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排序方式: 共有111条查询结果,搜索用时 15 毫秒
21.
22.
Highly heterogeneous Precambrian basement under the central Deccan Traps, India: Direct evidence from xenoliths in dykes 总被引:1,自引:0,他引:1
Ranjini Ray Anil D. Shukla Hetu C. Sheth Jyotiranjan S. Ray Raymond A. Duraiswami Loyc Vanderkluysen Chandramohan S. Rautela Jyotirmoy Mallik 《Gondwana Research》2008,13(3):375-385
Crustal or mantle xenoliths are not common in evolved, tholeiitic flood basalts that cover huge areas of the Precambrian shields. Yet, the occasional occurrences provide the most direct and unequivocal evidence on basement composition. Few xenolith occurrences are known from the Deccan Traps, India, and inferences about the Deccan basement have necessarily depended on geophysical studies and geochemistry of Deccan lavas and intrusions. Here, we report two basalt dykes (Rajmane and Talwade dykes) from the central Deccan Traps that are extremely rich in crustal xenoliths of great lithological variety (gneisses, quartzites, granite mylonite, felsic granulite, carbonate rock, tuff). Because the dykes are parallel and only 4 km apart, and only a few kilometres long, the xenoliths provide clear evidence for high small-scale lithological heterogeneity and strong tectonic deformation in the Precambrian Indian crust beneath. Measured 87Sr/86Sr ratios in the xenoliths range from 0.70935 (carbonate) to 0.78479 (granite mylonite). The Rajmane dyke sampled away from any of the xenoliths shows a present-day 87Sr/86Sr ratio of 0.70465 and initial (at 66 Ma) ratio of 0.70445. The dyke is subalkalic and fairly evolved (Mg No. = 44.1) and broadly similar in its Sr-isotopic and elemental composition to some of the lavas of the Mahabaleshwar Formation. The xenoliths are comparable lithologically and geochemically to basement rocks from the Archaean Dharwar craton forming much of southern India. As several lines of evidence suggest, the Dharwar craton may extend at least 350–400 km north under the Deccan lava cover. This is significant for Precambrian crustal evolution of India besides continental reconstructions. 相似文献
23.
Bibhash Nath Doris Stüben Sukumar Basu Mallik Debashis Chatterjee Laurent Charlet 《Applied Geochemistry》2008
The present study demonstrates the importance of hydrogeochemical characteristics (groundwater flow and recharge) of an aquifer in the release of As to groundwater. The study area (∼20 km2) is located in Chakdaha block, Nadia district, West Bengal, which hosts groundwaters of variable As content. The spatial distribution pattern of As is patchy with areas containing groundwater that is high in As (>200 μg L−1) found in close vicinity to low As (<50 μg L−1) groundwaters (within 100 m). The concentration of groundwater As is found to decrease with depth. In addition, the data shows that there is no conspicuous relationship between high groundwater As concentration and high groundwater abstraction, although the central cone of depression has enlarged over 2 a and is extending towards the SE of the study area. The river Hooghly, which forms the NW boundary of the study site, shows dual behaviour (effluent and influent during pre- and post-monsoon periods, respectively), complicating the site hydrogeology. The observed groundwater flow lines tend to be deflected away from the high As portion of the aquifer, indicating that groundwater movement is very sluggish in the As-rich area. This leads to a high residence time for this groundwater package, prolonging sediment–water interaction, and hence facilitating groundwater As release. 相似文献
24.
Experiments have been performed to study the thermal performance of a backfilled disposal vault in which the waste containers are emplaced in boreholes and surrounded by a bentonitesand buffer.
The near-field thermal regime in a buffer-backfill disposal vault in which the nuclear fuel waste is emplaced in canisters, is studied by one-eight scale experimental models. The thermal performance of the buffer and backfill are examined in a thermal drying environment prior to the ground water recharge of the disposal vault.
The formation of shrinkage cracks in the buffer due to drying and the response of these shrinkage cracks to the rewetting condition and their influence on the near-field thermal regime are assessed. 相似文献
25.
T.K. Mallik 《Marine Geology》1974,16(5):M85-M89
An underwater compass to collect oriented ocean-bottom samples was designed and tried in shallow depths of up to 112 m. The present design is very simple as compared to other types and can be fabricated at a very low cost. 相似文献
26.
27.
Bibhash Nath Sukumar Basu Mallik Doris Stüben Debashis Chatterjee Laurent Charlet 《Environmental Earth Sciences》2010,60(4):873-884
This study aims at finding out possible relation between lithology and spatial pattern of dissolved arsenic (As) in groundwater
around Chakdaha municipality, West Bengal, India. Satellite image, coupled with electrical resistivity survey and borehole
drilling helps to delineate surface and sub-surface lithological framework of the As affected alluvial aquifers. The satellite
imagery demonstrate that the high As area are presumably under active flood plain environment (low-lying areas), that constantly
receive organics due to periodic flooding. Thick low resistive (fine-grained) layer was observed at the top around the high
As areas, which, however, not found in low As areas. The result suggests that hydraulic properties of the surface/sub-surface
soil/sediment have an important control on the fate and transport of As in the aquifer. This study demonstrates that electrical
resistivity tools can be effectively used for the reconnaissance survey in characterizing the plausible lithological framework
of an alluvial aquifer containing As. 相似文献
28.
T. Radhakrishna D. G. Pearson John Mathai 《Contributions to Mineralogy and Petrology》1995,121(4):351-363
Agali–Coimbatore dolerite dykes constitute an important Proterozoic magmatic event that affected the south Indian shield.
Rb-Sr whole rock isotope data yield an “errorchron” of 2369±400 Ma (2σ error) which is within error of the reported 2030±65 Ma
K-Ar age. The dyke magmas were evolved Fe-rich tholeiitic melts produced by fractionation of clinopyroxene, orthopyroxene
and olivine in the initial stages. Plagioclase became a fractionation phase during the latter stages of crystallization. The
dykes characteristically have high 87Sr/86Sri (0.703–0.706) and are enriched in large-ion lithophile and light rare earth elements relative to primordial mantle values
and show negative Nb anomalies. These compositional characteristics are interpreted as source mantle characteristics whereas
some crustal effects are visible in some samples with high initial 87Sr/86Sr. Peridotite with minor hydrous metasomatic phases like amphibole (and phlogopite) within the shallow lithospheric mantle
could be a potential source material for the dykes. However, at this stage we cannot convincingly differentiate whether the
source of the parent magmas is solely lithospheric or a product of asthenosphere-lithosphere mixing. The δ18O values of the dykes range from +5.2 to +7.2 per mil (vs standard mean oceanic water). Initial Nd isotope values at the time
of dyke intrusion (ɛNd at t=2.0 Ga) range from −2.3 to −4.8. Whole rocks define a correlation on an Sm-Nd isochron plot with a slope equivalent to an
age of 3.15±0.53 Ga (2σ error); Sm-Nd crustal residence ages average at 2.87 Ga. The isochron age does not appear to be the
result of systematic mixing with an older crustal component. These results together with trace element geochemistry suggest that the south Indian
mantle lithosphere developed by addition of enriched melts/fluids at about 3.0 Ga synchronously with major crustal gene- ration
in the south Indian shield.
Received 20 June 1994/Accepted: 17 May 1995 相似文献
29.
I. V. Radhakrishna Murthy 《Pure and Applied Geophysics》1969,73(1):43-46
Summary A simple transformation called pseudo-magnetic transformation is introduced in which the horizontal derivatives of the observed gravity anomalies of any body will be calculated. These horizontal derivatives, called the pseudo-magnetic anomalies, are proportional to the vertical magnetic anomalies of the same body assumed to be polarised horizontally. It is shown that the interpretation of gravity anomalies of dykes is not unique except by computing the pseudo-magnetic anomalies, which will be interpreted by the logarithmic curve fitting technique indicated earlier [5]2). 相似文献
30.
B. S. R. Rao I. V. Radhakrishna Murthy S. Jeevananda Reddy 《Pure and Applied Geophysics》1970,80(1):5-26
Summary Second derivative coefficients for various grid systems are derived followingPeters' andElkins' approaches. Two simple formulae, one for three dimensional bodies and the other for two dimensional bodies are also suggested. These utilise the average of the anomalies around a single circle and the anomaly at the central point.All the coefficient sets and the proposed simple formulae are applied to gravity fields of spheres and cylinders. Coefficients ofElkins' type derived with a weightage of 1/r
4 to all the circles gave excellent results in both the cases of spheres and cylinders, whereasPeters' coefficients without any weightage to the circles gave reliable results in case of cylinders only. The simple formulae suggested gave more accurate results than any of the above coefficient sets. 相似文献