Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral-Fluid Equilibria and Petrogenetic Grids

The role of clinopyroxene in producing grandite garnet is evaluatedusing data from an ultrahigh-temperature metamorphosed calc-silicategranulite occurrence in the Eastern Ghats Belt, India. ‘Peak’pressure–temperature conditions of metamorphism were previouslyconstrained from associated high Mg–Al granulites as c.0·9 GPa, >950°C, and the rocks were near-isobaricallycooled to c. 750°C. Grandite garnet of variable compositionwas produced by a number of reactions involving phases suchas clinopyroxene, scapolite, plagioclase, wollastonite and calcite,in closely spaced domains. Compositional heterogeneity is preservedeven on a microscale. This precludes pervasive fluid fluxingduring either the peak or the retrograde stage of metamorphism,and is further corroborated by computation of fluid–rockratios. With the help of detailed textural and mineral compositionalstudies leading to formulation of balanced reactions, and usingan internally consistent thermodynamic dataset and relevantactivity–composition relationships, new petrogenetic gridsare developed involving clinopyroxene in the system CaO–Al₂O₃–FeO–SiO₂–CO₂–O₂in T–aCO₂–fO₂ space to demonstrate the importanceof these factors in the formation of grandite garnet. Two singularcompositions in garnet-producing reactions in this system arededuced, which explain apparently anomalous textural relations.The possible role of an esseneite component in clinopyroxenein the production of grandite garnet is evaluated. It is concludedthat temperature and fO₂ are the most crucial variables controllinggarnet composition in calc-silicate granulites. fO₂, however,behaves as a dependent variable of CO₂ in the fluid phase. Externalfluid fluxing of any composition is not necessary to producechemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidation–decarbonation equilibria     

Coal fire mapping of East Basuria Colliery,Jharia coalfield using vertical derivative technique of magnetic data

The present study deals with the coal fire mapping of East Basuria Colliery, Jharia coalfield, India, using the magnetic method. It is based on the fact that rise in temperature would result significant changes in magnetic susceptibility and thermo-remanent magnetization (TRM) of the overlying rocks. Magnetism increases slowly with the rise of temperature until the Curie temperature. Generally, rock/ overburden loses magnetization and becomes paramagnetic due to heating to Curie temperature, which results with significant reduction in magnetic susceptibility. However, magnetism increases significantly after cooling below the Curie temperature. Several data processing methods such as diurnal correction, reduction to pole (RTP), first and second vertical derivatives have been used for analysis of magnetic data and their interpretation. It is observed that the total magnetic field intensity anomaly of the area varies approximately from 44850 to 47460 nT and the residual magnetic anomaly varies approximately from ?1323 to 1253 nT. The range of the magnetic anomaly after RTP is approximately 1050–1450 nT. About 20 low magnetic anomaly zones have been identified associated with active coal fire regions and 11 high magnetic anomaly zones have been identified associated with non-coal fire regions using vertical derivative techniques.     

Geothermobarometry of mafic granulites and metapelite from the Palghat Gap, South India: petrological evidence for isothermal uplift and rapid cooling

The Palghat Gap region is located near the centre of the large southern Indian granulite terrane. at the northern edge of the Kodaikanal charnockite massif. The dominant rock types in the region are hornblende-biotite ± orthopyroxene gneisses and charnockites along with minor amounts of intercalated mafic granulite, metapelite and calc-silicate. The P-T estimates from garnetiferous mafic granulites and metapelite samples are generally in the range 9-10 kbar and 800-900 C using both conventional thermobarometric methods and the TWEEQU thermobarometry program. These P-T estimates, which should be taken as minimum values, are among the highest yet reported for South Indian and Sri Lankan granulites. The occurrence of orthopyroxene + plagioclase symplectites around embayed garnet grains in the mafic granulites and cordierite rims around garnet grains in metapelite suggest an isothermal decompression-type path. Similarly, a core-rim P-T trajectory indicates c. 3 and 7 kbar decompression at high temperature in the mafic granulites and metapelite, respectively. In both rock types, the key to the determination of the retrograde P-T path was the recognition of small amounts of second generation plagioclase with a more anorthitic composition than the matrix plagioclase. The preservation of high garnet-pyroxene temperatures in the mafic granulites (despite small garnet grain size) suggests rapid cooling of the terrane. Calculated minimum cooling rates range from 8 to 80 C Ma^-1. Such cooling rates are more rapid than those associated with normal isostatic processes and suggest that the terrane was tectonically exhumed at high temperature.     

ELECTRICAL POTENTIAL DUE TO A POINT CURRENT SOURCE OVER AN INHOMOGENEOUS ANISOTROPIC EARTH*

A mathematical expression for potential of a direct current point source in an inhomoge-neous anisotropic earth is derived. The coefficient of anisotropy is given by f= (σ_r/σ_z), where σ_r and σ_z are the conductivities parallel and perpendicular to the bedding plane. It is assumed that σ_z varies with depth, whereas σ_r varies transversely. This potential may be useful in interpretation of geoelectrical data in specified geological situations. Master curves for Wenner and Schlumberger configurations are presented     

SOME RESULTS OF EXPERIMENTAL GEOPHYSICAL SURVEYS FOR LOCATION OF ANCIENT GOLD WORKINGS,KOLAR, INDIA *

On an experimental basis, observations were made with various geophysical methods to locate the ancient gold workings in Kolar Gold Fields, Kolar, India. The results of experimental surveys indicated that two-electrode resistivity surveys with two spacings followed by limited trenching would be able to locate about 70 percent of the ancient workings, at 55-60 percent of the cost of conventional trenching.     

RESISTIVITY TYPE CURVES OVER OUTCROPPING VERTICAL DYKE—II *

The theoretical horizontal resistivity profiles Over an outcropping vertical dyke with two-electrode and Schlumberger electrode systems are discussed. The two-electrode array seems very useful in locating the highly conducting thick or thin vein, while the Schlumberger (gradient) array is suited to detect the conducting vein of low and moderately high resistivity contrast and as well as the resistive vein of all widths and resistivity contrasts. Besides, the inline gradient array has a distinction of establishing a clue to evaluate the resistivity contrast of the vein.     

Sedimentation in a closed trough north of the Iberia Abyssal Plain in the northeast Atlantic†

Mirrol Trough of the northeast Atlantic contains five NNE-SSW trending, en echelon, turbidite-filled basins deeper than 5500 m, each ranging from 4 to 10 km in width and 19 to 65 km in length. Trough deposition has consisted mainly of turbidites from adjacent hills and ridges as indicated by the physiography of the region, sediment isopach map, the nature of the sediments in the trough, and benthic foraminiferal depth indicator species. The sedimentation rate on abyssal hills and ridges, as deduced from palaeomagnetic evidence, is 1.36 cm/10³ years. Using this sedimentation rate, it is estimated that Mirrol Trough subsided relative to the surrounding area and began receiving sediments between 8.3 and 11.5 m.y. ago; and the deposition of the most recent turbidite has occurred sometime between 29,000 and 44,000 years b.p. Tilting of the base of the most recent turbidite with respect to the basin floor is observed, and this is attributed to relative sinking of the eastern margin of the trough after the deposition of the most recent turbidite.     

Instrumental Neutron Activation Analysis of Twenty-Nine International Geochemical Reference Samples

Results of measurements of twenty-seven elements, most of them in trace amounts, by Instrumental Neutron Activation Analysis (INAA) in twenty-nine international geochemical reference samples from Canada, U.S.A., Japan, France and South Africa, are reported and compared with available literature values. In general, our data agree well with literature values. The disagreement of our results (e.g., Zr in Canadian syenites and several trace elements in USGS ultrabasic rocks) with those reported by other workers is briefly discussed and ways of improving INAA are pointed out. The present report includes data on several trace elements for which existing data are rather scarce.     

Hydrogeological and geophysical study for deeper groundwater resource in quartzitic hard rock ridge region from 2D resistivity data

Electrical resistivity method is a versatile and economical technique for groundwater prospecting in different geological settings due to wide spectrum of resistivity compared to other geophysical parameters. Exploration and exploitation of groundwater, a vital and precious resource, is a challenging task in hard rock, which exhibits inherent heterogeneity. In the present study, two-dimensional Electrical Resistivity Tomography (2D-ERT) technique using two different arrays, viz., pole–dipole and pole–pole, were deployed to look into high signal strength data in a tectonically disturbed hard rock ridge region for groundwater. Four selected sites were investigated. 2D subsurface resistivity tomography data were collected using Syscal Pro Switch-10 channel system and covered a 2 km long profile in a tough terrain. The hydrogeological interpretation based on resistivity models reveal the water horizons trap within the clayey sand and weathered/fractured quartzite formations. Aquifer resistivity lies between ～3–35 and 100–200 Ωm. The results of the resistivity models decipher potential aquifer lying between 40 and 88 m depth, nevertheless, it corroborates with the static water level measurements in the area of study. The advantage of using pole–pole in conjunction with the pole–dipole array is well appreciated and proved worth which gives clear insight of the aquifer extent, variability and their dimension from shallow to deeper strata from the hydrogeological perspective in the present geological context.