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.

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ZusammenfassungDer Vulkangi~rtel yon Dalma liegt in der pr~ikambrischen Region Singhbhum in Ostindien. Aufgeschiossen ist der Dalma-Gi.irtel im mittleren Bereich eines geradlinigen Bekkens, das im Siiden durch Basement des Archaikums begrenzt wird. Die Laven befinden sich in einer geringmetamorphen Umgebung und weisen eine bimodale Zusammensetzung auf. An der Basis find Magnesium-reiche und darl.iber Kalium-arme Basalte entwickelt, die repr~isentativ fiir den Durchschnitt der Laven sind. Die ultrabasischen Laven enthalten niedrige Konzentrationen an immobilen nicht kompatiblen Elementen, deren Verhiiltnis wahrscheinlich yon den Eigenschaften der Lavaquelle gesteuert wird. Der geochemische Charakter der mafischen DalmaVulkanite entspricht weitgehend den heutigen Basaken der ozeanischen Riicken (MORB). Dabei dominieren leicht abgereicherte REE-Basalte sowie eisenreiche Basalte. Vergleicht man allerdings Etementverh~ilmisse wie das yon Th/Ta, wird eine Abweichung von dem typischen MORBCharakter hin zu Inselb6gen-Tholeiten deuthch. Berficksichtigt man diese Beobachtungen, scheinen die DalmaBasalte eine modiflzierte MORB-Zusammensetzung zu haben, die in ihrem Chemismus am ehesten den Basalten yon Back-Arc-Becken, z.B. in einem Gebiet einer SupraSubduktionszone, zu entsprechen. Die Bimodalit~t dew ultrabasischen und mafischen Laven erinnert an archaische komatiitische Regionen. Die geochemische Zusammensetzung der Dalma-Laven sowie der geologische Rahmen des Gebietes deuten auf ein randliches B-ecken hin, das sich w~hrend des Proterozoikums entwlekeh haben k6nnte. AbstractThe Dalma volcanic belt of the Singhbhum Precambrian terrain in eastern India is developed along the median zone of a linear basin flanked by Archaean cratonic basement to the south. The lavas, in a low grade metamorphic environment, preserve a strong compositional bimodality, with highly magnesian picritic volcanics developed at the base and low-K basalt flows above, constituting the bulk of the *) Author's address: Dr. M. K. BosE, Dr. M. K. CHAK-P, At3ARTI, Department of Geolog~ Presidency College, Calcutta-700 073, India and Dr. A. D. SAUNDEI~S, Department of Geology; University of Leicester, University Road, Leicester LE1 7RH, U.K. lava pile. The ultrabasic iavas have low concentrations of immobile incompatible elements, ratios of which are probably controlled by the source character. Dalma mafic flows are closely comparable in geochemical character to modern day mid-oceanic ridge basalts (MORB) with a dominance of light-REE-depleted basaks and ferrobasalts. However, in terms of certain element ratios (Th/Ta), deviation from MORB characteristics toward island arc tholeiites is apparent. In this respect the Dalma basalts appear to have a modified MORB composition and the closest chemical analogy may be basaks from back-arc basins, i. e. a supra-subduction zone environment. The bimodality of ultrabasic and mafic lavas is reminiscent of Archaean komatiitic provinces. The geochemical signature of the Dalma lavas, and the geological framework of the te...

Zusammenfassung AbstractIn the eastern part of the Indian shield covering Shinghbhum and adioining districts of Bihar, occurs the fossil marginal basin which opened up during the early Proterozoic, flanked by Arehaean continent to the south. Volcanism played a significant role in filling up the basin. The spine of the basin is occupied by a basaltic lava pile representing a narrow oceanic sector. BIF and possible 'marginal basin ophiolites' are other significant members of the basin cover. The structural features and sedimentary history of the basin are characteristic for marginal basins in general. Identification of this marginal basin emphasises the role of plate tectonics in the evolution of the Indian Precambrian shield. R@sum6Dans le Shingbhum et les districts voisins du Bihar de la pattie orientale du bouclier indien, apparalt un bassin marginal fossfle qui s'est ouvert au cours dn Prot6-rozo/que inferieur, flanqu6 au nord par un continent arch6en. Le volcanisme y a jou6 un r61e important en remplissant le bassin. Le centre du bassin est occup6 par un empilement de laves basahiques repr6sentant un secteur oc@anique 6troit. BIF et peut-~tre aussi des ophiolites de bassin marginal sont d'autres membres importants de la couverture de ce bassin. Les faits structuraux et l'histoire s6dimentaire du bassin sont caract6ristiques pour des bassins marginaux en g6n6ral. L'identification de ce basin marginal met en relief le r61e de la tectonique de plaque dans l'6volution du bouclier pr6cambrien indien.

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