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

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

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

Abstract

Seventeen trace elements in 37 total rock samples belonging to six magmatic units of Singhbhum Granite, and nearly as many trace elements in II individual minerals separated from these rocks were estimated by quantitative and semi-quantitative spectrographic analysis respectively. Uranium was determined fluorimetrically in all the 37 samples. The trace element distributions in these samples have been studied in relation to those of the major elements as computed from the modal analyses on the basis of optical and chemical data on the constituent minerals.

The trends of variation of the trace elements and of the diadochic element ratios in these rocks conform broadly to those expected under conditions of fractional crystallisation of volatile-rich granitic melts. Distinctive trends of variation of several trace elements and diadochic element ratios with increasing differentiation were found for the three distinct but closely related phases into which these intrusive units may be classified on structural and petrologic grounds. Multivariate analyses and t-tests on 12 trace elements in samples belonging to the three phases indicate that the three phases of granites have significantly different trace element compositions.

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A. V. Sankaran ¹, T. K. Bhattacharyya ¹, K. K. Dar ¹

Affiliations
1 Atomic Minerals Division, Department of Atomic Energy, New Delhi, IN

Abstract

Abundances of the lanthanides, yttrium, scandium and 12 other elements in pegmatitic and hydrothermal uraninites from India and Massif Central, France, are presented and compared with those available in literature. The pegmatitic uraninites exhibit a selective, and the hydrothermal types a complete rare-earth assemblage with Gd markedly high in both. The La-Nd: Sm-Ho: Er-Lu ratio may serve as an index of fractionation. Pegmatitic uraninites show an abnormal distribution of La in excess of Ce. An explanation is offered for this.

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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. Banerji ¹, A. K. Talapatra ², A. V. Sankaran ³, T. K. Bhattacharyya ³

Affiliations
1 Geology Department, Presidency College, Calcutta, IN
2 Geological Survey of India, Patna, IN
3 Petrology Laboratory, Atomic Minerals Division, New Delhi, IN

Abstract

Study of the trends of geochemical migration of some major and minor elements during progressive migmatisation within part of the Singhbhum shear zone supports the view that apatite-magnetite, uraninite and copper sulphide deposits and the associated wall rock alterations are related to processes of migmatisation therein. During migmatisation extensive mobilisation of elements took place when Si, Al, Na and small quantities of Ga, U and Zr were introduced into the pre-existing basic metavolcanics within the shear zone, and Fe, Mg, Ca, K, Ti, Mn, P, Ba, Rb, Sr, Li, Ni, Co, Cr, V, Cu, and Y were expelled from these rocks. The addition of the former group of elements brought about transformation of these rocks, into felspathic schists and gneisses (the soda-granites) while the expulsion of the latter group of elements from these rocks and their addition to the migmatising fluids caused progressive changes in the chemical composition of these fluids. At the closing stages of the process these fluids were transformed into mineralising solutions that gave rise to apatite-magnetite, uraninite and copper sulphide deposits and to extensive sericitisation, chloritisation, biotitisation and silicification within the shear zone. The various elements, present in traces, entered the crystal lattices of these minerals, in accordance with their geochemical preferences and in proportion to their concentration in the mineralising solutions. The resulting deposits should be regarded as 'metasomatic hydrothermal'.

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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. V. Sankaran ¹

Affiliations
1 Atomic Minerals Division, Department of Atomic Energy, Begumpet, Hyderabad, IN

Abstract

The important results of the application of trace element geochemistry to petrogenetic problems of the Indian Precambrian are reviewed. Such studies have gained importance only recently. Except for two major terrains comprising the Singhbhum granite in Eastern India and the Peninsular gneisses in Southern India, numerous rock complexes in other terrains have not been investigated in detail. The few available trace element data have provided insights into the emplacemental history of the Singhbhum and Soda granites and the associated mineralisation, and have also contributed to a better understanding of the nature of the Peninsular gneisses and some of the granites occurring within. The available data suggest a crustal heterogeneity in the Indian shield with respect to the granitophile elements.

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Mihir K. Bose ¹, A. V. Sankaran ², T. K. Bhattacharya ²

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

Abstract

Geochemical data for 20 igneous rock samples from alkali complex of Koraput, Orissa are presented and discussed. Trace element contents in 8 individual minerals separated from the rock members have been estimated. The abundances of rare alkali earth metals are distinctly higher in early basic and intermediate members compared to those in a calcalkali suite. Some diadochic element ratios reveal a pronounced change of the variation trends at the ultimate end of the fractionation process. The granitophilc elements are not so remarkably enriched in the late differentiates as expected in low melting temperature constituents. Unlike feldspathoidal syenites of agpaitic complex, the nepheline syenite of Koraput is poor in niobium and zirconium.

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