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1.
Summary Metamorphosed carbonatites and related skarn deposits, located in Fuerteventura Basal Complex, contain unusual Sr-rich minerals. Maximum SrO concentration in the following minerals are: calcite, 7.23wt%; apatite, 5.22wt%; epidote, 11.64wt%; clinozoisite, 1.25wt%; allanite, 5.63wt%; britholite, 4.11wt% and a Sr–Na aluminosilicate (probably stronalsite), 16.44wt% SrO. Calcite and apatite are chemically similar to those found in carbonatites and are therefore considered to be of igneous origin. Textural evidence indicates that the first skarn stage garnet+diopside+Sr–Na aluminosilicate formed as the result of chemical interaction between carbonatites and adjacent silicate rocks. The formation of Sr-bearing epidote/clinozoisite, allanite and britholite appears to be related to the release of Sr into the fluid phase from the breakdown of high temperature assemblages during the retrograde skarn stage. During the final evolution stages, further alteration of britholite by bastnäsite and törnebohmite took place. The occurrence of REE minerals shows that the fluids responsible for this metasomatism must also have transported significant quantities of REE.  相似文献   

2.
Summary ¶This study experimentally investigated the transformation kinetics of synthetic polycrystalline aragonite to calcite at four temperature/pressure conditions (330°C/200MPa, 380°C/325MPa, 430°C/580MPa, and 480°C/875MPa), close to the calcite-aragonite equilibrium phase boundary. The extents of transformation measured as a function of time in a synthetic system, using in-situ annealed, high purity samples, are consistent with the kinetic model for grain-boundary nucleation and interface-controlled growth. The growth rates are slightly lower than those previously determined for a natural polycrystalline sample at 330 and 380°C. The activation energy (158kJ/mol) for calcite growth from synthetic samples is lower than that (247kJ/mol) from natural samples, but is close to the previously reported value (163kJ/mol) from a single crystal aragonite. The extrapolation of our experimental data to natural conditions reveals unusually fast transformation rates, in contrast to those of natural samples. The presence of deformational strain, fractures, defects or impurities in natural samples, and other factors may account for the discrepancy. This study suggests that the retrograde metamorphism of aragonite to calcite may proceed in a wide range of rates also depending on other geological factors than temperature and pressure.Received July 15, 2002; revised version accepted May 15, 2003  相似文献   

3.
Résumé L'étude géochimique par dosages des éléments traces métalliques (Cu, Zn, Pb, Ni, Co, V, Cr, Mo, Sb, Li, Rb.) dans les roches éruptives du Sud-Péruvien a été menée afin d'établir une corrélation entre ces éléments traces et les minéralisations exprimées. Les variations des teneurs en éléments traces de certains types de roches suivent la répartition des gites métallifères correspondants. Des considérations géochimiques nous ont amené à diviser les Andes en deux provinces majeures, une province occidentale où dominent les effets du magmatisme en relation avec la subduction de la plaque de Nazca qui induit une zonalité transverse à l'orogène, et une province orientale où domine en revanche l'influence de zones de fusion crustales génératrices de minéralisations à dominante acide et poly-métallique. Une zonalité longitudinale à la chaîne andine se superpose à la précédente et s'explique par l'enfouissement graduel de la zone de fusion crustale vers le Nord.
The relationships between mineralisations in S-E Peru, and the eruptive country rocks have been studied by means of quantitative trace element determination (Cu, Zn, Pb, Ni, Co, V, Cr, Mo, Sb, Li, Rb). The variations of trace element content in the rocks is in good accordance with the repartition of the ore deposits. We have divided the Andean orogen in two principal provinces according to geochemical considerations: in the occidental one the Nazca plate subduction plays an important role in mineralizations zoning, but in the oriental one, the control of acid and poly-metallic mineralisations can be attributed to crustal fusion zones. Nevertheless, a longitudinal zoning appears in this zone and can be related to northward depth's increase of crustal fusion zone.
Zusammenfassung Die Analyse von verschiedenen Metallen (Cu, Zn, Pb, Ni, Co, V, Cr, Mo, Sb, Li, Rb) in magmatischen Gesteinen der süd-peruanischen Provinz zeigt eine Wechselbeziehung zwischen Geochemie und Lagerstättenbildung. Die Veränderungen der Gehalte folgen der Verteilung der entsprechenden Lagerstätten. Es folgt daraus, daß die Anden in zwei geochemische Provinzen einzuteilen sind. In der westlichen Provinz sind die Lagerstättenbildungen durch die Nazca Subduktion Platte kontrolliert; dieser Platte zugeordnet ist eine Einteilung in Zonen. In der östlichen Provinz haben die krustalen Aufschmelzungen einen sauren Charakter der Lagerstättenbildungen zur Folge. Eine weitere Teilung der östlichen Provinz in Zonen erklärt sich aus der Vertiefung der krustalen Aufschmelzungszonen in nördlicher Richtung.
Resumen El estudio geoquemico de Cu, Zn, Pb, Ni, Co, V, Cr, Mo, Sb, Li, Rb, en rocas igneas del Sur-Este del Peru ha llegado a relacionar estos elementos en trazas y los yacimientos explotados. Las variaciones de contenido en elementos metallicos de ciertos tipos de rocas igneas siguen la abundancia de los indicios superficiales corespondientes. Consideraciones geoquemicas permiten dividir la Cordillera de los Andes en dos provincias de major importancia: una provincia oriental donde esta predominante el efecto del magmatismo relacionado a la subduction de la placa de Nazca, lo que induce una zonalidad transversal a los Andes; una provincia oriental donde zonas de fusion hundidas dentro de la corteza continental producen en general paragenesis de tipo acido y polimetalico.Una zonalidad longitudinal a los Andes esta sobrepuesta a la precedente y se explica por el hundimiento progresivo de la zona de fusion hacia el Norte, entre Bolivia y el Peru del Sur.
, - : , Zn, Pb, Ni, , V, Cr, Mo, Sb, Li, Rb. . , . Nazca; . . , , , .
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4.
Résumé Après mise au point d'une technique permettant la comparaison des rapports isotopiques de potassium extrait de roches variées, à mieux que 0,2% près, on a recherché les variations possibles de l'abondance des isotopes 39 et 41. Certains résultats obtenus sur des roches volcaniques semblent indiquer des variations réelles. Une hypothèse est proposée pour l'interprétation de ces résultats, basée sur des phénomènes de différenciation crustale.
The relative abundance of 41 and 39 isotopes of potassium has been determined in various rocks. A brief account of experimental technique is given; the reproducibility of measurement is about 0.2%. Most samples do not show significant variations, except some volcanic rocks. A tentative interpretation of these results, based upon crustal differentiation, is given.
Zusammenfassung Dieser Bericht gibt die Ergebnisse der Isotopenanalysen des Kaliums, die an etwa vierzig verschiedenen Felsen-Probenstücken ausgeführt wurden. Die experimentelle Technik wird hier kurz beschrieben. Die meisten Felsarten, bis auf einige vulkanische Felsen, zeigen keine bedeutenden Veränderungen. Es wird eine Interpretation der Ergebnisse vorgeschlagen, die auf der Theorie der chemischen Differenzierung der Erdkruste beruht.
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5.
Summary On Diego de Almagro Island in Chilean Patagonia (51°30S), a convergent strike slip zone, the Seno Arcabuz shear zone, separates the Diego de Almagro Metamorphic Complex from very low grade metagreywackes in the east, which were intruded by Jurassic granitoids. The Diego de Almagro Metamorphic Complex is composed of a metapsammopelitic sequence containing blueschist intercalations in the west and (garnet) amphibolite lenses in the east. Peak metamorphic conditions (stage I) at 9.5–13.5kbar, 380–450°C in the blueschist and at 11.2–13.2, 460–565°C in the amphibolite indicate subduction and accretion at different positions within the deepest part of the accretionary wedge. A K–Ar age of 117±28Ma of amphibole approximately dates the peak of metamorphism in the amphibolite. The early retrograde stage of metamorphism occurred under static conditions and resulted in localized equilibration (stage II) at 6.3–9.6kbar, 320–385°C in the blueschist and 6.1–8.4kbar, 310–504°C in the amphibolite. Both P-T paths converge within a midcrustal level.In contrast, an orthogneiss of trondhjemitic composition occurring within the Seno Arcabuz shear zone is associated with a garnet mica-schist containing a high temperature/intermediate pressure assemblage formed at 4.9–6.5kbar, 580–690°C. A muscovite K–Ar age of 122.2±4.6Ma dates cooling after this event which is related to a concomitant magmatic arc. These rocks were overprinted by a mylonitic deformation, which is caused by convergent strike slip shearing and ends during formation of a retrograde phengite-chlorite-stilpnomelane assemblage at a minimum pressure of approximately 5.7kbar (at 300°C).Zircon fission track ages from rocks of the Seno Arcabuz shear zone are 64.9±2.7 and 64.9±2.7Ma; they record the end of shearing in the Seno Arcabuz shear zone that juxtaposed all rocks in the middle crust. Zircon fission track ages ranging from 78 to 105Ma in the South Patagonian batholith to the east indicate earlier cooling through 280°C. The rocks of the Diego de Almagro Metamorphic Complex were initially slowly exhumed and resided at a midcrustal level before being emplaced via shearing in the Seno Arcabuz shear zone. Apatite fission track ages (54±8Ma) from the Seno Arcabuz shear zone show that exhumation and cooling rates increased after this event. The incorporation of continental crust within the subduction system was a late process, which modified the Cretaceous accretionary wedge, resulting in considerable shortening of the convergent margin.  相似文献   

6.
Summary A large number of podiform chromitite bodies of massive, disseminated and nodular type have been located in ultramafic units, composed of depleted mantle harzburgite and dunite of the Marmaris Peridotite from Ortaca (Mula, SW Turkey). The chromite ore bodies are surrounded by dunite envelopes of variable thickness, exhibiting transitional boundaries to harzburgite host rocks. Chromitites, containing a large number of inclusions, i.e. silicates, base metal sulphides and alloys, and platinum-group minerals (PGM) have a wide range of chemical composition. The Cr# [Cr/(Cr+Al)] values of most chromitites are high (0.61–0.81) and Mg# [Mg/(Mg+Fe2+)] values range between 0.65 and 0.71 with TiO2 content lower than 0.24wt.%, which may reflect the crystallization of chromites from boninitic magmas in supra-subduction setting environment.Platinum-group minerals (PGM) such as laurite, erlichmanite and Os–Ir alloys, silicates such as olivine, clinopyroxene and amphibole, and base metal sulphides (BM-S), alloys (BM-A) and arsenides (BM-As) are found as inclusions in chromite or in the serpentine matrix. Platinum-group element (PGE) concentrations of the Ortaca chromitites (OC) are low in all samples. Total PGE (Ir+Ru+Rh+Pt+Pd) ranges from 63ng/g to 266ng/g and Pd/Ir ratios range between 0.23 and 4.75. PGE content is higher and the Pd/Ir ratio lower in Cr-rich chromitites compared to Al-rich ones. There is a strong negative correlation between the Cr# and Pd/Ir ratios (r=–0.930). The PGE patterns show a negative slope from Ru to Pt and a positive slope from Pt to Pd. The low PGE content in the majority of the OC may reflect a lack of sulphur saturation during an early stage of their crystallization. The laurite compositions show a wide range of Ru–Os substitution caused by relatively low temperature and increasing f(S2) during the chromite crystallization. The high Cr# of and hydrous silicate mineral inclusions in chromite imply that chromite crystallized in a supra-subduction setting.  相似文献   

7.
Zusammenfassung Beobachtungen gefügekundlicher und morphologischer Details an Bewegungsbildern einiger durch Hangbewegungen (Talzuschübe) charakteristisch verformter Isoklinalhänge in schiefrig-plattigem Quarzphyllit der Nördlichen Grauwackenzone NE des Gerlospasses (Salzburg, Österreich) waren Anregung und Grundlage zum Studium geeigneter Bewegungsmechanismen anhand gefügeäquivalenter ebener Modellsysteme.Bei großmaßstäblichen Böschungsversuchen mit Lochkartenpaketen als Modellmaterial haben sich den Gegebenheiten im Gelände entsprechende strukturelle Ergebnisse erzielen lassen.Aus unterschiedlichen Versuchsdurchführungen ergaben sich Hinweise auf die Beeinflussung des Ablaufs und des Ergebnisses des Vorganges durch den Zeitfaktor.Die Verhältnisse im Gelände werden dargestellt und die Modellversuche dazu erläutert.
NE of Gerlospass (Salzburg, Austria) slaty quartzphyllites with slope-parallel cleavage have been characteristically shaped by slope-movements (creep; TalzuschubStini).Based on observations of typical features concerning joint pattern and morphology in these high slopes, experimental studies on deformation-mechanisms, which have probably caused the well analysed structures, had been carried out with the help of two-dimensional idealized models, representing equivalent fabric with special respect to planes of slaty cleavage and joints.Using punch-cards as model material, large-scale stability model-tests have shown results corresponding to the actual movement-pictures.Different test-conditions allow conclusions about the effects of the time-factor on the deformation-process.The paper deals with the geological field-data and the corresponding model tests.
Résumé Des observations de détail, structurales et morphologiques, faites sur des figures de mouvement résultant de déformations suivant la pente subies par des phyllades quartziques de la »zone septentrionale de la grauwacke« au Nord-Est du col de Gerlos (Salzbourg, Autriche) ont fait l'objet de l'étude du mécanisme du mouvement sur des modèles plans de structure équivalente.On a utilisé des paquets de cartes d'ordinateur comme matériau de maquette pour ces recherches sur pente à grande échelle. Les résultats se sont montrées concordants aux données du terrain du point de vue structural.Les recherches, menées de plusieurs façons différentes, ont donné des renseignements sur l'influence du facteur temps sur le déroulement et les résultats du phénomène. Les conditions de terrains sont figurées et les essais sur modèles en fournissent l'explication.
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8.
Summary The Ni–Cu–Platinum Group Element (PGE) sulfide deposits of the Sudbury Structure have provided a major portion of the worlds total nickel production and their host rocks have been the subject of numerous research studies, yet a number of perplexing problems remain to be solved. On the one hand, studies seeking to explain the formation of the Sudbury Structure have now converged on a genetic model which proposes that the Main Mass and Offset Dykes of the Sudbury Igneous Complex (SIC) were produced by crystallization of an impact-generated melt sheet. On the other hand, these models have yet to be fully reconciled with the production of the very large volume of magmatic Ni, Cu, Co, and PGE-rich sulfide mineralization and the associated mafic rock types. This paper explores this problem using new precious metal data from the Main Mass and Offset Dykes. These data are used to understand the relationships between these rocks, and to provide constraints on how the Ni–Cu–PGE sulfide ore deposits fit into the geological evolution of the Sudbury Structure.In the two drill cores selected for study in this project, the Mafic Norite has 1–5 modal percent pyrrhotite plus chalcopyrite, and elevated Ni (40–1000ppm), Cu (40–1140ppm), and PGE (1.9–7.8ppb Pd, 1.8–7.3ppb Pt); this is overlain by Felsic Norite that contains pyrrhotite, and has a wide range in concentration of Ni (13–257ppm), Cu (7–328ppm), and PGE (<0.01–6.4ppb Pd, <0.01–5ppb Pt). For a similar range of MgO, the upper portion of the Felsic Norite unit has 5–10 times lower Ni and Cu abundances than within-plate basalts and local crustal rocks, and PGE abundance levels are mostly below analytical determination limits. Stratigraphic studies of other compositional profiles around the SIC demonstrate that this depletion signature of Ni, Cu, and PGE is widespread and developed not only above mineralized embayments and offsets, but also above barren sections of the lower contact of the SIC.The depletion of the upper part of the Felsic Norite in Ni, Cu and PGE is presumably due to equilibration of the magma with magmatic sulfide, and accumulation of this dense sulfide liquid. Results of modeling indicate that the parental magma giving rise to the Mafic and Felsic Norites had initial Ni and Cu contents of 210 and 110ppm, respectively. In addition, Ni, Cu and PGE tenors calculated in 100% sulfide from the Copper Cliff Offset average 13% Cu, 6% Ni, 18ppm Pd, and 19ppm Pt indicating that these sulfides had formed by fractionation from magmas that contained 310ppm Ni, 310ppm Cu, 18ppb Pd and 19ppb Pt. These values are factors of 3 to 5 higher than the Ni, Cu, Pd, and Pt contents of the Onaping Formation with average values of 55ppm Ni, 48ppm Cu, and 4.9ppb Pd as well as the marginal sulfide-poor phase of the Worthington Offset quartz diorite, which has average values of 61ppm Ni, 59ppm Cu, 2.8ppb Pd and 4.0ppb Pt. Both the Onaping Formation and the marginal quartz diorite are believed to represent the initial composition of a large component of the melt sheet. There is therefore a fundamental problem in reconciling the initial metal contents of the SIC magma as indicated by the marginal phases of the Offset dykes and that of the Onaping Formation with the composition of the SIC magma at the times of formation of the sulfides as indicated by their Ni, Cu and PGE tenors.It is proposed that because the SIC melt sheet was initially superheated with a temperature of 1700°C, it was able to dissolve 5 times as much S as it could at its liquidus temperature of 1200°C. It was also initially composed of an emulsion of mafic and felsic melts (Marsh and Zieg, 1999), which may have formed discrete magma cells. As the temperature of the melt sheet decreased, some of these magma cells became S-saturated and the resultant Ni–Cu–PGE sulfides settled downwards and on reaching magma cells lower in the melt sheet were re-dissolved thereby raising the Ni, Cu and PGE contents of the lower magma cells. It was from these enriched magma cells that precipitation of the ore-forming Ni–Cu–PGE sulfide melts eventually took place.The mineral potential of Offset and embayment structures appears to be empirically linked to the thickness of the overlying noritic rocks; for example, the most heavily mineralized embayments and Offset Dykes are located in areas where the Felsic Norite is thickest. It appears unlikely that the entire 1–3km-thick melt sheet was convectively mixing throughout its lateral extent, and so the heterogeneity in sulfide distribution was retained after crystallization and cooling.  相似文献   

9.
A Rb-Sr whole-rock investigation of suites of samples from two orthogneiss bodies in the Almendralejo area of the Oporto-Portalegre-Badajoz-Córdoba belt points to an Ordovician age for the granitic magmatism: about 470 Ma for the Almendralejo Gneiss (peralkaline metagranite) and about 425 Ma for the Ribera del Fresno Gneiss (leucocratic subaluminous metagranite), both with high initial87Sr/86Sr ratios. This points to a post-Ordovician (Hercynian) age for the metamorphism and tectonic deformation that affected the area, including the gneisses, and rules out the alleged Cadomian or even older age.
Zusammenfassung Die Rb-Sr-Gesamtgesteinsanalysen von Probeserien zweier orthogneisischer Körper der Almendralejo-Region des Oporto-Portalegre-Badajoz-Córdoba-Gürtels weisen auf ein ordovizisches Alter des granitischen Magmas hin: rund 470 Ma für den Almendralejo-Gneis (peralkaliner Metagranit) und rund 425 Ma für den Ribera del Fresno-Gneis (leukokratischer Metagranit), beide bei einem hohen initialen87Sr/86Sr. Diese Ergebnisse deuten auf ein post-ordovizisches (herzynisches) Alter der Metamorphose und der tektonischen Deformation hin, die das Gebiet, einschließ-lich des Gneises, beeinflußt haben. Sie schließen ein vermeintliches kadomisches oder älteres Alter aus.
Résumé Une étude par Rb-Sr sur roche totale de séries d'échantillons de deux massifs d'orthogneiss de la région d'Almendralejo, dans la ceinture Oporto-Portalegre-Badajoz-Cordoue, indique un âge ordovicien pour le magmatisme granitique: environ 470 Ma pour le gneiss d'Almendralejo (métagranite peralcalin) et environ 425 Ma pour le gneiss de Ribera del Fresno (métagranite leucocrate subalumineux), tous deux avec des rapports initiaux87Sr/86Sr élevés. Ces résultats indiquent un âge post-ordovicien (hercynien) pour le métamorphisme et la déformation tectonique qui ont affecté la région, y compris les gneiss, et éliminent l'hypothèse proposée par ailleurs d'un âge cadomien ou même plus ancien.
— — — Rb/Sr : 425 -- ( ) 470 ( ) 87Sr/86Sr . - ( ) , , . , .
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10.
Summary A detailed electron microprobe study of P, F, Ge and Ga-contents in rock-forming topaz was performed on a suite of Variscan granites at Podlesí in the western Kruné Hory Mts., Czech Republic. Topaz crystals from the relatively less evolved biotite- and protolithionite granites display homogeneous cathodo-luminescence (CL) intensities, whereas topaz from the marginal pegmatite, highly fractionated zinnwaldite granite and greisens show intense oscillatory zoning. Phosphorus contents reach 1.15wt% P2O5 in topaz from the zinnwaldite granite. Many topaz crystals are distinctly zoned with a maximum P content in the transition zone between core and rim. Phosphorus is incorporated into the topaz lattice by berlinite substitution: Al3++P5+=Si4++Si4+. The majority of analysed topazes are highly saturated in F, reaching 90–97% of the theoretical maximum saturation. Topaz from the marginal pegmatite only reaches 87–90% of F-saturation. There is a positive correlation between Ptopaz and Pwhole rock, but no correlation between Ftopaz and Fwhole rock. No difference has been found in P and F contents between magmatic and the hydrothermal (=greisen stage) topaz. Contents of Ge and Ga vary from around the detection limit (50ppm) up to 200ppm Ge and 100ppm Ga, respectively.  相似文献   

11.
Summary The paper investigates the role of primary magmatic phases in the fractionation and concentration of PGE in Archaean mafic and ultramafic systems. The composition of chromites and olivines in sulphur-poor (S<0.6wt%) komatiites from the Agnew-Wiluna Belt (Western Australia), and of chromite concentrated from komatiitic basalt, ferropicritic basalt and tholeiitic basalt from the Abitibi Belt (Canada) were analysed. The results of laser ablation ICP-MS analyses show that PGE-bearing alloys are not stable in crystallising komatiite and that ruthenium is soluble in chromite during crystallisation. Conversely, analyses of chromites separated from Theos Flow tholeiitic basalt indicate that Ir–Os–(±Pt) enrichments (>200ppb) reflect the presence of PGM. Chromites from Freds Flow komatiitic basalt contain Ir-rich clusters, whereas Pt enrichments (>370ppb) in Boston Creek ferropicritic basalt reflect the presence of Pt-rich compounds. The presence of PGE-bearing alloys in Theos Flow and Freds Flow is due to late S-supersaturation, whereas the presence of Pt-rich compounds in Boston Creek Flow reflects high state of melt oxidisation. The lack of PGE-bearing alloys in the olivines and chromites of komatiites can be explained by thermal instability of PGM, depletion in PGE at the mantle source, early S-supersaturation, the oxidisation conditions of the melt, or a combination of these factors.  相似文献   

12.
Summary The provenance of the Mid-Devonian clastic sediments in the Teplá-Barrandian Unit (TBU) of the Bohemian Massif was investigated by laser ablation ICP-MS U–Pb zircon dating, bulk sediment geochemistry and mineralogical study of the heavy mineral fraction. In contrast to the island arc provenance of the TBU Neoproterozoic sediments, the Early Palaeozoic sediments contain significant amounts of differentiated crustal material. The detrital zircon populations in the Barrandian Mid-Devonian siltstones and sandstones show ages ranging from Archaean (3.0Ga) to Early Palaeozoic (0.39Ga). Major age maxima are at 2.6Ga, 2.0–2.25Ga, 0.62 and 0.51Ga. The youngest identified zircons so far correspond to Lower and Mid-Devonian ages. The extensive mechanical abrasion of zircons having Archaean (3.0, 2.8 and 2.6Ga) to Paleoproterozoic ages (2.25–2.0Ga) suggest their provenance from recycled old sedimentary sequences. The relatively large number of zircons with ages between 2.0 and 3.0Ga may indicate the presence of relicts of the Archaean/Paleoproterozoic crust in the source areas of the studied Mid-Devonian sediments. The absence of detrital zircon ages between 0.9 and 1.2Ga and the presence of zircon ages of 2.0–2.25 and 0.5–0.8Ga correspond to the zircon age pattern from the Gondwana-related North African, rather than Gondwana-related South American and Baltic terranes. The material was entering the basin predominantly from the west and consisted primarily of detrital material of Cambrian granitoids and recycled material of Neoproterozoic meta-sedimentary sequences.  相似文献   

13.
The brittle-plastic transition and the depth of seismic faulting   总被引：8，自引：0，他引：8  
A simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow. The two are separated by an abrupt brittle-plastic transition, which is assumed to be indicated by the lower limit of seismicity. Experimental studies, however, as well as the deformation structures of mylonites, indicate that a broad transitional field of semi-brittle behavior lies between these extremes. This is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws. For quartzofeldspathic rocks the semi-brittle field lies between T1, the onset of quartz plasticity at about 300 °C and T2, feldspar plasticity at about 450 °C. A model is presented in which the transition T1 does not correspond to a transition to bulk flow but to a change from unstable, velocity-weakening friction to stable, velocity-strengthening friction. T1 thus marks the depth limit of earthquake nucleation, but large earthquakes can propagate to a greater depth, T3, (T32) which corresponds to the lower limit of dynamic frictional behavior in the semi-brittle field and approximately to the peak in strength. The zone between T1 and T3 is one of alternating behavior, with flow occurring m the interseismic period and with co-seismic dynamic slip occurring during large earthquakes. This zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting. The transition T1 is also marked by a change in the generation mechanism of fault rocks, from abrasive wear above which produces cataclastites, to adhesive wear below, which is proposed as an important generation mechanism of mylonites in the upper part of the semi-brittle field.
Zusammenfassung Ein einfaches rheologisches Modell für Schervorgänge in der Lithosphäre, das eine weite Akzeptanz erreicht hat, ist das Zweilagenmodell mit einer oberen spröden Zone, in der Deformation über Reibungsgleitung entlang diskreter Störungsflächen stattfindet, und einer unteren duktilen Zone. Hier erfolgt die Deformation durch plastisches Fließen. Beide Zonen werden durch einen abrupten spröd-plastischen Übergang voneinander getrennt, der vermutlich durch die untere Grenze der nachweisbaren Seismizität angezeigt wird. Experimentelle Untersuchungen wie auch die Deformationsgefüge in Myloniten zeigen hingegen, daß ein breites Übergangsfeld mit semisprödem Verhalten zwischen diesen beiden Extremen liegt. Hier befindet sich ein Bereich »gemischter« Deformation, deren Ausmaß beträchtlich über den aus der Extrapolation von Hochtemperatur-Fließ-Gesetzmäßigkeiten ableitbaren Werten liegen dürfte. Für Quarz-Feldspat-Gesteine liegt das halbspröde Feld zwischen T1 dem Beginn der plastischen Deformation des Quarzes bei ca. 300 °C, und T2, dem Beginn der Feldspatplastizität bei ca. 450 °C. Hier wird ein Modell vorgestellt, in dem der Übergang bei T1 nicht dem Übergang zum Gesamtfließen, sondern dem Wechsel von instabiler, geschwindigkeitsreduzierender Reibung zu stabiler, geschwindigkeitskonstanter Reibung entspricht. T1 markiert damit die Tiefengrenze der Erdbebenbildung, stärkere Beben können dagegen in größere Tiefe reichen T3 (T32), die dann der unteren Grenze dynamischen Reibungsverhaltens im semispröden Bereich und angenähert dem Stärkemaximum entspricht. Die Zone zwischen T1 und T3 zeigt alternierendes Verhalten mit plastischem Fließen während interseismischer Perioden und dynamischem Gleiten während größerer Erdbeben. Diese Zone wird charakterisiert durch Mylonite und dicht zwischengepackten Pseudotachyliten sowie anderen Anzeigern dynamischer Faltung. Hinzu kommt im Bereich des Überganges T1 ein Wechsel im Bildungsmechanismus gestörter Gesteine. Er geht von durchgreifender Ermüdung, die zu Kataklasiten führt, bis zu adhesiver Ermüdung, die als wichtiger Bildungsmechanismus für Mylonite im oberen Abschnitt des semispröden Feldes angesehen wird.
Résumé Le modèle rhéologique de cisaillement de la lithosphère le plus largement accepté comporte deux couches superposées: une couche supérieure cassante, siège de déformations le long de surfaces discrètes, les failles, et une zone inférieure ductile ou s'opère un fluage plastique d'ensemble. La transition entre ces deux domaines est brusque et considérée comme la limite inférieure de la séismicité.L'étude des structures mylonitiques ainsi que les mesures expérimentales indiquent cependant qu'un large champ de transition à caractère «semi-cassant» s'étend entre ces deux extrêmes. Ruptures et fluage plastique sont présents dans cette zone, dont la compétence peut être considérée comme bien supérieure à celle qui résulte de l'extrapolation des lois de fluage à haute température. Pour les roches quartzofeldspathiques, ce champ «semi-cassant» s'étend de T1, seuil de plasticité du quartz (environ 300 °C) à T2, celui du feldspath (environ 450 °C). Dans ce modèle, la température T1 ne correspond pas au seuil de fluage d'ensemble, mais à la frontière entre une région de frictions «instables» qui diminuent avec la vitesse, et une région de frictions «stables» qui augmentent avec la vitesse. Elle correspond donc à la profondeur limite de genèse des séismes. Les grands séismes peuvent cependant se propager jusqu'à une profondeur T3 (T32) qui correspond à la limite inférieure du comportement dynamique des forces de frottement dans le domaine «semicassant». T3 peut être considérée approximativement comme le point de résistance maximale de cette zone de transition. La zone comprise entre T1 et T3 peut donc être le siège alternativement soit de processus de fluage, soit de glissements le long de surfaces de rupture à l'occasion de séismes de forte magnitude. Cette zone se caractérise par des mylonites entremêlées de pseudotachylites et autres signes de rupture dynamique. La transition T1 est également marquée par une modification du mécanisme de transformation des roches dans les zones de faille. Audessus de cette limite, un mécanisme d'usure «abrasif» produit des cataclasites, par opposition à un mécanisme d'usure «adhésif» en-dessous. Ce dernier mécanisme est proposé comme fort probable lors de la genèse de mylomtes dans la partie supérieure du champ «semi-cassant».
() , . , , . . - , , . , , , , , . « » , , . - T1 — 300° T2 — 450°. , T1 , , , . . . T1 , , . — 3 (3 > 2), . . , . t1 2 . , . , t1 . , , ; .
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14.
Temperatures of the formation of mud-volcanic waters are determined based on concentrations of some temperature-dependent components (Na–Li, Mg–Li). Estimates obtained for the Taman and Kakhetia regions are similar and range from 45 to 170°, which correspond to depths of 1–4.5 km. The calculated temperatures correlate with the chemical (Li, Rb, Cs, Sr, Ba, B, I, and HCO3) composition of water and 13 (2) and 13 (CH4) values in spontaneous gases. The isotope values indicate that mechanisms of the formation of 13-rich gases, i.e., gases with high 13 values (up to +16.0 in 2 and –23.4 in CH4) in mud-volcanic systems of Taman and Kakhetia are governed by fluid-generation temperatures rather than the supply of abyssal gases. The 11 value was determined for the first time in mud-volcanic products of the Caucasus region. This value ranges from +22.5 to +39.4 in the volcanic water of Georgia, from –1.2 to +7.4 in the clayey pulp of Georgia, and from –7.6 to +13.2 in the clayey pulp of Taman. It is shown that the 11 value in clay correlates with the fluid-generation temperature and 11 correlates with 13 in carbon-bearing gases. These correlations probably testify to the formation of different phases of mud-volcanic emanations in a single geochemical system and suggest the crucial role of temperature in the development of isotope-geochemical features.  相似文献   

15.
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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16.
Zusammenfassung In vielen Fällen beeinflussen äußere Faktoren das Vorkommen der Ammoniten. Im Oberjura spielt die Wassertiefe eine große Rolle, daneben tritt die Wassertemperatur. Voraussetzungen der ökologischen Analyse sind bodenbezogenes Leben der Tiere und postmortale Autochthonie der Schalen. Methoden und Fehlerquellen werden besprochen. Aus der ökologischen Bindung vieler Ammoniten ergeben sich Folgen, vor allem für Stratigraphie, Phylogenie und Ontogenie.
In many cases external factors determine the occurence of the ammonites. In the Upper Jurassic the depth of the sea is of great importance, as well as the temperature of the water. Ecological analysis requires life of the animals close to the sea floor, and that the shells are autochthonous.Methods and sources of errors are discussed. The dependence on environment of many ammonites has consequences, especially for stratigraphy, phylogeny and ontogeny.
Résumé Souvent des facteurs extérieurs déterminent la présence des ammonites. Dans le Jurassique Supérieur la profondeur de la mer joue un grand rôle, aussi bien que la température de l'eau. L'analyse écologique doit supposer que les animaux vivent près du fond de la mer et que les coquilles sont autochtones. Les méthodes et les sources d'erreurs sont discutées. La dépendance des ammonites de leur milieu a des conséquences avant tout pour la stratigraphie, la phylogénie et l'ontogénie.
, . , . .
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17.
Summary Geochronological data (U–Pb, Rb–Sr and 40Ar/39Ar) are used to unravel the Late Alpine high-grade metamorphism, migmatisation and exhumation of Variscan granitoids within the core of the Central Rhodopean dome, Bulgaria. The age of the granitoid protolith is 300±11Ma, as determined by U–Pb analyses on single zircons selected from the core of the dome structure.Rb–Sr whole rock data define an errorchron with a large scatter of the data points due to the Late Alpine metamorphic overprint. The slope of the reference line indicates a Variscan magmatic event. Strontium characteristics are used to discriminate the samples most influenced by metamorphism from those, which reflect possible differences in the protolith age of the granitoids.Petrological-geochemical data, the initial strontium ratio of 0.708±0.001, and Hf zircon values ranging from –2.58 to –3.82 point to a mixed, but crust-dominated origin of the Variscan magmas; young crustal material and mantle fragments were sources for the I-type metagranitoids.The exhumation of the granitoids from depths greater than 20–25km to about 5km below the surface was a rapid geological process. It started with the formation of granitic eutectic minimum melts at the temperature peak of metamorphism. Monazite crystallisation at about 650°C continued during isothermal decompression to possible depths of about 10–12km. An age of 35.83±0.40Ma was determined using conventional U–Pb isotope methods on four multigrain monazite fractions. A maximum average age of 36.6–37.5Ma (assuming same error uncertainties) for crystallisation of the metamorphic monazites was calculated assuming 10 to 20% monazite resetting during the subsequent Oligocene volcanism and hydrothermal activity in the region of the Central Rhodopean Dome. The rocks were then cooled to about 350–300°C at 35.35±0.22Ma according to 40Ar/39Ar ages of biotites and below 300°C at 35.31±0.25Ma (Rb–Sr data), as indicated by crystallisation of adularia in an open vein subsequent to pegmatite intrusion. A minimum exhumation rate of 3–5km per 1 million years can therefore be calculated for the exhumation of the metagranitoids during the period from 38–35Ma.  相似文献   

18.
Deformation and stability of an elasto-plastic softening pillar   总被引：6，自引：0，他引：6  
Summary A model of rock pillar or coal seam is considered assuming linear elastic behaviour before reaching the maximum strength and post-peak behaviour characterized by the residual strength. The deformation and stress across the pillar height are assumed to be uniform and the interaction with overlying rock strata is treated assuming beam model of the strata. The elasto-plastic stress distribution within pillar and the onset of instability occurring for the critical opening span are determined. Comparison with a solution for a simplified spring model of pillar is also presented.  相似文献   

19.
Summary The Etive complex, one of the Caledonian Newer Granites of Scotland, is a ring complex of Devonian age, ranging in composition from pyroxene-diorite to leucogranite. Six samples, representing the major rock units in the southern parts of the Etive complex were chosen for mineral chemical studies and for estimation of the pressure and temperature conditions of magmatic crystallisation. Application of Al- in-hornblende barometry and crossite contents of amphiboles indicates a pressure <3kbar for the intrusion, in good agreement with published independent pressure estimates of 2kbar from mineral equilibria in metasedimentary hornfelses in the Etive thermal aureole. Thermometry, using ternary and binary feldspar systems, yields low temperatures, which probably reflect late-stage, post-magmatic re-equilibration of these minerals. Several geothermometers have been applied to the Quarry Diorite, the outermost intrusion of the complex. The highest temperature for the rocks comes from orthopyroxene–clinopyroxene solvus thermometry, and is 1000°C; this is interpreted to reflect the initial crystallisation of the diorite magma immediately after its emplacement. The maximum temperature from hornblende-plagioclase thermometer is 816°C, which probably reflects late-stage crystallisation of the magma.  相似文献   

20.
The current picture of the plate configuration in the western Mediterranean suggests that Africa and Europe are separated by a transcurrent fault striking east-west through the Strait of Gibraltar and the Alboran Sea. Such a fault has been generally assumed because the Azores transform fault approaches the Horseshoe Seamounts north of Madeira and therefore could possibly extend through the Strait of Gibraltar. However, detailed geological studies in the Arc of Gibraltar and the Alboran Sea reveal no major east-west fault transecting the crust there. This finding is used here to revise the boundary between the African and Eurasian plates. It is suggested that the northern boundary of the African plate in the western Mediterranean is formed by the Hayes-Atlas Fault instead of the Azores transform.
Zusammenfassung Nach gegenwärtiger Auffassung der Plattenkonfiguration im westlichen Mittelmeergebiet werden Afrika und Europa durch eine Ost-West verlaufende Transformverwerfung durch die Straße von Gibraltar und die Alboran See getrennt. Eine solche Verwerfung wird allgemein angenommen, da die Azoren-Transformverwerfung bis zum Horseshoe Seamount reicht und möglicherweise durch die Straße von Gibraltar fortgesetzt ist. Detailierte geologische Untersuchungen im Gibraltarbogen und der Alboran-See weisen jedoch nicht auf eine größere Ost-West Verwerfung hin, die dort die Kruste durchziehen könnte. Auf Grund dieses Befundes wird die Grenze zwischen der afrikanischen und der eurasischen Platte rediviert. Es wird vorgeschlagen, daß nicht die Azoren-Transformverwerfung, sondern die Hayes-Atlas-Verwerfung die nördliche Begrenzung der afrikanischen Platte im westlichen Mittelmeergebiet bildet.
Résumé Le modèle habituellement admis des plaques dans la partie ouest de la Mediterranée suggère que l'Afrique et l'Europe sont séparées par une faille transcurrente est-ouest passant par le détroit de Gibraltar et la mer d'Alboran. Cette conception se base sur le fait que la faille transformante des Açores atteint le »Horseshoe Seamount« au nord de Madère et qu'elle pourrait se prolonger au travers du détroit de Gibraltar. Toutefois, des études géologiques détaillées dans l'Arc de Gibraltar et la mer d'Alboran ne révèlent pas de faille majeure (est-ouest) qui affecterait la croûte dans cette région. Cette observation amène une nouvelle interprétation du contact entre les plaques africaine et eurasiatique: il est suggéré que la bordure nord de la plaque africaine dans la partie ouest de la Méditerranée est formée par la faille de «Hayes-Atlas» et non par celle des Açores.
, . , «» (Horseshoe Seamounts), , , . , - . . , , Hayes-Atlas.
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