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51.
Diptansu Sengupta Basab Mukhopadhyay Om Prakash Mishra 《Journal of the Geological Society of India》2018,92(6):661-670
The annual b-value fluctuation patterns in Burmese subduction zone and Andaman–Sumatra subduction zone are evaluated from earthquake data (January 1990 to June 2016; Mw ³ 4.3) to identify seismic cycles with sequential dynamic phases as described in the ‘elastic failure model’ of Main et al. (1989). Two seismic cycles have been identified in Andaman–Sumatra subduction zone, one started in 1990 and ended on 2004 with occurrence of great Sumatra earthquake (Mw 9.0) and the other started in 2005 and continuing till date with the phase of crack coalescence and fluid diffusion (3A&B). Similarly, the subduction zone of Burma shows evidence of one incomplete seismic cycle within 1990–2016 and presently undergoing the crack coalescence and fluid diffusion (3A&B) phase. The analysis has prompted to subdivide the area into thirteen smaller seismic blocks (A to M) to analyse area specific seismic trend and vulnerability analysis employing Hurst Statistics. Hurst plots with the dynamic phases of ‘elastic failure model’ of earthquake generation is compared to assess the blocks with high seismic vulnerability. The analysis suggest that north Andaman zone (block G) and north Burma fold belt (block M) are seismically most vulnerable. Moreover, the seismic vulnerability of Tripura fold belt and Bangladesh plain (block K) is equally high. 相似文献
52.
Maitrayee Chakraborty Sayan Biswas Pulak Sengupta 《International Geology Review》2015,57(11-12):1619-1632
The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern fringe of the Palaeoproterozoic North Singhbhum Fold Belt (NSFB) of the East Indian Shield. Repeated folding, ductile shearing, and accompanying hydrothermal activities in the SSZ during the late Palaeoproterozoic (ca. 1.66–1.60 Ga) orogeny resulted in a highly tectonized ensemble of rocks including a suite of peraluminous kyanite-rich quartzite (KQR). Near Kanyaluka village, the KQR shows millimetre- to decimetre-thick alternation of kyanite- and quartz-rich bands. The banded rock is intensely sheared and is cross-cut by weakly deformed to undeformed kyanite-quartz veins. In many places, kyanite-rich bands show sea-green coloured pods rich in lazulite. Textural studies reveal that deformed kyanite and quartz grains are sequentially replaced by augelite and lazulite (XMg > 0.97) at the terminal phase of shearing in the SSZ. Modelling of observed textures and mineral compositions with the C-Space program shows the following augelite- and lazulite-forming reactions:
1.829Kyanite + 0.998P + 1.5H2O + 0.001Ca = 1Augelite + 1.666Al + 0.001 Mg + 1.822SiO2 + 0.002Fe + 0.0002Na
1.778Kyanite + 0.667Augelite + 1.294P + 1.011 Mg + 0.011Fe + 0.0001Na = 1Lazulite + 2.833Al + 1.78SiO2 + 0.001 Ca
53.
The newer dolerite dykes around Keonjhar within the Singbhum Granite occur in NE–SW, NW–SE and NNE–SSW trends. The mafic dykes of the present study exhibit several mineralogical changes like clouding of plagioclase feldspars, bastitisation of orthopyroxene, and development of fibrous amphibole (tremolite–actinolite) from clinopyroxene, which are all considered products of hydrothermal alterations. This alteration involves addition and subtraction of certain elements. Graphical analyses with Alteration index and elemental abundances show that elements like Rb, Ba, Th, La and K have been added during the alteration process, whereas elements like Sc, Cr, Co, Ni, Si, Al, Fe, Mg and Ca have been removed. It is observed that in spite of such chemical alteration, correlation between major and trace elements, characteristic of petrogenetic process, is still preserved. This might reflect systematic Alteration (addition or subtraction) of elements without disturbing the original element to element correlation. It has also been established by earlier workers that the evolution of newer dolerite had occurred in an arc-back arc setting which may also be true for newer dolerites of the present study. This is evident from plots of pyroxene composition and whole rock composition of newer dolerite samples in different tectonic discrimination diagrams using immobile elements. The newer dolerite dykes of the Keonjhar area may thus be considered to represent an example of hydrothermal activity on mafic rocks in an arc setting. 相似文献
54.
Manish Mehta Nilton O. Renno John Marshall Anita Sengupta Jasper F. Kok Raymond E. Arvidson Mark T. Lemmon 《Icarus》2011,211(1):172-194
While steady thruster jets caused only modest surface erosion during previous spacecraft landings on the Moon and Mars, the pulsed jets from the Phoenix spacecraft led to extensive alteration of its landing site on the martian arctic, exposed a large fraction of the subsurface water ice under the lander, and led to the discovery of evidence for liquid saline water on Mars. Here we report the discovery of the ‘explosive erosion’ process that led to this extensive erosion. We show that the impingement of supersonic pulsed jets fluidizes porous soils and forms cyclic shock waves which propagate through the soil and produce erosion rates more than an order of magnitude larger than that of other jet-induced processes. The understanding of ‘explosive erosion’ allows the calculation of bulk physical properties of the soils altered by it, provides insight into a new behavior of granular flow at extreme conditions and explains the rapid alteration of the Phoenix landing site’s ground morphology at the northern arctic plains of Mars. 相似文献
55.
K. S. Banerjee R. Guin J. L. Gutierrez-Villanueva M. E. Charro D. Sengupta 《Environmental Earth Sciences》2012,65(7):2103-2110
The determination of the natural radioactivity has been carried out, by means of gamma-ray spectrometry, in exposed rock samples
collected from various geological formations across the Singhbhum Shear Zone of India. The spatial variation in the concentration
of primordial radionuclides reflects, in general, the geochemical variance among the analyzed rocks and affirms the earlier
views on genesis of uranium mineralization. This study further speculates new possibilities of thorium depletion during shearing
and metamorphism. All the obtained U-values were more than the worldwide average U-concentration (which is about 40 Bq/kg).
Interestingly, U-concentration in mica schists from Singhbhum group of rocks and in slates and argillaceous quartzites from
Iron Ore group of rocks was five to seven times higher than the global values. In contrast, the granites in study area showed
comparatively lower U-concentration. The overall results suggest the control of complex metamorphism and shearing on U-distribution
in the terrane. The contribution of 238U, 232Th and 40K to the total radiation dose in the study area was found to be significant ranging from 110 to 310 nGy/h in comparison with
the lower global, as well as Indian average values. 相似文献
56.
S. Sengupta 《Journal of the Geological Society of India》2009,74(5):597-600
Megalospheric form of a striate Nummulites, provisionally identified here as Nummulites sp. aff. chavannesi de la Harpe, is documented from the Early Oligocene rocks of SW Kutch, Gujarat. This striate Nummulites occurs in association with N. fichteli-clipeus Group, N. cf. fichteli, Heterostegina, Operculina and Gypsina. High abundance of microspheric tests of reticulate Nummulites and virtual absence of microspheric tests of striate Nummulites reflect contrasting success of growth and sexual reproduction of the two groups of sympatric Nummulites. 相似文献
57.
Jyothirmayi Palaparthi Ramananda Chakrabarti Santanu Banerjee Rasmohan Guin Shayantani Ghosal Sudha Agrahari Debashish Sengupta 《Arabian Journal of Geosciences》2017,10(9):201
A comprehensive study was carried out in order to determine the radioelement and rare earth element (REE) concentrations in beach placer deposits at selected locations along the eastern coast of Andhra Pradesh in India. This was done to evaluate the economic value of these deposits. The findings of this study suggest that high Th and low K concentrations delineate the prospective regions having REE deposits. The beach placers, in general, can be characterized by high thorium and moderate uranium concentrations. The concentrations of REEs vary in the following order: Ce > La> Nd > Pr > Sm > Gd > Dy. Rapid in situ thorium prospectivity coupled with laboratory-based techniques like ICP-MS, as proposed in this study, would help in the identification of prospective REE sources along the coastal placers. The development of indigenous resources of light rare earth elements (LREEs), medium rare earth elements (MREEs), and heavy rare earth elements (HREEs) would decrease the dependence on imports, which have a strategic hold on the production and supply of the REEs, globally. 相似文献
58.
The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite?+?clinopyroxene (mg#?=?71–73)?+?grandite (16–21 mol% Adr)?+?quartz?±?calcite, Type II: grandite (25–29 mol% Adr )?+?clinopyroxene (mg#?=?70)?+?calcite?+?quartz, and Type III: grandite (36–38 mol% Adr)?+?clinopyroxene (mg#?=?55–65)?+?epidote?+?scapolite?+?calcite?+?quartz. Type I skarn is 2–10 cm thick, and is dominated by wollastonite (>70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn1 related to the Fn1 folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn2 folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40–70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite?+?SiO2?→?wollastonite?+?V, 2. calcite?+?clinopyroxene?+?O2?→?grandite?+?SiO2?+?V, 3. scapolite?+?calcite?+?quartz?+?clinopyroxene?+?O2?→?grandite?+?V and 4. epidote?+?calcite?+?quartz?+?clinopyroxene?+?O2?→?grandite?+?V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (XCO2?<?0.15) led to production of large volumes of wollastonite in the wollastonite-rich skarn whereas mobility of FeO, SiO2 and CaO across the interface of marble and calc-silicate and infiltration of aqueous fluids (XCO2?<?0.35) were instrumental for the formation of grandite skarns. Composition of minerals in type II skarn indicates that Al2O3 was introduced in the host marble by the infiltrating fluid. Interpretation of mineral assemblages observed in the interlayered metapelites and the calcareous rocks in pseudosections, isothermal P-XCO2 and isobaric T-XCO2 diagrams tightly bracket the “peak” metamorphic conditions at c.9?±?1 kbar and 750°?±?30°C. Subsequent to ‘peak’ metamorphic conditions, the rocks were exhumed on a steeply decompressive P–T path. The estimated ‘peak’ P–T estimates are inconsistent with the “extreme” metamorphic conditions (>11 kbar and >950°C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn1 folding event at or close to the ‘peak’ metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks. 相似文献
59.
The Samchampi-Samteran alkaline igneous complex (SAC) is a near circular, plug-like body approximately 12 km2 area and is emplaced into the Precambrian gneissic terrain of the Karbi Anglong district of Assam. The host rocks, which
are exposed in immediate vicinity of the intrusion, comprise granite gneiss, migmatite, granodiorite, amphibolite, pegmatite
and quartz veins.
The SAC is composed of a wide variety of lithologies identified as syenitic fenite, magnetite ± perovskite ± apatite rock,
alkali pyroxenite, ijolite-melteigite, carbonatite, nepheline syenite with leucocratic and mesocratic variants, phonolite,
volcanic tuff, phosphatic rock and chert breccia.
The magnetite ± perovskite ± apatite rock was generated as a cumulus phase owing to the partitioning of Ti, Fe at a shallow
level magma chamber (not evolved DI = O1). The highly alkaline hydrous fluid activity indicated by the presence of strongly
alkalic minerals in carbonatites and associated alkaline rocks suggests that the composition of original melt was more alkalic
than those now found and represent a silica undersaturated ultramafic rock of carbonated olivine-poor nephelinite which splits
with falling temperature into two immiscible fractions—one ultimately crystallises as alkali pyroxenite/ijolite and the other
as carbonatite. The spatial distribution of varied lithotypes of SAC and their genetic relationships suggests that the silicate
and carbonate melts, produced through liquid immiscibility, during ascent generated into an array of lithotypes and also reaction
with the country rocks by alkali emanations produced fenitic aureoles (nephelinisation process). Isotopic studies (δ18O and δ13C) on carbonatites of Samchampi have indicated that the δ13C of the source magma is related to contamination from recycled carbon. 相似文献
60.
A refined garnet - biotite Fe−Mg exchange geothermometer and its application in amphibolites and granulites 总被引:4,自引:0,他引:4
Somnath Dasgupta Pulak Sengupta Dipayan Guha M. Fukuoka 《Contributions to Mineralogy and Petrology》1991,109(1):130-137
A new formulation of garnet-biotite Fe–Mg exchange thermometer has been developed through statistical regression of the reversed experimental data of Ferry and Spear. Input parameters include available thermo-chemical data for quaternary Fe–Mg–Ca–Mn garnet solid solution and for excess free energy terms, associated with mixing of Al and Ti, in octahedral sites, in biotite solid solution. The regression indicates that Fe–Mg mixing in biotite approximates a symmetrical regular solution model showing positive deviation from ideality withW
FeMg
bi
=1073±490 cal/mol. H
r
and S
r
for the garnet-biotite exchange equilibrium were derived to be 4301 cal and 1.85 cal respectively. The resultant thermometer gives consistent results for rocks with a much wider compositional range than can be accommodated by earlier formulations. 相似文献