藏南罗布莎铬铁矿床铬尖晶石矿物学与矿床成因研究

西藏罗布莎铬铁矿床是我国目前研究程度最高、规模最大、地幔橄榄岩相对新鲜的豆荚状铬铁矿床,主要工业矿体产于蛇绿岩壳-幔边界(即岩石莫霍面)以下方辉橄榄岩相带一定层位中,主要有块状、浸染状和豆状等矿石类型。罗布莎铬尖晶石成分变化范围大,依据铬尖晶石的化学成分与矿物学研究至少可识别出3个期次铬尖晶石:(1)成矿前期铬尖晶石,主要以熔蚀残斑晶、出溶晶及少量自形晶形式产于方辉橄榄岩中,以富Al₂O₃为特征,Cr^#值变化范围大(17.19~66.30),且大部分小于60,并与Mg^#值呈负相关关系,由出溶晶,残斑晶到自形晶铬尖晶石,总体表现向富Cr、Fe的方向演变;(2)成矿主期铬尖晶石,可分为早、晚2个阶段。早阶段铬尖晶石主要以它形晶产于不同类型铬铁矿石中,部分呈自形-半自形晶产于铬铁矿体的纯橄岩外壳中,主要以富铬为特征,矿石中Cr^#值变化范围小(70.08~87.03),均大于60,其中块状铬铁矿具有最高的Cr^#和Mg^#,由纯橄岩外壳中副矿物铬尖晶石向豆状、浸染状矿石以及块状矿石演变过程中,铬尖晶石化学成分总体向更富Cr、富Mg方向演变;晚阶段铬尖晶石:主要以自形-半自形晶产于具堆晶结构的纯橄岩相带中,成份上以更加富而贫Al₂O₃,且具有最低Mg^#(18.79~44.77)值为特征;(3)成矿晚期铬尖晶石,主要以网状集合体产于豆状-网脉状(眼眉状)矿石中,以更贫Al、富Fe为特征,具有最高的Cr^#值和低的Mg^#值。综合研究表明,罗布莎铬铁矿中的铬主要来自原始地幔岩本身,且主要来自于地幔橄榄岩中2种辉石的不一致熔融和对副矿物铬尖晶石的改造,原始富铬矿物可能来自地幔深部的八面体硅酸盐矿物。罗布莎豆荚状铬铁矿的成矿作用具有多期次、多成因、多种构造背景下成矿特征,成矿作用过程经历了由大洋中脊(MOR)扩张环境向岛弧体系俯冲环境的转变过程,洋内俯冲带之上(SSZ)的弧间盆地环境是形成冶金级豆荚状铬铁矿的最为有利构造环境。研究提出了罗布莎铬铁矿的"三阶段"成矿模式,即,经历了大洋中脊预富集阶段,俯冲带之上主成矿阶段及之后的构造抬升改造阶段。纯橄岩与方辉橄榄岩接触带之下的方辉橄榄岩相带是寻找较大规模铬铁矿床的有利地带。     

地幔橄榄岩中橄榄石的指示意义

橄榄石是地幔橄榄岩和辉石岩的主要组成矿物,但也经常以斑晶和捕虏晶的形式出现在玄武质岩石中。结合近年来在地幔橄榄岩的主要元素（如Mg和Fe）组成特征以及Li、Mg和Fe稳定同位素地球化学方面的研究成果,重点对橄榄石的地球化学特征与华北克拉通岩石圈地幔演化过程之间的联系进行了讨论,旨在加深对华北克拉通岩石圈地幔演化过程的理解。现有研究表明：地幔橄榄岩中橄榄石的矿物学特征、元素和同位素地球化学组成能够很好地指示岩石圈地幔的特征及其演化过程,因而具有重要的意义。对于克拉通地区的地幔橄榄岩来说,橄榄石的Mg#通常可以指示岩石圈地幔的属性,古老、难熔的地幔橄榄岩中的橄榄石一般具有较高的Mg#（〉92）,而新生的岩石圈地幔橄榄岩中的橄榄石则具有较低的Mg#（〈91）。因此,地幔橄榄岩中橄榄石的Mg#在一定程度上具有年龄意义。橄榄岩中橄榄石的Li、Mg和Fe同位素组成也可以明确指示岩石圈地幔的属性及其所经历的演化过程,正常地幔的δ7Li、δ26Mg和δ57Fe组成相对均一,如果上述同位素组成偏离正常地幔值,则说明岩石圈地幔经历了熔体/流体的交代作用。华北克拉通地区地幔橄榄岩捕虏体中橄榄石的Li、Mg和Fe同位素组成研究表明：该区的岩石圈地幔经历了多个阶段、不同来源的熔体/流体的改造过程。     

地幔中EM1端员成因的锂同位素制约

在四个地幔端员(DMM、HIMU、EM1、EM2)中,EM1的成因一直令人困惑。华北克拉通中部(太行山地区)地幔橄榄岩捕虏体的Li同位素组成特征为探讨这一问题提供了新的制约。橄榄岩捕虏体中主要组成矿物(橄榄石、斜方辉石和单斜辉石)的Li同位素组成特征表明,该区的岩石圈地幔中有再循环的、古老的洋壳物质。结合已有的地幔橄榄岩捕虏体的Sr-Nd同位素数据,可以推测再循环的古老蚀变洋壳对地幔EM1端员组分的产生有重要的作用。     

Fast exhumation of the ultrahigh-pressure Alpe Arami garnet peridotite (Central Alps, Switzerland): constraints from geospeedometry and thermal modelling

Previous studies suggest that the metamorphic evolution of the ultrahigh‐pressure garnet peridotite from Alpe Arami was characterized by rapid subduction to a depth of c. 180 km with partial chemical equilibration at c. 5.9 Gpa/1180 °C and an initial stage of near‐isothermal decompression followed by enhanced cooling. In this study, average cooling rates were constrained by diffusion modelling on retrograde Fe–Mg zonation profiles across garnet porphyroclasts. Considering the effects of temperature, pressure and garnet bulk composition on the Fe–Mg interdiffusion coefficient, cooling rates of 380–1600 °C Myr^?1 for the interval from 1180 to 800 °C were obtained. Similar or even higher average cooling rates resulted from thermal modelling, whereby the characteristics of the calculated temperature‐time path depend on the shape and size of the hot peridotite body and the boundary conditions of the cooling process. The very high cooling rates obtained from both geospeedometry and thermal modelling imply extremely fast exhumation rates of c. 15 mm yr^?1 or more. These results agree with the range of exhumation rates (16–50 mm yr^?1) deduced from geochronological results. It is suggested that the Alpe Arami peridotite passively returned towards the surface as part of a buoyant sliver, caused as a consequence of slab breakoff.     

Alteration processes recorded by back-arc mantle peridotites from oceanic core complexes,Shikoku Basin,Philippine Sea

We determined the mineralogical and petrological characteristics of ultramafic rocks dredged from two oceanic core complexes: the Mado Megamullion and 23°30′N non-transform offset massif, which are located within the Shikoku back-arc basin in the Philippine Sea. The ultramafic rocks are strongly serpentinized, but can be classified as harzburgite/lherzolite or dunite, based on relict primary minerals and their pseudomorphs. Strongly elongated pyroxene porphyroclasts with undulatory extinction indicate high-temperature (≥700 °C) strain localization on a detachment fault within the upper mantle at depths below the brittle–viscous transition. During exhumation, the peridotites underwent impregnation by magmatic or hydrothermal fluids, lizardite/chrysotile serpentinization at ≤300 °C, antigorite crystallization, and silica metasomatism that formed talc. These features indicate that the detachment fault zones formed a fluid pathway and facilitated a range of fluid–peridotite interactions.     

西南天山东德沟橄榄岩的地球化学特征及其地质意义

东德沟橄榄岩出露于中国新疆伊犁—中天山板块和塔里木板块之间西南天山造山带的北缘,含大量角闪石、金云母等含水矿物,显示全部遭受了不同程度的交代变质作用.岩石主量元素含量变化较大,其后期蚀变基本向富铝和富钙两个方向变化,其Nb/Ta和Zr/Hf值显示难熔地幔残余的特征.该区橄榄岩轻重稀土分馏较明显,稀土元素配分曲线总体为LREE富集的右倾型,且富集大离子亲石元素Rb、Ba、Sr和放射性元素Th,但岩石呈现不一致的解耦现象,即主量元素表现难熔而稀土元素、不相容微量元素体现富集,说明主量元素和微量元素具有各自的演化特点.研究认为东德沟橄榄岩是经历部分熔融的楔形地幔残余,微量元素呈现的特征是俯冲板块析出熔体或流体交代的结果.与其岩石矿物学研究结果一致,对东德沟橄榄岩地球化学特征的深入研究证明,它们来源于SSZ环境俯冲上板片地幔楔橄榄岩残片.     

A brief review of isotopically light Li – a feature of the enriched mantle?

Lithium isotope geochemistry is increasingly being used to trace deep-earth processes, reflecting the observed large variation of Li isotope ratios in mantle-derived rocks, including peridotite xenoliths associated with ancient continents. We briefly review the Li isotopic compositions of major geochemical reservoirs, the assumed mechanisms of Li isotopic fractionation, and, in particular, the origins of isotopically light Li in mantle-derived rocks based on the latest developments in Li isotope geochemistry. Comparison of Li isotope data with existing Sr-Nd isotope ratios reflects the subduction-recycling of ancient oceanic crust and the reappearance of Li in volcanic rocks. This circulation may play an important role in generating the isotopically light-Li component in the mantle – perhaps the enriched mantle end member defined by the Sr-Nd isotopic compositions of oceanic basalts.     

Garnet variety and zircon ages in UHP meta-sedimentary rocks from the Jubrique zone (Alpujárride Complex,Betic Cordillera,Spain): evidence for a pre-Alpine emplacement of the Ronda peridotite

The Ronda peridotite is a group of lherzolite slabs (1.5 to 2 km thick) in southern Spain. Despite clear evidence that pre-Alpine events affected pre-Permo-Triassic units from the Alborán domain (internal zone of the Betic-Rif Cordillera, Spain, and Morocco), numerous papers continue to emphasize Alpine metamorphic and structural evolution. Here, we evaluate the pre-Cenozoic evolution of the Ronda peridotite by reporting new petrographic and U–Pb SHRIMP zircon dating of meta-sedimentary rocks from the Jubrique zone (Alpujárride Complex, Betic Cordillera, Spain) directly overlying the Ronda peridotite. Field inspection and petrographical study revealed generalized migmatitic textures and a gradual transition mainly defined by garnet content (from ~30 to <3 wt.%) and size (from 1.5 cm to <0.5 mm) in the overlying granulite-gneiss sequence, suggesting that most garnet grew as a consequence of the peridotite emplacement. Garnet shows notable variations in composition and inclusion types, which are interpreted as reflecting different stages of garnet growth. Diamond-bearing garnets are only well-preserved in gneisses from the uppermost part of the sequence, whereas the large garnets from rocks overlying the peridotite mainly record later thermal events. SHRIMP zircon dating indicates two age peaks at 330 ± 9 and 265 ± 4 Ma. The oldest age characterizes rims overgrowing detrital cores and reflects an early Hercynian metamorphism; the younger age characterizes zircon with magmatic oscillatory zoning, reflecting anatexis. On the basis of these data and of previous dating of monazite included in the large garnets, we conclude that the peridotite was emplaced either shortly before or during early Hercynian times, ~330 Ma.     

Mineralogy,geochemistry, and geotectonic significance of the Neoproterozoic ophiolite of Wadi Arais area,south Eastern Desert,Egypt

ABSTRACT

The dismembered ophiolites in Wadi Arais area of the south Eastern Desert of Egypt are one of a series of Neoproterozoic ophiolites found within the Arabian–Nubian Shield (ANS). We present new major, trace, and rare earth element analyses and mineral composition data from samples of the Wadi Arais ophiolitic rocks with the goal of constraining their geotectonic setting. The suite includes serpentinized ultramafics (mantle section) and greenschist facies metagabbros (crustal section). The major and trace element characteristics of the metagabbro unit show a tholeiitic to calc-alkaline affinity. The serpentinized ultramafics display a bastite, or less commonly mesh, texture of serpentine minerals reflecting harzburgite and dunite protoliths, and unaltered relics of olivine, orthopyroxene, clinopyroxene, and chrome spinel can be found. Bulk-rock chemistry confirms harzburgite as the main protolith. The high Mg# (91.93–93.15) and low Al₂O₃/SiO₂ ratios (0.01–0.02) of the serpentinized peridotite, together with the high Cr# (>0.6) of their Cr-spinels and the high NiO contents (0.39–0.49 wt.%) of their olivines, are consistent with residual mantle rocks that experienced high degrees of partial melt extraction. The high Cr# and low TiO₂ contents (0.02–0.34 wt.%) of the Cr-spinels are most consistent with modern highly refractory fore-arc peridtotites and suggest that these rocks probably developed in a supra-subduction zone environment.     

祁连山玉石沟橄榄岩显微构造特征及演化

橄榄岩作为上地幔的主要成分,影响着上地幔的流变学行为,其显微构造记录了岩石形成发展过程中所经受的构造事件.通过对祁连山玉石沟橄榄岩样品的镜下显微构造观察、岩石组构测定及TEM位错分析,探讨了与变形相关的温度、围压、含水性和应变速率等因素,总结该岩石的变形机制和变形历史,并推断其形成发展时大地构造环境.研究表明,玉石沟橄榄岩产自上地幔,其变形改造经历了上地幔演化和脆-韧性变形2个阶段.上地幔演化阶段橄榄石发育明显的A型原生组构,电子背散射衍射技术（EBSD）测定的橄榄石显微组构表明,该地区橄榄岩形成于高温（＞ 1200℃）、低应力（＜350 MPa）、低应变速率、低含水量的地幔浅部环境条件下;脆-韧性变形阶段叠加改造了原生A型组构,而发育明显的D型次生组构.橄榄石变形主控因素为动态恢复作用,普遍发育亚晶粒、消光带和扭折显微构造等相关组构,并与透射电镜下所观察到的位错排对应,同时还发育以微破裂为主的一套脆性变形组构,表现出两组共轭剪破裂和另一组张性破裂等现象,揭示出与韧性动态恢复现象一致的应力场方向指示.