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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.
《地学前缘(英文版)》2022,13(2):101336
Continental Flood Basalts (CFB) occupy one fourth of the world’s land area. Hence, it is important to discern the hydrological processes in this complex hydrogeological setup for the sustainable water resources development. A model assisted isotope, geochemical, geospatial and geophysical study was conducted to understand the monsoonal characteristics, recharge processes, renewability and geochemical evolution in one of the largest continental flood basalt provinces of India. HYSPLIT modelling and stable isotopes were used to assess the monsoonal characteristics. Rayleigh distillation model were used to understand the climatic conditions at the time of groundwater recharge. Lumped parameter models (LPM) were employed to quantify the mean transit time (MTT) of groundwater. Statistical and geochemical models were adopted to understand the geochemical evolution along the groundwater flow path. A geophysical model was used to understand the geometry of the aquifer. The back trajectory analysis confirms the isotopic finding that precipitation in this region is caused by orographic uplifting of air masses originating from the Arabian Sea. Stable isotopic data of groundwater showed its meteoric origin and two recharge processes were discerned; (i) quick and direct recharge by precipitation through fractured and weathered basalt, (ii) low infiltration through the clayey black cotton soil and subjected to evaporation prior to the recharge. Tritium data showed that the groundwater is a renewable source and have shorter transit times (from present day to <30 years). The hydrogeochemical study indicated multiple sources/processes such as: the minerals dissolution, silicate weathering, ion exchange, anthropogenic influences etc. control the chemistry of the groundwater. Based on the geo-electrical resistivity survey, the potential zones (weathered and fractured) were delineated for the groundwater development. Thus, the study highlights the usefulness of model assisted isotopic hydrogeochemical techniques for understanding the recharge and geochemical processes in a basaltic aquifer system. 相似文献
24.
The Koyna River basin in India drew the attention of geoscientists after an earthquake (magnitude 7) in 1967. Since then,
detailed geological, tectonic, and seismic investigations of this river basin have been carried out by several workers. However,
very little study has been done on its hydrogeological framework. The present work aims at filling this gap. Basalts, laterites,
alluvium, soils, and talus deposits form shallow unconfined aquifers, with transmissivity of 27–135 m2/d and a regional specific yield of 0.012. In shallow basaltic aquifers, the lower part of the highly weathered and highly
jointed horizon above, and the poorly weathered and highly jointed horizon below, form the most potential zone for groundwater
occurrence. Well yields in the deeper basaltic aquifers are directly related to the occurrence of lineaments, whereas at a
shallower level they are related to geomorphic features. Spring discharges are highly dependent on their source aquifers and
areas of recharge. They have a mean winter discharge of 46 m3/d and a summer discharge of 28 m3/d.
Chemically, groundwaters are dominated by alkaline earths (Ca2+, Mg2+) and weak acids (HCO3
–, CO3
–); they are calcium-bicarbonate type (53%) and calcium-magnesium-bicarbonate type (27%) at shallower levels; and calcium-magnesium-bicarbonate
type (29%), sodium-bicarbonate type (24%), calcium-bicarbonate type (19%), and calcium-magnesium-sodium-bicarbonate type (19%)
in deeper aquifers. The Koyna River basin is characterized by both scarcity and abundance of groundwater. In the water-scarce
areas in the dissected plateaus, artificial recharge of aquifers through construction of several recharge structures at suitable
locations is highly recommended. In the water-abundant areas in the central valley, on the other hand, expanded consumptive
use of water resources is encouraged.
Electronic Publication 相似文献
25.
Kanchan?PandeEmail author S.?K.?Pattanayak K.?V.?Subbarao P.?Navaneethakrishnan T.?R.?Venkatesan 《Journal of Earth System Science》2004,113(4):755-758
We report here a40Ar-39Ar age of 66.0 ± 0.9 Ma (2σ) for a reversely magnetised tholeiitic lava flow from the Bhimashankar Formation (Fm.), Giravali Ghat, western Deccan province,
India. This age is consistent with the view that the 1.8–2 km thick bottom part of the exposed basalt flow sequence in the
Western Ghats was extruded very close to 67.4 Ma. 相似文献
26.
H. C. Sheth 《Journal of Earth System Science》2006,115(6):615-629
There is a growing interest in deciphering the emplacement and environmental impact of flood basalt provinces such as the
Deccan, India. Observations of active volcanism lead to meaningful interpretations of now-extinct volcanic systems. Here,
I illustrate and discuss the morphology and emplacement of the modern and active lava flows of Kilauea volcano in Hawaii,
and based on them, interpret the compound pahoehoe lavas of the Deccan Traps. The latter are vastly larger (areally extensive
and voluminous) than Kilauea flows, and yet, their internal architecture is the same as that of Kilauea flows, and even the
sizes of individual flow units often identical. Many or most compound flows of the Deccan Traps were emplaced in a gentle,
effusive, Kilauea-like fashion. Bulk eruption rates for the Deccan province are unknown, and were probably high, but the local
eruption rates of the compound flows were no larger than Kilauea’s. Large (≥ 1000 km3) individual compound pahoehoe flows in the Deccan could have been emplaced at Kilauea-like local eruption rates (1 m3/sec per metre length of fissure) in a decade or less, given fissures of sufficient length (tens of kilometres), now exposed
as dyke swarms in the province. 相似文献
27.
The concentration of dissolved Ba in a number of rivers having their drainage almost entirely in Deccan Trap basalts has been
measured. These results along with available data on the abundances of major elements in these waters, and on Ba and major
elements in bed sediments of these rivers provide a measure of (i) the relative mobility of Ba during chemical weathering
and erosion of basalts, particularly with respect to alkaline earths, Mg, Ca and Sr, and (ii) the flux of Ba out of the Deccan
and its global significance. The concentration of dissolved Ba ranges from 8 to 105 nM. The average Ba/Mg*, Ba/Ca* and Ba/Sr
(* is concentration corrected for atmospheric contribution) in waters is lower than the corresponding mean ratios in Deccan
basalts, though they overlap within errors. Majority of the water samples, however, have ratios less than that in basalts.
These findings can be interpreted as a cumulative effect of limited release/mobility of Ba during chemical weathering and
erosion of basalts and its reactive behaviour in waters which promote its association with clays and oxy-hydroxides of Fe.
These results also indicate that during chemical erosion of Deccan basalts, Ba is the least mobile among the alkaline earth
elements. The abundance of Ba in sediments and their Ba/Al ratios relative to basalts are consistent with the above conclusion.
Ba/Mg and Ba/Ca ratios in water and in sediments from the same location are strongly correlated; however, the mean ratios
in waters are far less than those in sediments. This is a result of limited Ba mobility, effectively 5–6 times lower than
that of Mg. The annual flux of dissolved Ba out of the Deccan Traps is ~1 × 107 moles, ~ 0.2% of its global riverine transport to oceans. The contribution of dissolved Ba from Deccan Traps, seem lower
than its aerial coverage, ~ 0.5% of the global drainage area; the potential causes for this could be the lower abundance of
Ba in basalts relative to “average continental crust”, and its behaviour during chemical weathering and erosion. 相似文献
28.
29.
Anil Kumar Y J Bhaskar Rao V M Padma Kumari A M Dayal K Gopalan 《Journal of Earth System Science》1988,97(1):107-114
Palaeomagnetic, geochemical and geochronological studies have been conducted on a set of dolerite dykes intruding the Peninsular
gneisses near Huliyurdurga town, Karnataka, as a reconnaissance survey indicated a Cretaceous age for them. The dykes are
mainly tholeiitic in composition with their 87Sr/86Sr ratios tightly clustered around 0·7045. Their palaeomagnetic data (D
m
=329°,I
m
=−55°) and the corresponding palaeopole coordinates (λ
p
= 34°S,L
p
=108°E) are strikingly close to those of the Deccan Traps to the north. Whole rock K-Ar ages of these dykes ranging between
69 and 84 Ma are also similar to the range of K-Ar ages of the Deccan basalts. The chemical, palaeomagnetic and temporal coherence
between the dykes and the Deccan basalts indicate that they may indeed be tectonically related events. 相似文献
30.
H M Iyer V K Gaur S S Rai D S Ramesh CVR Rao D Srinagesh K Suryaprakasam 《Journal of Earth System Science》1989,98(1):31-60
Analysis of teleseismicP-wave residuals observed at 15 seismograph stations operated in the Deccan volcanic province (DVP) in west central India points
to the existence of a large, deep anomalous region in the upper mantle where the velocity is a few per cent higher than in
the surrounding region. The seismic stations were operated in three deployments together with a reference station on precambrian
granite at Hyderabad and another common station at Poona. The first group of stations lay along a west-northwesterly profile
from Hyderabad through Poona to Bhatsa. The second group roughly formed an L-shaped profile from Poona to Hyderabad through
Dharwar and Hospet. The third group of stations lay along a northwesterly profile from Hyderabad to Dhule through Aurangabad
and Latur. Relative residuals computed with respect to Hyderabad at all the stations showed two basic features: a large almost
linear variation from approximately +1s for teleseisms from the north to—1s for those from the southeast at the western stations,
and persistance of the pattern with diminishing magnitudes towards the east. Preliminary ray-plotting and three-dimensional
inversion of theP-wave residual data delineate the presence of a 600 km long approximately N−S trending anomalous region of high velocity (1–4%
contrast) from a depth of about 100 km in the upper mantle encompassing almost the whole width of the DVP. Inversion ofP-wave relative residuals reveal the existence of two prominent features beneath the DVP. The first is a thick high velocity
zone (1–4% faster) extending from a depth of about 100 km directly beneath most of the DVP. The second feature is a prominent
low velocity region which coincides with the westernmost part of the DVP. A possible explanation for the observed coherent
high velocity anomaly is that it forms the root of the lithosphere which coherently translates with the continents during
plate motions, an architecture characteristic of precambrian shields. The low velocity zone appears to be related to the rift
systems (anomaly 28, 65 Ma) which provided the channel for the outpouring of Deccan basalts at the close of the Cretaceous
period. 相似文献