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

A. K. Saha ¹, T. K. Bandyopadhyay ¹

Affiliations
1 Department of Geology, Presidency College, Calcutta, IN

Abstract

The composition of biotite granodiorite in an area of 11 square miles around Kuyali has been studied on the basis of modal analyses of 41 systematically collected samples. The average mineralogical composition of the sampled area of the granodiorite has been determined by superposing a square grid pattern (size of grid 1 sq. mile) over the area and considering the average composition of each grid. Isopleths for quartz percentage, K feldspar percentage and mafics percentage have been drawn on the basis of the deviation of the grid averages from the average for the entire sampled area. By using a procedure of analysis of variance, it is shown that the local variations in composition are considerable and that they tend to mask the regional variation. Poor and generally non-significant correlations amongst the major minerals of the granodiorite have been found.

The compositional data support the field evidences of considerable assimilation of basic metamorphic rocks and argillites having been an important factor in the evolution of these rocks. K feldspar-rich residual liquid is shown to have migrated towards the low-pressure southern border region of the granodiorite body.

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P. K. Chatterjee ¹, A. K. Saha ¹

Affiliations
1 Presidency College, Calcutta, IN

Abstract

Analysis of variance and polynomial trend surface techniques are used to determine the levels of variability and geographic variability, of five modal attributes and specific gravity of the Dalima Biotite Granodiorite. It is found that modal analyses from one measurement area in one hand specimen collected from one exposure in the pluton gives a reasonable estimate of that exposure, but that more than one exposure per one-square-mile grid is needed to estimate the variability within the grid. There are significant differences between the samples from each one-square-mile grid. The degree 6 polynomial trend surface maps of the modal attributes and specific gravity, based on149samples, represent good appraisals of the regional variability; there is good correspondence between the trends of primary foliation and the compositional contours of the trend surfaces. The structural evidence of the existence of two distinct intrusive units within the pluton is supported by (a) comparison of the cumulative frequency distributions of the modal attributes, and (b) the distinctive patterns of the individual trend surfaces for the two units. Compositional and structural evidences suggest that the western unit is the older of the two.

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A. K. Saha ¹, M. K. Bose ¹, A. V. Sankaran ², T. K. Bhattacharyya ²

Affiliations
1 Department of Geology, Presidency College, Calcutta, IN
2 Petrology Laboratory, Department of Atomic Energy, New Delhi, IN

Abstract

An ultramafic body flanked on either side by tholeiitic dolerite occurs as a minor intrusion in the Singhbhum granite, southwest of Keshargaria (22°36' : 85°56'). The ultramafic body varies from pyroxene-rich harzburgite to olivine-rich poikilitic harzburgite, the latter being conspicuous in the central part of the intrusion. A crude planar structure defined by enstatite crystals is developed towards the contact walls. The constituent mafic silicates, arranged in paragenetic order, are: forsteritic olivine, enstatite, diopsidic augite (malacolite), phlogopite and plagioclase, the last three phases appear to represent intracummulus crystallisation.

The central part of the intrusion is relatively enriched in olivine whereas the marginal parts are in an advanced stage of differentiation relative to the central zone, indicated by lower Mg/Fe and Ba/Rb ratios in the former. This spatial variation in degree of differentiation is considered to have been induced during flowage of the crystal-iquid mush. The ultramafic intrusion is non-uniform in chemical and mineralogical compositions and does not represent unaltered mantle material. The significant deviation from the mantle composition is in higher aluminium and ferric iron and lower magnesium in the investigated peridotite. The Keshargaria ultramafic body which is one of several such bodies associated with the Newer Dolerite intrusions, represents a stage of fractionation of mantle derived magma.

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A. K. Saha ¹, A. V. Sankaran ², T. K. Bhattacharyya ²

Affiliations
1 Department of Geology, Presidency College, Calcutta, IN
2 Atomic Minerals Division, Department of Atomic Energy, New Delhi, IN

Abstract

Three petrogenetic groups are recognised from petrologic and geochemical data on the Precambrian Newer Dolerite dyke swarm. They are (a) accumulates (peridotites, norites and lamprophyres), (b) products of direct magmatic crystallization (two generations of dole rites and associated micropegmatitic dolerites and micro-granites, each with distinctive trends of major-and minor-element variations), and (c) products of partial melting (leuco-granophyre and xenolithic granophyre), Geochemically, the entire suite has abnormally high K/Rb, Ni, Co and abnormally low Rb, Ba and Sr. Linear discriminant function indicates that the dolerites are typically sub-alkaline. Source magma is inferred to have been picritic and derived by partial melting of the upper mantle.

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A. K. Saha ¹

Affiliations
1 Department of Geology, Presidency College, Calcutta, IN

Abstract

The term 'Mayurbhanj Granite' is proposed here for a batholithic body of biotite-amphibole-(stilpnomelane) granite grading to alkali-feldspar granite which covers nearly 1000 sq. km area in the northern part of Mayurbhanj district, Orissa. The batholith shows intrusive relationship with the adjoining Singhbhum Granite, Dhanjori Group gabbro-anorthosite and the Singhbhum Group metamorphites, and is made up of three units which were successively emplaced within the same magmatic cycle: (a) fine-grained, homophanous biotite-ferrohastingsite (-stilpnomelane) microgranite, in part granophyric, (b) coarse-grained, homophanous to well-foliated ferrohastingsite-biotite granite, which is porphyroblastic and gneissos in the east, and (c) 'biotite aprogranite injections within the porphyroblastic gneisses in the east. The second unit is by far the largest and was intruded synkinematically with respect to the second-generation folding in the Singhbhum Group metamorphites, probably about 1.1 × 10⁹ years ago.

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A. Roy ¹, A. K. Saha ², S. N. Sarkar ³

Affiliations
1 Geological Survey of India, ER, 12 Russel St., Calcutta-700071, IN
2 Department of Geology, Presidency College, Calcutta-700073, IN
3 95A, Ballygunge Gardens, Calcutta-700019, IN

Abstract

Correspondence analysis, using the concept of joint probability distribution of two random variables, individual samples and chemical variables, was applied to two sets of multi-metallic data from Mosaboni copper mines (18th, 21st & 25th revels) and Sargipalli lead-zinc mine falling within the Gangpur-Singhbhum metallo-tectonic belt. The analysis results factor loadings for individual samples and variables, which when plotted as points on two-dimensional coordinate (factorial) axes with the same origin recognize distinct geochemical domains characterised by natural groupings or clusters of samples and variables. The simultaneous variation of trace elements in these two data sets with sample locations (background/anomalous) seems to reflect overall trends of geochemical evolution of base metal mineralisation in the region.

Keywords

Statistical Studies, Economic Geology, Copper, Lead-Zinc, Sargipalli, Mosaboni, Bihar, Orissa.

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A. K. Saha ¹, K. C. Mahapatra ¹, S. Shome ², K. G. Bhoskar ¹

Affiliations
1 Geological Survey of India, Maharashtra, Nagpur - 440006, IN
2 Geological Survey of India, Central Palaeontological Laboratory, Kolkata - 700016, IN

Abstract

No Abstract.

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A. K. Saha ¹

Affiliations
1 Department of Geology, Presidency College, Calcutta 700073, IN

Abstract

Archaean granites and granite gneisses are known to occur in (a) the Iron Ore Province of Singhbhum-Orissa region in eastern India, (b) the Granite-Greenstone Belt of Karnataka and Andhra Pradesh, (c) the Granulite Belts of Tamil Nadu and Kerala and (d) the Bundelkhand-Rajasthan Precambrian province.

In the Iron Ore Province of Singhbhum region, the Older Metamorphic tonalite-gneiss (OMTG-c. 3200 Ma) has strong geochemical resemblance with the 'Ancient tonalite-gneiss' of Barberton. South Africa. The low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7018-703) for the total-rock Rb-Sr isochrons indicate derivation of the tonalitic melt from the mantle perhaps in a two-stage process. The magmatic units of the composite batholith of Singhbhum Granite (2900-3000 Ma) show distinctive trends of fractional crystallisation for the three successive but closely related phases. Multivariate analysis of variance of the trace-element data indicates statistically significant differences among the phases while Rsmode Factor analysis suggests operation of significantly different processes of trace-element distribution in the three phases. The geochemical relationship as well as the high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.711 ± .009 suggest that the three magmatic phases were derived by partial melting successively in the same source region of the crust. The Bahalda metasomatic unit of Singhbhum Granite shows certain abnormal trends of trace-element variations in the sequence: hornblende schist→hornblende diorite→shornblende (-epidote) granodiorite. These have been explained by assuming that during progressive granitisation, (a) ionic charge and electronegativity have been relatively ineffective and (b) certain trace-elements are preferentially incorporated in some preexisting minerals.

The Peninsular Gneiss and associated Archaean granitic bodies in the Granite Greenstone Belt of South India show wide compositional variations ranging from migmatitic and tonalitic gneiss through granodiorite to K-rich adamellite. These rocks show clusters of Rb-Sr isochrons at 3200-3300Ma, 2900-3000Ma and around 2600Ma. Enrichment of nickel and chromium in parts of these gneisses has been reported. Although a metasomatic origin of the Peninsular Gneiss has been suggested, the low initial ⁸⁷Sr/⁸⁶Sr ratios (0.701-0.704) do not support this view and instead indicate direct or multi-stage derivation of the granitic melt from the mantle. Occurrence of several abnormal U-Th enriched granite bodies within Peninsular Gneiss is of special interest.

The Bundelkhand Granite is a composite body of several granitic and gneissic units and appears to be dominantly adamellitic - monzonitic in composition (2555 ± 55 Ma), but has numerous xenoliths and rafts of migmatite and gneiss, mostly of granodiorite and tonalite composition.

It appears that in the Indian shield, the oldest gneisses and granites (> 3000 Ma) are dominantly tonalites while adamellitic granites become more and more abundant in the younger assemblages.

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