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

Mihir K. Bose ¹

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

Abstract

The 'Sivamalai Series' of Holland comprise ferrosyenite, perthite syenite and a group of nepheline syenites. The main intrusion of Sivamalai forms a thick lensoid body tilted to the northeast. The ferrosyenite is characterised by abundant mesoperthite whereas the mafic phases, olivine and clinopyroxene, are appreciably rich in ferrous iron. The clinopyroxene, a hypersolvus ferroaugite, unmixed during subsolidus cooling. The major petrographic types in Sivamalai intrusion are (1) hornblende nepheline syenite (2) biotite hastingsite nepheline syenite (3) perthite syenite (perthosite) (4) leucocratic nepheline syenite and pegmatite with clots of iron-titanium oxides. Alkali feldspar, unmixed to mesoperthite or antiperthite, is the dominant constituent of these hypersolvus alkali syenites. Chemical and X-ray data indicate the primary feldspar to be a sodium rich monoclinic phase. The nepheline in leucocratic syenite and pegmatite shows deviation from the ideal composition with respect to Si and K contents. With progressive fractional crystallisation, both amphibole and biotite show relative enrichment in iron, the latter particularly in ferric iron. The coexisting iron-titanium oxides indicate a low temperature of crystallisation for the late syenitic liquid.

Differentiation index of the Sivamalai series increases progressively from ferrosyenite to leucocratic syenite, the composition of the magma descends down from near the feldspar join towards the temperature minimum in the undersaturated part of Petrogeny's residua system. The index of agpaicity in these rocks is normally below unity except for the locally developed peralkalic facies. The trend of evolution of syenitic magma in Sivamalai series has been compared with that in adjoining alkalic complexes.

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Mihir K. Bose ¹

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

Abstract

Alkali basaltic magma fractionating towards the salic ends, in plutonic environment,. shows characteristic crystallisation behaviour for which both magma chemistry and physical conditions seem to be responsible. Both pressure and iron enrichment of the magma appear to inhibit reaction relation of olivine with the rest liquid. The hydrous magma may experience prolonged precipitation of chain-silicates whose field of crystallisation is extended at the expense of plagioclase and olivine. Both pyroxene and amphibole show parallel trend in compositional variations by inviting calcium, titanium and aluminium along with a progressive Fe-Mg diadochic replacement with fractionation. Compositions of the precipitating mafic phases highly influence the nature of residual liquid which may thus show wide variation in silica saturation. High pressure fractionation of an alkalic basic magma under favourable condition may even lead to a peralkalic silica rich differentiate.

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Mihir K. Bose ¹

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

Abstract

The late Pre-Cambrian intrusive suite of Koraput, Orissa comprises of alkali-gabbro-calc-alkali syenite-perthite syenite (perthosite)-nepheline syenite, arranged in the sequence of emplacement. Alaskitic granite develops as a direct offshoot from the hydrous alkali-gabbro magma during fractional crystallisation.

Chemical, optical and X-ray data for the rock forming minerals have been furnished, and the trend of mineralogical variation during fractionation of the magma has been traced. Structural states and geothermometric significance of the feldspars are investigated. The alkali feldspafs in the suite reveal a compositional variation tending to approach the temperature minimum on Ab-Or join. Nephelines both in syenite and pegmatite show a marked variation from the ideal composition. The major mafic silicates, namely pyroxene and amphibole, essentially show a parallel trend of variation being progressively richer in ferrous iron at the expense of magnesium while the entry of calcium in the chain structures is not much affected. The trend of increasing Fe/Mg ratio with fractionation is also imprinted on biotite. The iron-titanium oxides indicate the change of physicochemical conditions in fractionating magma.

Variations of major elements and of some trace elements as related to respective diadochic major elements have been discussed. The alkalic suite shows a progressive increase in alkali metals with advancing differentiation as reflected through increasing differentiation index though peralkalinity is never reached but in granite. Magmatic evolution in the development of the alkalic complex has been elucidated.

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M. K. Nagpal ¹, K. K. Nagpaul ¹, Mihir K. Bose ²

Affiliations
1 Physics Department, Kurukshetra University, Kurukshetra, IN
2 Department of Geology, Presidency College, Calcutta, IN

Abstract

Attempt has been made to date the alkaline suites of India on the basis of fission track analysis of apatite. The alkaline plutons of the Eastern Ghats Belt appear to be related to a late Precambrian tecto-igneous activity though all the intrusions may not be strictly synchronous. New age data on some alkaline rocks from the Deccan volcanic province indicate the related magmatism to be older than the available age level for the bulk of Deccan lavas. The apparently isolated occurrence of alkaline complex of Mundwara in Sirohi, Rajasthan has been related to Deccan volcanicity on the basis of apatite ages.

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Jayanta K. Sinha ¹, Mihir K. Bose ¹

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

Abstract

Anorthosite suite east of Bela in Gaya District, Bihar, comprises: Anorthosite (s. sir.); gabbroic anorthosite and (meta-) gabbro. The rocks occur as isolated blocks and pods in granitoids. Plagioclase in the anorthositic rocks has bimodal character, compositional range from Anso to An73, low structural state and high K/Rb ratio. The chief mafic constituent in the suite is hornblende. Fe-Ti oxide minerals (ilmenite and ilmenomagnetite) occur as pockets associated with the suite. The anorthosite suite may represent detached blocks of an original labradorite type massif. Some significant attributes of the investigated suite have been compared with those of the neighbouring Bengal anorthosite massif.

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Mihir K. Bose ¹

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

Abstract

Alkaline magmatism in the Deccan volcanic province is characterised by i) relative spatial restriction, being manifested in the western and northwestern parts of the province, ii) development of alkaline rock assemblages in plug like bodies and hypabyssal minor intrusions, iii) alignment of magmatic activities along three (or four) volcanotectonic belts meeting near the gulf of Cambay, iv) pronounced fractional crystallisation of parent alkali basaltic magma often leading to wide spectrum of rock composition and v) relative rarity of effusive alkaline rocks.

The ultimate derivative of fractionating alkaline basic magma is usually a hypersolvus foid syenite. Alkaline ultrabasic rocks (melteigite-ijolite) in association with carbonatites are developed in a few plugs. Alkaline minor intrusions of wide compositional variation, are largely associated with subvolcanic plugs and a few volcano tectonic belts. In comparison, effusive alkaline rocks are rare in the province. Manifestation of alkaline lavas together with picritic basalts and spilite in western India reflect unique structural setting of this terrain.

Available petrological, geomorphological and geophysical data all point to a plume activity in the present Cambay region and this appears to be the major controlling mechanism for alkaline magmatism.

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Anirvan Ghosh ¹, Mihir K. Bose ¹

Affiliations
1 Department of Geology, Presidency College, Calcutta - 700 073, IN

Abstract

The conglomerate horizon near Bisrampur in Singhbhum, Bihar, on the flank of Archaean (Singhbhum granite) platfonn, manifests a highly mature pebble assemblage in an argillaceous to arkosic matrix. The triaxial pebbles in conglomerate show readily recognizable down dip preferred orientation of longest axes and a common graving lineation on pebble surfaces. Strain analyses indicate a feeble relative dominance of flat type pebbles and a possible non-random orientation pattern of the primary deposited nonspherical pebbles. Calculation of finite strain also suggests that the axial parameters of pebbles cannot be related to a single phase of deformation. Thus, there may be two factors involved responsible for triaxiality of pebbles, one prior to deposition and a later modification by feeble deformation. Although spatially associated to a strongly tectonised domain to the north, the conglomerate largely retains the primary depositional character controlled by mass flow on the basin flank. This behaviour may be resolved by protective character of the channel filling on continental slope.

Keywords

Bisrampur Conglomerate, Deformation, Singhbhum, Bihar.

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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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Mihir K. Bose ¹, Manas K. Chakraborti ¹

Affiliations
1 Department of Geology, Presidency College, Calcutta-700 073, IN

Abstract

Geochemical characteristics of Dalma basalts from the Proterozoic Singhbhum basin of eastern Indian shield have been evaluated in the perspective of basalt geochemistry across the Archaean-Proterozoic Boundary (APB) in greenstone belt association. Most of the incompatible elements and element ratios in Dalma basalts clearly establish their depleted geochemical signature comparable to the late Archaean greenstone belt basalts. Like their late Archaean counterparts, the Dalma basalts have La/Yb = 2.50, La/Ta = 19, Ti/V = 18 and Zr/Y = 2.6. This ultrabasic and potash-poor tholeiitic association of Singhbhum basin differs remarkably from calc-alkali basalts-potassic felsic rocks assemblage of Proterozoic greenstone belts. Convincing lithotectonic association and geochemical behaviour are strongly indicative of a back-are setting for Dalma volcanism. However, due to partly modifying influence of subduction zone elements on primary primitive composition of Dalma basalts, an apparent are association is imposed in terms of certain element ratios. Taken together, the lithoassociation and geochemical data suggest that basalt magmatism in early Proterozoic Singhbhum basin is broadly similar to those found in many late-Archaean greenstone belts. Dalma basalts do not appear to fit a generalised pattern of secular m~mt1e evolution across the APB. Further, it is observed that geochemical signatures of basalts are controlled by local tectonics which persist across a preconceived discrete time plane.

Keywords

Archaean, Proterozoic, Geochemistry, Basalts, Dalma Volcanics, Singhbhum, Bihar.

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Monoj Maitra ¹, Mihir K. Bose ¹, Jyothisankar Ray ²

Affiliations
1 Department of Geology, Presidency College, 86/1 College Street, Kolkata - 700 073, IN
2 Department of Geology, Calcutta University, 35, Ballygunge Circular Road, Kolkata - 700 019, IN

Abstract

The ultramafic rocks of Chalk Hills near Salem, comprise dunite, wehrlite and olivine clinopyroxenite in association with mafic alkaline rocks. The constituent phases of the ultramafics have been analysed and their compositional data have been evaluated to assess petrogenetic factors. Dunite appears to be restite formed through mantle melting whereas wehrtite and olivine clinopyroxenite represent successive progressive cumulation products from transported high magnesian melt. Mineral chemistry of mafic phases and compositional interrelation between them indicates an equilibration temperature of 1022° to 1088°C for wehrtite and a relatively lower temperature level (ca 900°C) for olivine clinopyroxenite. It appears that the extracted high magnesian melt generating the wehrlite and olivine clinopyroxenicte cumulates, equilibrated at shallower level near base of the crust under ca 11 kb pressure.

Keywords

Mineral Chemistry, Mantle Restite, Wehrlite-olivine Clinopyroxenite, Magmatic Equilibration Temperature.

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Mihir K. Bose ¹

Affiliations
1 Department of Geology, Presidency College, 86/1 College Street, Kolkata - 700 073, IN

Abstract

The Singhbhum craton has a chequred history of mafic magmatism spanning from early Archaean to Proterozoic. However, lack of adequate isotopic age data put constraints on accurately establishing the history of spatial growth of the craton in which mafic magmatism played a very significant role. Mafic magmatism in the craton spreads from ca.3.3 Ga (oldest "enclaves" of orthoamphibolites) to about 0.1 Ga ('Newer dolerite' dyke swarms). Nearly contemporaneous amphibolite and intimately associated tonalitic orthogneiss may represent Archaean bimodal magmatism. The metabasic enclaves are appreciably enriched and do not fulfill the geochemical characteristics of worldwide known early Archaean (>3.0 Ga) mafic magmatism. The enclaves reveal compositional spectrum from siliceous high-magnesian basalt (SHMB) to andesite. However, the occurrence of minor depleted boninitic type within the assemblage has so far been overlooked. High magnesian basalt with boninitic character of Mesoarchaean age is also reported in association with supracrustals from southern fringe of the granitoid cratonic nucleus. The subcontinental lithospheric mantle (SCLM) below the craton is conjectured to have initiated during the early Archaean. Significantly, recurrence of depleted magma types in the craton is observed during the whole span of mafic igneous activity which has been vaguely related to "mantle heterogeneity" although the alternative model of sequential mantle melting is also being explored.

The Singhbhum craton includes the Banded Iron Formation (BIF) associated mafic lavas, MORB-like basic and komatiitic ultrabasic bimodal volcanism - documented as Dalma volcanics, Dhanjori lavas, and the Proterozoic Newer dolerite dykes. Three different types of REE fractionation patterns are observed in the BIF-associated mafic lavas. These are the REE unfractionated type is more depleted than N-MORB and some lavas with boninitic type of REE distribution. MORB-like basic and komatiitic ultrabasic (Dalma volcanics) are emplaced within the Proterozoic Singhbhum Basin (PSB). The vista of magmatism in the basin was controlled by a miniature spreading centre represented by the mid-basinal Dalma volcanic ridge. The volcano-sedimentary basinal domain of Dhanjori emerged at the interface of two subprovinces (viz. the mobile volcano-sedimentary belt of PSB and rigid granite platform) under unique stress environment related to extensional tectonic regime. Trace element distribution in Dhanjori lavas is remarkably similar to that in PSB minor intrusions and lavas (except a Ta spike in the latter). The Proterozoic Newer dolerite dykes within Singhbhum nucleus manifest an unusually wide spam of intrusive activity (ca 2100 Ma to 1100 Ma) and unexpectedly uniform mantle melting behaviour.

Keywords

Boninitic Magma, Siliceous High Magnesian Basalt (SHMB), Komatiite, N-MORB, Mantle Characteristics, Singhbhum Craton.

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Manas K. Chakraborti ¹, Mihir K. Bose ¹

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

Abstract

The theralite-melteigite-carbonatite association is being reported from Mundwara alkaline suite in the Sirohi district, Rajasthan. The assemblage occurs in an arcuate ring around the central plug of Mer. Theralite (composed mainly of plagioclase, pyroxene, and nepheline) and melteigite (composed mainly of pyroxene and nepheline) are feldspathoid bearing basic and ultrabasic rocks, having high colour index and are closely associated with fine to coarse grained carbonatites in the south eastern part of the ring.

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