The mesothermal gold-lamprophyre association: significance for an accretionary geodynamic setting,supercontinent cycles,and metallogenic processes     

R. Kerrich  D. A. Wyman 《Mineralogy and Petrology》1994,51(2-4):147-172

Summary Archean shoshonitic lamprophyres are cotemporal and cospatial with gold mineralization in the Superior Province of Canada, both being emplaced along translithospheric structures that demark subprovince boundaries. By analogy with geochemically similar Phanerozoic counterparts, the dikes are a product of specific plate interactions rather than a deep asthenosphere plume-initiated event, and their onset in the late-Archean at 2.7 Ga signifies that Phanerozoic style plate-tectonics was operating at this time. Fresh shonshonitic dikes are characterized by normal background gold contents of 3.9 ± 8.1 ppb (l), close to the value of 3.0 ppb for the bulk continental crust, and average abundances of As, Sb, Bi, W, TI, B, Cu, Pb, Zn, and Mo are also close to their values in bulk continental crust. Thus, fresh lamprophyres are not intrinsically enriched either in Au or elements affiliated with gold in mesothermal deposits, and accordingly do not constitute a special source rock. Platinum group element contents (Ir = 0.4 ± 0.58 ppb; Pt = 5.9 ± 26.5, Pd = 5.5 ± 1.8), in conjunction with Cu, Au, and Ni abundances, define approximately flat patterns on primitive mantle-normalized diagrams, consistent with derivation of the alkaline magmas from a depleted mantle source variably enriched by incompatible elements. Comparable abundances and ratios of Pd/Au, Os/Ir, and Ru/Ir in Archean lamprophyres, Archean komatiites, and Gorgona komatiites signify that the Archean and Phanerozoic upper mantle had similar noble metal contents, such that the prolific greenstone belt Au-Ag vein deposits cannot be explained by secular variations in upper mantle Au abundance alone. The lack of covariation between Au and light rare earth elements in lamprophyres rules out mantle metasomatism as a process generating intrinsically Au-rich magmas.Emplacement of the lamprophyres was diachronous from north (2710 Ma) to south (2670 Ma) in the Superior Province, as was the gold mineralization. Both were related to late transpressional tectonics during successive accretions of individual subprovinces. Alkaline magmatism and gold mineralization are temporally and spatially related because they share a common geodynamic setting, but they are otherwise the products of distinct processes. Much of Archean time was devoid of shoshonites and mesothermal gold deposits. The first widespread inception of this duality at 2.71–2.65 Ga in the Superior and Slave Provinces, Canada, and in India and Australia, may reflect one of the first supercontinent aggregations involving accretionary, Cordilleran style tectonics. Giant mesothermal gold provinces and shoshonites recur through time in the Palaeozoic and Mesozoic in this geodynamic setting.

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Die mesothermale Gold-Lamprophyrassoziation und ihre Bedeutung für Akkretionsgeotektonik, Superkontinent-Zyklen und metallogenetische Prozesse

Zusammenfassung Archaische schoschonitische Lamprophyre sind zeitlich und räumich mit Goldmineralisationen in der Superior Provinz Kanadas vergesellschaftet; beide sind an translithosphärische Strukturen, die die Grenzen von Subprovinzen markieren, geknüpft. Ein Vergleich mit geochemisch ähnlichen phanerozoischen Abfolgen weist darauf hin, daß die Gänge eher durch spezifische Platteninteraktions- und nicht durch Hot Spot initiierte asthenosphärische Prozesse entstanden sind. Ihre spät-archaische Alterseinstufung (ca. 2.7 Ga) belegt somit, daß bereits zu dieser Zeit plattentektonische Prozesse, wie sie für das Phanerozoikum typisch sind, funktionierten.Frische schoschonitische Gänge zeigen mit Durchschnittskruste vergleichbare Untergrundgehalte an Gold von 3.9 ± 8.1 ppb (l), Auch die Gehalte an As, Sb, Bi, W, Tl, B, Cu, Pb, Zn und Mo entsprechen Gehalten der durchschnittlichen Erdkruste. Frische Lamprophyre sind daher nicht an Gold bzw. an anderen für mesothermale AuLagerstätten typischen Elementen angereichert und stellen somit keine spezifischen Muttergesteine dar. Die Gehalte an Platingruppen-Elementen (Ir = 0.4 ± 0.58 ppb; Pt = 5.9 ± 26.5; Pd 5.5 ± 1.8) in Verbindung mit der Verteilung von Cu, Au und Ni definieren einen flachen Trend in auf primitiven Mantel normierten Diagrammen, was mit einer Herkunft der alkalischem Magmen aus einer abgereicherten Mantelquelle, die im unterschiedlichen Ausmaß an einzelnen inkompatiblen Elementen angereichert ist, hinweist. Ähnliche Elementverteilungen bzw. verhältnisse von Pd/Au, Os/Ir und Ru/Ir in archaischen Lamprophyren, archaischen Komatiiten und Gorgona-Komatiiten belegen, daß der archaische und phanerozoische obere Mantel ähnliche Gehalte an Edelund Buntmetallen aufweisen. Die Au-Ag Ganglagerstätten in Greenstone Belts können daher nicht ausschließlich mit einer Variation der Au-Gehalte des oberen Erdmantels erklärt werden. Die fehlende Korrelation zwischen Au und den leichten Seltenen Erden in den Lamprophyren schließt mantelmetasomatische Prozesse für die Bildung von Au-reichen Magmen aus.Die Platznahme der Lamprophyre in der Superior Provinz erfolgte zeitgleich von N (2710 Ma) nach S (2670 Ma) mit der Bildung der Goldmineralisationen. Beide stehen mit einer späten transpressionalen Tektonik während der sukzessiven Akkretion einzelner Subprovinzen in Beziehung. Alkalimagmatismus und Goldmineralisationen sind deshalb räumlich und zeitlich vergesellschaftet, weil sie innerhalb desselben geodynamischen Settings gebildet wurden. Sie sind sonst aber Produkte unterschiedlicher Prozesse. Über weite Zeiträume des Archaikums fehlen Schoschonite und mesothermale Goldlagerstätten. Das erste großangelgte Auftreten beider in der Superior und Slave Provinz Kanadas während 2.71-2.65 Ga und in Indien und Australien könnte eine der ersten Superkontinentaggregationen im Stile einer Cordillera-style Akkretionstektonik widerspiegeln. Riesige mesothermale Goldprovinzen und Schoschonite treten während des Paläo- und Mesozoikums immer wieder, gebunden an diese geotektonische Position, in Erscheinung.

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Geochemistry and origin of the Proterozoic ultrapotassic rocks of the Churchill Province,Canada     

Dr. T. D. Peterson  Dr. S. Esperança  Dr. A. N. LeCheminant 《Mineralogy and Petrology》1994,51(2-4):251-276

Summary Early Proterozoic ultrapotassic dikes, lava flows, and pyroclastic rocks of the Christopher Island Formation (CIF) erupted throughout an area 600 × 300 km within the Churchill Province of the Canadian Shield at 1.84 Ga. The rocks range from mafic lamprophyres (mg # 60; SiO₂ 47–54%, mean K₂O/Na₂O > 4) with phenocrysts of phlogopite + diopside + apatite ± olivine ± magnetite, to phenocryst-poor felsic rocks and sanidine porphyries (SiO₂55–69%). All samples have high incompatible element contents and display large depletions of high field strength elements relative to K, Rb, Sr, Ba, and Th. The CIF has geochemical and petrographic characteristics of both minettes and lamproites, but overall most closely resembles young Mediterranean lamproites. Felsic rocks of the CIF were produced by crystal fractionation and crustal contamination of mafic ultrapotassic magma, and include both high-silica lamproites strongly enriched in Zr, U, and Th, and weakly potassic to sodic rocks of trachytic composition. Flows and feeder dikes have relatively homogeneous _{Nd, 1840 Ma} (–6 to –11) but highly variable ES., 1840 Ma (–40 to + 100); samples classified as lamproites have higher average _Sr. Dike samples have highly variable present-day Pb isotope compositions, ranging from moderately to strongly nonradiogenic. Geochemical and isotopic data are consistent with contributions from depleted Archean lithospheric mantle, and OIB-type convecting mantle, both metasomatized by subduction-related processes during the Early Proterozoic. The lithospheric mantle probably contained Archean enriched domains as well. Proterozoic enrichment may have accompanied shallow underplating of subducted oceanic lithosphere beneath the Churchill Province during amalgamation of the Laurentian supercontinent. There are strong analogies in isotopic composition, and interpreted source region history, between the CIF and lamproites and minettes of the Wyoming Province and western Greenland, which suggest the existence of a Laurentian ultrapotassic superprovince.

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Geochemie und Entstehung der Proterozoischen ultrapotassischen Gesteine der Churchill Provinz, Kanada

Zusammenfassung Altproterozoische, ultrapotassische Gänge, Lavaströme und pyroklastische Gesteine der Christopher Island Formation (CIF), eruptierten in einem Gebiet von 600 × 300 km in der Churchill Provinz des Kanadischen Schildes vor 1.84 Ga. Die Zusammensetzung dieser Gesteine variiert von mafischen Lamprophyren (mg > 60; SiO₂ = 47–54%, durchschnittliches K₂O/Na₂O > 4) mit Phänokristallent von Phlogopit + Diopsid + Apatit + Olivin + Magnetit, bis zu phänokristallarmen felsischen Gesteinen und Sanidinporphyren (SiO₂ = 55–69%). Alle Proben zeigen hohe Gehalte an inkompatiblen Elementen und zeigen beträchtliche Verarmung an high field strength Elementen relativ zu K, Rb, Sr, Ba und Th. Die CIF hat geochemische und petrographische Eigenschaften sowohl von Minetten wie von Lamproiten, aber im allgemeinen ähnelt sie am stärksten jungen mediterranen Lamproiten. Felsische Gesteine der CIF wurden durch Fraktionierung und Krustenkontamination aus mafischen ultrapotassischen Magmen gebildet. Letztere umfassen sowohl siliziumreiche Lamproite, die deutlich an Zr, U und Th angereichert sind und schwach potassische bis sodische Gesteine von trachytischer Zusammensetzung. Lavenergüsse und zufuhrgänge zeigent ein relativ homogenes _{Nd, 1840 Ma} (–6 bis –11) aber ein sehr variables _{Sr, 1840 Ma} (-40 bis + 100); Proben die als Lamproite klassifiziert wurden, zeigent höhere durchschnittliche _Sr-Werte. Proben von Gängen haben sehr variable Bleiisotopen-Zusammensetzungen, die von mäßig bis stark nichtradiogen variieren. Geochemische und Isotopendaten weisen auf Beiträge aus verarmtem archaischen lithosphärischen Mantel und aus konvektierendem OIB-Typ Mantel hin, die beide während des Alproterozoikums durch Subduktions-Vorgänge metasomatisiert wurden. Der lithosphärische Mantel enthielt wahrscheinlich auch angereicherte archaische Domänen. Proterozoische Anreicherungsvorgänge dürften seichtes Underplating subduzierter ozeanischer Lithosphäre unter der Churchill Provinz während der Amalgamation des laurentischen Superkontinentes begleitet haben. Es gibt starke Analogien in der Isotopenzusammensetzung und in der interpretierten Geschichte der Ursprungsregion, zwischen den CIF und Lamproiten und Minetten der Wyoming Provinz, und des westlichen Grönland. Diese weisen auf die Existenz einer laurentischen ultrapotassischen Superprovinz hin.

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Dike rock generation and magma interactions in the Bir Safsaf igneous complex,south-west Egypt: Implications for the Pan-African evolution in north-east Africa     

D. Pudlo  G. Franz 《International Journal of Earth Sciences》1994,83(3):523-536

The geological setting, ages, petrography and geochemistry of late Pan-African ( 580 Ma) calc-alkaline and tholeiitic dike rocks in the Bir Safsaf igneous complex of south-west Egypt are discussed. These basaltic to rhyolitic dikes intruded contemporaneously and shortly after the intrusion of granitoids. The major and trace element data, Sr and Nd isotope relations, in combination with textural observations, confirm complex interactions between most of the intermediate calcalkaline dike melts and plutonic melts, with different degrees of mixing, assimilation, replenishment and tapping of magma chambers. Trachytic and rhyolitic dikes are strongly differentiated melts from the granitic pluton. The tholeiitic dikes evolved dominantly by fractional crystallization processes. It is inferred that open system and closed system processes operated in calc-alkaline magma chambers, and that the calc-alkaline melts came from a garnet-and amphibole-bearing mantle, modified by a subduction component. Tholeiitic rocks were formed later by fractional crystallization and assimilation processes. Magma ascent of both dike types took place in an extensional environment and the presumed subduction zone has to be seen in connection with the Atmur-Delgo suture zone.  相似文献   

Wind variability in intermediate luminosity B supergiants     

D. Massa  A. Fullterton  R. Prinja 《Astrophysics and Space Science》1994,221(1-2):77-85

We discuss wind variability in intermediate luminosity B supergiants. Because these are not the hottest, most luminous or just about other kind of -est stars, they have not attracted much attention. However, since their wind lines are well-developed, but unsaturated, they make ideal diagnostics for time dependent wind studies. We show that their wind profiles undergo large scale variability and that this phenomenon is quite common. We also discuss a time series for the B0.5 Ib HD 64760 which suggests the long sought after connection between wind and photospheric activity. Newly developed analysis techniques needed to extract physical information from the wind lines are discussed, and results are applied to the time series.  相似文献   

The project “spice”     

D. Rouan  N. Epchtein  M. De Muizon  F. Lacombe  P. Puget  D. Tiphene  F. Boulanger  X. Desert  G. Guyot  L. D'Hendecourt  J -M. Lamarre  A. Leger  F. Pajot  J -L. Puget  E. Caux  M. Giard  G. Serra  C. Cesarsky  L. Vigroux  A. Omont 《Astrophysics and Space Science》1994,217(1-2):41-44

Proposed to both the French and the European Space Agency as one possible small mission, SPICE is a project for a dedicated small satellite for a near-IR spectroscopic all-sky survey. The instrument would cover the spectral range 1.8 - 3.6µm, possibly extended to 1.8 - 7µm, at a resolution of 100, with pixels of 1 arc-min. The excellent sensitivity (0.02 MJy sr-1) results from: i) the quasi-zero level of background due to the efficient passive cooling of the whole experiment; ii) the use of large format arrays; iii) the non-stop observing mode (drift-scanning). The spectral domain, complementing the one of ISO, partially opaque from the ground, is specially rich in spectral features tracing stars and all components of the Interstellar Medium (molecular, atomic and ionized gas, dust). With a cooling below 80 K of the focal instrument, then it becomes possible to consider doubling the spectral domain and to cover the whole 1.8 - 7µm range.  相似文献   

A coronal loop model for the production of X-ray and radio flares in the RS CVn binary HR 1099     

D. F. Falla  L. M. Bartlett  M. H. L. Smith  R. R. Hillier 《Astrophysics and Space Science》1994,211(1):115-126

The RS CVn binary stellar system HR 1099 is a source of both X-ray and radio flares. We present here a model of the system in which the two types of flare are produced by the same population of mildly-relativistic ( 10) electrons, injected into a coronal loop. After reviewing possible radiation mechanisms we conclude that, given the probable conditions in the flaring region, the radio emission is gyrosynchrotron radiation and the X-ray emission is thermal bremsstrahlung. The thermal X-ray source must lie in the stellar chromosphere, but the apparent absence of plasma absorption at radio frequencies indicates that the radio source is located high in the coronal loop. Using the relationships given by Dulk and Marsh (1982) for the radio emission from a power-law electron energy spectrum,N() ( - 1)^–, we conclude that 3 7, with 30% of the electron population trapped in the radio source. Some implications of these results for one particular version of the model are discussed.  相似文献   

Rosat wide field camera data and the temperature of the interstellar medium     

J. J. Quenby  T. J Sumner  S. D. Sidher  S. Immler 《Astrophysics and Space Science》1994,216(1-2):213-217

The shadowing effects of the molecular clouds in the nearby interstellar medium on the soft x-ray background has been investigated, using ROSAT WFC data in conjunction with previous rocket B and C band surveys. Shadowing over a 5° extent occurs only for a few percent of the sky, but the mixed model of the ISM is supported.Detailed modelling of the Draco shadowing region shows little evidence for a multi-temperature, hot ISM component.  相似文献   

Cosmic-ray modulation and long-duration solar flare events     

K. Kudela  A. Antalová  D. Venkatesan  J. Rybák 《Solar physics》1994,154(2):371-376

The correlation between the long-term intensity variations of cosmic rays at neutron monitor energies and the LDE index measure of solar flares with long-lasting soft X-ray emissions is reported. Three subsequent solar cycles, 20–22, are taken into account and half-monthly data are analyzed. Possible explanation of this correlation is discussed in terms of the recent concepts of cosmic-ray modulation, in particular with merged interaction regions affecting the cosmic-ray intensity.  相似文献   

A photochemical model of the martian atmosphere     

Nair H  Allen M  Anbar AD  Yung YL  Clancy RT 《Icarus》1994,111(1):124-150

The factors governing the amounts of CO, O2, and O3 in the martian atmosphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller than observed. Good agreement between models and observations of CO, O2, O3, and the escape flux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constants for the reactions CO + OH and OH + HO2 within their specified uncertainties. Similar revisions have been suggested to resolve discrepancies between models and observations of the terrestrial mesosphere. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atomic hydrogen escape, it is much larger than recent calculations of the exospheric escape rate for oxygen. Weathering of the surface may account for the imbalance. Quantification of the escape rates of oxygen and hydrogen from Mars is a worthwhile objective for an upcoming martian upper atmospheric mission. We also consider the possibility that HOx radicals may be catalytically destroyed on dust grains suspended in the atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much higher than the observed quantity. We feel that there is no need at this time to invoke heterogeneous processes to reconcile models and observations.  相似文献   

Sea breeze activity at an inland station Kharagpur (India) — A case study     

D. Lohar  B. Pal  B. Chakravarty 《Boundary-Layer Meteorology》1994,67(4):427-434

Surfaces fluxes, turbulent kinetic energy and Flux Richardson number are calculated for three typical sea breeze days characterizing three types of sea breeze onset at an inland station Kharagpur (22°21 N, 87°19 E), 80 km inland, one of the sites for MONTBLEX (MONsoon Trough Boundary Layer EXperiment), in India. The sea breeze onset is associated with a decrease in momentum and heat fluxes and an increase in moisture flux. The turbulent kinetic energy shows quite a significant value even in the late afternoon. The surface layer becomes nearly stable even before sunset, due to the passage of the sea breeze.  相似文献