sapphirine granulites from the kakanuru area, eastern

American Mineralogist, Volume 73, pages 692-700, 1988

Sapphirine granulites from the Kakanuru area, Eastern Ghats, India

D. C. K,q.NrrNBNr*Geological Survey of Canada,601 Booth Street, Ottawa KlA 0E8, Canada

A. T. Rc.oDepartment of Geology, Andhra University, Waltair, India 530003

Ansrnrcr

Sapphirine in the granulitic terrane of the Eastern Ghats, near Kakanuru, India, hasthree modes of occurrence: (l) as a dominant mineral in a rock of possible regolithicorigin, (2) generally as rims around spinel and magnetite in khondalite, and (3) replacingspinel in pyroxenites. All three types have distinct compositional characteristics; the X.,tMg/@g * Fe'z+)l value increases from 0.770 to 0.964 from type I to type 3. The para-geneses ofvarious types are discussed.

Temperature and pressure estimates based on suitable mineral pairs and calibratedgeothermometers and geobarometers indicate extremely high grade metamorphic condi-tions. The temperature estimate, based on two-pyroxene and two-feldspar geothermom-eters, ranges from 840 to 1050 "C. The estimated pressure is about 7 .3 kbar. These P- ?"conditions are in accordance with the sapphirine paragenesis. The temperature estimatefrom the garnet-biotite geothermometer yields a lower value (=600 "C) that can be attrib-uted to a subsequent metamorphic event accompanied by hydration and metasomatism.

INrnoouctIoN

Sapphirine-bearing granulites are very useful in deduc-ing the metamorphic conditions of the lower crust. Thisis mainly because sapphirine extends over a wide rangeof pressure-temperature (P-Z) conditions, as documentedin experimental and theoretical investigations (Nevvton,1972; Seifert, 1974; Ackermand et al., 1975; Bishop andNewton, 1975; Hensen, 1986, 1987). The occurrence ofsapphirine-bearing rocks has been noted in the EasternGhats granulite terrane by many workers (see Grew, 1982).The Eastern Ghats granulite belt, oriented NE-SW alongthe east coast of India (Fig. l), forms prominent hill rangesand is mainly composed of khondalites and charnockites.The khondalites comprise a variety of metamorphic rocksof sedimentary protoliths, quartz + orthoclase + gar-net + sillimadls + graphite being the most common as-semblage. The charnockites, containing garnet + pyrox-ene-bearing assemblages, show a wide spectrum oflithology ranging from felsic to mafic end members. Inthe eastern part of the belt, the charnockites are structur-ally emplaced into khondalites along axial regions of thefolds, whereas in the western margin, they occur alongthrust sheets.

In this paper, petrographic characteristics, bulk rockcomposition, mineral chemistry, and metamorphic con-ditions of sapphirine-bearing assemblages from the Ka-kanuru area are described. The region is covered by thick

* Present address: Applied Geoscience Branch,Nuclear Research Establishment. Pinawa. ManitobaCanada.

0003-o04x/88/0708-o692$02.00

tropical forest and is in close proximity to the westernmargin of the Eastern Ghats. Geographically, the regionfalls in parts of the East Godavari and Visakhapatnamdistricts of Andhra Pradesh.

GnNnncJ, cEoLocY oF THE AREA

A briefaccount ofthe geology around Kakanuru, basedon the preliminary map of Singh and Augustine (1978),is depicted in Figure 1. The area is essentially composedofthree broad units: (l) khondalite, (2) charnockite, and(3) amphibolite. Khondalite is the most abundant mapunit containing the assemblage quartz + feldspars * gar-net * sillimanite + sapphirine + spinel. On some out-crops, this unit displays extreme alteration, due to trop-ical weathering, developing laterite and/or bauxite. Thecharnockite unit is dominated by pyroxenes + plagio-clase * oxides + garnet. Olivine may be present in minoramounts. Some charnockite outcrops tend to be anor-thositic owing to local abundance ofplagioclase, gneissicbanding is common in charnockites, and leucosomes andmelanosomes are generally concentrated along the band-ing because of migmatization. An amphibole unit, occur-ring concordantly to the general rock foliation, is locatedin the northwest corner of the map (see Fig. l). In addi-tion, lenses of spinel-bearing pyroxenite containing coarse-grained sapphirine occur as disseminations along thekhondalite-charnockite contact.

S,lnnpr,ns AND PETRoGRAPHY

Samples (S25 to S30), hereafter referred to as samples25,26,27,28,29, and 30, were selected for deta i led

WhiteshellROE lLO,

692

KAMINENI AND RAO: SAPPHIRINE GRANULITES

Fig. l. Geologic map of the Kakanuru area showing locations of samples studied. The location of Kakanuru in the Eastern

Ghats is shown in the index map.

693

petrochemical study. Five of the studied samples are sap-phirine-bearing; one was collected at a khondalite-char-nockite contact (sample 25), three from pyroxenite lenses(samples 26, 27, and 28), and the fifth from khondalite(sample 30). Sample 29 belongs to the charnockite unitand is devoid of sapphirine. The sample locations andmineral assemblages are shown in Figure I and Table l,respectively.

Sample 25 is essentially composed of sapphirine *magnetite + sillimanite (Fig. 2). Apatite and magnetiteare present as inclusions in some sapphirine grains. Fineacicular hematite and/or magnetite are concentrated incleavages of both sapphirine and sillimanite. A weak pre-ferred orientation is present, and coarse hematite needlescut across the long dimension of sapphirine and silliman-

ite. The sapphirine grains display light blue to dark bluepleochroism, and some large grains contain small inclu-sions of sapphirine with the same optical properties butnot in optical continuity. Sillimanite prisms are inter-spersed between large sapphirine grains throughout, andsome of them show strain.

Samples 26,27, and 28 are similar in terms of mineralassemblages and textures. The only distinction is that 26also contains spinel. All are coarse-grained and charac-teizedby the assemblage sapphirine * orthopyroxene +phlogopite * monazite + spinel. Zircon and rutile arepresent in minor amounts. Sapphirine occurs as largegrains displaying weak pleochroism from pale blue to col-orless. Rodlike inclusions of rutile are common in sap-phirine. In sample 26, sapphirine invariably occurs as a

Fig. 2. Photomicrograph showing the assemblage sapphirine(Spr) + sillimanite (Sil) + magnetite (Mag). Sample 25.

Fig. 3. Photomicrograph of spinel-bearing (Spl) pyroxenite.Note sapphirine (Spr) rim around spinel. Also note phlogopite(Phl) and orthopyroxene (Opx). Sample 26.

694 KAMINENI AND RAO: SAPPHIRINE GRANULITES

TneLe 1. Mineral assemblages of samples investigated (amountsin vol%)

Sample: s28 s29 s30

Fig. 4. Photomicrograph showing monazite (Mo), with radialmicrofractures around, in sapphirine-bearing pyroxenite. Notephlogopite (Phl) and orthopyroxene in the matrix. Sample 28.

thin rim around spinel (Fig. 3). Orthopyroxene shows twowell-developed cleavages and weak pleochroism frombrown to colorless. A few orthopyroxene and sapphirinegrains are fragmented and enclosed in phlogopite prisms,implying that they are early in the paragenesis. Euhedraland some semi-rounded grains of monazite are concen-trated as clusters in some domains. Some monazite grainsshow radial microfractures around them owing to meta-mictization (Fie. a).

Sample 29 is a coarse-grained rock with an abundanceofplagioclase (see Table l). The plagioclase grains showresidual strain in the form of kinked twin lamellae; somegrains show antiperthetic texture. Acicular rutile and spi-nel inclusions are generally aligned parallel to twin la-mellae in plagioclase. Pyroxenes, olivine, and biotite areconcentrated around plagioclase boundaries. Spinel grainsinvariably occur as inclusions in ferromagnesian min-erals. Minor alteration, concentrated along grain bound-aries and extensional fractures, produced quartz, carbon-ate, and chlorite.

Sample 30 is a khondalite that has minor amounts ofsapphirine (<2o/o). The most important textural charac-teristic of this sample is the presence of coronas: (l) sap-phirine rims around spinel and magnetite, which in turnare rimmed by garnet, and (2) sillimanite rims aroundoxides (spinel and magnetite), which in turn are alsorimmed by garnet (Fig. 5). Ilmenite lamellae are noted inmagnetite in polished sections. Biotite is sporadically dis-tributed throughout in amounts of <10/0. Plagioclase ispolysynthetically twinned and shows perthitic and me-soperthitic texture. Quartz and sapphirine are present, indirect contact, in one domain.

Bur,x nocx AND MTNERAL ANALysES

Bulk chemical analyses of pulverized rock samples wereperformed using the rcp method at the Analytical Labo-ratory ofthe Geological Survey ofCanada. FeO was de-

10-25

1 0-25

10-25

1-5

1-21-2

1 0-251-51-2

1-5

1-2

t - c

t - z

1-2

1-2

10-25 10-25

10-25 10-25

10-25 10-25

termined by the wet-chemical method. Rare-earth ele-ments (REEs) were determined by rcp on trace-elementsolutions that were preconcentrated by a factor of I 0 afterseparation of major elements through ion-exchange res-1ns.

Oxygen-isotope analyses were carried out on an aliquotof bulk rock powder. Oxygen was extracted with BrF, andconverted to CO, prior to spectrometric analysis, as de-scribed by Clayton and Mayeda (1963). Isotopic valuesare reported as d18O in per mil relative to standard meanocean water (SMOW). The results of the bulk rock chem-ical analyses and oxygen-isotope analyses are listed inTable 2.

Microprobe analyses of silicates and oxides were per-formed on a MAc electron microprobe, using C-coatedpolished thin sections. The instrumental conditions wereas follows: accelerating potential, 20 kV; specimen cur-rent, l0 nA; counting time, 100 s. Natural standards wereused for all the minerals analyzed. Sodalite was used forCl in biotite. The raw data were corrected for back-ground, nonresolution, and matrix effects using the M,c,crcv program (Colby, 1980). For each mineral, a number ofgrains were analyzed to assess the inhomogeneity be-tween grains and also a number of spots within a singlegrain to investigate possible zonation. As alkali feldsparcontains exsolution lamellae, its analyses were reinte-grated; magretite grains and their contained ilmenite la-mellae were analyzed separately. Monazites were ana-lyzed on a cAMEcA probe using REE II and REE IIIstandards (Drake and Weill, 1972).The mineral analysesare presented in Tables 3 to 8.

Bur,x nocr cHEMrsrRY

An examination of the chemical composition (Table 2)of the samples studied shows some distinct composition-al characteristics. Sample 25 is silica undersaturated, richin Al,O, with a high FerOr/FeO ratio (1.42). It containssignificant TiO, and a large amount of MgO. These char-acteristics can be related to a protolith of residual mate-

OuartzOrthoclasePlagioclaseOlivineOrthopyroxeneClinopyroxeneSapphirineSillimaniteMicaGarnetllmeniteMagnetiteSpinelPyriteMonaziteApatiteZiconRutile

1 0-251 0-25

1 0-2510-251 0-25

1-51-51-5

10-251-21-51-5

1-21-21-21-2

1-5

1-2 1-2

1-2

1-21-2t - z


TneLe 2. Whole-rock analyses

Sample:

69s

SiO, (wt%)Tio,AlrosFerO.FeOMnOMgoCaONa.OK"oHrO*Pro.

Total

17 .16 31 .262.35 1.58

55.35 32.7'l7.80 1 505.bu z,Ya0.06 0.02

11.23 26 600.02 0.050.00 0.010.00 2.500.11 0 .250.00 0.200 00 0.05

99 58 99.68

2.36 122.24.24 243.51 .87 9870.33 15 60.05 1.820.18 15 .310.04 3.420.15 4 950.13 2.260.62 14.800.11 2 .21

5.87 4.51

41.90 34.301.12 0 .80

20.15 30.611.65 1 .364.28 2.830.03 0.01

26.91 25.520.02 0.070.0s 0 002.25 2.410 55 0.720.16 0.340 . 1 0 0 . 1 0

99.17 99.07

173.4 836.8330.6 1591 .0142.8 602.826.5 58.12.68 4 98

18.36 37 414.15 8 .016.02 8.782.53 2.60

15.70 1 8.402.61 3.0s

4 96 8.40

49.00 69.201 6 6 1 . 1 5

15.20 16.502.20 2.208.00 4.800.19 0 .109.22 2 .15

10.77 0.922.40 0.501.04 2190.52 0.480.13 0 .030 30 0 10

100 63 100 32

7.9 31 .816.9 50.211 .4 15 .33.29 2.511.44 1 .384.52 3.410.67 0 300.71 0.490.29 0.251.89 1 .55o.27 0.26

6.58 6.41

Fig. 5. Photomicrograph of khondalite showing concentricrims of sillimanite (Sil) and garnet (Grt) around magnetite (Mag).Sample 30.

rial such as an aluminous regolith. A similar protolithhas been identified by Meng and Moore (1972) for somesapphirine granulites in Labrador, Canada. The chon-drite-normalized REE pattern shows depletion of inter-mediate REEs (see Fig. 6) and is distinct from any of therock-mineral patterns from continental crust (see Taylorand McClennan, 1985). The pattern represents a com-posite of the assemblage sapphirine + sillimanite + mag-netrte.

Samples 26, 27, and 28 have similar chemical charac-teristics as reflected in their mineralogy. They are low inSiO, with large amounts of AlrO. and MgO. The Fe,OrlFeO ratio is small (0.38 to 0.51). KrO is significantlyhigh. They all show steep LREE chondrite-normalizedpattems and an enrichment trend of the same from sam-ples 26 to 28 (see Fig. 7). This trend correlates with theincrease in monazite and the elimination of spinel.

Compositionally, sample 29 is comparable to the maficigneous rocks of tholeiitic nature that represent the t1p-ical mafic granulite-charnockite suite of the Eastern Ghats.The chondrite-normalized pattern of REEs (Fig. 6) israther flat, which again is a typical characteristic oftho-leiitic rocks. The moderate positive Eu anomaly can beattributed to plagioclase abundance.

Sample 30 is silica saturated and can be considered asa quartzofeldspathic rock of pelitic nature. The chon-drite-normalized REE pattern (Fie. 6) shows an abun-dance of LREEs, a positive Eu anomaly, and a moderateconcentration of LREE. High LREEs can be attributed tothe presence of the accessary minerals monazite and zir-con. Feldspars account for the Eu anomaly, whereas gar-net concentrates the HREEs by acting as a sink.

The d'8O values (Table 2) for all the samples, except28, are light and range from 4.51 to 6.58V*. Sample 28has a slightly greater value, 8.40V,n. These results comparewell with the generally low 6'80 values reported for gran-ulites from the Adirondacks. U.S.A. (Morrison and Val-

ley, I 986), Australia (Allen, I 98 I ; Wilson and Baksi, I 983,1984), and the Sahara (Fourcade and Javoy, 1973). Thed'80 values, in the present investigation, appear to reflectpregranulite metamorphic signatures, as the values ofsamples 26 and 27 fall within the range reported for pris-tine mafic and ultramafic nodules (Kyser, 1986). How-ever, sample 28, belonging to the same category but moreaffected by metasomatic alteration, has the largest value,suggesting that it is isotopically modified. The d'80 valueof mafic granulite and charnockite (sample 29) falls with-in the range compiled for mafic igneous rocks (Taylor andSheppard, 1986). The D'8O values of samples 25 and30probably represent the signatures of their sedimentaryprotoliths.

Tnele 3. Microprobe analyses of sapphirine

Sample: s25 s26 s27 s28Rim onFe-Tioxide Matrix

La (ppm)CeNdSmtsUGdTbHoTmYbLu

6180 (7-)

s30

sio,Alr03FeO,MnOMgo

I o lal

14.50 14 85 14 90 14.73 12.49 12.6857 63 61 .70 61 .27 61 .16 60.70 60.321 1 .92 3.65 3.22 2.77 10.80 1 1 .480.07 0.00 0.00 0.00 0.00 0.0s

16 47 1 9.68 20.78 20.97 15.94 1 5.50100.59 99.88 100.17 99.63 99.93 100.03

Number of cations on the basis of 20(O)1 .711 1 .735 1 .734 1 .721 1.512 1 .5394.285 4.265 4.266 4.279 4.488 4.4613.976 4.202 4.142 4.145 4.173 4.1350.313 0.063 0j24 0.134 0.315 0.3260.898 0 300 0.189 0.137 0.741 0.7460.006 0.000 0.000 0.000 0.000 0.0052.915 3.426 3.605 3.651 2.875 2.804

0.770 0.919 0.950 0.964 0.795 0.790

5 lrilAl

t6tAl

Fe3*

Fe.-

MnM9

Xvs


TneLe 4. Microprobe analyses of pyroxenes, olivine, and garnet

Orthopyroxene Clino-pyroxene

s29Olivines29

Garnets30Sample: s28

sio,Tio,Al2ooFeO,MnOMgoCaONaro

Total

53.320.036 3 18 1 50.01

31.950.000.00

99.77

1.8570.1430 . 1 1 60.0010.0260.2140.0001.6580.0000.000

0.886

s3.660.035 9 27 2 30.00

32.730.000.00

99.57

54.540.045.5J

5.740.02

34.190.000 0 0

100.06

51.050.052.88

21.780.53

22.920,000.31

99.52

51.420.645 .187.510 .16

12.1121.49

1 .0699.57

34.820.020.35

37.870.55

26.210.000.00

99.82

39.370.01

21.5524.230.65

12.521 5 80.00

99.91

5.9820.0183.8420.0010.0003.0790.084z,oJc

0.2470.000

0.479

Number of cations on the basis of 6(0) for pyroxenes, 4(O) for olivine, and 24(O) for garnetSirltAlr6tAl

TiFe3*Fe2+MnMgCaNa

Xus

1.8650.1 350.1 080.0010.0260.1850.0001.6950.0000.000

0.902

1.872o.1280.0970.0010.0290.1330.0041 7Rn

0.0000.000

0.929

1 . 9 1 00.0900.0350.0010.0730.6020.0111.2730.000o.220

0.679

1.8930.1070 . 1 1 80.1800.0300.2010.0050.7190.8470.076

0.781

0.8950.0131.1040.0000.000

0.552

0.9840.0120.0000.0000.000

MrNnnLr- cHEMrsrRY

Microprobe analyses of various minerals analyzed arcgiven in Tables 3 to 8.

The sapphirine chemistry fiable 3) indicates that thereare three populations: (l) an FeO-rich type with relativelylower AlrO, (sample 25), (2) an intermediate type con-taining moderate amounts of AlrOr, FeO, and MgO withlower amounts of SiO, (sample 30), and (3) a MgO-richtype with relatively lower FeO (samples 26,27, and 28).These differences are reflected in X* [Mg/(Mg + Fe'z*)]

TaeLE 5. Microprobe analyses of mica

Phlogopite

Sample:

values, which increase from 0.770 to 0.964 from type Ito type 3 (see Table 3). Type 2 sapphirine, i.e., sample30, shows compositional variation in different domains.For example, sapphirine rims around magnetite havehigher MgO and lower FeO, compared to sapphirine grainsimmersed within the quartzofeldspathic matrix. Fe3+ cal-culated according to the method of Higgins et al. (1979)shows values ranging from 0.063 to 0.326 (see Table 3).

The sapphirine compositions, except sample 25, plot(Fig. 8) close to the line defining the common substitution(Mg,Fe,Mn) + si : 2Al (Deer et al., 1978; Higgins et al.,1979). Deviation of 25 from the common substitutionline indicates that it may have been involved in othersubstitutions as well.

TreLe 6. Microprobe analyses of feldspars

Plagioclase

Biotites29

sio,Tio,Al2o3FeO,MnOMgoCaONaroKrO

Total

35.879.22

13.7915.760.00

1 1 . 4 60 . 1 10.058.88

95.14

5.4082.4510.0001.O441.9850.0002.5700.0180.0321.705

0.564

Sample:

41.42 42.102.85 2.90

14.50 13.382.68 2.030.00 0.00

24.21 24.740.05 0.110.06 0.529.82 9.43

95.59 95.21

41.08 39.333.45 4.14

15.07 14.302.43 7.680.00 0.04

23.89 20.720.01 0.070.01 0.00

10.21 10.0496.15 96.32

s29Orthoclase

s30

sio,AlrosFeO,CaONa.OKrO

Total

SiAIFE

NaK

AbAnOr

Q i

r.lAlr6tAlTiFe2*MnMgCaNaK

Xue

56.6127.610.13

10.265.300.42

100.33

58.69 65.3225.55 18.460.01 0.028.54 0.626.93 3.810.23 12.02

99.95 100.2sNumber of cations on the basis of 22(O)

5.787 5.885 5.715 5.6422.213 2.115 2.285 2.3580.175 0.088 0.186 0.0600.299 0.305 0.361 0.4460.313 0.238 0.284 0.9210 000 0.000 0.000 0 0055.040 5.152 4.955 4 3420.007 0.017 0.002 0.0100 016 0.141 0.000 0.0001750 1 .681 1 .812 1837

0.941 0.956 0.946 0.825

Number of cations on the basis of 32(O)10.3265.4240.0031.6482 4200.026

Molecular proportions (%)47.14 58.8950 45 40.432.41 0.62

10.1475.8320.0191.9701 842n no(

11 .9343.9780.0030.1 331.5082.558

35.913 . 1 8

60.91/Vote.' S30 contains 0 52'/" Cl.


TABLE 7. Microprobe analyses of oxides

697

Spinel Magnetite llmenite

Samole: 526 S29 S30 S25 S30 S25 S29 S30

Tio, 072 011 O.O2 15.86 16.04 48.30 47.34 50.20Al,o3 61.55 57.41 59.96 0.40 0.20 0.00 0,00 0.00Cr.O. 0.49 0.36 0.73 0.24 0.11 0.00 0.00 0.00FeO, 22.11 32.86 25.95 82.95 83.65 49.75 50.63 48.02MnO 0.03 0.23 0.02 0.04 0.00 0.16 0.08 0.04MgO 15.27 I 57 12.94 0.61 0.46 2.05 1.82 213

Total 100.17 99.54 99.62 100.10 100.46 100.26 99.87 100.39

Number of cations on the basis of 4(O) for spinel and magnetite and 3(O) for ilmeniteTi 0.014 0.002 0.000 0.448 0.452 0.928 0.913 0.952Al 1 .909 1 .899 1 .91 1 0.140 0.072 0.000 0.000 0.000Cr 0.010 0.008 0.015 0.056 0.022 0.000 0.000 0.000Fe3* 0.048 0.1 1 4 0.084 1 .069 1.148 0j25 0.1 14 0.122FeF. 0.447 0.653 0.s03 1 .406 't.422 0.937 0.965 0.900Mn 0.000 0.005 0.000 0.001 0.000 0.004 0.002 0.001Mg 0.599 0.463 0.521 0.042 0.033 0.078 0.070 0.080

Orthopyroxenes, in general, show high Xr, values (up and An50); no chemical zoning is present. Orthoclase con-to 0.93), except those in sample 29, which have a lower tains significant amounts of NarO.value (0.68). The high-magnesian nature of the orthopy- Spinels are essentially unzoned and dominated by theroxenes categorizes them as enstatite. However, the AlrO, spinel-hercynite solid solution, with little CrrOr. Com-content is quite high for a typical enstatite. Clinopyrox- pared to the composition of spinel in charnockite, theene is augite in composition, which is a typical mineral spinel in sapphirine granulites has more MgO and AlrO,in mafic charnockites. On the basis of the method of and less FeO. Calculation of Fe3+ following the procedureHamm and Vieten (1971), the pyroxenes show very little of Bohlen and Essene (1977) shows significant amountsFe3+. Olivine contains significant amounts of FeO as re- of Fe3+. Magnetite contains large amounts of TiO, andflected in its intermediate X*value (0.55). can be termed titanomagnetite. The ilmenite lamellae

Based on Xr, ratios, the mica analyses (Table 5) show contain relatively more MgO than the host magnetite.two populations: (l) mica from the sapphirine-bearing Monazite shows two compositional trends that can berocks invariably has high X** ratios (0.82 to 0.96) and is related to its paragenesis. The monazite grains in the as-termed phlogopite and (2) mica from sample 29 has a semblage sapphirine + enstatite + phlogopite are uranif-lower Xr, value (0.56) and is termed biotite. The biotite erous and contain greater amounts of SiOr. On the otheris exceptionally rich in TiO, and is the most titaniferousone known to the authors. The phlogopites contain rela-tively high SiO, and moderate TiO,. But phlogopite from 1oosample 30 shows compositional characteristics typical ofthat mineral from many sapphirine-bearing rocks (seeKamineni and Rao, 1988, and references therein).

Plagioclase feldspar, present in two samples (29 and30), has anorthite contents in the intermediate range (Anoo

TeeLe 8. Microprobe analyses of monazite

Samole s27 S28 S30

La,O. '1211 12.75 13.48Ce,Os 24.47 25.11 26.22Pr"O" 3.57 3.83 3.75Nd,o3 1 1 .06 11.21 10.18Sm,O" 2.08 2.18 1.86Gd,o3 3.62 3.60 3.32Dy.O" 0.86 0.80 0.64Er,O. 0.19 0.13 0.08Y,O. 3.93 3.64 3.24Tho, 9.03 7.71 6.31UO, 0.41 0.55 0.00CaO 0.03 0.05 1.08

a ' \ ;7si...1s2e

$o \ ._:- \ r : ! :

s2N a

\\

LACE Nd SMEUGd Tb HO TMYb LU

38 'l.It '2.L1 '1:33 Fig. 6. chondrite-normalized patterns of REEs for samples

rotal s'.s6 se.17 e8.31 z)'-ls,.a.ndlot l:E-*" ilTdtl:d::':9.'l:chondrite values

||l

EGozoIoYooG l

of Haskin et al. (1968) and Masuda et al. (1973).


U!Gozo-()YooE 1oo

5 0 A40

I SmEu Gd Tb Ho Tr

Fig. 7. Chondrite-normalized pattems of REEs for samples26,27, and 28. REE data normalized as in Fig. 6.

hand, monazite grains in khondalite are devoid of U andcontain greater amounts of CaO. These characteristicsmay suggest the substitution Sia+ + IJ4+ : 4Ca2+.

P-? osrrvr.lrn

Pressure-temperature conditions of metamorphism areestimated, wherever possible, using calibrated geother-mometers and geobarometers. Sample 30 provides tem-perature estimates from the two-feldspar and garnet-bio-tite geothermometers, whereas sample 29 yields atemperature estimate from the two-pyroxene geother-mometer. Pressure is also estimated, in sample 30, on thebasis of the plagioclase-garnet-sillimanite-quartz geoba-rometer. The results are listed in Table 9.

Sample 30 records extremely high temperatures (> 1000"C) from two-feldspar thermometry, regardless of wheth-er binary or ternary solutions are adopted in the estimate.

TeeLe 9. Temperature-pressure estimates

Temper- Pres-alure sure

Sample fC) (kbar) Reference

Two-pyroxene geothermometerWood and Banno (1973)Wel ls (1977)Kretz (1 982)

Twojeldspar geothermometerWhitney and Stormer (1977)Mora and Valley (1985)

Garnet-biotite geothermometer585 Thompson (1976)592 Ferry and Spear (1978)595 Perchuk et al. (1981)

Garnet-sillimanite-plagioclase-quartz geobarometer

7 .3 Newton and Haselton (1 981)

50Mol % Al2Or

Fig. 8. Composition of sapphirine from the Kakanuru area,projected in the system (MgO + FeO + MnO)-Al,O3-SiOr.

On the other hand, the garnet-biotite geothermometerfrom the same sample gives moderate temperatures (=600'C). This discrepancy can possibly be attributed to a laterrehydration event, at lower temperature, that stabilizedphlogopite.

The estimated temperature, based on feldspar ther-mometry, probably represents the maximum for sap-phirine granulites in the Kakanuru area. A minimumtemperature, however, can be inferred from coexistingsapphirine + quartz as they are stable together only at,or in excess of, 6 kbar and 850'C (Grew, 1982). Similarconditions have also been deduced for the sapphirine +quartz occurrence, near Yizianagaram town, located inthe eastern part of the Eastern Ghats (Kamineni and Rao,1988). Much higher temperature estimates are reportedfor sapphirine + quartz from Antarctica: 900'C (Grew,1980), 950 "C (Harley, 1985), 980'C (Ell is et al., 1980),and > 1000'C (Sandiford and Powell, 1986).

The pyroxene thermometry (from sample 29) yieldstemperatures between 836 and 940"C, suggesting that theminimum temperature assigned for sample 30 is not un-realistic.

The pressure estimate, i.e., 1.3 kbar, for sample 30 isconformable with the temperature estimates. The P-Tconditions, in general, are analogous to those of sapphi-rine granulites reported from Enderby Land (Grew, 1980).Development of mesoperthitic textures in K-feldspar isalso compatible with the extremely high grade nature ofmetamorphism.

PlucnNpsEs oF SAPPHIRINE

Sapphirine-bearing rocks of various parageneses werereported from the Precambrian terrane of India, thesouthern region in particular. Generally, sapphirine oc-curs in metasedimentary rocks containing the assemblagegarnet + sillimanite * orthopyroxene * cordierite andin pyroxene granulites in the charnockite region ofsouthIndia (Grew, 1982).

Walker and Collins (1907) described sapphirine as areaction product between spinel-bearing ultramafic mag-ma and sillimanite-bearing aluminous rock in the Ma-dugula area of the Eastern Ghats. Rao (1971) also sup-ported a similar origin for a sapphirine occurrence in

29 847836940

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KAMINENI AND RAO: SAPPHIRINE GRANULITES 699

bronzite pyroxenites from the Anakapalli area. In addi-tion, sapphirine is also reported from reaction zones be-tween chromite-rich ultramafic layers and anorthosite inthe Sittampundi complex (Janardhan and Leake, 1974)and in ultramafrc enclaves in amphibolites of southernKarnataka (Janardhan and Shadakshara Swamy, 1982).

In the Kakanuru area, the three types of sapphirinedistinguished by us on the basis of associated mineralsand chemistry have distinct parageneses. Type I sapphi-rine (sample 25), the most Fe-rich variety, is associatedwith sillimanite and iron oxides. The bulk compositionofthe host rock can be approximated as SiOr-AlrOr-FerOr-FeO-MgO, with TiO, being an additional component.Presumably, the silica-undersaturated character com-bined with high AlrO3 and MgO provided chemical con-ditions conducive for sapphirine development. The highFerOr/FeO ratio (1.42) indicates that the sapphirine-forming reactions were oxidative.

Type 2 sapphirine (in sample 30) shows textures thatare useful to deduce reactions responsible for its forma-tion. Sapphirine rims around spinel indicate that the lat-ter was a deflnite reactant in the formation of the former.Since quartz is present, but always shielded from spinelby sapphirine, the following reaction can be suggested:

2(MgFe)Al,O4 + SiO,: (MgFe),AloSiO,o. (l)Spinel Quartz Sapphirine

In addition, the rims of sapphirine around magnetiteindicate that iron oxides were also involved, and the re-action was of an oxidative type as evidenced by signifi-cant Fe3+ in sapphirine. Rims of garnet around sapphi-rine suggest that it subsequently reacted to produce thatphase.

Type 3 sapphirine, occurring in samples 26, 27, and28, is analogous to "enstatite type" paragenesis fromGreenland, described by Herd et al. (1969). The "ensta-tite type" contains the assemblage sapphirine + enstatite* spinel + phlogopite t pargasite, which is almost iden-tical to sample 26 of the present study. Herd et al. (1969)identified a Mg- and Al-rich spinel-bearing ultramafic rockas a protolith for a sapphirine-bearing assemblage. Ac-cording to these authors, the ultramafic rock had beenmetasomatized and replaced by sapphirine and otherminerals. This appears to be true in the case of type 3sapphirine as well. Originally, samples 26, 27, and 28were spinel-bearing pyroxenites, later modified pro-foundly by metasomatism. The most profound effects ofmetasomatism are the growths of phlogopite and mona-zite due to addition of K, LREEs, actinides (U and Th),and HrO. Sapphirine growth, however, appears to havepreceded the onset of K metasomatism, as it is commonlyenclosed in biotite prisms. Rims of sapphirine aroundspinel and concentrations of rutile needles in sapphirinesupport the interpretation that sapphirine grew at the ex-pense of spinel; the rutile needles can be interpreted tohave formed from liberation of TiOr, present in spinel,

which was not accommodated in the sapphirine. Evi-dently, SiO, must have been added to promote sapphi-rine growth, as indicated in Reaction l. The source ofsilica could be either external, introduced by metasoma-tism. or internal. released because of formation of otherminerals such as phlogopite. But the textural evidencemitigates against the latter possibility. The preferentialreplacment of spinel by sapphirine at pyroxenite-khon-dalite contacts provides evidence for addition of silicafrom the khondalite. Considering the steep chemical gra-dients between undersaturated pyroxenite and saturatedkhondalite, this process appears highly likely to have oc-curred. Subsequently, the spinel + sapphirine pyroxe-nites were affected by metasomatism resulting in growthof phlogopite and monazite. The source of elements in-volved in metasomatism can be linked to younger gra-nitic intrusions in the Eastern Ghats. Plutonic activity,during late Proterozoic time, comprising granitic bodiesas a major component, is well recognized in the EasternGhats covering parts ofnortheastern Andhra Pradesh andsouthern Orissa provinces (Halden et al., 1982; Perrajuet al., 1979). The present study area falls within theseregions.

Coxcr,uorxc REMARKS

The granulite metamorphic belt of the Eastern Ghatsaround Kakanuru contains sapphirine occurrences thathave three types of parageneses as well as compositionalcharacteristics. Type 1 and type 2 sapphirines formedisochemically in rocks of suitable bulk composition underextremely high temperatures ranging from 850 to 1000.C. Type 3 sapphirine also formed at the same time byreplacing spinel in pyroxenite lenses that are concentratedalong charnockite-khondalite contacts. This process re-quires addition ofsilica, which is interpreted to have beensupplied by khondalite occurring in the immediate vicin-itv.

Some of the sapphirine granulites were affected subse-quently by metasomatism that produced phlogopite andmonazite. The metasomatic effects are well pronouncedin type 3 sapphirine-bearing rocks, possibly because oftheir location. The charnockite-khondalite contact alongwhich type 3 sapphirine granulites are concentrated mayhave acted as the most favorable pathway for migrationof matter during the metasomatic event. The source ofthe constituents involved in metasomatism, namely K,LREEs, and actinides (U and Th), is linked to the graniticplutonic activity that was prevalent in the Eastern Ghatsduring late Proterozoic time.

AcxNowr-nocMENTS

We sincerely thank P. F. Augustine, Geological Survey of India, forhelpful discussions and Maurizio Bonardi, Geological Survey ofCanada,for providing microprobe facilities. Robert Kerrich, University of Sas-katchewan, kindly carried out the oxygen-isotope analysis. Critical com-ments by two referees helped to improve the quality of the paper. We alsothank Dr. J. Alex Speer, North Carolina University, for encouragementto revise the paper. Pat Lucas patiently typed the manuscript.


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M.rr.r-ncrur'r RECETvED Ocronpn 2l, 1986M,c.M;scnrF.r AccEprED Mencu 4. 1988

sapphirine granulites from the kakanuru area, eastern

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