Early Palaeozoic sub-arc chromitite-bearing peridotite in the Kudi ophiolite on the westernmost Tibetan Plateau

ABSTRACT

A chromite deposit was discovered in the Kudi ophiolite in the Palaeozoic western Kunlun orogenic belt. Chromite forms elongated (<2 m in width) and banded chromitite bodies (<0.1 m in width for each band) in dunite and podiform chromitite bodies (<1.5 m in width) in harzburgite. Dunite is classified into two types. Type I dunite hosting massive and banded chromitites shows low Fo in olivine (88.1–90.9), moderate Cr^# [=Cr/(Cr + Al), 0.47–0.56] in chromite, and a positively sloped primitive mantle-normalized platinum group elements (PGE) pattern, suggesting that it is a cumulate of a mafic melt. Harzburgite and type II dunite show olivine with high Fo (>91.1) and chromite with moderate to high Cr^# (0.44–0.61), and flat to negatively sloped primitive mantle-normalized PGE patterns, indicating that they are residual mantle peridotite after partial melting. Chromite in all three types of chromitites has relatively uniform moderate values Cr^# ranging from 0.43 to 0.56. Massive chromitite contains euhedral chromite with high TiO₂ (0.40–0.43 wt.%) and has a positively sloped primitive mantle-normalized PGE pattern, suggesting that it represents a cumulate of a melt. Rocks containing disseminated and banded chromite show overall low total PGE, < 117 ppb, and a negatively sloped primitive mantle-normalized PGE pattern. Chromite grains in these two types of occurrences are irregular in shape and enclose olivine grains, suggesting that chromite formed later than olivine. We suggest that chromite-oversaturated melt penetrated into the pre-existing dunite and crystallized chromite. The oxygen fugacity (fO₂ values of chromitites and peridotites are high, ranging from FMQ+0.8 (0.8 logarithmic unit above the fayalite-magnetite-quartz buffer) to FMQ+2.3 for chromitites and from FMQ+0.9 to FMQ+2.8 for peridotites (dunite and harzburgite). The mineral compositions and high fO₂ values as well as estimated parental magma compositions of the chromitites suggest that the Kudi ophiolite formed in a sub-arc setting.     

Diamond Discovered in High-Al Chromitites of the Sartohay Ophiolite, Xinjiang Province, China

In recent years diamonds and other exotic minerals have been recovered from mantle peridotites and high-Cr chromitites of a number of ophiolites of different age and different tectonic environments. Here we report a similar collection of minerals from the Sartohay ophiolite of Xinjiang Province,western China,which is characterized by having high-Al chromitites. Several samples of massive podiform chromitite with an aggregate weight of nearly 900 kg yielded diamonds,moissanite and other highly reduced minerals,as well as common crustal minerals. Thus far,more than 20 grains each of diamond and moissanite have been recovered from heavy mineral separates of the chromitites. The diamonds are all 100-200 μm in size and range in color from pale yellow to reddish-orange to colorless. Most of the grains are anhedral to subhedral octahedra,commonly with elongate forms exhibiting well-developed striations. They all display characteristic Raman spectra with shifts between 1325 cm-1 and 1333 cm-1,mostly 1331.51 cm-1 or 1326.96 cm-1. The moissanite grains are light blue to dark blue,broken crystals,50-150 μm across,commonly occurring as small flakes or fragments. Their typical Raman spectra have shifts at 762 cm-1,785 cm-1,and 966 cm-1. This investigation extends the occurrence of diamonds and moissanite to a Paleozoic ophiolite in the Central Asian Orogenic Belt and demonstrates that these minerals can also occur in high-Al chromitites. We conclude that diamonds and moissanite are likely to be ubiquitous in ophiolitic mantle peridotites and chromitites.     

西藏罗布莎蛇绿岩豆荚状铬铁矿中镁橄榄石的晶体结构及其意义

在西藏罗布莎蛇绿岩块的豆荚状铬铁矿中，存在包裹体矿物镁橄榄石，这些镁橄榄石为无色透明多晶面形成的自形晶，Fo值可达到97－98，具富镁端员组分，选取一代表性颗粒的单晶进行X单晶和衍射分析，表明其为斜方晶系，空间群Pbnm，镁橄榄石晶胞参数,a,b,c以及M1－0和M2－0键长均小于地幔包裹体和一般地幔岩中橄缆中的对应值，测试出的晶胞参数与人工合成镁橄榄石的一致，镁橄榄石具有橄榄石族中已知最小的晶胞参数，据此推断它是在超高压环境下结晶的。     

Possible sub-arc origin of podiform chromitites

Abstract The sub-arc mantle condition possibly favors the formation of podiform chromitites. The Cr/(Cr + Al) atomic ratio (= Cr#) of their chromian spinel frequently is higher than 0.7, which is comparable with the range for arc-related primitive magmas. This almost excludes the possibility of their sub-oceanic origin, because both oceanic peridotites and MORB have chromian spinel with the Cr# < 0.6. Precipitation of chromitite and associated dunite enhances a relative depletion of high-field strength elements (HFSE) to large-ion lithophile elements (LILE), one of chemical characteristics of arc magmas, for the involved magma. This cannot alter completely, however, the MORB to the arc-type magma, especially for Ti and Zr. The presence of chromitite xenoliths, similar both in texture and in chemistry to podiform chromitites of some ophiolitic complexes, in some Cenozoic alkali basalts from the southwest Japan arc indicates directly that the upper mantle beneath the Japan arcs has chromitites.     

Podiform chromitites in the lherzolite-dominant mantle section of the Isabela ophiolite, the Philippines

Abstract The Isabela ophiolite, the Philippines, is characterized by a lherzolite‐dominant mantle section, which was probably formed beneath a slow‐spreading mid‐ocean ridge. Several podiform chromitites occur in the mantle section and grade into harzburgite to lherzolite. The chromitites show massive, nodular, layered and disseminated textures. Clinopyroxene (±orthopyroxene/amphibole) inclusions within chromian spinel (chromite hereafter) are commonly found in the massive‐type chromitites. Large chromitites are found in relatively depleted harzburgite hosts having high‐Cr? (Cr/(Cr + Al) atomic ratio = ～0.5) chromite. Light rare earth element (LREE) contents of clinopyroxenes in harzburgites near the chromitites are higher than those in lherzolite with low‐Cr? chromite, whereas heavy REE (HREE) contents of clinopyroxenes are lower in harzburgite than in lherzolite. The harzburgite near the chromitites is not a residual peridotite after simple melt extraction from lherzolite but is formed by open‐system melting (partial melting associated with influx of primitive basaltic melt of deeper origin). Clinopyroxene inclusions within chromite in chromitites exhibit convex‐shaped REE patterns with low HREE and high LREE (+Sr) abundances compared to the host peridotites. The chromitites were formed from a hybridized melt enriched with Cr, Si and incompatible elements (Na, LREE, Sr and H₂O). The melt was produced by mixing of secondary melts after melt–rock interaction and the primitive basaltic melts in large melt conduits, probably coupled with a zone‐refining effect. The Cr? of chromites in the chromitites ranges from 0.65 to 0.75 and is similar to those of arc‐related magmas. The upper mantle section of the Isabela ophiolite was initially formed beneath a slow‐spreading mid‐ocean ridge, later introduced by arc‐related magmatisms in response to a switch in tectonic setting during its obduction at a convergent margin.     

西藏蛇绿岩地幔中的主要自然金属矿物

在西藏雅鲁藏布江蛇绿岩带的罗布莎蛇绿岩块的豆荚状铬铁矿床中 ,揭示出一个由 70～ 80种矿物组成的地幔矿物群 ,包括自然金属、合金、硫 (砷 )化物、氧化物和硅酸盐等。这些矿物呈包裹体或脉石产于铬铁矿石中 ,经人工重砂分析 ,自然元素矿物有自然硅、自然铁、自然锌、自然铅、自然铝、自然铬、自然锡、自然镍、自然钨、自然钛、自然锇、自然铱、自然钌、自然钯、石墨、金刚石、自然金和自然银等。文中选择一些自然元素矿物 ,探索这些地幔矿物特点以及蛇绿岩和铬铁矿的形成机制。根据共生矿物群以及罗布莎地幔橄榄岩为新鲜的未蛇纹石化的岩石 ,认为罗布莎自然元素矿物与蛇纹石化作用无关。它们可能是在地核形成时期滞留于地幔中的成核物质 ,抑或是核幔之间化学反应的产物 ,后来被铬铁矿矿浆捕获 ,并同铬铁矿一起由地幔柱作用和板块作用侵位于浅部并仰冲出露于地表。     

内蒙古贺根山蛇绿岩的铬铁矿中发现金刚石等深部地幔矿物

前人报道在西藏中生代和俄罗斯极地乌拉尔早古生代蛇绿岩地幔橄榄岩铬铁矿中发现了金刚石等深部地幔矿物,认为需重新考虑铬铁矿浅部成因的传统认识。为了查明不同造山带蛇绿岩的铬铁矿中金刚石等深部矿物的分布规律和豆荚状铬铁矿的成因,笔者开展了内蒙古贺根山晚古生代蛇绿岩中的铬铁矿床的人工重砂矿物学研究,本研究获得约2000 kg的内蒙古贺根山蛇绿岩铬铁矿石样品,对所采样品开展人工重砂选矿,表明该铬铁矿矿石样品中至少有金刚石、碳硅石及其他自然元素类、金属互化物类、氧化物类、硫化物类、硅酸盐类等30余种矿物。内蒙—大兴安岭造山带晚古生代的内蒙古贺根山蛇绿岩带铬铁矿石中,发现金刚石等深部地幔矿物表明,贺根山铬铁矿可能为深部成因。     

雅鲁藏布江缝合带东段杰莎蛇绿岩中铬铁矿特征及其构造背景

雅鲁藏布江缝合带东段加查县杰莎岩体主要由蚀变较强的方辉橄榄岩和纯橄岩、豆荚状铬铁矿组成。铬铁矿矿体呈东西向,倾向北西,矿体的围岩为纯橄岩及方辉橄榄岩,长20~40m,宽1~3m。镜下特征和电子探针分析结果显示铬铁矿中铬尖晶石的Cr#=67.9~88.5,Mg#值变化在64.6~68.2之间,TiO2含量为0.06%~0.18%,Al2O3含量为13.1%~16.5%,表明杰莎铬铁矿为高铬型铬铁矿。方辉橄榄岩中橄榄石、斜方辉石和单斜辉石的矿物化学特征表明杰莎岩体既具有深海地幔橄榄岩特征,也具有岛弧地幔橄榄岩的特点。并且依据铬尖晶石-橄榄石/单斜辉石的矿物化学成分,识别出杰莎岩体至少经历了2期过程,包括早期部分熔融(20%~30%)和晚期的岩石/熔体反应作用(35%)。因此,杰莎地幔橄榄岩和铬铁矿可能与雅鲁藏布江缝合带中其他岩体一样,经历了洋中脊及俯冲带的多阶段叠加的过程。     

豆荚状铬铁矿床的研究现状及进展

豆荚状铬铁矿床是工业上冶金级铬铁矿石的最主要来源,对于其成因研究依然是各国地质学家最为关注的热点之一。文章着重概述了近年来国内外地质学者对豆荚状铬铁矿床成因研究的现状和最新进展。最新研究表明,显生宙以来的豆荚状铬铁矿床具有一定的成矿专属性,主要赋存于蛇绿岩套底部(壳-幔边界,即岩石莫霍面)地幔橄榄岩中的一定层位中。世界上含矿的地幔橄榄岩普遍具有垂直熔融分带特征,即上部偏基性,下部偏酸性。豆荚状铬铁矿床与纯橄岩-方辉橄榄岩相密接相关,却很少见有豆荚状铬铁矿床产于二辉橄榄岩中。豆荚状铬铁矿的成矿作用经历了由洋中脊(MOR)扩张环境向岛弧体系俯冲环境的转变过程,而岛弧环境(岛弧、弧后盆地、弧前盆地等)是形成冶金级豆荚状铬铁矿的最为有利的构造环境。富铬铬铁矿与纯橄岩、玻安岩(Boninite)均为亏损地幔橄榄岩再次高度熔融的最终产物,而玻安岩普遍产于岛弧环境。虽然玻安岩不是铬的有效载体,但玻安岩的熔离促使铬铁矿达到进一步的富集。铬铁矿中的铬来自原始地幔,主要来自于地幔橄榄岩中两种辉石的不一致熔融及其对副矿物铬尖晶石的改造。随着部分熔融程度的增高,地幔橄榄岩逐渐向富镁方向演变,而对应的造矿铬尖晶石也逐渐向富镁、富铬方向演变。     

豆荚状铬铁矿铂族元素地球化学研究进展

豆荚状铬铁矿是蛇绿岩中特有的一类矿产，按其化学成分可分为高Cr型和高Al型两种。其中的PGE主要以RuS2和Os、Ir、Ru合金等包体形式存在，或以类质同像形式进入铬铁矿晶格。两种类型的铬铁矿均表现出负倾斜型PGE配分模式，其Pt、Pd含量相近；与高Al型铬铁矿相比，高Cr型铬铁矿有更高的Os、Ir、Ru含量，部分豆荚状铬铁矿表现出Pt、Pd相对富集的平坦到正倾斜型PGE配分模式。目前对豆荚状铬铁矿PGE含量及配分模式还缺少一个统一的解释，但其PGE地球化学可为豆荚状铬铁矿的成因及构造背景解释提供更多的信息。