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Mineral Compositions of the Charnockitic Rocks from Kondapalli and Correlations with the Whole Rock Compositions

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SHORT COMMUNlCATlON 659

MINERAL COMPOSITIONS OF TIIE CHARNOCKITIC ROCKS FROM KONDAPALLI

AND

CORRELATIONS WITH TIIE WIIOLE ROCK COMPOSITIONS

C

LEELANANDAM

Department of Geology Osrnanla Univers~ty Hyderabad

-

500 007 Emall: clcelandndarn @ redlfl r n d corn

This communication is aimed at evaluating the colnpositional relationships between the minerals and host charnockitic rocks from Kondapalli and to demonstrate that the composition of any particular ferromagnesian mineral in the granulites is controlled not by the whole rack chemistry alone but by the amount and actual composition of all other coexisting ferromagnesian minerals and also of the ore minerals,

The results obtalned fi om the chemical dlssectlon of the charnockitlc rocks from Kondapalll, by the classical methods in a clln~cal mode, are summarlzed In Table 1 The compositlons of orthopyroxene (Opx), cl~nopyroxene (Cpx),

hornblende (Hbl) and blot~te (Bt), and also of the~r host rocks, explessed i n terms of F ~ ~ + / M ~ ratlos and Mg#

[ 1 0 0 ~ g l ( ~ g + ~ e ~ + ) ] , a] e provlded SO,, FeO and MgO contents of the whole rocks (rn wt%), and compos~tions of the const~tuent piagloclase (Pl) feldspar [100An/(An+Ab)J are also included for a greater appreclat~on of the chemical make up of the studted samples (Leelanandam, 1965)

The spdttally associated ultrarnafic rocks (prev~ously referled to as "ultrabaslc charnochltes") llsted In Table 1

belong to vdned gl oups of d~sparate paragenes~s They are merely ~ncluded here to represent wider chem~cal coverage of the speclmens with alleged (now dlscredlted) charnoch tlc lineage, and also to hrghllght their chemlcal distinctiveness They ale genet~calty unrelated to the rnafic and "felslc" granul t tes (previously I eferred to as basic and, "~ntermedlate and m d " 'charnockltes respect1 vely) and hence are not further discussed i n this cornrnunlcatron

The rndfic granul~tes (exceptrng the sample A18) behave

as d n unusudlly coherent group with highly restricted

composltlonal range (ME# =55 60) In 5 out of the 8 mafrc granulltes, ~ e ~ + / h 4 ~ of the Opx is greater than that of the host rock, while the reverse I S true i n all the 5 f e l s ~ c granulltes The Mg# of Opx (given within the brackets) In the rnafrc g~anullte sdrnples 28 (58 80), G17 (56 86), 220 (56 15), 474 (55 36) and 382

(5

1 51) are sllghtly higher than ~ n ' contents ( [ ~ g l ( ~ g + ~ e ~ + + ~ e ' + + Mn)] reported in Leelanandam (2002), the normative En values are

enchantingly closer to Mg# than to ~ n ' (see rdble 1 In Leelanandam, 2002) However, in the samples 1'45 (53 90) and A18 (49 91), whereln En* values ale slightly h ~ g h e ~ thdn normat~ve En vaIues, the departure of the latter from Mg# (when compared to ~ n ' ) 1s mdrglnally enhanced

In the mafic granulites, the Fe2+/Mg of both Cyx and Bt

IS always signiftcantly less (and of Hbl not so slgnlftcantly

less) than that of the host rock The sequence of minerals wlth decreas~ng order of Fe2+/Mg ratto (or lncleaslng order

of

Mg#) In these granul~tes IS Opx-Hbl-Bt Cpx

The apparently elevated ac~dtty values (as measured by S10, contents) can be rntsleadlng in some cases The mafic granullte (lens) sample 382, wlth 62% SiO,, contains a rather high amount of "~nttoduced" quartz 1 he othe~ two mafic granullte lenses (samples 28 and 220) also contaln mlnor "reledsed" quartz, while the sample A1 8 (WI th 55 30% S10,)

contains considerable amount of both ~ntroduced and released quartz The sample A18 [w~th the highest F ~ ~ + / M ~ (1 01) and lowest Mg# (50) I n this group] is exceptional on

many counts (Leelanandam, 1967d), and such "speclmens are equally as interesting as those that are well behaved" (see Kretz, 1990, p 504)

The two rnaftc granulites A 18 ( F ~ " / M ~ = 1 01) and A7 ( F ~ ~ + / M ~ = 0 70) occur in the vlcinlty of pink gran~tic mater~al In the field, and ale 11ch In Hbl and Bt 1 he ~ e ~ + / Mg rat~os of the orthopyroxenes A18 and A7 are the highest (1 00) and lowest (0 56, from optics) In this group The

speclmen A18 was collected close to a local sheal plane, the mlnerals exhibit aberrant chem~cdl behaviour, and the plag~oclase 1s strongly zoned (AnS, ,,) These two samples depart wldely from other mafic granulites, wh~ch do not show sufficient spread in Fig 1

Amongst the mafic granulltes, a poor posltlve correlatlon

I S perceptible between the rock Opx composltlons (see

Fig 2) If the data polnts for the two &lbe~rant samples A 18 and A7 are not considered, then the correlatlon becomes weak and ambiguous F ~ g u r e 2 further shows that the patterns depicting the F ~ ~ + / M ~ and Fe# dlsh lbutlons between the rock and Opx are ident~cal Desp~te the pauclty of data, a vague negatlve correlation between the lock-CpxIWbl compositlons cannot be ruled out (see Fig I)

(2)

Table 1. Composlbons of the rmnerals and thetr host rocks from Kondapalll (Analyst C Leelanandam) The five-dlg~t numbers apply to the Harker Collection In the Department of Earth Saences, Cambndge (England) For field xelat~onships of the rock unlts, see Leelanarrdarn (1969a) Data for rocks and plag~oclase from Leelanandam(l965), rest from Leelanandam (1 967a, 1970) "Rock S1O3 and MgO from the analyses done by Dr K Govrndaraju (CRPC, Nancy, France), 'see Grasty and'leelanandam (1965), ! lOOAn/(An+Ab), " see Leelanandam (1967b) see Leelanandam (1 968), 'Detemuned from optzcs (see Leianandarn, 1970) D = dyke L = lens. HM = lull mass. End = (unusual) enderb~te (see Leelanandarn, 1969b) Sample No 48 [96243] 323 [96245] 472 [96249] G4 [96248] J22 [96247] Dl4 [96242] 431 [96250] A7 [962S3IW 28 [96256] Rock S10, FeO MgO (wt%) (wt%) (wt%)

(L) (D) (L) (L)

G17 [96258] 220 1962521 474 E962.571 P45 [96259] 382 [96273] A18 [96251] 62 [96275] 359 [96287] ti MI2 [96281] 84 [96282Iw Sl [96280] 322 [96293] Mg # = 100Mg/(Mg+Fe2+) Rock Opx Cpx Hbl Bt Fe2+/Mg Rock Opx Cpx Hbl Bt

P1 An%'

Ultramafic rocks 5328 964 2703 4075 1185 1785 5077 753 1368 4510 962 1300 57-6 1 81 " 5 5 56-73 59 5 7 7 1 5 9 57, 56 6 1 64 60 5 5 70 55 54 64 56 5 2 68 50 50 70 5 6 59 069 076 041 0 68 074 078 - 0 65 0 56 0 67 0 81 0 43 082 086 0 55 079 094 0 48 101 1 00 0 43 0 79 0 69

(End) (End)

(HM)

1

49 14 9 65 789 89 "

77 92

"

5

5 85

- 0 08 020 0 22 0 12 - 0 20 - (033)' - 0 29 0 24 037 034 - 0 23 031 0 54 0 27 0 47 042 058 - 0 31 0 16 Mafic granulites

(L) (HM) (D)E (L)

92

83

8

2

89 8 3

- (75) ' 78 8 1 7 3 74 8 1 76 65 79 6 8 7 1 64 77 86 4850 977 738 50 03 9 99 8 35 4709 1038 712 6200 647 458 5530 I100 612 Felsrc granulites 60 84-88 " 59 (64) ' 66 69 5 7 5 9 72 GI 4890 1055 848 5189 914 692 4 5 3 8

44 40 54

(3)

30 . 4 0 5 0 6 0

M ~ #

in

rock

(4)

66 2 SHORT COMM1)NICATlON

F ~ ~ + M ~

in rock

~ e #

in

mrk

Fig.2. Orthopyroxcnc-rock (mafic granulltc) compositional rclatronsh~ps. Fstf = 100 E;c2+ 1 ( F c l + + M g ) = 100 - Mgti I:OI d u t ~ \or

Table I .

The plagioclase in the hill mass type of mafic granulite

G I 7 (An,,-,,) and that in the Bt-rich (atypical) maf-ic

granulite lens 382 (An,,) are somewhat comparable. The

plagioclase in the lens 28 (An,, ,,) and Bt-rich dyke P45

(An,,) is unusually a l c i c and anomalously antiperthitic. Though Ca* of the PI correlates reasonably well with ~ a ' o f

the host rock (see Table 2 and Fig. 3), drastic difference is

expcctedly encountered in the Cpx- and I-Ibl-rich mafic granulites G 17 and A7 respectively.

rarity of Cpx. 'L'hough lirnlted In number, they exhlbit

enormous compositional var1;ition ( f : e " l ~ ~ = I 02-2.72;

Mg# = 50-27), whlch is curiously not reflected In the composition of the const~tumr O p n ( l ; e " / ~ ~ = 0 95 1.72;

Mg# = 5 1-43).

Increase in thc I : ~ " / M ~ of Opn I S crudcly C O I I . C I ~ I I : I ~ I C . with the exception o f M 12, is ~ t h :in 111crci1sc In the S I O , and ~ e " / ~ g of the host rock. It i s quccr that orthopyroxcncs

M 12 and B4 with rather similar i ; e ' ' / ~ ~ ratios (1.05 and

1.17 respectively) occur In rocks ifttrh h ~ g h l y d~ssirnllar

Felsic Granulites F ~ ~ + / M ~ ratios (2.58 and 1.18 respccttvcly). ?'he M g

T h e f e l s i c g r a n u l i t e s (60.99-7 1 . 4 8 % S i O , ) a r e numbers magnify this anomaly. 'fhe rock M11- (08.52'4

characterized by the ubiqu~tous absence of Hbl and extreme SiO,) with lower ME# ( 2 8 ) contalnr Opx with h ~ g h c r

Mg#(49), while the rock R4 (65 (>O(k S I O , ) w ~ t h highcr

ME# (36) contains Opx w ~ t h lo\s!cr ME# (36). Iiqually

Table 2. Plagioclase and host rock compositions l'ro~n Kond:!paEli (Analyst C. Lcclanandam)

Sarnplc Rock Ca" I OOCaI(Ca+Na)

No. CaO Na,O Rock 1'1

( f (w1P/o

Felsic granulites

Thc numbers givcn in the decimal places for the Ca%values have no significance.

ironically, the biotite in the samples S 1 (G7.39(31 SiO,) and

359 (63.80% Sic>,) has the same F e ? ' l ~ ~ ratio (0.7 I ) ,

despite stupendous variation in thc host rock I:;.'/M~ r ~ ~ t i o s

(1.09 and 0.82 respectively). K ~ r ~ i t b l e rirnounts ol' garnet

and lye-'l'i oxides complicate the problcm of correlation

between the mineral and r c ~ k compositiona. Iiurthcl-niore,

the acidic rocks (unusual cnderbitcs) I34 (05.9OOi SiO,)

and 322 (7 1.48%2 SiO,) contain pl:~gioclase with 44% and

54% An respectively, which is singularly exceptional. All the Cri in felsic granulitcs (jvhich arc practically devoid 01' Ca-bearing fernic minerals like Cpx and 1 Ihl) is ef'fcctivcly

locked up in the constituent PI; the Ca contrib~ition by

garnet and apatite is negligible. LJnderstandably thc Cat#

o f plrigioclase is olten slightly less than th~it o f thc host

rock (Table 2). and thc anticipated nice col-I-elation bcrivccn

them is quite remarkable (1:i.g. 3).

The two groups of Kondapalli granulites have d ~ s t i n c t ~ v c mineral-chemical charactcr~srics. Aa there is n o strict

(5)

SHORT COMMUNICATION

A

Mafic granulites

7 - r - -

30

40

50

60

70

80

90

~ a #

in rock

Fig.3. Plagioclase-rock compositional relationships. For data sac Table 2.

F ~ ~ ' / M ~ or Mg# of the rock (Table I), mafic and felsic

granulites are not considered as one entity. Systematic

correlations between the F ~ ~ + / M ~ ratios of the host rocks and constituent minerals are absent; contrary to general expectations, the correlations are no better even when

Mg numbers are considered (see Fig. 1). Though ill-

defined positive correlations between the Opx and host rock compositions are noticeable, consistently regular (and straight line) relationships are not observed in either group. Rocks with minor or no garnet have indistinguishable chemistries. Garnet (which has grown at the expense of other minerals), when present in considerable amount, can cause drastic changes in the composition of other coexisting femics (and also plagioclase). Hence, petrogenetic appraisals based on the

mineral compositional data alone are particularly vulnerable

to misinterpretations.

The mineral cornpc)sitions from the granulites of

relationships between granulite facics host rock and constituent minerals was primarily utilized as a criterion for delinking the genetic relationship between the rnafic and

felsic granulites of Madras (Subramaninrn. 1959); by the

same token, various members of the so-called differentiation series of the Madras "charnockite suite" (felsic granulites) also are to be viewed as genetically unrelated (Leelanandam,

1965). From the available data, the Kondapalli mafic granulites are construed as chemically consanguineous, with no apparent linkage with the felsic granulites; it is unclear whether or nut there is consanguinity among the members of the felsic group. The present study dernonstrr~bly reaffirms the notion that the composition of any particular ferromagnesian mineral in the granulites is controlled not

by the whole rock chemistry alone, but by thc amount and actual compvsition of a11 other coexisting ferromagnesian minerals and also of the ore minerals.

Konda~alli. as those of the tvae area near Pa1Iavaram (Howie d a

and Subramaniam, 1957). arc not in tune with the accepted A c k n o ~ . t ~ l r d ~ r r r l ~ ~ z t s : 'The writer owes a dcht o f gratitude to Prof. W.A. Deer FItS. f'rof. K.A. I lowic and Prof. R . Krctz whose facts of mineral variation i n a normal igneous differentiation

advice and suggestions. given in the past at ditr'ercnt times, are

series. As the rocks under study are reconstituted during now hclplirl

preparing ,his A n incisive review of this

metamorphic recrystallization under granulite facies commun,cation, during

-

revision, by Sri TM, Malladevan

conditions, simple rock-mineral chemical relationships are is gratefully acknowledged. Financia] assistance is from

not expected. the Department or Science and 'Technology (under- USERS scheme)

The absence of stringent straightforward chemical during the preparation of this communication.

(6)

SHORT C O M M U N I C A I I O N

G R A ~ T Y , R L and L ~ ~ ~ , A N A N D A M , C (1965) l s o t ~ p l c ages of the

basic chatnockite and khondal~tc born Kondapall~, Andhra

P~adesh, India M~neralog Mag , v 35, pp 529-535

HOW~E, R A and SURRAMANIAM, A P (1957) The palagenes15 of

garnct In chainocktte, e n d e l b ~ t e , and related granulrtes

Minerrtlog Mag , v 3 1, pp 565-586

KRETZ, R (1 990) U~otlte and garnct co~npositlonal var~atron and

mineral cqulllbr rd in G~cnvllle gnclcscs ot the Otter Lake area,

Quebec Jour Metarnorph~c Geol , v 8, pp 493-506

LECLANANDAW, C (1965) Thc m~ncralogy and petrology ol the

K o n d d p a l l ~ charnockites U n p u b l ~ s l i e d 1% D ' T l ~ c s ~ s ,

Unlvers~ty of Cambridge

LFI I-ANANDAM, C (1967a) Chem~cal study of pyroxenes fiom the

charnoch~t~c rock5 ot K o n d ~ p a l l ~ (Andlil a 1'1 a d c ~ h ) , Ind~a, wlth

e m p h a s ~ s on thc d ~ ~ t ~ ~ b u t i o n of elcments ~n cocxi$ting

pyroxenes Mincl alog Mag , v 36. pp 153-179

LEELAVANDAM, C ( 1 967b) O c c u ~ ~ c n c c ot anorthrtc dnd anllpertlilLlc

bytownltc fiom the CIUI nock~tlc locks of Kondapalli Curl

SCI , v 36 pp 293-294

LECLANAYDAM, C (1968) Zoncd p l a g ~ o c l ~ s c Ilorn the ~ h a ~ n o c h ~ ~ c s

of Kondapall~, KI ishna dirt1 lct Andhra P t ~ d c ~ h , I n d ~ a

M~nerdlog Mag

.

v 36, pp 805-813

LEELAYANDAM, C (1969d) Note5 on [he llcld reldtonshlp$ of thc

lock unrts In thc Konddpalll nl ca I3~1ll Gcol Soc Ind~d, v 6,

pp 109-112

LFCLANANDAM, C (1969b) Basic pldg10~13';cs 111 dcld rock5 of

Kundapall~ Bull Geol Soc 111d1d, v (I pp 41 -42

L ~ C L A ~ A N D A M C (I 970) Chcn~ical rnlnc~ dlogy ol hol nblcndcs

and b~otltcs lrolr~ thc charnock~t~c lock5 of Konddpa111 111dld

Joi~r 13etrology v l l pp 475 505

L E ~ L A N A U D A M , C (2002) k n ol ottIiopy~oxcne\ and A n ol

plagloclascs I ~ o i n Kondapall~ mmpm ~ s ~ o n s hclwccn the

cliem~c,ll and notmatltc v ~ l u e s Joul Gcol Soc l n d ~ d v 60,

pp 701-702

SUURAMANIAM, A 1) (1959) Cha!nock~tcs ol lhc lypc atcd ncal

Madla$ - d rc~ntclprctatlon Am J o u ~ SLI , v 257, pp 321-353

(Kecerved 7 Apt 11 200 7 Uevlsrd form ucccpted 12 J a r ~ i ~ r v 2004)

Figure

Fig.2. Orthopyroxcnc-rock (mafic granulltc) compositional rclatronsh~ps. Fstf = 100 E;c2+ 1 (Fcl++Mg) = 100 - Mgti I:OI d u t ~  \or Table I
Fig.3. Plagioclase-rock compositional relationships. For data sac Table 2.

References

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