The footwall gneisses beneath the western part of the Paleoarchean (3.8 Ga) Isua Greenstone Belt, southern West Greenland, are interpreted here in terms of a 3.64 Ga stack of mylonitic crystalline thrust-nappes, the oldest example known on Earth. In present coordinates, the kinematic history of the thrust-nappe stack is couched in terms of initial longitudinal (strike-parallel) thrusting towards the southwest, followed by transverse thrusting to the northwest, and subsequent extensional collapse of the thickened crust toward the southeast.Diorite and tonalite that form the western margin of granitoids, structurally overlying the western part of the Isua Greenstone Belt and its footwall, contain 3.5 Ga mafic dykes, some of which are deformed and/or truncated at fault contacts within the granitoids. Accordingly, a component of the deformation structurally above the Isua Greenstone Belt occurred after 3.5 Ga, significantly later than the formation of the underlying mylonitic nappes, probably during the Neoarchean.The structural regime of mylonitic thrust-nappe stacking is very similar to that in modern mountain belts. It would appear that the deformational behaviour, rheological constitution and overall strength of Paleoarchean and modern continental crust were similar. 相似文献
Four image analysis systems for measuring rock fragmentation: FragScan, PowerSieve®, Split and WipFrag, have been compared under conditions necessary to provide an objective though limited assessment of their capabilities. The analysis of results is based on a sample of ten photographs taken from a series of photographs of controlled artificial muckpiles. These were created from dumping a blended mixture of sieved samples of limestone aggregate, in order to create a range of near perfect Rosin-Rammler sieve size distributions. Results from the various systems are compared with sieved results using both histogram and cumulative forms, with and without fines corrections in the case of Split and Wipfrag. Statistical indicators are evaluated to examine the match between system prediction values and sieving values. Commentaries on the results by the inventors of each system have been incorporated. All four systems were found to perform both well in some cases and poorly in others. From a detailed examination of the results, some insight into the strengths and weaknesses of the various systems is presented. 相似文献
Abstract The Yanbian area is located in the eastern part of the Central Asian Orogenic Belt (CAOB) of China and is characterized by widespread Phanerozoic granitic intrusions. It was previously thought that the Yanbian granitoids were mainly emplaced in the Early Paleozoic (so-called 'Caledonian' granitoids), extending east–west along the northern margin of the North China craton. However, few of them have been precisely dated; therefore, five typical 'Caledonian' granitic intrusions (the Huangniling, Dakai, Mengshan, Gaoling and Bailiping batholiths) were selected for U–Pb zircon isotopic study. New-age data show that emplacement of these granitoids extended from the Late Paleozoic to Late Mesozoic (285–116 Ma). This indicates that no 'Caledonian' granitic belt exists along the northern margin of the North China craton. The granitoids can be subdivided into four episodes based on our new data: Early Permian (285 ± 9 Ma), Early Triassic (249–245 Ma), Jurassic (192–168 Ma) and Cretaceous (119–116 Ma). The 285 ± 9 Ma tonalite was most likely related to subduction of the Paleo-Asian Oceanic Plate beneath the North China craton, followed by Triassic (249–245 Ma) syn-collisional monzogranites, representing the collision of the CAOB orogenic collage with the North China craton and final closure of the Paleo-Asian Ocean. The Jurassic granitoids resulted from subduction of the Paleo-Pacific plate and subsequent collision of the Jiamusi–Khanka Massif with the existing continent, assembled in the Triassic. The Early Cretaceous granitoids formed in an extensional setting along the eastern Asian continental margin. 相似文献
The climatologies of daily precipitation and of maximum and minimum temperatures over western North America are simulated using stochastic weather generators. Two types of generator, differentiated only by their method of modeling precipitation occurrence, are investigated. A second-order Markov model, in which the probability of the occurrence of precipitation is modeled as contingent upon its occurrence on the previous two days, is compared with a spell-length model, in which mass functions of wet- and dry-spell lengths are modeled. Both models are able to reproduce the observed annual and monthly climatology in the region to a high degree of accuracy. However, there is considerable over-dispersion in annual precipitation, resulting primarily from an underestimation in the interannual variability of precipitation intensity. The interannual variability of temperatures is similarly underestimated, and is most severe for minimum temperatures. There is a severe problem in estimating minimum temperature extremes, which can be attributed to the negatively skewed distribution of daily minimum temperatures. Non-normality in the distribution of daily temperatures is shown to be a problem in simulating extreme temperature maxima as well as of minima. It is suggested that the normal distribution used in the generation of daily temperatures in the widely used Richardson (1981) generator, and its derivations, be supplanted by a more appropriate distribution that permits skewness in either direction. 相似文献
Volatile organic compounds (VOCs) emissions by vegetation present in the Mediterranean area are not well known. They may contribute with anthropogenic VOC emissions to the tropospheric ozone formation that reaches important level in the European Mediterranean region. The present work, carried out as part of the European ESCOMPTE project «fiEld experimentS to COnstrain Models of atmospheric Pollution and Transport of Emissions», adds a new contribution to the inventory of the main natural hydrocarbons sources likely to participate in the ozone production. The corresponding measurement campaign was conducted in La Barben, a site close to Marseilles (France), with the aim to quantify the terpenic emission pattern and the behaviour of Pinus halepensis, an important Mediterranean species slightly studied.The determination of biogenic emissions from P. halepensis was done by the enclosure of an intact branch in a Teflon cuvette. Main emitted monoterpenes were β trans-ocimene and linalool. The total monoterpenic emission rates thus recorded were found to reach maximum values around 30 μg gdry weight−1 h−1. The normalized emission rates calculated at 30 °C and 1000 μmol m−2 s−1 with Guenther's algorithm was 14.76, 8.65 and 4.05 μg gdry weight−1 h−1, respectively, for the total monoterpenes, β trans-ocimene and linalool. 相似文献
Four new specimens of Euscalpellum? coatesi Donovan and Davis-Strickland from the Maastrichtian of Jamaica are described; two of these occur in association with the right valve of the rudistid bivalve Chiapasella radiolitiformis (Trechmann). This demonstrates that E.? coatesi is not a barnacle, but rather represents an unusual preservational state of the left valve of C. radiolitiformis retaining only the cellular inner part of the shell. 相似文献
The Snake River Plain (SRP), a 90-km-wide topographic depression in southern Idaho, is a topographically anomalous feature in the western U.S. Previous seismic studies focused on the northeastern SRP to study its relationship with the Yellowstone hotspot. We present new teleseismic shear-wave splitting data from six broadband seismic stations deployed along the axis of the SRP from June 2000 to September 2001. We also analyze splitting at HLID, a permanent station of the National Seismic Network located ∼100 km north of the plain. Splitting of individual teleseismic phases is consistent at all stations within 2σ errors, and we favor the interpretation of anisotropy with a single horizontal fast axis, although a dipping-axis interpretation is statistically permitted at two of the stations. Our station fast directions, as well as shear-wave splitting data from numerous other stations throughout the Basin and Range, are best explained by a lattice preferred orientation of olivine due to horizontal shear along the base of the plate associated with the gravitational spreading of buoyant plume-like upwelling material beneath eastern Nevada into a southwestward flowing asthenosphere (with respect to a fixed hotspot reference frame). This parabolic asthenospheric flow (PAF) model for the Great Basin is attractive because it explains the observed high elevations, high mantle buoyancy, low-velocity anomaly beneath eastern Nevada, high heat flow, and depleted geochemistry of some erupted basalts. The lack of Pliocene-Recent major volcanism in eastern Nevada suggests that a significant amount of the buoyancy flux is due to compositional buoyancy. Our splitting station delay times vary in a way not predicted by the PAF model, and can be explained by: a zone of aligned magma-filled lenses and/or partially molten dikes beneath the SRP lithosphere, a depleted olivine-rich residuum underneath the sides of the eastern SRP, and/or the effect of lateral lower crustal flow from beneath the eSRP toward its adjacent flanks. 相似文献
We have obtained major and trace element data for a suite of rocks emplaced over an area of 45,000 km2 in the Eastern Goldfields Province (EGP), Yilgarn Craton, that are petrographically and mineralogically described as kimberlites, melnoites and carbonatites. Kimberlites dominate the rock types found in the west whereas carbonatites and melnoites are common in the east. Compatible element data from the carbonatites and melnoites tend to lie along trends that imply silicate–carbonate fractionation. The kimberlites exhibit a much greater amount of compatible element scatter as a result of the variable contribution from continental lithospheric mantle (CLM). When compared to southern African kimberlites, the EGP kimberlites have consistently lower MgO and Os concentrations at comparable Ni concentrations. The opposite is true for Ti–Ni variation where the EGP kimberlites have higher Ti than the southern African kimberlites at comparable Ni concentrations. These data are interpreted to suggest that the CLM beneath the province was either melt metasomatised some time prior to kimberlite emplacement, or that the EGP CLM is less refractory (melt depleted) than the Kaaapvaal CLM.
In contrast, the incompatible element ratios and initial Nd values are constant throughout the entire rock suite. Carbonate C and O isotope data show a broad positive correlation, consistent with magmatic-hydrothermal trends found in many carbonatite complexes. These incompatible element and isotope data link all of the rocks within the province to the same mantle source that was similar to modern-day mantle plume sources.
Re–Os data for the various samples, including oxide minerals from all of the petrographic types, xenocryst-poor kimberlites and melnoites yield a precise Re–Os isochron of 2025±10 Ma and an initial γOs of 5.3±3.1 (MSWD=5.8). These data support the conclusion based on incompatible element, stable isotope and Sm–Nd isotope data that the rocks are comagmatic.
Initial Os isotopic compositions and Re/Os ratios for the xenocryst-rich kimberlites are also correlated. However, the correlation does not pass through the calculated initial γOs and Re/Os of the isochron. The Re–Os data show that the incompatible element-enriched melt exhibited very little control on the Re–Os variations of the xenocryst-rich kimberlites.
Correlations between deep mantle seismic velocities, petrology and whole-rock compatible element geochemistry suggest that the rheology and history of the EGP CLM played a significant role in determining the petrographic characteristics of the magmas that were ultimately emplaced into the EGP crust. 相似文献
Mineral inclusions of corundum are reported from diamonds from alluvial deposits of tributaries of the Rio Aripuanã, Juina, Brazil. We present the first recorded occurrence of sapphire as an inclusion in diamond and expand on the database of ruby and white corundum inclusions. Ruby inclusions are found to occur both as isolated and touching grains with aluminous pyroxene and associated with ferropericlase. Mineral chemistry and phase relations place the origin of such ruby-bearing diamonds within the lower mantle at 770 km. Mineral associations indaving other corundum inclusions were not observed; hence, their depth of origin is less certain.
Compositions of corundum samples were characterised by electron and ion microprobe. Given the scarcity of literature data, corundum samples from a variety of other geological settings were also analysed. Samples comprised corundums associated with granitic emplacement, metasomatism, amphibolite-facies and granulite-facies rocks, gem and industrial synthetic origins and carmine-coloured corundums recovered from kimberlite drill cores.
In addition to variable amounts of Cr, Fe, Ti, Mg and Si, measurable quantities of other transition elements and high field strength elements were also detected. Corundums from similar geological settings show very similar compositions and are easily distinguishable from other settings. Irrespective of locality, rubies from Norwegian, Tanzanian and Kenyan amphibolite-facies rocks are compositionally indistinguishable. Additionally, corundums from metasomatised zones associated with contact metamorphism from Arizona and Japan were very similar, particularly characterised by unusually high abundance of mobile Zr and Nb (tens of ppm). All Juina inclusions are particularly distinguishable from other corundums by high concentrations of Ni (18–171 ppm weight), typically at least an order of magnitude enriched over the same corundum varietal types from elsewhere. Furthermore, the sapphire inclusion exhibited much larger ratios of Ga and Ge to HFSE elements compared to otherwise similar samples, and ruby inclusions are distinguished by high Mg/Fe ratios (0.27–1.56 by weight). Compositional differences between inclusions in diamonds and corundums from other settings in addition to corundum's physical and chemical durability suggest that with the employment of rapid identification tools such as energy dispersive spectrometry (EDS) and laser-ICPMS, corundum has promise as an indicator of diamond prospectivity. 相似文献
The van Cittert-Zernike theorem describes the Fourier transform relationship between an extended source and its visibility function. Developments in classical optics texts use scalar field formulations for the theorem. Here, we develop a polarimetric extension to the van Cittert-Zernike theorem with applications to passive microwave earth remote sensing. The development provides insight into the mechanics of two-dimensional inteferometric imaging, particularly the effects of polarization basis differences between the scene and the observer. 相似文献
