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

V. Ravikant ¹, Amitava Kundu ¹

Affiliations
1 Antarctica Division, Geological Survey of India, NH-5P, N.I.T., Faridabad - 121001, IN

Abstract

A retrograde path inferred for the Schirmacher Hills granulites involves near ITD path following post-peak granulite facies metamorphism (M1/D1), preserved only in enclaves, followed by near IBC path, M3, post dating M2/D2 granulites facies metamorphism and deformation. Final uplift and cooling of the terrain occurred in the upper amphibolite facies metamorphic conditions, M4, syntectonic with respect to D3 deformation and emplacement of large volume of syntectonic granitoids. Fluids released during cooling and crystallization of these granitoids probably caused the large scale retrogression of granulite facies rocks to amphibolite facies gneisses.

Keywords

Petrology, Granulites, Geothermobarometry, Antarctica.

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V. Ravikant ¹

Affiliations
1 Geochronology and Isotope Geology Division, Geological Survey of India, 15A&B Kyd Street, Kolkata - 700 016, IN

Abstract

Mafic and ultramafic enclaves from Schirmacher oasis preserve evidences for the earliest metamorphic event. In this study, this event has been constrained through reaction textures, mineral chemistry and themobarometry by application of conventional and multireaction methods. Mafic enclaves comprise melanocratic Pargasite-Bearing gabbro, gabbroic anorthosite and ultramafic enclaves are spinel- and Garnet-Bearing websterite, Ortho- and clinopyroxenite which occur as deformed enclaves within foliated norite. Deformed metanorite dykes and metagabbro pods occur within metasedimentary granulite. Most of the enclaves are massive, without chromite layering, while few preserve deformed modal layering at Centimeter-Scale. It is likely that the enclaves represent mantle restites during a partial melting event. Reaction textures within enclaves indicate a decompressive sense of the P-T path inferred from Garnet-Bearing websterite, deformed metanorite dyke and melanocratic Pargasite-Bearing gabbro, recorded by breakdown of blastic garnet to symplectites of orthopyroxene and highly calcic plagioclase and by the decomposition of pargasite in metagabbro to syrnplectites of pyroxenes and plagioclase. Thermobarometric calculations confirm that these enclaves were metamorphosed to Granulite-Facies conditions at -8, kbar and - 800°C, and underwent High-Temperature decompression to a Mid-Crustal level.

Keywords

Ultramafic And Mafic Granulites, Reaction Textures, Metamorphism, Schirmacher Oasis, East Antarctica.

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V. Ravikant ¹, Dipayan Guha ²

Affiliations
1 G&IG Division, Geological Survey of India, Kolkata - 700 016, IN
2 D.D.G.(CGL) Cell, Geological Survey of India, Kolkata - 700 016, IN

Abstract

We report here petrographic and bulk geochemical characters of an ultrapotassic dyke from the Ladakh batholith, NW Himalaya, for the first time, and attempt to compare it with similar dykes from SW Tibet which are typical of post-collisional magmatism. Postcollisional shoshonitic and ultrapotassic dykes have been reported from north Karakoram (Pognante, 1990) and potassic and ultrapotassic lavas from SW Tibet by Miller et al. (1999). They have been linked to large scale tectonic reactivation involving the lithospheric mantle during the continued India-Eurasia collision since ∼50 Ma.

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V. Ravikant ¹

Affiliations
1 Antarctica Division, Geological Survey of India, NH-5P, N.I.T., Faridabad-121001, IN

Abstract

The Gruber massif-type anorthosite, with marginal noritic anorthosite, represents a lopolithic magmatic intrusion within the orogen exposed in the central Dronning Maud Land (cDML). The meta anorthosite, emplaced during the Dlb deformation event into the adjacent charnockitic gneiss (which had undergone an initial deformation event (Dla) equilibrated along with the charnockitic gneiss during granulite facies conditions (M₁ = 787 ± 50°C, 7.4 ± 1kbar) followed by near isobaric cooling, M₂ event The gneiss and anorthosite were then deformed and partly retrogressed during a M₃ metamorphism (hydration during amphibolite facies conditions) accompanied by weak to moderate D₂deformation.

 

Cooling of the anorthosite pluton (using a parameterized model with high initial crystallinity, a lower crustal contact temperature of ∼800°C with an average radius of 17 km) from 1200 to 840°C would require c. 45 Ma, following which the cooling rate would fall to <1 °C Ma^-1. An initial transient elevated geotherm of interest which fits the extracted P,T conditions during the M₁metamorphism was calculated, simulating the equilibration of the interface. Next, cooling and uplift of this interface with realistic combinations of thermal properties of the crust, heat generation, low uplift rates and boundary conditions was made. Calculation of the cooling and uplift of the equilibrated anorthosite and chamockitic gneiss interface produces an initial near isobaric cooling path for c. 30 Ma at or near the depth of equlibration.

 

Using available geophysical data from the cDML orogen, the causes for the thermal perturbation during the granulite facies metamorphism are discussed and it is proposed that the granulite facies metamorphism was caused by a combination of crustal thickening and magmatic underplating during the collision (i.e. crustal thickening followed by extension) of the Kalahari (equivalent) Craton and a proposed East Antarctica Craton during the Grenvillian/Kibaran event.

Keywords

Petrology, Anorthosite, Chamockitic Gneiss, Thermal Modelling, Crustal Thickening, Magmatic Underplating, Antarctica.

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