Emplacement age and isotope geochemistry of Sung Valley alkaline–carbonatite complex, Shillong Plateau, northeastern India: implications for primary carbonate melt and genesis of the associated silicate rocks

The early Cretaceous (Albian–Aptian) Sung Valley ultramafic–alkaline–carbonatite complex is one of several alkaline intrusions that occur in the Shillong Plateau, India. This complex comprises calcite carbonatite and closely associated ultramafic (serpentinized peridotite, pyroxenite and melilitolite) and alkaline rocks (ijolite and nepheline syenite). Field relationship and geochemical characteristics of these rocks do not support a genetic link between carbonatite and associated silicate rocks. There is geochemical evidence that pyroxenite, melilitolite and ijolite of the complex are genetically related. Stable (C and O) and radiogenic (Nd and Sr) isotope data clearly indicate a mantle origin for the carbonatite samples. The carbonatite Nd (+0.7 to +1.8) and Sr (+4.7 to +7.0) compositions overlap the field for Kerguelen ocean island basalts. One sample of ijolite has Nd and Sr isotopic compositions that also plot within the field for Kerguelen ocean island basalts, whereas the other silicate–carbonatite samples indicate involvement with an enriched component. These geochemical and isotopic data indicate that the rocks of the Sung Valley complex were derived from and interacted with an isotopically heterogeneous subcontinental mantle and is consistent with interaction of a mantle plume (e.g. Kerguelen plume) with lithosphere. A U–Pb perovskite age of 115.1±5.1 Ma obtained for a sample of Sung Valley ijolite also supports a temporal link to the Kerguelen plume. The observed geochemical characteristics of the carbonatite rocks indicate derivation by low-degree partial melting (0.1%) of carbonated mantle peridotite. This melt, containing a substantial amount of alkali elements, interacted with peridotite to form metasomatic clinopyroxene and olivine. This process could progressively metasomatize lherzolite to form alkaline wehrlite.     

Effect of dust storm on ocean color and snow parameters

The present study clearly shows the influence of dust storms on chlorophyll bloom in the offshore region of the Arabian Sea, with a time lag of few days, during the pre-monsoon season. Various satellite derived parameters over the Arabian Sea, Himalayan and Tibet snow covered regions show large changes due to the influence of dust storms. The MODIS snow albedo gives unreliable values under the influence of dust storms due to increase in the aerosol loading over these regions and snow albedo product must be used in combination with snow pixel counts during the dust storm season. A detailed study is required for the quantitative evaluation of dust storms on the chlorophyll blooms in the Arabian Sea region and on the snow parameters in the Himalayan region.     

Response of the Laprak,Nepal, landslide to the 2015 Mw 7.8 Gorkha earthquake

Natural Hazards - Coseismic displacements estimated from comparison before-and-after satellite images suggest that the large and intermittently active landslide upon which the village of Laprak,...     

Use of sulfur-oxidizing bacteria for assessment of chromium-contaminated soil

In recent years, biological toxicity tests have been conducted for soil assessment of environmental pollutants to evaluate the environmental risk due to heavy metals. In this study, batch tests were conducted with soils contaminated with hexavalent chromium (Cr⁶⁺) using sulfur-oxidizing bacteria (SOB). For control soils (without Cr⁶⁺), the electrical conductivity (EC) increased linearly over time in all samples, indicating that no toxic substances were present in the soil. The initial EC varied between 6 and 7.8 mS/cm, and the final EC varied between 22 and 27 mS/cm after incubation for 65 h. For batch tests performed using Cr⁶⁺-contaminated soil, the EC increased slightly or remained stable in all the test samples after a few hours. Thus, the presence of toxic substance Cr⁶⁺ inhibited the SOB, which leads to no sulfuric acid formation and therefore, no change in EC. These results indicated that SOB can be employed as a test microorganism to assess the quality of heavy metal-contaminated soils.     

Geochemistry and petrogenesis of early Cretaceous sub-alkaline mafic dykes from Swangkre-Rongmil,East Garo Hills,Shillong plateau,northeast India

Numerous early Cretaceous mafic and alkaline dykes, mostly trending in N-S direction, are emplaced in the Archaean gneissic complex of the Shillong plateau, northeastern India. These dykes are spatially associated with the N-S trending deep-seated Nongchram fault and well exposed around the Swangkre-Rongmil region. The petrological and geochemical characteristics of mafic dykes from this area are presented. These mafic dykes show very sharp contact with the host rocks and do not show any signature of assimilation with them. Petrographically these mafic dykes vary from fine-grained basalt (samples from the dyke margin) to medium-grained dolerite (samples from the middle of the dyke) having very similar chemical compositions, which may be classified as basaltic-andesite/andesite. The geochemical characteristics of these mafic dykes suggest that these are genetically related to each other and probably derived from the same parental magma. Although, the high-field strength element (+rare-earth elements) compositions disallow the possibility of any crustal involvement in the genesis of these rocks, but Nb/La, La/Ta, and Ba/Ta ratios, and similarities of geochemical characteristics of present samples with the Elan Bank basalts and Rajmahal (Group II) mafic dyke samples, suggest minor contamination by assimilation with a small amount of upper crustal material. Chemistry, particularly REE, hints at an alkaline basaltic nature of melt. Trace element modelling suggests that the melt responsible for these mafic dykes had undergone extreme differentiation (∼ 50%) before its emplacement. The basaltic-andesite nature of these rocks may be attributed to this differentiation. Chemistry of these rocks also indicates ∼ 10–15% melting of the mantle source. The mafic dyke samples of the present investigation show very close geochemical similarities with the mafic rocks derived from the Kerguelen mantle plume. Perhaps the Swangkre-Rongmil mafic dykes are also derived from the Kerguelen mantle plume.     

Scalar field cosmology with polytropic and causal viscous fluids

In this paper, we have considered a model for FRW space-time in the presence of coupled scalar field φ and potential V(φ) with causal viscous fluid and polytropic fluid. We have shown that irrespective of fluid the causality theory provides late time acceleration of the universe. In all cases, the potential always decreases due to evolution of the universe.     

Topographic correction of ALOS-PALSAR images using InSAR-derived DEM

Topographic corrections of synthetic aperture radar (SAR) images over hilly regions are vital for retrieval of correct backscatter values associated with natural targets. The coarse resolution external digital elevation models (DEM) available for topographic corrections of high resolution SAR images often result into degradation of spatial resolution or improper estimation of backscatter values in SAR images. Also, many a times the external DEMs do not spatially co-register well with the SAR data. The present study showcases the methodology and results of topographic correction of ALOS-PALSAR image using high resolution DEM generated from the same data. High resolution DEMs of Jaipur region, India were generated using multiple pair SAR images acquired from ALOS-PALSAR using interferometric (InSAR) techniques. The DEMs were validated using differential global positioning system measured elevation values as ground control points and were compared with photogrammetric DEM (advanced spaceborne thermal emission and reflection radiometer – ASTER) and SRTM (Shuttle Radar Topography Mission) DEM. It was observed that ALOS-PALSAR images with optimum baseline parameters produced high resolution DEM with better height accuracy. Finally, the validated DEM was used for topographic correction of ALOS-PALSAR images of the same region and were found to produce better result as compared with ASTER and SRTM-DEM.     

Revisiting the Koyna-Warna seismic zone: strain budget,present-day potential and associated hazard

Journal of Seismology - The Koyna-Warna region in western India is well known around the globe for recurrent reservoir-triggered seismicity soon after the impoundment of the Koyna and Warna...     

Characteristics of landslide in Koshi River Basin,Central Himalaya

Koshi River basin, which lies in the Central Himalayas with an area of 71,500 km2, is an important trans-boundary river basin shared by China, Nepal and India. Yet, landslide-prone areas are all located in China and Nepal, imposing alarming risks of widespread damages to property and loss of human life in both countries. Against this backdrop, this research, by utilizing remote sensing images and topographic maps, has identified a total number of 6877 landslides for the past 23 years and further examined their distribution, characteristics and causes. Analysis shows that the two-step topography in the Himalayan region has a considerable effect on the distribution of landslides in this area. Dense distribution of landslides falls into two regions: the Lesser Himalaya(mostly small and medium size landslides in east-west direction) and the TransitionBelt(mostly large and medium size landslides along the river in north-south direction). Landslides decrease against the elevation while the southern slopes of the Himalayas have more landslides than its northern side. Change analysis was carried out by comparing landslide distribution data of 1992, 2010 and 2015 in the Koshi River basin. The rainfallinduced landslides, usually small and shallow and occurring more frequently in regions with an elevation lower than 1000 m, are common in the south and south-east slopes due to heavy precipitation in the region, and are more prone to the slope gradient of 20°~30°. Most of them are distributed in Proterozoic stratum(Pt3ε, Pt3 and Pt2-3) and Quaternary stratum. While for earthquake-induced landslides, they are more prone to higher elevations(2000~3000 m) and steeper slopes(40°~50°).