petrographic characteristics of metasedimentary limestone and the ...

International Journal of Latest Research in Science and Technology Volume 6, Issue 2: Page No.38-43,March-April 2017 http://www.mnkjournals.com/ijlrst.htm

ISSN (Online):2278-5299

PETROGRAPHIC CHARACTERISTICS OF METASEDIMENTARY LIMESTONE AND THE ASSOCIATED ROCKS FROM THE SOUTH ETTAYAPURAM TALUK OF TAMIL NADU 1 Jayant Kumar Padhi, 2G R Senthil Kumar* 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: [email protected] 1

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 Daubrée, 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 metasedimentary-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

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International Journal of Latest Research in Science and Technology.

II. GEOLOGICAL SETTING OF THE STUDY AREA The study area is located near the Podupatti Village of Ettayapuram Taluk, Thoothukudi District, Tamil Nadu. The study area is situated at a distance of 19 km from Ettayapuram Town in the north and 5 km from the Pasuvanthanai Village in the south. The chosen study area falls under Survey of India toposheet number 58G/16, with the coordinates of 9°1’59” to 9°2’12” N latitudes and 77°58’51” to 77°58’54” E longitudes. The crystalline limestone occurs as a band of about 320m length and extends in an N-S direction. The average width of the band is about 36m, however, the central portion of the band is showing a maximum width of around 54m, this may have resulted due to folding. The average dip of the formation is 72° E and the dip amount varies between 60° and 82°.In the study area, the limestone occurred in two different colours, the pink and greyish-white. They are encountered within the country rock of calc-gneiss. In many places, intrusions of quartzite and patches of hornblende gneiss are noticed. Geological map of the study area is shown in Fig.1. III. MATERIALS AND METHODS A. Specimen collection In the limestone and the associated rocks, representative specimens were collected from the study area for thin section preparation and petrographical studies. In the limestone band pink and greyish-white coloured specimens were collected separately and in the associated rocks; hornblende gneiss and calc-gneiss specimens were collected and packed carefully. The pictorial images of the hand specimens are shown in Fig.2. B. Preparation of thin section The collected representative specimens were dispatched to the lab for preparation of thin sections. The thin sections were prepared by standard procedures, for this study, thin sections were made from M/s. Lab Crystals, Lucknow, Uttar Pradesh. C. Microscopy The microscopic investigations of thin sections were done using a Euromax Holland petrographic microscope. The minerals were investigated in plane polarised light (PPL) and crossed polarised light (XPL). A detailed textural and the mineralogical studies were done using the petrographical microscope.

Fig.2 Photographs showing hand specimen of greyishwhite limestone (A), pink limestone (B), hornblende gneiss (C) and calc-gneiss (D), collected from the study area.

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Fig.3 Photomicrographs under cross nicols showing (A) thin section of greyish-white limestone with the presence of rounded diopside (Di), anhedral calcite (Cal) and quartz (Qtz) grains. (B) white limestone showing pearly blue interference colour of calcite (Cal) and fragmented quartz (Qtz) crystals with diopside (Di) grains of varying interference colours. (C) white limestone with calcite (Cal) grains showing pink and grey interference colours with quartz (Qtz) along with diopside (Di) with 3rd order blue interference colour. (D) pink limestone showing calcite with perfect rhombohedral cleavage and grey to yellow interference colours and diopside (Di) occurring along with quartz (Qtz), and (E) greenish to yellow interference colour of diopside (Di) occurring along with quartz (Qtz) and large anhedral calcite (Cal) grains.

IV. PETROGRAPHY A. Greyish-white limestone The greyish-white limestone found in the study area comprises of calcite, quartz and diopside. Calcite is the dominating mineral, which is completely recrystallized and thus producing a granoblastic texture. Calcite crystals occur as equigranular crystals and anhedral in shape. They are colourless in plane polarised light and have low relief. They show perfect rhombohedral cleavage and symmetrical ISSN:2278-5299

extinction to the cleavage planes (Fig.3A). The lamellar twinning in calcite usually shows a bright interference colour. The calcite itself has a grey to higher order bluish (Fig.3B) or pinkish (Fig.3C) interference colour. Quartz presented along with the calcite in a saccharoidal distribution. They occur as anhedral crystals with 1st order grey interference colour. The metamorphism imposed the quartz with undulatory extinction. The third mineral assemblage is diopside, which appears to be pale green in plane polarised light and has a 2nd order blue and yellow interference colours and an oblique 40


Fig.4 Photomicrographs under cross Nicol showing (A) hornblende gneiss with large anhedral hornblende (Hbd) grain showing 3rd order green and blue interference colour and diopside with 3rd order reddish yellow interference colour. (B) hornblende gneiss showing elongated grains of hornblende (Hbd) and plagioclase (Plg). (C) hornblende gneiss showing hornblende (Hbd), clinopyroxenes such as augite (Au) and diopside (Di), quartz and orthoclase (Oth). (D) calc-gneiss showing crystals of anhedral calcite (Cal) grain showing perfect rhombohedral cleavage, quartz (Qtz), hornblende (Hbd) and diopside (Di). (E) calc-gneiss rock showing the occurrence of diopside (Di) along with calcite (Cal), quartz (Qtz) and plagioclase (Plg), which shows lamellar twinning and first order grey interference colour. extinction angle of 38°. Diopside is present as anhedral grains and shows a lack of structure, generally appearing as rounded crystals (Fig.3A). Diopside typically occurs as an accessory mineral. The primary assemblages of the grayish-white limestone are calcite + quartz + diopside. Philpotts (1990) explained that, such mineral assemblages belong to amphibolite to granulite facies of metamorphism [13].

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B. Pink limestone Similar to the greyish-white limestone, the pink limestone also composed predominantly of calcite. The calcite shows grey to higher order yellowish interference colour. A perfect rhombohedral cleavage and symmetrical extinction suggest complete recrystallisation. The grains are anhedral and show granoblastic texture (Fig.3D). The quartz in pink limestone is less dominant as compared to the greyish-white limestone. The quartz shows first order grey interference colour along with the undulatory extinction imparted due to 41


metamorphism. The quartz occurs as anhedral scattered grains throughout the section. The pyroxenes attached to the rock are present as accessory minerals. The typical pyroxene occurring is diopside, shown by its higher second order interference colour. The pyroxenes are exhibiting cataclastic texture (Fig.3D and 3E). The mineral assemblage of the pink limestone is calcite + quartz + diopside, which also reflects amphibolite to granulite facies of metamorphism, similar to greyish-white limestone [13]. C. Hornblende gneiss In thin section, it shows a gneissose texture. Complete recrystallization of mineral grains was observed and the grains are anhedral in shape. Under the plane polarised light, most of the hornblende grains show high relief; pleochroism from dark green to yellowish green colours. The cleavage planes are quite deformed due to metamorphism, although careful observation reveals some cleavage planes. Under the crossed Nicols, the hornblende grains show interference colours varying from 3rd order green (Fig.4A) to yellow or orange (Fig.4C) interference colour. Elongated grains of plagioclase are seen in places having lamellar twinning (Fig.4B). The plagioclase present in the specimen is Labradorite with an extinction angle of 39°.Clinopyroxenes, such as augite and diopside are also associated with the rock. Augite shows 2nd to 3rd order green to blue interference colour with an extinction angle of 30°. The diopside shows 2nd order pink to yellow (Fig.4C) interference colour with an extinction angle of 15°. Twinning is generally absent among the pyroxenes. The hornblende gneiss also contains quartz which is detected by the characteristics of wavy extinction and low relief. Rare occurrences of orthoclase are also seen showing the simple twinning (Fig.4C). The mineral assemblages of the rock are hornblende + plagioclase + articles + clinopyroxene + Quartz. Such assemblages are implying amphibolite lower granulite facies of metamorphism [13, 17]. D. Calc-gneiss The calc-gneiss occurs as host rock for the crystalline limestone formation. As observed from the hand specimen (Fig.2D), the rock consists of alternating bands of light and dark coloured minerals, thus, showing a typical gneissic structure. In thin section as well, the calc-gneiss shows a gneissose texture, the grains are usually anhedral (Fig.4D). The mineral assemblage of the calc-gneiss is quartz, plagioclase, calcite and hornblende along with the clinopyroxenes. Distinct band among the minerals like hornblende and pyroxenes are observed in the thin section (Fig.4E). The quartz is generally anhedral showing its peculiar wavy extinction. The plagioclase mineral in the rock is bytownite, showing a twin lamellae and the extinction angle of 29°. The calcite shows first order grey to higher order blue interference colours. It has the diagnostic perfect rhombohedral cleavage and symmetrical extinction. The pyroxene present is typically diopside which shows a 3rd order greenish blue interference colour. The mineralogical assemblages of the rock are quartz + plagioclase + calcite + hornblende + diopside.

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V. DISCUSSION The present study has brought out the petrographical aspects of the crystalline limestone and the country rocks that occur near the Podupatti Village of Ettayapuram Taluk, Tamil Nadu. Two types of limestones were observed in the field classified as per their colour, namely pink and greyishwhite limestone. The studied carbonates are composed of medium to coarse grains, which are observed in both hand specimen and in thin sections. In metamorphic process, calcite is recrystallised to produce a coarse grain size [15]. In the crystalline limestone, diopside forms as an accessory mineral. The presence of diopside can be attributed to the parental composition of the rock. If the original composition of limestone is dolomitic, i.e., contains Mg, it reacts with silica to produce diopside and carbon dioxide as per the following reaction [14, 16]. Dolomite + 2Quartz = Diopside + 2CO2 The carbon dioxide being volatile gets liberated due to increasing grade of metamorphism. The crystalline limestone of the study area contains less dolomite as compared to the quartz. Thus, it can be inferred that the parent rock, Mg concentration can be less and due to increasing grade of metamorphism reacted with the quartz and entirely converted to diopside [14, 16]. Hornblende gneiss in the study area in hand specimen appears like diorite. However, in thin section it reveals the gneissic characteristics. The main component of the rock is hornblende which appears as a dark green pleochroic mineral under the plane polarised light. The gneissic structure results from the tendency of the dark minerals to be concentrated in bands due to metamorphic differentiation. The hornblende grains show preferred orientation as gneissic banding, growing along the direction of least stress. The plagioclase, typically bytownite is the next most abundant mineral. The other minerals present are clinopyroxene and quartz. The rocks show remarkable cataclastic effects such as granulation and elongation of grains of quartz and plagioclase, with simultaneous banding of twin lamellae and development of undulatory extinction. Turner (1939) explains that the origin of rocks might belong to any of the three categories. i. Primary gneissic diorites in which hornblende and plagioclase are essentially direct products of crystallization from dioritic magma. ii. Basic and semi-basic plutonic rocks that owe their present mineral composition to deep-seated regional metamorphism. iii. A series of basic lava, tuffs and interstratified calcareous sediments or greywackes that have undergone regional metamorphism at depth. The mineral assemblage of the hornblende gneiss rock is hornblende + diopside + plagioclase + quartz, which indicates amphibolites to lower granulite facies of metamorphism. [17] Calc-gneiss is the major host rock for the crystalline limestone body occurring in the study area. Carbonate being subordinate, the rock is enriched in Si, Al and Na-rich minerals. The calc-gneiss has a more complex mineral assemblage than the limestone. The major assemblage of the 42

International Journal of Latest Research in Science and Technology. 9. Mani G. and Basu Anjan Kumar, “Interim report on the investigation for Flux-Grade limestone in Ettimadai Area, Coimbatore District, Tamil Nadu.”, Progress report on field season 1972-73, GSI Report, Dec 1975. 10. Srinivasan J., “Regional re-assessment of limestone resources in Velur-Paramati Area, Salem District, Tamil Nadu. Progress Report for field season 1982-83.”, GSI Report, GSI, Tamil Nadu Circle, Madras. 11. Paranthaman S., “Regional reassessment of limestone resources of the Padmaneri-Perumalkulam and Singhikulam Areas of Nanguneri Taluk, Tirunelvelli District, Tamil Nadu.”, GSI Report, Field Season 1983-84. 12. Jayaprakash A.V., “Regional assessment of limestone resources in Madukkarai-Walayar Area, Coimbatore District, Tamil Nadu.”, GSI Progress report for the field season 1985-86,. VI. CONCLUSION 13. Philpotts Anthony R. and Ague Jay J., “Principles of igneous and metamorphic petrology”, 2nd ed., Cambridge Universty Press, 2010. The present study deals with the petrographic 14. Winter John D., “Principles of Igneous and Metamorphic rocks”, 2nd characteristics of the meta-sedimentary limestone and the ed.; Pearson education Ltd., 2014. associated rocks occurring near the Podupatti Village of 15. Yardley Bruce W.D., “Introduction to metamorphic petrology”, Ettayappuram Taluk, Tamil Nadu, has revealed that the Longman Earth science series, 1990 16. Bucher Kurt and Grapes Rodney, “Petrogenesis of metamorphic limestone found in the study area are crystalline in nature. rocks”, 8th ed.; Springer, eISSBN:978-3-540-74169-5 Two varieties of limestone observed based on their colour 17. Turner F. J. “Hornblende gneisses, marbles and associated rocks from they are pink and grayish-white limestone. The mineral Doubtful Sounds, Fiordland”; Transitions of the Royal Society of assemblage of the limestones consists of calcite and quartz. New Zealand; pp.570-598, 1939. 18. Onimisi Jimoh A., Ariffin Kamar S., Hussin Hashim B., Baharun Diopside occurs as an accessory mineral in both the Norlia B.T., “Petrographic and geochemical characteristics of limestone varieties. The rocks are completely crystallised and metacarbonate in north central Nigeria; Potential applications in formed coarse-grained, anhedral calcite grains due to industries” JGEESI, Vol.3(3), pp.1-10, Aug 2015 metamorphism. The associated rock hornblende gneiss is a 19. Vernon R.H. and Clarke G.L., “Principles of metamorphic petrology”, Cambridge University press, 2008 dark coloured country rock showing gneissic banding of 20. Kerr Paul F, “Optical Mineralogy”, 4th ed.; Mc Graw Hill, Mar 1977. hornblende and pyroxene with plagioclase and quartz, etc., it 21. Perkins Dexter, “Mineralogy”, 3rd ed.; Pearlson education inc., 2011. is deficient of any calcite content. The calc-gneiss forms the 22. Melfos V., Voudouris P., Papadopoulou L., Sdrolia S. and Helly B., major host rock for the crystalline limestone occurred in the “Mineralogical, petrographic and stable isotope study of ancient white marble quarries in Thessaly, Greece-II, Charanbali, Tempi, Atrax, study area. The calc-gneiss rock displays alternating bands Tisiaion Mountain; Bulletin of the Geological society of Greece”, of light and dark coloured. Hornblende and Diopside Congress proceedings of the 12th International congress, Patras., May constitute the darker bands and the lighter bands composed 2010. by quartz, calcite and plagioclase. It can be summarized that 23. 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Research, 1971, pp. 34-42. 26. Jalo Muhammad El-Nafaty, “Geology and petrography of the rocks around Gulani Area, North East Nigeria”, Journal of geology and REFERENCES Mining research, vol. 7(5), Apr 2015, pp. 41-57. 1. Daubrée, “Études synthétiques de géologie expérimentale”, Tome I, 27. Mathur Ramavati, Raj B. Udai and Balam V., “Petrographic 1867, pp. 236-46. characteristics of proterozoic Vempalle carbonates, Cuddapah Basin, 2. Bruce Foote, R. “Geology of Madurai and Tirunelveli District”, 1883, India and their implication”; Journal Geological society of India, Vol. vol.XX, Part-I, pp.1–103. 84(3), Sep 2014, pp.267-280. 3. Vijaya Chitra S., Thangadurai S., Rajeswara Rao R., Sankara 28. 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rock is quartz, plagioclase, calcite, hornblende and pyroxenes. Calcite is stable over a large range of temperature and pressure conditions [14]. At high temperatures and low pressures, calcite may reach with the silica present to form calc-silicates [15]. Yardley (1990) explains that the calcite crystals are susceptible to extensive textural changes due to recrystallisation of calcite to produce a coarse grain size and often a preferred orientation [15]. The gneissose structure of the study area calc-gneiss has formed due to such preferred orientation of mineral grains.

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