Calc-Silicate Skarns in the Granites and Gneisses at Jothwad 305 the Wollastonite-Rich Skarns Are Seen in Thin Sections (Fig

CALC-SILICATE SKARNS IN THE GRANITES ANDGNEISSES AT JOTHWAD, PANCHMAHAL DISTRICT, GUJARAT STATE

BY M. S. SADASHIVAIAH (Department of Geology, Karnatak University, Dharwar) Received September 27, 1963

(Communicated by B. P. Radhakrishna, l,.A.sc.)

]NTRODUCTION A REMARKABLZ assemblage of wollastonite-garnet-diopside-scapolite minerals occurs in a granite quarry at Jothwad hill, Panchmahal District, Gujarat State (22", 30'; 73", 44')(Fig. 1). Inliers of gondites and manganese

FIG. I. Loca/ion map of the Jothwad oxea showing the occurrence of the calc-silicate skams (shaded ponion) at Jothwad hill. 303 304 M.S. SADASHIVAIAH ore bodies acutely folded, with granites and gneisses and cut by pegmatites and tongues of granitic rocks are seen exposed in this hilL The Calc-silicate skarns are associated with the gondites of the Champaneer series. The area has been earlier surveyed and described by Fermor (1909) in this memoir on the manganese ore deposits of India. In this and other works frequent references are made to Calc-silicate minerals, but details of the type of wollastonite-scapolite-diopside-garnet association are not given. It is the purpose of the present paper to describe the association and mineralisation of the calc-silicate minerals of the Jothwad hill.

OCCURRENCE Crystalline limestone of the quarry is thick-bedded, pale pink in colour, coarsely crystalline in texture and confined to the margins of the granites and gneisses. The crystalline limestone situated at the contact of the granites and gneisses grades into calc-silicate assemblages containing abundant wollastonite. In the deeply cut portion of the central part of the quarry occurs calc-silicate skarns containing little or no wollastonite, abundant diopside, epidote, phlogopite and scapolite. Calc-silicate minerals often occur as bands along the bedding planes of the limestones.

PETROLOGY Wide variation is noticed in the distribution of the skarn minerals in the calc-silicate rocks of Jothwad hill. They can be grouped under the following petrological types :-- (i) Wollastonite-bearing rocks. (ii) Diopside-garnet-epidote-phlogopite skarns. (i) Wollastonite-bearing rocks.--These are milk-white in colour, coarsely crystalline and show numerous blade.s of wollastonite. The growth of the wollastonite has been facilitated along bedding planes, where it has developed as bunches, patches and veins. Wollastonite bearing skarns are restricted to the limestone granite contacts in the quarry. In thin section, the rock is coarsely crystalline containing abundant prisms, blades, plates and sheaf-like aggregates of wollastonite, often sieved, enclosing numerous granules of the quartz, calcite, diopside, garnet and phlogopite (Fig. 2 A). Invariably the wollastonite is surrounded by granular quartz anda few plates of calcite. Occasionally, thin veins containing quartz, garnet, phlogopite, diopside and scarce scapolite and calcite cutting through Calc-Silicate Skarns in the Granites and Gneisses at Jothwad 305 the wollastonite-rich skarns are seen in thin sections (Fig. 2 C). Granules and patches of quartz occurring within the wollastonite plates often show optical continuity. }

A B C |rnr~ 1~o. 2. Cale-silicates of Jothwad hill. (A) WoUastonite-rir skarn. (B) Diopside-phlogopite-epidote skarn. (C) Wollastonite-rich skarn cut by veins. (W, Wollastonite; C, Calcite; G, Garnet; S, Scapolite; Q, Quartz; D, Diopside; P, Phlogopite, E, Epidote; PL, Plagioclase-felspar.)

(ii) Diopside-garnet-epidote-phlogopite skarns.--Calc-silicate rocks containing predominantly di0pside, garnet, epidote and phlogopite with sub- ordinate tremolite ate found away from the wollastonite zone and occur more or less in the centre of the mass. They are dark greyish-green coloured compact rocks, hornfelsic in texture.

In thin section, the rock is granulitic showing skarn assemblages differing in relative proportions of the minerals even within the limits of a single slice (Fig. 2 B). The following associations of skarn minerals are seen: 1. Tremolite-diopside. 2. Diopside-garnet. 3. Diopside-epidote-garnet. 4. Epidote-garnet. Phlogopite, scapolite, quartz and calcite are invariably present in most of the skarn assemblages. Sieved nature of the skarn minerals and the occurrence of quartz in optical continuity are the salient features noticed in thin sections. Various other minerals enter the calc-silicate assemblages particularly plagioclase felspar, microcline, quartz and apatite and these minerals are found at the junction of the calc-silicates and the granites and gneisses and A4 306 M.S. SADASHIVAIAH

their formation is very much influenced by the proximity of the junction and the presence of the bedding planes of the original sedimentary formation.

M INERALOGY

The main calc-siliczte minerals of the Jothwad hill are wollaslonite, garnet, diopside, scapolite and epidole. Associated with these minerals occur phlogopite and sphene. Wollastonite occurs as white fibrous laths or blades elongated almost parallel to ' b" . Cleavages parallel to (100), (001), (101) and (201) are evident and twins both simple and polysynthetic on (I00) are frequent. Wollastonite encloses tiny granules of epidote, diopside, phlogopite, garnet and quartz.

TABLE I Optical characters of wollastonite

-- 2V -- 39 ~ (20 determinations by direct measurement)

XA C -- 31 ~ [Burri's method (1950). Nemato and Turner's method (Turner, 1942) on twins] n~ ----- 1.630 (Immersion method of Rittmann and Grª 1939) n~ ------1.632 . (Birefringence method of Hess, 1949) n~ ----- 1.619 (y -- e) = 0.013 (r - ~) = 0.002 Twinning: Parallel to (100) (Simple and polysynthetic) Cleavages: (100), (001), (101) and (201).

The plotted cleavages of wollastonite are transferred on a synoptic dia- gram and it is found that the poles of the cleavages are disposed in a zone, the axis of x~hich is the crystallographic axis ' b' and it is 4 ~ removed from "Y' indicating the triclinicity of wollastonite (Fig. 3). Larsen and Berman (Peacock, 1935) have come to similar conclusions about the triclinicity of wollastonite from Crestmore, California. T,hey have opined that it is the only way to deduce the triclinicity of wollastonite (on optical and micro- scopic observations). The oblique extinction of the triclinic mineral is real and appreciable. Similar observation has been made by Balasubramaniyam (1955) while studying the wollastonite from calc-gneisses of Ganguvarpatti. Calc-Silicate Skarns in the Granites and Gneisses at Jothwrd 307

Fio. 3. Synoptic stereogram of the poles of the cleavages of woltastonite.

Peacock (1935) who discussed and described the monoclinic and triclinic modifi- cations of wollastonite, has shown that the wollastonite of skarns is triclinic. Pure crop of wollastonite separated lrom the wollastonite-rich calc- silicate rock, but still containing Ihe tiny inclusions of diopside, scapolite, phlogopite and epidote within the wollastonite plates which could not be ~:eparated, constituting 16 "57o by volume of the wollastonite was chemically analysed and the results are given in Table II together with the calculated minerals. An inspection of the chemical analysis of wollastonite shows Ihat it has appreciable amounts of FeO, Fe203, MnO, MgO and K20. The calculated minerals from the chemical analysis, show diopside (17"327o), orthoclase (1.57~), magnetite (I-67~) and negligible amount of other constituents. Thin sections do show Ihat the wollastonite itself contains granules of diopside, epidote, scapolite and phlogopite, which could not be removed. 308 M. S. SADASH[VAIAH

TABLE 1 I Chemical analysis of wollastonitefrom Jothwad

Constituents % Minerals

SiO~ 50.06 Orthoclase 1.67

AI208 0.34 Wollastonite 74.24 FeOa t. 03 Diopside 17- 32

FeO 2.16 2CaO.SiOe 1-89 MnO 1.30 Magnetite 1.39

TiOz Tr. Na20.SiO2 0.61 CaO 41.11 K20.SiOz i. 32 MgO 1.28

NasO 0- 33 K20 1-20

HiO 1.25 I00.06

Analyst : M. S. Naik.

Givingallowance to these minerals in the chemical analysis, the chemical coml~osifion of wollastonite indicates a nearly pure iron-free wollastonite.

Diopside occurs as greenish grains, of sieved plates exhibiting the characteristic cleavages and often enclosing quartz. It is observed that within the limits of a single slice there are patches rich in diopside and patches poor or devoid of diopside. Diopside occurs associated chiefly with epidote, garnet, scapolite and phlogopite. The following are the optical characters :-- 4 2V = 58 ~ (Direct measuremenl) ZA C - 42 c~ n~ ---- l "698 n~ ---- 1"692 n,~ = 1-718 Calc-Silicate Skarns in the Granites and Gneisses at Jothwad 309

(~, -- c0 = 0.026 (~ -- ~) = 0.006 X = Green Y = Yellowisb green

Z ~ 9, The above optical properties conform to a diopside of the composition Di6a Hez7 (Hess, 1949). Scapofite occurs as glass-clear anhedral grains amidst diopside and garnet in the skarn zone often showing two sets of cleavages. It is uniaxial negative with n, = 1.545, n,, = 1 "580 and n,, -- he --= 0"035, indicating that the scapolite is of Mizzonite variety with Ma29 MeT~ (Larsen and Berman, 1934). Garnet.--Pale yellow granular garnet is seen often in the skarn zone along with the other calc-silicate. It is free from cracks and completely isotropic. It has the refractive index n = 1.805 suggesting that the garnet is a spessartite-almandine variety. Garnet of similar composition has been described from Jothwad by Sathe (1958). Epidote is seen generally as anhedral but rarely as subhedral yellow plates or granules, often sieved and showing occasionally colour zoning. Epi- dote is pleochroic with X-----colourless, Y = pale yellow and Z = lemon yellow. The following are the optical characters:-- -- 2V = 77 ~ (Direct measurement) XA C = lO nt~ = 1-742 n~ = 1" 754 n4 = I "715 (~, -- a) = 0 '039 (~'- B) = 0"012 The above optical properties are comparable with ah epidote containing nearly 20~ HCa~FeaSiaO~3 molecules (Winchell, 1961). Colourless or pale brownish phlogopite is seen as shreds, patches or plates with all the calc-silicate assemblages. It is uniaxial negative. Tremolite is seen very rarely in the skarns. Where observed it is bluish- green, prismatic and fibrous and is particularly associated with diopside. A5 3 t 0 M.S. SADASHIVAIAH

Ir is feebly pleochroic with X-----colourless, Y = pale yellowish-green and Z=pale bluish-green with --2V----75 ~ and ZAC=10 o. Granular and lozenge-shaped brownish-red sphene is seen occasionally in the calc-silicate rocks.

ORIGIN Any discussion on the origin of skarn minerals at Jothwad must take into account the following field and petrological characters :-- 1. The occurrence of wollastonite confined to the contact zones of the limestone and the granites artd gneisses and the presente of diopside, epidote, scapolite, phlogopite and garnet more or less in the centre of the skarn zone suggesting some sort of zonal distribution. 2. Rapid variation in the grain size of the minerals. 3. Occurrence of the skarn minerals in patches showing the following assembtages : wollastonite-calcite ; diopside-phlogopite-scapolite; epidote- garnet-scapolite; diopside-epidote-garnet (with small amounts of green tremolite). The rough zonal distribution of the skarn minerals is related to the falling temperature gradient probably at constant pressure. Ir is established by Eskola (1922) and Bowen (1940) that the metamorphism of magnesian limestone with excess of silica proceeds in a regular sequence of steps with increasing temperature, starting with tremolite, then diopside-tremolite assemblages and finally, rocks containiog diopside and wollastonite. But at Jothwad, tremolite-diopside assemblage is first formed, then diopside and finally, a wollastonite assemblage containing diopside. Introduction of fluorine and chlorine during metamorphism has led to the development of phlogopite and scapolite as mineral phases in all the miJmral assemblages of the Jothwad area. Similar occurrence of phlogopite has been noticed in the ca•c-silicate hornfeLes of the Marulan area of New South Wales (Osborrte, I931). The presence of MnO allows the appearance of spessartite almandine in skarns from which grossularite would otherwise have formed. It is possible to formulate a number of equations for the formation of tremolite and diopside, for these minerals may arise from talc asa result of chain reactions or by direct reaction between dolomite and quartz. This may depend on the rapidity of establishment of a particular temperature of formation. The following equations are suggested by Bowen ('1940) for the production of (a) tremolite from silaceous dolomite and (b) diopside from lremo]ite : Calc-SiIicate Skarns in the GranŸ and Gneisses at Jothwad 311

Low temperature High temperature (a) 5CaMg (CO3)~ -k- 8SIO2 q- HzO =(OH)2CaMgs(Si~Oll)-k-3CaCO3+7COz (b) (OH)2Ca2Mg~(Si4Oll)2-t-3CaCO3---=5CaMgSi206 § 3COz -t- H20. The second reaction takes place ata higher temperature than the first. The relic tremolite prisms which are seen occasionally show a rapid change over to diopside and it seems possible that the isotherm determining the formation of diopside was rapidly established allowing the formation of diopside directly from the dolomite portion of tb_e limestone simultaneously trans- forming the earlier tremolite to diopside. The formation of diopside moved rapidly towards the outer wollastonite zone because wollastonite-bearing assemblage contains quartz and calcite, which represents a definite and constant temperature. Further, the occurrence of diopside (with scapolite and phlogopite) inside wolla tonite took place at high temperature andat constant pressure. The occurrence of epidote in these skarn assemblages may be due to the narrowing and downward displacement of the field of diopside-bearing assemblages during the falling temperatures resulting from simultaneous substitution of epidote for calc-plagioclase formed earlier. In all the skarn assemblages of the Jothwad area quartz occurs as granules and plates which indicates that the original limestone was siliceous. Itis also noticed that the quartz is of two generations. The bigger plates of quartz probably represent the detrital silica of the sediments while the granules of quartz, associated with the skarn mineral in optical continuity, are possibly formed from the silica introduced from a magmatic source. Such a dual source of silica in skarns has been noticed and discussed by Pitcher (1950). Original silica in the limestone which is in the forro of detrital quartz reacts less readily during the metamorphism than the ionised silica introduced from the magmatic source. The concentration of introduced silica should fall with increasing distance from the magmatic source, for diffusion into the crystalline limestone depends on the temperature and pressure gradients which fall away from the igneous intŸ Formation of skarn minerals in a regu'ar order, namely wollastonite-diopsidc-tremolite, as noticed at Jothwad. may arise due to the resulting variation in the proportion of silica to magnesia and is related to decreasing dedolomitisation. Ti'~e increase in the grain size in the wollastonite zone when compared with the diopside-bearing assemblages in the inner zone is a feature connected more with the solubility of the limestone at different temperature ranges during skarn formation. In the initial stages of skarn formation there was rapid rise of temperature and limited diffusion of" silica across the contact, A6 312 M.S. SADASHIVAIAH

but de:reasing inwards, giving rise to diopside-bearing assemblages with small grain size, the early formed tremolite rapidly changing over to diop~ide. At high temperature when the reaction between CaCOa ar~d SiO~ is facilitated near about the contacts aided by the diffusion of silica in ionic condition, wollastonite-bearing skarn assemblages developed. As the growth of" wollastonite progressed with ah increase in grain size the early formed diopside-bearing skarns of smaller grain size became stable and were enclosed in the wollastonite.

It is seen from the above description and discussion that the sequence of formation of calc-silicate minerals proceeds in a regular order in keeping with the rise of temperature. Tremolite-bearing assemblages ate the first to forro and with rise in temperature diopside-bearing types developed with the concomitant transformation of earlier tremolite into diopside. With furthzr rise of temperature and at constant pressure, wollastonite-bearing calc-silicate rocks developed. The elevated temperature remained for a considerable amount of time with the approach of the later granite giving rise to coarse-grained wollastonite-bearing types with wollastonite enclosing the earlier formed diopside, garnet, phlogopite and scapolite. The temperature range -during the formation of the wollastonite-bearing assemblages has been between 500 ~ C. and 700 ~ C. (Bowen, 1940). These assemblages fall into the pyroxene hornfels facies of Eskola (Turner, 1948) and they resemble the hornfels class t0 of Goldschmidt (Turner, 1948) described from the Oolo region, derived from the metamorphism of the dolomitic limestones with silica.

SUMMARY

Cal--silicate rocks containing wollastonite, diopside, garnet, scapolite and phlogopite occur in the granites and gneisses at Jothwad, Panchmahal District, Gujarat State. There is some sort of zonal distribution of skam minerals. Wollastonite is r~stricted to the limestone granite contact and the diopside, garnet, scapolite and phlogopite are confined to the main skarn zone at the centre. Optical characters of the minerals have shown the triclinicity of wollastonite, diopside (Di63He37), ~capolite of the Mizzonite variety, garnet belonging to the spessartite-almandine type and epidote having 2070 pistacite molecules. Calc-silicate rocks exhibit variation in grain size and the patchy distribution of the skarn minerals. The formation of the skarn minerals and the various associated features have been brought about by the temperature pre~sure gradients and diffusion of volatiles and silica during the metamorphism of the limestone due to the granitic intrusion. Calc-Silicate Skarns in the Gran#es and Gneisses at Jothwad 313

ACKNOWLEDGEMENT The author wJshes to thank Shri M. S. Naik for chemically analysing the wollastonite.

REFERENCES

Balasubramaniyam, M. N .... Madras Unir. Jour., 1955, 25B2, 153. Bowen, N.L. .. Jour. GeoL, 1940, 48, 225-74. Burri, C. .. Das Polarisatons mikroskop, Verlag bª Base 1950. Eskola, P. .. Jour. GeoL, 1922, 30, 265-94. Fermor, L.L. .. Mem. G.S.L, 1909, 37, 330. Hess, H.H. .. Amer. Min., 1949, 39, 621-66. Larsen, E. S. and Berman, H. ,. BulL U.S.G.S., 1934, 848, 345. Osborne, C.D. .. Geo. Mag., 1931, 30, 209-33. Peacock, M.A. ., Amer. Jour. Sci., 1935, 30(5), 521. Pitcher, W.S. .. Min. Mag., 1950, 29, 137. Rittmann, A. and Grutter, O... Tabellen Zur Bestimmung Von Mineralien. Mineralogisch Petrographisches lnstitute. Der. Universitat, Basel, 1939. Sathe, R. V. Poona Unir. Jour., 1958, 12, 88. Turner, E J. Amer. Jour. Sci., 1942, 240, 578; Mero. GeoL Soc. Amer., 1948, 30, 72-75. Wincheli, A. N. Elements of Optical Mineralogy, John Wiley, 1961, Part II.