Geochronology of the Dong Tso Ophiolite and the Tectonic Environment

The wedge shaped Dong Tso ophiolitic block is distributed near the transition point from the western to the middle sub-belt of the Bangong-Nujiang suture zone.The ophiolite is characterized by well-developed cumulate rocks that are mainly composed of cumulate and massive gabbros.In the cumulate gabbros,the adcumulate amphiboles are distributed extensively around the plagioclase and residual pyroxene grains; hence,the rocks are named adcumulate amphibole-gabbro.In this study,the formation age of the ophiolite has been estimated to be 166 ± 4 million years （Ma） by the sensitive high-resolution ion microprobe （SHRIMP） Ⅱ U-Pb isotopic analysis of the zircons from the adcumulate amphibole-gabbro; the 40Ar/39Ar plateau age was estimated to be 148.19 ± 1.53 Ma,which should represent the emplacement time of the ophiolite,by isotopic dating of the pure amphibole mineral from the amphibole-schist.Two different suits of volcanic lavas have been recognized in this work.The purple colored pillow basalts have high TiO2 and P2O5 contents,and are rich in light rare earth elements （LREEs）,large-ion lithospheric elements （LILEs） and high-field-strength elements （HFSEs）,the characteristics that are the typical of the oceanic island basalt （OIB）.On the other hand,other massive basaltic andesites of celadon color are poor in MgO; rich in Fe2O3,LREEs,LILEs,and HFSEs; and especially characterized by negative Nb and Ta anomalies,the properties that establish the andesites as continental arc volcanic rocks.It is concluded that hotspots had developed in the old Dong Tso basin,the oceanic basin that had been developing from middle Jurassic （166 Ma） or even before and emplaced northward in late Jurassic （about 148 Ma）.     

大洋岛弧的前世今生

根据板块构造理论,板块的边界是地质作用最为强烈的地区,因而它们是当今固体地球科学研究的重点。依据应力性质的不同,地球上板块的边界类型有扩张的洋中脊、汇聚的俯冲带和调节板块运动差异的转换断层三种。就汇聚型板块边界而言,它又可进一步划分为洋-洋俯冲的大洋或洋内岛弧带（Intra-oceanic arc）、洋-陆俯冲的安第斯型活动大陆边缘带和陆-陆接触的大陆碰撞带三种。相对而言,大洋岛弧的研究程度最低。传统认为最典型的大洋岛弧——日本诸岛,已不再被认为是洋-洋俯冲的产物,因为已有研究显示它是从亚洲大陆裂解的碎块。根据目前的调查,现今的大洋岛弧主要集中在西太平洋地区,以太平洋与菲律宾板块间的Izu-Bonin-Mariana弧和太平洋-澳大利亚间的西南太平洋岛弧为代表。大洋岛弧研究的最重要问题是,洋洋之间如何产生了俯冲。目前多倾向于认为：大洋中的转换断层可使不同时代的大洋岩石圈相互接触,在这种情况下,较老的岩石圈由于冷却时间较长而密度相对较大,因而可下沉而俯冲到较年轻的岩石圈之下。这一模型也被誉为蛇绿岩形成的初始俯冲定律（Subduction Initial Rule,简称SIR）。但存在的问题是,目前全球还没发现有转换断层转变为俯冲带的实例。更何况,全球大洋中发育如此众多的转换断层,但为何只在西太平洋发育大洋岛弧?本文通过对资料的总结还发现,这些大洋岛弧基本都是从亚洲或者澳大利亚大陆东部边缘裂解的碎块,只是后期的弧后扩张作用使裂解的碎块发生强烈的改造,形成具有类似大洋岩石圈的特点。目前提出的洋-洋自发形成俯冲带的模型并没有理论基础,也没有实际地质事实的支持。但在加勒比海、斯科舍海和阿留申地区,大洋岛弧的出现与洋底高原诱发的俯冲带跃迁或俯冲极性反转有关。因此,板块构造理论中的洋洋初始俯冲模式需要进一步资料的验证。

     

Geochemistry of Khor Um-Safi ophiolitic serpentinites,central Eastern desert,Egypt: Implications for neoproterozoic arc-basin system in the Arabian-Nubian shield

Serpentinites in the Eastern Desert of Egypt are the most distinctive lithological unit in the Arabian–Nubian Shield (ANS) ophiolite sequence which associated with major suture zones. Khor Um-Safi (KUS) serpentinites represent dismembered fragments of ophiolitic rocks located in the central Eastern Desert (CED) of Egypt.KUS serpentinites exhibit affinity to the typical metamorphic peridotites with harzburgitic protolith compositions. Their opaque mineral assemblage (pentlandite, heazlewoodite and magnetite) is similar to that observed in oceanic serpentinites and implies serpentinization under highly reducing conditions. They have refractory major element compositions with Al₂O₃ contents comparable to oceanic and active margin peridotites as well as Pan-African serpentinites. The Cr and TiO₂ contents reflect evolution within a supra-subduction zone (SSZ) environment. This implication is confirmed by the Al₂O₃/SiO₂ and MgO/SiO₂ ratios which akin to ANS ophiolitic peridotites in fore-arc setting. Their enrichment in compatible trace elements (Cr, Ni and Co) reveals a depleted mantle peridotite protolith.Modelling trace elements indicates that they represent the mantle residues from 15 to 20 % melting of spinel peridotite at oxygen fugacity conditions of the QFM + 1 buffer. This range of melt extraction is consistent with the typical range of SSZ peridotite. Oxygen fugacity estimation suggests evolution under more oxidizing regime similar to modern fore-arc basin system. Moreover, this implication indicates that the KUS mantle represents arc lithosphere interacted with arc melt.     

http://www.sciencedirect.com/science/article/pii/S1674987114000401

In this paper, a new discrimination diagram using absolute measures of Th and Nb is applied to post-Archean ophiolites to best discriminate a large number of different ophiolitic basalts. This diagram ...     

Structural evolution of the Bayanhongor region, west-central Mongolia

Most of previous models suggest that the Central Asia Orogenic Belt grew southward in the Phanerozoic. However, in the Bayanhongor region in west-central Mongolia, volcanic arc, accretionary prism, ophiolite, and passive margin complexes accreted northeastward away from the Baydrag micro-continent, and hence the region constitutes the southwestern part of a crustal-scale syntaxis close to the west. The syntaxis should be original, because presumably reorientation due to strike-slip faulting can be ignored. It is reconfirmed that the Baydrag eventually collided with another micro-continent (the Hangai) to the northeast. A thick sedimentary basin developed along the southern passive margin of the Hangai micro-continent. This region is also characterized by an exhumed metamorphosed accretionary complex and a passive margin complex, which are both bounded by detachment faults as well as basal reverse faults which formed simultaneously as extrusion wedges. This part of the Central Asia Orogenic Belt lacks exhumed crystalline rocks as observed in the Himalayas and other major collisional orogenic belts. In addition, we identified two phases of deformation, which occurred at each phase of zonal accretion as D1 through Cambrian and Devonian, and a synchronous phase of final micro-continental collision of Devonian as D2. The pre-collisional ocean was wide enough to be characterized by a mid-ocean ridge and ocean islands. Two different structural trends of D1 and D2 are observed in accretionary complexes formed to the southwest of the late Cambrian mid-ocean ridge. That is, the relative plate motions on both sides of the mid-ocean ridge were different. Accretionary complexes and passive margin sediments to the northeast of the mid-ocean ridge also experienced two periods of deformation but show the same structural trend. Unmetamorphosed cover sediments on the accretionary prism and on the Hangai micro-continent experienced only the D2 event due to micro-continental collision. These unmetamorphosed sediments form the hanging walls of the detachment faults. Moreover, they were at least partly derived from an active volcanic arc formed at the margin of the Baydrag micro-continent.     

Ophiolite hosted chromitite formed by supra-subduction zone peridotite –plume interaction

Chromitite bodies hosted in peridotites typical of suboceanic mantle (s.l. ophiolitic) are found in the northern and central part of the Loma Caribe peridotite, in the Cordillera Central of the Dominican Republic. These chromitites are massive pods of small size (less than a few meters across) and veins that intrude both dunite and harzburgite. Compositionally, they are high-Cr chromitites [Cr# ​= ​Cr/(Cr ​+ ​Al) atomic ratio ​= ​0.71–0.83] singularly enriched in TiO₂ (up to 1.25 ​wt.%), Fe₂O₃ (2.77–9.16 ​wt.%) as well as some trace elements (Ga, V, Co, Mn, and Zn) and PGE (up to 4548 ​ppb in whole-rock). This geochemical signature is unknown for chromitites hosted in oceanic upper mantle but akin to those chromites crystallized from mantle plume derived melts. Noteworthy, the melt estimated to be in equilibrium with such chromite from the Loma Caribe chromitites is similar to basalts derived from different source regions of a heterogeneous Caribbean mantle plume. This mantle plume is responsible for the formation of the Caribbean Large Igneous Province (CLIP). Dolerite dykes with back-arc basin basalt (BABB) and enriched mid-ocean ridge basalt (E-MORB) affinities commonly intrude the Loma Caribe peridotite, and are interpreted as evidence of the impact that the Caribbean plume had in the off-axis magmatism of the back-arc basin, developed after the Caribbean island-arc extension in the Late Cretaceous. We propose a model in which chromitites were formed in the shallow portion of the back-arc mantle as a result of the metasomatic reaction between the supra-subduction zone (SSZ) peridotites and upwelling plume-related melts.     

东波超镁铁岩体:西藏雅鲁藏布江缝合带西段一个甚具铬铁矿前景的地幔橄榄岩体

东波超镁铁岩体产在雅鲁藏布江缝合带的西段,与周边白垩纪沉积岩地层和火山岩以断层接触.航磁资料显示该岩体约400km2规模,地表出露连续,地下有一定延深.超镁铁岩体由亏损的地幔橄榄岩组成,主要有高镁的方辉橄榄岩、纯橄岩和少量二辉橄榄岩.方辉橄榄岩和二辉橄榄岩中橄榄石和斜方辉石属高镁型,分别为Fo=89.5～91.5和Mg#=90～91.5.但二辉橄榄岩中的Al2O3和CaO含量明显高于方辉橄榄岩.方辉橄榄岩中单斜辉石Mg#=92～95,二辉橄榄岩的Mg#=92～93,两者的值也重叠.二辉橄榄岩中的Al2O3和CaO含量要明显高于方辉橄榄岩.这些均为阿尔卑斯型地幔橄榄岩的典型特征.纯橄岩中的橄榄石Fo=92～93.2,其斜方辉石和单斜辉石的Mg#=～93,但Al2O3和CaO的含量比方辉橄榄岩和二辉橄榄岩的低.三种岩石的成分变化规律,反映了地幔部分熔融程度的差异.二辉橄榄岩铬尖晶石的Cr#值20～30,反映为典型深海橄榄岩特征,指示MOR环境.与其不同的是,方辉橄榄岩的铬尖晶石的Cr#=20～75,指示MOR和SSZ两者兼有环境.岩石的原始地幔标准化的REE和微量元素蛛网图模式支持了上述的认识.东波地幔橄榄岩中的岩石学特征与产有大型铬铁矿床的罗布莎地幔橄榄岩可对比,岩体中已多处发现块状铬铁矿石,其铬铁矿的Cr2O3含量56％～59％,表明东波是寻找铬铁矿大矿和富矿甚具前景的一个超镁铁岩体.     

北阿尔金地区米兰红柳沟蛇绿岩的岩石学特征和SHRIMP定年

米兰红柳沟蛇绿岩是北阿尔金蛇绿岩带中发育和保留最好的蛇绿岩,主要由地幔橄榄岩、镁铁-超镁铁质堆晶杂岩、岩墙群和基性熔岩等组成.它们以规模不等的构造岩块产出,大者长十余km,宽近1km,组成一条近100km长的蛇绿混杂岩带.地幔橄榄岩以方辉橄榄岩为主,有少量纯橄岩,主要由橄榄石(Fo=91.2～92.7),斜方辉石(En=93-98)和少量单斜辉石(En=46)组成;副矿物尖晶石Cr#为43～69(平均55),Mg#为43～64(平均58),表现出深海橄榄岩(Abyssal peridotite)和俯冲带环境(SSZ)橄榄岩成分特点.深成堆晶岩主要由异剥橄榄岩-橄榄二辉石岩-(橄榄)辉石岩-辉长岩-斜长岩,该组合的堆晶岩通常被认为是SSZ构造背景的产物.席状岩墙群的岩石成分与熔岩一致,其TiO2(1%～1.5%)和低含量的K2O<0.3%和P2O5表明具有MORB型的岩石特征,并得到了不相容元素和LREE平坦型和亏损型的球粒陨石标准化模型等证据的支持.该地区另存在一套高Tj的洋岛型拉斑玄武岩.两类熔岩的存在,以及地幔橄榄岩和堆晶岩的不同特征,表明米兰红柳沟蛇绿岩组合可能来自不同构造背景.带中与洋壳俯冲有关的蓝片岩和榴辉岩组成的高压变质带的存在,以及与俯冲碰撞有关的不同类型花岗岩类的产出,表明米兰红柳沟蛇绿混杂岩带代表了一个复杂的板块缝合带.蛇绿岩中辉长岩的锆石SHRIMP年代为479±8Ma,这是获得的第一个北阿尔金蛇绿岩的锆石SHRIMP U-Pb同位素年龄,认为代表蛇绿岩的形成时代.因此,北阿尔金缝合带无论在年龄和特征等方面,均可以与阿尔金断裂带东部的北祁连缝合带对比,证实两个带曾经是一个带,被阿尔金断裂左旋错断了约400km.     

Refractory chromitites recovered from the Eretria mine,East Othris massif (Greece): Implications for metallogeny and deformation of chromitites within the lithospheric mantle portion of a forearc-type ophiolite

The chrome ores of the abandoned Eretria mine of the East Othris ophiolite occur within a pervasively serpentinized and sheared harzburgite body. They consist of massive chromitites with mylonitic fabric in imbricate shaped pods. Modal analyses of these ores average at about 90–95% chromian spinel (Cr-spinel) and 5–10% secondary silicates. Chromian spinel compositions vary in Cr# [Cr/(Cr + Al) × 100] and Mg# [Mg/(Mg + Fe²⁺) × 100] from 44 to 62 and from 59 to 81, respectively. Trace element (Ti, Ni, V, Mn, Zn, Sc, Co and Ga) contents in Cr-spinel do not show significant variations from grain cores to grain boundaries. However, Cr-spinel compositions show depletions in Ti, Zn and Sc when compared to the composition of accessory Cr-spinel from typical mid-ocean ridge basalts (MORB). Mineral inclusions hosted in Cr-spinel comprise a range of (hydrous and anhydrous) silicate and base metal (BM) minerals occasionally intergrown with phosphate minerals and rare intermetallic compounds. A number of these inclusions have Cr-spinel rims with higher Cr# (63–68) than those of the enclosing Cr-spinel grains.The absence of dunite sheaths around chromitites is interpreted as an artifact of dunite structural obliteration during prolonged ductile shearing within harzburgite. The microtextural characteristics of a number of inclusions in Cr-spinel imply that they were initially fully molten. Furthermore, primary hydrosilicate (amphibole, phlogopite) inclusions in Cr-spinel indicate that chromitites crystallized from a water-bearing melt. Chromian spinel rims around silicate inclusions probably represent early crystals generated from a primitive magma produced by melting of a depleted mantle source.Geochemical calculations demonstrate that the parental melts of chromitites had intermediate affinity between MORB and arc-related magmas. Our preferred hypothesis for the genesis of the Eretria chromitites is that they were formed from a melt originated within the hydrated mantle wedge beneath a nascent forearc basin during subduction initiation.     

矿产开发与环境保护

通过国内多个矿区实例，较系统地叙述了矿产资源开发导致的生态环境破坏和污染状况。表明矿产开发中对环境污染的途径包括河流、土壤、等。最后简要介绍了国内、国外几种对采矿污染比较有效的防治措施。