the ophiolite. In these ophiolite, the Kudi ophiolites are well preserved, mainly by the Buziwan Vall Ey) the magnesium iron ultramafic rocks and some volcanic clastic rocks at the Yixieke Valley. The mafic ultramafic rocks consist mainly of the oceanic crust and mantle fragments, including the peridotite, the pure carcite, the pyroxenite and the gabbro. In recent years, several ferrochrome ore bodies have been found in the ultramafic rocks. The orebodies are pod like and layered. Stratiform, distributed in the peridotite and the trumbrite rock mass, the ore body and the surrounding rock are integrated or unconformable contact. The volcanic and pyroclastic rocks include massive and occipital basalt, Bose, tuff, anamite breccia and block rock. The former study of the Kutan ophiolite is concentrated on the volcanic rocks and the magnesium iron ultramafic rock The research is weak, especially in the ultramafic rocks in recent years, and the importance of ultramafic rocks is highlighted. For this reason, this paper chooses the representative olivite, peridotite and chromite as the research object, and carries out the detailed petrography and geochemistry analysis of them, and probes into the genesis of the rocks, the formation of the environment and the formation of the environment. A detailed mineral inclusion study of the chromite was also carried out. The genesis of the mineral inclusions and the properties of the chromite parent magma were discussed. Through the study, we found that there are two main types of the olivite, one in the upper part of the peridotite layer and in the stratiform and stratiform, and the distance from the upper lamellar gabbro. Recently, there is a catastrophic contact with the peridotite and widely distributed (I type); the other is distributed in the olperidotite, with a lenticular form, a lump like distribution, a sudden change or gradual contact with the peridotite, and a relatively limited (II).I type of the whole rock of the olivine with a higher TiO2, MnO, Pt group of platinum group elements (PPGE:Pt, Pd), PPGE/IPGE ratio (IPGE:Os, Ir, Ru). Low Mg# (0.88-0.90) and lower CaO/Al2O3 value (0.12-0.47). Among them, the Fo value of olivine, NiO is lower, MnO content is higher, Cr spinel Cr# is lower and TiO2 is higher, which is a heap genesis.II type of pure olive whole rock. In the olivite, olivine has a high Fo value, NiO, lower MnO, the chromium spinel has a higher Cr# and a lower TiO2, which is the residual cause of the melting. The olivine in the olivine has a high Fo value, the trapezite and the monoclinopyroxene have a high Mg# value, and the chromium spinel has a moderately high Cr# value. The whole rock has a higher IPGE and a lower PPG. E, in the partition diagram of the platinum group element, shows a flat or negative slope, similar to the remnants of the peridotite. In the samples of the olidolite, some of the CR spinel particles are coarser and contain silicate minerals such as olivine and pyroxene, and have a high content of TiO2 (up to 0.22 wt.%), which is the result of the melts and peridotite reaction, which is with the whole rock. The geochemical characteristics of trace elements are consistent. Therefore, the peridotite is a depleted mantle rock with a certain melt metasomatism in the later period. There are three types of chromite: massive chromite, disseminated chromite and pulse chromite. The massive chromite is mainly composed of chrome spinel, olivine and monopyroxene, the chrome spinel is self shaped, and the content of TiO2 is more than that of the chromite. High (0.3 wt.%), full rock enrichment of PPGE, loss of IPGE, in the platinum group element matching diagrammatic diagrammatic slope, similar to the genesis of chromite rock. Dipping chromite and pulse chromite mainly composed of chrome spinel and olivine. The chromic spinel is irregular in shape along the boundary of olivine particles, TiO2 content is low (0.3 wt.%), full rock enrichment. IPGE, a loss of PPGE, a negative slope of the distribution pattern of the platinum group element, a reaction to the melts containing chromium spinel and the pure cite. The chromic spinel in the chromite contains a lot of mineral inclusions, including anhydrous silicate minerals (such as olivine, clinopyroxene), hydrated silicate minerals (such as amphibole, mica), oxides (such as chrome spinel), and carbonic acid. Salt minerals (such as calcite), phosphate minerals (apatite), base metallic sulphides (such as nickel pyrite, chalcopyrite, and square chalcopyrite). Among them, the silicate minerals such as olivine and clinopyroxene are formed in single particles. They are formed before the crystallization of chromium spinel and are wrapped in chromic spinel at the magma stage; hornblende, gold mica, and other minerals. Other minerals, together, are formed during the complex reaction of melts and early crystalline minerals during the recrystallization of chromium spinel. Sulphides are formed during the separation of magma containing sulfides. The chromite parent magma is similar to the melted basalt melt, and the chromium spinel contains a lot of water. The oxygen fugacity (FMQ+0.61-FMQ+2.27) of the sample and the mineral chemistry of the mantle peridotite all reflect the environment under arc and before arc. Because of the volcanic rocks in the beginning of the subduction stage, we think the ultramafic iron in the Kredi ophiolite is considered. Rock and chromite are also formed at the beginning of the subduction stage of the plate. They were initially formed under the arc (the volcanic front) environment, followed by the influence of the the mantle flows, which was dragged to the front of the arc basin.

【学位授予单位】：吉林大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P588.1

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