Calc-Silicate Rocks and Charnockites from Kasimkota Area, Visakhapatnam District, A.P

CALC-SILICATE ROCKS AND CHARNOCKITES FROM KASIMKOTA AREA, VISAKHAPATNAM DISTRICT, A.P.

BY A. V. R. SASTRY AND R. S. N. MURTY (Geology Department, Andhra University, Waltair) Received July 22, 1968 (Communicated by Dr. P. N. Ganapati, F.A.SC.)

ABSTRACT Khondalites, leptynites, charnockites, hypersthene-granulites, diopside- gneisses and calc-silicate rocks outcrop conformably in a small area, which is seven kilometers away in the north-west direction from Kasimkota Railway Station. The close association of charnockites with diopside-gneisses and calc-silicate rocks particularly in the Eastern Ghats is of great importance to the elucidation of the charnockite problem. The constituent minerals of these rocks are diopside-hedenbergite, diopside-aegirine, bronzite-hypersthene, plagioclase, garnet (grossularite), titanite, wollastonite and quartz. Apatite and biotite are present in accessory amounts. Rounded zircons and microcline are found only in a few rocks. During regional metamorphism of impure calcareous sediments, various stages of reconstitution of minerals are found. From the petrographic investigations, it is surmised that garnet, titanite and hypersthene are developed by the reaction between diopside and anorthite. Garnets of second generation are found to have grown with the earlier garnets as their neuclei and also as intergranular veinlets at the expense of scapolite. These reactions are represented by equations. Metatectic quartz and plagioclase, diatectic biotite and opaque ores, alkali metasomatism are considered to be the different facets of mani- festation of anatexis in this area.

INTRODUCTION

KASIMKOTA area occupies north-eastern portion of the toposheet 65 K/14 and is situated about 35 kin. from Visakhapatnam on the Calcutta-Madras trunk road. Khondalites, leptynites, charnockites, hypersthene-granulites, 213 214 A.V.R. SAS'IRY AND R. S. N. MURTY diopside gneisses and calc-silicate rocks with small patches of monomineralic and bimineralic rocks of this area offer interesting petrographic investigations particularly because of the close association of charnockites with diopside-gneisses and talc-silicate rocks. "l-he petrogenetic history of these rocks is also discussed.

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FiG. 1. Geological sketch map of Kasimkota area depicting the calc-silicate rocks and the other associated rocks.

GEOLOGICAL SETTING Geological sketch map of this area depicting the calc-silicate rocks and the other associated rocks is shown in Fig. 1. Calc-Silic.:te Rock:; end Ch rncckite~ 215

Charnockites of this area are conformable to khondalites and leptynites but in one outcrop south of the 4/6 milestone on the road from Kasimkota to Kundram, the foliation direction of charnockites makes an angle of 10°-15 ° to the foliation direction of leptynites and the contact zone is marked by ferruginous (hematitic) material. The charnockites from this outcrop are partly silicified.

Hypersthene-granulites appear prominent by the bouldery weathering and in the field they are distinguished from charnockites by the rock fracture and fine-grained nature. They are conformable to the leptynites and are closely associated with diopside-gneisses and calc-silicate rocks, which show ribbed weathering and folded pattern of outcrop. Quartz veins are frequently found parallel to the gneissic banding of calc-silicate rocks. Within these rocks there is localised concentration of microcline to such an extent that the rock is light pink in colour and may be very easily mistaken for a syenite.

Diopside-gneisses occur as lenticular patches within calc-silicate rocks. Weathering of these rocks results in bouldery outcrops. The boulders on their outer shell resemble very much like leptynites but the core is similar to the charnockite rocks except for mottled appearance due to the presence of white plagioclase feldspars. Mineral segregation is common in the calc-silicate rocks and diopside-gneisses, leading to the development of monomineralic or bimineralic nodules, which range in length from less than an inch to three feet.

PETROGRAPHY The petrography of these rock types--Calc-silicate rocks, diopside- gneisses, hypersthene-granulites, charncckites zrd lept3nites is descril:cd below :

Calc-Silicate Rocks

The rocks are leucocratic to mesocratic, coarse to medium-grained and are gneissic to granulitic. The essential minerals are diopside, plagioclase, scapolite, bronzite-hypersthene, wollastonite, garnet, titanite and quartz; apatite, bastite and biotite are found in minor amounts. In some, microcline and quartz are present in major proportions. They are grano- blastic while all the other minerals are elongated giving rise to gneissic appearance. Quartz and microcline show micro-pegmatitic intergrowth. 216 A.V.R. SASTRY AND R. S. N. MURTY

Garnet and titanite are found along the contact of diopside with either plagioclase or scapolite. Often the grain boundaries of garnet, which are within the diopside grains, are ill-defined and sometimes sutured, indicating its formation by the reaction of diopside and plagioclase. The margins of the coarse-grained wollastonite are also irregular, where they come in contact with the veinlets containing quartz, feldspar and garnet.

The optical properties of the constituent minerals are given below:-- Diopside-Hedenbergite 2 Vz = 540-67 ° Z A C --- 40°--44° Diopside-Aegirine 2 Vz = 660-76 ° ZA C = 45°-60 ° Bronzite-Hypersthene 2Vz----56°-70 ° ZAC=0 ° nz-nx ---- 0"0135-0.0115 Anorthite content of plagioclase 40--65~o Wollastonite 2 Vx = 40°-48 ° X A C = 270-42 °

Diopside-Gneisses These are melanocratic rocks with gneissic to granulitic texture. The essential mineral constituents and their optical properties are similar to the minerals found in calc-silicate rocks. Untwinned plagioclase feldspars are also present in these rocks. Rounded zircons and rutile inclusions in plagioclase are found rarely. Biotite, which is closely associated with opaque minerals is present in small amounts. The plagioclase twin lamellae are bent and deformed.

Diopside, garnet and wollastonite show the margins of contact with scapolite grains embayed irregularly and smaller grains with random optic orientation are found along the margins which may be due to the action of fugitive constituents liberated at the time of formation of scapolite. Diopside and scapolite form glomeroblastic aggregates. Garnet is fo~and to form kelyphitic borders around diopside. Titanite shows lamellar growth and is closely associated with plagioclase and diopside. Quartz and feldspar which show myrmekitic intergrowth, are associated with opaque ores, along whose borders biotite is present.

In calc-silicate rocks and diopside-gneisses occur small patches or nodules of monomineralic and bimineralic rocks, which have markedly uniform specific gravity of 3.3. Calc-Silicate Rccks and Charnock#es 217

(i) Monomineralic rocks.--The rock has granulitic texture and bronzite showing lameUar growth forms the major constituent. Plagioclase and biotite are present in accessory amounts. Bronzite 2Vx = 76o-80 ° Z A C = 0o--7° nz--nx = 0.011-0.0105 Anorthite content of plagioclase -----60-65~o. (it) Bimineralic Rocks.--According to the preponderance of the constituent minerals, the rocks may be named as scapolite-garnet, diopside-garnet and diopside-scapolite rocks. Scapolite-Garnet Rocks The rock is coarse-grained and pegmatitic in habit. Quartz and diopside are present in small quantities. The garnets, ~vhere they have embayed margins have radial cracks filled in with sca/:olite, ir~dicalir~g that the garnet is formed earlier than scapolite. Quartz must have been introduced subsequent to the formation of scapolite, which can be inferred from the recrystallised smaller grains of scapolite at the contact of quartz with scapolite. Garnet of second generation occurs in the interstitial space between scapolite grains in the form of intergranular veinlets along with diopside (aegirine bearing)"and quartz suggesting that it is develolzed later than scapolite (Fig. 2). Quartz and scapolite show undulose extir, ction ~,hich might be due to the subsequent deformation. Scapolite and garnet are separated and purified by handpicking. Scapolite has a specific gravity of 2.73. The refractive index (Niki- tin, 1936) of the mineral is 1 "575 and the birefringence is 0.021. The mineral contains 48~ marialite and 52~o meionite. Garfiet has a specific gravity of 3.54 slight birefringence and the mag- netic susceptibility'is zero. (Kruglyakova, 1964). The garnet is grossularite with a little andradite molecule.

Diopside-Garnet Rock In these rocks, the metacryst garnets enclose diopside, plagioclase, scapolite, titanite and quartz resulting in poikiloblastic texture. The diopside is dark-green in colour and slightly pleochroic in shades of yellow. The optical properties of the minerals are given below: Diopside-Aegirine 2 Vz = 60°-76 ° Anotthite content of plagioclase = 60--65~ ~3 218 A.V.R. SASTRY AND R. $. N. MURTBY Diopside-Scapolite Rock The constituent minerals are seapolite and diopside. Plagioclase, garnet, titanite, quartz and apatite are present in smalt quantitks. Diopside-Aegirine 2 Vz = 68 ° Anorthite content of plagioclase = 50-65%. Hypero~thene-Granulite The specific gravity of the rock is 3-1. It is fine,grained and granulitic. Mortar structure, bent-twin lamellae of plagioclase~ and undulose ex- tinction of the constituent minerals are the effects of defolmation. Bron- zite-hypersthene and plagioclase are the major constituents. Apatite and opaque minerals are present. Bronzite-Hypersthene 2Vx = 60°-80 ° Z A C = 0 ° nx-nx = 0.0i25- 0-0105 Anorthite content of plagioclase -- 60-62% The fine-grained nature of these rocks in contrast to the grain size of the associated rocks and its mode of occurrence suggest that these rocks must have had an igneous parentage with noritic affinities. Dykes of ncIi- tic composition intrusive into impure calcareous sedimentary rocks have been reported by several authors--FarIas (1958); Sen (1967), l'qaiayana Swamy and" Puma Lakshmi (1963) in connection v, ith the ¢harnockite problem. Charnockites There could be a difference of opinion in describing these rocks as, charnockites. Some prefer to describe these as pyroxene/hypersthene- granulites which will be in consonance with their opinions on the genesis of the rocks. The authors use the term " Charnockite" to connote the petrographic understanding and not in the petrogenetic sense, l~urther these rocks must be distinguished from the hypersthene-granulites descril:ed abo~e. While the essential mineral constituents are the same in both the rock types, there is a marked difference between the two in their appearance, weathering, rock fracture, mineral assemblage, and texture~which precludes them being considered as mere variants of hypersthene,granulites. The Charnockites are coarse-to medium-grained with granulitic to gneissic texture. Hypersthene, plagioclase and quartz are lcresent in major proportions. The twin Iamellae of plagioclase are bent. Untwinned Calc-Silicate Rocks and Charnockites 219 plagioclase is also present roughly in equal proportions. Rounded zircon grains are found. Ilmenite and biotite are present in minor proportions. Adjacent to a grain of bastite the apatite grain is crushed to pieces. Xeno- blastic hypersthene, plagioclase and quartz make the rock appear granc- blastic. Quartz is heterogranular. Mylmekitic intergrowth of plagioclase and quartz is a common feature and it is more pronounced adjacent to quartz veinlets oriented parallel to the gneissosity of the rocks. The optical properties of the minerals are given below:

Bronzite-Hypersthene 2 Vx -----560-88 ° Z A C = 0 ° nz-nx = 0.0135- 0.0099 Anorthite content of plagioclase :40-78% In a charnockite outcrop near hypersthene granulites lamellar hypersthene, diopside, twinned augite and plagioclase are present with the following optical characters. Diopside-Hedenbergite 2 Vz= 55 ° Z A C = 38 ° Diopside,-Aegirine 2 Vz : 66 ° Z A C : 48 ° From the diagram (Fig. 3) it i,s seen that the frequent 2 V-values for diopside-hedenbergite, diopside-aegirine and bronzite-hypersthene in these rocks are 54°-56 °, 65°-70 ° and 70°-75 ° respectively.

Leptynites The rocks are leucocratic, medium-grained. As in charnockites they are gneissic to granulitic. The essential minerals are quartz and feldspar. Hypersthene, garnet, titanite and apatite are in accessory amounts. The association of garnet and titanite is similar to those found in calc-silicate rocks and gneisses. Myrmekitic intergrowth is conspicuous in these rocks.

SOME REACTIONS INVOLVING MINERAL RECONST1TUTION The close association of grossularite and titanite with diopside and plagioclase suggests that grossularite and titanite are developed from diopside and plagioclase and the reaction may be represented as follows: 3 Ca (Mg Fe) Si~Oe + TiOl + Ca Al~Si~Oa (Diopside) (Rutile) (Anorthite) Caa A12SiaOl~ + Ca Ti SiOs + SiO2 -+- 3 (Mg Fe)SiO, (Grossularite) (Titanite) (Quartz) (Hypersthene) B4 220 A.V.R. SASTRY AND R. S. N. MURTY The reaction proceeds to the right with the increase of pressure because of decrease of molar volume of the products of the reaction.

IRONZITE, --HYPERST HENt 2Q"I 1

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Fr~,. 3. Frequency diagram of 2 V-values of Bronzite-Hype~sthene, Diopside-Aegirine an Diop3ide-Hedenbergite in eale-silicate roc~ s and other associated rocks.

The development of garnet rims around scapolite grain in the bimineralic rock-scapolite-garnet indicates that the garnet might have been formed from scapolite and the reaction can be represented as follows: 3 Ca AI~Si~OsCaCOa q- 3 SiO~ -b 5 (Ca Mg Fe) O --~ (Meionite) (Quartz)

3 C% AliSiaOlz + COs. (grossularite)

Part of the calcium, which is probably released during the replacement of Ca and Mg by Na and Fe +++ in diopside to form aegirine molecule, is utilised for the forrr.ation of garnet. The association of this secondary garnet with aegirine-bearing diopside and quartz supports the view that such reactions must have taken place. Calc-Silicate Rocks and Charnockites 221

The specific gravity of separated sample of garnet is 3.54. There is varia- tion in the intensity of brown colour of the garnets, which also suggests the presence of andradite molecule, in which the ferric iron might have been supplemented during metasomatic process. Such an association of grossularite and andradite has been reported by White (1959).

PARAGENESIS Diopside and plagioclase which constitute the primary mineral assemblage of calc-silicate rocks and diopside-gneisses, are the result of regional metamorphism of impure calcareous sediments. Under progressive metamorphism with increase of pressure, allophase neomineralisation might have taken place, resulting in the development of grossularite, titanite and hypersthene from diopside and calcic plagioclase. Subsequently scapolite (meio- hire) was formed at the expense of calcic-plagioclase. With further pro- gress of metamorphism the metatectic quartz and plagioclase and diatectic biotite, ilmenite and other opaque ores made their appearance in these rocks as well as in the associated charnockites and leptynites. About 2 miles north east of Kutsangi is a massive outcrop of granitic rocks and co- genetic with them, there could have been alkali and iron metasomatism resulting in the transformation of diopside into aegirine and at the same time scapolite into garnet of second generation.

SUMMARY AND CONCLUSIONS From the field relationship of the rock tyFes and petrographic investigations the following conclusions are drawn:-- (1) Hypersthene-granulites probably represent the metamorphosed igneous rocks of noritic composition, intrusive into impure calcareous sediments which were later metamorphosed into calc- silicate rocks and diopside-gneisses. (2) Various stages of anatexis phenomenon can be understood by the study of textural features recorded in all these rock types. (3) From the field observations and textural studies of the rocks it is suggested that the charnockites of this area are the resultant products of anatexis.

ACKNOWLEDGEMENTS One of the authors (R. S. N. M.) is grateful to the Andhra University for partial linancial assistance. 222 A. V. R.. SASTRY AND R. S. N. MURTHY

REFERENCES

1. Kraglyakova, G. L .. Aspects of Theoretical Mineralogy in the U.S.S.R., Edited by M. H. Bahey and S. I. Tomkeieff, Pergamon Press, 1964, 435. 2. Narayana Swamy, S. and "Charnockitic rocks of Tinnevelly District, Madras," .roar. PArna Laxmi Geol. Soc. India, 1967, 8, 38-49. 3. Nikitin, W. .. Die Fedorow-methode, Verlag Von Gebr~ider,, Borntlaeger, Berlin, 1936, 54. 4. Parras, K. .. "On the charnockites in the light of a highly metamorphic rock complex in So:Lth-Western Finland," Bull. Commi. Geol. Finlande, 1958, No. 181. 5. Sen, S. .. "Charnockites of Manbhum and the charnockite problem," Jour. Geol. Soe. Ind., 1967, 8, ~-17. 6. White, A. J. R. .. "Scapolite-bearing marbles and calc-silicate rocks from TungkiUo and Milendella, South Australia," Geol. Mag., 1959, 96, 285-306. A. V. R. Sastry and Proc. Ind. Acad. Sci., B, Vol. LXIX, Pl. VI R. S. N. IVlurty

FIG. 2. Scapolite-garr,et rock showing intergranular veinlets containing garnet, quartz and diopside, under partly crossed nicols (x S0). (Sc, Scapolite; Gar, Garnet; Qz, Quartz; Di, Diopside.)