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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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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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C. Srikantappa ¹, C. R. L. Friend ²

Affiliations
1 Department of Geology, University of Mysore, Manasagangotri, Mysore 570005, IN
2 Department of Geology and Physical Sciences, Oxford Polytechnic, Headington, Oxford, OX3 OBP, GB

Abstract

The occurrence of chromian edenite, confined to the degraded chromitite layers of the Sinduvalli ultramafic body is reported. Field, petrographic and chemical studies suggest that its development has occurred during the prograde metamorphism of the layered body. The enrichment of silicate phases in chrome-amphibole and chloritesis attributed to re-equilibration of chromite during the metamorphic episodes affecting the area and the formation of 'ferritchromit'.

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C. Srikantappa ¹

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

Abstract

Ultramafic rocks occurring within the high-grade schistose rocks from the Sargur region show evidences of recrystallization. A metamorphic temperature of the order of 700± 20°C is estimated based on coexisting olivine-spinel and orthopyroxene clinopyroxene mineral chemistry.

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C. Srikantappa ¹, M. Raith ², B. Spiering ²

Affiliations
1 Department of Geology, University of Mysore, Manasa Gangotri, Mysore 570006, IN
2 Mineralogisch-Petrologisches Institut, Universitat Bonn, Poppelsdorfer Schloss, 5300 Bonn, Fed Rep, DE

Abstract

The crustal segment south of the Achankovil zone in southern Kerala is built up by two litho-tectonic units: The Nagerkoil unit is represented by a series of acid to intermediate magmatogenic gneisses and charnockites associated with layered anorthositic to noritic complexes. The Pon Mudi unit consists of interlayered garnet-biotite gneisses (leptynites), garnet-biotite-sil1imanite gneisses (khondalites) and garnet-biotite-sillimanite-cordierite metatexites which represent a series of intensely deformed psammitic and pelitic sediments metamorphosed to upper amphibolite grade (700-750°C; 6-8 k bars). Subsequent to regional metamorphism in the POD Mudi unit, the leptynites have been partly transformed to massive orthopyroxene-garnet-bearing charnockites along a conjugate set of fractures (s1: N30E ilDd s2: S70E) and the foliation planes of the rocks (N30-60W).

The geochemical data and the results of geothermobarometry show that charnockitization was essentialJy isochemical and occurred at 750 ± 50°C and 6 ± 1 kbars lithostatic pressure. A complex but conformable development of tbe fluid phase composition during metamorphism is indicated by the occurrence of at least four generations of fluid inclusions (almost pure H₂O and CO₂ inclusions, mixed CO₂-H₂O inclusions and CH₄-N₂ inclusions) in both the leptynites and the charnockites. The common presence of graphite+pyrrhotite+ilmenite furthermore suggests internal buffering of the fluid composition at oxygen fugacities below those defined by the quartz-magnetite-ferrohypersthene assemblage and XCO₂>0.5.

The results of tbe present study indicate that charnockitization probably was not caused by the influx of CO₂-rich fluids of deep-seated origin, tbe presently favoured model of granulite genesis (Newton, 1984), but rather was induced by an isothermal decrease of fluid pressure relative to lithostatic pressure, due to migration of the pore fluid into the network of fractures and ascent into higher crustal levels.

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C. Srikantappa ¹, M. Raith ², B. Spiering ²

Affiliations
1 Department of Geology, University of Mysore, Mysore 570006, IN
2 Institute for Mineralogy and Petrology, University of Bonn, Poppelsdorfer Schloss, Bonn, 5300, West Germany, DE

Abstract

No Abstract.

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C. Srikantappa ¹

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

Abstract

Charnockites from the Nilgiri Hills show presence of abundant high-density carbonic (1.05 -1.10 g/cm³) inclusions. The pressure-temperature conditions recorded by fluid inclusions correspond with the P-T data obtained from mineral assemblages (T = 770 ± 60°C, P = 8-9 Kbar) indicating their entrapment near the peak metamorpbic conditions.

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G. R. Ravindra Kumar ¹, C. Srikantappa ²

Affiliations
1 Centre for Earth Science Studies, P. B. No. 7250, Akkulam, Trivandrum 695031, IN
2 Department of Geology, University of Mysore, Mysore 570006, IN

Abstract

The mineral reactions: garnet + quartz = orthopyroxene + plagioclase; garnet + clinopyroxene = orthopyroxene + pargasite + anorthite and [clinopyroxene = hornblende + quartz are seen as symplectitic intergrowths in the mafic granulites of north Kerala, suggesting a retrogressive evolutionary history. The estimate of physical conditions of metamorphism based on the equilibrium mineral chemistry of gar + cpx (+ plag + q + z ), suggest 800°C temperature and 11 kb pressure for the peak period of metamorphism. Uplift of the terrain, following metamorphism caused partial breakdown of garnet in the presence of clinopyroxene and quartz in producing retrogressive textures and mineral assemblages. P-T of 6 kb and 583°C is indicated for tbe development of syrnplectite of garnet and secondary orthopyroxene. The pressures document slow uplift from inferred depth of at least 30 krn during Archaean to a level of 20 km for the development of symplectites. The mineral chemistry and P-T suggest that the retrogressive metamorphism did not transcend but remained 'within' the transitional granulite facies conditions.

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C. Srikantappa ¹

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

Abstract

Peak-metamorphic carbonic pore fluids have been observed in granulite facies rocks of the Satnur-Halgur area. The CO₂ densities (1.045 to 0.900 g/cc) are consistent with the P-T metamorphic conditions, indicating syn-metamorphic nature of carbonic inclusions.

Keywords

Carbonic Inclusions, Granulites, Satnur-Halgur, Karnataka.

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Syed Ashfaq Ahmed ¹, A. M. Pathan ¹, C. Srikantappa ²

Affiliations
1 Department of Geology, Bangalore University, Bangalore 560056, IN
2 Department of Geology, Mysore University, Mysore 570006, IN

Abstract

A small and economically unimportant molybdenite mineralization is found in the Peninsular gneisses around Yegawakote. Pyrite and chalcopyrite are the other ore minerals found associated with molybdenite. These minerals are found as fracture fillings in quartz veins.

Fluid inclusion studies on quartz carrying ore minerals reveal the presence of carbonic, through CO₂-H₂O, to entirely pure aqueous saline fluids. Microthermometric data show that most of the aqueous inclusions are in the salinity range of 4 to 15 wt%. NaCl have formed at temperatures of around 200-370 °C.

Keywords

Molybdenite, Fluid Inclusions, Homogenization, Salinity, Yegawakote, Karnataka.

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C. Srikantappa ¹, B. N. Shankar ², V. N. Vasudev ³

Affiliations
1 Department of Geology, Manasagangotri, University of Mysore, Mysore 570 006, IN
2 Department of Mines and Geology, Krishnamurthypuram, Mysore, 570 004, IN
3 Department of Mines and Geology, Lalbagh Road, Bangalore 560 004, IN

Abstract

Ultramafic rocks of the Karighatta schist belt display spinifex texture and komatiitic chemistry indicating their volcanic origin.

Keywords

Paidotitic Komatiite, Karighatta Schist Belt, Karnataka.

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C. Srikantappa ¹, John W. Valley ²

Affiliations
1 Department of Geology, Manasagangotri, University of Mysore, Mysore, 570007, IN
2 Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, US

Abstract

Oxygen and carbon isotopic compositions for carbonate rocks from the Sargur Group in Kamataka and carbonates from the Madukkarai area in Tamil Nadu are reported. Carbonate rocks from both areas show preservation of sedimentary isotopic compositions. Bettadabidu carbonates have lower isotopic ratios (δ¹⁸0 = 15.8 SMOW ; δ¹³C = ·0.7 PDB) when compared to Madukkarai carbonates (δ ¹⁸0 = 20 SMOW; δ ¹³C = 2.0 POB). Oxygen and carbon isotopic values exhibit positive correlation indicating a progressive devolatilization process during regional metamorphism. The range in δ¹⁸0 values demonstrates that large quantities of a homogeneous metamorphic fluid have not pervasively infiltrated either terrane.

Keywords

Precambrian Carbonates, Oxygen and Carbon Isotopes, Kamataka, Tamil Nadu.

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C. Srikantappa ¹, L. Venugopal ¹, J. Devaraju ¹, B. Basavalingu ¹

Affiliations
1 Department of Geology, Manasagangotri, University of Mysore, Mysore 570006, IN

Abstract

Charnockites and basic granulites occur extensively around Coorg in Western Ghats. Minor metasedimentary rocks like kyanite/sillimanite bearing gneisses and ironstone also occur. A set of late dolerite dykes cross cut all the lithologies.

The Coorg granulites record a near-peak metamorphic conditions of 720 to 760° C at pressures of 7 lo 8.6 kb. Charnockites and basic granulites contain high-density ( 1.07 to 1.09 g/cc) carbonic inclusions. Micro-textures, mineral P-T and fluid inclusion data indicate "clockwise" P-T trajectory, pointing to a relatively rapid uplift/erosion of the Coorg granulites.

Keywords

Granulites, Thermobarometry, Fluid Inclusions, Igneous Petrology, Coorg District, Karnataka.

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G. R. Ravindra Kumar ¹, C. Srikantappa ²

Affiliations
1 Centre for Earth Science Studies, Akkulam, Trivandrum - 695 031, IN
2 Department of Geology, Manasagangotri, Mysore - 570 006, IN

Abstract

Exposures of in-situ charnockitisation of gneisses in widely varying geological associations, related to differing mechanisms of dehydration and prograde metamorphism, are recorded from the Palghat region, southern India. Mineral reactions in gneiss-charnockite prograde profiles indicate hornblende + biotite breakdown in the presence of quartz to produce orthopyroxene within structurally controlled and/or compositionally modified portions of the gneisses. Chemically the gneisses and arrested charnockite zones are granitic in composition whereas the associated banded charnockites (regional granulites) are tonalitic to granodioritic in composition, Geothermobarometric data indicate high temperature and high pressure early granulite-facies (regional granulite) metamorphism at 780 - 880°C and 9-10 Kbar. The fluid species in both types of charnockites and gneisses are characterised by CO, and mixed CO₂-H₂O inclusions.

The banded charnockites largely pre-date major deformation and the early Palaeozoic time of development of arrested charnockites. A definite time gap between the formation of banded charnockites and arrested charnockites is indicated. Arrested charnockites in the Palghat region are formed along (1) structurally controlled zones, (2) in compositionally favourable zones of modified paleosomes, (3) in melt portions of migrnatitic gneisses and (4) along margins of invading alkaline dykes. These are interpreted as different processes triggered by emplacement of igneous bodies and movement of CO₂, enriched melts through the crust, producing sequential interrelated events like dehydration, partial melting, metasomatism and wall-rock alteration, leading ultimately to charnockite formation. The present field dispositions of arrested charnockite are inferred as only an expression of varying response of rock types to changing physico-chemical conditions (e.g. PT and fluid regime) with time during metamorphism.

Keywords

Charnockite, Metamorphism, Patghat, Kerala.

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M. N. Malathi ¹, C. Srikantappa ¹

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

Abstract

The late Archaean Nilgiri enderbitic to charnockitic granulites (ECG) show evidence of ductile, ductile/brittle shear deformation and retrograde metamorphism, along the Bhavani Shear Zone (BSZ) and Malapuram-Gudalur Shear Zone (MGSZ) around Nilambur, Kerala. Biotite-hornblende gneiss, the common rock types in shear zones, represent retrogressed enderbitic to chamockitic granulites related to ductile/brittle and brittle shearing during Neo-Proterozoic time. Numerous Gold-Quartz veins of varying dimensions, often exhibiting sharp contact, occur either parallel or cross cut the regional foliation in the host biotite-hornblende gneiss. Gold occur as disseminated grains as well as invisible gold locked in sulphides.

Fluid inclusion studies of auriferous quartz veins have revealed presence of low to moderate density carbonic inclusions (0.98 to 0.78 g/cc), mixed C0₂-H₂O inclusions and low salinity aqueous inclusions. In contrast, ECG contain high density carbonic inclusions (1.05 to 1.03 g/cc). Moderate density carbonic inclusions (0.90 to 0.78 g/cc) with few mixed CO₂-H₂O inclusions characterize the Biotite-Hornblende gneiss.

Fluid inclusion chronology is established based on Micro-Textures and entrapment of C0₂-rich fluids of varying densities in minerals like garnet, plagioclase and quartz making use of the concept of Group of Synchronous Inclusions (GSI) and Trail Bound Fluid Inclusions (TBFI). High density C0₂-rich inclusions (1.03 to 1.05 g/cc) in porphyroblastic garnet and plagioclase are the earliest fluid inclusions recorded in granulites. During shear deformation and retrograde alteration of granulites, the early high density carbonic fluids, were released due to change in fluid pressure (Pf), related to shear deformation and have been trapped as low density C0₂-rich fluids (0.09 to 0.80 g/cc) in biotite-hornblende gneiss. The low density C0,-rich fluids (0.70 to 0.88 g/cc) in Gold-Quartz veins are interpreted to have been derived from metamorphic fluids, originated during shear deformation and retrograde metamorphism of granulites. These fluids have migrated along shears and fractures in Biotite-Hornblende gneiss. Gold was leached from granulites and transported as carbonyl complexes to upper crustal level and into the vein system. Mixing of low density gold bearing carbonic fluids of deep seated origin (derived from granulites) with low salinity aqueous fluids (derived from Pan- African granites) has destabilised the gold bearing complexes, leading to remobilisation and deposition of gold in quartz veins. Gold mineralization around Nilambur occurred at a late stage, during retrograde metamorphism (around 2- 3 kb and 300 to 450°C, M2), when compared to regional granulite facies metamorphism (7 to 8 kb and 750 to 800°C, MI) and is related to uplift of granulites along an ITD path. Gold mineralization took place during Pan-African event as a result of remobilisation caused by the pink alkali granite.

Keywords

Gold, Fluid Inclusions, Bhavani Shear Zone, Nilambur, Kerala.

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K. J. Suresha ¹, C. Srikantappa ¹

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

Abstract

Chamockites from the Southern Granulite Terrane were considered to be product of regional high-grade metamorphism, aided by C0₂-rich and alkaline fluids. Here, we report the occurrence of intrusive charno-enderbites and charnockites around Dindigul, which consist of pyroxenes, plagioclase, alkali feldspar, quartz, biotite, titanomagnetite, ilmenite and apatite. These rocks are characterized by higher abundance of K₂O, TiO₂, and P₂O₅, and lower contents of CaO with lower m* for a given SiO₂ content, relative to metamorphic charnockites from the Biligiri Rangan and Nilgiri granulites, and have the geochemical similarities with the Ardery charnockitic intrusions from Antarctica.

Keywords

Igneous charnockites, C-type magmas, Geochemistry, Dindigul, Tamil Nadu.

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C. Srikantappa ¹, H. M. Ramachandra ²

Affiliations
1 Department of studies in Geology, University of Mysore, Mysore, IN
2 AMSE Wing, Geological Survey of India, Bangalore, IN

Abstract

No Abstract.

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Subhash Sukumaran ¹, G. R. Ravindra Kumar ¹, C. Srikantappa ²

Affiliations
1 Centre for Earth Science Studies, Akkulam, Trivandrum - 695 031, IN
2 Department of Studies in Geology, University of Mysore, Mysore - 560 006, IN

Abstract

The Palghat region in southern India presents excellent examples of fluid controlled gneiss to charnockite transition zones. The paper constitutes results of detailed fluid inclusion studies carried out to understand the nature and role of fluids, across transition zones of such transformations and in associated charnoenderbite and pegmatite. Fluid inclusion petrographic studies suggest presence of at least three generations of fluid inclusions in different rock types viz , CO₂, CO₂-H₂O, and CO₂-H₂O-NaCl. The high-Density CO₂ inclusions (1 03 to 0 91 g/cc) are characteristics of charnoenderbites with moderate density CO₂ inclusions(0 92 to 0 70 g/cc) in hornblende gneiss and relatively low CO₂ density( 0 84 to 0 71 g/cc) inclusions in arrested charnockites. Salinities range from 2 to 35 wt% NaCl equivalent. Majority of the inclusions are low-Density CO₂, indicating preferential leakage and re-Entrapment of these inclusions under shallower conditions. In charnoenderbites, a distinct bi-Modal distribution in CO₂ densities is documented with early high-Density CO₂ inclusions, trapped under higher pressure, deep seated conditions and a late, low-Density CO₂ inclusions trapped under relatively shallower conditions. Occurrence of generally low-Density CO₂ inclusions in the arrested charnockites suggest release and re-Entrapment of CO₂ inclusions along structurally controlled weak zones. The higher concentration of low-Density monophase CO₂ inclusions in the arrested charnockites and pegmatite are interpreted as a result of selective removal of H₂O from the mixed CO₂-H₂O inclusions, during ductile and ductile-Brittle shear deformation. The CO₂ inclusions showing large density differences between the early charnockites (charnoenderbite) and late (arrested) charnockites are suggestive of entrapment at different periods and that the CO₂ densities of the charnoenderbite have been re-Equilibrated during the latest tectono-Metamorphic event. The fluids responsible for the formation of arrested charnockites are low P-T type late fluids, formed at a shallower, mid crustal levels. The study further illustrates major role of saline brines in the formation of arrested charnockites.

Keywords

Fluid Inclusion, Charnockites, Brines, Palghat, Southern India.

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C. Srikantappa ¹, L. Venugopal ¹

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

Abstract

Fluid inclusion studies in migmatites around Hunsur document the presence of a) CO₂-CH₄, CO₂ and CO₂-H₂O inclusions in paleosome, b) predominantly CO₂ inclusions in leucosome and c) CO₂ and low salinity aqueous inclusions in late pink granite.

Chronological studies of CO₂ inclusions in quartz grains from migmatites and their densities corresponding to Th maxima plus mineral P-T data for the regional metamorphism indicate that they are syn-migmatitic fluids. An internal origin of CO₂ inclusions during the process of migmatization is envisaged.

Keywords

Fluid Inclusions, Petrography, Migmatites, Karnataka.

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