CHAPTER I GEOLOGICAL SETTING GENERAL GEOLOGY OF THE EASTERN INDIAN SHIELD The Eastern Indian Shield is bounded by Mahanadi Graben and Sukinda thrust in the west and in the south by granulite terrain of Eastern Ghats and recent coastal alluvium. In the north and east it is masked by vast Gangetic alluvium and Quaternary sediments of Bengal Basin (Fig. 1). It has received adequate attention of geoscientists and almost all aspects of its geology have been discussed (Mukhopadhyay, 2001; Mahadevan, 2002). However, due to scarcity of reliable radiometric age data and structural complexities, stratigraphic sections and evolutionary models from difTerent parts are inconsistent from each other. Three geological provinces have been recognized in the Eastern Indian Shield: (i) Chotanagpur Granite Gneiss Complex (CGGC) (ii) Singhbhum Mobile Belt (SMB) and (iii) Singhbhum-Orissa Craton (SBOC). The tectonic and temporal relationships between these geological provinces and the stratigraphy within each province have been the subject of the debate (Mahadevan, 2002; Naqvi, 2005). Stratigraphic successions proposed by various workers are given in Table 1. These successions indicate that there are wide differences of opinion regarding the chronostratigraphy of various metasedimentary rocks of SMB and their relationship with the rocks of the SBOC. The general geological features of each of the above geological provinces are briefly discussed in the following sections. CHOTANAGPUR GRANITE GNEISS COMPLEX (CGGC) Chotanagpur Granite Gneiss Complex (CGCC) covers an area of about 80,000 km^ (Latitudes 23°00Tsr to 25^00?^; Lx)ngitudes 83°45'E to 87*'45'E) in parts of West Bengal and almost entire Jharkhand state, except the Singhbhum region. It is a composite mass consisting mainly of granite-gneisses, migmatites and massive
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CHAPTER I
GEOLOGICAL SETTING
GENERAL GEOLOGY OF THE EASTERN INDIAN SHIELD
The Eastern Indian Shield is bounded by Mahanadi Graben and Sukinda thrust
in the west and in the south by granulite terrain of Eastern Ghats and recent coastal
alluvium. In the north and east it is masked by vast Gangetic alluvium and Quaternary
sediments of Bengal Basin (Fig. 1). It has received adequate attention of geoscientists
and almost all aspects of its geology have been discussed (Mukhopadhyay, 2001;
Mahadevan, 2002). However, due to scarcity of reliable radiometric age data and
structural complexities, stratigraphic sections and evolutionary models from difTerent
parts are inconsistent from each other. Three geological provinces have been
recognized in the Eastern Indian Shield: (i) Chotanagpur Granite Gneiss Complex
(CGGC) (ii) Singhbhum Mobile Belt (SMB) and (iii) Singhbhum-Orissa Craton
(SBOC). The tectonic and temporal relationships between these geological provinces
and the stratigraphy within each province have been the subject of the debate
(Mahadevan, 2002; Naqvi, 2005). Stratigraphic successions proposed by various
workers are given in Table 1. These successions indicate that there are wide
differences of opinion regarding the chronostratigraphy of various metasedimentary
rocks of SMB and their relationship with the rocks of the SBOC. The general
geological features of each of the above geological provinces are briefly discussed in
the following sections.
CHOTANAGPUR GRANITE GNEISS COMPLEX (CGGC)
Chotanagpur Granite Gneiss Complex (CGCC) covers an area of about 80,000
km^ (Latitudes 23°00Tsr to 25^00? ; Lx)ngitudes 83°45'E to 87*'45'E) in parts of West
Bengal and almost entire Jharkhand state, except the Singhbhum region. It is a
composite mass consisting mainly of granite-gneisses, migmatites and massive
INDEX Kolhan Group
Gondwana Rocks
Metasediments
Volcanic Suites
Granitoid Complex
Banded Iron Formation
Undiferentiated Rocks
OMer Metamorphic Rocks
Granite - Gneiss Complex
Gangpur Group
/
(After, A. N. Sarkar. 1982)
Eastern Ghats
Newer Oolerite [Tykes
Lineaments
Thrust / Shear Zone
Fig. 1.1. Simplified geological map of Eastern Indian Shield (after Sarkar, 1982) illustrating the three geological provinces viz. Chotanagpur Granite Gneiss Complex (CGGC), Singhbhum Mobile Belt (SMB) and Singhbhum-Orissa Craton (SBOC). SSZ - Singhbhum Shear Zone.
granites with enclaves of para-and ortho-metamorphics, dolerite dykes and
innumerable veins of pegmatite, aplite and quartz (Naqvi and Rogers, 1987). From the
structural patterns, woiiced out in different parts of the CGGC, it is clear that the
region has undergone polyphase deformation producing distinctive folds and related
linear fabrics (Mahadevan, 2002). Reliable radiometric age data are rare in CGGC.
The available isotopic data (Mahadevan, 2002) indicate that the metamorphism,
migmatization or granitization continued for a long period fix)m 1600 to 635 Ma.
Geologic events older than 1600 Ma are not recorded because of the fact that efficient
and robust radiometric systons have not been applied to this geologic province.
Radhakrishna (1989) suggested that CGGC is a Paleoproterozoic Suspect Terrain.
Mahadevan (2002) based on the available structural, metamorphic, lithological and
radiometric data, has proposed that the CGGC may be classified into five east-west
trending belts, namely (i) South Palamau-Gumla-Ranchi Purulia belt, (ii) Daltonganj
(North Palamau)-Hazaribagh belt, (iii) North Gariiawa-Chatra-Gridih-Deoghat-
Dumka belt, (iv) Bihar Mica Belt and (v) Rajgir-Kharagpur belt. All these belts,
metamorphosed into greenschist to granulite facies, have variable proportions of
metasediments and metavolcanic rocks permeated by granitoids and pegmatites.
CGGC is cut into northern and southern parts by Damodar rif^ valley
containing rocks of Gondwana Supergroup. Anorthosite-gabbro plutons are found at
several places in CGGC. Some of the well known occurrences of the anorthosite-
gabbro plutons are Bela gabbro-anorthosite, Debaur anorthosite, Dumka anorthosite
and Bankura anorthosite. A smaller body of anorthosite at Kushinda is associated with
nepheline syenite (Naqvi, 2005).
SINGHBHUM MOBILE BEL T (SMB)
The formations occurring in between the Singhbhum Granitoid Complex
(SBGC) and Chotanagpur Granite Gneiss Complex (CGGC) are now collectively
recognized either as the Singhbhum Mobile Belt (SMB) or Singhbhum Group despite
of different opinions given regarding the tectono-stratigr^hic relationship between
the rocks of this belt and those lying across the well known Singhbhum Shear Zone
(SSZ) in the southern part of this belt (e.g., Dunn, 1929, 1940, 1966; Dunn and Dey,
1942; Iyengar and Anandalwar, 1965; Banerji, 1975,1984; Iyengar and Murthy 1982;
Saikar, 1982; Sarkar and Charkraborti 1982; Saha ,1994). The SMB (Fig. 2), has been
divided into five litho-stratigraphic domains (Saiicar et al., 1992; Gupta and Basu,
2000): (i) Low to medium grade volcano-sedimentary belt, between the Dalma
metavolcanic suite and Utte CGGC in the north, (ii) Dabna metavolcanic belt, (iii) the
metamorphic belt con^rising the schistose and high metamorphic grade Chaibasa and
Dhalbhum Formations, between the SSZ and the Dalma metavolcanic belt, (iv) the
rocks occurring in the SSZ and (v) the volcano-sedimentary formations, including the
disputed Dhanjori and/or the Oi^arbira metavolcanic rocks. These domains preserve
the transiHessional features of their evolution.
The low to medium grade volcano-sedimentary belt, between the Dalma
metavolcanic rocks and the CGGC in the north has been variously referred to as