Isotopic age determinations of crystalline rocks of the Upper Harz Mountains,Germany

Dating of crystalline rocks occurring in the Upper Harz Mountains was carried out by means of U-Pb isotope investigations on zircons and sphene as well as by Rb-Sr isotope measurements on whole-rock samples and biotites.U-Pb data of euhedral and rounded, detrital zircons of the allochthonous block of the Ecker gneiss point to an age (upper intercept) of the source area of the predominantly metasedimentary rocks of about 1.6 Ga or older. The lower intercept indicates a possible metamorphic event at ca. 560 Ma in the Ecker gneiss or in the source rock of the zircons of this complex. The concordant data point of a sphene fraction from a metavolcanic sample documents contact metamorphic influence on the Ecker gneiss by the Variscan intrusions of the Upper Harz Mountains.Emplacement of the intrusions of the Harzburg gabbronorite and the Brocken and Oker granites occurred contemporaneously 293–297 Ma ago within the analytical error limits. This points to a common geotectonic cause of the ascent of the magmas. Uplift of the Ecker gneiss block, now squeezed in between Harzburg gabbronorite and Brocken granite, was connected to these processes.The mineral ages of the plutonites are close to the intrusion ages indicating fast cooling processes in shallow crustal levels.

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Zusammenfassung Eine altersmäßige Einstufung der im Oberharz vorkommenden kristallinen Gesteine wurde durch U-Pb-Isotopenuntersuchungen an Zirkonen und Titanit sowie Rb-Sr-Isotopenmessungen an Gesamtgesteinsproben und Biotiten durchgeführt.U-Pb-Daten von idiomorphen und runden, detritischen Zirkonen der allochthonen Eckergneis-Scholle deuten auf ein Alter des Ursprungsgebietes der weitgehend metasedimentären Gesteine von etwa 1. 6 Ga oder älter hin und sie weisen ferner auf ein mögliches Metamorphose-Ereignis vor rund 560 Ma im Eckergneis oder im Liefergestein der Zirkone des Eckergneises hin. Der konkordante Datenpunkt bei ca. 295 Ma einer Titanit-Fraktion aus einer Metavulkanit-Probe dokumentiert die kontaktmetamorphe Beeinflussung des Eckergneises durch die variskischen Intrusionen des Oberharzes.Die Platznahme der Intrusionen der Gesteine des Harzburger Gabbronorit-Massivs sowie des Brokken- und Oker-Granits erfolgten innerhalb der analytischen Fehlergrenzen zeitgleich vor 293–295 Ma. Dies deutet auf eine gemeinsame geotektonische Ursache des Aufdringens der Magmen. Damit verbunden war der Aufstieg der Eckergneis-Scholle, die jetzt zwischen Harzburger Gabbronorit und Brockengranit »eingeklemmt« ist.Mineralalter der Plutonite liegen nahe bei den Intrusionsaltern und weisen auf rasche Abkühlungsvorgänge in seichtem Krustenniveau hin.

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Résumé Des mesures d'âge ont été effectuées sur des roches cristallines de l'Oberharz par les méthodes de l'U-Pb sur zircon et sphène et du Rb-Sr sur roches totales et sur biotites. La méthode U-Pb, appliquée aux zircons idiomorphes et détritiques arrondis de l'écaille allochtone des gneiss d'Ecker, fournit un âge (intersection supérieure) d'environ 1,6 Ga au moins pour la source des matériaux surtout métasédimentaires. L'intersection inférieure indique la possibilité d'un événement métamorphique à ± 560 Ma soit dans les gneiss d'Ecker, soit dans les roches-sources de leurs zircons. Un résultat concordant à ± 295 Ma fourni par le sphène d'une métavolcanite enregistre l'action du métamorphisme de contact engendré dans les gneiss d'Ecker par les intrusions varisques.La mise en place des intrusions du massif gabbronoritique du Harzburg et celle des granite de Brocken et d'Ocker ont été contemporaines: âges de 293 à 297 Ma, compris dans l'intervalle d'approximation des mesures. Ce résultat est en faveur d'une même cause géotectonique pour l'ascension de ces magmas. C'est en relation avec ce processus que s'est produit le soulèvement de l'écaillé des gneiss d'Ecker, actuellement coincée entre le gabbro du Harzburg et le granite de Brocken.Les âges minéraux des plutons sont voisins des âges d'intrusion, ce qui indique un refroidissement rapide dans un niveau crustal peu profond.

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天山托洋峰花岗质岩石的同位素地球化学特征

分析了西天山托木尔峰地区花岗质岩石和基底岩石的稀土元素,微量元素以及Ｏ、Ｐｂ、Ｓｒ、Ｎｄ同位素组成。本区北部伊犁地块基底岩石的Ｎｄ同位素模式年龄约为１．９Ｇａ,南部南天山褶皱带的约为０．７０５到０．７３３）表明本区花岗质岩石主要为地壳溶融产物。Ｏ、Ｐｂ、Ｓｒ、Ｎｄ同位素组成批示位于伊犁地块和南天山褶皱带的花岗岩源区岩石的Ｎｄ模式年龄都相当于伊犁地壳,但源区性质不同,前者源区δ＾１８Ｏ低,μ值低,Ｔ     

鄂西黄陵花岗岩基同位素年龄谱

采用Isoplot程序重新计算了鄂西黄陵花岗岩基主要岩套的Rb-Sr等时线年龄和锆石U-Pb一致曲线年龄,测定了各种类型脉岩的侵入年龄,最后获得该岩基岩浆活动的年龄谱是:主体太平溪英云闪长岩岩套和黄陵庙花岗闪长岩岩套的侵位年龄分别是(833±29)Ma和(819±7)Ma,大老岭石英二长花岗岩岩套是(786±54)Ma,脉岩套侵入年龄从花岗闪长斑岩的813 Ma开始,到770Ma的辉绿岩和772Ma的石英脉侵入,整个岩基抬升作用完成,岩浆活动基本结束.在印支期(217±7)Ma时,岩基上NNW和NNE向大断裂发生了强烈的再次活动.Nd,Sr同位素证据表明,除大老岭岩套外,其他三个岩套的岩浆演化是连续的,这些岩浆起源于晋宁期扬子板块北缘发生的大洋板块消减作用.     

Absolute age determinations of crystalline rocks of Manali-Jaspa region,Northwestern Himalaya

Comparison of “argon” absolute age determinations of crystalline rocks of the “Crystalline Nappe” (Kumar, 1971, unpublished Ph. D. thesis) and of the “root-zone” (Jaspa granite) has led to an important conclusion concerning the interpretation of the development of the “crystalline nappe” and its rejuvenation.     

Geochemistry of biotites from granitic rocks,Northern Portugal

The biotites from a series of rocks ranging in composition from tonalite to granite have been analysed for both major and trace elements.The relations between chemical composition and paragenesis of the biotites are studied. Most biotites co-exist with potassium feldspar and ilmenite. Variations in composition can be correlated with the occurrence of amphibole, primary muscovite and aluminosilicates in the rocks.Variation diagrams of the trace element contents and element ratios of biotite are compared to those of the host rocks. Fractionation of elements can be defined more accurately as the influence of other mineral phases is eliminated.Variations in the proportions of the octahedrally co-ordinated Al, Ti and Fe³⁺ are correlated with the conditions of crystallization and comparisons made with biotites from other suites of calc-alkali rocks.In the light of the experimental data available, the petrographic observations and the chemical data it is apparent that biotites crystallized from systems in which fO₂ was buffered, its values remaining close to that of the buffer FMQ. From the same data, a temperature of 800°C for fO₂ = 10^?14to 10^?15 bars is deduced as prevalent during the crystallization of the tonalites while for the granites, at a temperature of crystallization of 680°C, fO₂ = 10^?16to 10^?18 bars.A calc-alkali trend of fractionation is therefore apparent with decreasing fO₂ while fH₂O₂ remains relatively high.     

天山石炭-二叠纪后碰撞花岗质岩石地球化学研究

在对前人研究资料综合的基础上,重点对天山造山带不同构造单元中石炭—二叠纪后碰撞花岗岩地球化学特征进行了对比研究,认为天山石炭—二叠纪后碰撞花岗质岩石属高钾钙碱—钾玄岩系列,以钙碱系花岗质岩石类型为主。其中,发育在裂谷带中的石炭—二叠纪花岗质岩石较发育在古微地块中或其边缘的花岗质岩石富MgO、TiO2、Na2O,贫K2O、Nb、Th,两者具有不同的源区。裂谷带中的花岗质岩石多与火山岩同源,由含不同程度壳源组分的壳幔混合源岩浆形成,而古微地块花岗质岩石主要来自壳源。处于同一类构造位置但不同地区的花岗质岩石化学成分也不相同,如天山西端伊犁地块南缘的托木尔峰花岗质岩石较天山中东段南部的库鲁克塔格花岗质岩石富Fe、Mg、K、Nb、Sr,而贫Zr、Ba,它们均是壳源成因,反映了天山东、西地壳基底组分的不同。石炭—二叠纪裂谷带中的花岗质岩石在天山东、西段也有差别,东段的觉罗塔格和博格达花岗质岩石较天山西段伊犁裂谷中的花岗质岩石明显地富Nb、Th而贫Sr、Ba,反映出东、西天山石炭—二叠世花岗岩源区不同。花岗质岩石的正εNd值并不是来源于与基性、超基性岩浆活动有关的幔源组分,而是来自于幔源火山岩浆活动。     

Isotopic and morphological features of fracture calcite from granitic rocks of the Tono area,Japan: a promising palaeohydrogeological tool

This study aimed to develop a methodology for assessing the hydrochemical evolution of a groundwater system, using fracture-filling and fracture-lining calcite. Fracture calcite in deep (to ca. 1000 m) granitic rocks of the Tono area, central Japan, was investigated by optical and electron microscopy, and chemical and isotopic analysis. Coupled with geological evidence, these new data imply 3 main origins for the waters that precipitated calcite: (1) relatively high-temperature hydrothermal solutions, precipitating calcite distinguished by δ¹⁸O_SMOW from −3 to ca. 10‰, and with δ¹³C_PDB from ca. −18 to −7‰; (2) seawater, probably partly of Miocene age, which precipitated calcite distinguished by δ¹³C_PDB of ca. 0‰ and δ¹⁸O_SMOW > ca. 20‰; (3) fresh water, with a variable δ¹³C_PDB composition, but which precipitated calcite distinguished by δ¹³C_PDB that was significantly < 0‰ and as low as ca. −29‰ and δ¹⁸O_SMOW > ca. 17‰. Data for ¹⁴C suggest that at least some of the fresh-water calcite formed within the last 50 ka. The present day hydrogeological regime in the Tono area is also dominated by fresh groundwater. However, the marine calcite of probable Miocene age found at depth has shown no evidence for dissolution and many different calcite crystal forms have been preserved. Studies of other groundwater systems have correlated similar crystallographic variations with variations in the salinity of coexisting groundwaters. When this correlation is applied to the Tono observations, the calcite crystal forms imply a similar range of groundwater salinity to that inferred from the isotopic data. Thus, the present study suggests that even in presently low-salinity groundwater systems, calcite morphological variations may record the changing salinity of coexisting groundwaters. It is suggested that calcite morphological data, coupled with isotopic data, could provide a powerful palaeohydrogeological tool in such circumstances.     

Rare earth elements in granitic rocks

The younger granites in Finland contain more REE than the older ones. In the youngest, postorogenic rapakivi granites, the total REE concentration is highest, the light REE are more enriched, and the negative Eu anomaly is more pronounced than in the older granites. The enrichment of the light REE, the anomalous behavior of the extreme elements (La, Ce, and Lu) in normalized graphs, and the depletion of Eu indicate the degree of differentiation the rock has undergone. These features are usually more pronounced in large, homogeneous granites than in metamorphic or volcanogenic rocks. Silicic vein rocks usually contain less REE than the granites proper; the distribution pattern in many is as in granites, but in some the heavy elements are more enriched. The positive Eu anomaly in Precambrian metamorphic rocks is tentatively attributed to metamorphic differentiation and to the secretion of silicic material from the host rock.     

Phosphorus in granitic rocks of Colombia

One hundred and three samples of granitic rock taken systematically from the Andean part of Colombia reveal that the observed variation in phosphorus content reflects the variation observed for the wholerock chemical composition. This relationship was established using a form of trend analysis termed COMTRENA and information-theory statistics.     

The microstructures of microcline from some granitic rocks and pegmatites

Numerical simulations of the growth of a large crystal face of plagioclase in response to an instantaneous undercooling below the equilibrium temperature are presented for model granodiorite and basalt melts with varying water contents. The simulations suggest that the anorthite content of plagioclase decreases uniformly from the composition in equilibrium with the bulk melt as undercooling is increased, and that the water content in the melt has little influence on this result. Comparison of the simulations with sharp compositional changes in natural profiles suggests that undercoolings of tens of degrees C can be rapidly imposed on plutonic phenocrysts. Large changes of undercooling most likely result from chilling of the magma and local convection around growing crystals. The observation in experiments that growth rate does not increase rapidly with increasing water content in the starting melting composition can be attributed to the concentration of water at the crystal face during growth; the action of water to reduce liquidus temperature and undercooling has a greater effect on growth rate than its action to increase transport rates. Even at large undercooling, there is no significant increase in temperature at the interface caused by the release of heat of crystallization.Simulations are presented to illustrate how disequilibrium growth processes due to undercooling can modify the normal zoning profiles expected from fractionation. Assuming that an undercooling is necessary to cause nucleation, normal zoning can result if crystal growth takes place at constant or increasing undercooling, but reverse zoning can occur at decreasing undercooling. Undercooling during growth is controlled by the relative rate of cooling and the rate at which the liquidus temperature is decreased by the accumulation of residual components and volatiles in the melt. Consequently, normal zoning should be promoted by rapid cooling, contemporaneous crystallization of other phases, and absence of volatiles, while reverse zoning should be expected in phenocrysts grown in slowly-cooled melts or in melts where volatiles are concentrated. The zoning patterns found in many plutonic plagioclase crystals suggest that their compositions are in significant disequilibrium with the melt; consequently, they are unsuitable for use in geothermometers.Approximate calculations suggest that the amount of water concentrated at the surface of growing phenocrysts in plutons can promote local convection. Comparison of simulated and observed oscillatory zoning profiles suggests that oscillatory zoning is not explained by a re-nucleationdiffusion model (Harloff 1927), but is readily explained by periodic local convection.     

Sulfide and oxide minerals from S-type and I-type granitic rocks

In the Ohmine I-type and S-type granitic rocks of Miocene age, Kii peninsula, central Japan, ilmenite, rutile, pyrrhotite, pyrite and chalcopyrite are common, but no magnetite is present. The primary paragenetic relations of these minerals are determined by their modes of occurrence as inclusions within major plagioclase, orthoclase, quartz, hornblende and biotite, although their parageneses changed during cooling. Significant subsolidus reactions for sulfide minerals took place in both the groundmass and the inclusions until the rocks cooled below 300°C. The loss of S and Cu through silicate crystals was too easy to preserve the chemistry of sulfides included in silicates. However, the removal of Ni through silicate crystals as well as along grain boundaries was restricted, as indicated by the fact that the distribution frequency of Ni-bearing pyrrhotite is higher in inclusions than in the groundmass, and significantly higher in I-type cognate enclaves than in host I-type rocks. Nickel-bearing pyrrhotite is common in I-type but rare in S-type rocks, sedimentary enclaves, restates and country rocks. This eliminates the possibility that the Ohmine S-type rocks were formed by interaction of I-type magma and sedimentary rocks or by mixing between I-type and an ideal S-type magmas, as interpreted from the chemistry of biotite and the chronology of these rocks.On the basis of mineralogy, and isotopic data for S, O, Sr, C, and rare gases, it was shown that I-type granitic rocks were derived from partial melting of intermediate igneous rocks containing hornblende and plagioclase and S-type, from partial melting of sedimentary rocks containing biotite and orthoclase.     

Ammonium in biotite from metamorphic and granitic rocks of Japan

Ammonium contents of biotites from metamorphic and granitic rocks of Japan have been determined, and correlated with the ${}^{18}\text{O}{}^{16}\text{O}$ ratios of the rocks.NH₄ contents of biotites averaged 22 ppm in granitic rocks of non-metamorphic terranes, 67 ppm in granitic rocks in the Ryoke metamorphic belt, and 279 ppm in metamorphosed sedimentary rocks of the Ryoke belt. In granitic rocks, enrichment of NH₄ in biotites is a result of the interaction between the granitic magma and surrounding sedimentary rocks. In metasedimentary rocks, the high NH₄ content in biotites is due to inheritance from original organic material in sedimentary rocks.Biotites from migmatites of the Ryoke belt contain more NH₄ (average, 475 ppm) than those from metasedimentary rocks. This suggests the existence of a metamorphic fluid or anatectic magma enriched in NH₄.     

花岗岩与金铜及钨锡成矿的关系

文章从对国内外若干与金铜钨锡矿床有关的花岗岩Sr、Yb含量的统计出发,按照花岗岩新的分类,归纳了花岗岩与成矿的关系。指出金铜成矿与埃达克型和喜马拉雅型花岗岩有关,钨锡成矿与南岭型花岗岩有关。其原因主要取决于成岩和成矿的深度以及氧逸度条件。金铜和钨锡成矿的深度不同,因此,金铜和钨锡不可能在同时同地出现,但可以叠加在一起。作者认为,成岩和成矿是两回事,成岩基本上是一个物理过程,而成矿主要体现为化学反应;成岩需要热,而成矿需要热、流体以及合适的矿源3个条件,缺一不可。在一个地区,成岩作用可以很普遍,但是,成矿可能很局限。成岩与成矿有关不是成因有关而是时空有关。成矿与成岩同时、或成矿早于成岩、或晚于成岩,都是合理的,而区分含矿岩体和不含矿岩体可能是没有意义的。文中还讨论了金能否来源于围岩的问题及找矿思路的问题,指出就矿找矿仍然是行之有效的找矿方法。     

Wolframoixiolite and niobian ferberite from zinnwaldite granitic rocks of the Chukchi Peninsula

The finding of wolframoixiolite with inclusions of niobian ferberite is described from zinnwaldite granite and ongonite of the Severny pluton of the Chukchi Peninsula. The optical, morphological, and chemical properties of minerals are characterized and compared with their analogues from other regions. The petrologic and mineragenic index implications of the minerals are discussed with allowance for our contemporary mineralogical knowledge on W-bearing Ta-Nb minerals.     

Geochemistry of mineralized granitic rocks of northeast Queensland

Tin-bearing granitic rocks of northeast Queensland range in age from Precambrian to Permian. They are high-level granites or adamellites enriched in volatile elements, such as Li, Be, B, and F. Tin contents are significantly higher than those of non-stanniferous granites, but lower than values reported for many tin granites elsewhere. A distribution of tin in which a high proportion of samples have tin contents significantly higher than background values, appears to be a useful criterion of potential tin mineralization. An uneven distribution of tin in the crust (or possibly the upper mantle) would explain the difference in tin contents of granites associated with tin mineralization, compared with granites from areas which do not contain tin deposits.No correlation of granite geochemistry with lead/zinc or copper mineralization was found. In particular, granites associated with such mineralization do not show anomalous abundances of Pb, Zn, or Cu. If mineralization is regarded as an independent by-product of magma generation rather than the result of differentiation processes, then this lack of correlation is explicable.     

Deformation of granitic rocks across the brittle-ductile transition

A microstructural analysis has been carried out on mylonites and mylonitic gneisses of the Eastern Peninsular Ranges Mylonite Zone, which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies. Composite planar fabrics in the form of C and S planes are found at all metamorphic grades. Fractured feldspars, kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites. At mid-upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery. Biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns. At epidote-amphibolite grade and above, recovery processes and annealing recrystallization predominate in all minerals. Residual orthoclase porphyroclasts show strain-related myrmekite formation along those sides of the grains that face the instantaneous shortening direction. Myrmekite formation due to replacement reactions cannot explain this geometry. It is proposed that the myrmekites formed due to a combination of exsolution, replacement and strain-enhanced diffusion.     

Geochemical and Isotopic Constraints on the Origin of the Jurassic Dolerites of Tasmania

The Jurassic dolerites of Tasmania represent a small part ofthe widespread Mesozoic magmatism of Gondwana, and possess someof the most extreme isotopic and elemental signatures observedin tholeiitic rocks. This study documents the remarkable uniformityof the magma in major, trace element and isotopic compositionat the time of emplacement. Many of the trace element and isotopiccharacteristics compare more closely with those of continentalcrustal rocks than with typical mantle-derived materials, indicatingthe dominance of a crustal component in the petrogenesis ofthese tholeiites. Difficulties in reproducing the trace elementsignatures of the dolerites using calculations involving crustalassimilation, and the unreasonable isotopic compositions requiredof the contaminant, make such models unlikely. We argue that a more likely explanation involves the introductionof a small quantity of sediment (3 wt.%) into a depleted mantlesource by the process of subduction. It is envisaged that thetrace element and isotopic compositions of the mantle sourcewere overprinted by the crustal signatures owing to the lowabundance of incompatible trace elements originally present.These crustal signatures were subsequently inherited by magmasproduced during partial melting.     

Chemical distinction between replacement and magmatic granitic rocks

Utilising about 170 chemical analyses of granitic rocks of proved origin from different parts of the world, a new field is suggested in the Ca-Na-K (ionic weight per cent) trilinear diagram to distinguish the replacement from the magmatic type.     

福建东山变质岩LA-ICP-MS同位素年龄及其地质意义

LA-ICPMS锆石U-Pb法测得东山地区变质岩的年龄为176.8 Ma、203 Ma、230 Ma、1320 Ma、1810 Ma等,其中176.8 Ma为火山-沉积岩中的岩浆锆石,代表着地层形成的年龄;203 Ma、230 Ma为继承岩浆碎屑锆石,代表了晚三叠世源区火成岩的物源年龄;1810 Ma、1320 Ma为变质岩继承锆石,代表了基底物源年龄。地球化学分析结果表明:岩石具贫硅、高铝、铁,富钾,Co、Ni、V含量低,Ba、Rb、Zr、Hf、U、Th及La含量高,具壳源特征;稀土元素总量较高,轻稀土略富集,轻、重稀土元素分异不明显,无明显的负铕异常,表明变质岩的部分物质与地幔有关。福建东山地区亲营山组可能属早侏罗世晚期华夏陆块边缘进入裂陷期海陆交互相火山-沉积的产物。