Evidence of melt/rock interaction in the Cr-spinel bearing wehrlite rocks of Bangriposi,India: Implications for nature of the metasomatic agent

Cr-spinel bearing wehrlite rocks of Bangriposi are found within the multiply deformed metasedimentary rocks of Singhbhum Group belonging to North Singhbhum Mobile Belt of eastern India. Detailed mineralogical and geochemical studies reveal that these rocks have suffered a two-stage alteration involving a deeper level modal and cryptic metasomatism and a subsequent shallower depth pervasive hydrothermal alteration. Cryptic metasomatism is defined by elevated LREE contents of the wehrlite and its clinopyroxne grains. Metasomatism induced changes in the modal mineralogy of the rocks include the absence of primary orthopyroxene grains, presence of secondary diopside-phlogopite(now present as vermiculite) defining disequilibrium reaction textures and secondary orthopyroxene rims around serpentinized olivine. The mineralogical and geochemical changes due to the metasomatic event present a contrasting picture in regard to the metasomatic history of the rocks. Possible scenarios involving a single stage or multiple stage metasomatism events have been discussed while explaining the metasomatic reactions that took place. An attempt has been made to estimate the REE concentrations of the final equilibrating melt from REE contents of clinopyroxene grains of the wehrlite. The possibility of the LREE-enriched equilibrating melt of the wehrlite rocks(the deeper level metasomatic agent) being similar to residual melts from the E-MORB type parental melts of nearby gabbro suite has been ruled out by geochemical modeling. REE abundance patterns of several natural enriched melts have been compared with REE pattern of calculated LREE-enriched equilibrating melt of the wehrlite and most resemblance has been observed with calcic and potassic melts. It is therefore suggested that the Cr-spinel bearing wehrlite rocks of Bangriposi has been affected by a calcio-potassic melt in deeper level, prior to the shallow level serpentinization event.     

Characterizing groundwater recharge using oxygen and hydrogen isotopes: a case study in a temperate forested region,South Korea

In the cool temperate region of South Korea, oxygen and hydrogen isotopes of groundwater, lake water, and precipitation were studied to determine the season of groundwater recharge. All the groundwater samples, irrespective of season, on δ¹⁸O–δ²H scale plotted along the summer precipitation, suggesting summer precipitation largely modulates recharge. The deuterium excess values of groundwater (d-excess) show clear seasonal difference, higher in winter (> 18‰) and lower in summer (< 10‰). And its resemblance to the summer precipitation d-excess value further suggests dominant role of summer precipitation in groundwater recharge. Based on the mass balance equation, with end-member d-excess values of seasonal precipitation and groundwater as input variables, groundwater is composed of 66% summer and 34% winter precipitation. Despite the study area being heavily forested, summer rainfall contribution higher than winter suggests that evapotranspiration effect is minimal in the region; may be due to thin sand–gravel-based porous soil overlying highly weathered granitic rock system.     

The age of the Nodular Limestone Formation (Late Cretaceous), Narmada Basin,central India

The age of the marine Nodular Limestone Formation of the Bagh Group is refined at Substage level through ammonoid and inoceramid index taxa. The study is based on the fresh collections from three well-defined successive intervals (Lower Karondia, Upper Karondia and Chirakhan members) of this formation having excellent exposures in different localities of the Narmada Basin, central India. The first record of the widely distributed Turonian ammonoid genera Spathites Kummel and Decker and Collignoniceras Breistroffer from the Nodular Limestone Formation constrained its age exclusively to Turonian. The Early Turonian species Spathites (Jeanrogericeras) aff. revelieranus (Courtiller) and Mytiloides labiatus (Sclotheim) occur in the lower part, while the Middle Turonian marker Collignoniceras cf. carolinum (d’Obrbigny) and Inoceramus hobetsensis (Nagao and Matsumoto) occurs in the upper part of the Karondia Member. The record of the index species Inoceramus teshioensis (Nagao and Matsumoto) in association with Placenticeras mintoi Vredenburg from Chirakhan Member allows a definite Late Turonian age. The present contribution is an attempt to resolve the controversies in the age of the Nodular Limestone Formation and also demarcation of the three divisions (Early, Middle and Late) of the Turonian Stage in the Narmada Basin, central India.     

Occurrence of thorite and radioactive zircon in syenite pegmatite,North of Ashwaraopet,Khammam district,Andhra Pradesh,India

This paper reports the occurrence of thorite and confirms the earlier find of radioactive zircon by Subbarao, Murali and others from syenite pegmatite in the Vinayakapuram-Kunavaram alkaline belt in the Khammam district of Andhra Pradesh, India. Thorite has also been found as residual concentrations in soil overlying the pegmatite.     

Application of IRS-1A data for delineating buried channels in Southern part of Allahabad district of Uttar Pradesh

Two buried channels were identified in southern part of Allahabad district based on the visual interpretation of IRS-1 A LISS II FCC followed by detailed study of aerial photographs and cheks. It has been concluded on basis of the configuration of the channels that these were initially joined forming one channel which flowed from east to west although the present master slope of the area is from west to east. The present reversal of the drainage might have been caused by the neo-tectonic activity in the area. The buried channels provide potential ground water reservoirs in the area as proved by a few boreholes drilled on the channels. Some part of the area has become waterlogged due to the seepage of water from the canals cutting across one of the buried channels.     

Landsat mss data for land use and land cover inventory and mapping: A case study of Andhra Pradesh

This study deals with the technique of remote sensing and how far it helps in the rapid study of geographical phenomena especially land use within a very short time and accurate manner. It evaluates how well data from the Landsat - Multispectral Scanner (MSS) could be used to detect, identify and delineate land use features within the Andhra Pradesh State. The main objective was to prepare a small scale land use map from satellite imagery showing the broad distribution of land use patterns to serve as a base for monitoring land use change.     

Application of multidate satellite imagery for land use planning and management in the drought prone areas of Karnataka

Based on visual interpretation of Multidate Landsat Imagery, the spatial distribution of land use/land cover over 45,000 sq.km, spread over the three drought prone districts of Bijapur, Belgaum and Dharwar in NW Karnataka, has been mapped. The land use/land cover is classified into five Level-I and twelve Level-II classes. The pattern of change in land use/land cover during the period October, 1980 and January, 1982 has been one of decline in all the land use classes (except for agricultural use, which is more due to seasonal change) which highlight the land use/land cover changes in the drought prone area. An optimum land use plan requires that all the cropland should be zoned for cultivation while marginal lands like scrub land and mixed barren land (from the view point of cultivation) should be zoned for pasture/grazing and animal husbandary. There is a case for flexibility here, depending upon the pressure of population on land. The accuracy level of the ‘information base’ of the thematic map(s) obtained from Landsat imagery is 94 percent.     

Generation of Deccan Trap magmas

Deccan Trap magmas may have erupted through multiple centers, the most prominent of which may have been a shield volcano-like structure in the Western Ghats area. The lavas are predominantly tholeiitic; alkalic mafic lavas and carbonatites are rare. Radioisotope dating, magnetic chronology, and age constraints from paleontology indicate that although the eruption started some 68 Ma, the bulk of lavas erupted at around 65–66 Ma. Paleomagnetic constraints indicate an uncertainty of ± 500,000 years for peak volcanic activity at 65 m.y. in the type section of the Western Ghats. Maximum magma residence times were calculated in this study based on growth rates of “giant plagioclase” crystals in lavas that marked the end phase of volcanic activity of different magma chambers. These calculations suggest that the > 1.7 km thick Western Ghats section might have erupted within a much shorter time interval of ∼ 55,000 years, implying phenomenal eruption rates that are orders of magnitude larger than any present-day eruption rate from any tectonic environment. Other significant observations/conclusions are as follows: (1) Deccan lavas can be grouped into stratigraphic subdivisions based on their geochemistry; (2) While some formations are relatively uncontaminated others are strongly contaminated by the continental crust; (3) Deccan magmas were produced by 15–30% melting of a Fe-rich lherzolitic source at ∼ 3–2 GPa; (4) Parent magmas of the relatively uncontaminated Ambenali formation had a primitive composition with 16%MgO, 47%SiO₂; (5) Deccan magmas were generated much deeper and by significantly more melting than other continental flood basalt provinces; (6) The erupted Deccan tholeiitic lavas underwent fractionation and magma mixing at ∼ 0.2 GPa. The composition and origin of the crust and crust/mantle boundary beneath the Deccan are discussed with respect to the influence of Deccan magmatic episode.     

A curiosity regarding the relation between surface level humidity and precipitable water

We have examined the global relation betwen the mean monthly surface level humidity and the precipitable water (Q-W relation) developed by Liu (1986) for its applicability to instantaneous data sets as proposed by Hsu and Blanchard (1989) over the Indian oceanic region. Our analysis using regional instantaneous values of W from Monex-79 in-situ data sets gave an rms error of 5.66gm/kg. This is considerably larger than the rms error of 0.4 gm/kg as obtained by Liu using the global mean monthly relation, thus failing to support Hsu and Blanchard's conjecture. However, a fifth-order polynomial in W derived by us based on instantaneous data indicates an rms error of 0.92 gm/kg.     

Reappraisal of the structure of the Western Iron Ore Group, Singhbhum craton, eastern India: Implications for the exploration of BIF-hosted iron ore deposits

The middle to late Archean Iron Ore Group rocks occurring along the western margin (the Western Iron Ore basin) of the Singhbhum Granite massif in the Singhbhum craton were deformed during Iron Ore orogeny and are disposed in a horseshoe-shaped synclinal structure in the eastern part of the Indian shield. The Western Iron Ore basin hosts almost all the major high-grade iron ore deposits of eastern India. Contrary to the established view, present analysis emphasizes that the horseshoe fold in reality is a synclinorium consisting of a syncline–anticline fold pair which were later cross-folded along an east–west axis.

Structural analysis in the eastern anticline of the ‘horseshoe synclinorium’ suggests that the BIF hosting the high-grade iron ore bodies are disposed in three linear NNE–SSW trending belts, each showing an open synclinal geometry. Later cross folding produced development of widespread dome and basin pattern at the sub-horizontal hinge zones of these synclinal fold belts. The major iron ore deposits in the eastern anticline at the present level of erosion are preferentially localized within shallow elongated basinal structures only. The axis of the adjoining western syncline was similarly uplifted as partial culminations where cross-folded against E–W anticlinal axes. But here, the BIF-iron ore bodies are preferentially localized within elongated domal structures in contrast to the basinal sites in the adjacent eastern anticline. Such an inference based on structural analysis could probably be utilized as a potential tool for all future explorations, reserve estimation and recovery of the iron ore deposits in the terrain.