Application of anisotropy of magnetic susceptibility in deformed rocks and associated intrusives...

Prasanth RS et al Anisotropy of magnetic susceptibility in deformed rocks Application of anisotropy of magnetic susceptibility in deformed rocks and associated intrusives from Erattupetta, Madurai block, south India R S Prasanth, P Pratheesh and R B Binoj Kumar Department of Geology, University of Kerala, Trivandrum 695 5 81, Kerala, India E-mail: [email protected] m Abstract. Magnetic fabric studies in the regional country rock and mafic dykes of Madurai block as well as the successive deformational history of the area were portrayed in the present study. Magne tic foliation and linea tion in charnockit e and gneiss ic rocks stand for three major patterns, which embody the three major deformational episodes noticed in the structural analysis. Magnetic anisotropy in the country rocks is mainly controlled by both paramagne tic and ferromagn etic minera ls. The mafic dykes in the area also point to a similar magnetic fabric condition with the country rocks. The emplacement direction of these dykes point to a SE plunge, which is unsw erving with the regional mag netic lineation observed in the country rocks. Hence the mafic dyke episod es represent a later stage of deformatio n or the finit e strain localizat ion during the final deformation al episode. Thus the episod e was mainly control led by the regional tectoni cs. Analysis of a minor shear from the study area also reveals the application of anisotropy in petrofabric analysis. The study point towards the relevance of magnetic fabrics, which can be emplo yed as a petrofabric tool in the nonappearance of noticeable deformation fabrics in a rock, hence it is in consistence with the regional deformation episodes. Key Words: Madurai Granulite Block, Anisotropy of Magnetic Susceptibility, Dyke, Deformation Introduction Magnetic fabrics have long been considered as proxies for mineral fabrics in a wide range of materials (Hrouda 1982; Tarling and Hrouda 1993; Borradaile and Henry 1997). The success of magnetic fabric methods lies in their ability to measure the orientation of hundreds of grains within minutes, against the universal stage or electron backscattered diffraction methods ( 8 ≈ hours /sample). The anis otropy of magnetic susc eptib ilit y (AMS) measured in low fiel d has proved to be partic ula rly useful for studying the flow of igneous rocks, bot h vol canic and plutonic (e.g. Bouchez 1997). A recent study showed that this method could be a valuable tool for studying the petrofabric of migmatites and gneisses (Ferr´e et al. 2003). In these migmatites and gneisses, the low-field AMS fabric is largely dominated by the shape anisotropy of primary multidomain magnetite, a strongly ferrimagnetic phase, that masks the contribution of biotite. Magmatic fabrics in intrusive bodies usually result from the accumulated strain during final cryst allizati on are sens itive indicato rs of the evol ving inte racti ons between mag matic and tectonic processes in the Earth’s crust (Zak et al., 2005). Hence, timing of magma emplacement and fabric geometry are critical factors in understanding the tectonic evolution of the host rock and for the regional correlations of igneous activity. Present work is an attempt to demarcate the defor matio nal history in the regi onal country rocks along with empla cement natu re of the intrusive bod ies usi ng Aniso tropy of Ma gne tic Sus ceptib ili ty (AMS) da ta in con tra st wi th deformational fabrics along the part of Madurai block, one among the prominent granulite block in the Southern Granulite Terrai n (SGT) which having a uniq ue P- T cond it ions and deformational style. The present investigation has been performed with the following objectives. Analysis of the deformation fabric in the regional country rocks. Identificati on of the possible flow directions in the Dolerite dykes emplacement. Structural evaluation of regional deformation Shaji E & Prade epkumar AP (Eds) 2014 Mineral Resources of Kerala Trivandrum: Dept of Geology Univ of Kerala ISBN 978-81-923449-0-4 163

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