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331.
Pedro Filipe de Oliveira Cordeiro José Affonso Brod Matheus Palmieri Claudinei Gouveia de Oliveira Elisa Soares Rocha Barbosa Roberto Ventura Santos José Carlos Gaspar Luis Carlos Assis 《Ore Geology Reviews》2011,41(1):112-121
The Catalão I alkaline–carbonatite–phoscorite complex contains both fresh rock and residual (weathering-related) niobium mineralization. The fresh rock niobium deposit consists of two plug-shaped orebodies named Mine II and East Area, respectively emplaced in carbonatite and phlogopitite. Together, these orebodies contain 29 Mt at 1.22 wt.% Nb2O5 (measured and indicated). In closer detail, the orebodies consist of dike swarms of pyrochlore-bearing, olivine-free phoscorite-series rocks (nelsonite) that can be either apatite-rich (P2 unit) or magnetite-rich (P3 unit). Dolomite carbonatite (DC) is intimately related with nelsonite. Natropyrochlore and calciopyrochlore are the most abundant niobium phases in the fresh rock deposit. Pyrochlore supergroup chemistry shows a compositional trend from Ca–Na dominant pyrochlores toward Ba-enriched kenopyrochlore in fresh rock and the dominance of Ba-rich kenopyrochlore in the residual deposit. Carbonates associated with Ba-, Sr-enriched pyrochlore show higher δ18OSMOW than expected for carbonates crystallizing from mantle-derived magmas. We interpret both the δ18OSMOW and pyrochlore chemistry variations from the original composition as evidence of interaction with low-temperature fluids which, albeit not responsible for the mineralization, modified its magmatic isotopic features. The origin of the Catalão I niobium deposit is related to carbonatite magmatism but the process that generated such niobium-rich rocks is still undetermined and might be related to crystal accumulation and/or emplacement of a phosphate–iron-oxide magma. 相似文献
332.
G. Bellieni E. M. Piccirillo R. Petrini V. A. V. Girardi A. B. Menezes Leal W. Teixeira L. R. Bastos Leal A. De Min P. Comin Chiaramonti M. A. F. Tanner de Oliveira 《Contributions to Mineralogy and Petrology》1995,122(3):252-261
: Early Proterozoic (2.0–2.4 Ga), unmetamorphosed mafic dykes intrude Archean (3.1–2.7 Ga) terrains of the northern Sao Francisco
craton (Uaua, NE-Brazil). The dykes are composed of evolved [atomic Mg/(Mg+Fe2)<0.6] two-pyroxene quartz tholeiites, the compositional variations of which are compatible with gabbro fractionation from
different parental melts. The incompatible trace-element patterns indicate that the parental melts derived from partial melting
of chemically heterogeneous garnet peridotite sources. Geochemistry and Sr-Nd isotopes imply some contamination of the Uaua
dyke magma by continental crustal components during emplacement. Sr-isotopes suggest that the “uncontaminated” dykes may be
related to time-integrated depleted mantle materials, while Nd-isotopes suggest mantle sources similar to the bulk Earth composition.
The “uncontaminated” dykes have a positive Nb-anomaly and incompatible element contents which do not support appreciable enrichment
of large ion lithophile and light rare earth elements in the source(s) throughmantle metasomatism and/or crustal components
related to dehydration of a subducting slab, and an anorogenic ensialic emplacement is inferred. The Uaua dykes are chemically
and isotopically distinct from the Early–Middle Proterozoic (1.9–1.7 Ga) unmetamorphosed dykes from W-Uruguay (Rio de La Plata
craton) which are believed to have originated from different garnet peridotite sources. This suggests that Uaua–Uruguay compositional
differences may reflect different Archean mantle differentiation and evolution of the cratonization processes. If this possibility
proves to be correct, then magma genesis from various South American cratons would be by independent cratonic processes and
their imprints on the subcratonic mantle.
Received: 22 November 1994/Accepted: 24 July 1995 相似文献
333.
Observed boundary-layer circulations close to the confluence of the Negro and Solimões rivers near Manaus in the Brazilian equatorial Amazon forest were presented in Part I. These are shown through linear analysis and second-order turbulence modelling to be aspects of a river breeze superimposed on the basic flow. Linear analysis is presented to estimate the spatial structure and intensity of a breeze induced by a river with width and thermal contrast similar to that observed in the central Amazon. It is found that observed thermal contrasts are sufficient to produce a river breeze that can be perceived more than 20 km inland daily. A one-dimensional second-order closure model is used to show that observed nocturnal low-level wind maxima and diurnal surface wind rotation are aspects of a river breeze interacting with the seasonally-varying mean flow. At night, partial decoupling of the surface from the lower atmosphere allows the land breeze to be expressed as a low-level wind maximum. During the day, convective mixing communicates upper level winds to the surface during rapid morning boundary-layer growth. Rotation of the surface wind follows as the river breeze circulation is then superimposed. 相似文献
334.
Enhancement of accuracy of the direct integration methods used in structural dynamic response analysis is presented. The accuracy achieved is obtained at the same time as the cost of the problem solution is reduced. The method presented is based on Richardson's extrapolation technique. 相似文献
335.
Viviane S. Guerra Otávio C. Acevedo Luiz E. Medeiros Pablo E. S. Oliveira Daniel M. Santos 《Boundary-Layer Meteorology》2018,166(3):395-422
The study of the boundary layer can be most difficult when it is in transition and forced by a complex surface, such as an urban area. Here, a novel combination of ground-based remote sensing and in situ instrumentation in central London, UK, is deployed, aiming to capture the full evolution of the urban boundary layer (UBL) from night-time until the fully-developed convective phase. In contrast with the night-time stable boundary layer observed over rural areas, the night-time UBL is weakly convective. Therefore, a new approach for the detection of the morning-transition and rapid-growth phases is introduced, based on the sharp, quasi-linear increase of the mixing height. The urban morning-transition phase varied in duration between 0.5 and 4 h and the growth rate of the mixing layer during the rapid-growth phase had a strong positive relationship with the convective velocity scale, and a weaker, negative relationship with wind speed. Wind shear was found to be higher during the night-time and morning-transition phases than the rapid-growth phase and the shear production of turbulent kinetic energy near the mixing-layer top was around six times larger than surface shear production in summer, and around 1.5 times larger in winter. In summer under low winds, low-level jets dominated the UBL, and shear production was greater than buoyant production during the night-time and the morning-transition phase near the mixing-layer top. Within the rapid-growth phase, buoyant production dominated at the surface, but shear production dominated in the upper half of the UBL. These results imply that regional flows such as low-level jets play an important role alongside surface forcing in determining UBL structure and growth. 相似文献
336.
J. T. Oliveira C. J. P. Rosa Z. Pereira D. R. N. Rosa J. X. Matos C. M. C. Inverno T. Andersen 《Mineralium Deposita》2013,48(6):749-766
The lithostratigraphic sequence in the Rosário–Neves Corvo antiform comprises the Phyllite–Quartzite Group, whose top is of Famennian age, the Volcanic Sedimentary Complex, of Strunian to upper Visean age, and the Mértola Formation (the lower unit of the Baixo Alentejo Flysch Group) of upper Visean age. The volcanic sedimentary complex comprises a lower sequence of Strunian (Late Famennian) age and an upper sequence of lower to upper Visean age. Detailed mapping of the antiform towards NW of the Neves Corvo mine, supported by palynological dating, identified two new lithostratigraphic units: the Barrancão member (upper Famennian) ascribed to the Phyllite–Quartzite Group and made up of laminated dark shales with siliceous lenses and nodules, and the Ribeira de Cobres Formation of the Volcanic Sedimentary Complex, containing shales, siltstones and fine volcaniclastic rocks. Based on zircon U–Pb isotope dating, five discrete felsic magmatic events were identified at approximately 354, 359, 365, 373 and 384 Ma. This suggests that the volcanic activity in the area has extended for about 30 Ma, in a context of high regional heat flow as indicated by the geochemical signatures of the felsic volcanic rocks. The characteristics of magmatism and the depositional environment indicated by the sedimentary record should therefore have been highly favourable for massive sulphide formation. However, evidence of massive sulphide mineralization in the study area is still to be found. Moreover, reconstruction of the volcanic facies architecture demonstrated that the volcanic units in the Rosário area are strongly dominated by coherent facies typical of the inner part of thick lavas/domes. In fact, most of their external part, the more favourable location for possible massive sulphide mineralization, is missing. Palynological dating indicates a significant hiatus, recognised between the lower and upper sequences of the volcanic sedimentary complex, which implies erosion of the top of the volcanic centre, where VHMS deposits could possibly have formed. However, lateral areas of this volcanic centre, eventually preserved at depth, have good potential to host massive sulphide mineralization. 相似文献
337.
338.
Daniel T. C. Yao Waldyr Lopes de Oliveira‐Filho Xiao‐Chuan Cai Dobroslav Znidarcic 《国际地质力学数值与分析法杂志》2002,26(2):139-161
The consolidation and desiccation behaviour of soft soils can be described by two time‐dependent non‐linear partial differential equations using the finite strain theory. Analytical solutions do not exist for these governing equations. In this paper, we develop efficient numerical methods and software for finding the numerical solutions. We introduce a semi‐implicit time integration scheme, and show numerically that our method converges. In addition, the numerical solution matches well with the experimental result. A boundary refinement method is also developed to improve the convergence and stability for the case of Neumann type boundary conditions. Interface governing equations are derived to maintain the continuity of consolidation and desiccation processes. This is useful because the soil column can undergo desiccation on top and consolidation on the bottom simultaneously. The numerical algorithms has been implemented into a computer program and the results have been verified with centrifuge test results conducted in our laboratory. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
339.
Y. C. Unruh J.-F. Donati J. M. Oliveira A. Collier Cameron C. Catala H. F. Henrichs C. M. Johns-Krull B. Foing J. Hao H. Cao J. D. Landstreet H. C. Stempels J. A. de Jong J. Telting N. Walton P. Ehrenfreund A. P. Hatzes J. E. Neff T. Böhm T. Simon L. Kaper K. G. Strassmeier Th. Granzer 《Monthly notices of the Royal Astronomical Society》2004,348(4):1301-1320