TECTONIC HORSE: FIELD EVIDENCE FROM THE SOUTH PURULIA SHEAR ZONE, INDIA

The metamorphic rocks of Chhotanagpur Granite belt exposed in Manbazar area, Purulia district of West Bengal, India provides an opportunity to study the relation between the deformation as expressed by the complexity of structure and metamorphism in upper greenschist facies. The area forms a part of a large shear zone, north of South Purulia Shear Zone (SPSZ). The major rock types include phyllite, mica-schist, amphibolites, gondite, granite gneiss, augen gneiss, granite mylonite and tourmaline rich pegmatite. The southern part of the area is characterised by the presence of granite, amphibolite and quartz-mica schist. The granites and amphibolites are deformed to variable degrees from simple granite gneiss, through proto-mylonite to mylonite. The mylonitization may be related to the shearing along the WNW–ESE trending SPSZ. The schistose quartzite and amphibolites occurs as xenoliths within the deformed granite body. The nature of the parent rock of amphibolite is also debatable; it might be mafic igneous rock occurring as dyke, or impure calc-magnesium sediments. From the field relations it appears that at least some of the amphibolites were doleritic intrusive into the metasediments and older than the granite. Gondite is present as an isolated rock body surrounded by the granite. It is later refolded by the same deformation that has affected the granite. An early isoclinal fold (F1) is later shear folded (F2) to a hook shape interference pattern. The portion of the gondite exposed is totally un-sheared due to high competency and forms a horse within the deformed sheared rocks. The area has undergone polyphase deformation. Initial sedimentary rocks were deformed to form a schistosity (S1) which was later tightly folded with the development of crenulation cleavage (S2) forming the dominant E-W trending transposition structure. Local shearing affected S2 along the contact of metasediments forming S3. The entire area has later sinistrally folded by an N-S trending axial plane (S4). The consistent sinistral shape (looking from south) of the asymmetric folds (F4) in metasediments and granite, and the presence of S-C fabric in granite, quartz tourmaline rock and phyllite on the horizontal surface suggest a ESE-WNW strike slip movement with sinistral sense of shear on the sub-vertical planes.