INTRODUCTION. Basalt, the widespread igneous rock on the earth sur-face. today appear to have been equally important

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1 INTRODUCTION Basalt, the widespread igneous rock on the earth sur-face today appear to have been equally important in the geologic past. Basaltic rocks are mostly partial melting products of the upper mantle and thus they provide insights into crust and mantle interaction in terms o-f both petrology and tectonics. Geochemical and experimental studies on basaltic rocks have revealed that the depth of magma formation, degree of partial melting, physico-chemical conditions of the magma, its differentiation, contamination and/or mixing, all these factors leave their signatures on the end products (Yoder and Til ley, 1962; Ringwood, 1975; Dungan and Rhodes, 1978; Sun et al., 1979; Yoder, 1979; Perfit et al., 1980; Thompson et al., 1984; Huppert and Sparks, 1985; Sparks, 1986; E1 1 am and Hawkesworth, 1988). The compositional variations in different basaltic rocks have been successfully used to estimate the depth of magma genesis (Sagimura, 1968), the rate of lithosphere subduction (Sugisaki, 1976), the mantle heterogeneity <Langmuir and Hanson, 1980) and in identification of tectonic environments in plate tectonic parlance <Pearce and Cann, 1973; Pearce et al., 1975, 1977; Pearce, 1976, 1982; Rogers, 1V82; Mullen, 1983; Holm, 1985). Our knowledge of modern plate tectonics, involving the global motions and interactions of large, rigid lithospheric plates greatly influences our interpretation of Precambrian (Kroner, 1981). Numerous investigators revealed that

2 studied by many workers (e.g. Naqyi amd Hussain, 1973a, b; Condie and Baragar, 1974; Naqvi et al., 1974; Naqyi, 1979; Condie, 1985, 1989). Generally, these studies are based on the data which vary in time and space. The eastern part of Indian shield forms a coherent crustal segment in which geological history can be traced continuously throughout most of the iprecambrian starting from 3400 Ma ago (Moorbath and Taylor, 1988>. It therefore provides a classic area for the study of the different stages of the Precambrian crustal evolution. This part of Indian shield comprises the Chotanagpur granite-gneiss complex in the north, the Singhbhum-Orissa Iron Ore craton (referred to as Singhbhum craton in the present study) in the south and an E-U) trending Singhbhum orogenic belt (SarUar, 1982) between these two (Fig. 1). The terrain of Chotanagpur granitegneiss complex predominantly consists of granites and granite gneisses with the enclaves and blankets of supracrustal rocks. Singhbhum craton comprises of granites, banded iron formations (BIF), several type of metasedimentary and basic volcanic rocks. Singhbhum orogenic belt consists mainly of metamorphosed sedimentary and basic volcanic rocks. The Singhbhum craton occupying the southern part of eastern Indian shield hosts fairly extensive occurrences of basic volcanic rocks which are characteristically associated with metasedimentary rocks flanking the granite massif (Fig. 1). In variety and extent these volcanic rocks appear to be different from those of many Precambrian terrains of the

3 world as reviewed by Condie (1981, 1982a). However, these volcanic rocks of Singhbhum craton have not been studied in detail as yet. The stratigraphic positions o-f these volcanics are still not clear (Dunn, 1929, 1940, 1966; Dunn and Dey, 1942; Iyengar and Anandalwar, 1965; Banerji, 1974; Sarkar and Saha, 1977, 1983; Banerjee, 1982; Iyengar and Murthy, 1982; Gupta et al., 1985). Although the age given by Saha and Ray (1984) and Saha et al. (l98s) is questionable, the earliest phase o-f vol can ism in this region is evident from ortho-amphibolites which occur as enclaves within the Older Metamorphic Group. The next phase o-f volcanic activity is represented by Gorumahisani-Ukampahar mafic and ultramafic rocks, whose relationship with the associated metasediments (Badampahar Group; Iyengar and Murthy, 1982) is not clear. Banerjee (1982) suggested that this suite represents a sub volcanic assemblage, intrusive into and extrusive on the Gorumahisani- Badampahar BIF sequence. The late Archaean-early Proterozoic volcanism is represented by Bonai range metavol canics which consist mainly of variable amount of mafic flows and tuff with minor silicic volcaniclastic interbeds (Bose, 1982; Banerjee, 1982). The basic volcanics of Dhanjori, Ongarbira and Jagannathpur areas respectively occur on the north eastern, north western and western margins of the granite massif (Fig. l^. Dhanjori and Jagannathpur volcanic suites have been correlated with Dalma metavolcanics of Singhbhum orogenic belt with a possible age

4 of Ma (Sarkar et al., 1969; Sarkar and Saha, 1977, 1983). Howeyer, Banerjee <1982) on the basis o-f geochemical characters and tectonic style placed the stratigraphic position o-f Dal ma metavol canics above the Dhanjon, Ongarbira and Jagannathpur vol canics. Iyengar and Anandalwar (1965'>, Iyengar and Murthy (1982) and Sarkar and Saha a977, 1983) have correlated the lavas o-f Dhanjon and Simlipal basins. However, Rb-Sr isochron age o-f Ma <Iyengar et al., 1981) for Simlipal basin and structural studies (Sarkar and Saha, 1977, 1983; Mukhopadhyay, 1986) contradict this correlat ion. Ongarbira and Dhanjori volcanics have also been correlated with the Dal ma metavolcanics of Singhbhum orogenic belt (Dunn, 1929; De, 1964; Gupta et al., 1980; Iyengar and Murthy, 1982). Both of these volcanics, occurring in south of Singhbhum thrust, are associated with molasse type metasediments <Banerjee, 1982; Sarkar, 1982; Sarkar and Chakraborti, 1982). Dalma metavol cam cs, on the other hand, unconformably overlie the metasediments of Singhbhum orogenic belt <De et al., 1963; Sarkar and Saha, 1977, 1983; Uerma et al., 1978). The metavolcanics, ranging m composition from ultramafic to basaltic andesite, also contain fragments of dolente, epidionte, serpen t in i te, gabbro and pyroxenite <Sarkar, 1982) and have been considered as a greenstone belt in an in tracraton i c rift (Gupta et al., 1980; Bose and Chakraborti, 1981) and an ophiolite suite resulted by the collision of Chotanagpur and Singhbhum microplates (Sarkar, 1982). Naha and Ghosh (1960) and Banerjee (1982) have

5 considered it as an island arc suite. In order to achieve a better understanding of the Proterozoic geologic events in Singhbhum craton, geochemical investigations on the two most critical volcanic suites are presented here in this thesis. The volcanic suite occurring on the north eastern margin o-f carton is re-ferred to as Dhanjon volcanics (Dunn and Dey, 1942; Banerjee, 1982^, while the suite on the western margin is designated as Jagannathpur volcanics (Saha, 1964). Major, minor and trace elements including REE of various samples from these suites have been determined. Based on the data certain important aspects like comparability of these volcanic suites with those from Archaean, Proterozoic and Phanerozoic eras, their petrogenesis and tectonic implications are discussed. Chapter I contains general geologic frame work of the eastern Indian shield as well as the studied basic volcanic suites. An account of the mineralogic and petrographic characters of Dhanjori and Jagannathpur volcanics is given in Chapter II. Major and trace element distributions and their relationships are discussed in Chapter III and IKf respectively. In Chapter M magma classification is discussed. Under the head of petrogenesis in Chapter VI, the origin of Dhanjori and Jagannathpur volcanics is discussed in the light of geochemical data available in the literature. This will provide insights into the tectonic processes operating m the crust and upper mantle. Chapter UII presents a discussion of the geochemical as well as the

6 geological data to interpret the tectonic settings o-f these volcanics and associated tectonic events in the Singhbhum craton. Summary and conclusions are provided in Chapter VIII.