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_Journal of Structural Geology 32 (2010) 249–261_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Superposed deformation fabrics in the Precambrian metabasic rocks of the Iron Ore Group, Singhbhum craton, Eastern India: Evidences from anisotropy of magnetic susceptibility studies Gautam Ghosh a,* Snehalata Kumari a S.K. Patil b Joydip Mukhopadhyay a Arijit Raya a Department of Geology, Presidency College, Kolkata, India 700073 b IIGM, Allahabad, UP, India Article info Article history: Received 19 June 2009 Received in revised form 30 November 2009 Accepted 4 December 2009 Available online 11 December 2009 Keywords: Archaean Iron Ore Group AMS Superposed deformation Singhbhum craton Abstract Anisotropy of magnetic susceptibility (AMS) data have been utilized to build up the multiple deformation sequences and strain variation from an apparently undeformed metabasic lava sequence of the basal part of the Paleo-Mesoarchean greenstone succession of the Iron Ore Group rocks in the western Iron Ore basin, Singhbhum craton, eastern India. The basal lava crops out bordering the regional horseshoe shaped outcrop (the horseshoe synclinorium structure) of the overlying BIF rocks. AMS data of 892 cylindrical specimens drilled from 83 samples collected from different structural domains within the synclinorium structure are presented in the analysis. AMS orientation data have been analyzed in the light of superposed fold (Type-1 interference) fabrics developed within BIF as a result of successive deformation episodes (D1 and D2). It is found that the lava samples ubiquitously present two consistent magnetic fabrics homogeneous over smaller sub-areas, which have been successfully correlated with the D1 and D2 deformation fabrics from the BIF rocks. Moreover, a regional scale variation in intensity and nature of strain is noticed from the AMS data, which have been utilized to decipher folding mechanism as well as the changing deformation pattern during successive folding events. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Identification and analysis of separate deformation episodes from a multiply deformed terrane is an important aspect of structural geology. It involves recognizing planar and linear fabrics of each deformation episode and studying their mutual relationships to ascertain the exact chronology and kinematics of deformation phases (Ramsay, 1967; Turner and Weiss, 1963; Ramsay and Huber, 1987; Thiessen and Means, 1980; Ghosh, 1966; Ghosh et al., 1992, 1993; Simon, 2004). However, usually practiced methods of geometrical analysis in a superposed terrane require presence of multiple sets of mesoscopic structures commonly manifested by folds, faults, foliations and lineations. In the absence of such planar and linear structures, identifying the separate deformation episodes and establishing the sequence and kinematics of deformations is a challenging task. The anisotropy of magnetic susceptibility studies (AMS) have been utilized in recent times in such cases to infer the deformation kinematics from terranes lacking any obvious deformation imprints (Hrouda, 1993; Tarling and Hrouda, 1993; Borradaile and Henry, 1997; Borradaile and Jackson, 2004). Deformation studies using AMS data in rocks like granites that do not contain well developed mesoscopic lineations and foliations have been extensively carried out (Bouchez et al., 1990; Archanjo et al., 1994; Leblanc et al., 1996; Bouchez, 1997; Djouadi et al., 1997; Borradaile et al., 1998; de Wall et al., 2001; Gleizes et al., 2001; Ferre? et al., 2002; Neves et al., 2003; Sen et al., 2005; Parada et al., 2005; Sen and Mamtani, 2006). Besides, AMS studies have been successfully applied to analyse fold-thrust structures (Hrouda, 1978; Mamtani et al., 1999; Hrouda et al., 2000; Jayangondaperumal and Dubey, 2001; Mukherji et al., 2004) and record strain variations and strain gradients (Hrouda, 1993; Tarling and Hrouda, 1993; Borradaile and Henry, 1997; Mukherji et al., 2004) in deformed terranes. However, AMS studies have not yet been used for establishing superposed deformation from strata that do not apparently exhibit megascopic evidences for multiple deformations though being a part of multiply deformed terrane. We document here, for the first time, evidences of superposed deformation from AMS studies of the metabasic lavas that are devoid of any mesoscopic deformational features from the Archaean greenstone belt succession of the Iron Ore Group, Singhbhum Craton, eastern India (Fig. 1a). The metabasic rocks form the basal unit of the Iron Ore Group (IOG) of rocks in the western Iron Ore basin (Jones, 1934; Saha, 1994; Ghosh and Mukhopadhyay, 2007; Beukes et al., 2008) around Noamundi–Koira Valley of the eastern Indian states of Jharkhand and Orissa. These are overlain by manganiferous shale, BIF-iron ore and an upper shale unit (Jones, 1934; Saha, 1994; Beukes et al., 2008). The western IOG rocks are disposed in a horseshoe shaped synclinorium structure (Jones, 1934; Dunn, 1929; Saha, 1994; Ghosh and Mukhopadhyay, 2007). Two episodes of deformation have been established from the BIF and host shale units of the IOG synclinorium (Jones, 1934; Dunn, 1940; Sarkar and Saha, 1962; Ghosh and Mukhopadhyay, 2007). However, the basal metabasic lava in most of the outcrop belts lack any mesoscopic deformation fabrics that prompted some workers to assign a younger Proterozoic age to these rocks (Sarkar and Saha, 1962; Saha, 1994). The primary objective of this contribution is to assess the extent of deformation in these apparently undeformed metabasic lavas through AMS studies and correlate the deformation history with that established from the marker BIF-unit of the synclinorium. 2. Geological background The Singhbhum craton is one of the oldest Palaeo-Mesoarchean cratonic nuclei of the peninsular India (Saha, 1994). The craton is a composite mosaic of Archean greenstone-granitoids and Proterozoic supracrustal successions (Fig. 1a) (Acharyya, 1993; Saha, 1994; Sarkar, 2000; Mukhopadhyay, 2001; Naqvi, 2005; Misra, 2006). The Singhbhum Granite is the aerially most extensive unit, with three phases of emplacement between 3.3 Ga and 3.1 Ga (Saha, 1994; Misra, 2006), and encloses synformal keels of BIF-bearing greenstone belts, the Iron Ore Group (IOG) (Jones, 1934; Acharyya, 1993; Saha, 1994; Mukhopadhyay, 2001; Beukes et al., 2008), along its western (Noamundi–Koira), eastern (Gorumahisani–Badampahar) and southern (Malaygiri–Tamka–Daitari) peripheries (Fig. 1a), informally known as the western, eastern and southern IOGs, respectively. A minimum w3.2 Ga age was provided for the Iron Ore Group by granitoids intrusive into the three outcrop belts (cf. Paul et al., 1991; Misra, 2006). Recently, Mukhopadhyay et al. (2008) obtained 3506 ± 2 Ma U-Pb SHRIMP zircon age from the dacitic lava of the southern IOG around Daitari and Basu et al. (2008) reported w3.4 Ga age from the volcanics of the western IOG in the Noamundi–Koira Valley. 2.1. Western Iron Ore basin The western IOG (Jones, 1934; Dunn, 1940; Sarkar and Saha, 1962; Saha, 1994) includes from base to the top, thick, massive, or pillowed metabasalt, lower phyllitic shale–tuff (locally with thin carbonates and manganiferous units), BIF-iron ore, and upper phyllitic shale (Ghosh and Mukhopadhyay, 2007; Beukes et al., 2008). Two phases of deformation structures have been described by most of the workers from this belt (Jones, 1934; Dunn, 1940; Sarkar and Saha, 1962; Saha, 1994; Ghosh and Mukhopadhyay, 2007). The main regional (D1) structure comprises a low north-northeasterly plunging synclinorium, the ‘horseshoe synclinorium’ (Fig. 1a), overturned towards east (Jones, 1934; Dunn, 1940; Sarkar and Saha, 1962), which has been cross-folded (D2) along an east-west axis (Jones, 1934; Sarkar and Saha, 1962; Ghosh and Mukhopadhyay, 2007). Chatterjee and Mukherjee (1981) and Mukherji et al. (2004), however, recognized a three phase deformation history with the earliest phase (D1) represented by NW-SE to N-S trending very tight reclined to inclined folds that have been coaxially refolded by the regionally persistent N-S to NNE-SSW trending D2 folds and a third generation of E-W trending cross folds (D3 struc' Ключевые слова: early fold, primary layering, jde jde, magnetic anisotropy, metabasic lava, piper, science, jones, magnetic fabric, dunn, iron ore, scep, specimen, foliation, girdle represents, ramberg, prolate shape, naqvi, map, positive, ams fabric, superposed deformation, mukherjee mukherji, limb, gondwana, borradaile, gleizes, gandhamardan iron, susceptibility, mutual relationship, earth, allahabad india, superposed, scale, price, ghosh mukhopadhyay, oblique superposition, study area, acharyya, granite, journal structural, bouchez, pole, western iog, dacitic lava, hinge, regional scale, eld data, journal, multiple set, magnetic, tectonic origin, iog rock, domain-microlithon fabric, deformation fabric, high angle, geological map, orientation, magnetic susceptibility, basin pattern, degree, deformation episode, greiling, western belt, mukherji, fabric, ?eld, systematic change, bif-iron ore, principal, data, western syncline, analysis, jelinek, synclinorium, york, kinematics, geological, iog, ams data, ams, study, preferential occurrence, hrouda borradaile, singhbhum, jelinek plot, ghosh, tectonophysics, lava sample, london, average, susceptibility ellipsoid, gandhamardan, india, layering, axial, hrouda, axial surface, eastern limb, jd, overlying bif, sengupta, basal lava, mukhopadhyay, fold set, saha ghosh, e-w, syncline, pj, anisotropy, misra, jdw jdw, bif, anastasio, kj, angle, development, anticline, lava sequence, mamtani, fischer, ams study, structural, unit, kj kj, precambrian, developed, sarkar, deformation, core specimen, paramagnetic mineral, episode, geology, variation, emplacement, core, domain, regional fold, indian, elsevier, ramsay, horseshoe, indian institute, average orientation, structural geology, bif rock, zone, higher susceptibility, structural analysis, basal, structural domains, regional, mesoscopic fold, physics, sample, jdw, pattern, superposed buckling, higher strain, subtle variation, constrictional strain, lava, attening type, society, belt, thiessen, hinge zone, craton, structure, mesoscopic scale, eastern anticline, jagannathpur, second-order fold, strain, fold, site, sen, layer, western, group, outcrop, basin, western limb, iron, jde, bulk susceptibility, folding, ore, ghosh journal, overturned limb, mukhopadhyay beukes, second-order, crustal evolution, set, high, strain variation, saha, journal structural geology, direct bearing, structural domain, strain development, jd jd, sequence, magnetic foliation, area, rock, eastern, negative, magnetic data, table, singhbhum craton, fold hinge, tj, ferromagnetic mineral, geologists