华北地埠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；堆积带中侯榄石呈微细粒、鹇，仅发生网状蛇纹石化。岩石化学特征富MgO,CaO/Al2O3=1.04，用科马提岩分类图判别为超镁铁质科马提岩。     

桂北中元古代的科马提岩

桂北四堡群的科马提岩,自下而上分为堆晶带、鬣剌带和冷凝带。它们分别由辉石质科马提岩、具鬣刺结构的玄武质科马提岩和淬碎熔岩组成。与国外太古代绿岩带科马提岩对比,岩石化学成分、分带性、特有的鬣刺结构及区域变质特征、含矿性等方面亟相似。但我国华南元古代科马提岩也有自己的特点,产于元古代古大陆边缘,化学成分为超镁铁—镁铁质,且以镁铁质为主。     

山东蒙阴苏家沟科马提岩的特征及其意义

科马提岩作为太古代绿岩带的一个标志,一直受到广大地质工作者的重视.最近,在蒙阴县坦埠镇苏家沟村发现的具典型鬣刺结构科马提岩,进一步证实了鲁西地区是华北地台上乃至世界上最典型的太古代绿岩带之一,为鲁西地区太古代绿岩带及其矿产的研究提供了新的信息.苏家沟科马提岩呈透镜状残留体赋存于前寒武纪花岗质岩石中,主要由蛇纹石化橄榄科马提岩、透闪石岩、阳起透闪片岩、绿泥透闪片岩、黑云阳起片岩等组成,具典型的鬣刺结构和变余鬣刺结构.其岩石学、岩石化学及结构等特征与世界典型地区科马提岩极为相似,属橄榄科马提岩.     

山东蒙阴苏家沟科马提岩的特征及其意义

科马提岩作为太古代绿岩带的一个标志，一直受到广大地质工作者的重视，最近，在蒙阴县坦力家沟村发现的具典型鬣刺结构斜马提岩，进一步证实了鲁西地区是华北地台上乃至世界上最典型的太古代绿岩带之一，为鲁西地区太古代绿岩带及其矿产的研究提供了新的信息。苏家沟科马提岩呈透镜状残留体赋存于前寒武纪花岗质岩石中，主要由蛇纹石化橄榄科马提岩，透闪石岩、阳起透闪片岩，绿泥透闪片岩，黑云阳起片岩等组成，具典型的鬣刺结构和变余鬣刺结构，其岩石学，岩石化学及结构等特征与世界典型地区科马提岩极为相似，属橄榄科马提岩。     

大兴安岭吉峰地区中元古代科马提岩及成因类型

大兴安岭吉峰地区的橄榄质科马提岩具有良好的显微鬣刺结构,在橄榄石和辉石晶体间隙中充填有基质物质,说明其为火山岩.通过研究,对其初步划分了冷凝带和鬣刺带,它们构成一个不完整的冷凝单元.吉峰科马提岩在岩石化上表现了良好的科马提岩属性,与南非及西澳大利亚橄榄质科马提岩一致.岩石地球化学研究表明,科马提岩及与其伴生的玄武岩具有紧密的成因联系,它们的演化符合科马提岩-拉斑玄武岩的演化趋势,具良好的正相关特点.在主量元素、微量元素及REE特征上,本区科马提岩具有较好的II类科马提岩属性,与南非巴伯顿、印度、西格陵兰II类科马提岩极为相似.吉峰科马提岩的ε_Nd（T）=+7.51,说明源于亏损地幔,并且以LREE轻度富集为特征,显示它们系亏损地幔经较小程度部分熔融产生的.吉峰拉马提岩的形成时代为中元古代,其形成环境可能是地壳拉伸减薄,上涌的科马提质岩浆在淬火快速冷却的条件下形成.这一岩石类型的发现为研究大兴安岭地区元古宙大地构造演化提供了一个新线索.     

桂北四堡群中科马提岩系及其成因类型

桂北四堡群中许多镁铁质—超镁铁质岩体,实际上是火山岩或次火山岩。有的还具有科马提岩岩流的结构分层,其堆积带上部橄榄石和辉石晶体间隙中充填有基质物质。这些基质中发育了典型的橄榄石鬣刺结构。科马提岩与其伴生玄武岩属具成因联系的同一演化系列。在微量元素及REE特征上,区内科马提岩具有较好的Ⅱ类科马提岩属性,它们系亏损地幔经较小程度部分熔融产生。     

蒙阴县苏家沟科马提岩

苏家沟科马提岩呈透镜状残留体赋存于早元古代二长花岗岩中,主要由蛇方石化橄榄科马提岩、透闪石岩、透闪片岩、阳起诱闪片岩、绿泥透闪片岩、黑云阳起片岩等组成,具典型的鬣刺结构及变余鬣刺结构。该科马提岩的岩石学、岩石化学及结构特征与民办典型地区科马提岩极为相似,属橄榄科马提岩。     

非均匀温度场中熔体结晶作用与科马提岩结构成因

具有重要岩石学和构造学意义的科马提岩，最吸引人同时又最令人困惑的特征之一是它的鬣刺结构和分层构造。本文从喷出地表的岩桨冷凝时温度分布、熔体性质与结晶作用的密切联系，把科马提岩浆喷发形成的岩流作为一个冷凝整体。通过与金属铸锭浇注成形和矿石冶炼生产的比较，发现它们在冷凝固结过程中温度场特征、熔体的性质、结晶能力和结晶方式都有十分相似的特点，并以此为出发点详细解析了科马提岩的鬣刺结构和岩石分层构造的形成     

华北地台东段花岗绿岩带中和龙橄榄石质科马提岩

和龙科马提岩是作者于 1990年在研究“晚太古代夹皮沟—金城洞花岗绿岩带及成矿作用”时发现 ,而后又在 1995～ 1999年进一步工作才确定下来的。1　鉴别科马提岩的标志　　科马提岩是Ｖｉｌｊｏｅｎ兄弟专门用于指一种太古代超镁铁岩。这类岩石具有特殊的岩流顶部冷凝节理、鬣刺结构和富镁 (ＭｇＯ高达 32× 10 - 2 )、异常高的ＣａＯ/Ａｌ2 0 3比值 (>1)、低不相容元素的特点。 1977年Ａｒｎｄｔ、Ｎｉｓｂｅｔ、Ｓｃｈａｕ等在野外观察和研究成果的基础上 ,结合他人资料提出了被大多数地质学家采纳的鉴别标志 :(1)表示火山成因的特征 :冷…     

我国首次发现具鬛刺结构的太古代科马提岩

我院教师王仁民、陈能松和研究生李平凡于1982年7月在河北省遵化县东陵附近发现了深变质而保留良好的变余鬣刺结构的太古代科马提岩(照片)。这是一种产于太古代或元古代绿岩带的高度富镁而贫钛贫钾的超镁铁质为主的喷发熔岩。除了化学特征外,它以具有独特的由细长柱状橄榄石或辉石构成的鬣刺结构作为重要的成因特征和可靠的鉴别标志。这种岩石自从1969年首先在南非被发现和命名之后,立即引起了世界各国地质学家的注意并迅速在若干国家相继发现。但保留鬣刺结构者仅见于太古代的低级变质区或者在高级区而不超过角闪岩相的变质作用,我国遵化绿岩带曾遭受麻粒岩相变质作用而仍能保留鬣刺结构实属难     

A comparative study of olivine and clinopyroxene spinifex flows from Alexo,abitibi greenstone Belt,Ontario, Canada

A petrological and geochemical study of an olivine and of a clinipyroxene spinifex textured flow, from Alexo, indicates that the initial liquid in both flows probably came from the same mantle melting event and that the source was incompatible element depleted. The starting liquid of the clinopyroxene flow had experienced more olivine fractionation (10%) prior to its emplacement at Alexo, than the initial liquid of the olivine spinifex flow. The development of each of the textural and compositional zones in the flows can be modelled by means of crystal fractionation. In the case of the clinopyroxene flow the B-zone is formed by the fractionation of olivine, low-Ca pyroxene and chromite. An unusual feature of the Alexo clinopyroxene flow is presence of a peridotitic komatiite above the pyroxene cumulate layer, where a basaltic komatiite would usually be present. The presence of the peridotitic komatiite suggests an influx of new magma and hence a dynamic model for the flow. The composition of the clinopyroxene spinifex zone represents a mixture of clinopyroxene plus liquid, rather than simply a frozen liquid. This could happen if the clinopyroxene needles grew stalactitelike from the chilled upper surface of the flow into a flowing basaltic liquid. In the olivine spinifex flow the zones can be modelled as frozen liquids in the A₂-zone, as initial liquid which has fractionated 30% olivine in the A₃-zone and as liquid plus 50% olivine in the B-zone. But, if the clinopyroxene spinifex developed by stalactite growth of clinopyroxene needles into the a flowing liquid, the possibility that the olivine spinifex represent fractionated liquid plus stalactite olivines arises.     

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     

Partial melting and recrystallization of Archeaan komatiites by residual heat from rapidly accumulated flows

Spinifex-textured komatiites at Honeymoon Well, Western Australia, show evidence of partial melting and recrystallization of original igneous textures. Their textures and mineral compositions differ markedly from those typical of komatiites. Spinifex olivine plates are bent and broken, while interstitial space between spinifex and cumulus olivine is occupied by polygonal aggregates of clinopyroxene, orthopyroxene, minor olivine and plagioclase. Similar granular pyroxene-plagioclase aggregates occur as diffuse veins cutting spinifex zones and cumulate zones of the flows and, in places, form the matrix to a breccia containing corroded fragments of spinifex rock. Thermometry based on the two pyroxene assemblages yields temperatures of 1055° to 1141° C, just below the low-pressure komatiite solidus. Mineral compositions are different from those of typical komatiites: clinopyroxenes are Al-poor and Cr-rich, olivines are unusually iron-rich and depleted in Cr and Ca, and the low-Ca pyroxene is bronzite rather than the more typical pigeonite. We interpret these observations as the results of thermal metamorphism, partial remelting and subsequent slow crystallization of originally normal spinifex-textured komatiite flows. The rocks in question occupy a 40–70 m interval sandwiched between two olivine-rich units: an underlying 90 m-thick olivine adcumulate layer, forming part of the cumulate zone of a basal 160 m-thick flow, and an overlying 1 km-thick extrusive body composed mostly of olivine mesocumulate and adcumulate and capped in turn by spinifex-textured flows. Thermal modelling shows that a sinusoidal temperature profile of cool flow tops and hot flow centres would exist within this sequence shortly after eruption. Conductive thermal relaxation of this profile could reheat spinifex zones to the extent of inducing partial melting and textural reconstitution. Such reheating is largely dependent on the time interval between the emplacement of successive flows. Calculations suggest that at Honeymoon Well the emplacement interval must have been of the order of 10 years or less. Textural reconstitution may have contributed to the development of the thick orthocumulate sequences characteristic of komatiites in the Agnew-Wiluna belt. Present address: Geochemex Australia, P.O. Box 281, West Perth, 6005, Western Australia     

Differentiation of Komatiite Flows

Although layered komatiite flows with spinifex-textured upperparts and olivine-enriched lower parts are commonly thoughtto provide firm evidence of magmatic differentiation by olivinesettling, there are reasons to suspect that this may not betrue. Komatiite flows almost certainly convect vigorously asthey cool, with convective velocities that greatly exceed thesettling velocities of olivine grains. Other explanations forthe differentiation of komatiite flows into layers with differentolivine contents need to be investigated. One such explanation is provided by a detailed study of a komatiiteflow from Alexo, Ontario, Canada. This flow is about 16 m thickand has a spinifex-textured upper layer, and a lower layer,called the B-layer, which is composed of closely-packed, equantolivine phenocrysts. The composition of the initial liquid inthe flow is given by an upper chill sample, which has 28 percent MgO. In underlying spinifex lavas, MgO contents range from20 to 35 per cent MgO, and the B-layer has about 42 per centMgO. Olivines are most Fo-rich in the chill margin (Fo_94.1)and in the B-layer (Fo_93.8). In spinifex lavas, olivines aregenerally less magnesian, ranging from a high of Fo_93.8 in unusuallyMgO-rich chevron spinifex lavas, down to Fo₈₉ in the lower platespinifex lavas. It is believed that solidification of the upper part of theflow started only after it had become ponded, and that crystallizationproceeded with growth of spinifex olivines downwards from theroof of the flow. The manner in which the composition of silicateliquid within the flow changed during growth of the spinifexlayer can be calculated using the compositions of olivines inspinifex lavas. These calculations show that all the spinifexlavas are more magnesian than the liquids from which they formed:i.e. they all contain a component of excess olivine. Furthercalculations indicate, however, that liquid compositions inthe lower part of the flow changed more rapidly than can beexplained by accumulation of olivine in the spinifex lavas.This additional olivine must have crystallized in the lowerpart of the flow, and remained there as phenocrysts suspendedin the convecting liquid. As the spinifex-textured upper partcontinued to grow and thicken, olivine continued to crystallizewithin the flow, and the growing phenocrysts became more andmore concentrated in an ever-decreasing volume of liquid. Whenthe concentration exceeded about 50 vol. per cent, the viscosityof the lava became high enough to inhibit convection. The flowthen solidified completely, maintaining its olivine distributionwith a spinifextextured upper layer and an olivine-enrichedlower layer.     

Formation of Spinifex Texture in Komatiites: an Experimental Study

The formation of platy olivine spinifex, the texture that characterizeskomatiite lavas, has long been enigmatic. A major problem isthat the dendritic morphology of the olivine resembles thatof crystals grown in laboratory experiments at high coolingrates (>50°C/h), but at the position where these texturesform, up to several meters below the komatiite flow top, thecooling rate cannot have been greater than 1–5°C/h.We performed experiments that demonstrate that the platy habitof spinifex olivine or pyroxene is a consequence of slow coolingof ultramafic magma in a thermal gradient (7–35°C/cm).The charges were cooled at rates between 2 and 1428°C/hand, even at the low cooling rates, the thermal gradient ledto constrained growth and the development of preferentiallyoriented dendritic crystals with morphologies like those innatural platy spinifex-textured lavas. Under these conditions,olivine starts to crystallize at temperatures well below theequilibrium liquidus temperature (37°C < –T<56°C) depending on the composition of the starting material.When the cooling rate is high, the thermal gradient has a negligibleeffect on the texture and the crystals have a random orientation,like that in the upper parts of komatiite flows. KEY WORDS: komatiite; spinifex; cooling rate; experimental petrology; thermal gradient     

Komatiites and nickel sulfide ores of the Black Swan area,Yilgarn Craton,Western Australia. 3: Komatiite geochemistry,and implications for ore forming processes

The Black Swan komatiite sequence is a package of dominantly olivine-rich cumulates with lesser volumes of spinifex textured rocks, interpreted as a section through an extensive komatiite lava flow field. The sequence hosts a number of nickel sulfide orebodies, including the Silver Swan massive shoot and the Cygnet and Black Swan disseminated orebodies. A large body of whole rock analyses on komatiitic rocks from the Black Swan area has been filtered for metasomatic effects. With the exception of mobile elements such as Ca and alkalis, most samples retain residual igneous geochemistry, and can be modelled predominantly by fractionation and accumulation of olivine. Whole rock MgO–FeO relationships imply a relatively restricted range of olivine compositions, more primitive than the olivine which would have been in equilibrium with the transporting komatiite lavas, and together with textural data indicate that much of the cumulus olivine in the sequence was transported. Flow top compositions show evidence for chromite saturation, but the cumulates are deficient in accumulated chromite. Chromite compositions are typical of those found in compound flow-facies komatiites, and are distinct from those in komatiitic dunite bodies. Incompatible trace element abundances show three superimposed influences: control by the relative proportion of olivine to liquid; a signature of crustal contamination and an overprint of metasomatic introduction of LREE, Zr and Th. This overprint is most evident in cumulates, and relatively insignificant in the spinifex rocks. Platinum and palladium behaved as incompatible elements and are negatively correlated with MgO. They show no evidence for wholesale depletion due to sulfide extraction, which was evidently restricted to specific lava tubes or pathways. The lack of correspondence between PGE depletion and contamination by siliceous material implies that contamination alone is insufficient to generate S-saturation and ore formation in the absence of sulfide in the assimilant. Contamination signatures in spinifex-textured rocks may be a guide to Ni-sulfide mineralisation, but are not entirely reliable in the absence of other evidence. The widespread vesicularity of the sequence may be attributable to assimilated water rather than to primary mantle-derived volatiles, and cannot be taken as evidence for primary volatile-rich magmas. The characteristic signature of the Black Swan Succession is the presence of highly localised disseminated sulfide within a sequence showing more widespread evidence for crustal contamination and interaction with its immediate substrate. This has important implications for the applicability of trace element geochemistry in exploration for komatiite-hosted nickel deposits.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial handling: Peter Lightfoot     

Komatiitic and Iron-rich Tholeiitic Lavas of Munro Township, Northeast Ontario

Munro Township, in the Archean Abitibi greenstone belt of northeastOntario, contains volcanic and hypabyssal rocks of two magmaseries: (1) an Fe-rich tholeiitic series of basaltic to daciticlava flows (3–10 m thick), layered peridotite-pyroxenite-gabbroflows (120 m thick), and layered sills (700 m thick); (2) anultramafic-mafic komatiitic series, comprising discrete lavaflows of peridotitic to andesitic composition (1–17 mthick), layered peridotite-gabbro flows (120 m thick), and layeredsills (500 m thick). The komatiitie lavas form a successionabout 1000 m thick that is both underlain and overlain by thickersuccessions of tholeiitic volcanic rocks. Three types of komatiite are recognized: peridotitic, pyroxenitic,and basaltic komatiites. The most ultramafic are peridotiticcumulates rich in forsteritic olivine (Fo_89–94), at thebases of flows and sills. Many less mafic peridotitic komatiites(MgO: 20–30 per cent), which typically form the upperparts of flows and the marginal parts of small intrusions, exhibitspinifex textures indicative of their formation from ultrabasicliquids. Pyroxenitic komatiites (MgO: 12–20 per cent)also may contain olivine, but are dominated by clinopyroxene,usually in spinifex textures. Basaltic komatiites (MgO <12per cent) are composed mainly of clino-pyroxene and plagioclaseor devitrified glass, rarely in spinifex texture and more commonlyin equigranular textures. Peridotitic komatiite with roughly30 per cent MgO appears to represent a parental liquid fromwhich the more ultramafic komatiites formed by accumulationof olivine, and the less mafic types were derived by fractionationof olivine, joined and finally succeeded in later stages byclinopyroxene and plagioclase. Komatiites of Munro Township share many of the characteristicsof the komatiites from the Barberton Mountain Land, South Africa(Voljoen & Viljoen, 1969a and b), but lack the high CaO/Al₂O₃ratios that distinguish the Barberton rocks. The Munro komatiitesare identical in this respect to ultramafic volcanic rocks inAustralia, Canada, Rhodesia, and India. It is proposed thatthe definition of the term komatiite be broadened so that itincludes all members of this ultramafic-mafic rock series, notonly those from Barberton Mountain Land. The proposed criteriaare: (1) highly ultramafic compositions in noncumulate lavas;(2) unusual volcanic structures such as spinifex texture andpolyhedral jointing; (3) low Fe/Mg ratios at given Al₂O₃ valuesor high CaO/Al₂O₃ ratios; low TiO₂ at given SiO₂; and high MgO,NiO, and Cr₂O₃.     

Komatiites from west Shandong, North China craton: Implications for plume tectonics

The Mengyin komatiites are located at the base of the Taishan Complex ( 2.7 Ga), in the western Shandong greenstone belt of the eastern block of North China craton. The Mengyin komatiites have high-magnesian, low-titanium, and aluminum-undepleted chemical characteristics. Although the ultramafic rocks underwent amphibolite to greenschist facies metamorphism, they preserve remnant igneous spinifex textures. Some komatiite samples even preserve rare fresh olivine. According to the mass equilibrium between the olivine and melt, the potential eruption temperature of the Mengyin komatiites at 1 atm is about 1270 °C which might indicate a plume tectonic setting. It is inferred that komatiites in Mengyin, west Shandong may be part of a plume-related global magmatic event related to the break-up of a supercontinent that included the eastern block of the North China craton.     

Magnesium and iron isotopes in 2.7 Ga Alexo komatiites: Mantle signatures, no evidence for Soret diffusion, and identification of diffusive transport in zoned olivine

Komatiites from Alexo, Canada, are well preserved and represent high-degree partial mantle melts (∼50%). They are thus well suited for investigating the Mg and Fe isotopic compositions of the Archean mantle and the conditions of magmatic differentiation in komatiitic lavas. High precision Mg and Fe isotopic analyses of 22 samples taken along a 15-m depth profile in a komatiite flow are reported. The δ²⁵Mg and δ²⁶Mg values of the bulk flow are −0.138 ± 0.021‰ and −0.275 ± 0.042‰, respectively. These values are indistinguishable from those measured in mantle peridotites and chondrites, and represent the best estimate of the composition of the silicate Earth from analysis of volcanic rocks. Excluding the samples affected by secondary Fe mobilization, the δ⁵⁶Fe and δ⁵⁷Fe values of the bulk flow are +0.044 ± 0.030‰, and +0.059 ± 0.044‰, respectively. These values are consistent with a near-chondritic Fe isotopic composition of the silicate Earth and minor fractionation during komatiite magma genesis. In order to explain the early crystallization of pigeonite relative to augite in slowly cooled spinifex lavas, it was suggested that magmas trapped in the crystal mush during spinifex growth differentiated by Soret effect, which should be associated with large and coupled variations in the isotopic compositions of Mg and Fe. The lack of variations in Mg and Fe isotopic ratios either rules out the Soret effect in the komatiite flow or the effect is effaced as the solidification front migrates downward through the flow crust. Olivine separated from a cumulate sample has light δ⁵⁶Fe and slightly heavy δ²⁶Mg values relative to the bulk flow, which modeling shows can be explained by kinetic isotope fractionation associated with Fe-Mg inter-diffusion in olivine. Such variations can be used to identify diffusive processes involved in the formation of zoned minerals.