Sodalite-Cancrinite-Fluorite Syenite from the Granitic Terrain of Podili, Prakasam District, Andhra Pradesh
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RESEARCH NOTES 297 References KRUMBEIN, W. C., (1941) Measurement and geologic significance of shape and roundness of sedimentary particles, Jour. Sed. Pet., v. 11, pp. 64-72. PETTIJOHN, F. J., (1956) Sedimentary Rocks. Harper & Bros., 690 p. POWERS, M. C., (1953) A new roundness scale for sedimentary particles, Jour. Sed. Pet., v. 23, pp. 117-119. Address of the author SUJIT DASGUPTA, Geological Survey of India, Banamalipur, Agartala 799001. Sodalite-cancrinite-fluorite syenite from the granitic terrain of Podili, Prakasam District, Andhra Pradesh M. S. MURTY Abstract The present paper reports the occurrence of sodalite-cancrinite-fiuorite syenite from Podili. This felspathoidal syenite is inferred to be formed by the crystallisation of alk~line felsic magma, derived by the partial melting of deep-crustal rocks, enriched in volatiles. Introduction The occurrence of syenites and especially felspathoidal syenites is extremely rare in granitic terrains. The present paper reports the occurrence of sodalite-cancrinite fluorite syenite from the grey granites of Podili (15°38' 15"N-79°36'45"E), Prakasam District, Andhra Pradesh. The three Taluks - Kanigiri, Podili and Darsi in Prakasam District have attained some notoriety on account of the prevalence of an endemic disease called fluorosis caused by excess of fluorine in waters of the area. The ultimate source of fluorine is the fluorite found in the grey granites. Although several schistose rocks, grey granites, gabbros, pink granites and dolerites are present, none except the grey granite contains fluorite. The grey granite hosts the sodalite-cancrinite-fluorite syenite reported here. Petrography The syenite within the grey granite of the low-lying lands near Podili occurs in two patches, 100 metres apart, each measuring two metres by one metre. It is grey coloured and is conspicuous on account of the presence of blue-coloured sodalite in the form of veins. The rock exhibits hypidiomorphic-granular texture and has perthite and sodalite as major and plagioclase, nepheline, cancrinite, fluorite, biotite, muscovite and iron ore as minor constituents. Potash felspar is a perthite in the form of coarse grains. Plagioclase (An2o) is subordinate and is fine grained. Sodalite occurs as veins cutting across the felspars. Fluorite occurs as small veins and discrete grains. At times sodalite veins narrow down and merge with the fluorite veins forming it continuous vein. Both sodalite and fluorite are isotropic, the former has higher refractive index 298 RESEARCH NOTES (l.485) than the latter (1.434); the latter· shows cleavage and has a pitted appearance. Nepheline occurs as rectangular (idiomorphic) grains showing parallel extinction and low birefringence (0.004). Cancrinite is anhedral, unaltered and exhibits parallel extinction, and high birefringence (0.020). The rock chip when treated with (1 : 5) hydrochloric acid gives mild effervescence indicating the presence of cancrinite (no calcite in the rock). The scooped-out material from the blue-coloured portion of the rock when placed on a glass slide and treated with nitric acid (1 : 1), shows forma tion of cubic crystal of sodium chloride indicating the presence of sodalite (Winchell, 1938, p. 349). The high birefringence of cancrinite distinguishes it from nepheline. Biotite is pleochroic from light yellow to dark brown. Muscovite is colourless with thin laths and high polarisation colours. Iron ore is found invariably as euhedral grains. The modal composition of the syenite (Vol. %) is as follows:- Perthite 60.4 Soda lite 55.0 Nepheline 2.0 Cancrinite 2.2 Fluorite 2.8 Plagioclase 2.6 Biotite 2.4 Muscovite 1.0 Iron ore 1.6 Discussion The role of volatiles is c~sidered to be of prime importance in the genesis of felspathoidal rocks (Sorensen, 1974, p. 48). The absence of limestone as a formation in the present-area precludes the limestone syntexis hypothesis as a possible mechanism for the formation of syenitic rock. The association of felspathoidal syenite with the magmatic fluorite-bearing grey granite (host-rock) of the area makes one think that it too has a similar origin. The alkaline felspathic magma generated by partial melting of the deep crust influenced by the influx of volatiles and mobile elements from the underlying mantle due to the relief of pressure at depth may be responsible for the formation of the felspathoidal syenite. The volatiles and the mobile elements reduce the melting range of the deep crustal rocks, focus heat in active zones and give the felsic magmas their special alkaline character (Bailey in Sorensen, 1974, p.441). It is concluded that the sodalite-cancrinite-flourite syenite is formed by the crystallisation of the alkaline felsic magma enriched in volatiles like CI, S03, CO2 etc. as suggested by Sorensen. Acknowledgement: The author's thanks are due to Prof. M. G. Chakrapani Naidu, for providing facilities for the work. References SORENSEN, H., (1974) Alkaline Rocks. John Wiley & Sons. WINCHELL, A. N., (1968) Elements %ptical mineralogy, Part II, Wiley Eastern Private Limited~ New Delhi. Address 0/ the author M. S. MURTY, Department of Geology, S. V. University, Tirupati, Andhra Pradesh. .