Magnetic susceptibility mapping of felsic magmatic lithounits in the central part of Bundelkhand Massif,central India

Late Archaean to Palaeoproterozoic felsic magmatic lithounits exposed in the central part of the Bundelkhand massif have been mapped and their redox series (magnetite vs ilmenite series) evaluated based on magnetic susceptibility (MS) data. The central part of Bundelkhand massif comprises of multiple felsic magmatic pulses (∼2600–2200 Ma), commonly represented by coarse grained granite (CGG-grey granite, CPG-pink granite), medium grained pink granite (MPG), fine grained pink granite (FPG), grey and pink rhyolites and granite porphyry (GP). However, the pink colour of these felsic rocks is the result of hydrothermal fluid-flushing leading to potassic alteration of grey granites. MS values of CGG vary from 0.058 to 14.75×10⁻³ SI with an average of 6.35×10⁻³ SI, which mostly represent oxidized type, magnetite series (73%) granites involving infracrustal (igneous) source materials. CPG (av. MS=3.95×10⁻³ SI) is indeed a pink variety of CGG, the original oxidizing nature of which must have been similar to the bulk of CGG, but has been moderately to strongly reduced because of distinctly more porphyritic nature together with partial assimilation of metapelitic (supracrustal) materials, surmicaceous enclaves, carbonaceous material included in the source materials, and to some extent, induced by hydrothermal and later deformational processes. MPG (av. MS= 1.15×10⁻³ SI) as lensoidal stock-like bodies intrudes the CPG and represent both magnetite series (18%) and ilmenite series (82%) granites, which are probably formed by heterogeneous (mixed) source rocks. GP (av. MS=6.26×10⁻³ SI) occur as dykes (mostly trending NE-SW) intrudes the MPG, CPG and migmatites and bears the nature similar to oxidized type, magnetite series granite. FPG (av. MS= 0.666×10⁻³ SI) trending NE-SW occur as lensoid bodies including a large outcrop, is intrusive into both CPG and MPG, and is moderately to very strongly reduced type, ilmenite series granites, which may be derived by the melting of metapelitic crustal sources. FPG hosting microgranular (mafic magmatic) enclaves commonly exhibit high MS values (7.31–10.22×10⁻³ SI), which appear induced by the mixing and mingling of interacting felsic and mafic magmas prevailed in an open system. Grey (av. MS=10.30×10⁻³ SI) and pink (av. MS=6.72×10⁻³ SI) rhyolites represent oxidized type, magnetite series granites, which may have been derived from infracrustal (magmatic) protoliths. Granite series evaluation of felsic magmatic rocks of central part of Bundelkhand massif strongly suggests their varied redox conditions (differential oxygen fugacity) mostly intrinsic to magma source regions and partially modified by hydrothermal and tectonic processes acting upon them.     

Petrography and major elements geochemistry of microgranular enclaves and neoproterozoic granitoids of south Khasi,Meghalaya: Evidence of magma mixing and alkali diffusion

Neoproterozoic (690±19 Ma) felsic magmatism in the south Khasi region of Precambrian northeast Indian shield, referred to as south Khasi granitoids (SKG), contains country-rock xenoliths and microgranular enclaves (ME). The mineral assemblages (pl-hbl-bt-kf-qtz-mag) of the ME and SKG are the same but differ in proportions and grain size. Modal composition of ME corresponds to quartz monzodiorite whereas SKG are quartz monzodiorite, quartz monzonite and monzogranite. The presence of acicular apatite, fine grains of mafic-felsic minerals, resorbed maficfelsic xenocrysts and ocellar quartz in ME strongly suggest magma-mixed and undercooled origin for ME. Molar Al₂O₃/CaO+Na₂O+K₂O (A/CNK) ratio of ME (0.68–0.94) and SKG (0.81–1.00) suggests their metaluminous (I-type) character. Linear to sub-linear variations of major elements (MgO, Fe₂O₃ ^t, P₂O₅, TiO₂, MnO and CaO against SiO₂) of ME and SKG and two-component mixing model constrain the origin of ME by mixing of mafic and felsic magmas in various proportions, which later mingled and undercooled as hybrid globules into cooler felsic (SKG) magma. However, rapid diffusion of mobile elements from felsic to mafic melt during mixing and mingling events has elevated the alkali contents of some ME.     

Chemical weathering in the plain and peninsular sub-basins of the Ganga: Impact on major ion chemistry and elemental fluxes

Concentrations of major ions, Sr and ⁸⁷Sr/⁸⁶Sr have been measured in the Gomti, the Son and the Yamuna, tributaries of the Ganga draining its peninsular and plain sub-basins to determine their contribution to the water chemistry of the Ganga and silicate and carbonate erosion of the Ganga basin. The results show high concentrations of Na and Sr in the Gomti, the Yamuna and the Ganga (at Varanasi) with much of the Na in excess of Cl. The use of this ‘excess Na’ (Na∗ = Na_riv − Cl_riv) a common index of silicate weathering yield values of ∼18 tons km⁻² yr⁻¹ for silicate erosion rate (SER) in the Gomti and the Yamuna basins. There are however, indications that part of this Na∗ can be from saline/alkaline soils abundant in their basins, raising questions about its use as a proxy to determine SER of the Ganga plain. Independent estimation of SER based on dissolved Si as a proxy give an average value of ∼5 tons km⁻² yr⁻¹ for the peninsular and the plain drainages, several times lower than that derived using Na∗. The major source of uncertainty in this estimate is the potential removal of Si from rivers by biological and chemical processes. The Si based SER and CER (carbonate erosion rate) are also much lower than that in the Himalayan sub-basin of the Ganga. The lower relief, runoff and physical erosion in the peninsular and the plain basins relative to the Himalayan sub-basin and calcite precipitation in them all could be contributing to their lower erosion rates.Budget calculations show that the Yamuna, the Son and Gomti together account for ∼75% Na, 41% Mg and ∼53% Sr and ⁸⁷Sr of their supply to the Ganga from its major tributaries, with the Yamuna dominating the contribution. The results highlight the important role of the plain and peninsular sub-basins in determining the solute and Sr isotope budgets of the Ganga. The study also shows that the anthropogenic contribution accounts for ?10% of the major ion fluxes of the Ganga at Rajmahal during high river stages (October). The impact of both saline/alkaline soils and anthropogenic sources on the major ion abundances of the Ganga is minimum during its peak flow and therefore the SER and CO₂ consumption rates of the river is best determined during this period.     

The Central India Tectonic Zone: A geophysical perspective on continental amalgamation along a Mesoproterozoic suture

The Central India Tectonic Zone (CITZ) is a prominent divide and a major suture zone between the North Indian and South Indian crustal blocks. The resistive upper crust as modeled in the magnetotelluric data from CITZ suggests a dominant tonalite–trdondhjemite–granodiorite composition associated with an accretionary complex characterized by mainly felsic rock components. The highly conductive bodies in this zone might represent mafic/ultramafic-layered intrusives derived from a deeper reservoir of underplated basaltic magma related to the formation of the Cretaceous Deccan flood basalts. The uniformly thick mafic lower crust below the cratons on both sides of the suture is interpreted as the accreted remnants of Archaean and Paleoproterozoic subducted slabs. We redefine the nature of deep faults traversing the CITZ, which were described as steep and penetrating the Moho by previous workers, and classify them as listric faults with gentle dips at depth.Seismic reflection data from the eastern side of the suture suggest a northwestward subduction of the Bhandara Craton. Reflection data from the central part of the CITZ show northerly dip in the southern part suggesting northward subduction of the Dharwar Craton. However, an opposite trend is observed in the northern part of the suture with a southward dip of the Bundelkhand craton. Based on these features, and in conjunction with existing magnetotelluric models, we propose a double-sided subduction history along the CITZ. This would be similar to the ongoing subduction–accretion process in the western Pacific region, which possibly led to the development of paired collision-type and Pacific-type orogens. One important feature is the domal structure along the central part of the suture with a thick felsic crust occurring between mafic and intermediate crust. The high resistivity felsic domain suggests underplated sediments/felsic crust that would have caused the doming. Our model also accounts for the extrusion of regional metamorphic belts at the orogenic core, and the occurrence of high pressure–ultrahigh-temperature paired metamorphic belts within the suture.     

Conservation of biodiversity of the coastal resources of Sundarbans,Northeast India: an integrated approach through environmental education

The Indian Sundarbans, a diversified coastal wetland on the southern fringe of the State of West Bengal, harbors a luxuriant biodiversity and acts as a potential refuge of living marine resources. Girdled with thick mangrove foliage, this estuarine delta system offers an excellent nursery ground for most of the brackishwater finfish and shellfish. Since supply of hatchery-produced tiger prawn seed (Penaeus monodon) is highly inadequate in West Bengal, the aquaculture farms of this region largely depend on the supply from natural resources. Being motivated by a regular cash income, the majority of coastal people from Sundarbans have adopted prawn seed collection as their profession almost throughout the year as an important source of earning. The users are neither trained nor guided at any stage from collection to marketing and are fully dependent on traditional methods. They first sort out the tiger prawn seeds (mainly the postlarval stage PL 20) accounting only 0.25-0.27% of the total catch and thereafter the major portion of the haul are thrown away on the beach flats or the tidal mudflats. This wasted by-catch contains the juveniles of economic and uneconomic varieties of finfish and shellfish along with a bulk of holoplankters and meroplankters (non-target species). This practice causes several ecological and occupational consequences, namely, (i) the huge destruction of the pelagic biota that can lead to severe stock depletion as well as hamper the energy transference through the marine ecosystem food webs; (ii) constant dragging of nets along the coast and tidal creeks paves the way for soil erosion, uprooting the mangrove seedlings and saltmarsh vegetation; (iii) the water quality is deteriorating in the catchment areas due to mud erosion and (iv) due to constant contact with the seawater, the collectors are affected with waterborne diseases, skin infections, reproductive tract disease in women and many other contagious diseases.This paper, in addition to identifying the challenge to environmental quality and resource abundance, emphasizes the need for grass-root public education so that local people come to understand, support and implement sustainable resource conservation and environmental protection activities now and in the future. As a follow-up course of action, the authors have initiated a general awareness program for developing consciousness among the coastal people regarding proper use of natural resources. Initiatives are taken for educating coastal people by holding workshops and seminars with the use of educational resource materials. Exclusive awareness camps and grass root level training for coastal people are also being proposed as a future course of action by means of exhibitions, audiovisuals etc. It is proposed that local government bodies come forward to mitigate this problem with scientific and rational approaches and to take proper actions towards conservation.     

Study of dems derived from ERS-1/2 SAR and SRTM data

The DEM of the Bhuj earthquake affected area of 50 x 50 km was generated using the ERS-1/2 SAR tandem data (May 15—16,1996). Region growing algorithm coupled with path following approach was used for phase unwrapping. Phase to height conversion was done using D-GPS control points. Geocoding was done using GAMMA software. A sample data of DEM of Shuttle Radar Topography Mission (SRTM) of the Bhuj area is made available by DLR Germany. The intensity image, DEM and Error map are well registered. The spatial resolution of this DEM is about 25 m with height accuracy of a few meters. The DEM derived through ERS SAR data is prone to atmospheric affects as the required two images are acquired in different timings where as SRTM acquired the two images simultaneously. An RMS height error of 12.06 m is observed with reference to SRTM though some of the individual locations differ by as much as 35 m.     

Satellite remote sensing for detecting the temporal changes in the grazing lands

Traditional grazing lands in the Indian arid zone can be identified and mapped, reliably and reasonably within ±10% accuracy, through Landsat TM false colour composite of bands 2,3,4. Comparative study of Landsat TM and the Survey of India topographical maps revealed reduction in the areal extent of the grazing lands in the Jodhpur district up to 9 to 30% over a period of 28 years, between 1958 and 1986, due to the human activity like cultivation and urbanisation resulting in the desertification of the adjoining agriculturallands.     

Geochemistry of coexisting hornblende and biotite from the Ambalavayal granite,Kerala

Major and trace element geochemistry of coexisting hornblendes and biotites from the Ambalavayal granite, northern Kerala, are presented. The hornblendes correspond to edenitic composition, whereas the biotites correspond to annite. The hornblendes typically show high Al₂O₃ contents (9·69–11·89%) comparable with those from anorogenic granites. The biotites are characteristically low Mg-type, similar to those reported from alkaline rocks. The distribution coefficients calculated for all the major and trace elements are presented and an evaluation of the nature of variation indicate near-chemical equilibrium conditions during the crystallization of the two minerals. The hornblende-biotite tie lines in the Fe³⁺?Fe²⁺?Mg compositional triangle, lie parallel to those of buffered biotites, indicating crystallization in an environment closed to oxygen and well above the Ni?NiO buffer. It is inferred that thefH₂O increased towards the residual stage andfO₂ values were high, in the range of 10^?15 bars.