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141.
D. Gebauer J. Bernard-Griffiths M. Grünenfelder 《Contributions to Mineralogy and Petrology》1981,76(3):292-300
U-Pb data on zircons from the largest mafic-ultramafic body (6×2 km) of the French Central Massif (Sauviat-sur-Vige) yield the following age results: Primary magmatic crystallization of the gabbroic and peridotitic protoliths took place in the Cambro-Ordovician (496±25/17 m.y.). Variable transformation under eclogite facies conditions was Hercynian (320±29/36 m.y.). The same age pattern, derived by U-Pb monazite analyses, was found also for the immediate country rocks, i.e. kyanite bearing, coarse-grained metagranites occurring to the W and N of the Sauviat massif. Due to the fact that there is no regional Hercynian high-grade metamorphism in this part of the French Central Massif (e.g. Duthou 1977; Bernard-Griffiths 1975), both mafic-ultramafic complex as well as immediate felsic country rocks must have been emplaced tectonically into pre-Hercynian (Acadian±Caledonian) crustal rocks. The cause for such a Hercynian tectonism is thought to be due to continent-continent collision of the Spanish with the Armorican plate. Preliminary U-Pb zircon results on one eclogite sample taken about 50 km S of the Sauviat complex indicate also an Early Palaeozoic age for the magmatic protolith and a Hercynian transformation into eclogite, combined with and/or followed by tectonic emplacement. However, opposite to the continental Sauviat massif, we are probably dealing here with oceanic material, possibly deposited in an Early Palaeozoic marginal sea basin. Thus, subduction and tectonic emplacement of oceanic crust into continental crust terminated in this area also in the Hercynian. Concerning U-Pb systematics of zircon and monazite the following conclusions can be drawn:
- U-Pb systems of primary magmatic zircons of mafic and ultramafic rocks are only weakly disturbed during static eclogite facies metamorphism (T>820° C; P> 15 kbar);
- New- and overgrowth of zircon during eclogite facies metamorphism seems to be the major cause for the degree of discordance;
- Amphibolitization of metagabbros and eclogites had no effect on the degree of discordance of zircon;
- U-Pb systems of monazites remained undisturbed during intense weathering of the mother rock.
142.
In an attempt to find the premetamorphic discordance pattern of detrital zircons extracted from Central European metasediments, unmetamorphosed or only slightly metamorphosed sediments were collected from two areas: (1) from the Montagne Noire, the southernmost part of the French Central Massif and (2) from the innerbohemian Algonkian (= Proterozoic) in the CSSR.The generally accepted hypothesis that zircons from Central European metasediments must have plotted close to or at the corresponding upper intercept between discordia trajectory and concordia curve prior to the metamorphism of the host rocks could not be supported. The zircon populations from sediments of both areas are similar in discordance to those of the numerous populations extracted from metasediments of the Central European basement complexes. However, in contrast to the latter, the data points of size fractions scatter considerably and reliable intercept ages cannot be calculated.In the case of the Cambro-Ordovician sand- and siltstones of the Montagne Noire, the ages of detrital muscovites strongly argue for a Cadomian (550–700 m.y.) provenance of the detritus. Thus, the strong discordance of the analyzed fractions most probably is caused by zircons newly formed and/or partly or completely reset during a Cadomian event in the provenance of the detritus. In addition, lattice unit parameters indicate that the detrital zircons must have been recrystallized after their primary formation more than 1.7 b.y. ago.The Algonkian sediments of Bohemia (CSSR) can be taken as the very low-grade metamorphic equivalents of the Moldanubian paragneisses from which discordia trajectories between about 2 b.y. and 460–320 m.y. are known (Gebauer and Grünenfelder, 1974; Grauert et al., 1974). Nevertheless, all analyzed zircon fractions are strongly discordant indicating that they probably recrystallized during the Assyntian (=Cadomian) very low-grade metamorphism of the host rock loosing most of their accumulated radiogenic lead. If such an interpretation is correct, the low-temperature recrystallization model of Gebauer and Grünenfelder (1976) can be applied to metamict zircons in host rocks formed at temperatures as low as 300 ° C. In our 1976-paper we gave temperatures of 350–400 ° C for the maximum temperature necessary to recrystallize metamict zircons in chlorite-grade quartzphyllites in agreement with the experimental results of Pidgeon et al. (1973).In contrast to the zircons of the Montagne Noire it can be shown that the U-Pb systems of the Algonkian zircons must have been re-opened in post-Assyntian time, probably recently or in the Tertiary. However, no plausible explanation can be given to account for this. 相似文献
143.
V. Kárník D. Procházková Z. Schenková L. Ruprechtová A. Dudek J. Drimmel E. Schmedes G. Leydecker J. P. Rothé B. Guterch H. Lewandowska D. Mayer-Rosa D. Cvijanović V. Kuk F. Giorgetti G. Grünthal E. Hurtig 《Pure and Applied Geophysics》1978,116(6):1307-1313
Summary The isoseismal map of the North-Italian earthquake of 6 May 1976 was compiled using the contributions of the European countries lying within the shaken area. The characteristic feature of the macroseismic field is discussed. 相似文献
144.
A. Stettler P. Eberhardt J. Geiss N. Grögler 《Earth and Planetary Science Letters》1974,23(3):453-461
39Ar-40Ar ages and37Ar-38Ar exposure ages of samples representing four different lithologies of the Apollo 17 station 7 boulder were measured. The age of the dark veinlet material77015of3.98 ± 0.04AE is interpreted as representing the time of intrusion of this veinlet into the 77215 clast. The data obtained so far indicate that the vesicular basalt 77135 formed 100–200 m.y. later. However, this has to be confirmed by39Ar-40Ar investigations on separated mineral and/or grain-size fractions. A small clast enclosed in the 77135 basalt gives a well-defined high temperature age of3.99 ± 0.02AE. A sample of the noritic clast 77215 gave4.04 ± 0.03AE, the highest age found so far in this boulder. The39Ar-40Ar ages obtained are in agreement with the age relationships deduced from the stratigraphic evidence.Taking into account the shielding by the boulder itself, an average37Ar-38Ar exposure age of(27.5 ± 2.5)m.y. is obtained for the samples collected from the boulder. 相似文献
145.
146.
A RAM-economizing domain decomposition technique for regional high-resolution groundwater simulation
A major limitation of the use of high-resolution groundwater models on a regional scale for resource management by water companies
is the excessive RAM requirements of such models which surpass the capacity of today's PCs. A strategy is presented to overcome
this problem by using overlapping domain decomposition techniques. Furthermore, because very long computing time is the bottleneck
for the practical use of this technique for large groundwater models, an analysis is also presented of a number of methods
implemented to increase calculation performance. The approach presented here is characterized by a fairly simple structure
that represents a generalized relaxation algorithm. It can be adapted for use with finite element as well as with finite difference
methods.
Electronic Publication 相似文献
147.
Pierre Antoine Albert Marchiol Marcel Brocandel Yves Gros 《Comptes Rendus Geoscience》2005,337(16):444
Since 1990, a dense network of subvertical cracks has been observed at the upper part of the Albian sands of the site of radioactive waste storage operated by ANDRA. New studies carried out in 1999 during enlargement of the site of Soulaines and in 2003 on the nearby TFA site highlighted an especially well-developed network of periglacial features that are mainly of the sand-wedge type (subvertical stratified infill). These observations demonstrate the periglacial origin of these cracks, which are mainly superimposed on a dense network of former tectonic extension joints, and that rapidly disappear at the top of the underlying clays. In addition, these observations currently represent the only example of a large sand-wedge network developed during the Last Glacial in northern France. To cite this article: P. Antoine et al., C. R. Geoscience 337 (2005). 相似文献
148.
The Fe3+/ΣFe ratio of 104 MORB glasses from the Pacific, the Atlantic, the Indian, and the Red Sea spreading centers have been determined using wet chemical Fe2+ analyses and electron microprobe FeOtotal measurements. The data provide a new estimate for the MORB oxygen fugacity (fO2) of 0.41 ± 0.43 (1sigma, N = 100) log units below the fayalite-magnetite-quartz buffer (FMQ), equivalent to a Fe3+/ΣFe = 0.12 ± 0.02 (1sigma, N = 104). This new fO2 estimate is 0.8 log units more oxidized than the average fO2 proposed by Christie et al. (1986) (FMQ-1.20 ± 0.44; Fe3+/ΣFe = 0.07 ± 0.01; N = 87). This slight difference may be related in part to the 3.5% underestimation of the Fe2+ concentration determined by Christie et al. (1986) compared with this study. MORB oxygen fugacity does not display any significant difference between the three main oceanic domains, or between enriched and depleted MORB. Yet, the iron red-ox state ratio shows a broad increase during fractional crystallization. Detailed study of magmatic suites highlights the lack of systematic Fe3+/ΣFe ratio fractionation during differentiation. Despite the large variations of inferred partial melting degrees (from 5 to 20%), the present data set does not provide any evidence of Fe3+/ΣFe relationships with partial melting proxies such as Na8.0.Based on the Fe3+ systematics during partial melting, it is suggested that the oxidation state of MORB reflects a “buffered mantle melting process” resulting in the apparent compatible behavior of Fe3+ during partial melting, and in the relatively constant Fe3+/ΣFe ratio irrespective of the extent of melting. This result implies that partial melting processes may be open relative to oxygen. We propose a model where the Fe3+/ΣFe ratio in the melt is buffered during partial melting. The MORB Fe2O3 systematics can be accounted for by using a fO2 of FMQ-1 that is equivalent to the average fO2 reported for abyssal peridotites. 相似文献
149.
The meta-igneous granulite xenoliths from Kerguelen Archipelago: evidence of a continent nucleation in an oceanic setting 总被引:1,自引:1,他引:0
Michel Grégoire Jean Yves Cottin André Giret Nadine Mattielli Dominique Weis 《Contributions to Mineralogy and Petrology》1998,133(3):259-283
Xenoliths of meta-igneous origin occur as one of the two main types of ultramafic and mafic xenoliths entrained by alkaline
lavas of the Kerguelen islands. These are designated type II xenoliths and are subdivided into three mineralogical groups.
Subtype IIa and IIc xenoliths are interpreted as crystallisation products of basaltic melts that were emplaced near the crust-mantle
boundary during the early tholeiitic–transitional magmatic activity of the Kerguelen islands. Younger magmatism became more
alkaline and subtype IIb xenoliths were formed as high-pressure alkaline cumulates related to the last alkaline volcanic stage.
Subsequently, the plagioclase-bearing type II rocks have been re-equilibrated under granulite facies conditions. This addition
of mafic material around the crust-mantle boundary is consistent with seismic evidence for crustal thickening to 14–20 km.
Calculated compressional seismic velocities (Vp) for the basic granulites are consistent with the range of observed Vp in
the low-velocity region underlying the oceanic crust. Such growth in the thickness of the oceanic crust may be caused by intrusion
of basalts at different levels in the lithosphere and may provide the heat responsible for granulitic metamorphism in the
oceanic setting. This study suggests that basic granulites can account for the observed seismic characteristics of oceanic
plateaux and can be important components of Kerguelen oceanic lithosphere where there has been large-scale magma production.
Moreover we speculate that the Kerguelen islands and perhaps the surrounding plateau represent a continental nucleation process.
Received: 30 September 1997 / Accepted: 17 June 1998 相似文献
150.
Freshly cleaved (001) natural muscovite was chemically characterized by X-ray Photoelectron Spectroscopy (XPS). The investigated
muscovite crystals have composition ranging from pure end member to phengitic muscovite. XPS provides experimental evidence
that the chemical composition of the near-surface region differs from the bulk. This difference is not due to analytical problems,
but to extreme surface sensitivity of XPS. Depth profiling was also carried out after tilting of 60° or 70° of the sample
along an axis perpendicular to [001]. After tilting of phengitic muscovite, the concentration of Al increases and of K decreases,
and therefore Al is the topmost monolayer. Repulsion between the octahedral and tetrahedral sheets, stacking faults or more
probably interlayered phases (gibbsite-like) are responsible for the cleavage. Possible perturbations of surface chemistry
due to X-ray radiation and Ar+ ion sputtering have also been investigated. X-ray radiation is negligeable, but Ar+ ion sputtering introduces important structural modifications, perturbation of the chemical environment of elements and minor
chemical variations.
Received: 13 November 1996 / Revised, accepted: 21 March 1997 相似文献