Melilitite-carbonatite tuffs in the Laetolil Beds of Tanzania

The upper unit of the Laetolil Beds, 45 to 60 m thick, is about 80% wind worked or eolian tuff and 20% air-fall tuff. The air-fall tuffs comprise a phonolitic tuff and numerous thin tuffs of original melilitite-carbonatite composition. Most of the melilitite-carbonatite tuffs consist largely of sand-sized lava globules and crystals cemented by calcite. Evidence of former carbonatite ash is provided by calcite globules, fenestral textures, and high contents of Ba and Sr in early-deposited calcite. These air-fall tuffs were produced by volatile-rich eruptions of highly fluid magma. In a typical eruptive cycle, lava droplets were followed by crystals which increased in size during the eruption. Commonly the final event was an eruption of fine ash and carbonatite globules. Particularly violent explosions ejected blocks of lava and plutonic rock 10 to 15 cm in diameter for distances of 20 km.The climate was semiarid, and melilitite-carbonatite ash layers were first cemented by soluble salts such as trona resulting from incongruent solution of the carbonatite ash by rainfall. Repeated solution and crystallization of salts resulted in a polygonal fracture pattern in the thinner tuffs. Ash layers not cemented by soluble salts were eroded and redeposited by wind to form eolian tuffs. Subsequently both the air-fall and eolian tuffs were modified by several diagenetic stages, mostly in the vadose zone, to form rocks consisting principally of montmorillonite, phillipsite, and calcite. At an early stage calcium carbonate derived from carbonatite ash was precipitated as micrite both as a cement and replacement of organic matter. Glass, nepheline, and melilite were now weathered to clay, releasing components to form phillipsite. Calcite spar was precipitated last, as a replacement, cement, and pore filling. Unaltered glass, preserved in some of the eolian tuffs, has an unusually high content of Na, K, and Fe for a melilitite composition.These beds contain a rich fauna, notable for the excellent preservation of delicate fossils such as bovid dung, land snails, and bird eggs. This preservation is attributed, at least in part, to carbonatite ash. Carbonatite ash was also responsible for the preservation of footprints in one of the tuffs.     

火山碎屑密度流沉积机制研究——以松辽盆地东南隆起区九台地区白垩系营城组火山碎屑岩为例

火山碎屑密度流是一种危险的火山活动现象,也是一种重要的盆地物源供给方式,对其沉积机制的研究具有灾害预防和油气勘探的双重意义。松辽盆地东南隆起区九台营城煤矿地区白垩系营城组古火山机构保存良好,发育有典型的火山碎屑密度流沉积物。本文在精细刻画火山碎屑岩的岩石结构、沉积构造的基础上,运用薄片观察和沉积物粒度统计的方法,从物质来源、搬运机制和就位方式角度系统地分析了火山碎屑密度流的整个沉积过程,并结合国内外火山学、沉积学的研究进展探讨了不同浓度火山碎屑密度流的沉积机制。研究区内的火山碎屑密度流沉积物可以划分为五种微相:①块状熔结角砾凝灰岩微相;②无序含集块凝灰角砾岩微相;③逆粒序或双粒序角砾凝灰岩微相;④正粒序角砾凝灰岩微相;⑤韵律层理凝灰岩微相。第一种微相具有熔结结构,可能形成于高挥发分岩浆喷发柱的垮塌,火山碎屑密度流的就位温度较高;后四种微相具有正常火山碎屑岩结构,可能形成于火山口的侧向爆炸,火山碎屑密度流的就位温度中等。沉积块状熔结角砾凝灰岩微相的火山碎屑密度流具有黏性碎屑流的流体特征,沉积物整体冻结就位;沉积无序含集块凝灰角砾岩微相和逆粒序或双粒序角砾凝灰岩微相的火山碎屑密度流具有颗粒流的流体特征,沉积物整体冻结就位;沉积正粒序角砾凝灰岩微相和韵律层理凝灰岩微相的火山碎屑密度流具有湍流的流体特征,沉积物连续加积就位。火山碎屑密度流的颗粒浓度是一个连续变量,但流体性质可能会发生突变,稀释的火山碎屑密度流的沉积机制符合下部流动边界模型,稠密的火山碎屑密度流的沉积机制符合层流(碎屑流或颗粒流)模型。     

The behaviour of consolidated volcanic tuffs: weathering mechanisms under simulated laboratory conditions

Five volcanic tuffs ranging from dacitic tuffs of Hungary to rhyolite, phonolite and basaltic tuffs of Germany were consolidated under laboratory conditions. Prior to consolidation an anti-hygro, a hydrous consolidant, which reduces the swelling ability of clay minerals, was applied. The three consolidants, a silicic acid ester (SAE), an elastic silicic acid ester (eSAE) and an acrylate resin (PMMA) were applied on test specimens under vacuum. Petrographic characterisation (polarizing microscopy, XRD, SEM) provided data for fabric analyses and the mineral composition of the tuffs. Changes in fabric, effective porosity, density, tensile strength, ultrasonic wave velocity were evaluated after the treatment. Weathering simulation tests such as hygric dilatation and thermal dilatation aimed to prove the effectiveness of consolidation and the durability of consolidated tuff samples. More than 500 samples were analysed. The tests showed that SAE caused the highest increase in indirect tensile strength. The water absorption and the pore size distribution of the tuffs were modified by consolidation. The PMMA reduced the water absorption the most, whereas SAE modified it the least. All the tested consolidants increased the thermal dilatation of the tuffs. The changes in hygric dilatation were not uniform: for most tuffs SAE increased and PMMA decreased the hygric dilatation, although the clay-rich Habichtswald tuff showed the opposite trend. The changes in hygric and thermal behaviour of consolidated tuff require special care when specific consolidants are chosen. These products modify the physical properties of consolidated tuffs and change the behaviour of weathering.     

Evaluation of natural quartz and zeolitic tuffs for As(V) removal from aqueous solutions: a mechanistic approach

Naturally occurring pyroclastic materials, quartz-rich tuffs (Qz) and stellerite tuffs, were evaluated for their capacities to remove As(V) from aqueous solutions. The mechanism of As(V) uptake was evaluated using sequential extraction of As(V)-loaded adsorbent which entailed that the plausible removal mechanism is sorption to short-range ordered hydrous oxides of iron and aluminum. In addition, buffering effects of adsorbents could form favorable charges upon them through hydrolysis of amphoteric oxides, enhancing the performance of sorption. The influence of anions co-existing with As(V) in water such as carbonate, bicarbonate, nitrate, chloride, phosphate, and sulfate was studied in a batch sorption process. The impact of most anions on As(V) removal was found to be negligible except phosphate. The sorption behavior well fitted to Langmuir and Freundlich models. Estimated maximum sorption capacities of 0.42 and 0.23 mg/L were observed using quartz-rich tuffs and stellerite tuffs, respectively. As(V) concentration of 0.8 mg/L was easily decreased to below the drinking water standard of 0.01 mg/L using Qz adsorbent, whereas 0.1 mg/L As(V) decreased to below this limit upon the use of stellerite tuffs under similar conditions. The buffering capacity of quartz-rich tuffs and stellerite tuffs induced a pH increase to 5.76 and 5.40, respectively, from initial pH of 3.50, which will incur an important asset in real applications.     

Physical and stable-isotope evidence for formation of secondary calcite and silica in the unsaturated zone,Yucca Mountain,Nevada

Calcite and silica form coatings on fracture footwalls and cavity floors in the welded tuffs at Yucca Mountain, the potential site of a high-level radioactive waste repository. These secondary mineral deposits are heterogeneously distributed in the unsaturated zone (UZ) with fewer than 10% of possible depositional sites mineralized. The paragenetic sequence, compiled from deposits throughout the UZ, consists of an early-stage assemblage of calcite±fluorite±zeolites that is frequently capped by chalcedony±quartz. Intermediate- and late-stage deposits consist largely of calcite, commonly with opal on buried growth layers or outermost crystal faces of the calcite. Coatings on steep-dipping fractures usually are thin (⩽3 mm) with low-relief outer surfaces whereas shallow-dipping fractures and lithophysal cavities typically contain thicker, more coarsely crystalline deposits characterized by unusual thin, tabular calcite blades up to several cms in length. These blades may be capped with knobby or corniced overgrowths of late-stage calcite intergrown with opal. The observed textures in the fracture and cavity deposits are consistent with deposition from films of water fingering down fracture footwalls or drawn up faces of growing crystals by surface tension and evaporated at the crystal tips. Fluid inclusion studies have shown that most early-stage and some intermediate-stage calcite formed at temperatures of 35 to 85 °C. Calcite deposition during the past several million years appears to have been at temperatures <30 °C. The elevated temperatures indicated by the fluid inclusions are consistent with temperatures estimated from calcite δ¹⁸O values. Although others have interpreted the elevated temperatures as evidence of hydrothermal activity and flooding of the tuffs of the potential repository, the authors conclude that the temperatures and fluid-inclusion assemblages are consistent with deposition in a UZ environment that experienced prolonged heat input from gradual cooling of nearby plutons. The physical restriction of the deposits (and, therefore, fluid flow) to fracture footwalls and cavity floors and the heterogeneous and limited distribution of the deposits provides compelling evidence that they do not reflect flooding of the thick UZ at Yucca Mountain. The textures and isotopic and chemical compositions of these mineral deposits are consistent with deposition in a UZ setting from meteoric waters percolating downward along fracture flow paths.     

The crater-facies kimberlite system of Tokapal, Bastar District, Chhattisgarh, India

Discovery of diamondiferous kimberlites in the Mainpur Kimberlite Field, Raipur District, Chhattisgarh in central India, encouraged investigation of similar bodies in other parts of the Bastar craton. The earlier known Tokapal ultramafic intrusive body, located beyond the 19-km milestone in Tokapal village along the Jagdalpur–Geedam road, was reinterpreted as crater-facies kimberlite. Its stratigraphic position in the Meso-Neoproterozoic intracratonic sedimentary Indravati basin makes it one of the oldest preserved crater-facies kimberlite systems. Ground and limited subsurface data (dug-, tube-wells and exploratory boreholes) have outlined an extensive surface area (>550 ha) of the kimberlite. The morphological and surface color features of this body on enhanced satellite images suggest that there is a central feeder surrounded by a collar and wide pyroclastic apron. Exploration drilling indicates that the central zone probably corresponds to a vent overlain by resedimented volcaniclastic (epiclastic) rocks that are surrounded by a 2-km-wide spread of pyroclastic rocks (lapilli tuff, tuff/ash beds and volcaniclastic breccia). Drill-holes also reveal that kimberlitic lapilli tuffs and tuffs are sandwiched between the Kanger and Jagdalpur Formations and also form sills within the sedimentary sequence of the Indravati basin. The lapilli tuffs are commonly well stratified and display slumping. Base surges and lava flows occur in the southern part of the Tokapal system. The geochemistry and petrology of the rock correspond to average Group I kimberlite with a moderate degree of contamination. However, the exposed rock is intensely weathered and altered with strong leaching of mobile elements (Ba, Rb, Sr). Layers of vesicular fine-grained glassy material represent kimberlitic lava flows. Tuffs containing juvenile lapilli with pseudomorphed olivine macrocrysts are set in a talc–serpentine–carbonate matrix with locally abundant spinel and sphene. Garnet has not been observed, and phlogopite is very rare. Very limited microdiamond testing (two 18-kg samples) proved negative; however, the composition of chromite grains indicate crystallization in the diamond stability field.     

Gefüge und Entstehung der Ringdünen in den grauen Tuffen des Laacher Vulkans

The last products of the pleistocene Laach volcano (Eifel, about 40 kms SW of Bonn, Germany) are grey tuffs. Outcrops in the rim show characteristic cross-bedding in these tuffs. It is in discussion whether these structures are produced by wind or by phreato-magmatic outbursts. At all larger outcrops in the rim we measured numerous depositional elements of the tuffs, and analyzed their geometrical structure. So we are able to reconstruct shape, structure and orientation of dunes in the tuffs. They show transport vectors in centrifugal orientation, radially to the vent. The dunes are orientated concentrically to the vent, we intruduce the term “concentrical dune” for this dune type. This points out that the dunes in the Laach tuffs are of volcanic origin. But our geometrical results give no authorization to assume phreato-magmatic processes during the activity of the Laach volcano. We think that the grey Laach tuffs are no base surge deposits, but products of gas-rich eruptions in the phase of decreasing volcanic energy.     

Structure and products of subaquatic volcanoes in Iceland

Iceland provides a unique opportunity to study the mechanism of subaquatic eruptions. The Neovolcanic zone dissecting the country from NE to SW has been volcanically active since the end of the Tertiary, and a large number of eruptions have occurred under glaciers where melt water provided a subaquatic environment.The subaquatic volcanic piles have a regular structural sequence consisting of pillow lavas at the base covered by pillow breccias and glassy tuffs. This regularity is believed to result from external conditions rather than changes caused by variations in endogenic forces. The effective chilling of the lava surface as it enters the water prevents the natural degassing of the material under the prevailing hydrostatic pressure. The interior parts of a pillow continue to degass, however, creating gas pressures within each unit representing the amount of exsolved gases at magmatic temperature. The amount of exsolved gases is ultimately controlled by the hydrostatic pressure and pillow stability is thus a function of water depth above the volcanic vent. As the volcanic vent comes closer to the surface, a larger part of the dissolved gases will exsolve and pressures will be created within each glass encrusted unit which eventually are capable of exploding the material into fine-grained tuffs.A subaquatic lavaflow tends to form a thick pile instead of spreading out because of the effective chilling of flow fronts. This pile, consisting to a large extent of pillows, is eventually covered with layers of glassy tuffs. These tuffs are characteristically altered into palagonite. The alteration process neither can be related to immediate reactions between the hot melt and water, nor can this large-scale alteration be explained as a continuous weathering process under prevailing climatic conditions. It is suggested that a subaquatic volcanic structure is somewhat similar to a geothermal field during the period of cooling, where the pillow pile serves as a heat source and the tuffs provide the confining walls and cover. The palagonitization is thus considered to represent mild hydrothermal alteration in a short-lived thermal system.     

Deterioration mechanisms of tuffs in Midas monument

Slightly weathered white and pink tuffs of the Midas monument have deterioration problems. In this study, depths and characteristics of the weathering zones developed within the tuffs are investigated through optical microscopy, X-ray diffractometry (XRD), chemical analyses, scanning electron microscopy (SEM) and some index parameters. Accelerated weathering tests including wetting–drying, freezing–thawing and salt crystallization are performed, and durability assessment methods are used to predict the durabilities of the tuffs. The findings are compared with field observations. By examining quantitative weathering indices and comparing them with thin section studies, it is found that thin section analyses of the crystals, LoI, and WPI are good indicators to quantify the depth of weathering for the tuffs. However, thin section studies have limited value for fine-grained tuffaceous matrix. The chemical weathering of the tuffs produces weathered zones that are 4.5-cm thick within the white tuff and 2.5-cm thick within the pink tuff. Physical weathering causes scaling of outer layers of the tuffs and fracturing of feldspars along their cleavage planes. However, variations of the index properties of the tuffs due to weathering are not so significant to quantify the weathering depths in the tuffs. Among the accelerated weathering tests, salt crystallization is found to be the most destructive environmental condition. Pigeon droppings rich in NO₃¹⁻ are found to be the main source of soluble salt at the Midas monument. The salt transported up by capillary rise due to surface water causes spalling of the tuffs in the capillary zone. Surface water and salt of any kind in the close vicinity of the monument should be totally eliminated for the purpose of conservation. Field observations and the durability equations reveal that the white tuff is less durable than the pink tuff. Wet-to-dry strength ratio yields a better stone durability assessment among various durability methods used in this study.     

Dehumidification property of Eskisehir–Derbent Ciftligi white tuffs in Turkey

In Anatolia, throughout the centuries, caverns in tuffs have been used for food storage. The aim of this study is to investigate experimentally the dehumidification property of tuffs, and to confirm whether tuffs play any part in the humidity of the ecosystem in storage. According to tests carried out on the Eskisehir?CDerbent Ciftligi white tuffs and six different rock types from Turkey, it was found that the atmospheric moisture absorption ability of tuffs is relatively higher than those of other tested rocks. This suggests that the dehumidification property is a unique characteristic of tuffs. It may allow for the tuff walls to work as an air dehumidifier for storing food.     

Geology of the Mwadui kimberlite, Shinyanga district, Tanzania

The Mwadui pipe represents the largest diamondiferous kimberlite ever mined and is an almost perfectly preserved example of a kimberlitic crater in-fill, albeit without the tuff ring.

The geology of Mwadui can be subdivided into five geological units, viz. the primary pyroclastic kimberlite (PK), re-sedimented volcaniclastic kimberlite deposits (RVK), granite breccias (subdivided into two units), the turbidite deposits, and the yellow shales listed in approximate order of formation. The PK can be further subdivided into two units—lithic-rich ash and lapilli tuffs which dominate the succession, and lithic-poor juvenile-rich ash and lapilli tuffs. The lower crater is well bedded down to at least 684 m from present surface (extent of current drill data). The bedding is defined by the presence of juvenile-rich lapilli tuffs vs. lithic-rich lapilli tuffs, and the systematic variation in granite content and clast size within much of the lithic-rich lapilli tuffs. Four distinct types of bedding have been identified in the pyroclastic deposits. Diffuse zones characterised by increased granite abundance and size, and upward-fining units, represent the dominant types throughout the deposit.

Lateral heterogeneity was observed, in addition to the vertical changes, suggesting that the eruption was quite heterogeneous, or that more than one vent may have been present. The continuous nature of the bedding in the pyroclastic material and the lack of ash-partings suggest deposition from a high concentration (ejecta), sustained eruption column at times, e.g. the massive, very diffusely stratified deposits. The paucity of tractional bed forms suggest near vertical particle trajectories, i.e. a clear air-fall component, but the poorly sorted, matrix-supported nature of the deposits suggest that pyroclastic flow and/or surge processes may also have been active during the eruption.

Available diamond sampling data were examined and correlated with the geology. Data derive from the old 120 (37 m), 200 (61 m), 300 (92 m) and 1200 ft (366 m) levels, pits sunk during historical mining operations, drill logs, as well as more recent bench mapping. Correlating macro-diamond sample data and geology shows a clear relationship between diamond grade and lithology. Localised enrichment and dilution of the primary diamond grade has taken place in the upper reworked volcaniclastic deposits due to post-eruptive sedimentary in-fill processes. Clear distinction can be drawn between upper (re-sedimented) and lower (pyroclastic) crater deposits at Mwadui, both from a geological and diamond grade perspective.

Finally, an emplacement model for the Mwadui kimberlite is proposed. Geological evidence suggests that little or no sedimentary cover existed at the time of emplacement. The nature of the bedding within the pyroclastic deposits and the continuity of the bedding in the vertical dimension suggest that the eruption was continuous, but that the eruption column may have been heterogeneous, both petrologically as well as geometrically. Volcanic activity appears to have ceased thereafter and the crater was gradually filled with granite debris from the unstable crater walls and re-sedimented volcaniclastic material derived from the tuff ring.

The Mwadui kimberlite exhibits marked similarities compared to the Orapa kimberlite in Botswana.     

Thermal expansion on volcanic tuff rocks used as building stones: examples from Mexico

Three cities in the center of Mexico, declared as cultural heritage, were erected mainly of volcanic tuffs as main construction element. Many of the historic buildings of these cities show significant damage and deterioration. One of the causes of these damages can be attributed to a phenomenon poorly studied in volcanic tuffs, the thermal expansion. To understand the response of volcanic tuffs to thermal expansion, as well as to know their thermal expansion coefficient, thermal expansion test was performed on 12 representative tuffs of these localities. In the same way and to know which of their petrographic, petrophysical, mineralogical, and geochemical properties influence this phenomenon, several laboratory studies were carried out to determine which of these plays an important role in the expansion. The results of our tests showed that volcanic tuffs rocks have very varied thermal expansion values, ranging from 0.7 mm/m at their maximum expansion to even contraction in the order of ??0.5 mm/m. Equally varied were the thermal expansion coefficient values ranging from 10.1 to ??7.2 10^?6/K. The most dominant behavior type is the “anisotropic no residual strain behavior”. In general, the behavior of the tuffs in practically all the tests carried out showed a very important heterogeneity in the rocks; however, in spite of the non-homogeneous behavior of the tuffs, three main factors could be identified that play an important role in the thermal expansion of the studied rocks, the chemical composition, e.g., acid volcanic tuff rocks (rhyolitic composition) had greater expansions, while the more basic rocks (basaltic composition) expanded less, the textural and fabrics homogeneity, e.g., the most homogeneous rocks texturally have higher expansion values and the degree of crystallinity, and, e.g., the tuff rocks with the largest amount of glass showed smaller thermal expansions than the tuffs composed of microcrystals.     

测井曲线剩余变化的分维描述火成岩裂缝发育情况

大庆深层火成岩段岩性复杂，偿仅具有从基性到酸性的各种熔岩，还包括不同喷发期次的凝灰岩。岩性变化是影响各种测井曲线响应特征变化的最主要因素。裂缝的响应往往被岩性的影响所淹灭。为此从测井曲线中提取分维来描述裂缝发育程度，必须考虑岩性的影响。为了消除岩性的影响，对分辨率较高且对裂缝反映灵敏的微球形聚焦测井曲线进行滤波处理，获得原始曲线与滤波曲线间的剩余变化，再从中提取分维，从而突出裂缝的影响。实际处理表明，从剩余曲线中提取的分数维，比从原始测井曲线中提取的分维能更好地指示出复杂火成岩地层裂缝发育情况。     

Quantification of weathering depths in slightly weathered tuffs

Slightly weathered white and pink tuffs of Midas monument have deterioration problems. In this study, depths of the weathering zones developed in the tuffs are investigated through optical microscopy, XRD, chemical analyses, SEM and some index parameters. By examining quantitative weathering indices and comparing them with thin section studies, it is found that thin section analyses of the phenocrysts, LoI and WPI are good indicators to quantify the depth of weathering for the tuffs. However, thin section studies have limited value for fine-grained tuffaceous matrix. The chemical weathering of the tuffs produces weathered zones that are 4.5 cm thick within the white tuff and 2.5 cm thick within the pink tuff. Physical weathering causes fracturing of feldspars along their cleavage planes. Variations of the index properties of the tuffs caused by weathering are not significant enough to quantify the weathering depths in the tuffs.     

Recognition,Characterization and Implications of High-Grade Silicic Ignimbrite Facies from the Paleoproterozoic Bijli Rhyolites,Dongargarh Supergroup,Central India

A controversy regarding the distinction between the highly welded lava-like ignimbrites sometimes showing strongly rheomorphic characters, and the extensive silicic lava flow has been overwhelming in the recent literature. However, a rethinking, after Walker (1983), has brought into light the concept of ‘grade’ referring to the degree and extent of welding between the pyroclasts. Various parameters and characteristics were suggested for strengthening the idea of densely welded ignimbrites, which differentiate them from lava. Here, a comprehensive study on early Proterozoic acid magmatic rocks forming lower part of the Dongargarh Supergroup, central India, has been made to suggest extensive occurrence of high-grade welded rheomorphic tuffs. The possibility of their being welded ignimbrite rather than lava flow has been explored in the light of facies analysis as well as detailed microscopic evidences. Despite having overall monolithologic look various units bear distinction on account of their nature of welding, enrichment of phenocrysts and degree of stretching. The presence of vitroclastic texture, melt inclusions and radial fracturing of phenocrysts suggests pyroclastic nature of these deposits. Based on these characters four facies — A, B, C and D from bottom to the top respectively, have been identified from field studies around Salekasa. Facies-A and B represent clast-supported/matrix-supported welded pyroclastic flow deposits. Facies-C represents extremely welded thinly laminated rheomorphic tuffs while lava-like tuffs with an autobreccia carapace is represented by facies D. A complete gradation of facies A/B to D through C exists. High to extremely high-grade nature of welding in these deposits suggests a low column-height subaerial plinian to fissure eruption of a very high temperature silicic magma in a continental setting.     

西藏甲鲁朗地区叶巴组火山岩成因及其对新特提斯洋俯冲的约束

西藏冈底斯岩浆弧叶巴组火山岩对于新特提斯洋俯冲时限的制约存在着重要的意义.通过对甲鲁朗地区叶巴组凝灰岩的LA-ICP-MS锆石U-Pb年龄和岩石地球化学组成的研究,锆石测年获得3件凝灰岩样品206Pb/238U年龄加权平均值分别为207.8±1.6 Ma、204.8±1.7 Ma和209.3±3.4 Ma,结合古生物化石证据,表明其形成于晚三叠世;凝灰岩样品富集轻稀土元素和大离子亲石元素(Rb、K、Th、U、Pb),亏损重稀土元素和高场强元素(Nb、Ta、Ti、P),同时亏损Sr元素,Eu负异常较明显.岩石地球化学特征表明叶巴组中酸性岩可能为壳幔混染成因.综合前人研究成果,认为叶巴组形成于新特提斯洋俯冲环境下的大陆边缘弧,新特提斯洋的俯冲从晚三叠世就已经开始.本研究为叶巴组的形成时代以及动力学背景提供了新的制约.      

鄂尔多斯盆地延长组凝灰岩夹层展布特征及其地质意义

鄂尔多斯盆地延长组凝灰岩夹层分布广泛,通过众多凝灰岩夹层的实际岩心观察,归纳总结出凝灰岩夹层的常规测井曲线表现为具有相对低电位、高自然伽马、高电阻率、高声波时差值的特征,并且高值常略低于致密泥岩段;单井分析凝灰岩夹层与上、下部岩石整合接触,较少发育同沉积构造,以典型的火山尘灰大气降落沉积产物为主,但也偶见凝灰岩与砂泥岩一起发育扰动构造,明显经受了水动力改造;连井剖面分析说明延长组长1—长9皆有凝灰岩分布,横向上可对比性凝灰岩主要有4套,分别为长91、长73底、长72、长71期,以盆地西南部长73期最发育;凝灰岩平面展布特征整体呈北西向展布,由南西—北东向凝灰岩厚度逐渐变薄,并且与烃源岩、放射性异常展布形态一致,因此认为凝灰岩的沉积作用所引起的元素迁移和古环境变化,可能对延长组优质烃源岩的发育具有重要意义。     

Trap magmatism and mobilization of hydrocarbon fluids (Western Siberia)

Drill core material from the superdeep SG-6 and SG-7 wells (Koltogor-Urengoi Graben) has been examined. It has been established that the temperature conditions of alteration of sediments were non-linear and discontinuous. The basalts underlying these sediments are metamorphosed and metasomatically altered under the influence of high-temperature fluids associated with a superplume. The tuffs under the basalts have a phreatic origin. It is assumed that the intrusion of basaltic sills into the coal-bearing Permo-Carboniferous rock mass beneath the tuffs caused thermolysis of coals and the generation of hydrocarbon gases of a methane composition.     

Basaltoids and carbonatite tuffs of Ambinsky volcano (Southwestern Primorye): Geology and genesis

Geological-petrological data were first obtained on the Early Miocene basaltoids and spinel-fassaite carbonatite tuffs of the Ambinsky volcanic structure in southwestern Primorye. The geological study of Ambinsky volcano allowed the reconstruction of stratigraphic sections across lava and pyroclastic basaltic rocks and stratified carbonatite tuffs. The chemical compositions of rocks and mineral phenocrysts from basalts and carbonatite tuffs are reported. The basaltoids are classed with undifferentiated moderately alkaline within-plate basalts. Evidence of carbonate-silicate immiscibility was found in the basaltoids and carbonatite tuffs. It was suggested that the formation of the carbonatite melt associated with simultaneous basification and abundant crystallization of spinel, fassaite, as well as oversaturation of the silicate system in Ca was caused by limestone assimilation, subsequent transformation of the melt, and liquid immiscibility. Thermal decomposition of carbonates with dissolution of released CaO in magma and accumulation of CO₂ in a closed magmatic chamber gave rise to the autoclave gas effect and, correspondingly, heavy explosive eruptions atypical of such volcanic rocks. The genesis of carbonatite tuffs of Ambinsky volcano can serve as a model example of exsolution of carbonate melt in the moderately alkaline nonagpaitic basaltic system.     

The Mount Wright Arc: A Cambrian subduction system developed on the continental margin of East Gondwana,Koonenberry Belt,eastern Australia

The Mount Wright Arc, in the Koonenberry Belt in eastern Australia, is associated with two early to middle Cambrian lithostratigraphic groups developed onto the Late Neoproterozoic volcanic passive margin of East Gondwana. The Gnalta Group includes a calc-alkaline basalt-andesite-dacite suite (Mount Wright Volcanics), interpreted to represent the volcanic component of the arc. Volcaniclastic Gnalta Group rocks now buried in the Bancannia Trough represent the continental back-arc, developed immediately behind the arc in a manner analogous to the modern Taupo Volcanic Zone of New Zealand. East of the Gnalta Group is the Ponto Group, a deep marine sedimentary package that includes tholeiitic lavas (Bittles Tank Volcanics) and felsic tuffs, interpreted as part of a fore-arc sequence. The configuration of these units suggests the Mount Wright Arc developed on continental crust in response to west-dipping subduction along the East Gondwana margin, in contrast with some models for Cambrian convergence on other sections of the Delamerian Orogen, which invoke east-dipping subduction and arc accretion by arc-continent collision.This convergent margin was deformed by the middle Cambrian Delamerian Orogeny, which involved initial co-axial shortening followed by sinistral transpression, and oroclinal folding around the edge of the Curnamona Province.