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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A. T. Rao ¹, V. I. Fonarev ², A. N. Konilov ², M. Romanenko ²

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam-530 003, IN
2 Institute of Experimental Mineralogy, 1142432, Chernogolovka, Moscow District, RU

Abstract

Electron microprobe dating method is same as chemical aging method of U-Th minerals used in geochronology. The Th-U-Pb concentrations obtained from a single electron probe analysis allow calculation of geologically meaningful age. Based on this method monazite from spinel granulite of Visakhapatnam in the Eastern Ghats Belt (EGB) gave ∼1,000 Ma, corroborating the late Proterozoic metamorphism and charnockite emplacement, established in parts of the EGB and Antarctica. Microgeochronology of commonly occurring accessory mineral such as monazite in granulite variants is recommended for quick reconnaissance correlation in Gondwanian fragments in East Gondwanaland.

Keywords

Micro-Geochronology, Electron Microprobe Dating, Monazite, Eastern Ghats Belt, A. P.

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