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81.
Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil 总被引:1,自引:0,他引:1
S. A. Gibson R. N. Thompson R. K. Weska A. P. Dickin O. H. Leonardos 《Contributions to Mineralogy and Petrology》1997,126(3):303-314
High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser 40Ar/39Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (87Sr/86Sr?=?0.704 and 143Nd/144Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C. 相似文献
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88.
The study of REE distribution in the clay fraction of sedimentary rocks from two coalfields made it possible to distinguish
several types of REE distribution, which correlate with their mineral composition. It is shown that the REE fractionation
was related to the mineral reconstruction of the primary clay fraction and some detrital minerals in the course of postsedimentary
transformations of rocks during diagenesis, early catagenesis, and beginning of late catagenesis. These transformations were
governed by several factors, such as the composition of sediments; hydrochemical features of accumulation environment; the
chemical composition, dynamics, and feeding sources of pore solutions; the porosity and permeability of sediments and rocks;
and the content of organic matter and its reaction ability. 相似文献
89.
Summary Snow has been studied widely in hydrology for many decades whereas recent meteorological interest in snow is caused by increased
emphasis on high latitudes and wintertime in climate-change research as well as by the need to improve weather-forecast models
during these conditions. Ground-based measurements of snow properties are needed both to improve understanding of surface-atmosphere
exchange processes and to provide ground truth to new remote-sensing algorithms. This justifies a review of techniques to
measure snow in combination with establishment of criteria for the suitability of the methods for process studies. This review
assesses the state-of-art in ground-based snow-measurement techniques in the end of the 1990s in view of their accuracy, time
resolution, possibility to automate, practicality and suitability in different terrain. Methods for snow-pack water equivalent,
depth, density, growth, quality, liquid-water content and water leaving the snow pack are reviewed. Synoptic snow measurements
in Fennoscandian countries are widely varying and there is no single standard on which process-related studies can build.
A long-term, continuous monitoring of mass and energy properties of a snow cover requires a combination of point-measurement
techniques. Areally representative values of snow properties can be achieved through a combination of automatically collected
point data with repeated manual, areally covering measurements, remote-sensing data and digital elevation models, preferably
in a GIS framework.
Received August 27, 1999 相似文献
90.
Diurnal variation of deep cloud systems over the Indian region using INSAT-1B pixel data 总被引:1,自引:0,他引:1
Summary
Among the various time scales of deep clouds, diurnal variation is found to be prominent over the tropics. The present study
examines the diurnal variation in the number and area cover associated with the cloud systems over the tropical Indian region
using INSAT-1B pixel data. Three periods, namely, April–May 1988, July–August 1988, and January–February 1989 are considered.
The dependence of diurnal characteristics on the temperature threshold, life duration and size of cloud systems, and land-sea
contrast has been explored. The diurnal characteristics of cloud systems living for more than a day have been studied for
the first time. It is shown that cloud systems exhibit strong diurnal dependence at the coldest temperature threshold used
(201 K). Also, the diurnal variation is more for larger cloud systems and for longer living systems. In general, more deep
cloud activity is found from the satellite data during the pre-dawn and early morning hours. Precipitation is enhanced during
morning to early noon hours. Further, using data from a recent field experiment, clear evidence of diurnal variation in precipitation
over the Bay of Bengal is also presented.
Received March 20, 2000/Revised October 3, 2000 相似文献