Rb/Sr geochronology in the Eastern Alps

New Rb/Sr data on mineral and whole rock samples from in and around the south-east corner of the Tauern Window are presented. Pennine orthogeneisses yield an Rb/Sr whole rock age of 279±9 m.y., while orthogneiss samples from the Altkristallin Sheet near Innerkrems, Carinthia, yield an age of 381±30 m.y. by the same technique. The apparent mineral age break across the margins of the Tauern Window is investigated in an area of good structural and petrofabric control. The post-Palaeozoic history of the Eastern Alps is then discussed in the context of the available Rb/Sr data. It is argued that the bulk of the Katschberg Phyllites are of pre-Mesozoic age; that the major overthrusting movements of the Austroalpine Units were completed by 60–65 m.y.; and that the Peri-Adriatic intrusives can be little older than middle Tertiary.     

Li,F and Rb contents and Ba/Rb and Rb/Sr ratios as indicators of postmagmatic alteration and mineralization in the granitic rocks of the Banke and Ririwai Younger Granite complexes,Northern Nigeria

The Banke and Ririwai complexes have plutonic phases of igneous activity composed mainly of granitic rocks. These granitic ring complexes are associated with Sn-Nb mineralization and are characterized by high Li, F and Rb contents and Rb/Sr ratios, and low Ba and Sr contents and Ba/Rb ratios. — The altered and mineralized granites have variable Rb/Sr and Ba/Rb ratios differing significantly from those of fresh rocks. These ratios as well as the Li, F and Rb concentrations are good indicators of granitic rocks associated with postmagmatic alteration and mineralization providing valuable tools for Sn-Nb exploration within the Nigerian Younger Granite province.     

Metasomatic control of K/Rb ratios in amphibolites

Amphibolite facies metamorphism of gabbroic dykes, together with their quartzo-feldspathic, granulite facies country rock (Archean basement, west Greenland), transformed the gabbroic rocks into amphibolites with remarkably high K/Rb values (up to 2300, average 727), reflecting correspondingly high K/Rb values in the country rock. A similar dyke transformation in an amphibolite facies complex (Precambrian basement, southwest Sweden) produced amphibolites with more common K/Rb values (average 375) similar to those found in the country rock. As the protolith chemistry and the P-T conditions of origin for the two amphibolite occurrences are very similar, it is concluded that the difference in K/Rb distribution is caused by the difference in country rocks. The K and Rb values in the two amphibolite occurrences lie between protolith and country-rock values, suggesting an exchange of K and Rb between basic rock and quartzo-feldspathic country rock, probably through the hydrothermal fluids which assisted in the amphibolite formation.     

地质年龄测定中高Rb／Sr样品部分除铷处理方法

介绍了地质年龄测定中对高Ｒｂ／Ｓｒ样品，用吸附了六硝基二苯胺的阴离子交换树脂除去部分铷元素的新方法，从而实现样品在阳离子交换柱上的铷锶完全分离。该方法可解决Ｒｂ／Ｓｒ＞２０样品在质谱分析测定中￣（８７）Ｒｂ干扰￣（８７）Ｓｒ的问题。文章中对Ｒｂ／Ｓｒ＞２０的地质样品部分除铷法与其它方法进行了对照测试，结果一致。部分除铷法比其它方法具有节省时间、操作简便的优点     

大别造山带花岗岩类和正片麻岩的Rb／Sr分区

根据花岗岩类和片麻岩的Ｒｂ／Ｓｒ比值和其他岩相学和地球化学性质,大别造山带可以分为下列四个带：（１）北大别北带：是一个灰色片麻岩和基性、超基性岩带,其灰色片麻岩的Ｒｂ／Ｓｒ比值为０．０１～０．０９;（２）北大别南带：为中酸性岩浆活动和强裂混合岩化的区域,其片麻岩的Ｒｂ／Ｓｒ比值为０．１１％～０．４０,花岗岩类为０．３％～０．９;（３）南大别带：为一构造混杂带,超高压变质作用和岩浆活动和混合岩化均有     

Determination of Rb/Sr and 87Sr86Sr ratios of some standard rocks and evaluation of X-ray fluorescence spectrometry in Rb1bSr geochemistry

New isotope dilution results are presented for Rb and Sr concentrations in U.S.G.S. standard rocks and NBS-70a K-feldspar. The results (based on at least five analyses of each rock), are generally accurate to ± 0.5% and resolve discrepancies in previously published data. X-ray fluorescence analyses of the same samples yield Rb and Sr determinations which are only accurate to ± 5%, but Rb/Sr ratios which are as precise and in excellent agreement with the isotope dilution values. It is concluded that X.R.F. determination of Rb/Sr ratios is perfectly suitable for whole-rock Rb1bSr geochronology.${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios have been determined on G-2, GSP-1, BCR-1 and AGV-1 as well as the Eimer and Amend SrCO₃ standard.     

湖泊沉积物Rb/Sr比值在古气候/古环境研究中的应用与展望

Rb和Sr在赋存矿物和表生地球化学行为等方面存在显著差异,因而各种记录体中的Rb/Sr比值被广泛应用于古气候/古环境研究。湖泊沉积物中的Rb、Sr主要包括2部分来源：一是流域物理侵蚀直接带来的陆源碎屑组分,在沉积物中多以残渣态（碎屑矿物）形式存在;二是流域化学风化带来的溶解态物质,在湖泊水体中通过物理吸附、化学沉淀和生...     

A distribution-free alternative to least-squares regression and its application to Rb/Sr isochron calculations

A distribution-free estimator of the slope of a regression line is introduced. This estimator is designated S_m and is given by the median of the set of n(n – 1)/2 slope estimators, which may be calculated by inserting pairs of points (X _i, Y_i)and (X _j, Y_j)into the slope formula S _i = (Y _i – Y_j)/(X _i – X_j),1 i < j n Once S _m is determined, outliers may be detected by calculating the residuals given by R_i = Y_i – S_mX_i where 1 i n, and chosing the median R_m. Outliers are defined as points for which |R_i – R_m| > k (median {|R _i – R_m|}). If no outliers are found, the Y-intercept is given by R_m. Confidence limits on R_m and S_m can be found from the sets of R_i and S_i, respectively. The distribution-free estimators are compared with the least-squares estimators now in use by utilizing published data. Differences between the least-squares and distribution-free estimates are discussed, as are the drawbacks of the distribution-free techniques.     

U/Pb,Sm/Nd and Rb/Sr geochronological and isotopic study of northern Sierra Nevada ophiolitic assemblages,California

Distinct ophiolitic assemblages occur as oceanic basement within three of the four regional tectonic belts of the northern Sierra Nevada. New U/Pb zircon, Sm/Nd and Rb/Sr data are presented for each assemblage, providing critical geochronological and isotopic constraints on the petrogenesis and tectonic evolution of the ophiolitic and associated ensimatic assemblages. Ophiolitic assemblages include from west to east the Smartville complex, Central belt and Feather River belt. The Smartville complex represents an island arc volcanic-plutonic sequence with a major late-stage sheeted dike swarm. The Sm/Nd systems from a wide compositional spectrum of rocks record a 178±21 Ma petrogenetic age and an _Nd(T)=+9.2±0.6. Zircon U/Pb systems on an uppermost dacite yield a 164±2 Ma age, and on a number of plagiogranite screens and dikes from the sheeted complex 162±1 Ma ages. The Central and Feather River belts are structurally complex polygenetic assemblages. The U/Pb zircon and Sm/Nd systems record major 205 Ma and 315 Ma petrogenetic events respectively both involving depleted mantle derived magmas. Such magmatism probably occurred in marginal basin/transform systems developed within an older oceanic depleted mantle basement regime. Both Sm/Nd and U/Pb zircon systems show local components of Proterozoic sialic material. The sialic contaminants were probably introduced into the system as craton derived detritus. It is doubtful that any of the ophiolitic assemblages studied represent genetically related crust-upper mantle sequences generated during the development of new oceanic lithosphere. Integration of the geochronological data with geological relations reveals a pattern of petrogenesis and tectonics whereby progressively younger ensimatic terranes were added to the continental margin through time by plate convergence, and were ultimately welded into North American sial by a crosscutting batholithic belt. This accretionary pattern is reflected in both the protolith ages and deformation-metamorphic ages of each of the regional belts which progressively young westward. Crustal components of the accreted ensimatic terranes grew by mainly basaltic igneous activity within island arc, marginal basin and leaky transform systems adjacent to the continent edge prior to final tectonic accretion. Such complexities are suggested to be typical of Cordilleran-type ophiolites and representative of the circum-Pacific erogenic style.     

Rb/Sr dating of the Upper Proterozoic basement of Zambesia,Mozambique

The Proterozoic basement of the Province of Zambezia (Mozambique) is located near the southern end of the so called Mozambique Belt. Rb-Sr data derived from the analyses of 66 whole rocks and a few mineral samples bear witness of a magmatic and metamorphic episode referable to the Kibaran (ca. 1000 Ma) and helped to model the late-Proterozoic evolution as follows. At 1100–1050 Ma a pre-orogenic, calc-alkaline magmatism was responsible for the emplacement of now deformed granite and granodiorite. This phase was accompanied by rhyolitic volcanism. A subsequent peak of deformation and metamorphism occurred at 1000-950 Ma and was associated with a second generation of substantially post-tectonic granites. A thermal event resulting in the emplacement of a final generation of granites and pegmatites can be dated to around 500-450 Ma.The metamorphic basement has not preserved its protolith age, probably as a result of extensive isotopic homogeneization of Sr in Kibaran times. Its maximum time of differentiation from the mantle should be around 1600 Ma.The very low values of initial Sr isotopic ratios suggest the absence of Archean crustal precursors and support the rather young, yet pre-Pan-African evolution of the southernmost Mozambique Belt.

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Zusammenfassung Das proterozoische Grundgebirge der Provinz Zambesi (Mosambique) befindet sich in der Nähe des südlichen Endes des sogenannten Mozambique Belt. Rb/Sr Daten aus 66 Ganzgesteins- und einigen Mineralanalysen zeugen von einer magmatischen und metamorphen Episode die sich dem Kibaran (ca. 1000 Ma) zuordnen läßt, und wie sich im folgenden zeigt, hilfreich bei der Modellisierung der spätproterozoischen Entwicklung ist. Um 1100 bis 1050 Ma erfolgte ein präorogener kalkalkalischer Magmatismus, bestehend aus Graniten und Granodioriten, die heute deformiert vorliegen. Diese Phase wurde von rhyolithischem Vulkanismus begleitet. Von 1000 bis 950 Ma folgte der Höhepunkt der Deformation und Metamorphose, begleitet von einer bedeutenden 2. Generation posttektonischer Granite. Bedingt durch ein weiteres thermales Ereignis intrudierte die letzte Granit- und Pegmatitgeneration vor 500 bis 450 Ma.Im metamorphen Grundgebirge ist das Eduktalter vermutlich aufgrund der starken isotopischen Homogenisierung von Sr während der Kibara-Zeit nicht erhalten. Seine maximale Differentiationszeit vom Mantel muß vor 1600 Ma gewesen sein.Die sehr niedrigen Gehalte der initialen Sr-Isotopenverhältnisse lassen die Abwesenheit von archaischen Krustenvorläufern vermuten, und unterstützen die sehr junge, bis jetzt Prä-Pan-Afrikanische Entwicklung des südlichsten Mozambique Belts.

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Résumé Le socle protérozoïque de la province du Zambèze, au Mozambique, se trouve à proximité de l'extrémité sud du «Mozambique Belt». Des mesures Rb/Sr effectuées sur 66 roches totales et sur quelques minéraux témoignent d'un épisode magmatique et métamorphique attribuable au Kibarien (environ 1.000 Ma), ce qui permet de se représenter comme suit l'histoire de cette région au Protérozoïque supérieur. Il y a 1.100 à 1.050 Ma, la région a connu une phase de magmatisme calco-alcalin qui s'est traduite par la mise en place de granites et granodiorites, aujord'hui déformés. Cette phase s'est accompagnée d'un volcanisme rhyolitique. Elle a été suivie, entre 1.000 et 950 Ma, d'une activité tectono-métamorphique majeure, à laquelle est associée une deuxième et importante génération de granites, post-tectoniques. Ultérieurement, la région a connu encore un épisode thermique, daté à 500-450 Ma, responsable de la mise en place d'une dernière génération de granite et de pegmatite.Le socle métamorphique n'a pas gardé de témoins de l'âge de son protolithe, en raison probablement de la forte homogénisation isotopique du Sr à l'époque kibarienne. Lâge maximal de sa différenciation à partir du manteau doit se situer autour de 1.600 Ma.La valeur très basse des rapports isotopiques initiaux du Sr suggère l'absence de précurseurs crustaux archéens et incite à admettre une évolution relativement jeune — bien que pré-panafricaine — du «Mozambique Belt» méridional.

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() .. . , Rb/Sr 66 , , Kibara, 1000 , , , . 1100 1050 - - , , . . 1000 950 , - . , 500 450 . , , , Kibara. , , 1600 my . - .

     

Constraints on the origin of Damaran granites by Rb/Sr and δ18O data

The mean (⁸⁷Sr/⁸⁶Sr) and mean (⁸⁷Rb/⁸⁶Sr) ratios of the intrusive granites from the North and South of the Orogen's Central Zone plot on straight lines. These are interpreted as areal isochrons indicating the time of last Sr isotope homogenization 526 and 571 Ma ago in the respective source rocks.Initial (⁸⁷Sr/⁸⁶Sr) and mean (⁸⁷Rb/⁸⁶Sr) ratios of approximately coeval granites of the main magmatic pulses in the North (470 Ma) and in the South (520 Ma) line up along isochrons suggesting that in both cases 60 Ma passed after the homogenization in the protolith before large scale intrusions took place.The data require that the intrusions have preserved the Rb/Sr ratios of their source rocks permitting only very little assimilation or fractionation.The source rocks in the North and South had rather unradiogenic Sr 526 and 571 Ma ago, respectively. At R_i0.7066 all presently known Damaran metasediments and metavolcanics as well as the basement must be excluded as the protoliths. A hypothetical source with a large proportion of low (⁸⁷Sr/⁸⁶Sr) volcanic material is required. In the center, on the other hand, the Sr isotope ratios are more radiogenic and derivation from common Damaran metasediments is a distinct possibility.The total rock ¹⁸O values show an unusual spread from 7.1 to 15.2, the majority being very heavy. This excludes granulites and requires sediments or heavily altered volcanics as source rocks. A plot of ¹⁸O vs. initial Sr isotope ratios of the granites from the center which could have been derived from Damaran metasediments has a very clear negative slope. No trend is visible for the southern granites. Of the northern granites the older group shows a negative, the younger group a positive correlation. This is interpreted as indicating mainly altered volcanics (perhaps spilites) for the older and a mixture of volcanogenic and metasedimentary rocks as the source for the younger group. The high ¹⁸O values show that the granites are crustal remelts.     

Rb/Sr geochronology of granites and gneisses from the Mount Everest region,Nepal Himalaya

Nineteen samples of granites, orthogneisses and paragneisses from the High Himalaya basement nappe(s) of the Mount Everest region have been dated by the Rb/Sr method. The post-metamorphic tourmaline leucogranites of the upper Imja Drangka (Nuptse, Lhotse Glacier) have high initial Sr⁸⁷/Sr⁸⁶ characteristic of an anatectic origin from crustal material. A whole-rock isochron age of 52 m. y. (Early Eocene) has been obtained for the samples from the granite body of Lhotse Glacier; apparently Sr isotopic homogenization was not reached throughout the much larger Nuptse granite. The granite precursor of the migmatitic orthogneisses from the upper Dudh Kosi valley has an age of 550 ± 16 m. y. (whole rock isochron) and a high initial Sr⁸⁷/Sr⁸⁶ ratio indicating its origin from an older basement complex. The Rb/Sr data on paragneisses from the south face of Lhotse do not define an isochron, possibly reflecting isotopic hetero-geneity in the sedimentary protoliths and incomplete homogenization during a late Precambrian metamorphism. All the mineral ages fall in the time span from 15 to 17 m. y. They represent cooling ages reflecting a regional phase of major uplift in the Middle Miocene and post-dating the peak of the Himalayan metamorphism which the data from the Mt. Everest region place in pre-Eocene times.     

Rb/1bSr isochrons for the granitic plutons around Farsund,southern Norway

The granitic and charnockitic intrusions in the Precambrian migmatites around Farsund in southern Norway have previously been shown to be successively intruded as well as being geochemically different. Rb- and Sr-isotope data support the conclusion that they are intruded with a separate time interval and that the Farsund charnockite is younger (852±41 m.y.) than the Lyngdal hornblende granite (932±38 m.y.). Furthermore, the different initial ratios (0.7128 and 0.7054, respectively) disprove a comagmatic origin unless large-scale contamination has occurred.     

湖光岩玛珥湖非残渣态Rb/Sr比值研究及其古气候意义

湖泊沉积物Rb/Sr比值变化主要受控于Sr含量的变化,而真正反映流域化学风化强度的主要是迁移至集水盆地(湖泊)的溶解态Sr2+,在沉积物中主要以碳酸盐结合态和铁锰氧化物结合态等形式存在.通过对不同赋存状态的Rb/Sr比值与相应的Sr含量之间相关关系的对比分析发现,沉积物非残渣态Rb/Sr比值更好地反映了流域化学风化.根...     

Spatial and temporal variations of Rb/Sr ratios of the bulk surface sediments in Lake Qinghai

The Rb/Sr ratios of lake sediments have been suggested as indicators of weathering intensity by increasing work. However, the geochemistry of Rb/Sr ratios of lake sediments is variable between different lakes. In this study, we investigated the spatial and temporal patterns of Rb/Sr ratios, as well as those of other major elements in surface sediments of Lake Qinghai. We find that the spatial pattern of Rb/Sr ratios of the bulk sediments correlates well with that of the mass accumulation rate, and those of the terrigenous fractions, e.g., SiO₂, Ti, and Fe. The temporal variations of Rb/Sr ratios also synchronize with those of SiO₂, Ti, and Fe of each individual core. These suggest that Rb/Sr ratios of the surface sediments are closely related to terrigenous input from the catchment. Two out of eight cores show similar trends between Rb/Sr ratios and precipitation indices on decadal scales; however, the other cores do not show such relationship. The result of this study suggests that physical weathering and chemical weathering in Lake Qinghai catchment have opposite influence on Rb/Sr ratios of the bulk sediments, and they compete in dominating the Rb/Sr ratios of lake sediments on different spatial and temporal scales. Therefore, it is necessary to study the geochemistry of Rb/Sr ratio of lake sediments (especially that on short term timescales) particularly before it is used as an indicator of weathering intensity of the catchment.     

Rb/Sr isotopic and geochemical evolution of a recrystallized shear (mylonite) zone at Broken Hill

Relatively narrow (1 to 1,000 metres) planar zones of intense shear deformation and retrograde metamorphism (retrograde shist zones, RSZ) are common in the Willyama Complex around Broken Hill. A Rb/Sr isotopic study of one of these zones has revealed an unexpected isotopic pattern. All of the analysed points lie to the left of and above the 1665 Ma isochron of the Potosi Gneiss host. This result indicates a different isotopic composition of the zone as a whole rather than a chemical redistribution of Potosi Gneiss within a closed system. The isochron plot scatter is similar to that of ubiquitous metamorphic pegmatites in the basement of the Broken Hill region. The shear zone observation, however, is shown to be unrelated to the pegmatite segregation process and requires the introduction, by elemental exchange, of a more radiogenic strontium, whilst maintaining relatively fixed Rb/Sr ratios. Significant proportions of SiO₂, K₂O and Ba were removed at the same time. Na₂O, Al₂O₃, TiO₂, MgO, P₂O₅, Zr, Ni, Sc, Y, Nb, Rb remain essentially constant after allowing for volume loss of the removed elements. CaO and Sr increase slightly overall.This pattern of elemental exchange is consistent with the breakdown of K-feldspar to muscovite and the essentially isochemical recrystallization of the other major phases (quartz, plagioclase and biotite) that are observed petrographically. Silica produced by the K-feldspar breakdown and a significant amount of the free quartz must also have been removed. Large scale fluid transport is required to achieve these results.The total rock isochron gives no indication of the age of the retrograde events. However two biotite-total rock joins indicate that the last internal isotopic redistribution occurred 458 Ma ago, shortly after the time of a regional low grade metamorphism. ⁸⁷Sr/⁸⁶Sr v 100/⁸⁶Sr plots confirm that a mixing process took place at about this time and not at 1,490 Ma or the present day.The introduced fluid had an ⁸⁷Sr/⁸⁶Sr ratio of about 0.794. If this figure still represents the original source value then it has come from what was long established mature crust at 458 Ma and not the mantle. The large fluid volumes required for the silica dissolution preclude a source in the local rock porosity, and suggest a meteoric fluid which has reacted extensively with the Willyama Complex metamorphics.     

RbSr studies of CI and CM chondrites

RbSr whole rock analyses have been performed on 2 CI and 3 CM chondrites. Four of these stones (Ivuna, Orgueil, Cold Bokkeveld and Erakot) were previously studied in this laboratory and were shown to be discordant from a 4.6 Gyr isochron. The fifth, Murchison, was not previously studied. The new data support the discordance of the first four stones, and indicate that Murchison is also discordant. Studies of Sr isotope ratios in unspiked Orgueil show that the discordance is not due to inhomogeneities in the $\text{Sr}{}^{84}\text{Sr}{}^{86}$ ratio caused by incomplete mixing of nucleosynthesis products.In order to gauge the effects of weathering, two leaching experiments were performed on fresh, interior samples of Murchison; one for a period of 1.5 hr and the other for 117 hr. The results indicate that the relative solubility of nonradiogenic Sr is approximately twice that of Rb and radiogenic Sr is more soluble than the nonradiogenic Sr. This gives the residue a lower model age than the whole rock both by increasing the $\text{Rb}\text{Sr}$ ratio and by decreasing the $\text{Sr}{}^{87}\text{Sr}{}^{86}$ ratio. This result is in contrast to that expected from studies of ordinary chondrite finds, which generally show higher model ages than falls. The constancy of $\text{K}\text{Rb}$ and $\text{K}\text{Sr}$ ratios between the two leaching experiments, and their difference from the unaltered whole rock ratios suggest that the bulk ratios are produced by dissolution of a single phase, and the higher radiogenic Sr content by selective leaching of other phases.     

Rb and Sr partitioning between haplogranitic melts and aqueous solutions

Rubidium and strontium partitioning experiments between haplogranitic melts and aqueous fluids (water or 1.16-3.56 m (NaCl + KCl) ± HCl) were conducted at 750-950 °C and 0.2-1.4 GPa to investigate the effects of melt and fluid composition, pressure, and temperature. In addition, we studied if the applied technique (rapid and slow quench, and in-situ determination of trace element concentration in the fluid) has a bearing on the obtained data. There is good agreement of the data from different techniques for chloridic solutions, whereas back reactions between fluid and melt upon cooling have a significant effect on results from the experiments with water.The Rb fluid-melt partition coefficient shows no recognizable dependence on melt composition and temperature.For chloridic solutions, it is ∼0.4, independent of pressure. In experiments with water, it is one to two orders of magnitude lower and increases with pressure. The strontium fluid-melt partition coefficient does not depend on temperature. It increases slightly with pressure in Cl free experiments. In chloridic fluids, there is a sharp increase in the Sr partition coefficient with the alumina saturation index (ASI) from 0.003 at an ASI of 0.8 to a maximum of 0.3 at an ASI of 1.05. At higher ASI, it decreases slightly to 0.2 at an ASI of 1.6. It is one to two orders of magnitude higher in chloridic fluids compared to those found in H₂O experiments. The Rb/Sr ratio in non-chloridic solutions in equilibrium with metaluminous melts increases with pressure, whereas the Rb/Sr ratio in chloridic fluids is independent of pressure and decreases with fluid salinity.The obtained fluid-melt partition coefficients are in good agreement with data from natural cogenetic fluid and melt inclusions. Numerical modeling shows that although the Rb/Sr ratio in the residual melt is particularly sensitive to the degree of fractional crystallization, exsolution of a fluid phase, and associated fluid-melt partitioning is not a significant factor controlling Rb and Sr concentrations in the residual melt during crystallization of most granitoids.     

江南隆起带安徽宁国西坞口矿床铷的赋存状态及其成矿机制研究

西坞口矿床是江南隆起带安徽宁国市近年来发现的首个关键金属铷(Rb)矿床,与成矿有关的岩浆岩为花岗斑岩。矿床中发育两类铷矿体:矿体I赋存于花岗斑岩体内,矿体II产于花岗斑岩体顶部。两类矿体的矿物组合均为多硅白云母+石英。矿体I呈透镜体状产于花岗斑岩体顶部,多硅白云母含量可达70%,品位为0.25%;矿体II产于花岗斑岩体和地层接触带,品位为0.21%。前人对西坞口矿床的矿化特征、岩浆岩年代学、岩浆岩成因开展了研究,但关于铷的赋存状态及成矿机制尚不明确。本次工作分别对成矿岩体和两类矿体中的主要矿物开展了电子探针(EPMA)和激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)研究,分析结果显示花岗斑岩中斜长石、钾长石和黑云母中Rb的平均含量分别为3.10×10^-6、872×10^-6和1 328×10^-6,矿体I和II中多硅白云母的Rb平均含量分别为2 544×10^-6和2 686×10^-6。多硅白云母晶体化学计算表明,铷以Rb⁺→K⁺/Na⁺置换的方式进入多硅白云母晶格,在多硅白云母中以类质同象的形式赋存。结合矿床地质特征和区域岩浆岩对比分析,本文提出西坞口矿床成矿岩浆具有较高的Rb背景值以及氟(F)含量,矿物结晶作用使得Rb在残余岩浆含量升高,岩浆向热液过渡阶段的富F特征导致原先岩浆体系的架状硅酸盐结构变成层状,进而有利于结晶出大量富Rb的多硅白云母,并富集形成铷矿体。     

Die Datierung geologischer Eriignisse im Altkristallin der Gleinalpe (Steiermark) mit der Rb/Sr-Methode

Zusammenfassung Die Datierung von Gneislagen einer Amphibolit-Gneis-Serie aus dem Kern der Gleinalpe, die dem Altkristallin angehört, ergibt ein Isochronenalter von 500±45 Mio. Jahre mit einem initialen Sr-Verhältnis von 0,7044±0,0012. Das geologische Erscheinungsbild weist auf vulkanische Entstehung des Ausgangsmaterials der Serie an der Wende Kambrium/ Ordoviz hin. Andere Gneistypen granitischer Zusammensetzung ergeben variszische Alter, deren Bedeutung (Bildungsalter der Granite oder Sr Homogenisierung während der variszischen Metamorphose) noch offen ist. Glimmer aus diesen Granittypen ergeben Alter von 81±9 bzw. 76±3 Mio Jahre für Muskovit bzw. Biotit und unterstreichen den starken thermischen Einfluß während der alpidischen Metamorphose in dem untersuchten Gebiet.

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Rb/Sr dating of geological events in the Altkristallin of the Gleinalpe (Steiermark)

Summary Plagioclase-quartz-gneisses from a series consisting of amphibolite and gneiss layers from the core of the Gleinalpe, which forms part of the Altkristallin, yielded a Rb-Sr isochron age of 500±45 m years, the initial Sr isotope ratio being 0.7044±0.0012. The geological state points to a volcanic origin of the series at the Cambrian/Ordovician boundary.Granitic gneisses indicate a Variscan age which might represent the time of rock formation or Sr homogenisation during metamorphism. Biotite from these granites gives an age of 76±3 and muscovite 81±9 m years. Both ages stress the strong thermal influence of the Alpine metamorphism in the area under investigation.

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