Mineralogy of INDIAN CHARNOCKITES Ortllopyroxenes Are the Most Characteristic and Important Ferromagnesian Minerals in Rocks Of

JOUR. GEOLOGICAL SOCIETY OF INDIA Vol. 20. June 1979, pp. 257 to 276

MiNERALOGY OF INDIAN CHARNOCKITES

C. S. PICHAMUTHU

Abstract Considerable amount of investigation has recently been done on the minerals commonly present in the charnockites of India. In this paper a very concise account is given of the optical properties and chemical composition of orthopyroxene, clino­ PYroxene, hornblende, biotite, garnet, plagioclase, and potassium feldspars. Mention is made of some of the minor minerals such as monazite, magnetite, ilmenite, pyrites, PYrrhotite, apatite, zircon, spinel, and sapphirlne. The work done on the chemistry of coexisting pyroxenes, and the cation distri­ bution in the coexisting silicate minerals is also briefly reviewed.

ORTHOPYROXENES OrtllOpyroxenes are the most characteristic and important ferromagnesian minerals in rocks of the charnockite series, in which they occur exclusively or in asso­ ciation with diopside-salite clinopyroxenes, brown hornblende, biotite, or garnet. Composition: The orthopyroxenes in charnockites range in composition from eulite EnZ5 to hypersthene En64' They are ferrohypersthenes and eulites in enderbitic rocks, and typical hypersthenes in syenites and charnockites (Subramaniam, 1959). This varia­ tion in the FejMg ratio is not entirely in line with the behaviour of an igneous differentiation series (Howie, 1964a). As pointed out by Bhattacharyya (1977), though there is a general increase of Fe and decrease of Mg in the orthopyroxenes with increasing acidity of the host rocks, more acidic rocks often contain less Fe-rich orthopyroxene as in the Madras (Howie & Subramaniam, 1957) and Amaravathi (Ramaswamy & Murty, 1973) areas. This was one of the points made by Subramaniam (1959) to show that the mafic charnockite in the type area is not genetically related to his' charnockite suite'. The belief that the pyroxenes in chamcckitic and granulitic rocks contain high alumina is not generally true (Bhattacharyya, 1968; Sen, 1974). The alumina content of metamorphic pyroxenes appear to be related to the bulk chemistry; other things being equal, higher pressure increases their alumina content. According to Leelanandam (1967a) the pyroxenes in the ultra-mafic charnockites of Kondapalli are more magnesian than those in the Madras region (Howie, 1955; Subramaniam, 1962). Pleochtoism : Hypersthene is generally strongly pleochroic in shades of pink and green. According to Howie (1955), the pleochroism is not directly related to the content of ferrous or ferric iron, to the amount of manganese present, or to any particular trace element, though a combination of chemical factors may have some influence. He considers that the explanation of Kuno (1954) that there is some relationship between pleochroism and titanium content does not hold for the Madras rocks since in the most pleochroic bronzites the Ti02 content is very low; the pleochroism may be related to the oriented exsolution lamellae and schiller inclusions. The intensity of pleochroism is not controlled by the FejMg ratio but can be tentatively correlated with decrease in the cell dimensions, and with the entry of appreciable aluminium to I 258 C. S. PICHAMUTHU the octahedral position in the structure replacing the larger Mg and Fe2+ ions (Howie, 1964b). A variety of views, often contradictory, have been expressed regarding the causes for the pleochroism exhibited by orthopyroxenes as can be seen from the following: (l) Iron content is not responsible for the intensity of pleochroism (Turner, 1948; Howie, 1955; Subramaniam, 1959; Eskola, 1957); (2) Pleochroism is dependent on Ti02 content (Kuno, 1954; Hess, 1960; Murty, 1964a, b); (3) Pleochroism is un­ related to Ti02 content (Howie, 1955; Lovering, and White, 1964); (4) Pleochroism can be correlated with the presence of Ni or Cr (Subramaniam, 1962); (5) Pleo­ chroism is related to the A1203 content (Binns, 1962; Howie, 1964a, b); (6) Pleo­ chroism is not directly related to anyone particular clement but to a combination of chemical factors (Leelanandam, 1967e); (7) Intensity ofpleochroism can be correlated satisfactorily with Ti02 content (Bhattacharyya, 1969). Origin: Whether charnockites were formed by magmatic or metamorphic and meta­ somatic processes, hypersthene is the most stable phase that can exist in a •dry' system. The real problem is to determine whether the pyroxene is primary and formed during magmatic processes, or secondary due to the breakdown of a pre­ existing mafic mineral. Both processes are possible, and it is difficult to distinguish them. Hess (1952) has classified an orthopyroxene in a granulite from the type area of Pallavaram as metamorphic. Wilson (1955) considers that the hypersthenes of magmatic charnockites are constantly more ferriferous than those of gneissic char-

TABLE r. Chemical analyses of orthopyroxenes

2 3 4 5 6 7 8 9 ------~--- - Si02 46.17 51.17 49.50 49.76 49.60 50.81 51.69 50.08 48.77 AI203 3.22 1.97 2.01 1.56 2.51 3.20 2.51 1.34 1.06 Ti02 1.78 0.15 0.11 0.13 tr 0.38 0.43 0.12 tr FC203 1.16 0.59 1.31 1.66 1.78 0.14 1.29 0.75 FcO 33.17 23.01 32.60 30.96 27.48 24.36 21.41 35.44 34.55 MgO 12.24 20.7S 13.74 13.10 15.27 19.51 22.29 10.81 13.61 MnO 0.75 0.87 0.59 0.76 0.53 0.60 0.38 0.93 1.08 CaO 0.04 0.90 0.16 2.18 2.77 0.64 0.53 1.37 0.60 Na20 0.52 0.11 0.17 0.16 0.49 0.11 0.02 0.24 0.21 K20 0.34 0.53 0.07 0.03 0.03 002 H20+ n.d. n.d. 0.01 0.01 0.19 0.10 n.d. H20- 0.31 0.14 0.02 0.05 0.03

--- ~------~-_.<_-- Total 99.70 100.19 100.29 100.31 100.65 99.92 100.58 100.33 100.63

Original number, host rock. locality, and reference 1. 6436 Felsic charnockite, Meenambakam, Anal: R. A. Howie (1955) 2. 2270 Intermediate charnockite, Salem, Anal: R. A. Howie (1955) 3. 4642 A Mafic charnockite, Pallavaram, Anal: R. A. Howie (1955) 4. 2 Maficcharnockite, Type area, Madras, (Ray & Sen, 1970) 5. 4 Mafic charnockite, Type area, Madras, (Ray & Sen, 1970) 6. 28 Mafic charnockite, Kondapalli, Anal: (C. Leelanandam, 1967e) 7. DI4 Ultramafic charnockite, Kondapalli, Anal: (c. Leelanandam, 1967c) 8. 98 Mafic granulite, Saltora, W. Bengal, (Sen & Manna, 1976) 9. 335 Mafic granulite, Saltora, W. Bengal, (Sen & Manna, 1976) MINERALOGY OF INDIAN CHARNOCKITES 259 nockites (whether felsic or mafic). According to Rao (1967), hypersthenes do have a variable composition, but the variation appears to be dependant on temperature, and so may not reveal their true origin. Bhattacharyya (I97Ia) found that plots of weight percentage of (MgO + FeO + Fe203) vs weight percentage of Ah03 form the demarcation between igneous and metamorphic orthopyroxenes. Analyses of 12 orthopyroxenes from the ultramafic rocks of Gangineni and Kondapalli, and 29 orthopyroxenes from charnockites (18 from Kondapalli and 11 from Madras) were plotted on such a diagram by Sinha and Mall (1974). It was found that they fall into two distinct groups-the orthopyroxene analyses from the chromiferous ultramafics fall in the igneous field, and those from charnockites in the metamorphic field. It is obvious, therefore, that the ultramafic rocks of Kondapalli and Gangineni do not bear any genetic relationship to the associated charnockites.

CLINOPYROXENES The elinopyroxenes generally occur in irregular light green grains with faint pleochroism, sometimes with lamellae parallel to (100). The composition is typically in the salite or augite range. In the Ca-Mg-Fe compositional plot given by Leela­ nandam (1967e), the Kondapalli elinopyroxenes fall in the fields of endiopside, diop­ side, saIite, and augite. Unlike orthopyroxenes which occur throughout the charnockite series, the elino­ pyroxenes are normally confined to the less acid rocks such as basic granulites and pyroxenites. In the ultramafic rocks of the Madras charnockite series, the clino­ pyroxene has a diopsidic composition. Although in the less mafic rocks the clino- 260 c. S. PICHAMUTHU pyroxene is an augite and in the intermediate charnockite of Tirunelveli it is a ferro­ augite, the clinopyroxenes of this series are all characterised by a high calcium content, and their compositions lie close to the diopside-hedenbergite join (Howie, 1955). According to Howie (1955, Fig. 6), the clinopyroxene trend line for the char­ nockite series in the type area is displaced upwards nearer the diopside-hedenbergite composition line in comparison with typical igneous series i.e., these clinopyroxenes are richer in lime, and that this indicates that the Madras rocks were held at a relatively higher temperature over an extended period of time, allowing the two pyroxene phases to become almost completely exsolved. The low calcium content of the orthopyroxenes appears to support this.

HORNBLENDE In the charnockite rocks of Madras, hornblendes are common in the ultramafic and mafic rocks, but are less frequently present in the intermediate varieties, and are almost entirely absent in the felsic rocks of this region. No pyroxene granulite is completely free from hornblende in the type area (Sen, 1970; Ray, 1970). De Waard (1967) reports that in the Adirondack Highlands, another classic granulite facies area, pyroxene granulites free from hornblende and/or biotite, are rare. Hornblende can be considered as an essential phase of basic granulites. Hornblende in charnockite is optically olive-brown to green in colour, and strongly pleochroic. Sometimes there are fine lamellae parallel to (100).

Composition: The mineral, often surprisingly rich in fluorine, is almost always present in mafic and ultramafic charnockites. It is usually more aluminous than the hornblendes of normal igneous rocks. There is a general decrease of Ah03 with the decrease of basicity of the rocks of the charnockite series. TiOz is often rather high and MnO fairly low. Their fluorine contents may be around 1 to 1.5 per cent. These horn­ blendes appear to be in equilibrium with the other minerals and show a variation in composition as they are traced into less basic rocks; pyroxene is also always present, and so it is probable that the amount of volatiles rather than the bulk composition of the rock was the controlling agent for hornblende crystallisation (Howie, 1958). Ramberg (1949, 1952) is of the view that the hornblende of metamorphic enderbitic and basic rocks should be less magnesian than the co-existing ortho­ pyroxene, but Howie (1955) finds that for the co-existing hornblendes and ortho­ pyroxenes in the Madras charnockites, the hornblende is the more magnesian in every case. Sen (1970) has pointed out, on the basis of published analyses of hbl-opx-cpx triads, that XMg (i.e., molecular Mg/Mg} Fe) decreases in the sequence cpx-hbl-opx. Ramberg (1948) found that granulite facies hornblendes are rich in Al and con­ tain sizable amounts of Ti showing, in a general way, strong affinities to the pargasite-ferrohastingsite series. The Madras hornblendes have distinctly different alkali (mainly Na) and tetrahedral Al contents, and closely correspond to a 1: 1 mixture of edenite-ferroedenite and tschermakite-ferrotschermakite. The I: 1 Ed: Ts makes it more specific than the rather general remark of Ramberg, and such a com­ position is not very far from pargasite-ferrohastingsite. The composition EdsoTsso can be used as a good first approximation for these hornblendes (Sen and Ray, 1971a). There is a strong correlation between molecular MgO/(Mgo+FeO) ratios of rocks and hornblendes in an almost 1: 1 fashion. According to Sen (1970) and Ray (1970), this has two significant petrological implications: (1) that there is no additional MINERALOGY OF INDIAN CHARNOCKITES 261 compositional variance due to different Mg-Fe2+ ratios in the phases or hornblende breakdown equilibria, and (2) that the hornblendes are primary. Several petro­ graphical features such as concentration of released granules of ilIl1enite along borders of hornblende grains, intergrowth of pyroxene and ilmenite, and absence of hornblende-quartz intergrowth (as against common biotite-quartz intergrowths in acid rocks), do not support the belief that the majority of hornblendes in granulite facies are retrograde. The hornblendes in the Kondapalli mafic granulites have Na20 < K20, and low total H20, whereas in Madras and Saltora Na20> K20; the hornblendes from the orthopyroxene-bearing ultramafics of Kondapalli have, however, Na20? K20. Origin: Barth (1952) has stated that the transition of minerals in cbernockites is from biotite-s.augite-s-hypersthene, but according to Naidu (1963) it is just the opposite, for he considers that the amphiboles are always secondary after pyroxenes-a view not supported by recent investigations. Reference in literature to 'dry' assemblages is a relative term offield convenience since biotite and hornblende are ubiquitous, though not abundant, even in the type locality of Madras (Spooner and Fairbairn, 1970). In some instances, this is the result of retrograde metamorphism but equilibrium textures containing biotite and hornblende or both are also recognised (Griffin and Heier, 1969), reflecting local variations in the partial pressure of water during metamorphism. According to Winkler (1976), the presence or absence ofhornblende in mafic charnockites (pyroxene granulites) should be attributed to differences in original water content and not to differences in temperature under equal pressures. Investigations on mafic granulites in the type area have revealed that the thinner bands of mafic granulites and the borders of the wider bands interacted with a water­ bearing anatectic melt (Sen, 1970). Convincing evidences of anatexis exist in the Adirondack and many other granulite facies areas where quartzo-feldspathic rocks occur, and it is quite likely that water reached an 'equilibrium potential' in these areas through the formation and presence of anatectic granite-granodiorite melts dur­ ing granulite facies metamorphism. As hornblendes are fairly rich in Ti, and pyroxenes do not admit appreciable amounts of Ti in their structures, the movement in the reaction: hbl + qz ~-=' opx + calc pyrox + plag + opaques + water should be from left to right (Sen and Ray, 1971b). The hornblende-poor and horn­ blende-rich mafic granulite assemblages represent frozen stages of the above reaction. Interfingering of ilmenite and pyroxene, presence of opaques in higher modal propor­ tions in almost all the hornblende-poor mafic granulites, and spongy skeletal habit of pyroxenes in many hornblende-poor varieties-all these support a forward movement ofthe reaction. It is significant that hornblende-quartz symplektite, a good criterion in favour of a reverse movement (Katz, 1968), is absent in these rocks. The association of pyroxene granulites with or without hornblende is to be attri­ buted not to a difference in temperature under equal pressures but rather to different original water contents and to metamorphism under essentially closed system condi­ tions (Winkler, 1976). According to Buddington (1963), if a series of geosynclinal sediments with intr\lded sheets of gabbro is downfolded to a 'Loneof high temtper'At\l'ie and pressure, then the outer border of the gabbro sheets may be subjected to recrystal­ lisation under conditions of temperature, load pressure, and water pressure adequate to produce the mafic mineral assemblages hornblende+hypersthene+(c1inopyroxenc), 1** 262 c. S. PICHAMUTHU or the same plus garnet, while the inner portions were being recrystallised to a mineral assemblage characteristic of the pyroxene-granulite subfacies, In the inner portions no water has had access and no hydroxyl bearing minerals could form. In the outer portions, a little water has moved along grain boundaries, and some horn­ blende (and biotite) is formed as long as water was available. Therefore, orthopyro­ xenes may coexist with the hornblende and biotite that have been formed. Sen and Ray (l97Ib) have put forward the veiw that uSi02 played an important role in determining the presence of hornblende in the mafic granulites of Madras, and that the rock systems were open to H20 during metamorphism. According to Sinha and Mall (1974), the hornblendes and biotites in Kondapalli are compatible and not formed by retrograde metamorphism of pyroxene (Subra­ maniam, 1967), nor are the hypersthenes formed by prograde metamorphism of the hornblende and biotite of the original amphibolites as stated by Murty (1965). Leelanandam (1970a) has demonstrated perfect chemical equilibrium between existing pyroxenes, hornblendes, and biotites. All the Kondapalli rocks were finally meta­ morphosed together under plutonic conditions during or after emplacement of the acid

TABLE Ill. Chemical analyses of hornblendes

2 3 4 5 6 7

Si02 42.25 41.67 42.05 42.86 42.24 40.25 42.21 AI203 12.57 11.52 14.69 10.01 13.30 13.26 11.77 Ti02 1.83 1.72 1.48 2.34 1.88 2.10 0.74 Fe203 1.86 4.71 3.21 2.24 2.93 1.69 1.99 FeO 10.35 14.92 6.30 16.64 13.42 21.00 20.45 MgO 12.90 9.46 14.91 10.37 10.32 6.36 7.52 MnO 0.09 0.13 0.04 0.20 0.37 0.48 0.48 Cao 13.98 11.04 12.83 11.83 11.80 10.76 10.61 Na20 2.02 1.54 2.01 1.87 1.83 1.69 2.24 K20 0.90 2.13 0.65 0.82 0.49 0.97 0.70 H20+ 1.30 1.01 1.53 1.27 1.68 1.95 1.84 H20- 0.06 0.03 0.09 0.01 0.41 0.08 0.14 F 0.40 1.40 0.50 n.d. n.d. n.d. n.d.

Total 100.51 101.28 100.29 100.46 100.67 100.59 100.69

LesseaO 0.17 0.59 0.21

Total 100.34 100.69 100.08

Original number. host rock. locality, and reference 1. 2270 Intermediate charnockite, Salem, Anal: R. A. Howie (1955) 2. 4642A Mafic chamockite, Pallavaram, Anal: R. A. Howie (1955) 3. 3709 Ultramafic charnockite, Pammal Hill, Anal: R. A. Howie (1955) 4. 2 Mafic charnockite, type area, Madras, Anal: S. Ray (Ray & Sen, 1970) 5. 6 Mafic charnockite, type area, Madras, Anal: S. Ray (Ray & Sen, 1970) 6. 90 Mafic granulite, Saltora, W. Bengal (Sen & Manna, 1976) 7. 335 Mafic granulite, Saltora, W. Bengal (Sen & Manna, 1976) MINERALOGY OF INDIAN CHARNOCKITES 263 charnockites at temperatures of about 650cC when hornblende and biotite were stable in the presence of 'excess' of water (Sinha and Mall, 1974). Ray and Sen (1970) have shown that there is a close approach to equilibrium partitioning of Mg and Fe2+ in hornblende-pyroxene pairs.

BIOTITE Biotite is neither a common nor an abundant mineral in charnockites, but Howie (1955) has described an intermediate rock from Salem, Tamil Nadu containing 8 per cent biotite.

Composition: According to Howie (1964a), biotite when present in charnockites is a normal one containing the proper proportion of alkalies and the (OH, F) group, and not relatively low in these constituents as reported by Groves (1935) for a biotite from an Ugandan charnockite and suggested by him to be due to the' dry' conditions of crystallisation. The biotites in the charnockites of Visakhapatnam, vary in refractive index (Nz) from 1.628 to 1.637, which places them midway between annite-siderophyllite and phlogopite-eastonite (Murty, 1966). The biotite composition is controlled by TABLE IV. Chemical analyses of that ofthe host rock and ofco-existing phases. Biotites Recent chemical investigations of biotite from 2 3 charnockites and orthopyroxene granulites of India by Bhattacharyya (1970), Leelanandam Si02 38.66 38.52 40.52 (1970a), and Dhana Raju (1975), have shown AI203 13.96 16.02 13.75 that the biotites in the ultramafics of Konda­ Ti02 4.43 2.31 1.65 palli have much lower Fe203 and Ti02 than Fe203 1.54 0.21 1.79 those from the orthopyroxene-bearing felsic to FeO 13.15 14.67 14.85 mafic granulites of this area as well as from MgO 14.65 13.84 14.10 Madras and Srikakulam where Ti02 in biotites is generally above 4 per cent. The biotites MnO 0.11 tr 0.14 from the hypersthene-garnet-cordierite gneis­ CaO 0.60 0.20 1.90 ses of Karnataka have, however, low Ti0 Na20 0.71 1.68 0.70 2 content (Anantha Iyer and Kutty, 1975). 8.34 9.23 8.52 K20 The Kondapalli biotite is characterised + 2.96 2.06 2.00 H20 by absence of Ca, exceptionally high Cl and H20- 0.05 0.72 F, but low total water. The range of con­ F 0.30 n.d. n.d. -centration of Cr, Li, Ni, Co, V, and Rb, is Total 99.46 99.46 99.92 higher in Kondapalli biotites from felsic to intermediate granulites than in biotites from Less =:: 0 0.12 similar rocks in the Madras area. Ba is high Total 99.34 both in the Madras and Kondapalli biotites (Leelanandarn, 1970a). Original number, host rock, locality, and reference Howie (1955) has shown that Ga, Li, and 1 2270 Intermediate charnockite, V are more concentrated in the biotites from Salem, Anal: R. A. HOWIe (1955) the more felsic charnockites of the Madras 2 1 Enderbite, Pallavaram, area; Sc, Y, and Sr concentrated in those of Anal: B. S. Machigad (1967) the rocks intermediate composition, while the 3 1 Biotite norite, Maddilapalem highest content of Cr occurs in the more Quarry, Visakhapatnam, Anal: M. S. Murty (1966) magnesium-rich biotites. 264 c. S. PICHAMUTHU

GARNET Composition: Almandine is the predominant garnet in charnockites, followed by pyrope and other end-member molecules irrespective of the acidity of the host rock (Bhatta­ charyya et al., 1970; Leelanandam, 1970b; Anantha Iyer and Kutty, 1974). In almandines from mafic rocks, however, grossularite predominates over pyrope, The cell parameters of charnockitic garnet, like those from metapelites, show a positive correlation with the Ca content of the mineral (Bhattacharyya et al., 1970; Leela­ nandam, 1970b).

Origin: In many high-grade metamorphic terrains, garnet-bearing hornblende-pyroxene­ granulites and pyroxene-granulites are intimately associated with their non-garneti­ ferous counterparts. The causes for the selective occurrence of garnets is the main prolJlem that these rocks pose. The cornpiexiry of this problem is evident from the commonly reported unsystematic pattern of development of garnets (Buddington, 1965). The relatively large number of ferromagnesian phases in these rocks is also intriguing from the standpoint of equilibrium, and many consider them to be dis­ equilibrium assemblages-specially in view of the late overprint of garnets. According to Manna and Sen (1974), several evidences in the Saltora area (West Bengal) indicate that hornblende-pyroxene-granulites were the precursors of the garnetiferous granulites, such as, rimming of hornblende and pyroxene by garnet, and textural features indicating orthopyroxene to garnet transformation; garnetiferous mafic granulites having the same gross chemical composition as hornblende-pyroxene granulites; bands of hornblende-pyroxene granulites passing laterally into garneti­ ferous varieties, the amounts of pyroxene and hornblende diminishing while that of garnet increases; MgOj(MgO + FeO) ratios of hornblendes being matched by those of rocks even in the garnetiferous mafic granulites, thereby indicating that horn­ blendes ill these rocks are also primary. In the absence of any data pointing to differences of pressures between garneti­ ferous and non-garnetiferous granulites, the garnets must have been formed under more or less isobaric cooling in these rocks. By examining the chemistry of conti­ guous rock samples, Manna and Sen (1974) conclude that lower Mgj(Mg + Fe) ratio was the most important chemical factor in bringing in garnets. Among other controls, normative anorthite content was the more significant one, while silica saturation and especially alumina, have exerted an occasional and possibly lesser influence. The significant textural features of garnet-hornblende-pyroxene-granulites are: symplectic intergrowths of garnet and ilmenite, garnet and quartz, and clinopyroxene and garnet; rims ofquartz around garnets; coronas of garnets around hornblende and pyroxene; and presence of more albitic borders around plagioclases. It is seen that garnetiferous basic granulites were developed from hornblende­ pyroxene-granulites; that the metamorphic reactions producing garnets were combi­ nations of hornblende breakdown and orthopyroxene to garnet reactions; that garnet­ forming reactions ran selectively in compositions characterised by high Fej(Mg + Fe); that garnets were formed in the down temperature direction of the reactions; and that within the range of favoured iron-rich compositions the more magnesian ones pro­ duced garnets at lower temperatures (Manna and Sen, 1974). MINERALOGY OF INDIAN CHARNOCKlTES 265

TABLE V. Chemical analyses of garnets

2 3 4 5 6 7

~-- ---

S.'02 38.02 37.84 38.60 37.45 40.10 38.92 38.52 T102 0.03 0.78 0.37 0.30 tr tr A1203 21.02 20.58 19.03 21.18 21.92 20.17 23.32 Fe203 1.98 0.97 1.17 0.50 2.98 29.27 30.10 FeO 28.12 2909 29.76 30.87 24.52 MnO 0.64 1.94 1.24 087 063 0.35 0.28 MgO 7.87 3.86 2.32 2.20 6.93 7.84 7.43 CaO 2.25 6.53 7.28 6.67 2.70 2.70 0.78 Na20 0.11 K20 0.01 H2O+ H2O- 0.09

Total 100.14 100.81 100.18 100.11 100.08 99.25 100.43

Original number, host rock, locality, and reference 8 Enderbite, Pallavaram, Madras, Anal: R. A. Howie (Howie and Subramaniam, 1957) 2 83 Mane granulite, Salrora, W. Bengal (Sen & Manna, 1976) 3 90 Mafic granulite, SaItora, W. Bengal (Sen & Manna, 1976) 4 94 Mafic granulite, Saltora, W. Bengal (Sen & Manna, 1976) 5 F Charnocklte, Visakhapatnam, Anal: M. S. Murty (1966) 6 3 Enderbite, Pallavaram, Anal: B. S. Machigad (1967) 7 5 Fnderbrte, Pallavaram, Anal: B. S. Machigad (1967)

CATION DISTRIBUTION Partitioning of common elements, specially Mg and Fe 2+, among co-existing silicate minerals has attracted considerable attention in recent years. Bowie (1955) was probably the earliest to furnish chemical data on two pyroxene pairs from the type charnockite area near Madras. Much attention has been paid to tie-lines joining the compositional plots of co­ existing ortho- and cline-pyroxenes on a triangular Ca-Mg-Fe diagram, it being con­ sidered at one time that the points of intersection of the prolongation of these tie­ lines with the Ca-Mg line could be used to distinguish between igneous and meta­ morphic assemblages. It is now realised, using chemically analysed mineral pairs, that there is no appreciable variation in the intersection positions for any pair of pyroxenes which have crystallised together under conditions of equilibrium (Howie, 1964a). The partition coefficient for magnesium and ferrous iron between co-existing {Jfroxcnes has been used by Kretz (1961, 1963), and Bertholom« (1961) as an index of PoT conditions of crystallisation, and to distinguish igneous and metamorphic pyroxenes. Kretz (1963) found that the values for his distribution function Ko was 0.86 to 0.65 in igneous rocks, and 0.65 to 0.51 in metamorphic rocks. For the co- 266 c. S. PICHAMUTHU existing pyroxenes of the Madras charnockite series he showed that KD is 0.54, while for those in the Skaergaard igneous differentiation series it is 0.73. According to Ray and Sen (1970), KD decreases with increase of X~~x. Both Ca and Fe2+ pre­ fer Mu site in the pyroxene structure and, therefore, increase of Ca causes decrease of Fe2+ in clinopyroxene. Binns (1962) has used a different relationship in plotting the composition of co­ existing pyroxenes in the pyroxene granulites of Broken Hill, New South Wales, giving an intercept P which varies from 0.37 for the lowest grade to 0.17 for high grade assemblages. For the Madras pyroxene pairs P is approximately 0.12 (Howie, 1964a). According to Wilson (1960), there is a difference in the type of co-existing pyroxenes in different charnockites. The role of A1 203, and the differentiation of augite-bearing and diopside-bearing types have yet to be investigated. Many co-existing pyroxene pairs in the mafic charnockites of India have been studied (Howie, 1955; Howie and Subramaniam, 1957; Subramaniam, 1962; Naidu and Rao, 1967). The distribution co-efficients for all these pairs show similar values and was interpreted as indicating a temperature of about 600°C; this is, however, on the low side (vide infra). The K D values which are quite similar indicate a close approach to equilibrium for these rocks (Mall and Rao, 1970; Mall and Singh, 1972). Compositional data on co-existing pyroxenes (Kretz, 1963) suggest that for pyroxene pairs with similar Ca content, a difference in the distribution of Fe and Mg is due to a difference in the temperature of crystallisation. Concentration of Ca in pyroxenes usually varies with temperature, and mixing of Ca with Fe and Mg is generally non-ideal; this changes the simple correlation between temperature and Fe-Mg distribution. Saxena (1976) synthesised the compositional data and recent theoretical results on solution models into a tentative model for geothermometry that would provide a relative scale of temperature estimate. He applied this geo­ thermometer to the pyroxene compositions in charnockites from India (Howie, 1955; Leelanandam, 1967e), Varberg, Sweden (Saxena, 1968a), and in charnockitic rocks from Uusimaa, Finland (Saxena, 1969). The rocks studied by him are mafic to intermediate in composition and associated with magmatic activity. The temperatures ofcrystallisation vary from 700°C to 870°C. On the basis of experimental results and theoretical analysis of chemical compositions of co-existing minerals, Saxena (1977) has recently stated that the PoT field of the charnockites lies in the temperature range of 800°-900°C, and in the pressure range of 0.5 to 3kb, for it does not appear possible that the mineral assemblage Qz-Plag-Or-Bi-Opx-Cpx-Gar could survive in PoT conditions significantly different from these ranges. However, Weaver et al. (1978) concluded on the basis of Opx-Cpx and Opx-Gar relations, and feldspar thermo­ metry, that the poT conditions of recrystallisation of Madras charnockites were in the region nOo-840cC and 9-10 kb. The titanium contents of 8 pyroxene pairs from the Kondapalli charnockites and of 10 pairs from the Madras charnockites were studied by Leelanandam (1967c) with reference to the nature of distribution of titanium in the co-existing ortho- and clino­ pyroxenes. He found that the distribution points were scattered, more so in the Madras pyroxene pairs than in those ofKondapalli, and hence stated that no meaning­ ful purpose will be served by drawing any conclusions based on titanium distribution in the paired pyroxene phases. The orthopyroxenes have more Fe2+jMg, MnO, and Co, and less AI, Ti02, Fe203' NazO, K20, 100 Mgj(Mg + Fe2+ + Fe3+ + Mn), Cr, Li, Ni, V, Zr, Sc, and Y than in co-existing clinopyroxenes (Leelanandam 1966, 1968). MINERALOGY OF INDIAN CHARNOCKITES 267

The hornblendes generally contain more Ti02, FeZ03' Ga, Cr, Ni, V, Sc, Zr, Y, Sr, and Ba, and less Fe2+jMg and MnO than in co-existing orthopyroxenes; also, more Ti02, FeZ03, FeO, Fez+/Mg, Ga, Cr, Ni, Co, V, Sc, Y, Sr, La, and Ba, and less MgO, MnO, CaO, Li, and Zr than in co-existing clinopyroxenes (Bhattacharyya, 1977). There is a positive correlation between tetrahedral aluminium and KDr~g~~~) and KD(~;h¥~) values in the charnockites and granulites of Madras, Saltora, and Konda­ palli (Sen, 1970, 1973; Ray and Sen, 1970; Leelanandam, 1970a). The KD of co-existing pyroxenes-hornblende cannot be used to distinguish one metamorphic grade from the other. The garnets contain more Fez+jMg, FeZ03' MnO, and CaO than in co-existing orthopyroxenes, and more FeZ+jMg and MnO than in co-existing clinopyroxenes ·(Howie and Subramaniam, 1957; Sen and Sahu, 1970; Sen and Manna, 1976). Calcium and manganese in garnets modify KD values in both orthopyroxene-garnet and clinopyroxene-garnet pairs. Garnets contain Jess SiOz, Alz03, NazO, KzO, and more Fez+jMg ratio, MnO, and CaO, than in co-existing cordierite. The average KD value of five pairs of orthopyroxene-garnet from the Saltora basic granulites is 3.7 which is within the range of similar data from other granulite facies areas of the world, and the average KD of six pairs of clinopyroxene-garnet from the same area is 7.7 (Bhattacharyya, 1977). Saxena (1968b) obtained an average value of 7.2 for the clinopyroxene-garnet pairs from this area. The biotites have more TiOz and FeZ03' and less FeO, MnO, Fez+jMg, and CaO than in co-existing orthopyroxenes. According to Dhana Raju (1973), the ortho­ pyroxene-biotite pairs have a narrow range of K o from 1.0 to 2.0, thus indicating that in a great majority ofco-existing orthopyroxene-biotite phases, the orthopyroxene is comparatively richer in FeZ+ while the biotite is Mg-rich. There is some difference -of opinion regarding the nature of Mg-Fe-" distribution in orthopyroxene-biotite pairs. It is considered to be near equilibrium by Bhattacharyya (1970) and Leela­ nandam (1970a), but in disequilibrium by Dhana Raju (1973); according to Sen and Sahu (1970), it is in equilibrium under granulite facies conditions of metamorphism. The biotites contain more Ti, tetrahedral AI, K, total HzO, F, Li, Ni, Co, Ba, Rb, and Cu, and less Si, octahedral AI, Fez+jMg, Fe3+, Fe3+j(Fe3+ + Fez+), Mn, Na, Cl, Ga, Cr, V, and Zr than in the co-existing hornblendes (Howie, 1955; Leela­ nandam, 1970a). According to Saxena (1968 a and c) the KD value in hornblende­ biotite pairs is not affected by metamorphic grade, but changes with the variable con­ centration of tetrahedral Al in hornblende or biotite. The biotites contain less Ah03, FeO, Fez+jMg, MnO, and CaO, and more TiOz than in co-existing garnets; and more FeO, FeZ03' Fe2+jMg ratio, Ti02, and CaO, and less SiOz and Alz03 than in co-existing cordierite.

PLAGIOCLASE Twinning: Antiperthitic plagioclases occur in most of the acid intermediate charnockites. The prevalence in the charnockites ofplagioclase twinning on the uncommon albite-ala law has been noticed by Rajagopalan (1946), Naidu (1950), Bhaskara Rao and Srirama Rao (1953), Ramanathan (1954), Howie (1955, 1964a), and Subramaniam (1967). Both Gorai (195I) and Turner (1951) have tried to distinguish between plagio­ clase twinning in igneous and metamorphic rocks, but until more is known about the cause and mechanism of twinning, it is inadvisable to consider the plagioclase twin laws as diagnostic of the history of the rock (Howie, 1955; Naidu, 1963). 268 C. S. PICHAMUTHU The twinning in the plagioclases of charnockites of the Eastern Ghats show the following features (Bhaskara Rao and Srirama Rao, 1953). They are all twinned in the basic rocks, the grains being absolutely fresh and without any alteration cracks or undulose extinction. The commonest twins are of complex types with very few parallel twins, all of them being C-twins. Perfectly developed sub-individuals are numerous; this may be due to paragenetic twinning. All these are suggestive of an igneous origin for the basic rocks. In the intermediate rocks twinned plagioclases preponderate over untwinned ones. Almost all grains exhibit undulose extinction, cracks with alteration, and myrmekitic growths. Normal and parallel twin laws are common and hence, are C- and A-twins. The acid charnockites contain mostly untwinned plagioclases with a few twinned ones. That these rocks have suffered severe metamorphism is indicated by bent twin lamellae and wavy extinction, along with myrmekitic and microperthitic intergrowths. The twinned laths exhibit very fine lamellae. There are no sub-individuals. The twins come under the category of A- and C-twins as they mostly have the parallel twins dominating the normal twins. All these indicate that the intermediate rocks are products of metasomatic processes.

Composition: The plagioclase feldspars of the intermediate and felsic rocks of the granuhte facies commonly have compositions An30 to An35' although in some of the more mafic rocks labradorite occurs. In the Madras charnockites the MgO in plagioclases is fairly low, and FeO high. Fe203 is also fairly high and shows a tendency to rise with increasing Na20 content. The ferrous iron is more likely to give rise to the greenish colour in the feldspars possibly having been taken into the lattice under the high temperature and pressure conditions (Howie, 1955). The absence of zoning in the Madras plagioclases indicate, according to Howie (1955), that they were in almost perfect equilibrium during crystallisation which appears to be borne out by their composition, for the range is rather restricted usually lying between Ann and An4o, Though the composition varies from oligoclase to labradorite, it very commonly is in the sodic andesine range at around An35 Ab6s (Howie, 1964a). The plagioclases in enderbites also do not show zoning (Subra­ maniam, 1959), Mall and Singh (1972) find that in the mafic charnockites there are wide differ­ ences in the anorthite content of co-existing plagioclases even within one thin section while the other co-existing minerals show equilibrium conditions. It is perhaps due to a higher water content in rocks that plagioclases of low temperature formations (schists, gneisses, granites) are often homogeneous in contrast to plagioclases from relatively high-temperature formations which show strong heterogeneities (Mehnert, 1962). The co-existence of low temperature' intermediate' plagioclases of widely differing compositions in many plutonic and metamorphic rocks may partly be due to structural discontinuities and partly to difficult and slow substitution of Si/Al in tetrahedral sites; the presence of water facilitates homogenisation in plagioclase com­ position. These plagioclases are inhomogeneous though having an ordered structural state. The structural state of plagioclase is dependent on many factors such as rate of cooling, water pressure, composition etc. Sodic plagioclases get more easily ordered than calcic plagioclases (Eberhard, 1967). The plagioclases from mafic charnockites have failed to homogenise (in spite of the slow rate of cooling) because they are more calcic and formed in more or less' dry' conditions (Mall and Khanna, 1974). MINERALOGY OF INDIAN CHARNOCKITES 269 An optical study was made by Wenk (1965) of a labradorite from a mafic char­ nockite (Iabradorite-hypersthene-augite-amphibole granulite), and he found that it was more calcic (An 63.2%) than those described by Howie (An 58%), and Naidu, {An 56%). Leelanandam (1967b) recorded the occurrence of so calcic a plagioclase as anorthite, and of medium-calcic bytownites (some of them antiperthitic) in the mafic charnockites of Kondapalli-features which are either unknown or most unusual in -charnockitic and granulitic facies rocks. The An content (An/An + Ab) of the plagioclases from an ultramafic charnockitic lens and from a dyke is 92% (anorthite) and 89% (calcic bytownite) respectively. The ultramafic charnockites of Kondapalli always contain a small amount of plagioclase while those from the type area appear to be devoid of any plagioclase (Howie, 1955; Subramaniam, 1959). The An content of the plagioclase from a mafic charnockite lens and from a dyke is 88% (calcic bytownite) and 81% (medium bytownite) respectively; the latter is antiperthitic con­ taining a rather high Or content (2.2%). This is rather unusual, for though anti­ perthites have been reported in the plagioclases of mafic charnockites of Madras (Howie, 1955; Naidu, 1955), they are commonly restricted to the oligoclase-andesine range, and almost unknown in labradorite and bytownite (Deer, Howie, and Zussman, 1963). Unlike Ba and Sr, Fe and Ti are mostly contained in minerals other than feldspar, but Sen (1960) has shown that even the anorthites which are relatively purer than the other plagioclases, contain K, Ba, Sr, Fe, Ti etc. though in small quantities. From the common occurrence of exsolved needles of hematite or magnetite or rutile in plagioclases on a microscopic scale, it can be inferred that Fe was present in the struc­ ture in such cases. According to Sen, variations of Fe and Ti content in plagioclases are due to temperature and/or availability, but high availability alone cannot cause a higher Fe-Ti content in the plagioclase if temperature is unfavourable; neither can temperature cause an increase in Fe-Ti content if availability is low. Fe and Ti generally show an increase with increasing anorthite content in the granulite facies plagioclases. However, any conclusion that Fe (the trend of Fe is more marked than that ofTi) increases with anorthite content is unwarranted because: (1) variations of temperature and availability are extremely important in affecting the Fe and Ti content of the plagioclases, and (2) exsolution and migration effects, especially for the plagioclases with high Fe content, can bring about significant modi­ fications. Increasing temperature is the major causative factor for the increase of K content in plagioclases (Sen, 1959). Plagioclases in rocks formed at higher temperatures show greater Ba and Sr contents. In general, granulite facies rocks appear to be depleted in alkalies (Ramberg, 1951).

Antiperthite: Charnockitic rocks commonly contain antiperthites. Sen & Khan (1972)consider that in the mafic rocks of the Madras area they have been formed by the ingress of KAISi30 s material in plagioclase under granulite facies metamorphism followed by exsolution, the KAISi30s-forming material being mainly available from the anatectic melting of the' charnockite suite' rocks during the peak of granulite facies metamor­ phism. The exsolved KAISi30 s could have come out of the grain boundary forming replacement antiperthite in the adjacent grains. It would appear that the difference between exsolution and replacement origin may be a matter of scale (Sen and Khan, 1972). 270 c. S. PICHAMUTHU

POTASSIUM FELDSPARS The alkali feldspar of the granulite facies is often dark greenish to brown, or greenish-black in hand specimen, with a distinctive fine microperthitic structure, and hair-perthite is one of the most specific characteristics of these rocks (Eskola, 1952). The potassium feldspars are dominantly microc1ine perthites and microc1ine microperthites in the type area near Madras; they appear to be a variety intermediate between orthoclase and microcline (Howie, 1955), showing' shadowy and indistinct tartan twinning' (Howie, 1964a). The perthitic blebs are generally spindle-shaped, often fairly close, and in optical continuity. They exhibit undulatory extinction. 2V( -) varies from 62° to 78°. The majority of charnockites, especially the ender­ bites, carry perthites of the compositional range Or52 to Or65 (Subramaniam, 1959). In the potassium feldspars of Kondapalli charnockites, Leelanandam (1967e) did not find even incipient cross hatching and hence inferred that the feldspars are ortho­ clase and not microcline, the composition ranging from Or78.8 to Or84.7 (from chemical analyses). TABLE VI. Chemical analyses of feldspars ------Plagioclase Potassium Feldspars 2 3 4 5 6

Si02 60.07 59.51 58.10 65.53 64.42 63.68 Al203 24.84 25.60 26.44 18.90 18.49 19.57 Ti02 0.16 nil tr 0.02 0.02 0.01 Fe203 0.35 0.26 0.04 0.13 0.47 0.29 FeO 0.21 0.13 0.15 009 0.12 0.24 MgO 0.02 0.1l 0.03 0.33 0.31 0.05 MnO tr 0.01 tr tr 0.01 nil CaO 6.65 6.35 7.84 0.54 0.20 0.40 Na20 7.54 6.92 6.48 2.96 1.84 1.56 K20 0.34 I.II 110 11.59 13.20 14.21 H20+ 0.04 0.02 0.Q3 0.30 0.04 0.04 H20- 0.05 0.09 0.06 0.14 0.09 0.Q7

Total 100.27 100.11 100.27 100.26 99.21 100.12

Original number. host rock. locality, and reference 1. 6436 Felsic charnockite, Meenambakam. Anal: R. A. Howie (1955) 2. 2270 Intermediate charnockite, Salem, Anal: R. A. Howie (1955) 3. 4642A Mafic charnockite, Pallavaram, Anal: R. A. Howie (1955) 4. 4639 Felsic charnoekite, Pallavaram, Anal: R. A. Howie (1955) 5. 6436 Felsic charnockite, Meenambakam, Anal: R. A. Howie (1955) 6. 3705 Felsic charnockite, St. Thomas Mount, Anal: R. A. Howie (1955) X-ray study of a perthite from Visakhapatnam shows that it is an orthoclase cryptoperthite with zero triclinicity (Sriramadas and Murty, 1963). This also shows that the temperature was high and that the time necessary for ordering was shorter for the charnockites of this area than for the charnockites of other areas containing microc1ine (Murty, 1965). MINERALOGY OF INDIAN CHARNOCKITES 271 It is necessary to know the state of order in the co-existing potash and soda­ feldspars. Howie (1955) has furnished some data but they indicate that the potassium feldspar is not always an ordered microcline, and that orthoclase does also occur as a constituent. Myrmekite: A common feature in charnockitic rocks is the occurrence of myrmekite which has originated in different ways. In the Guntur district of Andhra Pradesh, Rama­ swamy and Murty (1972) find that myrmekite is common in felsic charnockites (granitic to dioritic), while it is scarce in mafic charnockites (pyroxene granulites). They believe that myrmekite is developed by exsolution, the two components of the intergrowth being derived from the potash feldspar. They had earlier reported (Ramaswamy and Murty, 1968) the rare occurrence of myrmekite in the pyroxene granulites of this area where the above explanation cannot hold good since these rocks do not contain any potassium feldspar. In such cases, they consider that the myrmekite is probably formed by the breakdown of the anorthite molecule of the plagioclase releasing the required silica and making the plagioclase less basic than be­ fore. Such a view was already expressed by Sarma and Raja (1958) for the origin of myrmekites in the felsic (granitic) rocks of the Hyderabad area. According to Bhattacharyya (1971 b), the presence or absence of myrmekite at the contact of plagioclase and K-feldspar is a function of stress, the higher stress favouring the formation of myrmekite. Elsewhere, it has been thought to be related to potash metasomatism (Macdonald, 1964).

QUARTZ The quartz grains in charnockites are usually clear .with abundant small acicular inclusions and shapeless red-brown plates. The fine needles show definite orientation, generally two sets intersecting at 72°. Undulose extinction is common. According to Howie (1955), the blue colour is not due to the inclusions alone. He heated a specimen of coarse charnockite at 950°C for one week and found that though the inclusions remained, the colour was lost; he considered that the blue colour may be due to very fine ultramicroscopic material exsolved involving Ti and Zr ions when the quartz was held at a moderately high temperature following its orginal crystallisation, or during subsequent metamorphism.

MINOR MINERALS Murty (1958) recorded the occurrence of monazite in the felsic and intermediate charnockites of Visakhapatnam. Earlier, it had been noticed in the pegmatites (Mahadevan and Sathapathy, 1948) and leptynites (Sastry, 1954) of this area. The mineral is usually found in association with zircon. The common opaque minerals are magnetite and ilmenite. Some pyrite occurs in the Salem rocks, and pyrrhotite has been noticed in the mafic charnockites in the type area. Apatite and zircon are found as accessories in some charnockites. Green spinel has been reported from Pammal, near Madras: its composition is intermediate between pleonaste and hercynite. Muthuswami (1949) has described sapphirine associated with cordierite, spinel, garnet, biotite, and anthophyllite, in bands of hypersthene granulite near Mathurai in South India. 272 CS PICHAMUTHU

ACKNOWLEDGEMENT The writer IS grateful to Professor SISIr K. Sen, Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, for cntically going through the manuscnpt and making many valuable suggestions.

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Address 0/ the author C. S. PJCHAMUTHU, President, Geological Society of India, I, De Souza Road, Bangalore 560025.

(Received: July 27, 1978 .. Revisedform accepted: Jan. 18. 1979)