The Trans-Amazonian cycle was an important rock-forming event in South America, generating voluminous juvenile and reworked fractions of continental crust. The Bacajá domain, in the southern sector of the Maroni-Itacaiúnas Province in the Amazonian craton, is an example of the Trans-Amazonian terranes adjacent to the Archean Carajás block. Zircon Pb-evaporation and whole-rock Sm–Nd analyses were carried out on representative samples of six lithological units, and allowed the proposal of a comprehensive tectonic-magmatic evolutionary sequence for the central and eastern parts of this domain, from the Neoarchean to the Rhyacian. Gneisses with ages of ca. 2.67 and 2.44 Ga are the oldest rocks recorded in the region, and probably represent remnants of island and continental arcs. The Três Palmeiras succession, emplaced between 2.36 and 2.34 Ga, hosts gold deposits and represents the first record of Siderian supracrustal rocks in the Amazonian craton. It was probably part of an island arc/ocean floor accreted to a craton margin. Rhyacian granitogenesis lasted for ca. 140 My (2.22–2.08 Ga), marking different stages of the Trans-Amazonian cycle. The first stage is represented by continental arc granitoids formed by melting of Archean crust at 2.22–2.18 Ga. The second is characterized by the production of juvenile material between 2.16 and 2.13 Ga. The third and final stage at ca. 2.08 Ga is represented by a large volume of granitoids originated from either juvenile material or reworked crust during compressive stresses. Nd isotopes reveal that juvenile rocks dominated in the northern part of the domain, whereas those formed from reworked crust predominate in the south. The present-day configuration of the Bacajá domain results from collision against the Archean Carajás block at the end of the Trans-Amazonian cycle. 相似文献
A new discrete fracture model is introduced to simulate the steady‐state fluid flow in discontinuous porous media. The formulation uses a multi‐layered approach to capture the effect of both longitudinal and transverse permeability of the discontinuities in the pressure distribution. The formulation allows the independent discretisation of mesh and discontinuities, which do not need to conform. Given that the formulation is developed at the element level, no additional degrees of freedom or special integration procedures are required for coupling the non‐conforming meshes. The proposed model is shown to be reliable regardless of the permeability of the discontinuity being higher or lower than the surrounding domain. Four numerical examples of increasing complexity are solved to demonstrate the efficiency and accuracy of the new technique when compared with results available in the literature. Results show that the proposed method can simulate the fluid pressure distribution in fractured porous media. Furthermore, a sensitivity analysis demonstrated the stability regarding the condition number for wide range values of the coupling parameter. 相似文献
Atmospheric air pollution turbulent fluxes can be assumed to be proportional to the mean concentration gradient. This assumption, along with the equation of continuity, leads to the advection–diffusion equation. Moreover, large eddies are able to mix scalar quantities in a manner that is counter to the local gradient. In this work we present a semi-analytical solution for the three-dimensional steady-state advection–diffusion equation, considering non-local turbulence closure using the Generalized Integral Advection Diffusion Multilayer Technique (GIADMT). We report some examples of applications of the new solution for two different datasets and for a water tank experiment. 相似文献
We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading
rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and
Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive
magma (~11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals
into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted
basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium
with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different
histories. Diffusion modelling shows that crystal residence times in the mush were <140 years, whereas the interval between
mush disaggregation and eruption was ≤1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa
Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some
crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration.
This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts
from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring
mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate-
and slow-spreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales;
(2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones
appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted
basalts. 相似文献
Bolide impacts on Mars, within the proposed ocean boundaries (“contacts 1 and 2”) in the northern lowlands, would certainly have generated ultra high energy waves similar to tsunamis on Earth. Impacts into putative Noachian and Hesperian seas of variable areal extents and depths would have experienced high-energy inundations (transgressions), which would have left an imprint in the stack of deposits adjacent to the proposed shorelines. On Earth, the principal influencing factors for tsunami-wave energy are the character of shoreline topography and coastal water depth, which control wave compression and shoreline friction. Shorelines with narrow embayments and steep offshore gradients produce wave compression and increased collision of grains within the carried load contrasted with linear shorelines and shallow offshore gradients that dissipate energy. Steep offshore gradients produce concentrated major wave friction with the bed engendering high kinetic energy in the wave during emplacement of tsunami-generated sediment, which differs from shallow offshore beds that produce lower frictional effects over a wider area and drawdown of wave energy. Thus, overprinting of transported quartz grains on Earth is greatest where wave energy is highest, attenuated down to minor or nil overprinting where wave energy is less. Such grain overprinting in the form of energy-induced microtextures would also be observed in other grain types such as olivine and plagioclase, as such mineralogies are expected to dominate the Martian landscape based on orbital and local field (lander and rover) perspectives. Kinetic energy variation in tsunamis is controlled more by the square of velocity than mass, the resulting collisional effects of which produce swarms of v-shaped percussion microfeatures on quartz and other silicate mineral surfaces when velocity and compression are highest. This work indicates that a valid test for the ocean hypothesis is targeting “coastal” areas adjacent to narrow embayments where offshore depths are known to be highest, as possible tsunami-emplaced sediments, especially those that have been protected from atmospheric conditions through relatively rapid burial, may reveal a high frequency of percussion cracks, features of which appear to be unique to such terrestrial environments. 相似文献
Owing to its five decades in the chemical industry, Estarreja is one of the most important industrial areas in Portugal. Intensive industrial activity along with both direct discharge of the effluents into natural water streams and uncontrolled waste disposal on the ground has, throughout the years, had strong impact on health and welfare. Recently an association between industry and local authorities was created — ERASE. The main goal for this association is to find, in co-operation with the Portuguese Environmental Ministry, a cost-effective solution to deal with the soil/sediments contamination and solid waste problem.
The ERASE association planned to build a landfill for both solid waste and contaminated soil/sediments disposal. In order to determine more accurately the volume of material to be disposed of in the landfill, a site investigation was carried out during September/November 1998. The site investigation consisted mainly of systematic soil sampling at shallow depths, both within the industrial area and along the natural water streams.
The site investigation results revealed high concentrations of toxic pollutants, mainly heavy metals (namely As, Hg, Pb and Zn), in the soil of the industrial area. Much higher concentrations were found in the sediments of the water streams, several kilometres away from the industrial complex (the pollution source area).
In most cases, concentration increases with depth, reaching groundwater. Therefore the site investigation programme carried out could not determine the full extent of the contamination. Consequently, further studies were strongly recommended, which should include a wider and deeper investigation area and groundwater sampling. 相似文献
We have studied through a multiresolution wavelet analysis the oscillations in a limb prominence. Intensity fluctuations in time and height corresponding to different lines of Siiv and Oiv observed with SUMER on board SOHO have been analyzed in the wavelet bands of J3= 1 min 36 s to 3 min 12 s and J4=3 min 12 s to 6 min 24 s. For all species, oscillations in the J4 band were dominant. We found relevant differences between the behavior of line D1 (1393.76 Å) corresponding to Siiv and the set D2 (1401.16 Å), D3 (1404.81 Å), D4 (1402.77 Å) corresponding to Oiv, Oiv and Siiv respectively. We also report the identification of a pulse in the intensity of the line D1 that appears in the range of 15–20 min. This disturbance seems to travel with a speed of about 170 km s–1. 相似文献