Metamorphic conditions in the Nilgiri Granulite Terrane and the adjacent Moyar and Bhavani Shear Zones: A reevaluation

Data were presented on pressure and temperature determinations from the Nilgiri Hills. About 70 samples were analyzed by probe and several calibrations of garnet-pyroxene thermometry and barometry applied. Most calibrations gave considerable scatter; however, a new calibration by Bhattacharya, Raith, Lal, and others, accounting for nonideality in both garnet and orthopyroxene, gave consistent results of 754 + or - 52 C and 9.2 + or - 0.7 kbar. On the regional scale, a pressure increase of 6.5 to 7 kbar in the SW to 11 kbar in the NE was related to block tilting. A continuous pressure gradient into the Moyar shear zone suggests that the zone is not a suture juxtaposing unrelated blocks

Gneiss-charnockite transformation at Kottavattam, Southern Kerala (India)

At Kottavattam, leucocratic granitic garnet-biotite gneisses (age less than 2 Ga) were partially transformed to coarse-grained charnockite along a system of conjugate fractures (N70E and N20W) and the foliation planes (N60 to 80W; dip 80 to 90 SW) about 550 m.y. ago. To examine and quantify changes in fabric, mineralogy, pore fluids and chemical composition associated with this process, large rock specimens showing gneiss-charnockite transition were studied in detail. The results of the present study corroborate the concept that charnockite formation at Kottavattam is an internally-generated phenomenon and was not triggered by the influx of carbonic fluids from a deep-seated source. It is suggested that charnockitization was caused by the following mechanism: (1) near-isothermal decompression during uplift of the gneiss complex led to an increase of the pore fluid pressure (P sub fluid greater than P sub lith) which - in a regime of anisotropic stress - triggered or at least promoted the development of conjugate fractures; (2) the simultaneous release of pore fluids from bursting fluid inclusions and their escape into the developing fracture system resulted in a drop of fluid pressure; and (3) the internal generation and buffering of the fluids and their, probably, limited migration in an entirely granitic rock system explains the absence of any significant metasomatic mass transfer

Syn-metamorphic carbonic inclusions in the Satnur-Halgur granulites, Karnataka, India

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

Fluid inclusions in migmatites from Hunsur, Karnataka

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

Oxygen and Carbon Isotopic Composition of Precambrian Carbonates from Karnataka and Tamilnadu, India

Oxygen and carbon isotopic compositions for carbonate rocks from the Sargur Group in Karnataka and carbonates from the Madukkarai area in Tamil Nadu am reported. Carbonate rocks from both areas show preservation of sedimentary isotopic compositions. Bettadabidu carbonates have lower isotopic ratios (delta O-18 = 15.8 SMOW ; delta C-13 = - 0.7 PDB) when compared to Madukkarai carbonates (delta O-18 = 20 SMOW; delta C-13 = 2.0 PDB). Oxygen and carbon isotopic values exhibit positive correlation indicating a progressive devolatilization process during regional metamorphism. The range in delta O-18 values demonstrates that large quantities of a homogeneous metamorphic fluid have not pervasively infiltrated either terrane

Synmetamorphic High-Density Carbonic Fluids in the Lower Crust: Evidence from the Nilgiri Granulites, Southern India

High-density CO2 inclusions occur abundantly in granulite facies rocks (age of metamorphism approximately 2.5 b.y.) of the Nilgiri massif, southern India. The chronology of carbonic inclusions in the widespread enderbitic granulites studied in relation to the development of micro-textures and mineral assemblages indicates that randomly oriented, negative-crystal-shaped CO2 inclusions (4-20-mu-m) in garnet and quartz grains (qtz I) armoured by garnet entrap syn-peak-metamorphic pore fluids. The more abundant trail-bound CO2 inclusions in the deformed, polygonized, and partially recrystallized matrix quartz grains (qtz II and III) and plagioclase grains were formed in connection with a stage of compressional deformation and subsequent annealing related to the development of the late-Proterozoic Bhavani shear zone. These inclusions resulted from local re-equilibration of the former peak-granulitic carbonic inclusions and re-entrapment of released fluids. The presence of pure CO2 in all the inclusions is confirmed by microthermometric data and laser-excited Raman microspectrometry. Temperatures of homogenization (liquid phase) are in the range of -50 to +20-degrees-C, and the corresponding CO2 densities are between 1.154 and 0.807 g/cm3. Mineralogical thermobarometry on the enderbitic granulites documents a continuous gradient of near-peak metamorphic conditions from 750-degrees-C, 9-10 kb in the northern part to 730-degrees-C, 7 kb in the southwestern part of the Nilgiri massif. Uniform P, T estimates (600-650-degrees-C, 6-7 kb) for late coronitic garnet + quartz assemblages in enderbites and metadolerites indicate that differential uplift of the massif to mid-crustal levels was accomplished before late compressional deformation. In conformity, carbonic inclusions in quartz II and III are characterized by uniformly high density (1.154-1.08 g/cm3). In contrast, early carbonic inclusions in garnet and quartz I preserve the density contrast reflecting the regional P, T gradient during near-peak metamorphic fluid entrapment. The fluid inclusion systematics indicate `near-isochoric; uplift of the northern high-P domain, but near-isobaric cooling of the southwestern low-P domain. The carbonic fluids are thought to have been derived either from internal sources during dehydration-melting processes or from freezing synmetamorphic intrusives into the lower crust

Spinifex-textured peridotitic komatiite from Karighatta, Chitradurga schist belt, Karnataka

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

Highly saline fluid inclusions in Chamundi granite, South India

Fluid inclusions in minerals and rock represent a potentially valuable source of information about the composition and density of fluids present during the formation and evolution of rocks. Our preliminary studies on Chamundi granite have indicated presence of highly saline inclusions (up to 50-60 wt.% Nacl equivalent) as well as low-salinity inclusions (8 to 22 wt.% NaCl equivalent). Data on temperture of homogenization suggest that they represent remnant fluids of magmatic origin trapped in minerals

High-grade regional metamorphism of ultramafic and mafic rocks from the Archaean Sargur terrane, Karnataka, South India

Deformed and metamorphic ultramafic to mafic rocks emplaced into the Archaean Sargur supracrustal series (>3.0 Ga) in Karnataka, southern India, represent layered igneous bodies. The terrane has been affected by several episodes of deformation and metamorphism in the time span from 3.4 to 2.5 Ga ago. During the regional metamorphism about 2.5 Ga ago the igneous bodies re-equilibrated partly or completely at conditions of the upper amphibolite to granulite facies. The development of sagvandites with enstatite + magnesite and anthophyllite + magnesite-bearing assemblages, and of mafic garnet-pyroxene charnockites indicates the presence of CO2-rich intergranular fluids (XCO2 ≥ 0.5) in these rocks during metamorphism. The physical conditions of metamorphism have been estimated by applying methods of geothermobarometry to the recrystallized ultramafic assemblages with olivine, pyroxenes and spinel and to the charnockitic assemblages with garnet, pyroxenes, plagioclase and quartz. A best temperature estimate of 700 ± 30°C was derived with the geothermometers of Evans and Frost (Ol-Spi), Fabriès (Ol-Spi), Wells (Opx-Cpx), Powell (Opx-Cpx), Ellis and Green (Gra-Cpx), Lal and Raith (Gra-Opx), and Danckwerth and Newton (Al2O3-content in opx). A mean pressure estimate of 8.6 ± 0.8 kbar has been obtained with the models of Perkins and Newton (Gar-Opx/Cpx-Plag-Qtz). The P-T data indicate a minimum crustal thickness of about 35 km at c. 2.5 Ga in this part of the South Indian Archaean craton