Petrography, Mineralogy, and Metamorphic Evolution of an Eclogite from the Island of Margarita, Venezuela

The petrography and mineralogy of a coarse-grained eclogitelens from the La Rinconada Group of northeastern Margarita Island,Venezuela, reflect the metamorphic evolution of the rock. Omphacite+ garnet + paragonite + barroisite + epidote + rutile/ilmenite+ quartz + pyrite + apatite represents the stable assemblagemarking the culmination of a single metamorphic episode. Electronmicroprobe analyses of the major minerals indicate an affinitywith Smulikowski's low-temperature ‘ophiolitic’type. Schreinemakers relationships among the mineral phasesallow the observed complex reaction relations to be explainedas late-stage depressurization effects. The crystallizationof eclogite instead of amphibole gneiss, as in the chemicallyidentical country rock, must be related to a lower _H2O in theaqueous fluid phase attending metamorphism. It is inferred frommineralogical stability data that the ‘culmination assemblage’was stable at approximately 450–525 °C and 11.5–13.5kb load pressure, and that depressurization to less than 5 kboccurred at slightly increasing metamorphic temperatures.     

The geology of northeastern Margarita Island,Venezuela: A contribution to the study of Caribbean plate margins

An unequivocal interpretation of the paleotectonic history of the Caribbean area, especially in modem plate tectonic terms, has not been possible despite recent advances in our geophysical and marine-geological unterstanding of the region. To fulfil this goal, a more detailed knowledge will be required not only of the nature of the crust and sediments of the Caribbean Sea, but also of the geology and metamorphic history of the surrounding continents and islands (i. e. “plate margins“). In this report, a detailed geological map of northeastern Margarita Island is presented, an island that occupies a key geological and tectonic position at the southeastern margin of the Caribbean Sea. The study focusses on a 2,000–3,000 meters thick sequence of metabasalts, herein defined as the La Rinconada Group, that conformably underlies Jurassic to Cretaceous metasediments of the Caribbean Mountain System of northern South America, but is not exposed on the mainland. These mafic rocks, which are interlayered with minor, siliceous, carbonaceous schists, have been metamorphosed under relatively highp vs. T conditions to amphibole gneisses containing barroisitic amphibole throughout and omphacite as well asin situ amphibole-paragonite eclogite in suitable bulk compositions in the nothern part of the area mapped. Found within the amphibole gneisses are many irregular, elongate masses of ultramafic rock from several meters to several kilometers in length, as well as numerous intrusions of granite to tonalite composition. The ultramafic rocks are highly serpentinized and deformed, and are surrounded by extensive talc, tremolite-actinolite, and chlorite metasomatic contact-reaction zones. Tonalités are the most abundant intrusives and are predominantly totally metamorphosed.     

Origin of the sparry magnesite deposits around Bauri,Almora district,Uttar Pradesh,India

Seven pockets of variable dimensions of strata-bound sparry magnesite within the Middle Proterozoic Gangolihat Formation around Bauri in the Almora district, Kumaun, Lesser Himalaya, have been investigated petrographically and geochemically. The lenses and pockets of megacrystalline, bladed, occasionally stellate, magnesite aggregates invariably enclosed by stromatolitic or massive dolostones, often exhibit a concordant relationship with the latter. Besides the sharp contrast in crystal-linity of magnesite and dolostones and the patches of the latter in the former, relict features such as layers of chert, cryptocrystalline silica veins and stromatolitic structures are discernible in the magnesite. There is a gradual increase in MgO and FeO with a corresponding decrease in CaO, and a striking depletion in Sr from dolostone to magnesite but no noteworthy variation in other major or minor elements nor in insoluble contents. Both the dolostones and magnesites are characterised by the same range of oxygen isotope ratios. However, a marked enrichment of lighter carbon isotopes in magnesites is noted. Based on these observations, it is inferred that the magnesite around Bauri is a product of diagenetic magnesitisation of penecontemporaneous dolomite in a restricted biohermal tidal flat environment.     

三江地区义敦岛弧安山岩成因

义敦岛弧是一个发育于张性薄陆壳基底上的三叠纪岛弧,以发育岛弧裂谷和成对的火山弧为特征。弧火山岩主要为钙碱性火山岩系,以安山岩为主,其岩石组合为玄武岩—安山岩—英安岩。岩相学、地球化学、相平衡及定量模型计算均证实,分离结晶作用是产生钙碱性火山岩系的主导作用,安山岩是钙碱性玄武岩浆在不同压力条件下发生多阶段分离结晶的产物。岩浆演化具有相对封闭系统特征。玄武岩浆在较大压力条件下发生Ol+Cpx+Pl±Mt±Ap结晶(F=0.46)后,派生岩浆发生Hb+Cpx+Pl+Mt结晶分离(F=0.25),产生安山岩浆。该岩浆可能在较低压力下再次发生结晶分异,从而派生出英安岩。角闪石在相对较高的压力条件下的大量结晶是安山岩形成的主要原因。     

Origin and evolution of Lord Howe Island,Southwest Pacific Ocean

Lord Howe Island is the eroded remnant of a large shield volcano. Tholeiitic lavas of the North Ridge Basalt comprise the main shield building phase and were erupted about 6.9 Ma ago. The Boat Harbour Breccia probably formed within the throat of the volcano and, together with the North Ridge Basalt, is intruded by numerous basaltic dykes, which grade into a cone sheet complex near the main vent. Large scale collapse of the summit area of the volcano produced a caldera which was filled rapidly by lavas of the Mount Lidgbird Basalt some 6.4 Ma ago, bringing to a close the volcanic history of Lord Howe Island. The shield volcano thus was built during a short interval in the late Miocene.

Palaeomagnetic data show that the North Ridge Basalt and the Mount Lidgbird Basalt were erupted during periods when the geomagnetic field had normal polarity, and that their formation was separated by at least one interval of reversed polarity when the dykes and cone sheets were emplaced. The directions of magnetisation for the lavas and intrusives are such that, palaeomagnetically, no movement of Lord Howe Island is detected since its formation.

Lord Howe Island is the subaerial part of a large seamount which lies at the southern end of a northerly‐trending line of volcanic seamounts extending for more than 1000 km. The Lord Howe seamount chain probably was produced by movement of the Australian lithospheric plate over a magma source or hot spot located below the plate within the upper mantle. Other data suggest that the Australian plate is moving N at about 6 cm/a and from this it is predicted that the seamount underlying Nova Bank, at the northern end of the chain, was constructed by volcanic activity about 23 Ma ago. Similarly, if volcanism were to occur now in the Lord Howe seamount chain we predict that its location would be about 400 km S of Lord Howe Island.     

Origin of the Rubian carbonate-hosted magnesite deposit, Galicia, NW Spain: mineralogical, REE, fluid inclusion and isotope evidence

The Rubian magnesite deposit (West Asturian—Leonese Zone, Iberian Variscan belt) is hosted by a 100-m-thick folded and metamorphosed Lower Cambrian carbonate/siliciclastic metasedimentary sequence—the Cándana Limestone Formation. It comprises upper (20-m thickness) and lower (17-m thickness) lens-shaped ore bodies separated by 55 m of slates and micaceous schists. The main (lower) magnesite ore body comprises a package of magnesite beds with dolomite-rich intercalations, sandwiched between slates and micaceous schists. In the upper ore body, the magnesite beds are thinner (centimetre scale mainly) and occur between slate beds. Mafic dolerite dykes intrude the mineralisation. The mineralisation passes eastwards into sequence of bedded dolostone (Buxan) and laminated to banded calcitic marble (Mao). These show significant Variscan extensional shearing or fold-related deformation, whereas neither Rubian dolomite nor magnesite show evidence of tectonic disturbance. This suggests that the dolomitisation and magnesite formation postdate the main Variscan deformation. In addition, the morphology of magnesite crystals and primary fluid inclusions indicate that magnesite is a neoformed hydrothermal mineral. Magnesite contains irregularly distributed dolomite inclusions (<50 μm) and these are interpreted as relics of a metasomatically replaced dolostone precursor. The total rare earth element (REE) contents of magnesite are very similar to those of Buxan dolostone but are depleted in light rare earth elements (LREE); heavy rare earth element concentrations are comparable. However, magnesite REE chondrite normalised profiles lack any characteristic anomaly indicative of marine environment. Compared with Mao calcite, magnesite is distinct in terms of both REE concentrations and patterns. Fluid inclusion studies show that the mineralising fluids were MgCl₂–NaCl–CaCl₂–H₂O aqueous brines exhibiting highly variable salinities (3.3 to 29.5 wt.% salts). This may be the result of a combination of fluid mixing, migration of pulses of variable-salinity brines and/or local dissolution and replacement processes of the host dolostone. Fluid inclusion data and comparison with other N Iberian dolostone-hosted metasomatic deposits suggest that Rubian magnesite probably formed at temperatures between 160 and 200°C. This corresponds, at hydrostatic pressure (500 bar), to a depth of formation of ~~5 km. Mineralisation-related Rubian dolomite yields δ ¹⁸O values (δ ¹⁸O: 12.0–15.4‰, mean: 14.4±1.1‰) depleted by around 5‰ compared with barren Buxan dolomite (δ ¹⁸O: 17.1–20.2‰, mean: 19.4±1.0‰). This was interpreted to reflect an influx of ¹⁸O-depleted waters accompanied by a temperature increase in a fluid-dominated system. Overlapping calculated δ ¹⁸O_fluid values (~+5‰ at 200°C) for fluids in equilibrium with Rubian dolomite and magnesite show that they were formed by the same hydrothermal system at different temperatures. In terms of δ ¹³C values, Rubian dolomite (δ ¹³C: −1.4 to 1.9‰, mean: 0.4±1.3‰) and magnesite (δ ¹³C: −2.3 to 2.4‰, mean: 0.60±1.0‰) generally exhibit more negative δ ¹³C values compared with Buxan dolomite (δ ¹³C: −0.2 to 1.9‰, mean: 0.8±0.6‰) and Mao calcite (δ ¹³C: −0.3 to 1.5‰, mean: 0.6±0.6‰), indicating progressive modification to lower δ ¹³C values through interaction with hydrothermal fluids. ⁸⁷Sr/⁸⁶Sr ratios, calculated at 290 Ma, vary from 0.70849 to 0.70976 for the Mao calcite and from 0.70538 to 0.70880 for the Buxan dolostone. The ⁸⁷Sr/⁸⁶Sr ratios in Rubian magnesite are more radiogenic and range from 0.71123 to 0.71494. The combined δ ¹⁸O–δ ¹³C and ⁸⁷Sr/⁸⁶Sr data indicate that the magnesite-related fluids were modified basinal brines that have reacted and equilibrated with intercalated siliciclastic rocks. Magnesite formation is genetically linked to regional hydrothermal dolomitisation associated with lithospheric delamination, late-Variscan high heat flow and extensional tectonics in the NW Iberian Belt. A comparison with genetic models for the Puebla de Lillo talc deposits suggests that the formation of hydrothermal replacive magnesite at Rubian resulted from a metasomatic column with magnesite forming at higher fluid/rock ratios than dolomite. In this study, magnesite generation took place via the local reaction of hydrothermal dolostone with the same hydrothermal fluids in very high permeability zones at high fluid/rock ratios (e.g. faults). It was also possibly aided by additional heat from intrusive dykes or sub-cropping igneous bodies. This would locally raise isotherms enabling a transition from the dolomite stability field to that of magnesite.Editorial handling: F. Tornos     

The Origin of Amphibole in Lherzolite Xenoliths from Nunivak Island, Alaska

Ten lherzolite xenoliths collected on Nunivak Island, Alaska,contain interstitial chromian pargasitic amphibole. Of 2000lherzolite nodules examined from the maars of the island, however,50 per cent contain relics of such an amphibole in the formof fine-grained zones of euhedral diopside, olivine and spinelin a porous Al-rich glass. These nodules are believed to havebeen amphibole-bearing fragments of the upper mantle, that partiallymelted during their ascent to the surface in the Nunivak basalts.The textural intimacy and chemical zoning exhibited by the interstitialamphibole and spinel in the lherzolite xenoliths indicate thatalthough the amphibole predates the Nunivak basalts, it is asecondary aluminous phase. The amphibole was formed in the uppermantle during a pervasive metasomatic event caused by risingtemperature and the infiltration of alkali-rich fluid, priorto the introduction of basaltic magmas.     

石碌铁矿成矿流体特征及成因

石碌大型—超大型铁矿赋存于石碌群第六层中段条带状透辉石透闪石岩、铁质千枚岩和铁质砂岩中。铁矿体形态总体呈层状-似层状,沿地层走向连续分布,受石碌群第六层和北西向北一复向斜联合控制。矿床可划分为5期:海底喷溢沉积期、区域变质期、矽卡岩期、石英-硫化物期和表生期,石碌铁矿主要形成于喷溢沉积期和区域变质期。其中区域变形变质作用与古特斯洋沿着昌江-琼海断裂带向南俯冲及随后的与印支地块的碰撞作用有关,区域变形变质作用形成于印支期(约243Ma)。喷溢-沉积期包裹体为气液二相包裹体,均一温度多集中在150～210℃,盐度变化于0.71%～3.06%NaCl。矽卡岩早期阶段石榴子石、绿帘石和石英中包裹体均一温度变化范围宽,介于150～497℃,峰值出现于230～310℃、370～410℃和450～470℃3个区间,包裹体盐度变化于1.23%～22.31%NaCl,出现4%～7%NaCl、11%～13%NaCl和21%～22%NaCl 3个峰值区间。矽卡岩期晚期阶段石英和方解石中气液二相包裹体均一温度变化于155～286℃,峰值出现于170～210℃和230～250℃2个区间,包裹体盐度变化于1.40%～7.17%NaCl。含CO2三相包裹体均一温度变化于218～533℃,明显比气液二相包裹体高,包裹体盐度变化于4.98%～8.35%NaCl。石英-硫化物期石英和方解石中均一温度变化于151～462℃,峰值出现于170～250℃、290～310℃和370～390℃3个区间,包裹体盐度变化于1.05%～16.53%NaCl,有1%～4%NaCl和14%～17%NaCl二个区间。矽卡岩期早期阶段和晚期阶段石英同时出现气液二相包裹体、富CO2包裹体、含CO2三相包裹体,由流体不混溶作用所引起,成矿流体来源于岩浆流体。虽然矽卡岩期成矿流体能导致磁铁矿交代赤铁矿,但对铁矿的富集不起决定作用,相反导致矿石的贫化。     

Origin of the Torlesse Terrane and Coeval Rocks,North Island,New Zealand

On the basis of differing areal extent, age, petrographic modes, and bulk chemical composition, the sandstones of the northern quarter of the Torlesse terrane are subdivided into four new petrofacies. A comparison of these petrofacies with existing South Island Torlesse classifications indicates continuation of the Triassic Rakaia subterrane and Late Jurassic–to–early Cretaceous Pahau subterrane into the central part of the North Island (as Axial-A and Axial-B petrofacies, respectively). The Waioeka petrofacies defines a new and provisional Late Jurassic-to–early Cretaceous Waioeka subterrane that is not present in the South Island. The Omaio petrofacies is common to deformed Albian basement sequences in the Torlesse of both islands, and in the Houhora Complex of Northland.

The composite Torlesse terrane evolved by Early Jurassic accretion of allochthonous Rakaia rocks followed by parautochthonous deposition of Pahau and Waioeka sandstones. Waioeka sandstones are compositionally similar to sandstones in the coeval eastern Waipapa terrane, but may have been dextrally displaced from their original depositional site by up to 300 km since the middle Cretaceous.     

Origin and Evolution of Primitive Island Arc Ankaramites from Western Epi, Vanuatu

The petrography, mineralogy, and geochemistry of a suite oflavas from the northwestern part of Epi Island in the VanuatuArc, southwest Pacific Ocean, are described. The more primitivemembers of this suite are rich in clinopyroxene phenocrystsand are strikingly similar to primitive lavas from MerelavaIs. in the same arc. These primitive, clinopyroxene-rich lavasare designated arc ankaramites to differentiate them from primitive,olivine-rich arc picrites which also occur in this arc system.The primitive Epi lavas are shown to have evolved from low-Kprimary melts which were saturated in both olivine and clinopyroxene.The most Mg-rich olivine (mg-number 92?2) and clinopyroxene(mg-number 94?4) in the ankaramites represent cotectic crystallizationwith Cr-rich spinels. Initial plagioclase (An₉₄) crystallizedin equilibrium with olivine (mg-number 78–80) and theplagioclase-olivine cotectic path extends to mg-number 50 andAn₅₈. The ankaramitic parent magma composition is calculated fromthe most primitive olivine phenocryst composition and the liquidline of descent, and has 14?5% MgO, 11% A1₂O₃, 14?8%CaO, 0?29%K₂O, and flat REE patterns. The origin of this parent magmahas been modelled with Ghiorso & Carmichael's (1985) programSILMIN. An assimilation model involving a clinopyroxenite orwehrlite assimilate and a low-K picrite host requires ca. 90%assimilate to match the phase chemistry and bulk-rock chemistryof the parental ankaramite. The required degree of superheatingnecessary to achieve this, and the apparent restriction of low-Kpicrites to Anatom Island in the far south of the arc, rendersthis model unsatisfactory. Partial melting models involvingtypical upper mantle lherzolite also fail to give satisfactoryresults, but partial melting of a wehrlite source (mg-number87-88) with < 10% normative (mol.) orthopyroxene, at 5?10kband 1325?C, closely matches the parental ankaramite composition.These results can be reconciled with melting of lower crustalcumulates by an ascending peridotite diapir, a hypothesis whichaccounts for the very low Ni contents of the parental meltsand primitive phenocrysts. The more evolved lavas define two distinct assemblages: a relativelytight grouping of high-K andesites straddling the high-K-‘shoshonite’boundary, characterized by low Zr/Rb (2?2) and high K₂O/Na₂Oratios (1?3–0?9), and a relatively coherent fractionationpathway to dacites straddling the ‘calc-alkaline’-high-Kboundary, with Zr/Rb = 2?9 and K₂O/Na₂O=0?6. Numerical modellingdemonstrates that the dacite trend is compatible with fractionationfrom an ankaramite parent, whereas the high-K andesites areincompatible with open- or closed-system fractionation fromankaramitic or picritic sources and may represent fractionated,hybrid magmas, largely derived from melting of lower crustalgabbros.     

Origin of giant stratabound deposits of magnesite and siderite in Riphean carbonate rocks of the Bashkir mega-anticline, western Urals

In the Bashkir mega-anticline (western Urals) stratabound magnesite, siderite, fluorite and base-metal deposits are hosted by a sequence of Riphean sediments with a thickness of more than 12 km. The giant deposits of siderite (Bakal) and sparry magnesite (Satka) belong to the largest known mineral deposits of this type on Earth but are still disputed with respect to their origin. Both the Fe- and Mg-carbonate ores are clearly characterized by mimetic preservation of sedimentary and diagenetic textures of the host carbonate sediments, giving evidence of epigenetic metasomatic replacement. In the stratiform magnesite deposits of Satka, O- and C-isotopes, REE pattern and the lithostratigraphic position of the ore, point to the reflux of early diagenetic Mg-rich brines being responsible for the selective replacement of brecciated dolomite. The Bakal siderite deposits are hosted by Lower Riphean carbonate rocks and are controlled by a Lower to Middle Riphean unconformity marked by deep erosion and subsequent transgression-related sedimentation of coarse clastics. Their independence of carbonate lithofacies and their trace element distribution are indicative of metasomatic processes. Fe-bearing fluids have probably been generated by low-grade metamorphic (catagenetic) devolatization from underlying argillites, causing the metasomatic formation of large siderite ore bodies in the Bakal carbonates due to the focusing of the fluid flow by the overlying Mid-Riphean coarse clastics.     

平潭岛岚湖沉积环境和成因的研究

在中全新世后阶段，当海岸风沙正处于始发状态，平潭西南的岗湖还是一个由多叉的山谷组成的洼地。到全新世晚期，由于全球气候的变化和优势的北北东方向风力的逐渐增强，河谷洼地与海湾相联结的地区长时间风沙的堆积而形成岚湖。     

岛弧火山岩成因和地球化学特征

岛弧火山岩主要为俯冲带的俯冲板片脱水形成的富大离子亲石元素流体交代地幔楔,并使其发生部分熔融,产生岛弧岩浆作用而形成的,岩石组合通常为玄武岩—安山岩—英安岩—流纹岩及相应侵入岩组合。它以Al2O3、K2O高,低Ti O2,且K2ONa2O为特征,相对富集LILE,亏损HFSE,特别是Ti、Nb、Ta等。本文主要从岛弧岩浆作用的起因着手,分析流体和熔体对地幔楔的交代作用,以及岛弧岩浆作用过程,进而分析岛弧火山岩的地球化学特征。     

海南抱伦矿区构造变形、成矿时代与金矿化关系研究

抱伦金矿床位于海南岛西南部乐东县境内,是一个以石英脉型为主的大型金矿床。野外地质调查表明,矿区大致经历了三期构造变形:早期(D1)NE向褶皱,中期(D2)NNW向褶皱和剪切变形,晚期(D3)脆性断层和节理。其中,中期的NNW向右行剪切变形为金矿体的赋存提供了主要空间,金矿化主要发生于三个不同的阶段,以第一阶段(自然金-Q2石英阶段)最为重要,形成了大量自然金,为主要矿化阶段。对切割矿体的细晶岩脉中锆石的定年研究表明,抱伦金矿化主要与印支期尖峰岭花岗岩浆活动有关,而与燕山晚期岩浆活动关系不大。     

Magma Origin and Evolution of White Island (Whakaari) Volcano, Bay of Plenty, New Zealand

White Island is an active composite stratovolcano in the Bayof Plenty, New Zealand, that comprises many small volume (<0·1km³) andesite–dacite lava flows and pyroclastic depositswith phenocryst contents of     

香港坪洲岛坪洲组中钙芒硝假晶的确定及其成因探讨

本文通过野外地质调查、显微鉴定分析、EPMA测试和 X射线分析 ,确定香港坪洲岛坪洲组地层中的方沸石、锥辉石和次生方解石等矿物部分是交代原始盐类矿物钙芒硝形成的 ,它们多以集合体方式构成钙芒硝假晶 ,具有钙芒硝的棱面体形态 ,并可以和区域上含钙芒硝的盐系地层进行对比。本文认为 ,原始钙芒硝为上述矿物的形成提供了 Na2 O和 Ca O,热液活动带来了部分 Si O2 ,通过交代蚀变作用形成了丰富的钙芒硝假晶。这一研究成果表明 ,坪洲组是一盐系地层 ,大鹏湾盆地在晚白垩世—古近纪时期是一演化程度较高的硫酸盐型蒸发岩盆地。     

从矿物学角度探讨芝罘岛黄土状黄土成因及古气候变化

沉积矿物是记录物源信息和古气候变迁的良好载体。通过显微镜碎屑矿物鉴定和黏土矿物X射线衍射峰能谱分析对芝罘岛黄土状黄土2 个剖面13 个样品进行处理, 并对数据进行数学统计分析。结果表明, 芝罘岛黄土状黄土矿物种类、矿物形态特征、磁铁矿和岩屑的高含量等都揭示了黄土来源的多源性和近源性, 末次冰期时的海底平原沉积物以及洪积、残积物是芝罘岛黄土状黄土的重要物源, 而西北内陆沙漠吹来的细颗粒物质相对较少。普通角闪石、绿帘石含量较高的黄土堆积后期是以远源沉积为主, 且以风力搬运为主; 磁铁矿、岩屑含量较高的早、中期是以近源沉积为主, 搬运动力较复杂。根据黄土状黄土的剖面层序特征, 碎屑矿物以及黏土矿物垂相变化特征, 说明黄土堆积的早期, 气候是相对温暖湿润的, 之后变得寒冷干燥, 后来又逐渐回暖, 然后再转冷, 呈现出末次冰期该地冷暖交替的气候旋回。     

Geochemical record of anthropogenic impacts on Lake Valencia,Venezuela

Bulk geochemical parameters and organic matter biomarkers in a short, high resolution gravity core (Lake Valencia, Venezuela) were examined to reconstruct anthropogenic impacts on the lake’s conditions. During the period of ca. 1840–1990, sedimentary organic matter was characterized by high contents of total organic C (TOC) and total N (TN), low TOC/TN values as well as relatively enriched δ¹³C and δ¹⁵N signals, suggesting a primary autochthonous (algae and macrophytes) organic matter origin. The occurrence of large amounts of C₂₃ and C₂₅ relative to C₂₉ and C₃₁n-alkanes indicated substantial inputs from submerged/floating macrophytes. The variations of C₃₂ 15-keto-ol, tetrahymanol, diploptene, C₃₂ bishomohopanol, 2-methylhopane, dinosterol and isoarborinol concentrations over the investigated period record changes in the planktonic community structure, including Botryococcus braunii, bacteriavore ciliates, cyanobacteria, Eustigmatophytes and dinoflagellates. A principal shift occurred in the 1910s when cyanobacteria and dinoflagellates became more abundant at the expense and decline of B. braunii and Eustigmatophytes, likely related to increasing anthropogenic activity around the lake. A second shift (less obvious) occurred in the 1960s when cyanobacteria became the sole predominant planktonic class, coinciding with further deterioration of lake conditions.     

辽东半岛海城-大石桥菱镁矿矿床成因探讨

辽东半岛古元古界辽河群大石桥组三段地层中赋存有全球最大工业储量的菱镁矿矿床.在总结和整理已发表的辽东半岛晶质菱镁矿床地质和地球化学数据的基础上,本次研究进行了Mg和B同位素测试及研究工作,获得以下认识:① 菱镁矿矿体呈北东东走向,产于大石桥组三段,但不同矿床的矿体厚度差异较大(30～300 m),菱镁矿矿体与顶、底板高...