Formation and chemical evolution of magnesium chloride brines by evaporite dissolution processes—Implications for evaporite geochemistry

The evolution of magnesium chloride brines with high bromide contents via a multistage reaction and dissolution process has been studied in brine seeps of a German potash mine. The observed chemical trends and phase equilibria can be modeled and interpreted in terms of a NaCl solution (cap rock brine) infiltrating into a potash zone characterized by the metamorphic mineral assemblage kieserite + sylvite + halite + anhydrite. Establishment of a persistent, stable equilibrium assemblage and constant fluid composition in the invariant point IP1 of the six component (Na-K-Mg-Ca-Cl-SO₄-H₂O) system of oceanic salts is prevented by the perpetually renewed input of NaCl-brine and by the intermittent exposure of incompatible kieserite. Instead, the solutions develop towards the metastable invariant point IP1(gy), with the mineral assemblage carnallite + polyhalite + sylvite + halite + gypsum, where gypsum takes the place of anhydrite (stage I). The temporary exposure of kieserite and the ensuing formation of polyhalite effectively buffer the solutions along the metastable polyhalite phase boundary during stages II and III. Eventually, in stage IV, polyhalite becomes depleted and admixture of more NaCl brine leads to low sulfate solution compositions, which are now only constrained by carnallite + sylvite + halite, and the once hexary system degenerates to a quaternary one (Na-K-Mg-Cl-H₂O) in point E. Bromide in brines shows equilibrium partitioning with respect to the wall rock minerals. The pattern of evolving brine compositions may serve as a model for similar brine occurrences, which in some cases may have been misinterpreted as remains of fossil, highly concentrated and chemically modified seawater. Similar magnesium chloride brines of salt lakes (e.g., Dead Sea, Dabusun Lake) show subtle differences and are constrained by fewer mineral equilibria (more degrees of freedom), and their low sulfate contents are due to gypsum precipitation, driven by calcium chloride input from dolomitization reactions. Finally, the observed reaction sequence is generalized, and a model for the formation of magnesium sulfate depleted, chloride-type potash salts and bischofite deposits by leaching of sulfate-type evaporites is proposed.     

柴达木盆地别勒滩杂卤石矿床特征及其沉积环境研究

别勒滩是青海察尔汗盐湖固体钾矿的主要分布区段,也是企业固转液开采和生产钾盐的主要矿区,该区段分布着大面积的杂卤石矿(K₂Ca₂Mg[SO₄]₄2H₂O)。文章利用2022年察尔汗盐湖储量核查项目获得的岩芯样品和钻孔资料,通过系统的分析测试,开展别勒滩杂卤石成矿特征及沉积环境研究。文章首次查明别勒滩区段杂卤石矿床分布面积超过150 km²,平均厚度0.8 m,w(K₂SO₄)平均大于5%,计算获得K₂SO₄资源量2089.79×10⁴t。研究认为,别勒滩杂卤石形成于钾石盐、光卤石析出阶段的高浓缩盐湖相沉积环境,周期性淡化带入的大量Ca²⁺和SO₄^2-与卤水中含量较高的K⁺、Mg²⁺离子反应形成了杂卤石矿层。别勒滩杂卤石矿层具有较好渗透性和给水...     

自生伊利石同位素年代学在钾盐矿床中的应用展望

伊利石是一种层状硅酸盐矿物,层间主要为钾离子,可利用K-Ar法、Ar-Ar法和Rb-Sr法等相关的测年方法来分析和限定与自生伊利石有关的地质事件的时代,例如盆地演化历史、热液活动、构造活动和油气运移。本文讨论了利用钾盐矿床中自生伊利石进行蒸发岩地层年代学研究的可能性。虽然多数研究表明温度是控制自生伊利石形成的主要控制因素,即自生伊利石一般形成于温度较高的条件下,但一些研究表明在地表温度的盐湖环境中也可形成自生伊利石,此种环境下控制自生伊利石形成的主要因素为流体成分。这构成了利用自生伊利石定年来研究蒸发岩地层时代的理论基础。然而在实际研究过程中仍存在很多问题,需进行进一步的研究。     

青海察尔汗盐湖别勒滩区段杂卤石成因及其成钾指示意义

别勒滩区段位于青海察尔汗盐湖西部,前人认为察尔汗盐湖固体钾盐矿物以光卤石为主,在别勒滩区段西部发现有杂卤石,但并未引起足够重视,对杂卤石成因未做详细研究。本文通过对别勒区段七个钻孔样品的矿物鉴定及地球化学分析,发现别勒滩区段杂卤石以原生和准同生交代方式赋存,且以后者为主,多交代石膏等硫酸盐矿物产出,少量出现交代石盐现象。杂卤石主要分布于别勒滩中西部地区,整个别勒滩区段自西向东出现了以杂卤石为主渐变为以钾石盐—光卤石为主的分布规律。结合别勒滩区段晶间卤水的补给条件,研究认为别勒滩区段杂卤石形成是涩聂湖硫酸镁亚型湖水与先期形成的盐类矿物发生交代反应的结果。别勒滩地区杂卤石作为典型的陆相成因,对研究海相蒸发岩矿床中普遍存在的杂卤石成因具有一定的对比借鉴意义,海相蒸发岩矿床中的杂卤石可能是盆地形成的内陆盐湖湖水与早期沉积的盐类矿物发生交代反应形成,对寻找海相钾盐有一定的指示意义。     

川东盆地长寿地区三叠系杂卤石的发现及其成因研究

笔者先后2次对川东盆地盐矿实地考察,并系统采集了样品。水溶化学分析结果表明,长寿地区T1j4-2地层中岩盐w(K+)较高,最高可达3.25%,明显高于其他盐矿点(合川盐矿、云阳盐矿及万州高峰盐矿)。岩盐Br-含量也较高;Br×103/Cl值相对较高,一般为0.26~0.36,最大可达0.41,已达到晚期石盐沉积,甚至是钾镁盐早期沉积阶段。而后又对K+含量较高的样品(不含KCl)进行配矿分析,同时结合岩矿鉴定及XRD分析验证,首次发现和确定长平3井的主要含钾矿物为杂卤石,样品中不含钾石盐(KCl)。通过对样品薄片镜下观察,杂卤石为自形-半自形的板状和柱状及叶片状形态,结合杂卤石与石盐的接触关系及Br×103/Cl值分析,认为长平3井杂卤石为原生沉积成因,而非如前人所认为的四川盆地其他地区的杂卤石成因——后期含钾镁较高的热卤水交代硬石膏而形成。这一认识对该区寻找可溶性钾盐具有一定的启示作用。     

中国三叠纪钾盐沉积——以四川为例

中国三叠系蒸发岩分布于华南,面积超过100×10⁴km²,发现硫酸钾盐及富钾卤水,钾盐矿物有杂卤石、无水钾镁矾、硫锶钾石及多钙钾石膏等,富钾卤水矿化度255~382g/l,含钾量4~49g/l。蒸发盆地的发展受控于古构造及古地理,经历了开放台地一局限台地一堰塞湖一盐湖的发展阶段。钾盐沉积后,经历多阶段的复杂演变。富钾卤水中的钾主要来自硫酸钾盐的深滤,其次来自蒸发浓缩卤水及火山灰吸附钾的释放。杂卤石与富钾卤水的钾同位素测年值分别为210~150Ma及264~150Ma,说明固液相钾盐同源。从地质背景及成矿环境预测,三叠系的钾盐类型只能是硫酸钾盐,主要目的层为T₁j^5~2及T₂l^4~2,主要成矿带为构造分异及钾异常明显的四川盆地东、西部,富钾卤水的储集取决于蒸发岩地层的含钾性、碳酸盐夹层的裂隙发育程度及构造控矿条件。找矿模型应是油气钾卤兼探。     

The characteristics,formation and exploration progress of the potash deposits on the Khorat Plateau,Thailand and Laos,Southeast Asia

The giant potash deposit on the Khorat Plateau is one of the most promising targets for exploitation of potassium salts. So far, many researches and geologic survey have been conducted on the giant potash deposits. Hence, it is necessary to make an overall review on the potash deposits. The potash deposit on the Khorat Plateau was formed during the Middle to Late Cretaceous, during which seawater was enriched in Ca²⁺ and depleted in SO₄^2- compared with those of modem seawater. In addition to seawater, continental water and hydrothermal fluids could have affected the evaporite basins. The seawater was probably derived from Tethys ocean, and the brine should have evaporated to some extent before entering into the basin systems based on the evidence of absence of carbonates and unproportionate sulphate compared with chloride salts. The paleo-climate during Middle to Late Cretaceous was characterized as high temperature and extremely arid environment, which is favourable for deposition of potassium-magnesium saline minerals. The major saline minerals are of anhydrite, halite, camallite, sylvite and, tachyhydrite, with trace amounts of borates. The resources of the potash deposit on the Khorat Plateau could be approximately as much as 400×10⁹ t of camallite and 7×10⁹ t of sylvite. The evaporite sequences have been deformed and altered by postdepositinal processes, including tectonic movements and chemical alteration. Salt domes were formed in the postdepositional processes. Based on the analyses of geophysical surveys and drilling projects, high-quality sylvinite ores are commonly found at the flanks of those salt domes due to incongruent dissolution of camallite. The future potential prospecting areas for the high-quality sylvinite ores would be on the edges of the Khorat Plateau.     

Geochronology of high-pressure rocks on Sifnos (Cyclades,Greece)

Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M ₁ phengite.K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2M ₁+3T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon.K-Ar and Rb-Sr ages on 2M ₁ phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks.     

Progress in the investigation of potash resources in western China

Through the study of the geological conditions of potash deposits in China from recent years, a new understanding of potash theories has arisen that appropriate Chinese geological features. Important progress and substantial breakthroughs have been gained in the direction and management of potash prospecting: (1) Important breakthroughs in continental potassium prospecting: The “Quaternary gravel type deep potassium rich brine metallogenic model in western Qaidam” ensures Quaternary deep potassium rich brine prospecting will grow new KCl resources by 350 Mt, providing a resource guarantee for meeting the Chinese demand for sylvite. (2) The Marine facies potash prospecting shows good prospects: the determination of the new type of Triassic polyhalite potash ore deposits in Sichuan provide an important scientific basis for the establishment of exploration planning and the selection of exploration target areas for polyhalite minerals in the Sichuan Basin; The “two-storey potash deposits model” in southwestern Yunnan has been confirmed, which indicates prospects for the exploration of potash in the deeper Marine facies in southwestern Yunnan are likely to be successful. The discovery of a high concentration of rich bromite salt and potash salt in the Paleogene of the Kuqa depression and the southwestern Tarim region provides strong support for the likelihood large-scale potash deposits exist in these regions.     

湖北江陵凹陷科探井现场分析技术及其应用

江陵凹陷是江汉断陷盆地的一个面积宽广、矿产资源丰富的次级凹陷。通过对比钾盐等蒸发岩的分析技术,X射线荧光光谱技术(简称XRF)具有分析灵敏度高、样品制备简单、分析速度快等特点,有效的避免了岩盐和钾盐的易溶性和变化性所带来的影响。现场采集江陵凹陷科探井(SKD2)蒸发岩发育层段(327.11~437.70m)的岩屑样品,经处理、测定,分析结果与蒸发岩系盐类矿物成分具有较好的一致性。开展钻井实时化学监测技术研发、应用,为及时发现目标层、停钻取心提供数据支撑,对于蒸发岩勘查及钾盐找矿具有重要意义。     

显生宙全球海水化学成分演化及其对蒸发岩沉积的约束

通过搜集显生宙以来不同地质时期内海相碳酸盐岩鲕粒及胶结物矿物成分、钾盐矿床矿物种类及组合特征、蒸发岩盆地中石盐流体包裹体成分,并利用这些资料与人工海水模拟实验得到的石盐中Br分配特征的对比,得出海水成分在5.5亿年以来的显生宙期间,经历了五个阶段:其中晚元古代至寒武纪早期、二叠纪早期至中生代早期、新生代早期至现今,这些时期的原始海水组成特征系数m(SO_4~(2-))+m(HCO_3~-)/2m(Ca~(2+)),为Na-Mg-K-SO_4-Cl型海水,此期间沉积的钾盐矿床的钾镁盐矿物主要为钾盐镁矾、无水钾镁矾、杂卤石、硫酸镁石等含MgSO_4矿物,海相鲕粒和碳酸盐胶结物矿物成分为文石;而寒武纪早期至石炭纪、中生代早期至新生代早期,原始海水组成特征系数m(Ca~(2+))m(SO_4~(2-))+m(HCO_3~-)/2,为Na-Mg-KCa-Cl型海水,此期间沉积的钾镁盐矿物主要为光卤石和钾石盐,甚至含有溢晶石,海相鲕粒和碳酸盐胶结物矿物成分为方解石。根据石盐流体包裹体成分计算得出:显生宙期间,海水K+含量大部分时间变化幅度较小,为9.3～11.5mmol/kg H_2O(除了石炭纪和晚元古代),平均为10.55mmol/kg H_2O。Mg~(2+)含量在早寒武世≥67mmol/kg H_2O、晚志留世至中泥盆世31～41mmol/kg H_2O、晚古生代≥48mmol/kg H22O、晚白垩世34mmol/kg H_2O和现代55.1mmol/kg H_2O。Ca~+含量在晚元古代至古生代早期≤11mmol/kg H_2O、古生代早期至石炭纪22～35mmol/kg H_2O、石炭纪至中生代早期≤17mmol/kg H_2O、中生代早期至新生代早期19～39mmol/kg H_2O及新生代早期至今7～21mmol/kg H_2O。SO_4~(2-)含量在晚元古代至古生代早期≥23mmol/kg H_2O、古生代早期至石炭纪5～17mmol/kg H_2O、石炭纪至中生代早期13～22mmol/kg H_2O、中生代早期至新生代早期5～19mmol/kg H_2O及新生代早期至今12～29.2mmol/kg H_2O。海水Ca~(2+)与SO_4~(2-)含量的相对变化是控制海相钾盐矿床钾镁盐矿物类型的基本因素。同时,利用以上数据计算得到的显生宙各时期海水[m(Mg~(2+))+m(SO_4~(2-))]/[m(K~+)+m(Ca~(2+))]的变化与各时期海相蒸发岩系石盐层底部的Br含量变化具有同步性,进一步验证了显生宙期间海水成分是不断变化的,是约束海相蒸发岩钾盐矿物类型的主要因素。海水成分变化的控制因素为洋中脊热液和陆地水,其中洋中脊热液起主要作用,而控制这些因素变化的根本原因为板块构造运动。     

宣达地区中、下三叠统含杂卤石蒸发岩测井响应及岩性识别

钾盐主要用于农业中钾肥的生产,中国是钾盐消费和进口大国,但中国钾资源少、产量低,因此钾盐勘探对中国粮食安全有重要意义。四川盆地是中国找钾的重要区域,文章运用岩芯样品、测井等资料,通过X射线粉晶衍射、元素含量测试等实验对宣达地区中、下三叠统嘉陵江组-雷口坡组含盐系典型矿物特征及岩石测井响应特征进行分析,并基于测井解释方法原理对杂卤石进行识别。结果表明,研究区钾盐矿物包括氯化盐矿物、硫酸盐矿物、碳酸盐矿物3类,固态钾盐矿物主要为杂卤石,常与石盐、硬石膏共存,具有高电阻率、高密度、高伽马、高K的特征,主要分布于T₁j^4-2、T₁j^5-2层段。     

Dating the events of metamorphism and granitic magmatism in the Alpine orogen of Naxos (Cyclades,Greece)

An isotopic dating investigation (66 K-Ar and 34 Rb-Sr analyses) provided the geochronological framework for the Alpine events of metamorphism and granitic magmatism on Naxos. The oldest phase of high-pressure/medium-temperature metamorphism, M1, was dated by Rb-Sr and K-Ar analyses of paragonites, phengitic muscovites and muscovites at 45±5 Ma (Middle Eocene). Most of the record of the M1 phase has been eraded by a second phase of medium-pressure/high-temperature metamorphism, M2, which induced a metamorphic zonation with anatectic melting in the highest-grade part, the migmatite dome. Most K-Ar dates of M2 hornblendes, muscovites, biotites and tourmalines range from about 21 Ma in the lower-grade part (biotite-chloritoid zone) to about 11 Ma in the migmatite dome. From the pattern of K-Ar mineral dates it is concluded that the M2 phase took place 25±5 Ma ago (Late Oligocene/Early Miocene) and was followed by a prolonged cooling history until about 11 Ma ago (Late Miocene), when the ambient temperature in the migmatite dome had decreased to below 400–360 °C. A Rb-Sr whole-rock isochron analysis of a granodioritic mass dated the intrusion (and the associated M3 phase of contact-metamorphism) at 11.1±0.7 Ma (Late Miocene), with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7112 ±0.0001. A local phase of low-grade retrograde metamorphism, M4, probably related to Late Alpine overthrusting, was dated at about 10 Ma (Late Miocene).     

Limited mobility of argon in a metamorphic terrain

Excess ⁴⁰Ar in biotite from some relatively anhydrous charnockitic rocks in the Appalachian Piedmont indicates limited mobility of argon. Biotite from the Arden pluton of the granulite-facies Wilmington Complex apparently formed as a retrograde product at the expense of pyroxene and K-feldspar. Rb-Sr ages of biotite from all rocks are approximately 365 Myr. The same micas have apparent K-Ar ages which range from about 365–590 Myr, six of which clearly exceed the Sr isotope whole-rock date of 500 Myr. They contain variable amounts of excess⁴⁰Ar incorporated during crystallization or recrystallization of biotite at about 365 Myr ago. None of the other minerals appears to contain significant amounts of excess argon. The K-Ar apparent ages show strong, positive correlation with whole-rock K concentrations. These relations yield a correlation between excess argon in the biotite phase and rock potassium. This suggests that excess ⁴⁰Ar in biotite is of local derivation and is due to an imprint of the local argon activity. If the amount incorporated is roughly proportional to the prevailing argon partial pressure then substantial differences in p_Ar existed. Argon did not have a uniform chemical potential over large rock volumes. Analysis of closely spaced samples suggests different argon activity over the scale of less than 10m. This implies restricted transport of Ar and is probably due to very low effective permeability of the anhydrous assemblages.     

Mössbauer characteristics,mineralogy and isotopic age (Rb-Sr,K-Ar) of Upper Riphean glauconites from the UK Formation,the southern Urals

Comprehensive mineralogical analysis, Mössbauer spectroscopy and isotopic-geochronological study have been carried out for globular phyllosilicates (GPS) of glauconite group from the Uk Formation, the second one below the top of the Upper Riphean stratotype in the southern Urals. Glauconites have been sampled in the Kurtaza and Kulmas sections remote from each other in the Alatau anticlinorium that corresponds to western facies zone of the Bashkirian meganticlinorium. As is shown, size and density monomineral fractions of globules are represented by Al-glauconite according to established structural formula. The isotopic (Rb-Sr and K-Ar) dating of glauconites from the Uk Formation is performed for the first time along with computer simulation of cation arrangements in their crystal lattice and comparison of the results obtained with data of Mössbauer spectroscopy. It has been assumed by simulation that origin and transformation of the Rb-Sr and K-Ar systems in glauconite are concurrent to stages in structural evolution of this mineral, which have been controlled by geological and geochemical events in the history of sedimentary successions. The approach has been aimed at recognition of stratigraphically meaningful isotopic dates corresponding to the glauconite formation at the stage of the early diagenesis close to sedimentation time and the “rejuvenated” dates characterizing ages of subsequent geological events. The comparison of simulated cation arrangements with data of Mössbauer spectroscopy shows that the Rb-Sr (663 ± 9 Ma) and K-Ar (669 ± 16 Ma) dates established for glauconites correspond to the time of early diagenesis in their host sediments, being suitable for age assessment of the Uk Formation. The dates obtained are of interregional and wider significance, as they must be taken into consideration when constructing the general curve of Sr isotope variations in the Late Riphean oceans.     

中亚盆地钾盐矿床盐类矿物Rb-Sr同位素体系研究及其意义

中亚盆地钾盐矿床的形成时代目前被限定为晚侏罗世至早白垩世,较为宽泛.盐类矿物沉积之后若未被改造,可测定其形成时代;若被改造则可利用同位素测年研究其沉积后作用.为了得到中亚盆地钾盐矿床的形成时代和/或了解盐类矿物受到的后期改造过程,利用Rb-Sr同位素定年对矿床中的盐类矿物进行了测定.结果表明,无法形成可靠的Rb-Sr等...     

Isotope geochronological study of Late Palaeozoic granitic rocks in the Nanling Region,South China

Fourty-four isotopic ages have been determined by K-Ar and U-Th-Pb methods for Late Palaeozoic granitic rocks in the Nanling Region, South China. All dating values vary within the range of 231–348 m.y. From the obtained dates, further evidence has been found that there do exist Late Palaeozoic granitic rocks, which can be subdivided into Late Devonian and Permian granitic rocks. Within a Late Devonian terrain, there is a granitic pluton, namely granodiorite with a zircon U-Th-Pb age of 348 m.y., while ten granitic plutons have been recognized within a Permian terrain where granites are predominant, yielding biotite K-Ar ages of 236–289 m.y. (λ _β =4.72×10^?10yr.^?1,λ _K=5.57×10^?11yr.^?1) and zircon U-Th-Pb ages ranging from 231 to 280 m.y., respectively. It is obvious from the dates that intrusive activity of granitic magma extensively took place in the Nanling Region during Late Palaeozoic, although no records of orogenie movements have been found, indicating that the faults are the main factor controlling the activity of granitic magma, whereas the orogenic movements are not the only prerequisite for the formation of granitic magma and the intrusive activity.     

Classification and Recognition of Polyhalite in Chuanzhong Based on Support Vector Machine

Polyhalite is mainly solid mineral potassium in Sichuan Basin, The most of polyhalite layer in Sichuan region is impurity, and usually accompanied by layers of gypsum, anhydrite, rock salt, and even deposited in the same layer. Conventional logging interpretation method can only roughly identificate polyhalite layers. Based on the theory of Support Vector Machine and logging interpretation methods,this paper creates prediction model with the input of logging curves, and discriminates the polyhalite reservoirs in the lower-middle Triassic strata. Compared with logging data, the accuracy rate of the discrimination results reaches 90%. According to the prediction model, identification model can be established with the curve features of polyhalite to discriminate pure polyhalite reservoirs, gypsiferous polyhalite reservoirs and polyhalite-gypsum reservoirs, the accuracy rate is 91.78%. The study demonstrates that Support Vector Machine is superior to the method of logging interpretation, and it has broad prospects in potash exploration.     

Isotopic evidence of middle proterozoic magmatism from bombay high field: Implications to crustal evolution of western offshore of India

Precambrian granitic basement rocks obtained from well BH-36 of Bombay High Field, western offshore of India has been studied both by Rb-Sr and K-Ar dating methods. Seven basement samples chosen from two cores have yielded whole rock Rb-Sr isochron age of 1446 ± 67 Ma with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7062 ± 0.0012. This age has been interpreted as the formation/emplacement time of the granite. Two biotite fractions of different grain size separated from a sample CC6B2T have yielded Rb-Sr mineral isochron age of 1385 ± 21 Ma. However, these fractions when studied by K-Ar dating method have yielded slightly higher but mutually consistent ages of 1458 ± 43 Ma and 1465 ± 43 Ma, respectively. Further, two biotites separated from additional samples CC5B9T and CC6B3B have yielded K-Ar ages of 1452 ± 42 Ma and 1425 ± 40 Ma with an overall mean age of 1438 ± 19 Ma. This mean K-Ar age is indistinguishable from whole rock Rb-Sr isochron as well as mineral isochron age within experimental error. The similarity in the whole rock and biotite ages obtained by different isotopic methods suggests that no thermal disturbance has occurred in these rocks after their emplacement/formation around 1450 Ma ago. The present study provides the evidence for the existence of an important Middle Proterozoic magmatic event around 1400-1450 Ma on the western offshore of India which, hitherto, was thought to be mainly confined to the eastern Ghats, Satpura and Delhi fold belt of India. This finding may have an important bearing on the reconstruction of Proterozoic crustal evolution of western Indian shield.     

The age of the Kagenfels granite (northern Vosges) and its bearing on the intrusion scheme of late Variscan granitoids

In the Saxothuringian part of the Vosges (France), a first series of Variscan plutonic rocks (diorites to granites) has been intruded by several younger granites. Rocks of both the older generations have been cross-cut by the late orogenic Kagenfels granite. The averages of the hitherto published mineral ages of the earlier rock generations are 331 and 334 Ma, respectively, whereas Rb-Sr and K-Ar dates around 290 Ma have been reported for the Kagenfels granite. Because of the unlikely large age hiatus, a redetermination of the intrusion age of the Kagenfels granite formation appeared to be irrevocable. The newly obtained mineral ages on the Kagenfels granite (K-Ar and ⁴⁰Ar/³⁹Ar biotite ages as well as single zircon radiogenic ²⁰⁷Pb/²⁰⁶Pb data: 331 ± 5 Ma) are about 40 Ma older than the previous results. They are interpreted as giving the time of emplacement of the Kagenfels granite during the latest Visan. The mineral ages of the earlier plutonic rocks in this part of the Variscan Orogeny in all probability are not significantly different from their ages of intrusion. Therefore the age concordance of all three granitoid generations constrains a rather narrow time interval of orogenic magmatism close to the Lower-Upper Carboniferous boundary.