Aluminous Augite in the Pyroxenite of the Charnockite Rocks of Visakhapatnam District, Andhra Pradesh

ALUMINOUS AUGITE IN THE PYROXENITE OF THE CHARNOCKITE ROCKS OF VISAKHAPATNAM DISTRICT, ANDHRA PRADESH

BY A. SRIRAMADAS, K. S. R. RAO AND A. T. RAO (Department of Geology, .4ndhra University, F/altair) Received October 29, 1968 (Communicated by Dr. B. P. Radhaltrishna, F.A.Se.) ABSTRACT Aluminous augite from an augite-pyroxenite occurring as a lens in the pyroxene granulites of the charnockite rocks, interbedded with garnet- siUimanite gneisses, quartzites and calc-granulites, is described. The presence of high aluminium in the augite indicates high temperature and pressure conditions of erystaUisation and suggests that the ultrabasie and basic members of the charnockite rocks of Visakhapatnam have crystallised from an olivine tholeiitic magma.

INTRODUCTION ALUMINOUS augite which has not been reported in the charnockite rocks, adds a new concept of an earliest phase of the tholeiitic magma affinities in the charnockite rocks. A perusal Of the previous literature shows that the aluminous augite has been reported from alkali olivine basalt, Takasima, Japan (Kuno, 1964); aluminous titan augite from alkaline rocks of Japan (Aoki, 1964) and in a nepheline jacupirangite from Hesserean hill, Oka, Quebec (Peacor, 1967).

LOCATION The aluminous augite occurs in a pyroxenite near Aganampudi, (83 ° 8' 19":17 ° 42' 20"), 14 kin. south-west of Visakhapatnam.

GEOLOGICAL SETTING The area is in the Eastern Ghats belonging to the manganese ore mar- ble province of the charnockite region of Fermor (1936). The rock types encountered are quartz-feldspar-garuet-siUimanite-gneisses, garnetiferous quartzites and calc-granulites of the khondalite series; hypersthene granite, 15 16 A. SRIRAMADAS AND OTHERS diorites, plagioclase-hornblende-pyroxene-granulites, pyroxenites and anaphi- bolites of the charnockite rocks and the granites, garnetiferous granites and porphyritic granite gneisses. The aluminous augite pyroxenite occurs as a lens in the plagioclase- hornblende-pyroxene granulite of the charnockite rocks which is interbedded with quartz-feldspar-garnet-sillimanite gneisses and calc-granulites of the khondalite series and garnetiferous granites.

PETROGRAPHY OF ALUMINOUS AUGITE PYROXENITE AND PLAGIOCLASE- HORNBLENDE-PYROXENE GRANULITE Aluminous augite pyroxenite is pinkish-black in colour, hard and com- pact, medium to coarse-grained with Specific Gravity of 3.330 (-4-0.005). The aluminous augite shows poikilitic texture by enclosing plagioclase grains irregularly. Very small grains of spinel are enclosed in aluminous augites. Spinel is green in colour and isotropic. Most of the plagioclases are untwinned, showing undulose extinction and bending of cleavages. The optic axial angles vary from 74 ° to 88 ° over X. Some of the grains showing twinning on albite law giving anorthite content between 82~ and 8870 which corresponds to calcic bytownite. Secondary calcite is developed from plagioclase. The aluminous augite is pleochroic with X--light pink, Y--dark pink and Z--pinkish-pale green. 2V over Z varies from 47 ° to 49 ° and the Z over e varies from 42 ° to 46 °. Aluminous augites show one set of cleavages; two sets of cleavage are not uncommon. Plagioclase-hornblende-pyroxene-granulite is greyish-black, medium- grained rock with Specific Gravity 3.24. Plagioclase, augite, hornblende and hypersthene are the essential minerals with garnet, iron ores and apatite as accessories. The texture is granulitic and the minerals show bent cleavages and twin lamellae and undulose extinction. The plagioclases are mostly untwinned showing 2 V varying from 86 ° to 88 ° over X (bytownite) and also 88 ° over Z (labradorite). A few grains are twinned on albite law, indicating the anorthite content between 65% and 85~ which corresponds to labradorite and calcic bytownite in composition. Augite is very feebly pleochroic in shades of green. 2 V over Z is varying from 53 ° to 54 ° and Z over c from 40 ° to 42 °. Most of the augites show development of the hornblende needles along cleavages and also along Aluminous Augite in Pyroxenite of Charnockite Rocks 17 outer margins. The refractive indices of augite are ~--1.699; /?,---1.705; and 7--1.723.

Hornblende shows pleochroism with X--light yellow, Y--dark brown and Z--yellowish-brown. 2 V over X shows from 83 ° to 86 ° and Z over c from 10 ° to 15 ° which corresponds to the properties of common hornblende. Non-pleochroic dark brown biotite flakes are developed along the cleavages and a few hornblende grains are studded with iron ore inclusions. Hypersthene shows pleochroism X--pale pink, Y--colourless and Z--pale green. 2 V over X is from 54 ° to 56 °. The refractive indices are a--1.703; /3--1.713; and ~,--1.717. Pale pink garnet is mostly developed as a rim between the plagioclase and hornblende.

MINERALOGY

Pure fractions (95~o) of the minerals were obtained by using Frantz Isodynamic Separator. The remaining impurities were removed by hand- picking and also by means of heavy liquid separation. The /3-indices were determined by using immersion liquids on (100) flakes (Hess, 1949). Maxi- mum probable errors of the index determinations is considered to be -(- 0.002. ~ and y were inferred by determining birefringence in thin section by means of Bereck's compensator.

ALUMINOUS AUGITE

The aluminous augite is pleochroic with X--fight pink, Y--dark pink and Z--pinkish-pale green. 2 V over Z varies from 47 ° to 49 ° and Z over c ranges from 42 ° to 46 °. The X-ray diffractogram of the aluminous augite is shown in Fig. 1 and the unit cell parameters are given in Table I.

TABLE I

Ca Mg Fe a "° b ~° cA° /~ a sin/3

52.3 34.9 12.8 9.731 8.872 5.274 74° 5' 9"358

The chemical composition in terms of Ca : Mg : Fe as calculated from the chemical analysis is also given along with the unit cell parameters (Table I). 18 A. SRIRAMADAS AND OTHERS

°~

0,~

t/)

r Aluminous Augite in Pyroxenite of Charnockite Rocks 19

Sakata (1957) and Coleman (1963) have stated that in synthetic diop- sides the addition of A1 in experimental conditions resulted in a decrease in a, b and a sin/3 and increase in c. Lewis (1967) and Peacor (1967) have observed reduction in b, a sin/3 and increase in c for natural clinopyroxenes containing considerable amounts of A1, Fe +s and Ti. The aluminous augite of the present study with appreciable amounts of A1, Fe+s and Ti has also indicated reduction of a, b and a sin/3 and increase of c parameter within the range given for synthetic and natural clinopyroxenes investigated.

The chemical analysis of aluminous augite, augite, and hypersthene from the pyroxene granulite are given in Table II, together with the number of cations on the basis of 6 (0) along with some optical properties.

TABLE II Composition and properties of pyroxenes

1 2 3

SiOe 45 "48 50" 16 49" 56 AlaOa 11 "06 3"31 2"56 Fe,O8 2.03 2" 45 1.84 FeO 5.08 9.83 25.43 MnO 0.35 0.48 0.47 MgO 11.08 12.54 19.01 CaO 23.13 20.07 0.43 Na~O 0.21 0.26 0.19 K20 0.11 0.08 0.10 Cr~Oa 0.03 0.02 0.02 TiO2 1" 64 0.45 0.55 H~O+ 0"31 0"22 0'16 H~O- 0" 13 0"09 0"06 Total ... 100.64 99-96 100.38 2Vz 47° to 49 ° 53 ° to 54° 124° to 126° Z over c 42 to 46 40 to 44 ... ¢ 1 "720 1 "699 1 "703 /8 1 "728 1 "705 1 "713 7' 1 "741 1 "723 1 "7175 7....¢ 0.021 0.024 0"0145 20 A. SRIRAMADAS AND OTHERS TABLE II (Contd.) Number of metal atoms on the basis of 6 (0)

1 2 3

Si 1- 693)~ 1-896~ 1 • 892~ A1 0.307)" 2.00 0-104J 2.00 0-108j 2.00 A1 0.1781 0.0441 0.007 J Ti 0.046 1 0.014 1 0-016 I Fe+3 0.056 1 0.070 ~ 0.053 Fe+2 0.158 [ 0.311 [ 0.812 Mn 0.011~ 2"01 0.016k 1"996 0.015}2.02 Mg Ca 0.922 0.813 0.017 Na 0.018 I 0.016] 0"0141 K 0"004 J 0"004J 0"005J Ca .. 52.3 42.5 0.9 Mg .. 34.9 36.8 54.7 Fe .. 12.8 20.7 44.5

1. Aluminousaugite from pyroxenite of the charnockite ") rocks of Visakhapatnam. ~ Analyst: G. Damodara Rao, 2. Augite from plagioclase-hornblende-pyroxene-granulite( T.C.R. Corporation, Madras. of charnockite rocks of Visakhapatnam. ( Verifiedby: 3. Hypersthenefrom plagioclase-hornblende-pyroxene-gra- I K. Sriramaehandra Rao. nulite of charnockite rocks of Visak_hapatnam. ..J

The proportions of pyroxene components of aluminous augite and augite according to the methods of Kushiro (1962) are given in Table III.

TABLE Ill Proportions of pyroxene components of aluminous augite from aluminous augite pyroxenite and augite from plagioclase-hornblende-pyroxene granulite (Molecular per cent) Aluminous augite Augite

(Na,K)Fe+sSizO6 ... 1.16 1.23 NaAlSi206 .. CaTiAl20 e .. 2 ~67 0:~5 CaFe+sA1SiOe 2.09 2"52 CaA12SiO6 ... 10.37 1"95 CaFe+aSiaOe .. CaSiO3 ... 38 ~30 38:31 MgSiOs ... 35.63 37.75 FeSiOa ... 9 "79 17.51 Total .. 100.01 100.12 Aluminous Augite in Pyroxenite of Charnockite Rocks 21 Total (Na, K) is being consumed in forming (Na, K)Fe+3Si~Oe. (Na, K) Fe+s Si~Oe and CaFe+3A1SiOe have consumed all the available Fe +3. Kushiro (1962) inferred this as characteristic of igneous clinopyroxenes. Accord- ing to Kushiro (1962) CaA12SiO n in clinopyroxenes from basaltic rocks is 1.7~o, from charnockites is 5.5~ and from olivine nodules in basaltic rocks is 8"3~o. The CaA12SiO e from the aluminous augite is 10"37~o which is more than those of the clinopyroxenes from basaltic rocks.

DISCUSSION Field Relations Aluminous augite constitutes 87~ of the pyroxenites. This occurrence of pyroxenite as a lens in the plagioclase-hornblende-pyroxene granulite shows that the pyroxenite is genetically related to the plagioclase-hornblende- pyroxene granulite. The pyroxenite lens might have been crystallised as an earliest phase from the parent gabbroic magma from which the plagioclase-hornblende-pyroxene granulite had originally crystallised under conditions of decreasing temperature and pressure. Mineral Chemistry The aluminium content in the augite investigated is unusually high. The aluminium content is expected to be minimum in early stages of crystallisation of ferromagnesians at normal temperature and pressure conditions. However, the synthetic experiments have proved that the pyroxenes crystallising from a basalt magma at high pressure contain more alumina than those crystallising at low pressures from the same magma (Clark, Schairer and de Neufville, 1962; Tilley and Yoder, 1964). The entrance of aluminium into the tetrahedral position of pyroxenes is attributed to the higher temperature conditions (Thompson, 1947; Buer- ger, 1948; and Kushiro, 1960). To ascertain the parentage, the atomic proportions of Si and A1 of the aluminous augite of pyroxenite and augite of plagioclase-hornblende-pyroxene granulite are plotted in the diagram (Fig. 2) of Kushiro (1960). Aluminous augite falls in the region of the felspathoid-bearing alkalic rocks whereas the augite falls in the tholeiitic field. A reference to the diagram of Lebas (1962) also showed that the aluminous augite fell on the peralkaline field and augite of the pyroxene granulite fell on the non-alkaline field. As the aluminous augite from pyroxenite contains more CaAlzSiO 6 molecule in it, it is located in the felspathoid-bearing alkaline rock region whereas augite from pyroxene granulite is located in tholeiitic magma field. Kushiro (1962), Clark, Schairer A. SRIRAMADAS AND OTHERS and de Neufville (1962) have stated that the rising pressure increases the solubility of CaA12SiOe in clinopyroxenes and similar conclusion has been arrived by Hess (1960) for clinopyroxenes erystallising in plutonie conditions. The decrease of aluminium in the tetrahedral position in aluminous augite of pyroxenite compared to augite of pyroxene granulite shows tholeiitic magma affinity as observed by Kushiro (1960), Lebas (1962), Brown (1957) and also corroborated by the observation of Kuno (1955) who has described augite phenocrysts crystallising in the earliest stage from tholeiitic magma with high Al~O3 when compared to ttiat of the later crystallising groundmass clinopyroxenes.

tO ,

0.4 ¸

0.'~ |. o q~ O O

o •

o • I 8 :.

9.1

0 t-6 1.7 |.6 I-9 ~'.0 $i FIG. 2. Relation between the atomic proportions of Si and AI in the clinopyroxenes from the three different rock groups. Solid circles : clinopyroxenes from tholeiitic rocks. Open circles: clinopyroxenes from feldspathoid-free alkali rocks. Half solid circles: clinopyroxenes from feldspathoid-bearing alkal~c rocks. (Atomic proportions are calculated on the basis of six oxygen atoms.) (After I. Kushiro, 1960.) i~11 Al-minous augite from aluminous augite pyroxenite of Aganampudi, Visakhapatnam. I~ Augite from plagioclase-hornblende-pyroxene granulite of Aganampudi, Visakhapatnam. (By the authors) The presence of remarkable poikilitic texture and the absence of exsolution lamellae in the aluminous augite does indicate that the aluminous augite crystallised directly as an earliest phase from the magma. The com- positional difference from calcic bytownite in pyroxenite to bytownite in plagioclase-hornblende-pyroxene granulite may also be encountered due to fractionation during crystallisation. Aluminous Augite in pyroxenite of Charnockite Rocks 23

Based on the above facts the chemistry of the aluminous augite denote that the aluminous augite pyroxenite has been crystallised as an earliest phase from tholeiitic magma under high temperature and pressure conditions and the rest of the magma has been subsequently crystallised as gabbro under lowered conditions of temperature and pressure.

Petroehemistry The chemical analysis, C.I.P.W. norm and mode of atuminium augite pyroxenite and plagioclase-hornblende-pyroxene granulite are given in Table IV. On comparison, leucite, nepheline, olivine and calcium orthosilicate found in the pyroxenite norm, are absent in the mode. The amount of plagioclase in the mode is less than that of norm. The leucite, nepheline,

TABLE IV

Wt. ~ C.I.P.W. Norms Modes

1 2 1 2 1 2

SiOz 44.86 47.16 or .. 1"11 Plagioclase 12.05 34.03 AlaOa 14"28 15"82 ab .. 15.20 Aluminous augite 87.38 ... Fe~O3 1.32 1-72 an 36.97 34.47 Augite .. 25.83 FeO 4-98 9.98 le 0.44 .. Hypersthene .. 9" 59 MgO 9.68 7.88 ne 1.70 .. Hornblende 0-15 25.28 CaO 21.98 12-78 di 43.18 23.96 Garnet .. 3.47 Na~O 0.41 1- 82 hy .. 8.16 Calcite 0" 14 .. K~O 0.08 0.21 ol 7.23 11.32 Iron ores 0.04 1" 60 TiOs 1-26 1.18 es 4-87 .. Spinel 0.24 .. MnO 0.31 0.38 nat 1.86 2.55 Apafite .. 0.20 P~O5 0.91 0.31 il 2.43 2"28 COs 0.20 .. ap 0.34 0" 67 CrsOs 0.05 .. ce 0.50 ... H~O+ 0.41 0.52 H~O- 0.13 0.18

Total 100.14 99.94 Total Total 100.00 100.00

1. Aluminous augito pyroxenite. 2. Plagioclaso-hornblcadc.pyroxonogranulit¢. 24 A. SRIRAMADAS AND OTHERS olivine, calcium orthosilicate and plagioclase of the norm are represented by the aluminous augite and spinel of the mode. In plagioclase-hornblende-pyroxene granulite olivine is found which is absent in the mode. Plagioclase is less than that of the norm. Horn- blende, augite and garnet are present in the mode and not in the norm. Olivine, excess plagioclase and diopside of the norm are represented by hornblende, augite and garnet of the mode. The presence of olivine and nepheline, in the norm of aluminous augite pyroxenite and olivine and hypersthene in the norm of plagioclase-hornblende-pyroxene granulite, suggests affinity to alkali basalt and olivine tholeiitic magmas repectively as sugges- ted by Yoder Tilley (1962). Metamorphism The values of Ca : Wo : En for the coexisting ortho--and clino-pyroxenes of plagioclase-hornblende-pyroxene granulite are plotted in Hess's triangular diagram and the extended tie line cuts at En26 Wo74 point and is well within the range ascribed for igneous rocks (Hess, 1941; Howie, 1955; Brown, 1957). But recently this demarcation is found to be not clear cut. Based on the distribution of certain elements between coexisting pyroxene pairs, the metamorphic equilibrium conditions particularly tempera- tures could be assessed (Kretz, 1961; DeVore, 1951; Barthlome, 1962; Engel, Engel and Havens, 1964). The calculated 'Kp' value 1.703 of the present coexisting pair when recast with Barthlome (1962) and Engel, Engel and Havens (1964) diagrams gave a temperature of 750 ° C. indicating that the coexisting clinopyroxene and orthopyroxene in plagioclase-hornblende- pyroxene granulite has attained equilibrium during granulite facies of metamorphism. The brown hornblende in the plagioclase-hornblende-pyroxene granulite is mostly developed along cleavages and outer margins of augite which clearly indicates that it has been formed later from augite during metamorphism. The garnet is mostly corona-like in between the contact of hornblende and plagioclase. Similar relationship has been found by Buddington (1952) in the case of Adirondack gabbroic rocks. The presence of less calcic plagioclase adjacent to the hornblende and garnet may indicate that the lime and alumina of the anorthite molecule might have been utilised in the formation of hornblende and garnet during metamorphism. The metamorphism could only reconstitute the gabbroic mass into plagioclase-hornblende-pyroxene granulite without much change in bulk composition whereas the pyroxenite has retained its original poikilitic texture and was unaffected. Aluminous ~tugite in Pyroxenite of Charnoekite Rocks 25

CONCLUSIONS Aluminous augite is reported for the first time in the pyroxenite of charnockite rocks. The aluminous augite shows pleochroism, X--light pink, y--dark pink, and Z--pinkish-pale green, 2 V over Z: 470-49 ° and Z over c: 42°-46% The unit cell parameters are a : 9.731, b : 8.872, c: 5.274; /3 : 74 ° 5' and a sin/3: 9.358. All these characters are that of aluminous augite and is also confirmed by 11.06~ of A1203. The aluminous augite occurs in the pyroxenite which is a lens in the plagioclase-hornblende-pyroxene granulite. The high aluminium content with appreciable aluminium in tetrahedral position in aluminous augite of pyroxenite shows its early crystallisation under high temperature and pressure conditions from olivine tholeiitic magma. This is also confirmed by hypersthene and olivine in the norm of plagioclase-hornblende-pyroxene granulite. The remarkable poikilitic texture and absence of exsolution lamellae in aluminous augite point out that the pyroxenite lens might have crystallised as an earliest phase from parent gabbroic magma. The granulite facies of metamorphism has converted the gabbro to granulite leaving the pyroxenite lens unaffected.

ACKNOWLEDGEMENTS

The authors wish to thank P. Krishnamurthy for having taken the X-ray pattern of aluminous augite at McGill University.

REFERENCES

Aoki, I. K. .. "Clinopyroxenes from alkaline rocks of Japan," Amer. Min., 1964, 49, 1199-1223. Barthlome, P. .. "Iron-magnesium ratios in associated pyroxenes and olivines," Petrological Studies, Buddington Volume, 1962, pp. 1-20. Berman, H. .. "Constitution and classification of natural silicates," Amer. Min., 1937, 22, 342--408. Brown, G.M. •. "Pyroxenes from the early and middle stages of fractionation of the Skaergaard intrusion, East Greenland," Min. Mag., 1957, 31, 511-43. l~uerger, M. H. .. "The role of temperature in mineralory," ,4met. Min., 1948, 33, 101-21. Clark. S. P., Schairer, J. F. and Phase Relations hi the System CaMgSi2Oe-CaAl~SiO~-Si02 de NenfviUe, J. at Low and High Pressure, Carnegie Inst., Wash., Yb. 61, 1962, pp. 59-68. B3 26 A. SRIRAMADAS AND OTHERS

Deer, W. A., Howie, R. A. and Rock Forming Minerals, Longmans, London, 1963, 2. Zussman, J. Danold R. Peacor • . "Refinement of the crystal structttre of a pyroxene formula M1Mz (Si16A10r6)O,''. Amer. Min., 1967, 52, 31--41. Engel, A. E. G., Engel, G. C. and "Mineralogy of the amphibolite interlayers in the gneiss Havens, R. G. complex, N.W. Adirondock Mountains, New York," dour. GeoL, 1964, 72, 131-56.

Fermor, L L. .. "An attempt at the correlation of ancient schistose rocks of Peninsular India," Mem. GeoL Surv. India, 1936, 70. Hess, H.H. .. "Pyroxenes of common marie magmas," Amer. Min., 1941, 26, 545-55. .. "Chemical composition and optical properties of ctino- pyroxenes," Ibid., 1949, 34, 620-66. .. "Stillwater igneous complex, Montana," Geol. Soc. Amer. Memo., 1960, 80.

Howie, R. A. .. Geochemistry of the Charnockite Series of Madras', India, Transactions of Royal Soc. Edin., 1955, 62, Pt. III, 18, 725-69. John, F. Lewis .. "Unit-cell dimensions of aluminous natural clinopyroxenes." Amer. Min., 1967, 52, 42-54. Kretz, R. .. "Coexisting pyroxenes," Geol. Mag, 1961, 98, 31-32. Kuno, H. .. "Ion substitution in the diopside-ferropigeonite series of clinopyroxenes," Amer. Min., 1955, 40, 70-94.

.. "Aluminian augite and bronzite in alkaliolivine basalt from Takasima, North Kyusyu, Japan," Advancing Frontiers in Geology and Geophysics, M. S. Krishnan, 1964, pp. 205-220.

Kushiro, I. .. "Si-A1 relation in clinopyroxenes from igneous rocks," Amer. dour. Sci., 1960, 258, 548-54.

.. "Clinopyroxenes solid solutions. Pt. 1. CaA12SiO~ com- ponent," Japan dour. Geol. Geogr., 1962, 33, 213-20. Lebas, M.J. .. '~The role of AI in igneous clinopyroxenes with relation to their parentage," Amer. dour. Sci., 1962, 260, 267-88. I.,¢ Maitre, R. W. .. "Petrology of volcanic rocks, Gough Islands, South Atlantic," GeoL Soc. Amer. Bull., 1962, 73, 1309--40. Muir, I. D. and Tilley, C. E. .. "The composition of the coexisting pyroxenes from the metamorphic assemblages," GeoL Mag., 1958, 95, 403--408. Ramberg, H and Devore, G.W... "The distribution of Fe++ and Mg++ in coexisting olivines and pyroxenes." dour. GeoL, 1951, 59, 193-210. Aluminous Augite in Pyroxenlte of Charnockite Rocks 27

Thompson, J.B. .. "Role of aluminium in the rock-forming silicates," GeoL Soc. Amer. Bull., 1947, 58, 1232. Tilley, C. E. and Yoder, H. S. (Jr.) Pyroxene Fractionation in Marie Magma at High Pressures and Its Bearing on Basalt Genesis, Carnegie Inst. Wash., Yb. 63, 1964, pp. 114--20. Yoder, H. S. (Jr.) and Tilley, "Origin of basalt magmas: an experimental study of C.E. natural and synthetic rock systems," Jour. Petro., 1962, 3, Pt. 3, 342-532.