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

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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 clea- vages 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 vary- ing 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 O °~ 0,~ O O 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 num- ber 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.
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