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1.
Sm-Nd systematics for nine whole-rock samples of hornblende norites, pyroxenites and a lamprophyre from various parts of the Cortlandt Complex were analyzed. Six of these samples from the central and eastern parts of the complex give an isochron age of 430±34 (2) Ma with an Nd value of –2.9±0.5, and the other three samples from the western part, including the lamprophyre, define a similar age of 394±33 (2) Ma but with a distinctly different Nd value of –1.4±0.4. The two different initial 143Nd/144Nd ratios corresponding to these -values are interpreted to reflect continental crustal contamination of the lamprophyric parental liquid prior to final emplacement and crystal fractionation to produce the different rock types of the complex. The intrusion age of 430 Ma for the complex clearly post-dates the major metamorphic event of the Taconic orogeny. The Nd-isotopic data also suggest a relationship between the Cortlandt Complex and a belt of lamprophyric dike rocks to the west, known as the Beemerville trend, which cuts across the metamorphic trends of the Taconic (Ratcliffe 1981).  相似文献   

2.
Young volcanic rocks from different sections of the Aleutian Islands-Alaska Peninsula Arc have been measured for 87Sr/86Sr, 143Nd/144Nd and some trace elements. We found the 143Nd/144Nd to be highly restricted in range ( Nd=6 to 7) and low as compared to midocean ridge ba-salts (MORB). This indicates that the source of the Aleutian Arc magmas is different from MORB and remarkably isotopically homogeneous with respect to Nd. The range reported here for arc rocks is substantially smaller than found by other workers. However, the Sr isotope ratios vary considerably ( Sr=–24 to –14). Those samples from small volcanic centers north of the main arc (second arc) are characterized by low Sr. Our data in combination with previous studies suggest that there are slight geochemical differences between discrete sections of the arc. The general uniformity of Nd isotope ratios are thought to be the surface expression of an efficient mixing or homogenization process beneath the arc plate, but which still causes a wide dispersion in Sr isotopic composition.To relate the arc rocks to the broader tectonic setting and to identify possible sources of arc magmas, measurements were done on volcanic and sedimentary rocks from the North Pacific/Bering Sea area. Alkali basalts from the back-arc islands St. George, Nunivak and St. Lawrence and alkali-rich tholeiites from the fore-arc have Nd=+4 to +9 and are correlated on the Sr- Nddiagram parallel to the mantle array but shifted to lower Sr. These samples are thought to be isotopically representative of the mantle transported to that region. A tholeiitic basalt from the Kamchatka Basin ocean floor (back-arc), however, yielded typical MORB values ( Nd=10, Sr=–24). Composite sediment samples were made from DSDP cores in the Aleutian Abyssal Plain, Gulf of Alaska and the Alka Basin which represent mixtures of continentally and arc-derived materials. These composites have intermediate Nd isotopic ( Nd= –2 and +2) and high Sr isotopic values ( Sr=+9 and +37). These data show that possible source materials of the Aleutian Arc volcanics are isotopically different from and much more heterogeneous than the arc rocks themselves.On the basis of this study and of literature data, we developed a set of alternative models for volcanic arc magma generation, based on the restricted range in Nd and the wider range in Sr for arc rocks. Different isotopic and trace element characteristics found in different arcs or arc sections are explained by varying mixing proportions or concentrations in source materials. The basic observations require rather strict mixing ratios to obtain constant Nd. The preferred model is one where the melting of subducted oceanic crust is controlled by the amount of trapped sediment with the melting restricted to the upper part of the altered basaltic layer. Homogenization within the upper part of the oceanic crust is brought about by hydrothermal circulation attending dewatering of the slab during subduction and possibly some oxygen exchange of the magmas on ascent.Division Contribution Number 3849 (411)  相似文献   

3.
Ijolites from the type locality at Iivaara, Finland, form a continuous series of magmatic rocks ranging from urtites to melteigites. Both Ni and Cr, but also the large ion lithophile light-rare-earth elements, Zr, Hf, Nb, Rb, Sr and Ba are low in concentration. The Nd contents equal those of the neighboring fenites, Sr is distinctly less abundant, and there is no significant Eu anomaly. The 143Nd/144Nd and 87Sr/86Sr of the ijolites demonstrate a systematic covariation between the data of carbonaties from the Kola Alkaline Province (Sr – 13.8, Nd + 5.6) and those of the fenites at Iivaara (Sr + 132.9, Nd – 24.7) with Sr varying from +0.3 to +23.9 and Nd varying from-9.2 to-19.3. The trace element abundances and the isotopic data give evidence for a crystallization of the rocks from a liquid generated by melting (rheomorphism) of high-grade fenitized country rocks rather than from a primary mantle-derived magma which was contaminated at crustal levels. The fenitization of wall rocks preceding the ijolite magma formation was clement selective. Mixing of elements during the fenitization process between the designated components carbonatite (or derivative fenitizing fluid) and wall rock should have been dynamical depending on the stability of the wall rock mineral assemblages in contact with the fenitizing fluids, the migration velocity of these fluids, and their capacity of the respective elements. Such dynamical mixing explains best the variation of the isotope ratios withont systematic covariation of the respective element concentrations.  相似文献   

4.
Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km3), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initial Nd values (1 Nd unit), higher87Sr/86Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similar Nd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higher Nd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from low Nd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The low Nd magmas most likely formed via the incorporation of low 18O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higher 18O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have low Nd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing basification of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.  相似文献   

5.
The ca. 2.7–2.5 Ga Slave Province is a granitegreenstone terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks. The Nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km, east-west, transect at 65°N across the structural grain of the province. Initial Nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province, as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust. Supracrustal sequences (ca. 2.71–2.65 Ga) from the central and eastern parts of the province have positive Nd(1) values (+0.3 to +3.6), consistent with juvenile sources and formation remote from significantly older crust. Syn to late-deformation (ca. 2.63–2.60 Ga), mantle-derived diorites and related tonalites (type I) from the central and eastern parts of the province have similar initial Nd values (-0.1 to +2.7). In contrast, samples from the westernmost plutons, which intrude exposed pre-3.1 Ga crust, have much lower Nd(1) values (-1.0 to4.6) suggesting contamination of these magmas by older crust. The Nd(1) values of post-deformation granites (s.s.) (type II) also vary systematically across the province: values for granites west of longitude 110°30W range from-0.2 to -5.3; those to the east range from +0.6 to +3.7. These data suggest mixed crustal sources dominated by Mid to Early Archean material ( Nd-2.6 to- 17 at 2.6 Ga) for the western granitoid rocks and juvenile sources for the eastern granites. The Nd isotopic data are consistent with the geology of the province in that exposures of Mid to Early Archean crustal rocks, predating the principal 2.7–2.5 Ga orogenic event are restricted to the western part of the province. The asymmetric pattern defined by the Nd isotopic data indicates the presence of distinct crustal rocks beneath the Slave Province. Similar isotopic variations observed across Phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin. This process may also have been an important mechanism in the cratonization of the Slave Province.  相似文献   

6.
Initial Nd isotopic ratios are reported for 23 samples representing magmatic crustal components in the Svecokarelian terrain of South Finland. U-Pb zircon ages are determined for all geologic units, involving 21 separate upper concordia intercept ages based on more than 100 UP-b analyses. The ages range for all the rocks from 1.90 Ga for primitve plutonic rocks to 1.79 Ga for post-tectonic intrusions. The well-known gabbro-diorite-tonalite-trondhjemite association of the Kalanti district appears to consist of components with different ages: trondhjemites are probably 1.90 Ga or older, diorites/tonalites belong to the main Svecokarelian plutonic eposide at 1.89-1.87 Ga, and at least some gabbro has a post-tectonic age of 1.80 Ga. Nd (T) values range between +2 to +3 for meta-andesites, large gabbros and primitive granitoids to –0.5 for more evolved granitoids. A magma source with Nd of at least +2 to +3 was available during 1.90 to 1.87 Ga, but evolved granitoids have Nd close to zero. The preferred interpretation is that depleted mantle with Nd=+4 to +5 was present beneath the Svecokarelian crust forming during 1.9-1.8 Ga, and that all rocks have been affected more or less by addition of an Archean crustal component with Nd=–9 to –13. The primitive rocks with Nd=+2 to + 3 were only slightly affected, while granitoids with Nd close to zero include a 10% Archean component. The widespread nature of the Archean addition and the distance of up to 500 km to actual exposed Archean crust make it most realistic that the Archean component was added to the form of sediments delivered by subducting Proterozoic ocean crust. The plutonic rocks of the Finnish Svecokarelian crust in areas away from Archean cratons consist of 90% newly mantle-derived material.  相似文献   

7.
Initial 87Sr/86Sr and 143Nd/144Nd ratios of Phanerozoic granitoids and related intrusions of the New Zealand block display a mixing-type array indicative of the involvement in their sources of old continental crustal material, most likely of Proterozoic age. Sr(T) values range from –4 to +273 (87Sr/86Sr=0.7041–0.7233), while Nd(T) ranges from +2.7 to –11.0. Preexisting metasedimentary rocks have generally higher Sr and lower Nd (ranging to present-day values of +646 and –15.0, respectively), and, particularly for the Mesozoic intrusives, are isotopically appropriate mixing end-members. The widespread, early Paleozoic Greenland Group graywackes, which are derived from Proterozoic sources, are modeled as the source of the crustal end-member mixing with mantle-derived mafic magmas to produce the intrusive rocks. Four different types of models are applied to the isotopic and trace-element (Rb, Sr, Ba, REE) data: simple mixing; mixing with a partial melt of the metasedimentary rock, with or without isotopic equilibrium; and assimilation-fractional crystallization. Based on these models, some constraints may be applied on petrogenesis (e.g., the lack of high Rb concentrations points to the presence of biotite, and HREE depletion points to the presence of garnet); however, the models fail to adequately explain all the data. The New Zealand granitoids show similarities in isotopic character not only to rocks from offshore islands on the New Zealand block, but also to similar-aged granitoids in adjacent regions of Antarctica and Australia. This points to similarities in crustal character between continental blocks formerly proximal in Gondwanaland. We note an overall increase in Nd and decrease in Sr in felsic magmas from the Paleozoic to the Mesozoic to the Cenozoic in New Zealand, indicative of a decrease over time in the level of influence of recycled continental crust in subduction-related magmatism.Division Contribution No. 4538 (582)  相似文献   

8.
The Etendeka Formation volcanics consist of a bimodal association of basalts and quartz latites. Forty three new whole rock oxygen isotope analyses are reported for all the major magma types. All the rocks except a minor suite of dolerites have higher 18O values than normal mantle. The basic rocks (average of 29=8.8) have significantly different 18O to the acid rocks (average of 10=14.4) These data are apparently consistent with previously published petrogenetic models, which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived. However, mineral oxygen data show that there is significant oxygen isotopic disequilibrium between phenocryst and whole rock, the latter being significantly higher in most cases. One of the basic magma types (the Tafelberg basalts) shows mutual positive correlations between 18O, SiO2 and Sr. If these correlations are due to crustal contamination, then as much as 45% contamination is required by material having a 18O value of 15 which is the maximum observed value in the Damaran basement rocks. In the absence of pyroxene phenocryst 18O data for the high Sr Tafelberg basalts (they are aphyric), it is not possible to confirm that contamination has taken place. An alternative explanation is that the correlation between Sr and SiO2 resulted from assimilation coupled with fractional crystallization (AFC) (before emplacement). Post-eruption alteration resulted in a correlation between SiO2 18O because the material with the most Si-O bonds was able to concentrate 18O more effectively. The limited mineral data for the quartz latites suggests that there is some source heterogeneity. A pyroxene 18O value of 10% for a southern Etendeka quartz latite is consistent with a crustal source.  相似文献   

9.
Porosities of rock samples obtained by modal analysis and by the gas expansion method are compared. Results indicate that modal analysis is a suitable method for determining porosities of rock samples, especially well cuttings.
Zusammenfassung Vergleiche von Gesteinsporositäten, die einerseits durch die Gasexpansions-methode und andererseits durch das Punktzählverfahren ermittelt wurden, zeigen, daß das Punktzählverfahren dazu geeignet ist, Porositäten von Gesteinen, besonders von Spülproben, zu ermitteln.
Résumé Deux méthodes pour obtenir la porosité des roches sont comparées. Les résultats indiquent que l'analyse intégratrice se compare bien à la méthode par expansion de gaz, spécialement si la méthode intégratrice est appliquée aux particules des formations détruites pendant les opérations de forage.
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10.
Information on tectonic evolution can be obtained from the succession of magmatic types represented (I-type and S-type granitoids and volcanics, basic dykes and alkali complexes). The earliest Proterozoic magmatic activity resulted in the formation of greenstone belts within the Karelian continent area, probably in the age range 2500-2100 Ma. This was followed by an event similar to Andinotype orogeny at around 1900 Ma with I-type granitoids and calc-alkaline volcanics. This progressed to an event of Hercynotype which was concluded by about 1750 Ma. Subsequent magmatic activity (I-type granitoids and volcanics) suggests a new phase of Andino-type orogeny until about 1630 Ma (S and C Sweden) or 1500 Ma (N Sweden). Tension-related magmatism at around 1550 Ma (basic dykes, alkaline complexes and gabbro) may relate to rifting prior to the independant evolution of SW Sweden which includes intrusion of I-S-type granitoids, metamorphism, followed by continental rotation and collision with N America at about 1050 Ma. Basic dykes and S-type granites are related to subsequent uplift 950-850 Ma ago.
Zusammenfassung Information über die tektonische Entwicklung kann aus der Folge der dargestellten Magmentypen (I-Typ und S-Typ granitähnlicher Gesteine und Vulkanite, basische Gänge und Alkalikomplexe) erhalten werden. Die früheste proterozoische magmatische Aktivität bildete die Formation von Grünsteingürteln innerhalb des Gebietes des Karelian-Kontinents, wahrscheinlich in der Zeit vor 2500-2100 Ma. Vor etwa 1900 Ma folgte daraufhin ein Ereignis, ähnlich der Andinotypen-Orogenese, mit I-Typ-Granitoiden und Kalk-Alkali-Vulkaniten. Dies schritt weiter fort zu einem Hercynotyp-ähnlichen Ereignis, welches vor ungefähr 1750 Ma abgeschlossen war. Nachfolgende magmatische Aktivität ruft eine neue Phase Andinotyp-ähnlicher Orogenese bis ungefähr 1630 Ma (Süd- und Mittelschweden) oder 1500 Ma (Nordschweden) hervor. Zerrspannungsabhängiger Magmatismus um ungefähr 1550 Ma (basische Gänge, Alkali-Komplexe und Gabbro) können zu Rissen führen vor der unabhängigen Entwicklung SW-Schwedens, welche die Intrusion von I-S-Typen granitähnlicher Gesteine, Metamorphose, gefolgt von kontinentaler Rotation und Kollision mit N-Amerika vor ungefähr 1050 Ma einschließt. Basische Gänge und S-Typ-ähnliche Granite hängen mit der nachfolgenden Hebung vor 950-850 Ma zusammen.
Résumé L'information portant sur l'évolution tectonique peut être déduite de l'étude de la succession des types magmatiques représentés (Séries granitiques de type »I«ou»S« et volcanites, dykes basiques, complexes alcalins etc.). La première manifestation magmatique du Protérozoique est représentée par les ceintures de roches vertes, au sein du continent »Karélien«, probablement mises en place entre 2500 et 2100 Ma. Elle a été suivie par un événement orogénique à caractère «Andinotype» et qui correspond à l'intrusion de granites (Iato sensu) et de roches volcaniques, datée d'environ 1900 Ma. Cet événement fut suivi par un autre, de caractère «Hercynotype» qui se termina au tour de 1750 Ma. L'activité magmatique subséquente peut être caractérisée comme nouvelle phase Andinotype; elle est terminée à environ 1630 Ma pour la Suède méridionale et centrale, et à environ 1500 Ma pour le Nord du pays. Un épisode magmatique, en relation avec une phase d'extension, pourrait être dû à la formation de rift. Il est caractérisé par l'intrusion de dykes basiques, de complexes alcalins et de gabbros et précède l'évolution indépendante du sud-ouest de la Suède comprenant des granites de type «I» et «S» ainsi que leur métamorphisme ultérieur. Ceci fut suivi par une rotation continentale ainsi que par la collision avec l'Amérique du Nord à environ 1050 Ma. Enfin, des dykes basiques ainsi que des granites de type «S» se sont mis en place en relation avec une phase de soulèvement ultérieure datée d'environ 950-850 Ma.
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11.
Eclogites from the Roberts Victor mine, Kaapvaal craton are classic examples of subducted Achaean oceanic crust brought up as xenoliths by kimberlite. New in situ trace element and oxygen isotope data (18O=3.09–6.99 SMOW) presented here reemphasise their origin from seawater-altered plagioclase-rich precursors. Their Hf–Nd isotopic compositions are not in agreement with compositions predicted by geochemical modelling of the isotopic composition of aged subducted oceanic crust. Instead, Hf isotopic compositions are very heterogeneous, varying between 0.281625 and 0.355077 (–37.8 and +2561 Hf) at the time of kimberlite emplacement (128 Ma) in keeping with equally variable Nd isotopic compositions (0.511124–0.545092; –26.3 to +636 Nd). However, most samples plot on the terrestrial array. The isotopic compositions of some samples are too extreme to play a major role in mixed peridotite-eclogite melting in basalt source regions, whereas the isotopic composition of other samples is reconcilable with a contribution of up to ca. 15% of eclogite partial melt to the MORB source. Most importantly, our results show that ancient subducted oceanic crust is not isotopically homogeneous and should not be treated as a component or reservoir during geochemical modelling. The heterogeneity reflects radiogenic in-growth starting from small compositional heterogeneities in gabbroic protoliths, followed by modification during sea-floor alteration, subduction and emplacement into the subcratonic lithosphere.  相似文献   

12.
Some plutons of the Central-Extremadura batholith show lateral widening at the final stage of their emplacement. Consequently, various fracture systems have developed, generally filled by aplites that appear both in the plutonic mass and in the metamorphic host rocks.The spatial arrangement of fractures may be interpreted in a simple model in which an important role is played by fluid pressure, as it determines the value of the acting effective stress and, naturally, the different failure mechanisms reflected in different dike systems.
Zusammenfassung In einigen Granit-Plutonen des Zentralextremadura-Batholithen konnte eine laterale Ausdehnung in den Endetappen ihrer Platznahme festgestellt werden. Dabei wurden verschiedene Bruchsysteme entwickelt, die im allgemeinen mit Aplit gefüllt sind und sowohl im Inneren der plutonischen Masse als auch im metamorphen Nebengestein auftreten.Die spezielle Anordnung der Bruchsysteme konnte anhand eines einfachen Modells interpretiert werden. Dabei spielt der Durchströmungsdruck eine wichtige Rolle, da er den Wert der effektiven Spannung bestimmt und verschiedene Bruchmechanismen, die sich in verschiedenen Gangsystemen widerspiegeln.
Résumé Quelques plutons du batholite de l'Estremadure centrale ont donné lieu à une expansion latérale au stade final de leur mise en place. Plusieurs systèmes de fractures se sont ainsi développées; elles sont généralement occupées par des aplites et apparaissent aussi bien à l'intérieur de la masse plutonique que dans l'encaissant métamorphique.La distribution spatiale de ces fractures a pu être interprétée par un modèle simple dans lequel la pression des fluides joue un rôle important: cette pression détermine la valeur de la contrainte effective et conditionne les divers mécanismes de facturation, qui se traduisent dans différents systèmes de filons.
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13.
Graphite occurs in two distinct textural varieties in syntectonic granitoids of the New Hampshire Plutonic Series and in associated metasedimentary wall rocks. Textural characteristics indicate that coarse graphite flakes were present at an early stage of crystallization of the igneous rocks and thus may represent xenocrystic material assimilated from the wall rocks. The range of 13C values determined for flake graphite in the igneous rocks (–26.5 to –13.8) overlaps the range for flake graphite in the wall rocks (–26.0 to –16.7), and spatial correlation of some 13C values in the plutons and wall rocks supports the assimilation mechanism. The textures of fine-grained irregular aggregates or spherulites of graphite, on the other hand, indicate that they formed along with secondary hydrous silicates and carbonates during retrograde reactions between the primary silicates and a carbon-bearing aqueous fluid phase. Relative to coexisting flake graphite, spherulitic graphite shows isotopic shifts ranging from 1.9 higher to 1.4 lower in both igneous and metasedimentary samples.The observed isotopic shifts and the association of spherulitic graphite with hydrous silicates are explained by dehydration of C-O-H fluids initially on or near the graphite saturation boundary. Hydration of silicates causes dehydration of the fluid and drives the fluid composition to the graphite saturation surface. Continued dehydration of the fluid then requires coprecipitation of secondary graphite and hydrous silicates and drives the fluid toward either higher or lower CO2/CH4 depending upon the inital bulk composition. Isotopic shifts in graphite formed at successive reaction stages are explained by fractionation of 13C between secondary graphite and the evolving fluid because 13C is preferentially concentrated into CO2 relative to CH4.Epigenetic graphite in two vein deposits assiciated with the contacts of these igneous rocks is generally enriched in 13C (–15.7 to –11.6) relative to both the igneous and wall-rock 13C values. Values of 13C vary by up to 3.4 within veins, with samples taken only 3 cm apart differing by 2.0 These variations in 13C correlate with textural evidence showing sequential deposition of different generations of graphite in the veins from fluids which differed in proportions of carbon species or isotopic composition (or both).  相似文献   

14.
Sulfur isotope ratios have been determined in 27 selected volcanic rocks from Iceland together with their whole rock chemistry. The 34S of analyzed basalts ranges from –2.0 to +0.4 with an average value of –0.8 Tholeiitic and alkaline rocks exhibit little difference in 34S values but the intermediate and acid rocks analyzed have higher 34S values up to +4.2 It is suggested that the overall variation in sulfur isotope composition of the basalts is caused by degassing. The small range of the 34S values and its similarity to other oceanic and continental basalts, suggest that the depleted mantle is homogeneous in its sulfur isotope composition. The 34S of the depleted mantle is estimated to be within the range for undegassed oceanic basalts, –0.5 to +1.0  相似文献   

15.
Most sedimentary basins contain saline pore water. Saline formation waters can form during burial diagenesis as the result of normal processes of water/rock interaction involving incongruent halite dissolution, bittern salt destruction, and albitization of detrital plagioclase. The kinds of Na-Ca-Cl saline formation waters typical of sedimentary basins can also result from modification of surficial brines formed by the precipitation of NaCl from evaporated seawater. As the porosity of rocks is reduced during burial, discharge of saline formation waters contributes to crustal chloride cycling, and helps explain riverine chloride loads. During burial, the dissolution of metastable detrital minerals derived from crustal rocks in corrosive, saline water transfers incompatible elements such as Li and B from the igneous crust to the sedimentary crust. Similarly, albitization transfers Ca from the crustal silicate (igneous) mineral reservoir to the crustal carbonate and aqueous (sedimentary) reservoirs. Metamorphism and then melting of albite-enriched rocks accounts for the elevated sodium contents of igneous rocks relative to sedimentary rocks. In this way average sediments have become enriched through time in Ca, Cl, Br, S, Li, and B, and depleted in Na relative to average igneous crust.  相似文献   

16.
Zusammenfassung Die Radioaktivität vier hercynischer, granitischer Gesteine schweizerischer Zentralmassive (Mont-Blanc-Granit, Vallorcine-Granit, Giuv-Syenit und Zentraler Aaregranit s. l.) wurde im Gelände (Oberflächen- und vor allem Stollen-Messungen) und im Labor (gammaspektrometrische und fluorimetrische Bestimmungen) untersucht. Jeder Gesteinskomplex zeigt ein spezifisches Uran-Thorium-Variationsfeld. Das Th/U-Verhältnis nimmt mit steigenden Th-Gehalten ab. Die Variation der U- und Th-Gehalte ist petrologisch kontrolliert: es ist eine Zunahme in den Randzonen der Granitkörper und (bei Intrusionsfolgen) in den jüngsten Gesteinen, vor allem in Aplitgranitstöcken, festzustellen. Mehrere Gesteine sind besonders uran- und thoriumreich: Giuv-Syenit, Mittel aus 44 Bestimmungen, 22 ppm U und 66 ppm Th; Mont-Blanc-Granit (45), 19 ppm U/31 ppm Th; Zentraler Aaregranit s. str. (10), 13 ppm U/32 ppm Th. Die Gesteine enthalten zum Teil hohe Anteile leichtlöslichen Urans (bis 90%), wobei hohe Gesamt-Urangehalte in der Regel auch einen hohen Prozentsatz leichtlösliches Uran bedingen.In allen Gesteinskomplexen sind eigentliche Uranmineralisationen (meist mit Pechblende) gefunden worden. Sie sind ausschließlich an Scherzonen, Mylonite und Klüfte gebunden und liegen vor allem in den Randzonen der Gesteinskörper oder in der Nähe besonders uranreicher Gesteine wie Aplitgranite. Die Assoziation Granite/Uranvererzungen zeigt, abgesehen von der alpinen überprägung, große ähnlichkeiten mit den französischen Vorkommen des Massif Central.
The radioactivity of 4 Hercynian granitic rock units of the Swiss Central Massifs (Mont Blanc granite, Vallorcine granite, Giuv syenite and Central Aare granite s. l.) has been measured in the field (on the surface and in tunnels) and in the laboratory (gamma ray spectrometry). Each rock unit shows a clearly defined field of variation on a U-Th plot. The Th/U ratios decrease with increasing Th contents. The variation is petrologically controlled: the U and Th contents increase in the border zones of the granitic bodies and — in the case of intrusive sequences — in the younger derivates, especially in the aplitic phase. Three of the units show unusually high U and Th contents: Giuv syenite (mean of 44 determinations), 22 ppm U and 66 ppm Th; Mont Blanc granite (45), 19 ppm U/31 ppm Th; Central Aare granite s. str. (10), 13 ppm U/32 ppm Th. The fraction of easily leacheable uranium is, according to fluorimetric determinations, unusually high (up to 90%) in some granites. Generally, the higher the U content, the higher the leacheable fraction.In all rock units uranium mineralisations (mainly pitchblende) have been found. They are confined to shear zones, mylonites and fissures and occur mostly in the border zones of the granitic bodies or in the vicinity of especially uranium-rich rock types, such as aplites. Apart from the effects of the Alpine orogeny the association of uranium mineralisations with granites shows close similarities to the occurrences in the French Massif Central.
Résumé La radioactivité de quatre massifs granitiques hercyniens des massifs cristallins centraux des Alpes suisses (granites du Mont Blanc, de Vallorcine, et de l'Aar et syénite du Piz Giuv) a été étudiée sur le terrain et en laboratoire (spectrométrie gamma et fluorimétrie). La variation des teneurs en U et en Th est spécifique pour chaque type des roche. Le rapport Th/U diminue lorsque la teneur en Th augmente. L'Uranium s'enrichie d'une part dans les faciès bordiers des massifs granitiques et d'autre part (dans des successions magmatiques) vers les roches les plus jeunes et les plus acides (granites aplitiques). Quelques-unes de nos roches ont un fonds géochimique d'U et de Th élevé: syénite du Piz Giuv 22 ppm U/66 ppm Th (moyenne de 44 déterminations); granite du Mont Blanc 19 ppm U/31 ppm Th (45); granite de l'Aar 13 ppm U/32 ppm Th (10). En mÊme temps, on constate des teneurs élevées en Uranium facilement soluble (jusqu'a 80-90% de l'Uranium total).Dans tout les complexes se trouvent des minéralisations d'Uranium (sans importance écononique actuelle). Elles sont situées surtout aux bords des granites ou dans des roches avec beaucoup d'Uranium soluble («fertiles») et sont essentiellement liées à des structures tectoniques. A l'exception de la tectonique et du métamorphisme alpin et d'une forte érosion post-tertiaire notre association granites/mineralisations ressemble beacoup à celle du Massif Central franÇais.
( , Vallorcine, Giuv s. 1.), ( , , ), ( ). . Th/U . : ( ) , , , : Giuv, 44 22 ppm U 66 Th; (45) 19 U, 31 ppm Th; s. 1. (10) 13 ppm U, 32 ppm Th. ( 90%). , , . , . , , -: . / Massif Central, , , .
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17.
The relative size of glass rims coating crystals in the matrix ash provides a semi-quantitative measure of abrasion of ash grains in pyroclastic flows. Median abrasion indices (= areacrystal / areaglass rim) are 8.4 to 18.5 in Laacher See pyroclastic flow units but only 4 to 6.3 in assocciated fallout, showing stronger abrasion of ash particles in the pyroclastic flows. All pyroclasts undergo strong attrition in the vent but clasts in pyroclastic flows undergo a second major phase of abrasion during high-energy near-vent flow. Abrasion of ash particles is weaker during lower-energy higher-strength motion further downstream, suggesting that high contents of fine ash in distal deposits are due to diminishing elutriation rather than high rate of attrition.
Zusammenfassung Die relative Größe von Glassäumen um Kristalle in der Matrix-Asche kann als semi-quantitatives Maß für den Abrieb von Aschepartikeln in pyroklastischen Strömen benutzt werden. Median-Werte des Abriebs-Index (= Flächekristall / FlächeGlassaum) betragen 8,4 bis 18,5 in pyroklastischen Fließeinheiten am Laacher See. Abriebs-Indices stratigraphisch assoziierter Bims-Fallablagerungen liegen bei nur 4 bis 6,3; dies belegt einen höheren Abrieb von Aschepartikeln in den pyroklastischen Strömen. Alle Pyroklasten erfahren gemeinsam starken Abrieb im Schlot, aber Partikel in pyroklastischen Strömen durchlaufen eine zweite Phase starken Abriebs während des hochenergetischen Transports im schlotnahen Bereich. Bei niedriger-energetischem Fließen mit wachsender Bingham-Festigkeit weiter talabwärts wird der Partikelabrieb schwächer. Hohe Gehalte an feiner Asche in distalen Fließeinheiten sind also eher auf schwächer werdende Elutriierung als auf stärkeren Abrieb zurückzuführen.
Résumé La dimension relative des enduits vitreux qui enrobent les cristaux dans les dépôts de cendres volcaniques peut être utilisée comme mesure semiquantitative de l'abrasion des particules dans les courants pyroclastiques. Au Laacher See les indices d'abrasion (surface du cristal/surface de l'enduit vitreux) sont de 8,4 à 18,5 dans les coulées pyroclastiques et seulement de 4 à 6,3 dans les sédiments formés par retombée, qui leur sont interstratifiés; les particules de cendre ont donc subi une abrasion plus forte dans les coulées. Tous les pyroclastes subissent une forte abrasion dans le cratère; mais dans les coulées pyroclastiques, ils subissent une deuxième action d'abrasion au cours de leur transport en milieu de haute énergie, à proximité du cratère. Dans les conditions de plus basse énergie qui régnent plus en aval, l'abrasion est moins forte; on en déduit que la forte teneur en fines dans les dépôts distaux résulte d'une diminution de l'élutriation plutôt que d'une forte action d'abrasion.
, . ( / ) 8,4 18,5 . . , . , . ( ), .
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18.
The Mariánské Lázn complex (MLC) is located in the Bohemian Massif along the north-western margin of the Teplá-Barrandian microplate and consists of metagabbro, amphibolite and eclogite, with subordinate amounts of serpentinite, felsic gneiss and calcsilicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone between the Saxothuringian rocks to the north-west and the Teplá-Barrandian microplate to the south-east. Sm-Nd geochronology of garnet-omphacite pairs from two eclogite samples yields ages of 377±7, and 367±4 Ma. Samples of eclogite and amphibolite do not define a whole rock Sm-Nd isochron, even though there is a large range in Sm/Nd ratio, implying that the suite of samples may not be cogenetic. Eclogites do not have correlated Nd values and initial 87Sr/86Sr ratios. Five of the eight eclogite samples have high Nd values (+10.2 to +7.1) consistent with derivation from a MORB-like source, but variable 87Sr/86Sr ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower Nd values (+ 5.4 to –0.8) and widely variable 87Sr/86Sr ratios (0.7033 to 0.7096). Such low Nd values are inconsistent with derivation from a MORB, source and may reflect a subduction or oceanic island basalt component in their source. The MLC is an important petrotectonic element in the Bohemian Massif, providing evidence for Cambro-Ordovician formation of oceanic crust and interaction with seawater, Late Devonian (Frasnian-Famennian) high- and medium-pressure metamorphism related to closure of a Saxothuringian ocean basin, Early Carboniferous (Viséan) thrusting of the Teplá terrane over Saxothuringian rocks and Late Viséan extension.  相似文献   

19.
The Chibougamau area, Québec, is characteristic of the internal zone of the Archean Abitibi Orogenic Belt. The paleogeographic, paleotectonic and magmatic history of the Archean sequence in the Chibougamau area is subdivided into three stages.In the first stage a submarine volcanic chain formed mainly by the effusion of submarine lava flows composed of primitive, potash-poor, tholeiitic basalt. The volcanic chain gradually grew to sea level. In the second stage, volcanic islands emerged and grew. Mainly pyroclastic eruptions of strongly differentiated, calc-alkaline andesite and dacite concentrated on the volcanic islands, whereas effusion of basalt continued at first in the surrounding basin. A felsic volcaniclastic apron was deposited around the volcanic islands. In the third stage, the volcanic islands were uplifted and were eroded to the level of their subvolcanic plutons. The debris derived from this volcanic-plutonic terrain was deposited in downfaulted marine and continental basins. The contemporaneous volcanism was shoshonitic.The first paleogeographic stage is interpreted as the growth of an immature island arc. During the second stage, the island arc became mature and its crust was thickened by accretion of plutonic material. The third stage is a period of back-arc extension.
Zusammenfassung Das Gebiet von Chibougamau, Québec, ist characteristisch für die interne Zone des Archaischen Abitibi Orogens. Man kann seine paleogeographische, paleotectonische und magmatische Geschichte in drei Phasen gliedern.Eine submarine Vulkankette formte sich in der ersten Phase, hauptsächlich durch Effusion von submarinen Lavaergüssen aus primitivem, kaliarmen, tholeiitischem Basalt. Die Vulkankette wuchs langsam bis zum Meeresspiegel. Vulkanische Inseln bildeten sich und wuchsen während der zweiten paleogeographischen Phase. Vorwiegend pyroklastische Eruptionen von stark differenzierten, kalk-alkalischem Andesit und Dazit konzentrierten sich mehr und mehr auf den Inselvulkanen, während die Effusion von Basalt zunächst in den Becken noch stattfand. Ein Mantel aus felsitischen vulkanoklastischen Gesteinen wurde um die Inselvulkane abgelagert. Die dritte Phase begann mit einer Hebung der Inselvulkane und mit ihrer Erosion bis zum Niveau ihrer subvulkanischen Plutone. Der Detritus dieses vulkanisch-plutonischen Geländes wurde in marinen und kontinentalen Verwerfungsbecken abgelagert. Der gleichalte Vulkanismus ist shoshonitisch.Wir deuten die erste paleogeographische Phase als Wachstumsphase eines primitven Inselbogens. Während der zweiten Phase reifte der Inselbogen und seine Kruste verdickte sich durch Akkretion plutonischen Materials. Die dritte Phase ist eine Periode der Dehnung im Hinterland eines Inselbogens.
Résumé La région de Chibougamau, Québec, est caractéristique de la zone interne de la ceinture orogénique archéenne de l'Abitibi. Son évolution paléogéographique, paléotectonique et magmatique se subdivise en trois phases.Lors de la première phase paléogéographique, une chaîne sous-marine de volcans se formait, essentiellement par l'émission de coulées de lave composée de basalte primitif, hypopotassique, tholéiitique. Graduellement cette chaîne volcanique s'élevait jusqu'au niveau de la mer. A la phase suivante, des îles volcaniques émergeaient et croissaient. Des éruptions essentiellement pyroclastiques d'andésites et de dacites calco-alcalines et fortement différenciées se concentraient sur les îles tandis que l'effusion de laves basaltiques continuaient dans le bassin. Un manteau de roches volcaniclastiques felsiques se déposait autour des îles volcaniques. Lors de la troisième phase, les îles volcaniques furent soulevées et furent érodées jusqu'au niveau des masses plutoniques sub-volcaniques. Le débris de ce terrain volcano-plutonique fut déposé dans des bassins de faille marins et continentaux. Des shoshonites dominaient le volcanisme contemporain.Nous interprétons la première phase paléogéographique comme une phase de croissance d'un arc insulaire immature. Lors de la deuxième phase, 1'arc insulaire devenait mature et sa croûte s'epaissît par accrétion de matériel plutoni-que. Enfin, la troisième phase est une période d'extension en arrière d'un arc insulaire.
Chibougamau, Quebec, Abitibi. , . , . . , , . . , . - , , . . . . , , . , — . — . .
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20.
Rocks metamorphosed in two or more different facies are not necessarily polymetamorphic and are termed plurifacial rocks. The following age sequence of metamorphic facies of alpine age is reported: (1) glaucophane-schist facies; (2) albite-epidote-amphibolite facies; (S) almandine-amphibolite facies. There was a transition from kinematic to static conditions. The alpine metamorphism described seems to have proceeded under a pile of overthrust sheets. The change in metamorphic facies was in part due to a considerable rise of the temperature.
Zusammenfassung In mehreren verschiedenen Fazies metamorphosierte Gesteine brauchen nicht polymetamorph zu sein, und werden als plurifazielle Gesteine bezeichnet. Folgende zeitliche Abfolge metamorpher Fazies alpinen Alters wird beschrieben: 1. Glaukophanschieferfazies, 2. Albit-Epidot-Amphibolitfazies, 3. Almandin-Amphibolitfazies. Es gab einen Übergang von kinematischen zu statischen Bedingungen. Die beschriebene alpine Metamorphose scheint unter einer Deckenlast vor sich gegangen zu sein. Der zeitliche Fazieswechsel wurde z. T. von einer beträchtlichen Temperatursteigerung bedingt.
Résumé Les roches montrant plus d'un seul faciès métamorphique ne sont pas nécessairement polymétamorphiques et sont dénommées plurifacielles. Quant au métamorphisme alpin, l'ordre suivant de succession chronologique des faciès fut constaté: (1) le faciès à glaucophane, (2) le faciès amphibolite à albite-epidote, (3) le faciès amphibolite à almandin. Des conditions de métamorphisme cinématiques ont été suivies par des conditions statiques. Il paraît que le métamorphisme alpin décrit s'est effectué dans un géosynclinal de nappes. Le changement de faciès métamorphique dans le temps était partiellement dû à une augmentation considérable de la température.
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