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

V. Rajamani ¹, E. J. Krogstad ², G. N. Hanson ², S. Balakrishnan ¹

Affiliations
1 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, IN
2 Department of Earth and Space Sciences, Slate University of New York, Stony Brook, New York 11794, US

Abstract

Structural and geochemical evidence including major and trace elements, and Pb-isotopic data on K-feldspars, froin the granitoid rocks adjoining the Kolar Schist Belt are presented to show (1) that the Patna Granite an the NW of the belt is dissimilar to any granitoid rock E of the belt and (2) that the main granitoid gneiss immediately E of the belt, the Karnbha Gneiss, is nearly identical to the Bisanattam Granite of Narayanaswamy er al (1960).

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Jayaram Sahoo ¹, S. Balakrishnan ¹

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee 247667, IN

Abstract

The Kolar Schist Belt and adjoining areas of Dharwar Craton are considered to be accreted terranes. Two distinct sets of dolerite (diabase) dykes, trending E-W and N-S are found in the area. The major minerals in the dykes are plagioclase feldspars (50-65 mol. % anorthite) and c1inopyroxenes (augite, sub-calcic augite and magnesian pigeonite).

The Kolar dyke samples are essentially tholeiitic except for one sample which resembles a basaltic komatiite. Mineral, as well as, whole rock chemistry of the dyke samples indicate an iron enrichment trend. The modelling of major and trace elements of Kolar dykes suggest that their parental magmas must have been derived from Fe- enriched sources with higher Fe/Mg ratios than primitive mantle. Source for parental magma to the basaltic komatiite dyke has been inferred as garnet lherzolite, whereas, for all others it could be komatiite with variable Fe/Mg ratios.

The Kolar dyke samples are enriched in light rare earth elements (LREE) and have variable abundances of heavy rare earth elements (HREE) relative to chondrites. The modelling of major, trace, and rare earth element data indicate that the generation of parental magmas took place from sources similar to komatiites, by low percentages (<30%) of partial melting, within the lithosphere at pressures less than 3GPa. Magmas representing the dyke samples could have undergone small extents of fractional crystallisation and assimilation of granitoids. It is suggested that the partial melting of lithospheric sources is related to its stretching and elevation of geothermal gradient, possibly, as a result of intrusion of picrite/kornatiite melts into the lithosphere.

Keywords

Dolerite-Dyke, Geochemistry, REE, Igneous Petrology, Kolar Schist Belt, Dharwar Craton, Karnataka.

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S. R. Basir ¹, S. Balakrishnan ²

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee - 247 667, IN
2 Department of Earth Sciences, Pondicherry University, Pondicherry - 605 014, IN

Abstract

A study of the distribution of REE in sphene from three granodiorite samples around NNW-SSE trending Hutti-Maski schist belt has been described here. Two distinct sets of sphene-melt K_d, values for REE have been observed among the three sphene-whole rock pairs studied. The chondrite normalized REE pattern of sphene from east (Gajalagatta) and northeast (Uti) granodiorites show LREE enriched and HREE depleted patterns with slight or no negative Eu anomaly. These REE patterns are parallel to the respective whole rock samples. But the chondrite normalized REE pattern of sphene from the sample to west of the schist belt (Ankasdoddi) has markedly depleted Ce and Nd with slight or no negative Eu anomaly and steeply fractionated HREE, resulting in a convex-up pattern. The variation of REE K_d, values of sphenes is not due to differences in the composition of host magmas (because they are all granodiorites with identical major element abundances), but could be a function of temperature of crystallization. Small degrees of fractional crystallization of sphene from granodiorite magmas would result in residual magmas having lower REE abundances either with parallel REE patterns or HREE depleted fan shaped REE patterns, depending on the K_dvalues.

Partial melting of granitoid rocks, leaving minor amounts of sphene in the residue gives rise to melts with lower REE abundances than the source. The REE patterns of the melts are parallel to the source if one set of REE K_d sphene-melt values are considered. If the other set of K_d, values are used, the partial melts have (fan shaped) REE patterns with more depletion for heavy REEs.

Keywords

Geochemistry, Granodiorite, Sphene (Titanite), Hutti-Maski Schist Belt, Dharwar Craton, Karnataka.

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S. Balakrishnan ¹, R. Madusudanan ², S. Bhattacharjee ³, S. T. Narahari ³, R. Ananthanarayana ³, T. C. Devaraju ⁴

Affiliations
1 Geological Survey of India, Bangalore - 560 078, IN
2 Geological Survey of India, Bangalore - 560078, IN
3 Petrology Division, GSI, Hyderabad - 500 078, IN
4 'Rajamangala', Saptapur, Haliyal Road, Dharwad - 580001, IN

Abstract

No Abstract.

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Rm. P. Nachiappan ¹, Bhishm Kumar ¹, Rm. Manickavasagam ², S. Balakrishnan ³

Affiliations
1 National Institute of Hydrology, Roorkee - 247 667, IN
2 Institute Instrumentation Centre, Indian Institute of Technology, Roorkee - 247 667, IN
3 Department of Earth Sciences, Pondicherry University, Pondicherry - 605 014, IN

Abstract

Hydrochemical characteristics of the groundwater occuring in and around the Nainital lake basin indicate that Ca²⁺/Mg²⁺ ratios are less than one. Statistical analyses reveal that the variation in Ca²⁺/Mg²⁺ ratios is mainly due to variation in Ca²⁺. The groundwater is geochemically undersaturated with respect to calcite and dolomite and has a tendency to dissolve the calcareous rocks more readily than it is expected in ideal dolomite terrain. The higher infiltration rates and lower surface runoff indicate that the groundwater will play a major role in the hillside stability. Further, the groundwater movement will enhance dissolution of carbonate rocks and lead to enlargement of fractures. Hence, the quantum of groundwater seepage and their hydrochemical character seem to play a crucial role in the landslide hazards as experienced by the lake basin frequently.

Keywords

Hydrochemistry, Dolomite, Landslide Hazard, Groundwater, Nainital, Uttaranchal.

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J. Sahoo ¹, T. S. Giritharan ², S. Balakrishnan ², V. Rajamani ³

Affiliations
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN
2 Department of Earth Sciences, Pondicherry University, Pondicherry- 605 014, IN
3 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi - 110067, IN

Abstract

The Banded Gneissic Complex (BGC) occurring around Masuda consists of granitoids, migmatized mica schist, calc-gneiss and amphibolites. The amphibolites (few cm to tens of metre wide) are co-folded with the granitoid and calc- gneisses. The ortho-amphibolites are classified into high-Mg and tholeiitic amphibolites and they have distinct chondrite normalized Rare Earth Element (REE) patterns and Ce/Nd ratios. Modeling of their geochemical data suggests that magmas representing the high-Mg amphibolites were derived by low extents of partial melting of mantle sources more Fe-enriched than lherzolite whereas magmas representing the tholeiitic amphibolites were derived by low-extents of partial melting of sources formed by mixing of high-Mg magmas with lherzolite. The composition of this source is similar to the komatiites. The tholeiitic amphibolites have geochemical and petrogenetic characteristics of the basalts formed in island arcs. They are different from that of amphibolites found in the basal part of Aravalli Supergroup and mafic enclaves found within the BGC occurring east of Nathdwara and Udaipur.

Keywords

Geochemistry, Petrogenesis, Amphibolites, Banded Gneissic Complex, Aravalli Craton, Rajasthan.

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