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

C. Narasimha Murthy ¹, C. Naganna ¹, M. S. Sadashivaiah ¹

Affiliations
1 Department of Geology, Karnatak University, Dharwar, IN

Abstract

The quartz-dolerite dykes occurring in the Shirahatti area have been chemically analysed and a detailed petrological and mineralogical investigation has been made. The mineralogical investigation of the feldspars and pyroxenes has been done on the 4-axes Universal stage after the methods of Reinhard, Burri, Hess, Nemato, Turner, and others. The nature of the feldspars and the twin laws confirm the views of Gorai. The optical study of the pyroxene reveals the presence of enstatite, pigeonite and sub-calcic augite. The petrochemical. investigation indicates that the basic magma which has given rise to these dolerites had undergone fractional crystallization resulting in the enrichment of acidic residuum and volatiles. The deuteric and hydrothermal alterations seen in the dolerite dykes are due to the action of volatiles in the final stages of the consolidation of the basic magma.

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T. C. Devaraju ¹, M. S. Sadashivaiah ¹

Affiliations
1 Department of Geology, Karnatak University, Dharwar, IN

Abstract

The pyroxene-quartz-magnetite rocks occurring in association with the charnockites and other rocks of the Satnur-Halaguru area, which were regarded by many of the early officers of the Mysore Geological Department as members belonging to charnockite series have been reinvestigated. The investigation has brought to light the occurrence in these rocks, of iron-rich minerals like, ferrohypersthene, eulite, hedenbergite, ferroactinolite, iron-rich garnet and the unusual occurrence of pyroxmangite and orthoclase, It is suggested from a detailed study of the field relation, petrology, mineralogy and petrochemistry that the pyroxene-quartz- magnetite rocks form a separate entity, representing metamorphosed iron-rich sediments, which except for their formation under similar metamorphic conditions namely, pyroxene granulite facies, have no relation whatsoever with the intimately associated charnockites of the area.

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T. C. Devaraju ¹, M. S. Sadashivaiah ¹

Affiliations
1 Department of Geology, Karnatak University, Dharwar, IN

Abstract

Orthopyroxene-bearing rocks viz., quartz-norite, cordierite-biotite-hypersthene granulite, leptynite and hypersthene-biotite quartzite containing minerals like hypersthene (aluminous), diopside, brown hornblende, biotite, garnet, cordierite, plagioclase and quartz, occur constituting an integral part of the high-grade metapelites of the area. The field and petrological observations suggest that these orthopyroxene-bearing rocks are variants of the metapelites, derived by the metamorphism of appropriate sediments and have no connection with the associated charnockites of the area.

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M. S. Sadashivaiah ¹, M. Ikramuddin ¹

Affiliations
1 Department of Geology, Karnatak University, Dharwar, IN

Abstract

Textural, mineralogical and petrochemical variations observed across the width of the differentiated hypersthene-olivine dolerite dyke situated near Harohalli have been described and discussed. The salient features of the dyke are (i) the occurrence of mafic minerals rich in iron, and plagioclase less rich in anorthite content at the borders, with a progressive decrease of iron content in the mafic minerals and increase in anorthite content of the plagioclase towards the central zone and (ii) the textural variation from the flow and hyalopilitic at the margins, intersertal, intergranular, ophitic and subophitic at the intermediate zones and poikilitic at the centre. The chilled margins show a higher content of iron, alkalies and silica and a lower content of MgO, CaO and Al₂O₃. than the central portion. These characters indicate that the differentiation is due to thermodiffusion as described by Wahl as the observed features cannot be explained by any other known mechanisms of differentiation.

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M. S. Sadashivaiah

Abstract

No Abstract.

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