Evaluation of phase chemistry and petrochemical aspects of Samchampi-Samteran differentiated alkaline complex of Mikir Hills,northeastern India     

Abhishek Saha  Sohini Ganguly  Jyotisankar Ray  Nilanjan Chaterjee 《Journal of Earth System Science》2010,119(5):675-699

The Samchampi-Samteran alkaline complex occurs as a plug-like pluton within the Precambrian granite gneisses of Mikir Hills, Assam, northeastern India and it is genetically related to Sylhet Traps. The intrusive complex is marked by dominant development of syenite within which ijolitemelteigite suite of rocks is emplaced with an arcuate outcrop pattern. Inliers of alkali pyroxenite and alkali gabbro occur within this ijolite-melteigite suite of rocks. The pluton is also traversed by younger intrusives of nepheline syenite and carbonatite. Development of sporadic, lumpy magnetite ore bodies is also recorded within the pluton. Petrographic details of the constituent lithomembers of the pluton have been presented following standard nomenclatorial rules. Overall pyroxene compositions range from diopside to aegirine augite while alkali feldspars are typically orthoclase and plagioclase in syenite corresponds to oligoclase species. Phase chemistry of nepheline is suggestive of Na-rich alkaline character of the complex. Biotite compositions are typically restricted to a uniform compositional range and they belong to ‘biotite’ field in the relevant classification scheme. Garnets (developed in syenite and melteigite) typically tend to be Ti-rich andradite, which on a closer scan can be further designated as melanites. Opaque minerals mostly correspond to magnetite. Use of Lindsley’s pyroxene thermometric method suggests an equilibration temperature from ∼450°–600°C for melteigite/alkali gabbro and ∼400°C for syenite. Critical assessment of other thermometric methods reveals a temperature of equilibration of ∼700°–1350°C for ijolite-melteigite suite of rocks in contrast to a relatively lower equilibration temperature of ∼600°C for syenite. Geobarometric data based on pyroxene chemistry yield an equilibration pressure of 5.32–7.72 kb for ijolite, melteigite, alkali pyroxenite, alkali gabbro and nepheline syenite. The dominant syenite member of the intrusive plug records a much higher (∼11 kb) equilibration pressure indicating a deeper level of intrusion. Major oxide variations of constituent lithomembers with respect to differentiation index (D.I.) corroborate a normal magmatic differentiation. A prominent role of liquid immiscibility is envisaged from field geological, petrographic and petrochemical evidences. Tectonic discrimination diagrams involving clinopyroxene chemistry strongly suggest within plate alkaline affinity for the parental magma which is in conformity with the regional plume tectonics.  相似文献   

Vanadium bearing titaniferous magnetite ore bodies of Ganjang,Karbi-Anglong District,Northeastern India     

Abhishek Saha  Sohini Ganguly  Jyotisankar Ray  Avik Dhang 《Journal of the Geological Society of India》2010,76(1):26-32

A new occurrence of (syenite-hosted) Vanadium bearing titaniferous magnetite ore body has been reported from Ganjang (26°09′35″ N: 93°20′ E), Karbi-Anglong, Northeastern India. The magnetite ore bodies have lumpy and sporadic occurrences within the host syenite pluton intrusive into gneissic country rocks. Ore microscopic studies reveal that magnetite is often associated with haematite and ilmenite depicting different textural patterns. Critical consideration of several elemental patterns suggests magmatic differentiation to be main ore-forming process. The ore body is suggested to have been formed as late stage segregation from a differentiating alkaline magma in a fluid enriched milieu.  相似文献   

Some features of winter aerosol and water vapor characteristics over Agra, a semi-arid location in India     

S. K. Saha  P. C. S. Devara 《Meteorology and Atmospheric Physics》2010,109(3-4):139-147

Reported in this article are the results of the analysis of extensive observations of aerosol optical, micro-physical characteristics, and precipitable water content (PWC) that have been carried out using compact, multi-band solar radiometers, over a semi-arid station, Agra (27°10′N, 78°05′E, 169 m AMSL) during winter 2004. The aerosol optical depth (AOD), Angstrom wavelength exponent (α), and PWC show, higher values on hazy- and foggy-days and lower values on clear-days. The turbidity coefficient (β) shows higher values for smaller values of α and vice versa. The aerosol size spectra exhibit bi-modal distribution with abundance of accumulation-mode particles during fog and haze occasions, and relatively coarse-mode particles on clear-days. The above features have been explained with co-located PWC and surface-level meteorological parameters. The NOAA HYSPLIT five-day back trajectories indicate the influence of trans-boundary pollution transport over the experimental station during the study period.  相似文献   

The NOAA Goes-12 Solar X-Ray Imager (SXI) 1. Instrument, Operations, and Data     

S. M. Hill  V. J. Pizzo  C. C. Balch  D. A. Biesecker  P. Bornmann  E. Hildner  L. D. Lewis  R. N. Grubb  M. P. Husler  K. Prendergast  J. Vickroy  S. Greer  T. Defoor  D. C. Wilkinson  R. Hooker  P. Mulligan  E. Chipman  H. Bysal  J. P. Douglas  R. Reynolds  J. M. Davis  K. S. Wallace  K. Russell  K. Freestone  D. Bagdigian  T. Page  S. Kerns  R. Hoffman  S. A. Cauffman  M. A. Davis  R. Studer  F. E. Berthiaume  T. T. Saha  G. D. Berthiume  H. Farthing  F. Zimmermann 《Solar physics》2005,226(2):255-281

The Solar X-ray Imager (SXI) was launched 23 July 2001 on NOAAs GOES-12 satellite and completed post-launch testing 20 December 2001. Beginning 22 January 2003 it has provided nearly uninterrupted, full-disk, soft X-ray solar images, with a continuous frame rate significantly exceeding that for previous similar instruments. The SXI provides images with a 1 min cadence and a single-image (adjustable) dynamic range near 100. A set of metallic thin-film filters provides temperature discrimination in the 0.6 – 6.0 nm bandpass. The spatial resolution of approximately 10 arcsec FWHM is sampled with 5 arcsec pixels. Three instrument degradations have occurred since launch, two affecting entrance filters and one affecting the detector high-voltage system. This work presents the SXI instrument, its operations, and its data processing, including the impacts of the instrument degradations. A companion paper (Pizzo et al., this issue) presents SXI performance prior to an instrument degradation that occurred on 5 November 2003 and thus applies to more than 420000 soft X-ray images of the Sun.  相似文献   

Quantitative discrimination between magmatic units of the singhbhum granite     

A. K. Saha  S. V. L. N. Rao 《Mathematical Geology》1971,3(2):123-133

The weight-percent values of four mineralogic variables (quartz, K feldspar, color index, and muscovite) for 10 sets of granitic rocks (20–50 samples in each set) from magmatic units of the Singhbhum granite were used for (1) computation of the Mahalanobis' generalized distance functions (D ²) between all pairs of the 10 sets, (2) testing significance of the difference between the multivariate means, and (3) computation of the linear discriminant functions between all possible pairs of the sets. The 10 data sets are for six magmatic units which belong to three successive but closely related phases of emplacement. The multivariate means for all sets are significantly different except for those between two of the sets of phase I. Cluster analysis on the basis of theD ² values enables the 10 sets to be placed into four distinct groups. Group A includes two subgroups, one of which consists of two sets representing typical members of phase I; the other subgroup includes two sets which are typical of phase II. Group B includes two sets which are typical of phase III. The other four sets do not group with the typical representatives of the three phases, probably because of certain special conditions of their emplacement. A separate series ofD ² computation from the same data, but excluding the color index, was unsuccessful in making the four aberrant sets group with the typical members of the respective phases. Efficient LDF's could be determined for discrimination between most pairs of the 10 sets of granite rocks.  相似文献   

Subsurface sediment accumulation patterns and their relationships with tectonic lineaments in the semi-arid Luni river basin, Rajasthan, Western India     

V. N. Bajpai  T. K. Saha Roy  S. K. Tandon 《Journal of Arid Environments》2001,48(4):603

Sediment accumulation patterns in the Luni basin have been studied on the basis of tubewell lithologs and Bouguer gravity anomaly profiles. East–West geologic transects using these data reveal several sediment-filled graben depressions, for example, the Digrana-Bhawal graben in the northern part, the Mangta-Sindari, Sindari-Bhimgoda, and Bhimgoda-Juna Motisara grabens in the central part; and the Dungari-Ratanpura and Ratanpura-Khanpur grabens in the southern part. Maximum sediment accumulation of more than 300 m is located in the southwestern part of the Luni basin, more specifically in the Ratanpura-Khanpur graben. Minor depressions also occur towards west of Jodhpur and east of Bhadrajun.Sediment filling in these graben depressions commenced with thick clay deposits followed by multistoried fining up sand and gravel sequences. Sediment accumulation centres are coincident with major tectonic lineaments and thus indicate that important loci of sediment accumulation are, possibly, a response to synsedimentary tectonics.  相似文献   

Soil erosion assessment and mapping of the algar river watershed (uttar pradesh) using remote sensing technique     

SK Saha  BM Singh 《Journal of the Indian Society of Remote Sensing》1991,19(2):67-76

This study was undertaken to prepare an inventory on soil erosion of a hilly river watershed — the Aglar watershed, part of Tehri Garhwal and Dehradun districts (U.P.), using terrain physiography and soil survey data obtained from interpretation and analysis of Landsat TM FCC (1:62,500 scale) and limited ground investigations. The watershed is divided into four broad physiographic units viz. higher Himalayas (> 2000m elevation); lower Himalayas (< 2000m elevation); river terraces and flood plains. Each physiographic unit has been further divided into subunits on the basis of aspects and landuse. Three major orders of soils viz. Inceptisols, Mollisols and Entisols were found in different physiographic units. Soil, and land properties of soilscape units viz. soil depth, texture, structure, slope, landuse and soil temperature regime were evaluated for soil-erosion hazard. The results indicate that in the whole watershed 19.13%, 45.68%, 26.51% and 7.92% areas have been found to be under none to slight, moderate, severe and very severe soil erosion hazard categories, respectively.  相似文献   

Phytoplankton Distribution in an Oligotrophic Pond and a Eutrophic Pond     

L. C. Saha  D. E. Wujek 《洁净——土壤、空气、水》1989,17(4):407-416

Summary: An oligotrophic pond and a eutrophic pond of Bhagalpur, India, were studied for phytoplankton distribution and to establish the relationship between this distribution and the physico-chemical properties of the water. Altogether, 114 algal taxa were identified. In Rekabganj pond, diatoms constituted 54.4 % of the algal species followed by green algae (29.4 %), blue greens (11.7 %) and euglenoids (2.9 %). In Ramna pond, blue greens were the most dominant comprising 35.7 % of algal species followed by green algae (32.8 %), diatoms (18.5 %) and euglenoids (8.5 %). The members of Dinophyceae, Chrysophyceae and Xanthophyceae were poorly represented in both the water-bodies. Aphanizomenon flos-aquae, Microcystis aeruginosa, Chlorella vulgaris, Cladophora crispata, Volvox aureus and Euglena spp. were recorded only in Ramna pond, indicating its higher nutrient status. Dinobryon calciformis, many pennate diatoms and desmids, indicators of oligotrophy, were observed in Rekabganj pond only. Measurements of water properties in the two ponds showed that transparency, dissolved oxygen and silicate were higher in Rekabganj pond than in Ramna pond, while total alkalinity, hardness, total dissolved solids, dissolved organic matter, different cations, phosphate, nitrate and chloride were lower in Rekabganj pond than in Ramna pond.  相似文献   

Flow laws in polymineralic aggregates deformed by a combination of diffusion creep and dislocation creep     

Dilip Saha 《Journal of Earth System Science》1997,106(4):221-224

Microstructures in naturally deformed rocks in the upper crust demonstrate that creep strain in nature may be accommodated by a combination of dislocation creep, diffusion/dissolution processes and microcracking. A theoretical approach towards deriving an aggregate flow law is presented, where the strain in the constituent phases is assumed to occur by simultaneous operation of diffusive mass transfer and crystal plastic mechanisms (dislocation creep). Both uniform stress and uniform strain rate situations are considered.  相似文献   

Clay mineralogy of archaeological soil: an approach to paleoclimatic and environmental reconstruction of the archaeological sites of the Paharpur area,Badalgacchi upazila,Naogaon district,Bangladesh   总被引：1，自引：0，他引：1  

A. K. M. Masud Alam  Shucheng Xie  Dilip Kumar Saha  Sifatul Quader Chowdhury 《Environmental Geology》2008,53(8):1639-1650

A geo-archaeological examination was carried out in and around the Paharpur area, Badalgacchi upazila of Naogaon District in the north-western part of Bangladesh. Clay mineralogy and global paleo-temperature curves were used to determine the paleoclimatic and depositional environmental conditions of the Holocene archaeological soils (Barind clay residuum) of the Paharpur archaeological spot. Clay mineralogical data show that the dominant clay mineral is illite with subordinate amounts of kaolinite and chlorite. The high illite and low kaolinite content implies a marine or estuarine deposit. The clay mineral ratio log diagram, constructed by plotting the 7–10°Å integrated intensity ratios versus depth, supports this assumption. The presence of illite, together with quartz and feldspar, suggests high detrital input under generally cool to temperate and dry climates. Chlorite is another common by-product of weathering reactions with low hydrolysis, which is typical for cool to temperate and dry climates. The results suggest the existence of a marine–estuarine influence under a generally cool to temperate and dry climates around Paharpur and the surrounding region throughout the Pala dynasty around mid-century 700 a.d. to early 1100 a.d. Furthermore, this observation is consistent with the global paleo-temperature curves.  相似文献