Recent investigations on the Deccan Traps reveal many new findings of alkaline rocks more commonly occurring as minor intrusions than as lava flows. In comparison to the vast extent of the Deccan Traps, the alkaline rocks are negligible in their volume and are confined to tectonic belts in parts of Western India. The rocks exhibit no systematic variation in their petrographical and chemical characters thereby suggesting that they were not derived from a primary alkali olivine-basalt magma. The possibility of derivation of alkaline magma locally along the rift zones is proposed. Some of the alkaline rocks are shown to have been formed due to the effective role of volatiles in bringing dissociation of feldspar in certain cases, and alkali metasomatism in others. The syntexis of the pre-Deccan Trap carbonate rocks along the Narmada rift zone is also responsible for some occurrences. 相似文献
Geological studies made in the area of Mara Rosa in Goias State, Brazil, where small occurrences of gold are known in river placers, have revealed an interesting and extensive “lateritised gravel bed” overlying an inexpressive nodular lateritic crust horizon. The gravel has pebbles of vein quartz, quartzites and gneisses, which are interlocked and compacted by intense lateritisation with brown hydrous Fe-oxide (goethite). The underlying nodular crusts grade from deep brown to mottled, and occasionally have cavities. They possibly represent an interface between the bed rock and the gravel bed. The intensity of lateritisation is attributed to the existence of volcanogenic mafic sequences in the rock. The presence of pebbles of the same nature in the small placers with Au, suggest that Au has been dissolved and precipitated during the lateritisation process. Its abundance is mainly restricted to the lateritic gravel bed, which in turn has contributed Au to the stream placers. Thus, it is considered as a new guide horizon for lateritic Au. 相似文献
Two examples are given for comparing applications and limitations of four methods which can be used to deal with error propagation in geochemical calculations.The examples indicate that the Monte Carlo method can also be employed to evaluate the effect of covariance.A special function of the method for covariance matrix shown here can reveal the correlations of middle variables relative to the independent primary variables. 相似文献
The distribution of temperature and salinity in the upper 500 m of the northwestern Bay of Bengal, adjoining the east coast
of India, during the retreat of southwest monsoon (September) of 1983 is presented. This study reveals coastal upwelling (limited
to the upper 40 m) induced by the local winds. Waters of higher surface salinity near the coast characterize the upwelling.
The freshwater influx near the head of the Bay diluted the surface salinity to as low as 26.0 × 10−3. The surface circulation was weak and led to a net transport of 2.0 × 106m3.s−1 directed towards northeast. 相似文献
Carbon Preference Index (CPI values) of higher plant-derived long-chain n-alkanes extracted from 62 surface soil samples in eastern China exhibited a specific pattern of variations, namely gradual
increase with the increasing latitudes. Such regular variations existed in both forest soil and grassland soil. Our data implied
that CPI values of higher plant-derived long-chain n-alkanes had a certain connection with climatic conditions, and such a connection was not influenced by vegetation types.
Together with previous data from marine sediments, loess/paleosol sequences, tertiary red clay and modern plants, our observation
made us conclude that CPI values of higher plant-derived long-chain n-alkanes may be used as an excellent proxy for paleoclimatic studies. 相似文献
This study examines the effect of loss of apparent cohesion from rainwater infiltration upon the stability of partly saturated, allophanic soil slopes of Dominica (West Indies). The parent material of the Dominican allophanic soils are the andesitic and dacitic volcanic rocks from ten volcanic centres of mainly Pleistocene age. Although simplifying assumptions are made to assess the depth of wetting front and magnitudes of true and apparent cohesion values of the partly saturated allophanic soils, certain inferences of practical significance emerge from the study.
Matric suction contributes to the effective stress of unsaturated soils and increases the shear strength of these soils by imparting them an apparent cohesion strength component. This apparent cohesion strength component of the partly saturated Dominician allophanic soils is considered to be related to the matric suction term (ua−uw) by a parabolic relationship.
The partly saturated allophanic soil slopes of Dominica would invariably fail from loss of apparent cohesion upon saturation of the soil mantle by the infiltrating water front only if (a) the average slope angle (β) is greater than or equal to the drained shear strength parameter of the unsaturated soil (Φd) (slopes with β ≥ Φd are referred to as category 1 slopes in this study) and (b) the drained cohesion parameter (cd) belonging to the partly saturated soil of the category 1 slope is mainly contributed by matric suction induced apparent cohesion (capp) and contribution from true cohesion (c′) is absent. However, the possibility of the category 1 slopes (for the case 1 situation) failing in the saturated condition from effective stress reduction due to rise in ground water table is shown to be an unfeasible proposition. In contrast, for the case 1 situation, the partly saturated category 2 slopes (β < Φd) are insusceptible to failure from reduction in effective stress due to loss of matric suction but fail in the saturated condition from effective stress reduction due to rise in ground water table. If the allophanic soils were to possess a sufficient magnitude of true cohesion (c′), even the partly saturated category 1 slopes possessing slope angles (β) much in excess of their Φd values (β − Φd = 15.1°) would remain stable upon total elimination of the matric suction induced cohesion. However, partly saturated category 1 slopes possessing a true cohesion component and insusceptible to failure from reduction in effective stress upon loss of matric suction, invariably fail in the saturated condition from reduction in effective stress due to rise in ground water table.
Using the methodology developed in this study, two previously reported case histories pertaining to rain induced translational landslides in the residual soil areas of Brazil are re-examined and the results obtained in this study are found to be in agreement with the findings of the previous researchers. 相似文献
The basement gneiss of the Shillong plateau and the overlying rocks of the Shillong basin have been dissected by a number of mafic dyke swarms represented by older Proterozoic dolerite dykes and sills named as Khasi greenstone and the younger set of Cretaceous dykes. The older dolerites dykes of Shillong basin are partly metamorphosed and have undergone low-grade metamorphism compared to the fresh unmetamorphosed Cretaceous dykes dominantly exposed in the BGC of West Garo hills region. The Khasi greenstones are tholeiitic in composition and range from basalt to basaltic andesite. Most of the metamorphosed mafic dykes indicate continental nature with some amount of overlapping oceanic tectonic setting. Palaeomagnetic study of the metadolerite dykes show a direction of magnetization of Dm=29, Im=38 (α95 = 28.84; k = 48.33; N = 2) with a palaeolatitude of 21.3° N to the Indian sub-continent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from IRM studies is magnetite in the range of psuedosingle to multi domain (MD) states with minor contribution from hematite. 相似文献