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Geochemistry of Archaean Volcanic Rocks from Iron Ore Supergroup, Singhbhum, Eastern India

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Geochemistry of Archaean Volcanic Rocks from Iron Ore Supergroup, Singhbhum, Eastern India Geochemistry of Archaean volcanic rocks from Iron Ore Supergroup, Singhbhum, eastern India S SENGUPTA*, S K ACHARYYA*, J B DESMETH** * Geological Survey of India, 27, Jawaharlal Nehru Road, Calcutta 700016 ** ITC, Kanaalweg 3 2628EB, Delft, The Netherlands Mafic-ultramafic rocks of Archaean age constitute a significant component of the Eastern Indian Craton. These occur in two different modes. In the eastern belt these occur as a long, linear enclave within the Singhbhum granite and the primary banding in them is subvertical. In the more extensive western belt along the periphery of the Singhbhum granite, the disposition of the primary banding is subhorizontal. The major rock type in both the belts is meta-basalt with minor peridotitic komatiite and basaltic komatiite occurring in the eastern belt. Rare ultramafic rocks with cumulate textures are present in both the belts. The larger volume of the basaltic rocks preclude the possibility of their being derived by fractional crystallization of the high-MgO components. On the basis of trace element and REE characters the rocks may be classified into three groups. One of the groups shows a tholeiitic trend and include samples mostly from the eastern belt while the second consisting mostly of samples from the western belt shows a calc-alkaline trend. The third group includes samples having elemental ratios intermediate between these two groups. Zr/Nb ratios for the tholeiitic and calc-alkaline samples are different suggesting their sources to be different. The tholeiitic samples have been generated from a source having chondritic REE characters, while the calc-alkaline samples have been generated from a source with LREE enriched character. The high-MgO components in both the groups are suggested to represent high degrees of melting compared to the basalts in each group. It is further suggested that the tholeiitic basalts have been generated relatively early from a chondritic source. Down-buckling of this material has added LREE enriched melts to the source, thereby changing its character into a LREE enriched one. Melting of a source with such changed character has subsequently produced the calc-alkaline melts. Rocks with variable but intermediate characters between these two groups have been generated as a result of contamination between these two groups. 1. Introduction The chemical characters of these Archaean mafic- ultramafic volcanic rocks provide information about Mafic-ultramafic rocks with minor volcanogenic sedi- magma generation processes and the possible tectonic ments and banded iron formation, together with setting of their emplacement. Such data are vital for several ovoid granitoid batholiths of Archaean age, understanding the crustal evolution in any Archaean constitute a significant component of the Eastern craton. In the Eastern Indian Craton petrogenesis of Indian Craton. Dunn (1940) identified the supracrus- the sialic rocks have been studied in detail. (Baksi et al tal rocks as belonging to one stratigraphic unit named 1987; Sengupta et al 1983; Sengupta et al 1991; Saha as the Iron Ore Series. Presently this association is 1994; Sharma et al 1994; Sengupta et al 1996). Com- included in the Iron Ore Supergroup of rocks. Some of pared to them, data on mafic-ultramafic rocks are the granitoid batholiths intrusive into the rocks of the meagre and only recently attention has beeen focussed Iron Ore Supergroup have been dated to be 3.2 Ga on them (Acharyya 1993) but systematic chemical and hence these are of Archaean age. data on these mafic-ultramafic rocks are still lacking. Keywords. Geochemistry; Archaean volcanic rocks; Iron Ore Supergroup; Singhbhum. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 106, No. 4, December 1997, pp. 327-342 Printed in India 327 328 S Sengupta et al 85 ~ @6o I 23 ~ 22 ~ ~]~BAULA" 21 ~ Figure 1. Generalized geological map of the Eastern Indian Craton (modified from Iyengar and Murthy 1982), showing sample locations 1. Iron Ore Group of Badampahar-Gorumahishani area; 2. Iron Ore Group of Gua-Noamundi area; 3. Phyllite shale and BIF of Koira valley; 4. Granitoids of Singhbhum Bonai and Kaptipada batholiths 5. Ultramafic rocks of Sukinda and Nausahi; 6. Sediments and volcanics belonging to Simlipal, Dhanjori and Dalma Groups; 7. Intracratonic metasediments equivalent to Dhalbum Formation; 8. Unclassified cover sediments and gneissic rock 9. Sandstone and conglomerate occurring as younger cover (equivalent to Kolhan); 10. Intrusive granite, granophyre and associated rhyolite. In this paper we present chemical data for mafic- 2. Geological set up and location of samples ultramafic rocks from all the major supracrustal successions of the Eastern Indian Craton. Using field The mafic-ultramafic rocks together with other observations and textural data in combination with members of the supracrustal package occur in two chemistry, the petrogenesis of these rocks are dis- different modes within the Eastern Indian Craton. cussed along with the relationship between the One type occurs within the Singhbhum Granite volcanic rock suits of different composition. batholith, near its eastern margin. A long linear belt Geochemistry of Archaean volcanic rocks 329 of mafic-ultramafic rocks occurs as enclave extending Samples have been collected from the eastern belt to from south of Tatanagar in the north to Badampahar assess the compositional variation along its length and in the south. This major supracrustal succession across the stratigraphic section. Seven samples have covering territories within both Bihar and Orissa been included from Suriagora and Sulaipat. Fifteen state is known as the Gorumahishani-Badampahar more samples from the remaining part of the belt. South of Badampahar this belt bifurcates and Gorumahishani-Badampahar belt have been included both arms continue, one to the west and the other in this study. One sample has been selected from an south, towards Baula-Nansahi as trains of smaller isolated xenolith southwest of Badampahar for the xenoliths within the Singhbhum granite, (figure 1). sake of comparison with the rocks of the eastern belt. The other belt, which is more extensive, occurs along From the western belt samples have been collected the western and southern fringe of the Singhbhum representing the entire exposed extent of the mafic Granite. This extends from Gua~Barajamda up to rocks. Twelve samples have been collected from Sukinda where it often crops out from below an four boreholes drilled up to ll0.00m, northwest of unconformable cover of grits and sandstone (figure 1). Sukinda. This displays an interlayered sequence According to earlier studies, these rocks are folded of mafic and ultramafic rocks underlying the BIF of into a synclinorium popularly known as the Iron Ore Daitari hill (figure 1). Ten samples have been collec- Synclinorium (Dunn 1940), defined by the U-shaped ted from the south eastern part of the synclinorium, pattern of the banded hematite jasper (BHJ) band in representing the volcanic pile underlying BIF of the map (figure 1). Gandhamardan, west of Keonjhar. Ten more samples The primary banding in the mafic-ultramafic rocks have been collected from regions south of the of the eastern belt and the bedding in the associated synclinorium, where the best preserved subhorizontal banded iron formation are subvertical. The rocks are primary layering has been recorded. Two samples metamorphosed and often a schistosity is developed, have been selected from regions west of the synclinor- which is always parallel to the primary banding. As a ium and underlying the BIF unit of the Gua ranges. result of the subvertical nature of the primary band- The differences in the general geological setting of ing a considerable part of the stratigraphic column is the two belts have been interpreted by several workers exposed even in the narrow width of the belt. In the to mean a difference in age of the iron ore hosting Gorumahishani-Badampahar sector of the belt, three rocks of the two belts (Acharya 1984; Chakraborty separate volcanic flows have been recorded; the bot- and Majumdar 1986). According to another school the tom part of each flow is marked by ultramafic rocks eastern belt is correlated with a part of the western and the top is represented by volcanogenic sediments belt, while some lava exposures in the western belt are and banded iron formation. The actual number of interpreted to be of Proterozoic age (Saha 1994). flows in the entire belt is likely to be much more. However, we have observed no difference among the Complete succession is not preserved due to the lavas of the western belt. They can be mapped conti- intrusive nature of the granitoids on either side of the nuously and the contact between the supposedly older belt. At a few locations in the belt, ultramafic rocks and younger lavas could nowhere be deciphered. The with cumulate texture is observed which indicates broad petrological characters in the two belts are the that crystal settling took place in the volcanic flows. same. The only significant difference is in the struc- The width of the western belt is much larger tural disposition of the rocks. In both the belts, at (figure 1). The BIF occurrences in this belt occupy the several locations, granitoid rocks of trondhjemitic and crests of the high hills, and overlie the mafic rocks tonalitic composition are observed to intrude the occupying the low ground. The volume of BIF in this supracrustals. Granitoids from three such locations belt is significantly larger than in the eastern belt. The have been isotopically dated to be 3.2 Ga (Moorbath BIF is everywhere gently dipping. The mafic rocks are et al 1986; Sengupta et al 1991; Sharma et al 1994). metamorphosed as indicated by the absence of any Thus all the rocks in the two belts are older than primary mineralogy, but a metamorphic foliation has 3.2 Ga. Whether there is any difference in the relative not been noticed. At a few locations subhorizontal flow age of the rocks in the two belts cannot be ascertained banding defined by vesicular and nonvesicular layers with the present data.
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