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

V. I. Fonarev ¹, A. S. Janardhan ², A. N. Konilov ¹

Affiliations
1 Institute of Experimental Mineralogy of the Russian Academy of Sciences, Chernogolovka, Moscow district, 142432, RU
2 Department of Geology, University of Mysore, Mysore 570 006, IN

Abstract

A spinel phase with unusually high ZnO (8.8-11.0) and Cr₂O₃ (13.2-15.8%) contents is found in garnet-cordierite-sillimanite bearing migmatised pelitic bands interbedded with tonalitic charnockites of Malai Mahadeswara (MM) Hills. The textural and microprobe data indicate that this spinel phase is associated with late iron-rich garnet (rim)-cordierite-sillimanite assemblage.

P-T conditions of cordierite-garnet assemblages vary from peak metamorphic conditions of greater than 8 kbar and 750°C to 6.7±1.4 kbar and 685 ±20°C. The last retrograde phase of metamorphism took place at P of 4.5 kbar and at T of 580°C. The formation of Zn and Cr rich spinel phase must have taken place during this waning stages of metamorphism by the activity of hydrothermal fluids.

Keywords

Geothermobarometry, Mineralogy, Metamorphism, MM Hills, Karnataka.

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K. Francis Anto ¹, A. S. Janardhan ¹, B. Basavalingu ¹

Affiliations
1 Department of Geology, University of Mysore, Manasa Gangotri, Mysore-570006, IN

Abstract

Kambam valley forms an integral part of the Southern Granulite Terrain. The valley represents a deep crustal break hosting high temperature alkaline intrusives and also a carbonatite body. Common rock types encounterd are charnockite and granulite grade calc-silicate and metapelite. Calc-silicate lithologies are interbanded with mafic granulites on cm scale. Wolfastonite-scapolite-grandite garnet-clinopyroxene indicate high temperature (900°C). Corroborative high temperatures are recorded from associated rnetapelites. Textural features such as the formation of coronal garnet and garnet-quartz double rinds at the expense of calcite, scapolite, wollastonite, clinopyroxene suggest a late isobaric cooling at shallower depths. The cooling event was associated with domainal variation in fluid species. Fluid inclusion studies confirm a late carbonic fluid flux consistent with the carbonation reactions that followed.

The associated metapelitic lithologies in the area preserve textural evidence for a rapid high temperature decompression event. The Kambam valley granulites, thus witnessed high temperature metamorphism, rapid decompression and late isobaric cooling. The above metamorphic history is consistent with those reported from other parts of the Southern Granulite Terrain (SGT). Thermal perturbation associated with extensional thinning and alkaline magmatism explain the high temperature metamorphism, decompression and subsequent cooling.

Keywords

Petrology, Calc-Silicate, High Temperature Granulites, Metamorphic History, Southern Granulite Terrain, Tamil Nadu.

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A. S. Janardhan ¹, H. M. Ramachandra ¹

Affiliations
1 Department of Geology, University of Mysore, Manasagangotri, Mysore, IN

Abstract

Charnockites have been noticed in the canal sections near Hullahalli, south of Mysore city. From their field, petrographic and chemical character, it is argued that they were formed as a result of granulite grade metamorphism and migmatization of pelites and intercalated earlier basic igneous rocks and pelites of the Sargur schist complex.

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A. S. Janardhan ¹, C. Srikantappa ¹

Affiliations
1 Department of Geology, University of Mysore., Mysore 6, IN

Abstract

Carbonate-orthopyroxene-chrome tremolite rocks and their variants occur in close association with the ultramafics emplaced into the high grade metamorphic terrain of Sargur schist complex, around Mavinahalli village. The country rocks exhibit mineral assemblages typical of upper amphibolite to lower granulite facies of metamorphism and show signs of extensive migmatisation. The above mineral assemblages are very much similar to the rock types termed as 'Sagvandites', originally reported from Norway. Field evidences, petrography and petrochemistry of these rocks clearly show that they represent original ultramafic rocks metasomatically recrystallized due to circulating CO₂ (+Si0₂) rich solutions, at metamorphic temperatures of 630°C and at pressures of at least 6 kb.

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A. S. Janardhan ¹, H. M. Ramachandra ², G. R. Ravindra Kumar ¹

Affiliations
1 Department of Geology, University of Mysore, Manasa Gangotri, Mysore-570006, IN
2 Palaeomagnetic Division, Dy. D.G.'s Cell, Geological Survey of India, Central Circle, Nagpur, IN

Abstract

Results of structural investigations in Sargur schist complex around Sargur, Dodkanya and Gundlupet indicate four major fold episodes and associated fault movements. These episodes have resulted in the formation of antiform-synform structures(F₂), extensive interleaving and dome-basin patterns (F₂ and F₃). Tectono-metamorphic studies have brought out the polymetamorphic (with late retrogressive) and polydeformational nature of the terrain. At least two migmatization events and (anatectic) granitic intrusions (associated with F₂ and F₃) have been recognised. A classification of gneisses of the area has been made and reasons and criteria for the recognition of 'Kabini gneiss' are given.

The tectonic relations between the Sargurs, Dharwars and 'Khondalite-Char- nockites', point to structural continuity of the Sargur terrain with the 'Khondalite-Charnockite' terrain to its south.

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A. S. Janardhan ¹, H. M. Ramachandra ¹

Affiliations
1 Department of Geology, Manasagangotri, Mysore-570 006, IN

Abstract

The note records the occurrence of gabbroic anorthosites in the Kabini right bank canal sections, south of Hullahalli, close to the spot from where the same authors had originally described charnockites, and discusses the origin of these gabbroic anorthosites, with particular reference to their occurrence in Sargur type high grade terrains of Karnataka.

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M. N. Viswanathiah ¹, M. A. Shankara ¹, A. S. Janardhan ²

Affiliations
1 The Mineralogical Institute, Manasa Gangotri, Mysore - 570006, IN
2 Department of Geology, Manasa Gangotri, Mysore-570006, IN

Abstract

The green garnets found in the calc-silicate horizons around Bandihalli, is a chrome grossular (∼7% Cr₂O₃) with appreciable amount of Mn content. Thus, a partial miscibility between uvarovite and spessartite end members appears possible. The occurrence of thin band of graphite bearing quartzite within the calc-silicate horizons points to its metasedimentary origin. The presence of another variety of garnet (pyrope-almandine rich) in the associated cordierite bearing rock of the area shows that the composition of garnet may not be wholly dependant on grade of metamorphism.

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T. A. Selvan ¹, A. S. Janardhan ²

Affiliations
1 47, Park Road, East, Shenoynagar, Madras-600030, IN
2 Department of Geology, Manasa Gangotri, Mysore-570006, IN

Abstract

The note describes ultramafic and associated earlier gneissic enclaves within charnockite in a quarry near Bhavanisagar. The continuation of the charnockite fabric into the quartzo-feldspathic gneiss patches and the recrystallized olivine+opx+cpx assemblage of the ultramafic enclaves indicate a late granulite event.

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A. S. Janardhan ¹, N. Shadakshara Swamy ¹, G. R. Ravindra Kumar ²

Affiliations
1 Department of Geology, University of Mysore, Manasa Gangotri, Mysore 570006, IN
2 Centre of Earth Science Studies, PB No. 2235, Trivandrum 695010, IN

Abstract

Petrography and chemistry of hitherto unreported manganiferous horizons and associated recrystallised ultramafics (sagvandites) - key horizons in the Precambrian high grade terrain around Gundlupet are discussed. The occurrence of spessartine-rich garnets from an area of dominant upper amphibolite grade of metamorphism suggests that bulk chemistry was the most important controlling factor. In their mineralogy, and decrease of MnO% in garnets during subsequent migmatisation, the manganiferous horizons exhibit remarkable similarity to the khondalites of the Eastern Ghats. The sagvandites and the other lithologies, occurring now, as huge enclaves in the granite/gneissic terrain are similar to the Sargur Supracrustals.

Structural studies show that the area has suffered from three deformational episodes. The early N-S episode of the Sargurs are rotated to N 60° E and then to E-W. The dominant N 60° E episode is possibly connected with large scale granite emplacementfmigmatisation and the E-W shears are due to the prominent Moyar-Bhavani shear.

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C. Srikantappa ¹, C. R. L. Friend ², A. S. Janardhan ¹

Affiliations
1 Department of Geology, Manasa Gangotri, University of Mysore, Mysore 570006, IN
2 Dept. of Geology & Physical Sciences, Oxford Polytechnic, Headington, Oxford OX3 OBP, GB

Abstract

Mineralogical and chemical studies of chromite deposits of Sinduvalli ultramafic body, Mysore District, show definite evidence of layering and other crystal settling processes. Stretching of occluded olivines, chain structures in chromite and rotation of chromite layers indicate their participation in the deformational episodes which affected the area. Structure and chemistry of chromites classify the host ultramafics as belonging to stratiform type. Presence of gabbroic anorthosites confirms this and indicates that they represent layered complexes characterizing the Sargur high-grade terrain.

The presence of recrystallised orthopyroxenes and development of chrome amphibole indicate their formation under metamorphic conditions. Olivine-spinel geothermometry suggests re-equilibration at sub-solidus temperatures during high grade metamorphism. The formation of 'Ferritchromit' zones is connected to magnesite formation.

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B. M. Ravindra ¹, A. S. Janardhan ²

Affiliations
1 Department of Mines and Geology, Kankanadi, Mangalore - 575002, IN
2 Department of Geology, Manasa Gangotri, Mysore - 570006, IN

Abstract

Occurrence of syenite with the mineral assemblages: aegerine-augite-ferroedenite, brownish biotite and mesoperthite is described. From the available data of the associated rocks, it is concluded that the syenite intrusion is late and is associated with major EW linears.

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A. S. Janardhan ¹, Ph. Vidal ²

Affiliations
1 Department of Geology, University of Mysore, Manasa Gangotri, Mysore 570006, IN
2 Institut de Geology, Universite de Rennes, 35031, Rennes, FR

Abstract

No Abstract.

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A. S. Janardhan ¹, D. Gopalakrishna ¹

Affiliations
1 Department of Geology, Manasa Gangotri, Mysore 570006, IN

Abstract

Chemistry of co-existing mineral pairs of opx-cpx ; opx-gt and cpx-gt from several metabasic rocks consisting of two pyroxene assemblages and recrystallised ultramafics from Hullahalli and Dodkanya of Sargur terrain and Sathyamangalam, along with data taken from recent literature are utilized to estimate the Pressure-Temperature conditions of metamorphism. Temperature estimates are around 767±50°C and average pressures of 8-9 kbs indicate the peak of metamorphism which operated in the Sargur terrain. This suggests a crustal thickness of 30-35 krn, which was attained prior to the onset of granulite facies metamorphism.

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A. S. Janardhan ¹, Robert A. Wiebe ²

Affiliations
1 Department of Geology, Manasa Gangotri, Mysore 6, IN
2 Department of Geology, Franklin and Marshall College, Lancaster (Penn.), U. S. A, US

Abstract

Preliminary studies of the Oddanchatram anorthosite body, the largest of the bodies lying along a 100 km linear; on the northern slopes of Patni hilI ranges, show that it has all the chemical and mineralogical characters of Proterozoic massif anorthosites. Plagioclase compositions reveal two types: a) primary plagioclase of An₆₀ and b) secondary plagioclase with An₉₄ occurring as breakdown product of garnet. Estimates based on primary pyroxenes indicate intrusive temperatures between 960°C-1000°C; pressures of 5.3 kb. based on garnet breakdown products suggest a steep geothermal gradient. The chemistry suggests that the parental magma was unusually feldspathic; primary hornblende indicates a higher H₂O content of the magma.

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A. S. Janardhan ¹

Affiliations
1 Department of Geology, Manosa Gongotri, IN

Abstract

No Abstract.

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A. S. Janardhan ¹, N. Shadakshara Swamy ², R. Capdevila ³

Affiliations
1 Department of Geology, University of Mysore, Mysore 570006, IN
2 Department of Geology, Bangalore University, Bangalore 560056, IN
3 Institute of Geology, University of Rennes, 35031-Rennes, FR

Abstract

High-grade Archaean Sargur supracrustals are made up of quartzites, pelites, carbonates, BIF and manganiferous horizons and have been metamorphosed to upper amphibolite facies. The BIF and manganiferous horizons form thin but persistent bands. Iron-manganese-carbonate association is characteristic. Ten samples of BIF and spessartine-rich manganiferous horizons were analysed for major, trace and rare earth elements. Low trace and REB abundances characteristic of these bands agree well with the data of Archaean iron-formations elsewhere. Chemistry of Sargur BIF is distinctly different from that of the Algoma and Superior types. The positive Eu anamoly shown by the iron-formation indicates oxygenic conditions. The low content of some trace elements indicates shallow water environment of deposition. The REE concentrations of manganiferous-horizons are relatively higher than that of the BIF with anomalous behaviour of Ce and Eu. The present data show that the iron and manganese separation had taken place even in the Early Archaean.

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B. Manjunatha Reddy ¹, A. S. Janardhan ¹

Affiliations
1 Department of Geology, University of Mysore, Manasagangotri, Mysore 570 006, IN

Abstract

The optical and chemical characters of litanoclinohumite occurring in the scrpentinised dunites of Salem are described. The association of the mineral with serpentine veins and mobilised veins points to its secondary origin. Temperature of titanoclinohumite formation is estimated around 450-500°C based on indirect approach of olivine-spinel thermometry.

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A. S. Janardhan ¹, N. Shadakshara Swamy ², R. Capdevila ³

Affiliations
1 Department of Geology, University of Mysore, Mysore 570 006, IN
2 Department of Geology, Bangalore University, Bangalore 560 056, IN
3 Institute of Geology, University of Rennes, Rennes, FR

Abstract

Metapelites form one of the important supracrustal lithounits of the quartzite-pelite-carbonate association in the Sargur high-grade terrain of Southern Karnataka. These pelites have low ferromagnesium trace elemental concentrations, common to the pelites of the Archaean high-grade terrains elsewhere. Though concentration of Sc and U are comparable with the Archaean rocks, the Th concentration and majority of the trace and rare earth elemental ratios are more akin to post-Archaean levels. The REE pattern indicates a mixed provenance dominated by acid igneous rocks. Significant variability in the REB pattern advocates a lithologically complex but definitely a bimodal provenance and also inefficient mixing of source rocks.

Keywords

High-Grade Terrain, Karnataka, Pelite, REE, Sargur Supracrustal, Trace Element, Geochemistry.

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Saleem Ahmed Khan ¹, A. S. Janardhan ²

Affiliations
1 2348/1, M. H. Road, Mysore 570021, IN
2 Department of Geology, Manasagangotri, Mysore 570006, IN

Abstract

Field and mineralogical studies, supplemented by chemical analyses of charnockite and its derivative gneisses from one of the quarries between Ramalingapuram and Seshanchavadi along Attur valley have revealed retrogression of charnockite to biotite gneiss and onto ankerite-dolomite-bearing gneiss, involving addition of potassium and loss of iron, magnesium and sodium. The presence of two kinds of plagioclase and biotite in the retrogressed gneiss is indicative of chemical inequilibrium with a tendency to recrystallize into a lower grade mineral assemblage. The retrogression could have taken place at about 350°C and a pressure of 4 kb, with the influx of fluids richer in H₂O and CO₂. The co-existence of biotite and muscovite also requires fluids richer in H₂O, and CO₂ to a limited extent. The CO₂- and H₂O rich fluids are probably linked to major carbonatite-alkaline syenite intrusions emplaced along NNE and ENE lineaments.

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P. Sivasubramanian ¹, R. Natarajan ², A. S. Janardhan ¹

Affiliations
1 Department of Geology, University of Mysore, Mysore 570006, IN
2 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

A new find of sapphirine-bearing granulite rocks from Perumalmalai area, 15 km from Kodaikanal is reported. These rocks, contain in addition to sapphirine, cordierite-garnet-orthopyroxene-spinel-biotite-plagioclase. Among the pairs, garnetcordierite gives pressure of 8.5 kb at temperature of 550°C; garnet-orthopyroxene gives 6 kb at 800°C and cordierite-garnet-sapphirine gives 7.2 kb at 700°C. The sapphirineforming reaction probably took place after the main charnockite event. Evidences of decompression of garnet breaking down to orthopyroxene-spinel-plagioclase-biotite at rather high temperatures indicates rapid uplift.

Keywords

Sapphirine, Perumalmalai, Palani Hills, Tamil Nadu, Geobarometry, Granulites.

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A. S. Janardhan ¹, M. Jayananda ², M. A. Shankara ¹

Affiliations
1 Department of Geology, University of Mysore, Manasagangothri, Mysore-570006, IN
2 Department of Geology, Bangalore University, Bangalore 560 056, IN

Abstract

The granulites of the Biligiri Rangan (BR) and Niligiri Hill ranges are characterised by distinct field, geochemical and isotopic signatures. The B.R. Hill granulites contain a significant metasedimentary component (pelite-BIF-Mn-horizons), whilst metasediments are rare in the Niligiri granulite terrain. Lithologic and structural continuity between the two terrains cannot be demonstrated as they are separated by E-W running Moyar shear zone. Isotopic data suggest that B.R. Hills and Niligiri granulites have had different pre-crustal histories. Sm-Nd model TDM ages and U-Pb zircon concordia upper intercept age suggest that the protoliths of B.R.Hill quartzo-feldspathic charnockites accreted at 3.4Ga. On the other hand, Sm-Nd model TDM ages indicate that the magmatic protoliths of Niligiri granulites differentiated from mantle at 2.6 Ga.

Geochemical data from the B.R. Hills charnockites, particularly low-Mg numbers (0.2-0.3), low content of transition and HFS elements and strongly fractionated REE patterns with HREE depletion, suggest that their magmatic protoliths were derived by partial meltingof young, hydrated oceanic crust, in which garnet and amphibole were important residual phases. The high content of transition elements together with low total LREE and less-or unfractionated REE patterns of the B.R. Hills mafic granulites imply the derivation of their protoliths from long term depleted mantle sources. On the other hand the high Mg numbers (0.5-0.6), high content of transition and HFS elements, moderately fractionated REE patterns with HREE enrichment suggest that the protoliths of the Niligiri granulites are interpreted to have formed by melting of metasomatized mantle wedge with olivine and spinel as residual phases.

The Niligiri terrain was accreted onto the Dharwar Craton at C.2.6 Ga and the two terrains witnessed a common major granulite event at 2.51 Ga.

Keywords

Granulites, Isotopes, Geochemistry, Tectonics, B. R. Hills, Niligiri Hills, Karnataka, Tamil Nadu.

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N. S. Krishnaraj ¹, A. S. Janardhan ¹, B. Basavalingu ¹

Affiliations
1 Department of Geology, University of Mysore, Manasagangotri, Mysore-570 006, IN

Abstract

Garnet-kyanite-sillimanite bearing pelitic and garnetiferous mafic granulites exposed at several places along the Jalsoor-Mercara and Bavali shear zones preserve evidences of deep burial of atleast 30 km. Estimation of P-T conditions of metamorphism of these granulites gives T of 725-800°C and P of 7 to 8.6 kb. Increase of pressure form core to rim in the garnet-kyanite assemblages and kyanite mantling garnet are the most significant aspects of these lithologies, suggesting long residence in the deep crust and possible thrust tectonics.

Moreover, fibrolitic sillimanite overgrowing kyanite in pelites and the developement of opx-calcic plag symplectites around garnet in the mafic rocks indicate decompression/uplift, subsequent to crustal thickening. This uplift could be part of a later event unrelated to the burial episode.

Significantly, syenite and anorthosite bodies of late Proterozoic age occur along these shear zones indicating that these lineaments were reactivated again in late Proterozoic times.

Keywords

Granulites, Jalsoor-Mercara, Metamorphic Petrology, Kanataka.

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Fareeduddin ¹, M. A. Shankara ², B. Basavalingu ², A. S. Janardhan ²

Affiliations
1 H.408, Ramesh marg, C Scheme, Jaipur 302 001, IN
2 Department of Geology, Univeisity of Mysore, Manasagangotri, Mysore-570 006, IN

Abstract

A brief account of the petrology and structure of medium grade pelitic granulites and associated charnockites occurring as slivers in the Anasagar migmatitic gneiss is given. These granulite slivers have been traced along strike up to 40 km and may cover a much wider area. Cordierite bearing pelitic granulites associated with charnockites around Chinwali were chosen for estimating metamorphic P-T conditions. The pelitic granulites are represented by the assenlblage opx-cord-gt-bio--silli-plag-k felds- qtz. Biotite-fibrolite intergrowths grow on cordierite which has opx inclusions. Garnet is often smeared with graphite. Metamorphic conditions of petite show peak pressures of 7.5 kb at 640°C. Charnockites generally record retrogressive pressure conditions of 5.7 - 4.0 kb. The mineralogy of the Chinwali granulites and P-T condition compare well with that of Sandmata granulites. The presence of these granulites, north of Ajmer, in basement migmatitic gneisses suggest the presence of an older deep crustal fragment (ca. 1.7 Ga) along the northwestern margin of the Aravalli Delhi Mobile Belt (ADMB) and belies the prevailing concepts of eastward crustal thickening and westward age polarity in the western Indian shield.

Keywords

Granulite, Charnockite, Petrology, Ajmer, Chinwali, Rajasthan.

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J. Devaraju ¹, A. S. Janardhan ²

Affiliations
1 Department of Geology, University of Mysore, Manasagangotri, Mysore - 570 006
2 Department of Geology, University of Mysore, Manasagangotri, Mysore - 570 006, IN

Abstract

No Abstract.

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K. Sriramguru ¹, A. S. Janardhan ¹, Sharana Basava ¹, B. Basavalingu ¹

Affiliations
1 Department of Geology, University of Mysore, Manasa Gangotri, Mysore - 570 006, IN

Abstract

Sapphirine and kornerupine bearing assemblages in Mg-AI- rich pelites have been reported in and around the Kodaikanal hills and in the Kambam valley, forming part of the 550 Ma old Southern Granulite Terrain (SGT). Prismatine and sapphirine are found in a new association with garnet-orthopyroxene-cordierite-spinel-plagioclase-perthite bearing pelitic enclaves in granitic bodies intruding the charnockites at Rajapalaiyam, located on the southeastern slopes of the Varushanad hills and east of the Kambam valley. Prismatine with boron > 0.5 apfu is either associated with orthopyroxene or armoured by cordierite, suggesting its early formation. Association of prismatine with orthopyroxene and quartz is significant. Boron for the formation of prismatine must have come from the sedimentary protoliths, and fixed in the prismatine during fluid activity connected with earlier granulite grade metamorphism. Sapphirine bearing assemblages occur as mm-sized specks in garnetiferous granite close to the pelitic bands. This type of sapphirine is different from the general occurrence of sapphirine-orthopyroxene-cordierite forming as a breakdown product of garnet in SGT. Sapphirine bearing assemblages of SGT generally have formed during high grade, granulite facies metamorphism, with peak at 850-1000°C at P of 8.2-8.8 Kbar and with a dominant isothermal decompression path as shown by the assemblages orthopyroxene-cordierite+-plagioclaswe hich give T of 650°C at P of 6.5 Kbar. Orthopyroxene-cordierite association after garnet gives T of 900°C at P of 7.5 Kbar in Rajapalaiyam area.

Keywords

Prismatine, Sapphirine, Boron, Fluorine, Southern Granulite Terrain, Tamil Nadu.

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A. S. Janardhan ¹

Affiliations
1 Department of Geology, University of Mysore, Manasa Gangotri, Mysore - 570 006, IN

Abstract

No Abstract.

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