Early Proterozoic 87Rb-86Sr Model Ages of Pegmatitic Muscovite from Rare Metal-bearing Granite-Pegamtite System of Kawadgaon, Bastar Craton, Central India     

Yamuna Singh  T. Chabria 《Gondwana Research》2002,5(4):889-893

Seven muscovite mica separates from the rare metal pegmatites of Kawadgaon, Bastar Craton, Central India, give model 87Rb-86Sr ages ranging from 2330 to 1850 Ma. The oldest age of the muscovite almost overlaps within 20 error with the age (2497k152 Ma) of the parent fertile granites. The data suggest possible derivation of pegmatites shortly after the emplacement of Kawadgaon granites at ca. 2500 Ma. Most of the muscovite ages (n = 6) indicate tectonomagmatic ages after pegmatite injections. The 87Sr/86Sr ratio (0.7142) of granites suggests their derivation from crustal material.  相似文献   

Deccan traps mantle degassing in the terminal Cretaceous marine extinctions     

Dewey M. McLean 《Cretaceous Research》1985,6(3):235-259

Mantle degassing continually releases gases onto the earth's surface. Over geologically long time intervals, a general equilibrium probably exists between mantle CO₂ release and uptake by surficial sinks. However, during periods of rapid plate movement, or continental flood basalt volcanism, the increased rate of mantle CO₂ release may exceed that of uptake, leading to CO₂ accumulation in the atmosphere and the marine mixed layer (top 50–100 m). This in turn triggers chemical changes in the mixed layer, climatic warming, and bioevolutionary turnover. The Cretaceous/Tertiary ($\text{K}\text{T}$) transition at 65 Ma seems to have been a time of major mantle degassing which induced a perturbation of the carbon cycle. During the $\text{K}\text{T}$ transition, Deccan Traps volcanism, perhaps the greatest episode of continental flood basalt volcanism in the Phanerozoic, flooded an estimated 2.6 × 10⁶ km² of India with basaltic lavas, releasing 5 × 10¹⁷ moles of CO₂ into the earth's atmosphere over a duration 0.53–1.36 Ma at the rate of 3.9 × 10¹¹ to 9.6 × 10¹¹ moles CO₂ per year. The modern mean annual rate of mantle CO₂ release from all sources is 4.1 × 10¹² moles CO₂ per year; assuming a comparable rate of release prior to the Deccan Traps volcanism, the Deccan Traps addition would have elevated the rate of mantle CO₂ release by 10–25%. Sluggish marine circulation and warm, deep, oceans (14–15°C) would have exacerbated CO₂ buildup in the atmosphere, accounting for the Cretaceous to Tertiary drop in oxygen-18 via climatic warming, and, in the marine mixed layer (top 50–100 m), explaining the selective nature of the terminal Cretaceous marine extinctions via a pH change. The extinctions were most severe amongst the calcareous microplankton of the mixed layer; calcareous microplankton (planktonic foraminifera and coccolithophorids) begin to have pH problems at 7.8 and 7.5, respectively. Failure of the coccolithophorids would have disrupted the Williams-Riley pump (algal productivity-gravity pump of CO₂ from the atmosphere and mixed layer into the deep oceans) producing dead ocean conditions (severely reduced photosynthesis and CaCO₃ production). Failure of the Williams-Riley pump is reflected in the extinctions themselves, and in the loss of biogenic CaCO₃ to the sea floor, causing the $\text{K}\text{T}$ boundary hiatus and (or) the $\text{K}\text{T}$ boundary clay. Failure of the pump today would elevate atmospheric pCO₂ severalfold; the $\text{K}\text{T}$ failure would have responded comparably. Dead ocean conditions would, in themselves, have produced a major CO₂ buildup. Early Tertiary “Strangelove” conditions in the mixed layer, characterized by a dominance of the thoracosphaerids, braarudosphaerids and small planktonic foraminifera, were coeval with the main pulse of Deccan Traps volcanism. Overall, the record is one of gradual $\text{K}\text{T}$ bioevolutionary turnover during a period of disequilibrium between the rate of mantle CO₂ degassing and uptake by sinks. Mantle degassing during the Deccan Traps volcanism unifies the $\text{K}\text{T}$ biological and physicochemical records.  相似文献   

Late Quaternary geomorphic evolution of the lower Narmada valley, Western India: implications for neotectonic activity along the Narmada–Son Fault     

L. S. Chamyal  D. M. Maurya  S. Bhandari  Rachna Raj 《Geomorphology》2002,46(3-4)

Geomorphic data combined with stratigraphic studies provide significant information to constrain timing and amount of fault movement. The lower Narmada valley lies astride the Narmada–Son Fault (NSF), an important ENE–WSW-trending tectonic element responsible for the current intraplate seismicity being experienced in the central part of the Indian plate. Varying nature and degree of tectonic movements along the NSF during Late Pleistocene and Holocene have produced four geomorphic surfaces in the lower Narmada valley: the alluvial plain (S₁), ravine surface (S₂), a gravelly fan surface (S₃) and the valley fill terrace surface (S₄). Two major phases of tectonic movements in a compressive stress regime are recorded along the NSF: slow synsedimentary subsidence of the basin during Late Pleistocene due to differential movement, followed by inversion of the basin during the Holocene marked by differential uplift along the NSF. The study suggests that the inversion of the basin is in response to the significant increase in the intensity of compressive stresses in the Indian plate mainly during the Early Holocene. The present incisive drainage and recent seismic activity indicate that the compressive stresses continue to accumulate along the NSF due to continued northward movement of the Indian plate.  相似文献   

Melt-producing and melt-consuming reactions in the Achankovil cordierite gneisses, South India   总被引：7，自引：0，他引：7  

B. Cenki  L. M. Kriegsman  I. Braun 《Journal of Metamorphic Geology》2002,20(6):543-561

Migmatitic cordierite gneisses within the Achankovil Zone (AZ) of southern Pan‐African India record melt‐producing and subsequent melt‐consuming mineral reactions. Early mineral assemblages Bt‐Sil‐Qtz and Bt‐Sil‐Spl, deduced from inclusion textures in garnet prophyroblasts, break down via successive dehydration melting reactions to high‐T phase assemblages (e.g. Grt‐Crd‐Liq, Opx‐Liq, Spl‐Crd‐Liq). Later back reactions between the restite and the in situ crystallizing melt resulted in thin cordierite coronas separating garnet from the leucosome, and partial resorption of garnet to Opx‐Crd or Crd‐Bt‐Qtz symplectites. Leucosomes generally display a moderate (low‐strain gneisses) to strong (high‐strain gneisses) depletion of alkali feldspar attributed to mineral‐melt back reactions partly controlled by the degree of melt segregation. Using a KFMASH partial petrogenetic grid that includes a melt phase, and qualitative pseudosections for microdomains of high and low Al/Si ratios, the successive phase assemblages and reaction textures are interpreted in terms of a clockwise P–T path culminating at about 6–7 kbar and 900–950 °C. This P–T path is consistent with, but more detailed than published results, which suggests that taking a melt phase into account is not only a valid, but also a useful approach. Comparing P–T data and lithological and isotopic data for the AZ with adjacent East Gondwana fragments, suggests the presence of a coherent metasedimentary unit exposed from southern Madagascar via South India (AZ) and Sri Lanka (Wanni Complex) to the Lützow–Holm Bay in Eastern Antarctica.  相似文献   

Variations in crustal structure related to intraplate deformation: evidence from seismic refraction and gravity profiles in the Central Indian Basin     

Keith E. Louden 《Geophysical Journal International》1995,120(2):375-392

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Assessment of the impact of industrial effluents on water quality in Patancheru and environs, Medak district, Andhra Pradesh, India     

K. Subrahmanyam  P. Yadaiah 《Hydrogeology Journal》2001,9(3):297-312

The effects of multiple industrial-pollutant sources on the groundwater system were evaluated in the Industrial Development Areas (IDAs) of Medak district, Andhra Pradesh (AP), India. The quality of groundwater in the region has been affected negatively due to the discharge of effluents on open land and into ponds, tanks, and streams. Water samples from surface-water bodies, dug wells,and bore wells were analyzed for their major ion concentrations. The high values of electrical conductivity (EC) and concentrations of Na⁺, Ca²⁺, Cl^–, and HCO₃ ^– indicate the impact of industrial effluents. Based on the hydrochemistry, the groundwater is classified into various types, such as sodium-chloride, sodium-bicarbonate, calcium-chloride, and magnesium-chloride, and its suitability for drinking and irrigation has been assessed. The present studies made it possible to demarcate areas of contaminated groundwater and those prone to contamination in the near future. Water in the area has deteriorated all along Nakka Vagu up to a maximum distance of 500–700 m from the eastern bank. The groundwater quality in and around Patancheru (to a depth of 30 m) has become hazardous. Some possible remedial measures are suggested. Electronic Publication  相似文献   

Multistage reaction textures in xenolithic high-MgAl granulites at Anakapalle, Eastern Ghats Belt, India: examples of contact polymetamorphism and infiltration-driven metasomatism   总被引：1，自引：0，他引：1  

K. Rickers  M. Raith  S. Dasgupta 《Journal of Metamorphic Geology》2001,19(5):563-582

High‐MgAl rocks occur as xenoliths (up to 2 m in diameter) in mafic granulites at a newly discovered locality near Anakapalle. Following an early phase of deformation, ultrahigh‐temperature (UHT) metamorphism and near‐isothermal decompression, the rocks were intruded in a lit‐par‐lit manner by felsic melts (charnockite), which caused local‐scale metasomatism. A subsequent deformation produced isoclinal folds and the distinct gneissic foliation of the charnockite still at granulite facies conditions. The sequence of multiphase reaction textures in the high‐MgAl xenoliths reflects the changes of physico‐chemical conditions during the polyphase evolution of the terrane; UHT metamorphism (stage 1, > 1000°C, c. 10 kbar) is documented by relics of extremely coarse grained domains with the assemblage orthopyroxene (opx)₁ + garnet (grt)₁ + sapphirine (spr)₁ + spinel (spl)₁ + rutile (rt). A subsequent phase of near‐isothermal decompression in the order of 1–2 kbar (stage 2) resulted in extensive replacement of grt₁ and opx₁ megacrysts by lamellar (opx₂ + spr₂) symplectites. The intrusion of felsic melt (stage 3) led to the development of a narrow metasomatic black wall reaction zone (bt + sil + plg₃ + opx_2,3 + rt) at the immediate contact of the xenoliths and in melt infiltration zones to the partial replacement of (opx₂ + spr₂) symplectites by biotite and sillimanite and/or plg₃, mainly at the expense of orthopyroxene, with concomitant coarsening of the intergrowth texture. The subsequent deformation (stage 4) further modified the symplectite textures through polygonization, recrystallization and grain‐size coarsening. The deformation was followed by a period of cooling and decompression (stage 5, c. 800°C, 4–7 kbar) as indicated by local growth of late garnet (grt₅) at the expense of (opx + spr + plg) domains at static conditions. Recently published isotope data suggest that the multistage evolution of the high‐MgAl granulites at Anakapalle followed a discontinuous P–T trajectory that may be related to heating of the crust through magmatic accretion culminating in deep‐crustal UHT metamorphism at 1.4 Ga (stage 1), fast uplift of the UHT granulites into mid‐crustal levels as a consequence of extensional tectonics (stage 2), emplacement of felsic magmas in the Grenvillian (at c. 1 Ga, stage 3) resulting in reheating of the crust to high–T conditions followed by a phase of compressional tectonics (stage 4) and a period of cooling to the stable geotherm (stage 5) still in the Grenvillian.  相似文献   

Fluorine distribution in waters of Nalgonda District, Andhra Pradesh, India   总被引：5，自引：0，他引：5  

N. V. Ramamohana Rao  N. Rao  K. Surya Prakash Rao  R. D. Schuiling 《Environmental Geology》1993,21(1-2):84-89

Geochemical and hydrochemical studies were conducted in Nalgonda District (A.P.), to explore the causes of high fluorine in waters, causing a widespread incidence of fluorosis in the local population. Samples of granitic rocks, soils, stream sediments, and waters were analyzed for F and other salient chemical parameters. Samples from the area of Hyderabad city were analyzed for comparison. The F content of waters in areas with endemic fluorosis ranges from 0.4 to 20 mg/l. The low calcium content of rocks and soils, and the presence of high levels of sodium bicarbonate in soils and waters are important factors favoring high levels of F in waters.  相似文献   

Crustal trajectory of sapphirine-bearing granulites from Ganguvarpatti, South India: evidence for an isothermal decompression path   总被引：7，自引：0，他引：7  

A. MOHAN  B. F. WINDLEY 《Journal of Metamorphic Geology》1993,11(6):867-878

Abstract Ganguvarpatti is part of a Precambrian terrane characterized by granulite facies rocks, including charnockites, mafic granulites, sapphirine-bearing granulites, leptynites and gneisses. A sequence of reactions deduced from the multiphase reaction textures provide information on the metamorphic history of this area, as they formed in response to decompression during uplift. Geothermobarometry and constraints from reaction textures define a segment of a P–T path traversed by the granulites of Ganguvarpatti. Near-peak metamorphic conditions of c. 800°C and 8 kbar were succeeded by a symplectitic stage at a significantly lower pressure ( c. 700°C and 4.5 kbar), documenting a nearly isothermal decompression P–T path and rapid uplift ( c. 12 km) followed by cooling. The presence of many fluid inclusions of extremely low density in the charnockites is consistent with a nearly isothermal uplift path. Attainment of a maximum pressure of c. 8 kbar indicates c. 27 km depth of burial during metamorphism. This would imply a total crustal thickness of c. 65–70 km at 2.6–2.5 Ga. Such a profound crustal thickness and a clockwise decompressive P–T path is interpreted as a consequence of tectonic thickening of crust, accomplished by collision tectonics of the southern granulite terrane against the Dharwar craton along the Palghat–Cauvery shear zone via northward subduction.  相似文献   

Delineation of effluent contaminated zones by electrical surveys at two industrial sites in Visakhapatnam,India     

C. Subbarao  N. V. Subbarao 《Environmental Geology》1994,24(4):281-286

The industrial effluents sent through open channels or closed pipes with leaks contaminate the surface layer and infiltrate to add salinity to groundwater. Extents of contaminations are delineated through isoresistivity maps. Vertical electrical sounding gives the contaminated resistivities of the second layer. The technique is used for two industrial sites: polymer and zinc smelter plants of Visakhapatnam. The results of the studies are compared.  相似文献