Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

G. V. Anantha Iyer ¹, A. R. Vasudeva Murthy ¹

Affiliations
1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, IN

Abstract

No Abstract.

Full Text

   

G. V. Anantha Iyer ¹

Affiliations
1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, IN

Abstract

The Precambrian of S-E Mysore belongs to the non-charnockitic province of Fermor and the amphibolite facies of Pichamuthu, and include the rock formations between the longitudes 77°E and 79°E. Along the southern and the eastern margins of the formations in the State are the transitional amphibolite-charnockite facies rocks.

Field features of the metamorphic rocks across the prevailing N-S regional trend of the formations suggest a correspondence between the K-feldspar structural state and metamorphism.

The structural state of K-feldspars determined both by X-ray and infrared absorption techniques reveals (i) partially disordered feldspars occur in the transitional facies zone and completely disordered feldspars in the charnockites of Halagur, (ii) a decrease in the intensity of metamorphism as the Closepet belt is approached from the south-west margin and (iii) ordered feldspars of high obliquity are characteristic of amphibolite facies of granitic complex between the Closepet belt in the west and the Kolar schist belt in the east.

Full Text

     

T. R. Narayanan Kutty ¹, G. V. Anantha Iyer ¹

Affiliations
1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, IN

Abstract

X-ray, infrared spectra and chemical data reveal that the pink mineral from the calc-silicate rocks of Sakarsanahalli, Kolar, is a mixed phase of zoisite and clinozoisite. It is generally cloudy due to fine inclusions of plagioclase. Polymorphic transformation relations could be observed in it. The mixed crystals are found to be stable when the replacement of Al by Fe³⁺ is less than 10 atom per cent. The yellow-green to green mineral, on the other hand, is monophasic epidote, with 30 to 35 atom per cent Al being substituted by Fe³⁺. Hydrothermal experiments show that zoisite-clinozoisite transforms to zoisite-epidote (iron-rich clinozoisite) pair at higher temperature and pressure. However, larger Fe³⁺ activity upsets this transformation relation, where the monoclinic epidote phase alone is stabilised.

Full Text

     

G. V. Anantha Iyer ¹, T. R. Narayanan Kutty ¹

Affiliations
1 Indian Institute of Science, Bangalore, IN

Abstract

Field features and the mineralogical relations of rocks in the Precambrian of South Karnataka indicate progressive increase in intensity of metamorphism from greenschist facies in the north-west through amphibolite facies in the middle to granulite facies in the southern and south-western parts of the State. Chemical analyses of fifty garnets developed in the amphibolite and granulite facies rocks of Karnataka are furnished. The cell edges of these garnets have been determined accurately by X-ray powder diffraction, using the extrapolation technique of Nelson and Riley. The changing pattern of chemical composition and cell edges of garnets enable recognition of successive zones of progressive metamorphism in the amphibolite facies terrain in the Precambrian of South Karnataka.

Full Text

     

M. Ramakrishnan ¹, T. R. Narayanan Kutty ², G. V. Anantha Iyer ²

Affiliations
1 Jayanagar, Bangalore, IN
2 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, IN

Abstract

An alteration product of kyanite from Kudinirkatte near Holalkere, Karnataka State, is found to be a hydrous alumino-silicate with a chemical composition corresponding to that of muscovite. Thermal, X-ray diffraction, and infrared spectral data show distinct variation from the known polymorphs of muscovite. On hydrothermal reactions with potassium-rich solution, the alteration product converts to 1M muscovite, and in sodium-rich solution, it converts to a mixture of muscovite and paragonite. When seeded, it converts to 2M muscovite at a higher temperature and pressure. The alteration product of kyanite has a 1M muscovite structure with randomly distributed montmorillonite domains.

 

Full Text

     

T. R. N. Kutty ¹, G. V. Anantha Iyer ¹, E. De Poli ¹

Affiliations
1 Indian Institute of Science, Bangalore 560012, IN

Abstract

Petrography, mineral chemistry, major elements as well as Rb, Sr, Ba and REE geochemistry of pink granulites and the associated charnockites have been studied. Pink granulites are prograde rocks metamorphosed under granulite facies condition. There is no genetic relation between potassic pink granulites and tonalitic charnockites and their geochemical evolutionary trends are distinctly different. K/Rb ratios are in the same range, although abundance level of K and Rb in pink granulites is high. Charnockites have higher Sr and lower Rb/Sr. Pink granulites have variable Sr level, high Rb/Sr and variable Ba/Sr indicating heterogeneity in the parent rocks. REE abundance level in pink granulites shows wide variation; non-uniform but negative Eu anomaly, strong fractionation with respect to LREE, positive slope after Gd and lower ∑LREE/∑HREE ratio (5 to 26). Charnockites have coherent REE patterns, with strongly fractionated LREE and slight positive Eu anomaly and higher ∑LREE/∑HREE (30-70) ratios. Field relation, mineral paragenesis, whole rock chemistry, Sr-isotope ratios and REE geochemistry strongly support sedimentary parentage for the pink granulites. The highly fractionated REE patterns of the associated charnockites reflect not only igneous inheritance but also the effect of granulite grade metamorphism.

Full Text

     

P. Venkataramana ¹, T. R. N. Kutty ¹, G. V. Anantha Iyer ¹

Affiliations
1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, IN

Abstract

Previous experimental studies on magnesite formation which are confined to temperatures greater than 250°C, do not explain all the field and petrographic observations in natural situations where magnesite is associated with ultramafic complexes, as at Dodkanya, Mysore district, Karnataka.

A series of experiments were conducted using natural samples (rock and mineral powders) as starting materials. The experimental temperatures were varied from 35° to 300°C at intervals of approximately 50°C and normal and elevated pressures (500 and 1000 bars). At higher pressures, the X_CO2 in the fluid phase was also varied.

These studies have shown that magnesite and quartz can be formed directly by the interaction of CO₂-bearing water vapour and ferromagnesian minerals (olivine, orthopyroxene) without passing through the intermediate serpentine stage. The temperature conditions at which these reactions take place is between 150 and 250 °C at 500 bars total pressure with X_CO2 in the fluid phase greater than 0.015. The presence of Na⁺ salts enhances these reactions.

The presence of calcite-aragonite-quartz-aluminous hydroxide gel-limonite (birbirite) cap rock intersected by a network of magnesite veins in Dodkanya and surrounding areas has been explained on the basis of experiments with natural hornblende, the major Ca²⁺-bearing mineral in hornblende-peridotites. Hornblende, under hydrothermal conditions, breaks down to calcium silicate hydrate (CSH). CSH, being unstable in the presence of CO₂, inverts to CaCO₃ (calcite/aragonite) and quartz. Mg²⁺ in trace quantities stabilises aragonite.

Full Text

     

Yu. G. Safonov ¹, A. D. Genkin ¹, V. N. Vasudev ², B. Krishna Rao ³, G. V. Anantha Iyer ⁴

Affiliations
1 Institute of Geology of Ore Deposits, Academy of Science USSR, Moscow, RU
2 Department of Mines of Geology, Bangalore 560001, IN
3 Department of Geology, University of Mysore, Mysore 570005, IN
4 Indian Institute of Science, Bangalore 560012, IN

Abstract

Twenty-five ore minerals have been identified from a study of ore-bearing samples from different reefs of the Kolar Goldfields. The mineral association includes fifteen minerals being reported for the first time: altaite, hessite hedleyite, tzumoite, volynskyite gudmundite, ullmannite, brethauptite, tetrahedrite, hawleyite, cotunite, electrum and native bismuth. Besides, two new minerals, as yet unnamed, have been determined through electronprobe - Pb₃Te₉Cl₄S₂ and PbTeCI₂ They occur along with cotunite (PbCI₂) and altaite.

The study of time relations of different ore and vein mineral assemblages in the Kolar gold-quartz reefs has revealed six stages of mineralization: 1) Early quartz, 2) quartz-feldspar (Pegmatite), 3) scheelite, 4) early sulphides with gold, 5) sulphide-gold-tellurides (with quartz) and 6) quartz-carbonate. Fluid inclusions indicate that the mineral association of quartz-feldspar stage has developed under high TP-conditions of 250-300°C and 1.8 and 3.5 kb. Isotope composition of sulphur in the sulphides indicate the juvenile nature of sulphur. Available data suggest that the quartz of first stage is of metasomatic origin, derived possibly from host amphibolites; gold-sulphide and gold-telluride mineralization took place through hydrothermal processes.

Full Text

     

G. V. Anantha Iyer ¹, V. N. Vasudev ²

Affiliations
1 Indian Institute of Science, Bangalore 560012, IN
2 Department of Mines and Geology, Bangalore 560001, IN

Abstract

Massive volcanic-hosted sulphide deposits, within the Jogimardi volcanic suite (JVS) are Cu-rich with minor Zn and Pb and form an important early phase in the evolution of the late Archaean Chitradurga belt. The deposits are concordant stratabound lenses in the bedded mafic tuffs emplaced as submarine volcaniclastites. The JVS is an integral part of the Chitradurga antiform flanked by synclines on either side. The stratiform massive and vein-type sulphide deposits of the JVS are confined to the eastern limb of the antiform.

The volcanic rock succession in the JVS consists of 85% tholeiitic basa1ts; 12% low silica variolitic andesites and 3% of rhyolites. The basalts are quartz-normative tholeiites, not known to occur in MORB environment. Unlike those of MORB, in their major, minor and REE geochemistry, the basalts related to massive sulphide deposits are almost identical to back-arc basalts generated in an ensialic marginal basin environment.

The thermal and chemical environment of ore formation are determined based on fluid inclusions data and δ³⁴S pyrite values of individual sulphide phases. These data indicatce that the ores were deposited under anoxic chemical environment at a temperature of 350°C within a pH range of 5 to 7 corresponding to proximal facies conditions. The stratiform sulphide ore body in the area is located in the vicioity of a fracture controlled stringer/stock-work ore zone - 'a hydrothermal discharge vent' in the pillowed basalts of tholeiitic composition which confirms the proximal nature of the deposit.

Uranogenic and thorogenic lead isotope studies on the galenas of Ingladhal stratiform sulphide bodies indicate that they are not of mantle origin but derived from the basement gneisses, dated at 3 b.y.

Geologic setting, preservation of quench plagioclase textures in the pillowed basalts and volcanogenic synsedimentary nature of the stratiform sulphide deposits indicate that the sulphides were deposited initially in the JVS prior to folding and regional greenschist facies metamorphism.

Full Text

     

A. G. Menon ¹, G. V. Anantha Iyer ²

Affiliations
1 Instrumentation and Services Unit, Indian Institute of Science, Bangalore 560 012, IN
2 Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, IN

Abstract

Details of a new method to mode1 trace element distribution in hydrothermal ore deposits is outlined. The behaviour of Pb²⁺ at 100°, 200° and 300°C has been highlighted as a function of solution parameters f_s2 f_o2, and pH.

Full Text

     

T. R. N. Kutty ¹, S. R. N. Murthy ¹, G. V. Anantha Iyer ¹

Affiliations
1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, IN

Abstract

REE geochemistry of plagioclase-free, plagioclase-bearing and titanoclinohumite-bearing ultramafic suite of Chalk Hills is presented. The rare earth elements are determined by spark source mass-spectrography combined with a preanalytical chemical concentration reaching ± 4 accuracy and 0.01 ppm detection limit. Plagioclase-free ultramafics are 10 times LREE depleted. while HREE abundance is 1-2 times those of chondrite. Since the samples are not serpentinised, these REE patterns are considered to be primary and compare well with those of high temperature peridotites, particularly on Ce_N-Yb_N diagram. The plagioclase-bearing ultramafics and the gabbros have LREE contents 4-22 times chondritic, while the HREE varies from 2-12. The REE patterns of ultramafics from the smaller body show uniform increase in (La/Yb)_N ratios from 7 to 15. Higher abundance levels of incompatible alkali elements which do not correlate with the MgO contents, may suggest metasomatic introduction of these elements into the smaller body.

Petrogenetic modeling, based on partial melting of spinel peridotites as the source rocks with 2 times chondritic abundance, points towards the residual nature of the plagioclase-free ultramafics after extracting 8-10% of the melt. On the other band, the observed REE patterns of the plagioclase-bearing ultramafics and the gabbros not only indicate the absence of rocks more siliceous than syenogabbros but also suggest that they represent compositions of a depleted parent melt from which they originated. Thus the ultramafic association of Chalk Hills is distinctly different from the igneous suites associated with ophiolites and alpine peridotites.

Full Text

     

V. Swamy ¹, G. V. Anantha Iyer ², A. G. Menon ³

Affiliations
1 Department of Inorganic and Physical Chemistry, I. I. Sc., Bangalore - 560 012, IN
2 J. N. Centre for Advanced Scientific Research, I. I. Sc., Campus, Bangalore - 560 012, IN
3 Mass Spectrometry Laboratory, I. I. Sc., Bangalore - 560 012, IN

Abstract

This paper presents a numerical simulation of the well-documented, fluid-controlled Kabbal and Ponmudi type gneiss-charnockite transformations in southern India using a free energy minimization method. The computations have considered all the major solid phases and important fluid species in the rock - C-O-H and rock - C-O-H-N systems. Appropriate activity-composition relations for the solid solutions and equations of state for the fluids have been included in order to evaluate the mineral-fluid equilibria attending the incipient charnockite development in the gneisses. The C-O-H fluid speciation pattern in both the Kabbal and Ponmudi type systems indicates that CO₂ and H₂O make up the bulk of the fluid phase with CO, CH₄ H₂ and O₂ as minor constituents. In the graphite-buffered Ponmudi-system, the abundance of CO, CH₄ and H₂ is orders of magnitude higher than that in the graphite-free Kabbal system. Simulation with C-O-H -N fluids of varying composition demonstrates the complementary role of CO₂ and N₂ as rather inert dilutants of H₂O in the fluid phase. The simulation, carried out on available whole-rock data, has demonstrated the dependence of the transfonnation X(H₂O) on P, T, and phase and chemical composition of the precursor gneiss.

Keywords

Charnockitization, Thermodynamic Modelling, High-Grade Metamorphism, Mineral-Fluid Equilibria, Granulites.

Full Text

     

V. Swamy ¹, A. G. Menon ², G. V. Anantha Iyer ¹

Affiliations
1 Department of Inorganic and Physical Chemisty, Indian Institute of Science, Bangalore-560 012, IN
2 Instrumentation and Services Unit, Indian Institute of Science, Bangalore-560 012, IN

Abstract

High-temperature reactions (Ca 900°C) involving albite, K-feldspar or plagioclase and K, Ba-or K, Sr chlorides were experimentally studied. These experiments reveal that the reaction between K-exchanged albite, potash feldspar, or plagioclase and Ba-chloride/Ba-K chloride results in the formation of celsian by the breakdown of the starting feldspar structure above 800°C. Sr-feldspar does not form under similar conditions. A size-effect of the large M-site cation appears to be responsible for the formation of celsian. The reaction between K-feldspar and barium chloride may be used as a method for synthesizing celsian.

Keywords

Albite, K-Feldspar, Plagioclase, Celsian, Igneous Petrology.

Full Text

     

V. Swamy ¹, G. V. Anantha Iyer ², A. G. Menon ³

Affiliations
1 Division of Minerals, CSIRO, Clayton South,. Victoria 3169, AU
2 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore - 560 012, IN
3 Department of Instrumentation, Indian Institute of Science, Bangalore - 560 012, IN

Abstract

The structural state of K-feldspars in the quartzofeldspathic gneisses, charnockites, metapelites and pegmatites from the southern Karnataka, northern Tamil Nadu and southern Kerala high-grade regions of southern India has been characterized using petrographic and powder X-ray diffraction methods. The observed distribution pattern of structural state with a preponderance of disordered K-feldspar polymorphs in granulites compared to the ordered microclines in the amphibolite facies rocks is interpreted to reflect principally the varying H₂O contents in the metamorphic-metasomatic fluids across metamorphic grade. The K-feldspars in the pegmatites of granitic derivation and in a pegmatite of inferred metamorphic origin also point to the important role of aqueous fluids in their structural state.

Keywords

X-ray Diffraction, Amphibolite Facies, Granulites, Metamorphic-Metasomatic Fluids.

Full Text

     

M. Ramakrishnan ¹, S. Moorbath ², P. N. Taylor ², G. V. Anantha Iyer ³, M. N. Viswanatha ¹

Affiliations
1 Geological Survey of India, Gujarat Circle, Ashram Road, Ahmadabad - 380014, IN
2 Department of Geology and Mineralogy, University of Oxford, Oxford, GB
3 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, IN

Abstract

Rb-Sr and Pb-Pb whole rock isotopic systematics and petrochemical data for two suites of Peninsular Gneiss immediately underlying the first order regional unconformity at the base of the main greenstone successions of Dharwar Supergroup in Bababudan and Chitradurga belts are presented.

The Chikmagalur granite (s.l.) and associated migmatite gneisses (CMG suite) underlying the Bababudan greenstones define an eleven point Rb-Sr isochron of 3080 ± 80 m.y. (MSWD4.5) with an initial ⁸⁷Sr/⁸⁶86 isotope ratio (I.R.) of 0.7016 ± 0004. Pb isotope data for the same samples yield a 3185 ± 60 m.y. isochron (MSWO 7.0) with single stage 238_U/204_Pb (μ₁) ratio of 7.99. The crust-forming event in this area spans a range of 3000-3200 m.y. within the analytical error of the isochrons. The mantle-type I.R. and μ₁ values preclude interaction with or reworking of earlier continental crust in their genesis. Petrological and chemical studies of the CMG suite show that the Chikmagalur 'granite' is a biotite granodiorite and the intruded gneisses are low alumina trondjhemites having a calc-alkaline trend. It is significant that the CMG suite is located within a low strain zone in Central Karnataka.

The gneisses and granitoids close to the western margin of the Chitradurga belt (CDG Suite) give a Rb-Sr age of 2970 ± 100 m.y. (MSWD 11) with I.R. of 0.7035 ± 0.0013. The corresponding Pb-Pb isochron defines a date of 3044 ± 150 m.y. with μ₁ = 7.69. The μ₁ and I.R. values suggest the possible involvement of previous sialic crust in their generation during the gneiss-forming event around 3000 m.y. ago. The CDG suite is heterogeneous, but all its components are regarded as a part of the Peninsular Gneiss. The composition of the CDG suite has a wide range from tonalite to granite, with a typical calcalkaline granitic trend.

Recent intensive geochronological studies in central Karnataka suggest a widespread crust-forming event around 3000 m.y. together with older gneissic components of Ca 3300 m.y. The present data bridge the time gap between 3000 and 3300 m.y. suggesting that continental crust was being generated in Karnataka craton from 3300 to 3000 m.y. semi-continuously. The unambiguous field relations in central Karnataka suggest that this event occurred prior to the deposition of main greenstone piles of Dharwar Supergroup.

Full Text

     

G. V. Anantha Iyer ¹, T. R. Narayanan Kutty ¹

Affiliations
1 Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore, IN

Abstract

Amphiboles from the Precambrian of Karnataka were analysed for major elements by wet chemical methods. Their cell edges were determined by X-ray powder diffraction technique, using Cr K α radiation. The chemical composition of the amphibole is correlatable with the grade of regional metamorphism. Chemical compositions of the Ca-amphiboles from the northern parts show low alkali and Al⁺ characteristic of tremolite-actinolite group corresponding to greenschist facies. The amphiboles of the middle region are common hornblendes with higher alkali and Al⁺, characteristic of amphibolite facies. Amphiboles from the southern parts are aluminous and alkali-rich with higher titanium and Fe³⁺/Fe²⁺ ratios. They coexist with ortho-or clinopyroxenes, suggesting their formation under granulite facies conditions. The restricted occurrence of sodic amphiboles in the Bababudans, and of cummingtonite-grunerite series in Kudurcmukh iron formations is due to their narrow P-T fields of stability and compositional fields. Anthophyllites from certain horizons of the Precambrian may be regarded as products of metasomatism.

Full Text