Petrochemistry of the lavas from proterozoic dalma volcanic belt, Singhbhum, eastern India

Abstract

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...

∼1.6 Ga ultrahigh‐temperature granulite metamorphism in the Central Indian Tectonic Zone: insights from metamorphic reaction history, geothermobarometry and monazite chemical ages

“…The majority of these models envisage an early N-S extension, leading to the opening of a volcano-sedimentary basin (cf. NSMB) either in ensialic (Gupta et al, 1980;Mukhopadhyay, 1984;Sarkar et al, 1992;Ghosh et al, 2006;Acharyya et al, 2006), or marginal basin (Bose and Chakrabarty, 1981;Bose et al, 1989) settings, and succeeded by compressional deformation, causing basin inversion following either intraplate subduction (Sarkar and Saha, 1977), microcontinental subduction (Sarkar, 1982) or collision (Ghosh et al, 2006;Mahato et al, 2008). Available geochronological data, however, do not support an intracontinental rift related setting for the evolution of the NSMB, as Archaean ancestry of the CGC, which is required for a uniform cratonic basement across the NSMB, has not been established.…”

Precambrian mafic magmatism in the Singhbhum craton, eastern India

“…the supracrustals) of Singhbhum craton cut across the conventional Archaean-Proterozoic boundary (Mahadevan, 2002). However, the Proterozoic Singhbhum Basin (PSB) accreted on the northern fringe ( Fig.3) of the Singhbhum nucleus, has the characteristic bimodal volcanic geochemistry (represented by midbasinal Dalma volcanic belt) and related tectonosedimentary features of Archaean greenstone belts (Bose and Chakraborti, 1981;Bose et al 1989;Condie, 1990;Bose, 2000).…”

“…Reconstruction of palaeotectonic setting (Chakraborti and Bose, 1985) establishes PSB to be sited on a rifted continental margin generated in an extensional tectonic regime (Bose, 1994). The most distinctive feature is the mid-basinal picrite/ komatiite and basalt bimodal volcanism (Bose et al 1989) along the Dalma volcanic belt (Fig.3). Geochemical signature in Dalma volcanics marks a transition across the Archaean -Proterozoic boundary (APB) (Bose and Chakraborti, 1992).…”

Sm-Nd age and mantle source characteristics of the Dhanjori volcanic rocks, Eastern India.

“…While the sample T3 falls very close to the lower boundary of the plume array (∆Nb ~ 0), ∆Nb for R5 is ~-0.14 occurring quite close to the N-MORB along the N-MORB-Crust array. The nega- Bose et al, 1989;Saha, 1994;Roy, 1998); Primitive mantle and average N-MORB data from McDonough and Sun (1995); Average lower, middle and upper crust data from Rudnick and Fountain (1995)…”

“…We have also plotted in Fig. 7 Nb/Y-Zr/Y ratios of mid-Proterozoic (~1.6 Ga; Roy et al, 1999Roy et al, , 2002a) Dalma basalts, komatiites and ultrabasic intrusives all of which have been found to be depleted mantle products (in terms of both REE and ε Nd , data taken from Bose et al, 1989;Saha, 1994;Roy, 1998, Roy et al, 2002a. Interestingly, all these units of Dalma belt also fall within the plume array and together they indicate that the plume magmatism was quite active during the Proterozoic of the EIC.…”