STRUCTURAL EVOLUTION OF DHANJORI GREENSTONE BELT, DISTRICT SINGHBHUM, BIHAR, INDIA._

Abstract

The area under investigation forms NW-SE strip of Precambrian Dhanjori metavolcanics and associated metasediments (between longitudes 86°20'-86°30'E and 22°25'-22°40'N) lying at the southeastern margin of copper belt shear zone in Singhbhum district of Bihar, India. The metavolcanics represented , by a suite of mafic to ultramafic amygdaloidal extrusives and occasional intrusives range structurally between massive to intensively foliated types. The amygdules mostly elliptical in shape are composed of secondary quartz. Other rock types exposed in the area include mylonitized quartzites, phyllites and tuffaceous metasediments. Presence of excellent pillow structures indicate that Dhanjori volcanics, at least in part were errupted under sub oceanic or marine conditions. Systematic structural analysis on mesoscopic and macroscopic scales 'together with structural maps suggest the presence of the three phases (Fl, F2 and F3) of deformation in the area. Fl folds developed on quartz veins and relatively larger (>2.5 cm) amygdules (So) are preserved as intrafolial folds and rootless hinges showing tight to isoclinal and reclined geometry. iv The axial planes (Sl) of these folds strike NW-SE and dip with amounts varying from 36 to 48° towards Northeast. The F1 axis, as a rule is parallel to the long axis of amygdules and striping lineations. The second phase deformed the So I I S1 surfaces into {, F2 folds which are mostly coaxial, and broadly Coplanar to F1 folds and interfere with early folds (Fi) to develop type-3 (hook shaped) interference pattern. The third phase includes conjugate set (F3 and F3 ) of structures on S0,IS1IIS2 surfaces. F3 folds are upright plunging, open with NE-SW striking subvertical axial planes. The fold axis, parallel to lineations and S1/S3 intersections, plunges (30 to 40°) towards North-east . F are symmetric to asymmetric kinks and puckers with NW-SE striking axial planes and subhorizontal axis trending NW-SE. The variation in the attitude, of F1-axial planar surfaces (Sl) alongwith mesoscopic and macroscopic fold geometries, indicate that Dhanjori synform is a tight to isoclinal, reclined fold developed on Sl (II S0) layers during F2 phase. Present work attempts to order the chronological relationships between the deformation phases (F1, F2 and F3) and multiple events of shearing along CBSZ. V Two phases of shearing events have been deciphered An early .Pre-Fl followed by a late syn to post F2. F3 and F structures post date both the shearing events. It is also suggested that CBSZ is not only a zone of intense compression but a zone of intense shearing too.