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

S. S. Bhat ¹, A. Bhattacharya ², H. C. Dasgupta ², D. Dasgupta ²

Affiliations
1 Geological Cell, GM Ofice, Barkakana Area, PO: NTS Barkakana-829103, Bihar, IN
2 Geology and Geophysics Department, IIT, Kharagpur-721302, IN

Abstract

Petrology and mineral chemistry of garnetiferrous chlorite schists belonging to Deogiri formation in the Sandur schist belt have been studied. Mineral assemblages and chemical data on muscovite, biotite, chlorite, garnet from the Deogiri chlorite schists indicate low grade regional metamorphism under greenschist facies condition. Mineralogy of the metabasics associated with Deogiri chlorite schists and with the overlying Donimalai formation in this region, except near the peripheral contacts, also suggest greenschist facies metamorphism for the schist belt. Mineralogy and chemistry of Deogiri metabasalts indicated ocean floor type of alteration/metamorphisrn for the Deogiri metabasalts. Rocks close to the granite contact have seemingly undergone an episode of thermal metamorphism which is distinct from the regional metamorphism of the entire schist belt and possibly isolated both in space and time.

Keywords

Mineral Chemistry, Metamorphism, Sandur Schist Belt, Karnataka.

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V. V. Sambasiva Rao ¹, H. C. Dasgupta ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721302, IN

Abstract

Clay mineralogy of the Precambrian argillaceous rocks of the Iron Ore Group from the Noamundi basin, is represented mainly by kaolinite and illite. In general. the kaolinite/illite ratio, the illite crystallinity and the (060) peak position and (002)/(001) peak height ratio of illite change systematically from the stratigraphically higher upper shales to lower shales and suggest diagenetic neoformation of illite in these rocks. Dominance of 2M₁ polytype of illite in both the upper and lower shales, the range of illite crystallinity values and the absence of pyrophyllite in the assemblage indicate a maximum of epizone condition of metamorphism (-360°C/2kb) for these sediments. Growth of illite during diagenesis and paucity of chlorite indicate that the detrital sediments which accumulated in the basin were rich in kaolinite but poor in smectite group of minerals. The overwhelming dominance of kaolinite in the initial detrital physil input to the basin. its prolonged persistence and the non-opaque heavy mineral assemblage comprising mainly tourmaline, hornblende and zircon in the sand fraction suggest a granitic provenance and a non-marine condition of deposition for the sediments.

Keywords

Clay Mineralogy, Precambrian, Noamundi Basin, Singhbhum Granite.

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H. C. Dasgupta ¹, Somen Mishra ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 302, West Bengal, IN

Abstract

Hypersthene- to minor olivine-normative low K-quartz tholelites occurring in Hudupahar-Gitigarh area of Chhotanagpur Granite Gneissic Complex appear to have formed at low pressure (<10Kb) conditions. Small range and mutually coherent, regular patterns of distribution of the major and trace elements in Harker-Type and other variation diagrams and tight clustering of all data points in Pearce Element Ratio plots indicate unaltered and uncontaminated and comagmatic nature of the melt for the (meta)-basic rocks subjected to clinopyroxene, plagioclase and minor olivine fractionation. Multielement spidergrams and trace element modelling for a number of Compatible-Incompatible pars suggest that these rocks are products of partial melting of depleted mantle which possibly has been metasomatically slightly enriched in some but depleted in other elements prior to melting. Approximately -15% partial melting of the mantle followed by fractional crystallization of up to 20-30% in the melt appears to have been the probable mechanism for the generation of these tholeutes.

Keywords

Metabasics, Fractionation, Partial Melting, Modelling.

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