International Journal of Latest Research in Science and Technology ISSN (Online):2278-5299 Volume 6, Issue 2: Page No.38-43,March-April 2017 http://www.mnkjournals.com/ijlrst.htm ISSN:2278-5299 38 PETROGRAPHIC CHARACTERISTICS OF METASEDIMENTARY LIMESTONE AND THE ASSOCIATED ROCKS FROM THE SOUTH ETTAYAPURAM TALUK OF TAMIL NADU 1 Jayant Kumar Padhi, 2 G R Senthil Kumar* 1 M.Sc Geology, Dept of Earth Sciences, Annamalai University, Chidambaram, Tamil Nadu, India. 2 Associate Professor, Dept. of Earth Sciences, Annamalai University, Chidambaram, Tamil Nadu, India. * Email ID: gr_senthilkumar@yahoo.com Abstract- The present work describes the petrographical characteristics of the crystalline limestone and its associated rocks occurred in the southern Ettayapuram Taluk of Tamil Nadu. In the study area, crystalline limestone occurs in pink and greyish-white colour. The chief associated rocks are hornblende gneiss and calc-gneiss. The petrographical investigations on these rocks were carried out using a high-resolution petrographical microscope. The limestone shows a majority of recrystallized calcite along with the presence of quartz and diopside. Diopside occurs as rounded crystals within the limestone. In pink limestone, diopside appears as deformed grains. The hornblende gneiss consists hornblende and clinopyroxenes, which tend to form the dark colouration of the rock. It has a scarcity of calcite content. The gneissic bands (white) were formed by minerals such as plagioclase, orthoclase and quartz. The calc-gneiss is the most abundant country rock in the study area and it contains more silicate minerals than the calcite. The light coloured bands of the calc-gneiss rock were formed by calcite, quartz and plagioclase whereas; the dark bands are formed by the minerals like hornblende and diopside. Keywords - Metasedimentary rocks, petrography, crystalline limestone, hornblende gneiss, calc-gneiss I. INTRODUCTION According to DaubrØe, 1867, crystalline limestones are formed by recrystallisation of limestone as a result of metamorphism [1]. Bruce Foote (1883) explained about the crystalline limestone occurrences in southern Tamil Nadu [2]. The Tamil Nadu State ranks seventh in India in terms of production of limestone. There are 12 major cement plants functioning in the State the total limestone reserves are about 1,473 million tonnes. The crystalline limestones of Tamil Nadu are perhaps the oldest 2660 million years of limestones in the world. The crystalline limestones in Tamil Nadu are found associated with quartzite, calc-silicate rocks and garnetiferous-sillimanite-gneiss [3]. The oldest limestone seen in India occur all along with the other meta- sedimentary-migmatite formation as part of the Archaean Basement Complex. The increased temperature and pressure with time period had altered the original textural, mineralogical and structural characteristics of the calcareous sediments to coarse crystalline limestone with other metamorphic mineral assemblages. Often gneisses, quartz veins, pegmatites and charnockitic rocks also cut across the crystalline limestone [4]. Previous investigations on crystalline limestones of Tamil Nadu were carried out by various geologists (Narayanaswami, 1942-43 [5]; Narayanaswami and Gopal, 1947-48 [6]; Narasimhan, 1960 [7]; Dharmaraj, 1966 [8]; Mani and Basu, 1975 [9]; Srinivasan, 1982-83 [10]; Paranthaman, 1983-84 [11]; Jayaprakash, 1985-86 [12]). Fig.1 Geological map of the study area