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991.
In the southern Pannonian Basin, the Variscan Barrovian- and overprinted Abukuma-type progressive metamorphic sequences of the South Tisia in Slavonian Mts. (SM) and Mt. Moslava)ka Gora (MG) are interlayered with orthoamphibolites. These sequences represent part of a disrupted Variscan belt that extends southeastwards of the Bohemian Massif through the Carpathians to the Caucasus. Orthoamphibolites contain hornblende (Mg-hornblende, tschermakite, pargasite, and edenite), plagioclase An48-98 (MG) and An26-35 (SM), biotite, grossular (MG) and almandine (SM) enriched garnet, diopside (MG), with accessory ilmenite, titanite, and clinozoisite. Based on major and trace element analyses and CIPW norms, these orthoamphibolites, which originally were olivine tholeiites to slightly alkalic basalts, can be correlated with tholeiites of consuming plate margins. '18O of orthoamphibolites varies from 5.8 to 7.1, the measured 87Sr/86Sr ratio from 0.70435 to 0.70665, which indicates slight continental crust contamination, and the calculated ratio from 0.70262 to 0.70535, indicating an upper mantle origin of the original basaltic melts. For associated penecontemporaneous I-type granitoids, the calculated initial 87Sr/86Sr ratio also have upper mantle values ranging between 0.70293 and 0.70368. Geochemical data for orthoamphibolites and the associated penecontemporaneous I-type granites, and the occurrence of alpine-type ultramafic bodies within the Barrovian sequence, indicate that they have many features in common with orogenic associations related to recent subduction-related settings. Some characteristic element ratios suggest back-arc basin environments and the Ce/K vs. Ce diagram suggests a pargasite-lherzolite source. Its partial melting gave primitive basaltic melts of olivine tholeiite to slight alkalic affinities that produced at first differentiated mafic rocks by olivine fractionation (future orthoamphibolites) and, afterwards, a strongly differentiated suite of I-type granitoids by amphibole fractionation. Magma emplacement and subsequent AFC processes took place in subduction environments preceding the main Variscan deformational metamorphic event during which orthoamphibolites were generated. This interpretation is compatible with geodynamic modeling of this part of the Paleotethys. 相似文献
992.
Fluid-mobile trace element constraints on the role of slab melting and implications for Archaean crustal growth models 总被引:11,自引:0,他引:11
Balz S. Kamber Anthony Ewart Kenneth D. Collerson Michael C. Bruce Graeme D. McDonald 《Contributions to Mineralogy and Petrology》2002,144(1):38-56
We use published and new trace element data to identify element ratios which discriminate between arc magmas from the supra-subduction zone mantle wedge and those formed by direct melting of subducted crust (i.e. adakites). The clearest distinction is obtained with those element ratios which are strongly fractionated during refertilisation of the depleted mantle wedge, ultimately reflecting slab dehydration. Hence, adakites have significantly lower Pb/Nd and B/Be but higher Nb/Ta than typical arc magmas and continental crust as a whole. Although Li and Be are also overenriched in continental crust, behaviour of Li/Yb and Be/Nd is more complex and these ratios do not provide unique signatures of slab melting. Archaean tonalite-trondhjemite-granodiorites (TTGs) strongly resemble ordinary mantle wedge-derived arc magmas in terms of fluid-mobile trace element content, implying that they did not form by slab melting but that they originated from mantle which was hydrated and enriched in elements lost from slabs during prograde dehydration. We suggest that Archaean TTGs formed by extensive fractional crystallisation from a mafic precursor. It is widely claimed that the time between the creation and subduction of oceanic lithosphere was significantly shorter in the Archaean (i.e. 20 Ma) than it is today. This difference was seen as an attractive explanation for the presumed preponderance of adakitic magmas during the first half of Earth's history. However, when we consider the effects of a higher potential mantle temperature on the thickness of oceanic crust, it follows that the mean age of oceanic lithosphere has remained virtually constant. Formation of adakites has therefore always depended on local plate geometry and not on potential mantle temperature. 相似文献
993.
REE Geochemistry of the Lead—Zinc Ores from the Jinding Lead—Zinc Deposit,Lanping,Yunnan 总被引:3,自引:1,他引:3
Nineteen volcanic and magmatic rock samples were collectecd from the Jinding leadzinc deposit and its surrounding areas in Yunnan.The ICP and AES analyses,feferred to the previous results,show that the metal minerals and altered rocks in the Jinding lead-zinc deposit display a decreasing trend of ∑REE from the early to late stages of mineralization,and similarities in REE distribution patterns,indicating that the ore fluids are characterized by high LREE enrichment,markedly negative δCe anomaly and slight δEu anomaly.These REE distribution paterns exhibit striking similarities to those of the Pliocene trachyte in the study area,both of which are similar in ∑REE,LREE/HREE ratio,δEu and δCe.The ore fluids besides the basin fluids in the deposit are also closely related to those associated with Pliocene trachyte magmas. 相似文献
994.
Hydraulic Characteristics of Rough Fractures in Linear Flow under Normal and Shear Load 总被引:19,自引:3,他引:19
Summary
A hydro-mechanical testing system, which is capable of measuring both the flow rates and the normal and shear displacement
of a rock fracture, was built to investigate the hydraulic behaviour of rough tension fractures. Laboratory hydraulic tests
in linear flow were conducted on rough rock fractures, artificially created using a splitter under various normal and shear
loading. Prior to the tests, aperture distributions were determined by measuring the topography of upper and lower fracture
surfaces using a laser profilometer. Experimental variograms of the initial aperture distributions were classified into four
groups of geostatistical model, though the overall experimental variograms could be well fitted to the exponential model.
The permeability of the rough rock fractures decayed exponentially with respect to the normal stress increase up to 5 MPa.
Hydraulic behaviours during monotonic shear loading were significantly affected by the dilation occurring until the shear
stress reached the peak strength. With the further dilation, the permeability of the rough fracture specimens increased more.
However, beyond shear displacement of about 7 to 8 mm, permeability gradually reached a maximum threshold value. The combined
effects of both asperity degradation and gouge production, which prohibited the subsequent enlargement of mean fracture aperture,
mainly caused this phenomenon. Permeability changes during cyclic shear loading showed somewhat irregular variations, especially
after the first shear loading cycle, due to the complex interaction from asperity degradations and production of gouge materials.
The relation between hydraulic and mechanical apertures was analyzed to investigate the valid range of mechanical apertures
to be applied to the cubic law.
Received June 12, 2001; accepted February 26, 2002 Published online September 2, 2002 相似文献
995.
The chronological data obtained by a variety of dating methods have confirmed that the sodium enriched metavolcanite series
in the area of the Longbohe copper deposit at Jinping, Yunnan Province, was formed during the Proterozoic (its Pb−Pb isochron
age=1595±75 Ma), corresponding to the Dahongshan Group sodium enriched metavolcanites. The Sm−Nd isochron age of 1336±46Ma
should represent the time at which this series of volcanic rocks experienced spilitization in response to sodium metasomatism,
equivalent to the time of sedimentation of the Ailaoshan Group and Yashan Group rocks. The Rb−Sr ages of the volcanic rocks
and the Pb−Pb ages of copper ores have recorded the events of strong dynamic metamorphism and of strong reworking-metallogenesis
of copper during the Jinningian period. Similarities in forming age, rock assemblage and isotopic characteristics between
the sodium enriched metavolcanite series and the Dahongshan Group sodium enriched metavolcanite series have provided new clues
to the exploration of the Dahongshan-type copper deposits in this area.
Granted jointly by the National Natural Science Foundation of China (Nos. 49702022, 40073001) and the State 973 Program (No.
G1999043215). 相似文献
996.
Several types of anhydrite-bearing rocks have been found in the amphibolite-facies metamorphosed rocks at the north-eastern margin of the Moldanubian Zone. Anhydrite either forms monomineralic bands up to 40 cm thick, or occurs in the form of disseminated grains in surrounding calc-silicate gneiss together with feldspar, scapolite, amphibole, pyroxene, epidote and pyrite. The isotopic composition of sulphur ('34S=30.6 to 32.3) and strontium (87Sr/86Sr=0.70797 to 0.70781) in anhydrite may indicate a marine source of sulphate. The isotopic ratio of strontium is in the same range as that of metamorphosed strata-bound barite-sulphide ores, which have been previously described in the same area. The '34S values of coexisting pyrite range from 21.4 to 22.5, the (34Sanhydrite-pyrite corresponding to the metamorphic temperature of 600 to 660 °C. In contrast to many submarine-exhalative deposits, the oxygen isotopic compositions of anhydrite ('18O=9.3 to 10.2) are lighter than that of barite ('18O=10.4 to 13.8). This indicates that the both minerals are not in isotopic equilibrium. Therefore, it is probable that anhydrite and barite from the Roná district were deposited from fluids that contained different proportions of seawater and hydrothermal fluids or from hydrothermal fluids that underwent variable extent of oxygen isotope exchange with seafloor rocks. The '13C values in calcite ('13C=-17.2 to -18.7) from anhydrite-bearing rock are lower than those in distant marbles. As graphite is absent in anhydrite- and calcite-bearing rocks, impoverishment in the 13C isotope cannot be attributed to the graphite-carbonate isotopic exchange during metamorphism. It is proposed that low '13C values in carbonates are caused by pre-metamorphic oxidation of organic matter in course of hydrothermal processes. Anhydrite and anhydrite-bearing calc-silicate gneiss from the north-eastern part of the Moldanubian Zone are interpreted to be the high-grade metamorphosed analogue of anhydrite-rich exhalites commonly found in submarine-exhalative hydrothermal deposits. 相似文献
997.
998.
999.
1000.
Landslide risk assessment and management: an overview 总被引:29,自引:0,他引:29
Landslides can result in enormous casualties and huge economic losses in mountainous regions. In order to mitigate landslide hazard effectively, new methodologies are required to develop a better understanding of landslide hazard and to make rational decisions on the allocation of funds for management of landslide risk. Recent advances in risk analysis and risk assessment are beginning to provide systematic and rigorous processes to enhance slope management. In recent years, risk analysis and assessment has become an important tool in addressing uncertainty inherent in landslide hazards.This article reviews recent advances in landslide risk assessment and management, and discusses the applicability of a variety of approaches to assessing landslide risk. Firstly, a framework for landslide risk assessment and management by which landslide risk can be reduced is proposed. This is followed by a critical review of the current state of research on assessing the probability of landsliding, runout behavior, and vulnerability. Effective management strategies for reducing economic and social losses due to landslides are described. Problems in landslide risk assessment and management are also examined. 相似文献