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771.
Pedro Oyhant?abal Siegfried Siegesmund Klaus Wemmer 《International Journal of Earth Sciences》2011,100(2-3):201-220
A review of the lithostratigraphic units in the Río de la Plata Craton and of new and previously published geochronological, isotopic and geophysical data is presented. Sm?CNd TDM model ages between 2.6 and 2.2?Ga characterize the Piedra Alta Terrane of this craton. Crystallization ages between 2.2 and 2.1?Ga for the metamorphic protoliths and 2.1?C2.0?Ga for the post-orogenic granitoids indicate juvenile crust, followed by a short period of crustal recycling. Cratonization of this terrane occurred during the late Paleoproterozoic. Younger overprinting is not observed, suggesting it had a thick and strong lithosphere in the Neoproterozoic. A similar scenario is indicated for the Tandilia Belt of Argentina. Sm?CNd TDM model ages for the Nico Pérez Terrane show two main events of crustal growth (3.0?C2.6?and 2.3?C1.6?Ga). The crystallization ages on zircon ranges between 3.1 and 0.57?Ga, which is evidence for long-lived crustal reworking. The age for cratonization is still uncertain. In the Taquarembó Block, which is considered the prolongation of the Nico Pérez Terrane in southern Brazil, a similar scenario can be observed. These differences together with contrasting geophysical signatures support the redefinition of the Río de la Plata Craton comprising only the Piedra Alta Terrane and the Tandilia Belt. The Sarandí del Yí Shear Zone is regarded as the eastern margin of this Craton. 相似文献
772.
Malte Drobe Mónica López de Luchi André Steenken Klaus Wemmer Rudolf Naumann Robert Frei Siegfried Siegesmund 《International Journal of Earth Sciences》2011,100(2-3):631-657
Whole-rock geochemical analyses using major and trace elements in combination with the Sm–Nd and Pb–Pb isotope systems, together with SHRIMP age dating on metasedimentary rocks from the Sierras de Chepes, the Sierras de Córdoba, the Sierra Norte and the San Luis Formation in the Sierra de San Luis, have been carried out to unravel the provenance and the geodynamic history of the Eastern Sierras Pampeanas, Central Argentina. The geochemical and the Sm–Nd data point to a slightly stronger mafic and less-fractionated material in the provenance area of the Sierras de Córdoba when compared to the other units. The TDM model ages from the Sierras de Chepes (~1.82 Ga) and the Sierra Norte (~1.79 Ga) are significantly older than the data from the Sierras de Córdoba (1.67 Ga). The Pb data are homogeneous for the different units. Only the 208Pb/204Pb ratios of some samples from the Sierras de Córdoba are higher. A late Pampean detrital zircon peak around 520 Ma from the Sierras de Chepes is in accordance with the new data from the San Luis Formation. This is similar to the literature data from the Famatina Belt located to the northwest of the Sierras de Chepes and also fits the detrital zircon peaks in the Mesón group. These maximum depositional ages were also reported from some locations in the Puncoviscana Formation but are absent in the Sierras de Córdoba. An improved model for the development of the Eastern Sierras Pampeanas in the area between the Sierras de Córdoba and the Puncoviscana Formation is provided. This gives new insights into the late Pampean development of the Sierra de San Luis and the complex development of the Eastern Sierras Pampeanas. This new model explains the younger detrital ages in the Puncoviscana Formation compared with the older ages of the Sierras de Córdoba. Another model of the Sierra de San Luis explains the younger depositional ages of the Pringles Metamorphic Complex and the San Luis Formation when compared to the Nogolí Metamorphic Complex and the Conlara Metamorphic Complex. Additionally, the rather fast change of the high-grade metamorphic conditions in the Pringles Metamorphic Complex and the low-grade metamorphic conditions in the San Luis Formation is explained by extension, the ascent of (ultra) mafic material and later folding and erosion. 相似文献
773.
Andr�� Steenken M��nica G. L��pez de Luchi Carmen Mart��nez Dopico Malte Drobe Klaus Wemmer Siegfried Siegesmund 《International Journal of Earth Sciences》2011,100(2-3):465-488
The Eastern Sierras Pampeanas were structured by three main events: the Ediacaran to early Cambrian (580?C510?Ma) Pampean, the late Cambrian?COrdovician (500?C440?Ma) Famatinian and the Devonian-Carboniferous (400?C350?Ma) Achalian orogenies. Geochronological and Sm?CNd isotopic evidence combined with petrological and structural features allow to speculate for a major rift event (Ediacaran) dividing into two Mesoproterozoic major crustal blocks (source of the Grenvillian age peaks in the metaclastic rocks).This event would be coeval with the development of arc magmatism along the eastern margin of the eastern block. Closure of this eastern margin led to a Cambrian active margin (Sierra Norte arc) along the western margin of the eastern block in which magmatism reworked the same crustal block. Consumption of a ridge segment (input of OIB signature mafic magmas) which controlled granulite-facies metamorphism led to a final collision (Pampean orogeny) with the western Mesoprotrozoic block. Sm?CNd results for the metamorphic basement suggest that the T DM age interval of 1.8?C1.7?Ga, which is associated with the less radiogenic values of ??Nd(540) (?6 to ?8), can be considered as the mean average crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a younger T DM age and a more positive ??Nd(540) value. Pampean pre-540?Ma granitoids form two clusters, one with T DM ages between 2.0 and 1.75?Ga and another between 1.6 and 1.5?Ga. Pampean post-540?Ma granitoids exhibit more homogenous T DM ages ranging from 2.0 to 1.75?Ga. Ordovician re-activation of active margin along the western part of the block that collided in the Cambrian led to arc magmatism (Famatinian orogeny) and related ensialic back-arc basin in which high-grade metamorphism is related to mid-crustal felsic plutonism and mafic magmatism with significant contamination of continental crust. T DM values for the Ordovician Famatinian granitoids define a main interval of 1.8?C1.6, except for the Ordovician TTG suites of the Sierras de Córdoba, which show younger T DM ages ranging from 1.3 to 1.0?Ga. In Devonian times (Achalian orogeny), a new subduction regime installed west of the Eastern Sierras Pampeanas. Devonian magmatism in the Sierras exhibit process of mixing/assimilation of depleted mantle signature melts and continental crust. Achalian magmatism exhibits more radiogenic ??Nd(540) values that range between 0.5 and ?4 and T DM ages younger than 1.3?Ga. In pre-Devonian times, crustal reworking is dominant, whereas processes during Devonian times involved different geochemical and isotopic signatures that reflect a major input of juvenile magmatism. 相似文献
774.
Ming‐Yue Hu Xing‐Tao Fan Brigitte Stoll Dmitry Kuzmin Ying Liu Yongsheng Liu Weidong Sun Guang Wang Xiu‐Chun Zhan Klaus Peter Jochum 《Geostandards and Geoanalytical Research》2011,35(2):235-251
Four silicate glasses were prepared by the fusion of about 1 kg powder each of a basalt, syenite, soil and andesite to provide reference materials of natural composition for microanalytical work. These glasses are referred to as ‘Chinese Geological Standard Glasses’ (CGSG) ‐1, ‐2, ‐4 and ‐5. Micro and bulk analyses indicated that the glasses are well homogenised with respect to major and trace elements. Some siderophile/chalcophile elements (e.g., Sn, Pt, Pb) may be heterogeneously distributed in CGSG‐5. This paper provides the first analytical data for the CGSG reference glasses using a variety of analytical techniques (wet chemistry, XRF, EPMA, ICP‐AES, ICP‐MS, LA‐ICP‐MS) performed in nine laboratories. Most data agree within uncertainty limits of the analytical techniques used. Discrepancies in the data for some siderophile/chalcophile elements exist, mainly because of possible heterogeneities of these elements in the glasses and/or analytical problems. From the analytical data, preliminary reference and information values for fifty‐five elements were calculated. The analytical uncertainties [2 relative standard error (RSE)] were estimated to be between about 1% and 20%. 相似文献
775.
Erosion of continental margins in the Western Mediterranean due to sea-level stagnancy during the Messinian Salinity Crisis 总被引:1,自引:1,他引:0
Janna Just Christian H��bscher Christian Betzler Thomas L��dmann Klaus Reicherter 《Geo-Marine Letters》2011,31(1):51-64
High-resolution multi-channel seismic data from continental slopes with minor sediment input off southwest Mallorca Island,
the Bay of Oran (Algeria) and the Alboran Ridge reveal evidence that the Messinian erosional surface is terraced at an almost
constant depth interval between 320 and 380 m below present-day sea level. It is proposed that these several hundred- to 2,000-m-wide
terraces were eroded contemporaneously and essentially at the same depth. Present-day differences in these depths result from
subsidence or uplift in the individual realms. The terraces are thought to have evolved during one or multiple periods of
sea-level stagnancy in the Western Mediterranean Basin. According to several published scenarios, a single or multiple periods
of relative sea-level stillstand occurred during the Messinian desiccation event, generally known as the Messinian Salinity
Crisis. Some authors suggest that the stagnancy started during the refilling phase of the Mediterranean basins. When the rising
sea level reached the height of the Sicily Sill, the water spilled over this swell into the eastern basin. The stagnancy persisted
until sea level in the eastern basin caught up with the western Mediterranean water level. Other authors assigned periods
of sea-level stagnancy to drawdown phases, when inflowing waters from the Atlantic kept the western sea level constant at
the depth of the Sicily Sill. Our findings corroborate all those Messinian sea-level reconstructions, forwarding that a single
or multiple sea-level stagnancies at the depth of the Sicily Sill lasted long enough to significantly erode the upper slope.
Our data also have implications for the ongoing debate of the palaeo-depth of the Sicily Sill. Since the Mallorcan plateau
experienced the least vertical movement, the observed terrace depth of 380 m there is inferred to be close to the Messinian
depth of this swell. 相似文献
776.
Mitra has suggested that the Superrotation of the upper atmosphere is caused by a deposition of meteoroids. The meteoroids are assumed to impart to the atmosphere the excess of their orbital angular momentum per unit mass over the Earth's angular momentum per unit mass. The process is to take place in the height region above 150 km. Only above this height is a Superrotation of the atmosphere observed. In this report the forces that tend to make the atmosphere corotate with the Earth are analysed. It is shown that the most important of these forces is ion drag, and not viscous drag as postulated by Mitra. As the net angular spin momentum imparted by the meteoroids seems to be less than Mitra's estimate and its main part is applied to the atmosphere at altitudes much lower than 150 km, the hypothesis that meteoroids provide a significant contribution to the Superrotation is rejected. 相似文献
777.
Sebastian?Staude Wolfgang?Werner Teresa?Mordhorst Klaus?Wemmer Dorrit?E.?Jacob Gregor?MarklEmail author 《Mineralium Deposita》2012,47(3):251-276
The Wittichen Co–Ag–Bi–U mining area (Schwarzwald ore district, SW Germany) hosts several unconformity-related vein-type mineralizations
within Variscan leucogranite and Permian to Triassic redbeds. The multistage mineralization formed at the intersection of
two fault systems in the last 250 Ma. A Permo-Triassic ore stage I with minor U–Bi–quartz–fluorite mineralization is followed
by a Jurassic to Cretaceous ore stage II with the main Ag and Co mineralization consisting of several generations of gangue
minerals that host the sub-stages of U–Bi, Bi–Ag, Ni–As–Bi and Co–As–Bi. Important ore minerals are native elements, Co and
Ni arsenides, and pitchblende; sulphides are absent. The Miocene ore stage III comprises barite with the Cu–Bi sulfosalts
emplectite, wittichenite and aikinite, and the sulphides anilite and djurleite besides native Bi, chalcopyrite, sphalerite,
galena and tennantite. The mineral-forming fluid system changed from low salinity (<5 wt.% NaCl) at high temperature (around
300°C) in Permian to highly saline (around 25 wt.% NaCl + CaCl2) at lower temperatures (50–150°C) in Triassic to Cretaceous times. Thermodynamic calculations and comparison with similar
mineralizations worldwide show that the Mesozoic ore-forming fluid was alkaline with redox conditions above the hematite–magnetite
buffer. We suggest that the precipitation mechanism for native elements, pitchblende and arsenides is a decrease in pH during
fluid mixing processes. REE patterns in fluorite and the occurrence of Bi in all stages suggest a granitic source of some
ore-forming elements, whereas, e.g. Ag, Co and Ni probably have been leached from the redbeds. The greater importance of Cu
and isotope data indicates that the Miocene ore stage III is more influenced by fluids from the overlying redbeds and limestones
than the earlier mineralization stages. 相似文献
778.
Origin and extent of fresh groundwater, salty paleowaters and recent saltwater intrusions in Red River flood plain aquifers, Vietnam 总被引:1,自引:0,他引:1
Luu T. Tran Flemming Larsen Nhan Q. Pham Anders V. Christiansen Nghi Tran Hung V. Vu Long V. Tran Hoan V. Hoang Klaus Hinsby 《Hydrogeology Journal》2012,20(7):1295-1313
A model has been established on the origin and extent of fresh groundwater, salty paleowaters and saltwater from recent seawater intrusions in the Red River flood plain in Vietnam. This was done with geological observations, geophysical borehole logging and transient electromagnetic methods. Salt paleowater is present up to 50–75?km from the coastline, with occurrence controlled by the Holocene transgression. A density-driven leaching of salty porewater has occurred from high-permeability Holocene sediments into underlying Pleistocene deposits, whereas diffusion has dominated in low-permeability layers. In the Pleistocene aquifer, the highest content of dissolved solids is found below two intrinsic valleys with Holocene marine sediments and along the coastline. Recent intrusion of saltwater from the South China Sea is observed in shallow groundwater 35?km inland, probably a result of transport of salty water inland in rivers or leaching of paleowaters from very young near-coast marine sediments. The observed inverted salinity profile, with high saline water overlying fresher groundwater, has been formed due to the global eustatic sea-level changes during the last 8,000–9,000?years. The proposed model may therefore be applicable to other coastal aquifers, with a proper incorporation of the local geological environments. 相似文献
779.
780.
Wave‐induced fluid flow at microscopic and mesoscopic scales arguably constitutes the major cause of intrinsic seismic attenuation throughout the exploration seismic and sonic frequency ranges. The quantitative analysis of these phenomena is, however, complicated by the fact that the governing physical processes may be dependent. The reason for this is that the presence of microscopic heterogeneities, such as micro‐cracks or broken grain contacts, causes the stiffness of the so‐called modified dry frame to be complex‐valued and frequency‐dependent, which in turn may affect the viscoelastic behaviour in response to fluid flow at mesoscopic scales. In this work, we propose a simple but effective procedure to estimate the seismic attenuation and velocity dispersion behaviour associated with wave‐induced fluid flow due to both microscopic and mesoscopic heterogeneities and discuss the results obtained for a range of pertinent scenarios. 相似文献