JOURNAL GEOLOGICAL SOCIETY OF Vol. 26, June 1985, pp. 426 to 428

EUHEDRAJoJ AUTHIGENIC TOURMALINE FROl\1 THE KALADGI SANHSTONE, JAMKHANDI, DISTRICT, R. K. SUKHTANKAR Department of Geology, University of Poona, Pune 411 007.

Abstract PI:trographical studies of the KaJadgi sandstone of Precambrian age from the Bijapur district, Karnataka, revealed the presence of detrital and authigenic tourmaline. The presence of authigenic lOurmaline in the form of individual crystals is used to deci· pher the post-depositional geochemical changes. The increasing silica content and the alka1inl~ pH of the interstitial fluids have favoured the formation of authigenic tourma­ line.

Introduction The lithology and structure of the Precambrian Kaladgi basin have been des­ cribed previously (Foote, 1876; Nautiyal, 1966; Gokhale, 1977; Viswanathiah~ 1968, 1977). Petrographical studies of these rocks around lamkhandi (16°, 30' N; 75°, IS' E), Bijapur district,. Karnataka, were undertaken to evaluate the post-depo­ sitional changes in them. During the studies, the presence of authigenic tourmaline as individual grains has been recognized in the Kaladgi sandstone.

Geology The Kaladgi Group, represented by conglomerate, sandstone and shale of Pre­ cambrian age overlies basement granites and gneisses and is, in turn, overlain by the Deccan ba~,a1t flows of Late Cretaceous to Miocene period. Sandstone is the major. litho-unit of the Kaladgi Group in the area investigated. Field studies indicate the presence of an easterly plunging syncline (Fig. 1).

Petrogra pllY Quartz, feldspar and lithic fragments are the major detrital minerals. In some rnicrosections, tourmaline and clay minerals are present as accessories. Cement is argillaceou8, ferruginous, calcareous or siliceous in composition. Sandstone is classi­ fied as quartzose arenite to subfe:ldsarenite to feldspathic wackes, as per the classi­ fication of Okada (1971). It is c.bserved that each type of cement has replaced detrital minerals, j.e.~ quartz and feldspar to varying degrees, indicating post-depositional changes (Plate I, photo I). Clay minerals are devl~loped in sandstones with argillaceous cement, due to authigem!sis (Plate I, photo 2). Tourmaline is present in two. clearly distinct forms, exhibiting differences in colour, grain size, habit and mode of occurrence. One type is pleochroic in shades of brown and green and shows abrasional rounding of corners and edges. It shows development of overgrowths on its curved borders (Plate I r, Fig. 1). In contrast, the second ltype shows pleochroism in shades of blue and green, and parallel extinc­ tion. It oc:curs in the form of acicular, prismatic needles, with an average size range between 0.02 mm - 0.1 mm (Plate II, photo 2). It occupies intergranular or interstitial spaces and is free from any overgrowths and is prescnt only in sandstone with carbonate cement.

0016-7622/85/26-6-426/$ 1.00 © Geol. Soc. India. JOUR. GEOL. SOc. INDIA, VOL. 26, No. 6 PLATE r

1 Figure I. Authigenic overgrowth on potash feldspar (i ndicated by an arrow), bar scale 0.5 mm .

2

Figure 2. Replacement of detrital feldspar and quartz by calcareous cement; note the presence of tourmaline in the cement, bar scale 0.4 mm. JOUR . GEOL. SOc. INDIA. VOL .. 26, No. 6. PLATF II

1

Figure I. Authigenic overgrowth on detrital, rounded tourmaline (indicated by an arrow), bar scale 0.04 mm .

2.

Figure 2. Development l,f authigenic tourmaline at the expense of the cement, bar scale 0.03 mm. RESEARCH NOTES 427

10"

30 .. . ~. 15' . ..,..... 10· . . ""r' ...,.. . 10'. ...,.. .. \0 _ . ~~::: ...... 10' : : '. ~ : : : I------t H~n~ht~af.t! lif' /I {\ ~ 1\ BASALT

SHALE D- " 1\ 1\ " 1\ A 1\ 1\ II. SANDSTONE /I 1\ /I :" /I 1\ 1\ 1\ 1\ I\~II GRANITE 1\ 1l..:;"..:/I "(10~ 1\ •. /1 1\ 1\ /I 75 15 E 1\ 1\ 1\ OIP~ STRIKE Figure 1. Geological Map of the area around Jamkhandi Bij apur District, Karnataka.

These features are taken to mean that there are two generations of tourmaline in the Kaladgi sandstone. Discussion

The KaJadgi sandstone exhibits diagenetic changes, such aS 1 ferruginous coating on detrital grains, authigenic overgrowths on detrital quartz, feldspar, zircon and tourmaline, and replacement of quartz and feldspar in varying degrees by different types of cement. Of these, overgrowths on the cores of detrital feldspar, tourma­ line and zircon have been reported earlier (Govinda Raju]u and Gowda, 1968; Govioda Rajulu and Nagaraja, 1966, 1967, 1969). The feldspathization of the Kaladgi sandstone from Bilgi, Bijapur district has been reported by Viswanathiah and Murthy (1977). G0vinda Rajulu and Nagaraja (1969) have reported 'authigenic tourmaline' from the Kaladgi sandstone from the lamkhandi area. However, their description relates to authigenic overgrowths on the cores of detrital tourmaline grains and not ,on authigenic tourmaline grains sensu stricto. From the figure showing overgrowths on tourmaline, presented by Govinda Rajulu and Nagaraja (op. cit.), it appears that their observations are based on the separated heavy mineral fractions and not on microsections of the Kaladgi sandstone. The presence of overgrowths on tourmaline ,grains have led them to conclude the discrete crystals of tourmaline from the Jam­ khandi sandstone as part of detrital matrix rather than authigenic. J n the present studies, the second variety of tourma1ine, to which an authigenic {)rigin is being suggested, is in association with carbonate cement. This suggests synchroneity. The petrographic observations also reveal the replacement of detrital quartz grains by carbonate cement. The presence of carbonate cement suggests that the nature of the interstitial fluids was changing from acidic to alkaline. The 428 RESEARCH NOTES replacement of detrital quartz (i.e., dissolution of silica) by carbonate cement jndi-· cates the increasing concentration of silica in the interstitial fluids. The geochemical changes were thus characterized by increasing silica content and alkaline pH. The geochemicaJ changes - replacement of quartz and precipitation of carbonate cement - took place in alkaline solutions having pH greater than 9 (Friedman and Sanders,. 1978). According to the report made by Gautier (1979), the possibility of the for-· mation of tourmaline by authigenesis cannot be ruled out. Conclusion The petrographical studies of the Kaladgi sandstone from the lamkhandi area reveal the presence of two generations of tourmaline - detrital and authigenic. The geoch(:mical changes characterized by increasing silica content and alkaline pH in the interstitial fluids, during post-depositional period, appear to have favoured the formation of authigenic tourmaline. The presence of ferruginous cement suggests reducing conditions, thereby indi­ cating the restricted nature of the ·basin. The change from acidic to alkaline pH and replac(:ment of detrital quartz by carbonate cement suggests that the nature of the Kaladgi Basin, in this part of the area, changed from a restricted to an open basin.

Acknowledgements: The author is thankful to Prof. K. B .. Powar, Head of the Geology Department, Ponna University, for providing facilities and useful sugges­ tions; to rv1frs A. P. Gokhale for her help during the course of the present work and to Mr. A. 13. Chakranarayan for going through the manuscript.

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