Contrasting parageneses in the manganese silicate-carbonate rocks from Parseoni,Sausar Group,India and their interpretation

Mn silicate-carbonate rocks at Parseoni occur as conformable lenses within metapelites and calc-silicate rocks of the Precambrian Sausar Group, India. The host rocks are estimated to have been metamorphosed at uppermost P-T conditions of 500–550°C and 3–4 kbar. The Mn-rich rocks contain appreciable Fe, reflected in the occurrence of magnetite₍₁₎ (MnO 1%), magnetite₍₂₎ (MnO 15%) and magnetite₍₃₎ (MnO 10%). Two contrasting associations of pyroxmangite, with and without tephroite, developed in the Mn silicate-carbonate rocks under isothermal-isobaric conditions. The former assemblage formed in relatively Fe-rich bulk compositions and equilibrated with a metamorphic fluid having a low X _{CO 2} (<0.2), and the latter equilibrated with a CO₂-rich fluid. Rhodochrosite+magnetite₍₁₎+quartz protoliths produced the observed mineral assemblages on metamorphism. Partitioning of major elements between coexisting phases is somewhat variable. Fe shows preference for tephroite over pyroxmangite at the ambient physical conditions of metamorphism. Oxygen fugacity during metamorphism was monitored at or near the QFM buffer in tephroite bearing domains, and the fluid composition was buffered by mineral reactions in respective domains. As compared to other metamorphosed Mn deposits of the Sausar Group, the Mn silicate-carbonate rocks at Parseoni were, therefore, metamorphosed at much lower f _{O 2} through complex mineral-fluid interactions.     

Groundwater arsenic contamination and its health effects in India

During a 28-year field survey in India (1988–2016), groundwater arsenic contamination and its health effects were registered in the states of West Bengal, Jharkhand, Bihar and Uttar Pradesh in the Ganga River flood plain, and the states of Assam and Manipur in the flood plain of Brahamaputra and Imphal rivers. Groundwater of Rajnandgaon village in Chhattisgarh state, which is not in a flood plain, is also arsenic contaminated. More than 170,000 tubewell water samples from the affected states were analyzed and half of the samples had arsenic >10 μg/L (maximum concentration 3,700 μg/L). Chronic exposure to arsenic through drinking water causes various health problems, like dermal, neurological, reproductive and pregnancy effects, cardiovascular effects, diabetes mellitus, diseases of the respiratory and gastrointestinal systems, and cancers, typically involving the skin, lungs, liver, bladder, etc. About 4.5% of the 8,000 children from arsenic-affected villages of affected states were registered with mild to moderate arsenical skin lesions. In the preliminary survey, more than 10,000 patients were registered with different types of arsenic-related signs and symptoms, out of more than 100,000 people screened from affected states. Elevated levels of arsenic were also found in biological samples (urine, hair, nails) of the people living in affected states. The study reveals that the population who had severe arsenical skin lesions may suffer from multiple Bowens/cancers in the long term. Some unusual symptoms, such as burning sensation, skin itching and watering of eyes in the presence of sun light, were also noticed in arsenicosis patients.     

Planar, non-planar and refolded sheath folds in the Phulad Shear Zone, Rajasthan, India

Progressive ductile shearing in the Phulad Shear Zone of Rajasthan, India has produced a complex history of folding, with development of planar, non-planar and refolded sheath folds. There are three generations of reclined folds, F₁, F₂ and F₃, with a striping lineation (L₁) parallel to the hinge lines of F₁. The planar sheath folds of F₁ have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices. L₁ swerves harmoniously with the curving of F₁ hinge line. There is a strong down-dip mineral lineation parallel to the striping lineation in most places, but intersecting it at apices of first generation sheath folds. Both the striping and the mineral lineation are deformed in U-patterns over the hinges of reclined F₂ and F₃. Folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non-planar sheaths. The reclined folds of all the three generations were deformed by a group of subhorizontal folds. Each generation of fold initially grew with the hinge line at a very low angle with the Y-axis of bulk non-coaxial strain and was subsequently rotated towards the down-dip direction of maximum stretching. The patterns of deformed lineations indicate that the stretching along the X-direction was extremely large, much in excess of 6000 percent.     

Deformation of footwall rock of Phulad Shear Zone,Rajasthan: Evidence of transpressional shear zone

Phulad Shear Zone (PSZ) of Delhi Fold Belt in Rajasthan is a northeasterly striking ductile shear zone with a well developed mylonitic foliation (035/70E) and a downdip stretching lineation. The deformation in the PSZ has developed in a transpressional regime with thrusting sense of movement. The northeastern unit, i.e., the hanging wall contains a variety of rocks namely calc-silicates, pelites and amphibolites and the southwestern unit, i.e., the footwall unit contains only granitic rocks. Systematic investigation of the granites of the southwestern unit indicate a gradual change in the intensity of deformation from a distance of about 1 km west of the shear zone to the shear zone proper. The granite changes from weakly deformed granite to a mylonite/ultramylonite as we proceed towards the PSZ. The weakly deformed granite shows a crude foliation with the same attitude of mylonitic foliation of the PSZ. Microscopic study reveals the incipient development of C and S fabric with angle between C and S varying from 15 ^° to 24 ^°. The small angle between the C and S fabric in the least deformed granite variety indicates that the deformation has strong pure shear component. At a distance of about 1 m away from the PSZ, there is abrupt change in the intensity of deformation. The granite becomes intensely foliated with a strong downdip lineation and the rock becomes a true mylonite. In mesoscopic scale, the granite shows stretched porphyroclasts in both XZ and YZ sections indicating a flattening type of deformation. The angle between the C and S fabric is further reduced and finally becomes nearly parallel. In most places, S fabric is gradually replaced by C fabric. Calculation of sectional kinematic vorticity number ( W_n) from the protomylonitic and mylonite/ultramylonite granites varies from 0.3 ± 0.03 to 0.55 ± 0.04 indicating a strong component of pure shear. The similarity of the geometry of structures in the PSZ and the granites demonstrates that the deformation of the two units is broadly synchronous and the deformation in both the units is transpressional.     

Cosmic microwave background power spectrum estimation with non-circular beam and incomplete sky coverage

Over the last decade, measurements of the cosmic microwave background (CMB) anisotropy have spearheaded the remarkable transition of cosmology into a precision science. However, addressing the systematic effects in the increasingly sensitive, high-resolution, 'full' sky measurements from different CMB experiments poses a stiff challenge. The analysis techniques must not only be computationally fast to contend with the huge size of the data, but the higher sensitivity also limits the simplifying assumptions which can then be invoked to achieve the desired speed without compromising the final precision goals. While maximum likelihood is desirable, the enormous computational cost makes the suboptimal method of power spectrum estimation using pseudo-C _l unavoidable for high-resolution data. The debiasing of the pseudo-C _l needs account for non-circular beams, together with non-uniform sky coverage. We provide a (semi)analytic framework to estimate bias in the power spectrum due to the effect of beam non-circularity and non-uniform sky coverage, including incomplete/masked sky maps and scan strategy. The approach is perturbative in the distortion of the beam from non-circularity, allowing for rapid computations when the beam is mildly non-circular. We advocate that it is computationally advantageous to employ 'soft' azimuthally apodized masks whose spherical harmonic transform die down fast with m . We numerically implement our method for non-rotating beams . We present preliminary estimates of the computational cost to evaluate the bias for the upcoming CMB anisotropy probes  ( l _max∼ 3000)  , with angular resolution comparable to the Planck surveyor mission. We further show that this implementation and estimate are applicable for rotating beams on equal declination scans, and can possibly be extended to simple approximations to other scan strategies.     

Anisotropy of magnetic susceptibility analysis of deformed kaolinite: implications for evaluating landslides

Plane strain tests were performed on seven kaolinite blocks, each of which developed shear bands. Anisotropy of magnetic susceptibility (AMS) analysis of the kaolinite reveals a threshold degree of magnetic anisotropy (P′) exceeding which shear bands develop. Since P′ is a strain-intensity gauge and soils are known to develop shear bands prior to landsliding, it is concluded that soil in every landslide-prone region must have its unique threshold P′ exceeding which it develops shear bands before failing. Therefore, AMS monitoring of soil in landslide prone regions is proposed as a potential tool in the management of natural hazard zones.     

Dynamic Behavior of Small Scale Nailed Soil Slopes

The shaking table tests are conducted on small scale nailed embankment slopes to study their behavior under dynamic conditions. Medium grained local sand with a water content of 3% is used in the study. The embankment slope is constructed using the controlled-volume compaction method. Three slope angles, 30°, 35° and 40° with a constant slope height of 18 cm are considered for the embankment. Each slope is reinforced with six number of hollow aluminum nails in two rows. The ratio of the length of nail to slope height (0.82) is same for all model slopes. The nails are inserted at three different inclinations. Three strain gauges are glued to each nail to obtain local strains during shaking. The accelerations at the base and the crest of the model slopes are monitored to find the acceleration responses of the embankments during the input ground motions. The numerical simulation of the model tests is performed by a commercial program called FLAC. The results of the numerical analyses are found to be reasonably close to the corresponding experimental results.     

Fractal analysis of major faults in India on a regional scale

The seismicity of a region is implicit of the causal faulting mechanisms and geodynamic diversity of the subsurface regime nucleating earthquakes of different magnitudes, several of which may be as devastating as ones historically reported in global perspective of tectonic complexity as in the case of India. Fractal analysis using box-counting method for the major fault networks across the country estimates fractal dimension, D_f, values to be varying between 0.88 and 1.36. The fault segments in parts of northwest Himalayas, northeast India and Indo-Gangetic plains, are observed to be associated with higher D_f values implicating high seismicity rates. On the other hand, low D_f values in the peninsular India indicate isolated pattern of the underlying faults. The fractal dimension is observed to be indicative of predominant faulting types — higher values conforming to thrust faulting mechanism while lower to strike slip tectonism.     

Significance of mineral chemistry of syenites and associated rocks of Elagiri complex,Southern Granulite Terrane of the Indian shield

The Elagiri complex (12°31′ N: 78°35′ E) represents one of the important silica — oversaturated syenite plutons of the Southern Granulite Terrane of the Indian shield. This article for the first time reports the mineral chemistry of the Elagiri complex and brings out important petrogenetic significance. The litho-members of Elagiri complex are intrusive into high grade country rocks viz. granite gneiss, amphibolite and pyroxene granulite. The country rocks are foliated bearing evidences of multiple folding and deformation. On the other hand, the constituent litho members of the Elagiri complex (syenites, gabbro and later intrusives marked by lamprophyre and carbonatite) show preservation of igneous layering in terms of discernible parallelism of the constituent minerals. The Elagiri complex shows presence of sharp contacts among litho members and marked absence of chilled facies peripheral to the margin. Electron microprobe data have been critically used to systematize the constituent mineral-phases of the different lithomembers of the complex. Geothermobarometric data indicate a temperature of equilibration in the range of ∼700° to 500°C at ∼2.0 to 5.2 kb which corresponds to shallow level (cf. 18.2 km) equilibration-depth of the complex. Field observations and mineral chemistry data suggest that liquid immiscibility plays an important role during the evolution of the Elagiri complex.     

Petrology of the mafic sill of Narshingpur-Lakhnadon section,Eastern Deccan volcanic province

Deccan volcanism with a tremendous burst of volcanic activity marks a unique episode in Indian geological history and covers nearly two third of Peninsular India. Occurrences of mafic sill in the continental basalts are rather rare throughout the flood basalt provinces and only few sporadic reports have been described from different Continental Flood Basalts of the world. In the present article, petrology of mafic sill from the Narshingpur-Lakhnadon section of Eastern Deccan province of India has been presented. The mafic sill in the field is found to occur in a relatively deep valley amidst Gondwana rocks, which occur as the basement of the extrusion. The sill is spatially associated with three initial flows viz. flow I, II and III of adjacent Narshingpur-Harrai-Amarwara section. The sill in its central part is a medium grained rock and petrographically corresponds to dolerite containing augite, plagioclase and rare olivine grains; the chilled facies of the sill is characterized by phenocrysts of olivine, plagioclase and augite that are set in groundmass consisting predominantly of plagioclase, olivine and glass. Mineral chemistry indicates that olivine phenocrystal phase is magnesian (Fo61). Plagioclase phenocrystal composition ranges from An 51 to An 71 whereas the same variation of the groundmass plagioclase composition corresponds to An 31 to An 62. The overlap in the compositions for groundmass and phenocrystal plagioclase may be explained due to fluctuating PH2O condition. The pyroxene compositions (both groundmass and phenocryst) in majority of the cases are clubbed well within the augite field, however, in a few cases, groundmass compositions are found to fall in the sub-calcic augite and pigeonite field. Some zoned pyroxene phenocrysts, characteristically display different types of zoning patterns. Opaque minerals in the mafic sill are found to be magnetite and ilmenite and this coexisting iron-oxide composition helps to constrain the prevalent fO2 condition in the parent magma. The geochemistry of the mafic sill and associated basaltic lava flows indicates close genetic link amongst them. Critical consideration of trace elements indicates a distinct enriched mantle source (EM-I/EM-II/HIMU) for the parental magma. Trace element modeling indicates that equilibrium batch-melting of plume source followed by fractionation of olivine, clinopyroxene and plagioclase and subsequent heterogeneous mixing of melt and settled crystals can very well explain the genesis of the mafic sill and the associated basaltic flows.