發(fā)布時間：2018-04-28 22:27

  本文選題：豆莢狀鉻鐵礦 + 鉑族元素��； 參考：《吉林大學》2017年博士論文

【摘要】：I庫地-其曼于特蛇綠巖帶是青藏高原最北緣的一條縫合帶,多數(shù)學者認為是原特提斯洋在早古生代閉合留下的遺跡,其中奧依塔格、柯崗、庫地、其曼于特等地發(fā)育蛇綠巖。在這些蛇綠巖中,庫地蛇綠巖保存較好,主要由不孜完溝(Buziwan Valley)處的鎂鐵-超鎂鐵巖以及一些克溝(Yixieke Valley)處的火山巖、火山碎屑巖組成。鎂鐵-超鎂鐵巖主要由大洋殼及地幔殘片組成,包括方輝橄欖巖、純橄巖、輝石巖及輝長巖。最近幾年,在超鎂鐵巖中發(fā)現(xiàn)了數(shù)條鉻鐵礦體。礦體呈豆莢狀、層狀、似層狀,分布于方輝橄欖巖和純橄巖巖體中,礦體與圍巖呈整合或不整合接觸�；鹕綆r及火山碎屑巖包括塊狀及枕狀玄武巖、玻安巖、凝灰?guī)r、安山質角礫巖和集塊巖。前人對庫地蛇綠巖的研究主要集中在火山巖方面,對鎂鐵-超鎂鐵巖的研究較弱,尤其是近些年在超鎂鐵巖中發(fā)現(xiàn)了鉻鐵礦體,更突顯了超鎂鐵巖的重要性。為此,本文選擇代表性的純橄巖、方輝橄欖巖及鉻鐵巖作為研究對象,對它們進行了詳細的巖相學、地球化學特征分析,探討了巖石的成因、形成環(huán)境及形成時的氧化還原狀態(tài)。對鉻鐵巖還進行了詳細的礦物包裹體研究,探討了礦物包裹體的成因及鉻鐵巖母巖漿性質。通過研究,我們發(fā)現(xiàn)純橄巖主要有兩種:一種分布在方輝橄欖巖層位的上部,呈層狀、似層狀,與上部的堆晶輝長巖距離較近,與方輝橄欖巖呈突變接觸,分布廣泛(I型);另一種分布在方輝橄欖巖中,呈透鏡狀、團塊狀分布,與方輝橄欖巖呈突變或漸變接觸,分布較為局限(II型)。I型純橄巖全巖具有較高的TiO2、MnO、Pt族鉑族元素(PPGE:Pt,Pd)、PPGE/IPGE比值(IPGE:Os,Ir,Ru),較低的Mg#(0.88-0.90)及較低的CaO/Al2O3值(0.12-0.47)。其中橄欖石的Fo值、NiO較低,MnO含量較高,鉻尖晶石Cr#較低,TiO2較高,為堆晶成因。II型純橄巖全巖虧損易熔組分(如Al2O3、CaO、PPGE),富集難熔組分(如MgO、IPGE),其鉑族元素配分模式為負斜率。此外,II型純橄巖中,橄欖石具有較高的Fo值、NiO,較低的MnO;鉻尖晶石具有較高的Cr#、較低的TiO2,為熔融殘余成因。方輝橄欖巖中橄欖石具有較高的Fo值,斜方輝石和單斜輝石具有較高的Mg#值,鉻尖晶石具有中等偏高的Cr#值。全巖具有較高IPGE,較低的PPGE,在鉑族元素配分圖解中,呈平的或負的斜率,與殘余成因的方輝橄欖巖類似。方輝橄欖巖樣品中,部分鉻尖晶石顆粒較粗并包含橄欖石、輝石等硅酸鹽礦物顆粒,且具有較高的TiO2含量(可達0.22 wt.%),是熔體與橄欖巖反應的結果,這與全巖微量元素地球化學特征一致。因此,方輝橄欖巖為虧損的地幔巖,后期受到一定的熔體交代作用。鉻鐵巖有三種:塊狀鉻鐵巖、浸染狀鉻鐵巖及脈狀鉻鐵巖。塊狀鉻鐵巖主要由鉻尖晶石、橄欖石和單斜輝石組成,鉻尖晶石呈自形晶,TiO2含量較高(0.3 wt.%),全巖富集PPGE,虧損IPGE,在鉑族元素配分圖解中呈正斜率,與堆晶成因的鉻鐵巖類似。浸染狀鉻鐵巖和脈狀鉻鐵巖主要由鉻尖晶石和橄欖石組成。鉻尖晶石形態(tài)不規(guī)則,沿橄欖石顆粒邊界彼此連接,TiO2含量較低(0.3 wt.%),全巖富集IPGE,虧損PPGE,鉑族元素配分模式呈負斜率,為飽含鉻尖晶石的熔體和純橄巖反應而來。鉻鐵巖中的鉻尖晶石含有很多礦物包裹體,包括無水硅酸鹽礦物(如橄欖石、單斜輝石)、含水硅酸鹽礦物(如角閃石、金云母)、氧化物(如鉻尖晶石)、碳酸鹽礦物(如方解石)、磷酸鹽礦物(磷灰石)、賤金屬硫化物(如鎳黃鐵礦、黃銅礦、方黃銅礦)等。其中,橄欖石、單斜輝石等硅酸鹽礦物呈單顆粒形成存在,它們形成于鉻尖晶石結晶前,在巖漿階段被鉻尖晶石包裹;角閃石、金云母等礦物,主要和其他礦物共生在一起,是在鉻尖晶石重結晶作用過程中,由熔體和早期結晶的礦物經(jīng)過復雜的反應而來。硫化物則是在含硫化物的流體從巖漿的分離過程中形成的。鉻鐵巖母巖漿成分與拉斑玄武質熔體類似,且鉻尖晶石中含有很多含水硅酸鹽礦物包裹體,母巖漿應為含水的拉斑玄武質熔體。樣品的氧逸度值(FMQ+0.61-FMQ+2.27)及地幔橄欖巖的礦物化學成分均反映了弧下和弧前的環(huán)境。由于庫地火山巖具有俯沖開始階段火山巖的特征,因此我們認為庫地蛇綠巖中的超鎂鐵巖及鉻鐵巖也形成于板片的俯沖開始階段,它們最初形成于弧下(火山前緣)環(huán)境,之后受到地幔角流(the mantle flows)的影響,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.

【學位授予單位】：吉林大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：P588.1

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