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51.
52.
Proterozoic terrains in South India and Madagascar provide important clues in understanding the Gondwanaland tectonics, especially the assembly of this mega-continent during the Pan-African period. The Archaean terrains in both Madagascar and India are characterized by N-S trending greenstone belts occurring within gneissose granitic rocks in the northern part. Extensive development of K-rich granitic rocks of ca. 2.5 Ga is also characteristic in both areas. Such a broad age zonation of younger Dharwar (ca 2.6–3.0 Ga) in the north and the older Sargur (ca 3.0–3.4 Ga) in the south as in South India remains to be identified in future studies from Madagascar. The occurrence of greenschist facies rocks in the northeastern part and higher grade rocks in most of other parts in the north-central terrain of Madagascar is comparable with the general tendency of increasing metamorphic grade from northwestern to southern areas ranging from greenschist to granulite facies in South India. The Proterozoic crystalline rocks in both continents show pronounced lithological similarity with the wide occurrence of graphite-bearing khondalite in association with charnockitic rocks. While the Archaean-Proterozoic boundary is well defined in southern India by the Palghat-Cauvery or the KKPT shear zones as recently identified, this boundary is ill-defined in Madagascar due to extensive Pan-African overprinting, as well as the development of the Proterozoic cover sequence, the Itremo Group. There is also a possible general correlation between the Mesoproterozoic cover sequences in Madagascar and India, such as between the Itremo Group of west-central Madagascar and the Kaladgi and Cuddapah sequences of South India. The Pan-African granulite facies metamorphism of ca. 0.5 Ga extensively developed in both India and Madagascar is generally comparable in intensity and extent. P-T conditions and P-T-t paths also appear comparable, with the general range of ca. 700–1000°C and 6–9 kb, and near-isothermal decompressional paths. A-type granite plutons and alkaline rocks including anorthosites and mafic plutonic rocks of ca. 500–800 Ma develop in both terrains, provide strong basis for the correlation of both terrains, and define a Pan-African igneous province within East Gondwanaland. Major shear zones in both continents are expected to play a critical role in the correlation, albeit are still poorly constrained. Detailed elucidation of the tectonic history of the shear zones, and the timing of various events along the shear zones would provide important constraints on the correlation of the two continental fragments. 相似文献
53.
Lalmani Babu Madhu Kaul Kumar Rajou Singh Rajesh Singh Krishna Kumar 《Earth, Moon, and Planets》1999,84(3):151-162
The propagationmechanism of low latitude daytime whistlers is investigated on the basis of ground measurements made continuously
during daytime in North India at Jammu (geomag. lat. 22°26°N;L = 1.17). On February 14, 1998 extremely small dispersion (ESD)
whistlers with dispersion varying from 5–10 sec1/2 in surprisingly large numbers were recorded at Jammu during daytime in the late afternoon. The results of a study of the
characteristics of ESD whistlers are presented and the discussion indicates that ESD whistlers recorded are the VLF waves
radiated from the return stroke of the lightning discharge launched at the ionosphere with different initial wave normal angles,
propagated upwards under eitherquasi-longitudinal conditions or pro-longitudinal whistler mode, turned around at different
heights due to quasi-transverse propagation and received at Jammu with the dispersion of the order of 5–10 sec1/2. The validity of this suggestion has been tested by performing actualray-tracing computations in thepresence of equatorial
anomaly model.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
54.
Ocean Dynamics - The complicated pattern of the chaotic ocean surface depends strongly on the interaction between wind and waves. An accurate representation of momentum and energy exchange at... 相似文献
55.
Pramod?SoniEmail author Sachchida?Nand?Tripathi Rajesh?Srivastava 《Theoretical and Applied Climatology》2018,132(1-2):115-134
The Weather Research and Forecasting model with Chemistry (WRF-Chem) is utilized to examine the radiative effects of black carbon (BC) aerosols on the Indian monsoon, for the year 2010. Five ensemble simulations with different initial conditions (1st to 5th December, 2009) were performed and simulation results between 1st January, 2010 to 31st December, 2010 were used for analysis. Most of the BC which stays near the surface during the pre-monsoon season gets transported to higher altitudes with the northward migration of the Inter Tropical Convergence Zone (ITCZ) during the monsoon season. In both the seasons, strong negative SW anomalies are present at the surface along with positive anomalies in the atmosphere, which results in the surface cooling and lower tropospheric heating, respectively. During the pre-monsoon season, lower troposphere heating causes increased convection and enhanced meridional wind circulation, bringing moist air from Indian Ocean and Bay of Bengal to the North-East India, leading to increased rainfall there. However, during the monsoon season, along with cooling over the land regions, a warming over the Bay of Bengal is simulated. This differential heating results in an increased westerly moisture flux anomaly over central India, leading to increased rainfall over northern parts of India but decreased rainfall over southern parts. Decreased rainfall over southern India is also substantiated by the presence of increased evaporation over Bay of Bengal and decrease over land regions. 相似文献
56.
Jason E. French Larry M. Heaman Thomas Chacko Rajesh K. Srivastava 《Precambrian Research》2008,160(3-4):308-322
A newly recognized remnant of a Paleoproterozoic Large Igneous Province has been identified in the southern Bastar craton and nearby Cuddapah basin from the adjacent Dharwar craton, India. High precision U–Pb dates of 1891.1 ± 0.9 Ma (baddeleyite) and 1883.0 ± 1.4 Ma (baddeleyite and zircon) for two SE-trending mafic dykes from the BD2 dyke swarm, southern Bastar craton, and 1885.4 ± 3.1 Ma (baddeleyite) for a mafic sill from the Cuddapah basin, indicate the existence of 1891–1883 Ma mafic magmatism that spans an area of at least 90,000 km2 in the south Indian shield.This record of 1.9 Ga mafic/ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south-eastern margin of the south Indian shield is a widespread geologic phenomenon on Earth. Similar periods of intraplate mafic/ultramafic magmatism occur along the margin of the Superior craton in North America (1.88 Ga Molson large igneous province) and in southern Africa along the northern margin of the Kaapvaal craton (1.88–1.87 Ga dolerite sills intruding the Waterberg Group). Existing paleomagnetic data for the Molson and Waterberg 1.88 Ga large igneous provinces indicate that the Superior and Kalahari cratons were at similar paleolatitudes at 1.88 Ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a Limpopo-like 2.0 Ga deformation zone in the Superior Province. The widespread occurrence of 1.88 Ga intraplate and plate margin mafic magmatism and basin development in numerous Archean cratons worldwide likely reflects a period of global-scale mantle upwelling or enhanced mantle plume activity at this time. 相似文献
57.
Rajesh K. Srivastava 《Mineralogy and Petrology》1997,61(1-4):47-66
Summary The Ambadungar (Amba Dongar) alkaline carbonatite complex is emplaced in the Deccan traps igneous province. A wide range of carbonatites and alkaline rocks are exposed around Ambadungar. The alkaline rocks have been classified as tinguaite, phonolite and/or phononephelinite, melanephelinite, and syenite and/or nepheline syenite whereas carbonatites vary from calcio-carbonatites to ferro- and silicocarbonatites. The enrichment in large-ion lithophile elements (LILE), P, and rare-earth elements (REE) in carbonatites is considered to result from fractionation of a mantle derived magmatic liquid, i.e. nephelinitic magma, by liquid immiscibility which also produced melanephelinite and/or phononephelinite with high field strength elements (HFSE) such as Ca, Mg, Fe, and Mn in the alkaline silicate liquid fraction. The La:Lu ratios of the carbonatites are typical of igneous rocks and vary between 590 and 1945, similar to many known magmatic carbonatites. The 13C concentration varies between –2 and –8 whereas 18O-values vary between 7.7 and 26.8. The 13C concentration is typical of primary igneous carbonatites but 18O enrichment is thought to be the result of post-magmatic processes such as interaction with meteoric water and re-equilibration with hydrous fluids at low temperatures.
with 11 figures 相似文献
Petrologie, Geochemie und Genese der riftgebundenen Karbonatite von Ambadungar, Indien
Zusammenfassung Der Ambadungar (Amba Dongar) Alkalikarbonatit-Komplex liegt in der magmatischen Deccan Provinz. Er umfaßt eine Vielzahl von karbonatitischen und alkalischen Gesteinen, die in der Umgebung von Ambadungar aufgeschlossen sind. Die Alkaligesteine sind als Tinguaite, Phonolite und/oder Phononephelinite, Melanephelinite, Syenite und/oder Nephelinsyenite zu klassifizieren, die Karbonatite als Calcio-, bis Ferro- and Silicokarbonatite. Die Anreicherung an LIL-Elementen und Seltenen Erden in den Karbonatiten werden als das Ergebnis der Fraktionierung von Mantelschmelzen, i.e. eines nephelinitisches Magmas, infolge von Nichtmischbarkeit interpretiert. Melanephelinite und/oder Phononephelinite and hohe Gehalte an HFS-Elementen (Ca, Mg, Fe and Mn) in der alkalisch-silikatischen Schmelzfraktion sind ebenfalls das Ergebnis dieser Prozesse. Die La/Lu-Verhältnisse sind typisch für magmatische Karbonatite and variieren zwischen 590 and 1945. Die 13C Konzentrationen variieren zwischen -2 and -8 %o, die 18O Werte zwischen 7.7 and 26.8 %0. Während die 13C Konzentration typisch für primär magmatische Karbonatite ist, ist die 18O-Anreicherung mit postmagmatischen Prozessen, wie etwa die Interaktion mit meteorischen Wässern and die Reequilibration mit niedrig temperierten wäßrigen Fluiden, erklärbar.
with 11 figures 相似文献
58.
The Sargipali sulphide deposit, hosted by a sequence of metasediments of the Precambrian Gangpur Series, represents the stratiform and stratabound Pb-Cu mineralization. Lead-isotope analyses obtained from six galena samples of different forms and stratigraphical levels, yielded a model Pb age of 1665 Ma, which predates the episodes of deformation and metamorphism ( 850 Ma) as well as syn-to late-tectonic magmatism. The model Pb age, on the contrary, shows a relationship to the closing phase of the Gangpur sedimentation (1700 Ma) in a shallow-water marine environment. The notable paucity of the interlayered volcanic rock in the deposit and its Pb-rich nature possibly point to its being the sedimentary exhalative (SEDEX) type. Documentary evidence for the Pb component from the geological environment of the upper continental crust is revealed by an exceptionally high (= 238U/204Pb) and the slightly above average W(= 232Th/238U) values. These indicate that the source rock was initially rich in U-Th-Pb concentration. For the Sargipali ore Pb, the parent is assumed to be late Archean-lower Proterozoic soda granite. 相似文献
59.
N. V. Chalapathi Rao A. Dongre G. Kamde Rajesh K. Srivastava M. Sridhar F. V. Kaminsky 《Mineralogy and Petrology》2010,98(1-4):313-328
The Siddanpalli kimberlites constitute a newly discovered cluster (SKC) of Mesoproterozoic (1090 Ma) dykes occurring in the granite-greenstone terrain of the Gadwal area in the Eastern Dharwar Craton (EDC), Southern India. They belong to coherent facies and contain serpentinized olivines (two generations), phlogopite, spinel, perovskite, ilmenite, apatite, carbonate and garnet xenocrysts. A peculiar feature of these kimberlites is the abundance of carbonate and limestone xenoliths of the eroded platformal Proterozoic (Purana) sedimentary cover of Kurnool/Bhima age. Chemically, the Siddanpalli dykes are the most magnesium-rich (up to 35 wt.% MgO) and silica-undersaturated (SiO2?<?35 wt.%) of all kimberlites described so far from the Eastern Dharwar Craton. The La/Yb ratio in the Siddanpalli kimberlites (64–105) is considerably lower than that in the other EDC kimberlites (108–145), primarily owing to their much higher HREE abundances. Since there is no evidence of any crustal contamination by granitic rocks we infer this to be a specific character of the magmatic source. A comparison of the REE geochemistry of the Siddanpalli kimberlites with petrogenetic models for southern African kimberlites suggests that they display involvement of a wide range in the degree of melting in their genesis. The different geochemical signatures of the SKC compared to the other known kimberlites in the EDC can be explained by a combination of factors involving: (i) higher degrees of partial melting; (ii) relatively shallower depths of derivation; (iii) possible involvement of subducted component in their mantle source region; and (iv) previous extraction of boninitic magmas from their geological domain. 相似文献
60.
In this paper we focused on understanding the isostatic compensation of the Ninetyeast Ridge in the overall context of the
Bay of Bengal oceanic lithosphere and the interaction of the ridge system with the north Andaman subduction zone from north
of 7–18°N. This region is characterized by the initial interaction of the Kerguelen hotspot with the Bay of Bengal oceanic
lithosphere. We used satellite altimeter-derived marine geoid, as it should comprehensively reflect the compensations caused
by large spatial wavelength dominated deeper anomaly sources in a hotspot affected lithospheric load like the Ninetyeast Ridge.
Our analyses of the geoid-to-topography ratio (GTR), residual geoid, gravity-to-topographic kernel and upward continuation
of anomalies show the existence of two different types of source compensation bodies beneath the northern (12–18°N) and southern
(7–12°N) Ninetyeast Ridge. In the northern region, the geoid to topography ratio varies from 0.63 ± 0.05 to 0.44 ± 0.03, while
in the southern region it ranges from 1.34 ± 0.09 to 1.31 ± 0.07 which resulted in a north to south increase in the apparent
compensation depth from ~9 to 28 km. The presence of a shallow Moho, low GTR, broader gravity to topography kernel and the
absence of a ridge anomaly from the mantle density dominated upward continued anomaly at z = 300 km indicates that at the northern segment the underplated low density crustal melt is the dominant isostatic compensating
body. However, at the southern ridge segment the high GTR, strong gravity-to-topography kernel and the subsistence of the
anomaly at long wavelengths, even at z = 300 km represents the existence of large volumes of hotspot related underplated dense material as the source of compensation.
The proximity of the dense source compensating body of the southern Ninetyeast Ridge to the Andaman subduction zone affected
the regional mantle driven density gradient flow, as observed from the z = 300 km continued gravity anomaly. The existence of a southern Ninetyeast Ridge in such a transpressional regime has caused
the formation of a forearc sliver at its eastern flank, which is a major crustal deformational structure developed as a result
of ridge-trench collision. 相似文献