Komatiite Flows from the Reliance Formation, Belingwe Belt, Zimbabwe: I. Petrography and Mineralogy

The 2.7-Ga Reliance Formation of the Ngezi Group, Belingwe GreenstoneBelt, Zimbabwe, contains extremely fresh komatiite lavas. Detailedfield mapping and a 200-m deep drill-hole, with excellent corerecovery, demonstrated the existence of a suite of lava flows.Each major flow is 10 m thick and characteristically exhibitschilled top and bottom margins, a spinifex zone dominated byrandom spinifex, a B1 zone, and a thick cumulate zone that typicallycomposes two-thirds of the flow thickness. Preservation of olivineand pyroxene mineralogy is superb by Archaean standards, tothe extent that even the tips of skeletal crystals survive.The matrix, although devitrified, is well preserved. Detailedstudy of two flows shows that skeletal grains from the spinifexzone have maximum Fo contents of 91.4. The Fo contents of microphenocrystsfrom the cumulate zone range from Fo_91.2 to Fo_91.6, but rarelarge phenocrysts ( 5 vol.%) have maximum Fo contents of 93.6.The Fo contents of the cumulate olivines do not vary with stratigraphicheight, implying that the cumulate zone formed rapidly, by accumulationof transported crystals. The cumulate zones contain 42–57%modal olivine and display reverse size grading of the olivinemicrophenocrysts. This grain-size variation is believed to resultfrom adcumulus growth within a cumulate pile formed by the formedby the gravitational settling of clusters of olivine crystals.Textural relationships indicate that the final part of the flowto start to crystallize was the lowermost part of the spinifexzone. Reprint requests.     

Alteration of a Komatiite Flow from Alexo, Ontario, Canada

To investigate the ability of komatiites to preserve duringalteration a record of the geochemical and isotopic compositionof the Archean mantle, we studied the petrology and geochemistryof a komatiite flow from Alexo, Ontario. Although this flowis relatively well preserved, two main types of alteration arerecognized: (1) hydration of variable intensity throughout thewhole flow; (2) local rodingitization. In samples only slightlyaffected by hydration, the alteration was essentially isochemical;where more intense, hydration was accompanied by the mobilityof TiO₂, Zr and the light rare-earth elements (LREE). The mobilityof these reputedly immobile trace elements is influenced bytexture and secondary mineral assemblages. Rodingitization wasaccompanied by still greater mobility. An Nd isochron with anage of 272698 Ma and Nd_(T) =+ 2.5 is defined by wholerock samples.Because the spread of Sm-Nd ratios is far greater than can beexplained by magmatic processes, the age is interpreted as thatof the main alteration event during or soon after emplacement.Scattered initial Sr isotopic ratios and Nd _(T) (+ 3.6 to +0.7)provide evidence for several younger events. The Nd (T) of magnaticclinopyroxene ( plus;3.8) is higher than that of mafic tofelsicrocks and pyroxenes from the Abibiti belt (Nd _(T) +2.5). Thisresult implies: (1) the komatiites exchanged Nd with surroundingrocks during waterrock interaction; (2) the isotopic compositionof komatiites and their mantle source differed from that ofthe more common basalts of the Abitibi belt. KEY WORDS: alteration; Archean; Komatiite; trace elements; radiogenic and stable islotopes ^*Present address: Ore Genesis Research Group, VIEPS Department of Earth Science, Monash University, Clayton, Vic 3168, Australia. Telephone: (61) 3 9905-3881. Fax: (61) 3 9905–4903. e-mail: ylahaye{at}artemis.earth.monash.edu.au     

山东蒙阴苏家沟科马提岩岩石学特征

针对山东省镜内发现的科马提岩,阐述了其岩石学、矿物学、岩石化学、副矿物等特征,并与国内外资料对比,度为该岩石为一套典型的超镁铁质科马提岩。     

Intrusion and Crystallization of a Spinifex-Textured Komatiite Sill in Dundonald Township, Ontario

Although komatiite has been defined as an ultramafic volcanicrock characterized by spinifex texture, there is a growing recognitionthat similar textures can also form in high-level dykes andsills. Here, we report the results of a petrological and geochemicalinvestigation of a 5 m thick komatiite sill in Dundonald Township,Ontario, Canada. This unit forms part of a series of komatiitesand komatiitic basalts, some of which clearly intruded unconsolidatedsediments. The komatiite sill is differentiated into a spinifex-texturedupper part and an olivine cumulate lower part. Features characteristicof the upper sections of lava flows, such as volcanic brecciaand a thick glassy chilled margin, are absent and, instead,the upper margin of the sill is marked by a layer of relativelylarge (1–5 mm) solid, polyhedral olivine grains that gradesdownwards over a distance of only 2 cm into unusually large,centimetre-sized, skeletal hopper olivine grains. This is underlainby a 1 m thick zone of platy spinifex-textured olivine and coarse,complex, dendritic, spinifex-textured olivine. The texture ofthe olivine cumulate zone in the overlying unit is uniform rightdown to the contact and a lower chilled margin, present at thebase of all lava flows, is absent. The textures in the silland the overlying unit are interpreted to indicate that thesill intruded the olivine cumulate zone of the overlying unit.Thermal modelling suggests that soon after intrusion, a narrowinterval of the overlying cumulate partially melted and thatthe liquid in the upper part of the sill became undercooled.The range of olivine morphologies in the spinifex-textured partof the sill was controlled by nucleation and crystallizationof olivine in these variably undercooled liquids. KEY WORDS: komatiite; intrusion; spinifex texture; olivine     

华北地块东段和龙超镁铁质科马提岩的发现及特征

和龙岩体赋存于新太古代夹皮沟—金城洞花岗绿岩带中,与围岩发生同变形变质。岩体具有典型的科马提岩冷凝结构分层和鬣刺结构、冷凝多面体节理。上述特征是作者1990年发现确认的。该科马提岩的矿物学特征:玻璃质脱玻形成铁皂石等矿物;斜方辉石和橄榄石鬣刺呈中空骸晶状,大部被滑石、铁白云石、绿泥石、磁铁矿等取代,但仍保留完好的长柱状假像;堆积带中橄榄石呈微细粒、新鲜,仅发生网状蛇纹石化。岩石化学特征富MgO,CaO/Al_2O_3=1.04,用科马提岩分类图判别为超镁铁质科马提岩。     

The Perseverance Ultramafic Complex, Western Australia: The Product of a Komatiite Lava River

The Perseverance ultramafic complex is a body of olivine-richkomatiitic rocks spatially associated with the Agnew nickeldeposit, in the Agnew-Wiluna greenstone belt of the ArchaeanYilgarn Block in Western Australia. The complex consists ofa central lenticular body, up to 700 m thick, of olivine adcumulates,flanked by laterally extensive sheet-like bodies of olivineorthocumulates and spinifextextured komatiite flows. Rocks progressivelyfurther away from the central lens have chemical compositionsreflecting higher original proportions of komatiite liquid tocumulus olivine. Parent liquids had MgO contents between 25and 32% MgO, approximately chondritic Al/Ti ratios and HREEpatterns, and moderate depletion in LREE. Olivines within the adcumulate lens show a progressive increasein forsterite content from Fo₉₃ at the bottom to Fo_94?5, atthe top. Calculated original olivine compositions in the flankingrocks are similar to those at the base of the central lens.Original olivine nickel contents show a symmetrical variationfrom maximum values of 3500 ppm at the top of the central lens,through minimum values of 1000 ppm at the base and margins ofthe central lens to intermediate values in the distal rocks.The complex as a whole shows evidence for nickel depletion relativeto other komatiite suites. These observations are explained in terms of prolonged eruptionand flow of komatiitic lava down a major flow channel or lavariver. Adcumulates crystallized on the floor and sides of thecentral channel, which was formed at an early stage by thermalerosion of floor rocks. Episodic overflow of the central channelproduced distal ‘flood plain’ rocks consisting ofolivine orthocumulates and layered flows. Lavas became moremagnesian and nickel-rich with time, giving rise to the observedspatial variation in primary olivine composition. Nickel depletionof the earliest lavas is attributed to pre-eruption segregationof large volumes of immiscible Fe-Ni-sulfide, which were concentratedto form the underlying Agnew nickel deposit.     

Komatiite hosted Hunters Road nickel deposit,central Zimbabwe: physical volcanology and sulfide genesis

The nickel sulfide bearing Main Flow at Hunters Road is a thick komatiite unit situated at the base of a well developed lava channel complex overlying a prominent banded iron formation that caps a thick sequence of felsic lavas and volcaniclastic rocks intruded by a probably comagmatic feeder sill. The 300–350 m thick inner flank comprises a 200–250 m thick central olivine meso to adcumulate, relatively narrow lower and upper ortho to mesocumulates and a 2–10 m thick olivine spinifex zone at the top. Approximately 700 m towards the 90 m thick outer flank, the spinifex zone is up to 30 m thick, the central meso to adcumulate lenses out between the upper and lower mesocumulates, and the lower orthocumulate is more rich in clinopyroxene. In places, the flanks are directly overlain by rubbly hyaloclastite. The less well preserved, 500 m thick central axis coincides with a floor rock embayment of demonstrably primary origin, which is 200 m deep and at least 800 m wide. Nickel sulfide mineralisation is disseminated, texturally cocumulus to olivine and confined to the meso to adcumulate, the highest nickel grades being located towards the top. Large floor rock xenoliths occur in the mineralised meso to adcumulate of the inner flank within reaction envelopes of barren, pyroxene bearing, olivine ortho to mesocumulate. Formation of the embayment and xenoliths (clear evidence of large scale thermomechanical floor rock erosion) and of the olivine sulfide meso to adcumulate (the sulfur probably derived by the assimilation of sulfidic wall rocks adjacent to the feeder sill) is attributed to prolonged focused flow of hot turbulent lava close to the vent. The Main Flow is interpreted as the product of a thick channelised sheet flow comprising: (i) a turbulent lava river, open along the central axis and partly tubed over along the inner flanks; and (ii) a tubed over levée facies along the outer flanks accommodating overflow from the central axis by inflationary growth under laminar flow conditions.     

Komatiite within Chhotanagpur Gneissic Complex at Semra,Palamau District,Jharkhand: Petrological and geochemical fingerprints

Komatiite near Semra village, southwest of Daltonganj in Palamau District of Jharkhand, occurs within tremolite actinolite schist of ultramafic parentage. The fragmented olivine phenocrysts show mutually parallel as well as angular alignment, representing relict spinifex texture. Mineralogically the Semra ultramafic is lehrzolite in composition. The cumulates lack visible deformation suggesting original magmatic crystallization of these ultramafic rocks. The present occurrence of Spinifex Textured Peridotitic Komatiite (STPK) in Chhotanagpur Gneissic Complex (CGC) at northwestern part of Eastern Indian Shield is of great significance as it opens up a possibility of the presence of Archaean rock in CGC, which is yet to be established.     

Flows above oscillatory ripples

The flows above ripples of glass beads and ripple models have been visualized in an oscillatory water tunnel. Some ripples were observed to form without flow separation. Two types of vortices, the standing vortices without flow separation and the separation vortices have been observed, and their similarity is discussed.     

浅水流动的可压缩流数学模拟

利用浅水流动的可压缩流比拟来认识浅水方程组的特性及其有关物理现象,过去的讨论多限于矩形断面棱柱形明渠的最简单情况。本文首先将该模拟推广到任意断面非棱柱形明渠的一般情况,给出了等价绝热指数公式γ=H/h.(H为水力水深,h_c为形心水深),并指出γ=2为矩形断面的特例.然后着重阐明这一数学模拟对计算水力学具有普遍实用价值,并将计算气体动力学中的两种通量向量分裂(FVS)算法通过比拟应用于浅水流动计算。最后,本文讨论了浅水流动可压缩流模拟与一般完全气体可压缩流物理特性的异同点,据此提出正确应用这一模拟所应遵循的原则。     

Empirical Relationships for Debris Flows

The assessment of the debris flow hazard potential has to rely on semi-quantitative methods. Due to the complexity of the debris-flow process, numerical simulation models of debris flows are still limited with regard to practical applications. Thus, an overview is given of empirical relationships that can be used to estimate the most important parameters of debris-flow behavior. In a possible procedure, an assessment of a maximum debris-flow volume may be followed by estimates of the peak discharge, the mean flow velocity, the total travel distance, and the runout distance on the fan. The applicability of several empirical equations is compared with available field and laboratory data, and scaling considerations are used to discuss the variability of the parameters over a large range of values. Some recommendations are made with regard to the application of the presented relationships by practicing engineers, apart from advocating field reconnaissance and searching for historic events wherever possible.     

Residual Exchange Flows in Subtropical Estuaries

Observations of residual exchange flows at the entrance to four subtropical estuaries, two of them semiarid, indicate that these flows are mainly tidally driven, as they compare favorably with theoretical patterns of tidal residual flows. In every estuary examined, the tidal behavior was that of a standing or near-standing wave, i.e., tidal elevation and tidal currents were nearly in quadrature. The pattern of exchange flow that persisted at every estuary exhibited inflow in the channel and outflow over the shoals. Curiously, but also fortuitously, this pattern coincides with the exchange pattern driven by density gradients in other estuaries. The tidal stresses and the residual elevation slopes should be the dominant mechanisms that drive such tidal residual pattern because the Stokes transport mechanism is negligible for standing or near-standing waves. Time series measurements from the semiarid estuaries showed fortnightly modulation of the residual flow by tidal forcing in such a way that the strongest net exchange flows developed with the largest tidal distortions, i.e., during spring tides. This modulation is opposite to the modulation that typically results in temperate estuaries, where the strongest net exchange flows tend to develop during neap tides. The fortnightly modulation on tidal residual currents could be inferred from previous theoretical results because residual currents arise from tidal distortions but is made explicit in this study. The findings advanced herein should allow the drawing of generalities about exchange flow patterns in subtropical estuaries where residual flows are mainly driven by tides.     

兰州市城区泥石流危险性评价

泥石流危险性的研究对于兰州市城区泥石流治理、防灾减灾对策的确定具有重要意义.笔者通过调查的12条典型沟道资料,运用灰色关联分析的方法,取得了评价兰州市城区泥石流灾害的定量指标,并计算了各指标的权重,在此基础上得出了兰州市泥石流危险度评价模型,以此来对兰州市城区泥石流进行危险性评价,并计算出了12条沟道的危险度作为检验,评价结果与实际情况较吻合,结果表明本方法可靠、简便和实用.     

某低频泥石流沟堆积物特征研究

低频泥石流是我国分布最广、数量最大、危害性极大的一类泥石流,因其特有的隐蔽性,它所造成的人员伤亡和财产损失均居各类泥石流之首。通过对新疆某水电站坝后一低频泥石流沟堆积物的研究,探讨了该类泥石流的发育特征和研究方法。由于低频泥石流形成周期较长,一般几十年甚至上百年,从而后期的流水、暴雨有足够的时间通过侵蚀、置换作用,在各期泥石流堆积物的顶部形成一个显著的“粗化层”,这为判定泥石流活动的期次、规模、发展趋势提供了重要的依据。     

Fractionation Trends of Basalt Magmas in Lava Flows

Chemical compositions of segregation veins in basalt flows arecompared with those of their host rocks for tracing fractionationtrends of magmas after extrusion. In some tholeiite of Hawaiiand Japan and in Warner high-alumina basalt of California, totaliron increases markedly during fractionation while SiO₂ is nearlyconstant. In tholeiite near Catania, Sicily, both total ironand SiO₂ increase during fractionation. In high-alumina basaltof Huzi Volcano, Japan, and in some basaltic andesites of thehypersthenic rock series or calc-alkali rock series of Californiaand Japan, total iron is nearly constant or decreases slightlywhile SiO₂ increases. The trend with marked increase of ironis seen in flows with lower Fe₂O₃/FeO ratio, whereas that withconstant iron is seen in flows with higher Fe₂O₃/FeO. Thus thedifferent trends may arise largely from difference in oxygenpartial pressure of the magmas during fractionation. The higheroxygen pressure may be caused by higher initial water contentof the magmas. It is also noticed that residual liquids havingthe chemistry and mineralogy of the hypersthenic rock seriesoriginate only from magmas which separate orthopyroxene duringextrusion. This fact is also explained as due to the high watercontent of the magma of this series by which the crystallizationtemperature is lowered.     

Laboratory Granular Flows generated by Slope Failures

Summary. A series of flume experiments has been set up to analyze the behavior of water sand mixture flows, as a consequence of slope failures induced by water table raising and rainfall. The flows initially accelerated but at a certain stage of the process, unsteady deposition of the sand occurred, preceded by the transformation of the movement from flow to sliding. The comparison of such phenomena with other flume experiments, performed in similar conditions, showed that differences in flow behavior are dependent on a variety of aspects that are difficult to take into account. A numerical modeling of laboratory flows has been performed, using a cellular automata model based on the grain inertia regime. Despite the relatively good agreement between modeled and experimental phenomena, the study shows the difficulties of taking the actual behavior of granular flows into account. Author’s address: Chiara Deangeli, Politecnico di Torino, Ditag, corso Duca degli Abruzzi 24, 10129 Torino, Italy     

粘性泥石流的平均运动速度研究

粘性泥石流是泥石流类型中最常见也是危害最大的类型,泥石流的运动速度是泥石流的动力学参数中最重要的参数,因此准确而简洁地计算粘性泥石流的运动速度就显得非常重要。不同的泥石流地区的泥石流阻力有很大的不同:有的地区阻力较大,属于高阻力地区,泥石流运动速度较低;有的地区阻力较小,属于低阻力地区,泥石流运动速度较高。目前的粘性泥石流平均速度公式还不能兼顾计算所有地区的不同阻力类型的泥石流速度。泥石流的不均匀系数在不同的泥石流地区有很大的不同：不均匀系数小的地区阻力大,而不均匀系数大的地区阻力小,因此可以用不均匀系数划分泥石流沟的阻力特征,从而得到能兼顾所有不同地区的泥石流阻力规律。由一系列野外观测资料得到的由泥石流不均匀系数、泥石流运动底部纵比降和水力半径计算的粘性泥石流运动平均速度经验公式,能适应各种类型的泥石流沟,与其它系列的观测资料对比有很好的一致性,与粘性泥流的观测资料对比也很接近。由流体流动的福劳德数可以确定流动的缓急程度。一般的粘性泥石流都是急流,少数是缓流,极少数是运动速度非常缓慢的容重过大的粘性泥石流。粘性泥石流运动平均速度经验公式用于一般急流的粘性泥石流的速度计算结果很好,但不适用于容重过大的缓慢流动,对于缓流粘性泥石流速度计算偏大。在对泥石流的评估和治理中,平均速度公式可以用于泥石流堆积扇上游渠道中的粘性泥石流速度计算,对泥石流堆积扇上的粘性泥石流速度计算偏大,不适用于缓慢流动粘性泥石流,但在对泥石流的危害评估和治理中可以忽略缓慢流动的发生。     

异重流沉积过程和沉积特征研究

异重流是一种与河口相连、密度大于周围水体(湖泊或海洋)沿水底流动的流体,通常在干热气候、少植被的地区洪水期产生,也是一种持续型浊流。综合资料表明,异重流在流动过程中流量振荡频繁,但整体表现为先增强、后衰减的特点。由异重流形成的沉积岩被称为异重岩,其识别标志包括其垂向上为复合韵律,交错层理和块状层理砂砾岩的交替出现,层面富含有机质和陆源植物碎屑以及内部冲刷面发育等。异重流沉积垂向叠加厚度达数十米,平面主要表现为以具有水道、舌状体和堤岸沉积单元的点物源富砂型或富泥型海(湖)底扇为特征。由于异重岩具备形成优质储层的良好条件,因而具有重要的油气地质意义,可以作为有利的勘探目标。     

洪水混沌特性初步研究

混沌与随机是本质上不同的两种特性。区分产生洪水过程系统的混沌性和随机性是进行洪水模拟和预测的基础。从洪水时间序列出发,通过非线性动力系统重构和信息提取识别其基本特征。初步结果表明,洪水时间序列具有混沌动力系统的一些特征,即存在具有较低分维值、正的Lyapunov指数和Kolomogolov熵的奇怪吸引子,从而表明看起来异常复杂的洪水现象可能是由内在的非线性动力系统演化的结果。     

异重流沉积动力学过程及沉积特征

异重流(hyperpycnal flows)是指在汇水盆地水深足够条件下,由于携带大量沉积物颗粒,导致流体密度大于稳定环境水体的密度,流体受浮力影响小,沿盆地底部流动的高密度流体,在海水环境中形成异重流的临界沉积物密度为36~43 kg/m~3(体积浓度1.3%~1.7%)。异重流主要受洪水触发形成;从河口到汇水盆地经过回流区、深度有限流区、潜入区的演化,最终形成异重流;主要经历"早期沉积—侵蚀过路—晚期沉积"的沉降过程,在沉积近端以侵蚀充填沉积为主,远端以持续沉积为主;异重流的形成及其沉积主要受地形、气候、物源的控制,地形高差大、半干旱气候条件、丰富的细粒悬浮沉积物供给有利于异重流形成。异重流沉积特征及沉积序列受"源—汇"系统的控制,以流水成因交错层理、层内突变接触面或侵蚀接触面、碳质碎屑和植物碎片为区别于其他深水重力流沉积的典型沉积构造;沉积序列由沉积近端到沉积远端可能依次发育厚层序列、逆正粒序序列、薄层细粒序列。异重流研究具有十分重要的意义,能够丰富和完善深水重力流理论、探究古气候变化和古洪水作用规律、合理解释深水重力流沉积和砂体分布特征、指导深水常规和非常规油气的勘探。