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Geochemistry of Dharwar ultramafics and the Archaean mantle
Geochemistry of Dharwar ultramafics and the Archaean mantle V. D I V ~
RAO, K. SATYANARAYANA, S. M. NAQVI & S. M. HUSSAIN
LITHOS
O
h o , V~ Sttmmmyam. L N~vi, & M. & Humim .~ M. l~S: of INmwzr dtmnafim and the Argheanmantle, Lithoz 8, 77-91. G e ~ and p e ~ studies on serpmtinised u l u ~ of r , ~ Mysore State, India, indicate that the Archaean nnntle beneath the In~aa Shield wa~ of ]m~Jotitic in nature. This p r o ~ e in the Archaean period was uruJffmm,tiated ~ ¢oaceatrathms of dderolddle and tramitional dements. The address of all authors is: National Geophys~d Research lmdtute, Hydrabad - 500007, ind~
Geochemical data, when interpreted in terms of modern concepts on the relationship between magmas, mantle and crust, provide a basis for investigation of early Precambr/an geotectonic pattern and the origin of shields (gingwood & Green 1966, Green & ginswood 1967). There are four approaches for estimating the composition a n d mineral fly of materhl within the earth, ~ y the mantle, (I) Use of data from extraterrestrial sources and formulation of physical and geochemical models for the origin of the Solar system including the earth; (2) study of ultramafic and basaltic rocks supposed to have been derived from the mantle; (3) compar/son of various physical properties such as density, elastic properties, etc. obtained by geophysical methods; and (4) working out the mineral assemblages and phase transitions in the interior from the above compositional and physical properties. Due to the strong fractionation of crustal rocks, emphasis cannot be laid on extraterrestrial evidence for bulk composition, but examination of igneous rocks derived from the mantle does place limits on the composition of •
6 - - Lithos 2/75
•
•
0
the upper mantle. In view of their importance in this regard, much work has been done on the geochemistry of ultramafic rocks such as peridotites and dunites, which are supposed to be the nearest to mantle composition (Hess 1964, ginBwood 1966). It is now clear that the major rock type of the upper w~ntle is peridofite with olivine and lesser orthopyroxene as its major mineral phases (Green 1972). Closer characterisation of the nature of the upper mantle is possible from the examination of the products of natut~ processes in the upper mantle. The composition of the prim/five CrtL~ and its formation along the continents have long constituted a fundamental problem in geology. The nature of the primitive undifferentiated mantle, its composition, mode, time and rate of its differentiation; all these queries are unanswered and investigations are in progress to
relatively high degrees of partial melting of l~ridotitic upper mantle at shallow depths
78
DivakaJ'a R a o er al. i f-
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Ultrabasic Rocks Chromite Bearing Dharwar Schists
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Geological map of Mysore, Peninsular India. Square in the diagram indicates area under study.
(Kushiro et al. 1968, Green ~1971). Concurrently with the geophysical revolution of the last decade, a resurgence of interest in Precambrian terrains has brought to light import.ant differences between the continental crust of present day and Archaean times (Goodwin 1968, Engel 1963, Anhaeusser et al. 1969, Glikson 1972, Naqvi & Hussain 1973a). From the viewpoint that similar studies in shield areas on Archaean ultramafics and younger mafic rocks may reveal the nature of Archaean mantle and its development through geological time, investigations have been carried out on the Archaean ultramafics of Peninsular India by the authors.
General geological setting and description of the rock types The zone of serpentinised ultramafic rocks associated with gneisses, granites, metasediments and metavolcanics of the Dharwar system (Precambrian), Peninsular India forms an 'en echelon' belt nearly 320 km long and extends from Sindhuvalli in the south to Shivani in the north (Fig. 1). The ultramafics includ(: peridotites, dunites (mostly or partially serpentinised), pyroxenites and amphibolites (Fig. 2). Peridotites are not identified in the field, but are classified by microscopic study, and hence not represented in Fig. 2. These are
Geochemistry of Dharwar ".,ltrameyics 79 |
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Detailed 8eological map of the area under ~tudy, showing the distribution of different ultramaflcs.
Alpine type ultramafics (Varadarajan 1970, Karunakaran 1970), which occupy the axial zones of the Dharwar anticlinorium (Pichamuthu 1962). Individual lenses vary in size from a few metres to kilometres. Pyroxenite occurs as small patches, whereas amphibolites occur as small patches in dunite as well as individual lenses. This belt runs parallel to the structural axis in the region, steeply dipping and elongated in the trend of regional foliation, of approximate north--south to N N E SSW direction. Partial serpentinisation of dunite h ~ given rise to antigorite. Magnesite veins cut across dunite and are mined at many places (Dodkanya: Fig. 2). At ~he contact with gneisses the serpentinite is altered to talcschist. Peridotites occur to a lesser extent, distributed along the outer parts of dunite bodies, Chromite occurs in sizeable deposits in the dunites. Secondary alteration is responsible for the extensive deposits of magnesite and
talc. Asbestos mineralisation is also noticed, though less c~,mmon. Field relationships between the dunites and the Peninsular gheiss and the following field observations suggest that the dunites are probably older than the gneisses: (a) Veins of Peninsular gneiss cutting across dunite. (b) Enclaves of dunite in Peni_~ular gneiss. (c) Schistosity in the associated pyroxenites and dunites parallel to regional schistosity direction. (d) Alignment of many lenticular ultramafic lenses along the schistosity (Fig. 2). (e) Gravity surveys over the ultramafic lenses have revealed that they are connected to a large mass below under the cover of thin gneissic rock (Qureshy et al. 1970). (f) Lack of any high temperature metsmorphi.~n of the gneisses due to intrusion of the ultramaflcs.
It is possible that these ultramafi~ may belong to the missing greenstone sequence (Naqvi et al. 1974) older than ~ t i c activity around 2400 m.y. (Crawford 1969). The Kolar schist
80
Divakara Rao .et al,
Table 1. Major and trace element analysis of ultramsfl¢ rocks.
Wt%
1
2
3
4
5
6
SiO, TiO, A!tOº Fe, O,
39.33 0.18 1.59 7.29
39.00 0.07 1.32 6.48 2.12 1.57 47.28 0.48 0.02 0.12 0.33 640 590 1500 48 -
38.27 0.07 1.59 5.55 2.40 2.17 48.29 0.48 0.06 0.23 0.43 365 1050 1050 55 23
38.29 tr. 0.42 7.40 4.06 1.23 45.13 0.60 0.09 0.02 0.93 375 1500 1200 64 60
38.80 tr. 0.92 6.74 4.10 2.!}8 44.'18 1.i5 0.10 0.02 1.?1 2~'0 1200 11,':0 :;8 ~3
FeO
CaO MgO Na~O KsO MnO PsOs Co (ppm) Ni Cr Cu V
-
1.94 44.45 0.76 0.06 0.01 tr. 375 800 1000 64 18
7
8
9
4'.),44 I).19 1.28 1.25 3.00 2.26 49.24 0.94 0.02 0.01 0.27 440 600 900 59 -
42.34 0.14 0.95 1.96 3.94 1.08 48.39 0.62 0.02 0.03 0.30 550 2200 580 37 43
40.65 0.40 1.36 2.19 3.92 2.15 46.70 0.58 0.06 0.02 0.29 425 2600 2000 48 29
38.93 0.35 0.95 4.41 3.72 1.16 48.25 0.17 0.02 0.02 0.21 330 1400 1000 54 26
"
Table 1 (cont.).
Wt%
19
20
21
22
23
24
25
26
27
SiOs TiOs AlsOs Fe20 s FeO CaO MgO Na~O KgO MnO PgOs Co (ppm) Ni Cr Cu V Ga Y Zr
.~1.93 0.17 4.12 3.84 4.80 1.29 31.12 0.48 0.01 0.05 0.15 235 860 720 91 255
51.61 0.29 4.18 0.92 5.72 4.64 30.38 0.68 0.02 0.02 0.36 220 250 625 70 -
41.99 0.67 12.26 2.79 8.94 2.07 28.65 0.40 0.25 0.02 0.49 560 43 715 70 -
56.56 0.67 7.09 1.88 5.68 9.77 17.02 0.48 0.08 0.03 0.13 220 520 800 64 -
41.17 0.97 13.68 9.04 1.00 7.82 22.56 2.28 0.37 0.28 0.21 N.D. N.D. N.D. N.D. N.D.
47.91 0.35 8.53 3.65 7.30 2.72 27.29 1.50 0.10 0.02 0.71 50 450 435 35 220 8 20 45
50.99 1.07 9.42 2.38 9.94 8.17 12.18 2.70 0.25 0.02 0.61 44 410 430 74 230 15 14 60
47.68 0.32 10.30 1.92 1.56 3.60 29.51 2.44 1.73 0.19 0.22 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.
48.97 2.10 6.41 1.60 9.22 6.60 19.78 4.60 0.25 0.06 0.20 66 240 440 38 240 10 16 60
tr.=trace; ---not traceable; N.D.=not determined. 1 to 4=Peridotites; 5 to ll=Dunites; 12 to 23=Pyroxenites; 24 to 34=Amphibolites.
belt in the neighbouring area ~ a typical greenstone sequence and similar formations in this area are yet to be identified. These ultramafics thus m a y belong to the older greenstone sequence, which has been m e t a m o ~ h o s e d a n d gmnitised to form the vast surrounding gneissic terrain (Divakara R a o et al. 1974a). Pyro~enites alre fresh and mostly ~composed
of o r t h o p y r v x e n e (enstatite) 'with little clinopyroxene. H o r n b l e n d e and iron ore are the accessories. A p a t i t e and plagioclase occur sporadically. Amphibolites are m o s t l y hornblendites (mono-mineralic), occasionally containing a liittle plagioclase a n d magnetite. Peridotite is a harzburgite variety.
G e o c / ~ m b t ~ o f D h m ' u ~ u/wama/ic:r
10
11
12
13
14
15
38.17 1.23 1.76 6.57 2.44 2.29 43.75 1.08 0.44 0.02 0.15 N.D. N.D. N.D. N.D. N.D.
39.73 0.14 1.57 633 2.10 2.74 44.40 0.91 0.02 0.20 0.34 370 760 1300 70 130
46.26 0.03 4.98 5.23 4.10 2.93 34.11 1.20 0.10 0.01 0.98 320 710 480 48 255
50.16 0.55 6.49 6.88 4.50 10.80 16.92 1.40 0.10 0.04 0.31 170 250 1200 38 -
52.67 tr. 3.71 4.75 3.62 7.79 25.34 1.32 0.15 0.01 1.08 310 600 580 58 -
49.67 0.67 532 2.89 8.02 2.08 26.10 2.60 0.15 0.02 G.63 N.D. N J). N.D. N.D. N.D.
28
29
30
31
32
33
34
49.90 0.64 5.29 10.18 4.38 11.66 13.05 4.50 0.10 0.05 0.14 32 440 430 34 180 7 21 55
45.87 1.16 15.64 2.17 6.10 7.82 12.15 2.52 1.08 0.12 0.27 85 180 195 66 210 7 20 56
52.28 0.59 6.21 0.92 8.92 7.77 20.01 0.74 0.32 0.01 0.36 50 415 370 24 305 8 15 62
47.31 0.70 4.~.2 4.~.4 6.80 6.12 27.52 0.31 0.06 0.03 0.36 78 600 490 54 240 9 16 40
48.52 1.58 13.25 2.01 9.30 8.42 12.84 3.25 0.40 0.02 0.50 64 680 550 72 250 8 19 70
52.75 0.54 7.59 0.22 8~6 10.84 14.93 1.50 0.25 0.03 0.46 51 240 430 42 250 13 18 70
52.32 0~0 9.39 3.57 5.34 15.0~ 11.$9 1.30 0.17 0.04 0.08 62 330 480 34 185 9 14 52
Method of analysis and chemistry of the rocks Fresh ultramaf/cs, selected after microscopic examination of th~,n sections, have been ~_~_lysed for their major, minor and trace-elements. Major elements have been determined by the ra~d methods of Shapiro & Brannock (1962), alkalis by flame-photometry, and CaO and MgO by titration. Trace-elements have been determined by n~n$ Pd and In as internal standards
16 50.79 0.35 5.32 1.88 8.54 3.96 27.57 1.15 0.15 0.02 0.33 , N.D. N J). N.D. N.D. M.D.
81
17
18
44.80 0.03 2.89 4.16 4.48 12.17 28.64 0.68 0.02 033 0.15 250 1000 490 43 300
49.32 0.12 6.75 2.77 7.10 3.12 23.88 4.70 0.10 0.01 0.68 250 700 600 43 -
with a Hilser & Watts large quartz, Littrow spectro8raph. Base and standards ~ different concentrations for various elements were prepared in the iab6~t~,~,. and the procedure is reported elsmvhere (Nsqvi & Hussain 1972). lib and Cs have been determined in a few rumples by neutron-actlvation analysis (Gansadharam & lieddy 1969). Major aud ~ice-element concentrations in different samples are 8:iven in Table I. OHvine was separated from a few sawples with the help of Frantz's lsodynmni¢ .~.lmrat~r and analysed (Table 2).
82 Divakara Rao et at. Table 2. Chemical composition of olivine,. Wtc/~
~
2
3
4
5
6
7
8
9
10
11
SiO. TiO~ Al~O~ Fe:O~ FeO CaO MgO Na.O K ~O MnO i'~O~ .~Ag/Fe
38.68 tr. 0,76 3.50 3.80 0.19 51.85 0.16 tr. 0.01 0.15 5.80
39.43 0.15 0.52 4.32 2.32 0.24 52.04 0.18 tr. 0.06 tr. 6.51
40.44 tr. 0.52 6.88 0.36 0.18 51.20 0.19 t r.
40.80 0.30 0.20 3.84 2.16 0.2 ! 52.08 0.16 tr. O.01 O.13 ":.20
41.88 tr. 0.16 2.59 4.00 0.22 50.60 0.14 tr. 0.01 0.17 6.20
39A8 0,22 0.69 ::.41 ;.44 026 52.2~ ~, ; ~ tr. (.01 0.08 6.42
40,11 0.11 0A7 4A3 2.35 0.22 51.66 0.16 tr, 0.02 0.11 6.34
41.~;5 it. 0.0rl 0.36 8.22 0.04 49.76 0.03 0.04 0.08 . 4.52
41.70 0,01 0,23 5,98 0,.:9 $1,67 0.I7 0.¢ I 0.10 . . 6.7 !
40.75 0,18 0,68 10,05 0,00 48,01 N,D, N.D. 0.13 . 3.70
40A3 tr. 0,91 0.78 8,3 I 0.00 49,05 0.05 0.05 0.16
tr.
0.13 606
4.23
i to 6=analysis of authors; 7=Average of the authors: 8-composition of olivine from Mysore (Varadarajan 1970); 9=composition of olivine from u r , ~ r mantle (Carter 1970); 10=olivine compos!tic, n (Green ."964); l l=olivin¢ composition (G,lynethe & Challt'~ I%.Q.
]'he .<1 values [~] = 100 (FeO + Fe203)/FeO + + Fe20;;+ MgO] are plotted again.,.t different oxides and trace-elements in Figs. 3 & 4. Pyroxenites and amphibolites clearly indicate a differentiation trend which may suggest that ~he different marie rock types of this area are the products of differentiation from a single magma. This can also be correlated with the composition and proportions of different mir.era~ assemblages in the rock types. SiO 2, Al20 3, CaO, Na20. K20 and TiO 2 increase with increasing A, whereas MgO decreases, There is random variation from sample to sample in a given rock type for different oxides with iv.crease in A. FeO and FeTO3 show a|most the reverse trend with increase in A. Ferric iron is more abundant in dunites and peridotites and shows a decreasing trend in pyrosenites and amphibolites. Breakdown of olivine to magnetite due to serpentinisation ar.~ oxidation probably accounts for the high fe~'ric content in dunites z..nd peridotites, whereas the fresh py~oxenites and amphibolit,~s have much less ferric iron. Ferrous iron is lower in the former ar, d increases to 9% in pyroxenites and amphiboiites. MgO is fairly high (48%) in dunites and peridotites when compared to many reported values (Table 3). Random variation in different oxides frown s~mple to sample in a given rock type reflects the inhomogeneous nature of the pare~.t magma or in this case the mantle below. All these ultramafics are highly enriched in alkalis, especially K, Rb and Cs, this is attributed 1~
post- or (Divakara ever, the of alkalis out.
syntectonic crustal contamination Rao & Satyanarayana 1973), Howexistence of higher concentrations in parts of the mantle is not ruled
Trace-elements
Thoagh the concentration of major elemeats in the upper mantle appears to be relatively well established, larger uncertainties remain abo~,~t the concentration and distribution of trace-elements. Secondary alteration, which may appear insignificant in the petrographic sense., may cause serious contamination with respect to whole rock values for elements like kb, C~ and K (Wyllie 1971) Co, Ni, Cr and Cu decrease with increasing /I (Fig..4), V increaees with 3 in dunites and peridotites but decreases in amphibolites. Ni is low in these ultra~nafics when compared to ,:he reported value~ of 1700 ppm for dunites and 1100 ppm for pyroxenites (Edelshtein 1963); 4400 ppm in uitramafic inclusions (Wedepohl 1963); 2600 ppm in serpentinites (Hess & Of:alora 1964). It is within the range of 1000 to 4000 ppm given by Goles (1967), As the olivine lattice admits large amounts of Ni, there is a distinct tendency for this element to become enriched in the early products of differentiation, and a sympathetic variation of Ni with Mg can be expected, which is exhibited ~n the~e ultramafics (Fig. 5). Serpentinisation in these dunites should be taken into account when con-
Geochemistry of Dharwar ultrama[ics
.,.,
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of o-xid~ |n DL~rw~r ultmm~flc~ with diff~,~'~nt~tlon b~d~x LA),
83
84 Dlvakara Rao et al,
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Fig. 4.
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Diagram showing variation of trace elements In Dharwar ultramaflcs with differentiation index (A).
Table 3. Comparison of Dharwar mantle composition with otheE compositions. Wt%
1
2
3
4
5
6
7
8
9
SiOs Tie s AlsOs Fe,O, Fe¢) CaO
38.72 0.08 1.23 6.68 2.14 1.72
39.86 0.35 1.25 4,16 3,31 1.96
48.74 0.41 6.40 3.92 5.54 5.70
49.87 0.84 8.74 2,97 7,01 8.07
44.29 0.42 4.40 4.43 4.50 4.34
38.68 0.03 0.91 3.06 5.35 0,30
40.49 0,04 1.49 2.58 5.05 0.99
43.60 0.04 2.40 1.00 7.80 2.50
45.16 0,71 3.54 0.48
MgO Na,O KjO
46.29 0.60 0,06
46.51 0.78 0.09
26.02 1.45 0.12
18.26 2.30 0,43
34.52 1.28 0.17
44.84 0.05 0.03
41.3J. 0.08 0.03
41.50 0.32 0.01
37.47 0.57 0.13
MnO P,O, CraO .
0.09 0.42 0.28
0.05 0.38 0.47
0.07 0.46 -
0.03 0.35 -
0.07 0.40 0.18
0.12 . 0.38
0.10 . -
0.14
0.11 .
. 0.33
1. Average perldotite (authors); 2. Average dunite (authors); 3. Average pyroxenite (authors); 4. Average amphibolite (authors); 5. Averages of 1, 2, 3, and 4; 6. Average composition of dunite rocks for Alpine association (Velensky et al. 1970); 7. Average composition of peridotito for Alpine association (ibid.); 8, Type 'C' pefldotlte (Hess 1964); 9. Pyrolite (IUngwood 1966); 10. Pyrolite (Green & Ringwood 1963); 11. Average of 27 peridotite nodules (Reay 1965); 12, Average of 3 garnet-peridotite nodules (ibid.); 13. Dominant ultramafic rock composition (White 1967); 14. Mantle composition (Mason 1966). i5. Estimated upper mantle composition from which volatiles have been lost without any basalt removal by partial fusion (Nicholls 1967); 16. E~timated upper mantle composition (Carter 1970); 17. Average peridotite komatiite - Barberton area South Africa (Viljoea & Vil]oen 1969); 18. ~)mposition of basaltic komatiite from Singhbhum, India CYiswanathan & Sankaran 1!)73); 19, Hornb!ende--enstatite--peridotite, IUchughuta, Bihar, Indh (Gho~h 1970).
8.04
3.08
-
Geochemistry of D ~
well w i t h t h e r e p o r t e d velum for upper mantle
I
2500
x Periclotite o Dunite • Pyroxenite
I I I I I
A Amphiboite
& Turekian 1961, Turekian & C a r t
i
1500
n_ n
rocks f r u r e t m & W e d e p ~ ~96~, vt- ,~ck,v 1962). Y, Ga and Cu show little variation but are within acceptable ranges (Bodse~o 1963}. Co is si~aificantly high comlmod to the values reported elsewhere (Smales et aL 1957, (3trr
l I I
2~0
x
•
~ 0!
Z
-
•
/
-
•
k
0 I0
• s.., "SS ~...,--
~-O b •
500 - .
l
1961,
Steuber & Goles 1967), but higher concentrations of this element are reported in these parts Of the Shield in amphg)olites and metabasalts (Naqvi & Hussain 1973a, Satyanarayana et al. 1974).
&
I000 "
ullramaflcz 85
I~
X 0
Ib
•
,
'5
a
,
20
30
40
50
Discussion The ch.aracteristics of distinctive ultramafic and basic m g m u s of Archaean 8reenstone belts are consistent with melting of a mantle compmieon close to that of pyrolite but
require conditions of melting different from
MgO%
FIE. $. Diagram showing the variation of Mg with NI in Dharwar ultramafics.
those operating to yield modern basalts (Green 1972). It can be suggested from the chemistry of the Mysore ultramafLs that a magma
similar to per/dmitk komatltte (FIs. 6) can give rise to these rocks, where the dunito.sidering the trace-element values (Faust 1963). The average V values of 25 ppm and 40 ppm in the peridotites and dunites of Mysore agree
i
peridottte-pyroxenite and the amphlbollte are the differentiation products of the same masma. The main perldotite-dunite zone was
U l
i
.
|i
10
11
12
13
14
15
16
17
18
19
42.71 0.47 3.30 1.57 6.51 2.11 41.41 0.49 0.18 0.13
43.40 0.08 1.80 2.70 6.70 1.80 42.40 0.21 0.01 0.16
45.70 0.13 2.70 1.60 5.70 2.00 41.50 0.22 0.03 0.12
48.00 0.13 3.00 13.00 2.30 31.00 1.10 0.13 0.43
0.06
0.04
0.02
0.33
0.35
45.10 0.50 4.10 ZOO 7.90 2.30 36.70 0.60 0.02 0.20 0.10 0.30
42.86 0.33 6.99 0.36 8.97 4.37 35.07 0.45 tr. 0.04 0.18
46.63 0,34 3~)2 1~0 9.63 4.79 3433 0.15 0.03 0.18 -
50.66 0.63 7.16 4.78 10.72 12.60 10.51 1.68 0.28 0.06 0.11 -
42.56 0,38 11.12 3,48 10.02 6.03 19.04
0.45
4430 2.50 8.20 2.23 41.70 035 0.01 0.17 -
0.34
0.55
3A7 0.53 0.17 0.40 -
86 Dlvakara R a o et al. MgO
r 12 -
•
:',,,.-" 4 - ,p
/
•
,< o8 o U
4
;Zi I
t
4
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8
I
t
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I
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20
Ca0
AI 2 0 3
I. FIELD OF KNOWN PERIDOTITE CLASSES 2. F I E L D OF PERIDOTITE KOMATIITES 3 , 4 8 k 5 , FIELDS • BULK
Fig. 6.
OF BASALTIC KOMATIITES
CHEMICAL COMPOSITION OF DHARWAR
ULTRAMAFICS
(A) Diagram exhibiting the high Ca, low AI nature of duni~es and peridotites from Dharwar. (B) Ternary diagram for MgO-CaO-AI=Os indicating the peridotitic komatiite composition of average Mysore ultramafic magma.
the first unit to form, with much of the olivine having been derived from xenocrysts carried in the liquid peridotite magma from depth. With a decrease in temperature, olivine ceased to crystallise and was followed by orthopyroxene to give enstatite-rich pyroxenite. Clinopyroxene and plagioclase must have crystallised simultaneously with further decrease in temperature. Monomineralic amphibolite must have formed in the final stages with change in pressure,:, and temperature or the depth of the magma chamber. The intrusive relationships of amphibolite to dunite suggest late-stage crystallisation of amphibole. At present there are two major hypotheses for the origin of Alpine peridotites. One is that the peridotites originated by partial fusion, leaving over ultramafic residue after removal of more easily fusible components as mafic melts. This is suggested for the origin of basaltic magmas (Ringwood 1962, Green & Ringwood 1967, CarsweU 1968, Nicolas 1968, 1969). Dickey (1970) has recently presented evidence that lherzolite of the alpine type can yield grabbroic liquid by pa~ial fusion, leaving dunite and harzburgite as depleted residues, The second major hypothesis is that alpine peridotites originate from basic magmas by cumulus processes. Thayer (1969, 1970) in particular has advocated this origin for the
TOTAL F E M I C M O L E o/o RESIDUAL CRYSTALS
b,
X
60
,.o.
•
6s
( "-'o..
x
'~
O
.
~q~.
e,s,.,
DUNITES
/AOoOo~ " ' -I
:
,x .,,.
0
~'~I'°
0
• • •of
%
ss
•
•
I, 3S
,,,, 40
I, 45
50
Fig. 7. SiOR mol% vs total femlc mol% diagram ex-
hibiting the residual nature of Mysore dunlte from an ultramafic magma from which the basalt must have been removed.
harzburgite variety. The Mysore ultramafics thus fit into the first type (Fig. 7) where basaltic liquid must have been removed leaving a peridotite-dunite residue. The Mysore ultramafics are supposed to have been intruded in the solid state at cornparatively low temperature (Varadarajan 1970, Karunakaran 1970). It is generally accepted
Oeochem~y of Dh~rwar u l ~ ~ that peridotite material initially derived from the upper mantle could b e in one of several physical states OVy,ie 1967):
87
fall in the peridotitic komatiite field. Even in the mantle compositions suggested by different invest~tors, there are differences in Alz03, MgO, CaO and ~ Peridotitic komatiite • as a complete or nearly complete fusion melt of from Singhbhum Oriswanathan & Sankaran upper mantld, which could yield a mobile ultramafia 1973) and amphfl3el/te d/kes from Bihar magma; • as a more refractory crystalline residue composed (Ghosh 1970) also differ in chemical composilargely of olivine and pyroxene left behind after tion from these Mysore ultramafics; these partial fusion of the mantle, which yielded a basal- differences may be due to the degree of partial tic magma; • partial fusion of the upper mantle yielding a basal- melting and to movement of material through tic or picritlc liquid with an olivine crystal mush. different rock types. However, it is also posTransportation of thi~ mush and subsequent crystal. m~le that they represent to some extent chemIisatlon yielding ultranu~c rocks. This can be al- ical differences in the original magma and the most solid with little interstitial liquid; Archaean mantle of Penin.~mr India. The • upper mantle itself could be exposed. slightly lower silica and excess MgO in Mysore The Mysore ultramafics, from their lack of ultramafi~ compared to the Singhbhum rocks metamorphism, the alteration along their is reflected in the forsteritic ofivine in Mysore boundaries, and their strained olivine, appear and the more fayalite-rich variety in $inghto fit into the second type above. bhum. Partial fusion of peridotite or pyrolite According to Narayanaswamy (1970) a mass under different P - T conditions also may give of ultramafic peridotite liquid must have in- rise to these chemical differenceL truded first into the lower crust; during later A comparison of the composition of olivines orogeny this was intruded, cold, into the upper from fresh dunites and peridotites of Mysore crust. Gravity studies over these ultramafics, with those reported for other ultmmafics in as mentioned earlier, have indicated that these this area (Vm'adm'ajan 1970) and the upper lenses are connected below to a large mass, mantle olivine compositions of Green (1964), which supports the above view. Of primary Gwynethe & Challis (1965), and Carter (1970) importance in connection with the upper is given in Table 2. There is not much variamantle studies is the bulk composition of the tion from sample to sample and all oxides are parent magma which gives rise to differen- similar to the reported compositions in the tiated bodies. An important method of de- proposed mantle rocks. Total iron is almost termbJing this is the summation of the identical though ferric iron is higher in the chemical compositions of all the differen- Mysore olivines, due either to the level of tiated bodies combined in proportion. Another oxidation or the depth of generation of the method is to determine the chemical composi- magma from which these rocks have formed. tion of chilled borders of intrusions. A com- MgO is similar in all olivines, whereas CaO parison of various hypothetical and estimated is slightly higher in the Mysore olivines. The probable mantle compositions with those of Mg/Fe ratio is greater in the latter compared the average composition of Mysore ultramafic to reported values. rocks is given in Table 3. The interrelationships between different The average dunite and peridotite of Mysore trace-elements (Fig. 8) demonstrate the posihave less silica, and the pyroxenites and tive correlation between Cr and Co. In dunites amphibolites slightly more than any of the and peridotites their concentrations are higher reported mantle compositions. The average than in the amphibolite. Cr and Ni show compiled from the Mysore ultramafics (Table random variation though their concentrations 3, column 5) resembles in the concentrations are higher in dunite--peridotite, with similar of many oxides the pyrolite of Ringwood variation in Co-Ni. This distr£oution of trace0966) and Reay (1965), the peridotitic koma- elements suggests that their concentration is trite from Africa (Viljcen & Viljoen 1969), and higher in the parent magma, which was more the estimated mantle of Carter (1970). Perido- heterogeneous at the time of formation of the titic komatiite from Singhbhum (Viswanathan dunite-peridotite. A definite differentiation & Sankaran 1973) has !ess MgO, FeO and trend had set in when pyroxenites and amphibolites had formed. These dunites and peridom o r e S i O 2. These ultramafics belong to the high Ca, low AI group (Fig. 6) and distinctly tites may belong to the residual crystal mush
88 Divakara Rao et al, II+00
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Diagram showing ,'he interrelationshipbetween different trace elements.
of peridotitic komatiite magma from which probably basaltic liquid may have been separated (Fig. 7). Glikson (1971), on the basis of his studies of isocficmically metamorphosed Archaean igneous suites from Transvaal, Rhodesia, Western Australia and Canada, has suggested that the Archaean mantle had higher concentrations of siderophile and certain transitional elements than the pre~ent mantle. This also has been suggested in the case of Peninsular India (Naqvi & Hussain 1973b). Hart et al. (1970) suggested that the Archaean mantle was less depleted in Rb, a possibility also supported by data from Canada, Western Australia (Hallberg 1972) and the Mysore ultramafics (Divakara Rao & Satyanarayana 1973). Regional geochemical investigations in the Canadian Shield have revealed important variations between Archaean and Proterozoic terranes, mainly in relatively high Na, Ca, Mg, Ni and Cr and low K, Ti, U end Th for Archaean terranes (Fahr|g & Eade 1968). Work on the Archaean amphibolites of Mysore
State has also indicated higher concentrations of Co, Ni and Cr than in younger oceanic basalts (Satyanarayana et al. 1974) suggesting higher concentrations of these elements in the Archaean mantle. Table 4 gives a comparison of Permian Alpine type ultramafics from the Canyon Mountain Complex of northeastern Oregon (Thayer & Himmelberg 1968) and Tertiary amphibolites from Kumaon Himalayas (I)ivakara Rao et al. 1974b) with those of Archaean ultramafics of Mysore. If these rocks ere similar in origin, the differences in composition shown in ".'able 4 may reflect differences in mantle composition during these periods. A~chaean mantle would then appear to be less differentiated than the Permian mantle. The excess of SiOa, MgO, Na20, KzO and the abnormal concentrations of Rb and Cs in the Archaean ultramafics of Mysore indicate that the Archaean mantle may have contained more siderophile and transitional elements than at present.
Geochemis~y of D
~
ultram~i~
89
Table 4.
~SiOs "/'iO. Als~, ee~s CaO MSO Na.O K,6 MnO P,O,
Tertiary
Permian
Himalayan amphibolite
Dunitc
Peridotite
AmphiboHte Dmlite
Peridotite
AmphiboHte
47.13 0.57 13.69 2.31 8.62 8.63 14.05 1.19 0A9 0.14 0.37
33'85 0.01 1.58 4.59 2.79 0.00 41.69 0.00
41.53 0.03 1.73 2.29 6.48 1.36 41.01 0.02
52.80 1.90 14.70 0.06 10.40 7.80
38.72 0.08 1.23 6.68 2.14 1.72 46.29 0.60
49.87 0.84 8.74 2.97 7.01 8.07
4.40
39.86 0.35 1.25 4.16 3.31 1.96 46.51 0.78
0.00
0.00
0.90
0.09
0.06
0.43
0.I0 0.00
0.14 0.00
0.20 0.25
0.05 0.38
0.09 0.42
0.05 0.35
Ardmean
Conclusions Geochemical studies on A r c h a e a n ultramafics of Alpine-type from Mysore, Peninsular India indicate t h a t A r c h a e a n m a n t l e beneath t h e shield was peridotitic in n a t u r e and had h / s h e r concentrations of siderophile and transitional elements t h a n at present. Acknowledgements. - The authors are 8fateful to the Director of the National Geophysical Research Institute for his encouragament in work ,rod p m n l ~ o n to publish the data. The help rendered by Mr. V. Dlleap Kumar In the preparation of the manuscript is thankfully acimowledsed.
References Anhaeusser, C. R., Mason, R., VilJoen, M. J. & VilJoen, R. P. 1969: A reappraisal of some aspects of Precambrian 8eoloffy. Geol. See. Am. Bull. 80, 21752200. Borisenko, L. F. 1963: Some characteristics of the cllstrlbutlon of gallium In uitramaflc rocks. Geolchi. mya, 746-7$3. Cart, M. H. & Tureklan, K. K. 1961: The 8eochendstry of cobalt. Geochim. Cmmoehim. Aeta 23, 9-.60. Carswell, D. A. 1968: Possible primary upper mantle peridotlte in Norwe81an basal 8nel~ Lithos 1, 322355. Carter, N. L. 1970:Mlneralot7 and 8eochemistry of the upper mantle based on partial fusion, partial crystallis~ tlon model. Geol. See. Am. Bull. 81, 20212034. Crawford, A. R. 1969: Reconnaissance Rb-Sr datin8 of the Precambrlan rocks of southern Peninsular India. I. Geol. So¢. India 10, 117-166. Dickey, J. S. 1970: Partial fusion products in Alpinetype perldm:,es, Serranla de la Ronda and other examples. PiTth anniversary Sym. Min. See. Am. Sp. paper 3. Dlvakara Rao, V. & Satyanarayana, K. 1973: Anoma.
4.30
18.26
2.30
lously hI8h K, Rb and Cs concentrations In the serpentinised ultrmidlcs of Kadakola, Mysore and their significance. Chem. Geol. 12, 51--60. Dlvakara Rao, V., Naqvi, S. M., Satyanarayana, K. & Hussaln, S. M. 1974a: Geochemistry and oH8in of Penin-qular 8neiss, Mysore State, India. A Geul. &~c. India I$, 270-277. Dlvakara Rao, V., Naqvl, S. M., Satyanarayans, K. & Hussaln, S. M. 1974b. Geochemistry and petrosenesis of basic rocks from Bhowall, U.P. Geoi~y:. ReJ. Bull. 12, 63-74. Edelshtein, I. I. 1963: Petrolo8y and nickel content of ultrabatlc Intrudom In the Tobol-Bunyktal area of the southern Utah. Masmatlzm, Metamortlzm, Metallollertlya Urala. Akad. Nauk. $SSR Uralsk Filial, Petrolr. 319.-323. En8el, A. B. J. 1963: Geolo8ic evolution of North America, Science 140, 143~.152. Fahri8, W. F. & Fade, K. E. 1968: The chemical evolution of the Cenndlan Shield. Can. L £arth Sol. 5, 1247-1:52. Faust, G. T. 1963: Minor elements In serpentine additional data. GeocMm. Cmmochim. Acta 2?, 665-668. Gangadharam, B. V. & Reddy, O. R. 1969: Estimation of Potassium. rubidium and caesiem in standard and ultramafic recks ~ neutron-activation. Radio. cMm. A cta 11, 90..93. Gheah, N. C. 1970: Petrochemistry of the perldotito dykes from gichushuta, Blhar with special reference to the composition o f the upper mantle. Prec. 2rid Sym. U.M.P. Hyderabgul, India, 249-.360. Glikson, A. Y. 1971: Primitive Archaean element clistrlbution patterns: Chemical evidence and 8cotectonic sisniflcance. Earth & Plant. Sci. Lett. 12, 309--320. Gllkson, A. Y. 1972: Early Precambrlan evidence of a primitive oceanic crust and island nuclei of mclic granite. Geol. See. Am. Bull. 83, 3323--3344. Goles, G. C. 1967: Trace elements in ultramafie rocks. Pp. 352--362 In Wyllle, P. 1. (Ed.), Ultramafics and Related Rocks. John WHey & Sons, Goodwin, A. M. 1968: Archacan proto-continental growth and early crustal history of the Canadian shield. 23rd Int. Geol. C,ongr., Prague, Section I, 68-89.
90
Dlvakara R a t et al,
Earth and Terrestrial Planett. S l . Int, Sd. New Green, D, H, 1964: The petrogenests of the high York. temperature perldotite intrtaion in the Lizard area, Nicolas, A. 1968: Relations structurales entre le mauig Cornwall. L Petrol..5, 134--188, ultrabaxlque de Lanzo, seat satellites et la zone de Green, D. H. 1971: Compositions of basaltic magmas serla Lanzo. Bull. Seine. Min. Pdtrog. 48, 145as indicators of conditions of oils/n, application to 156, oceanic volcanism. Phil. Tran#. Roy. gee. l.ond. Nicolas, A. 1969: Ut~ rue unltaire concermmt l'odSino Set. A. 268, 707-725. de massifs ultrabaslques des Alpes occidenteles InGreen, D. H. 1972: Magmatic activity as the major ternes.,Acad. Sci. tempter Rendl~ 269, 1831-1834. process in the chemical evolution of the earth's crust Plchamuthu, C. S. 1962: Some obaervatlmm on the and mantle. Tectonophysics 13, 47-71. structure, metamorphim! and Ipmlogical evolution of Green, D. H. & gingwood, A. E. 1962: Mineral assemblages in a model mantle composition. I. Peninmlar India. I. Geol. See. India 3, 106-118. Qureshy, M. N., Bhatla, S. C. & Dlvakara Rao,~'V. Geophys. Rea. 68, 937-945. Green, D. H. & Ringwood, A. E. 1967: The g,~tblUty 1970: Preliminary Vavity studies over the ultrafields of aluminous pyroxene i~ddotite and garnet marie,s of gadakola, Mysore (abstrac0. 2rid Syrup. peridotite and their relavance in upper mantle U.M.P., Hyderabad, lndla, 130. structure. Earth & Planet. gel Left. 3, 151-160. Reay, A. 1965: Mantle composition and partial fusion Gwynethe, A. & EhaP~, G. A. 1965: The origin of of possible mantle material. Ph.D. Thesis, Univ. of Newzealand ultramaflc intrusives. I. Petrol. 6, 322Leeds, Leeds, England. 364. Rlngwood, A. E. 1962: A model for the upper mantle. Hallberg, J. A. 1972: Geochemistry of Archaean volI. Geophys. Reg. 67, 857-868. canic belts In the eastern Goldfield region of Ringwood, A. E. 1966: The chemical composition and Western AustraUa. I. Petrol. 13, 45-56. origin of the earth. Pp. 287-356 in Hurley, P. M. Hart, S. R., Brooks, C., Krogh, T. E., Davis, G. L. Mineralogy o/ the Mantle. M.I.T. Press. Cambridge, Mass. & Nova, D. 1970: Ancient and modern volcanic rocks; a trace.element model. Earth & Planet. $ci. Ringwood, A. E. & Green, D. H. 1966: An experiLett. 10, 17-28. mental investigation of the gabbro-eclogite transHess, H. H. 1964: The oceanic crust, the upper mantle formation and some geophysical consequences. Te¢. and the Mayaguez serpentinised perldotlte. Pp. 169tonophysl¢8 $, 383-427. 175 in Burk, C. A. (Ed.), A Study of Serpentinlte.,... Satyanarayana, K., Naqvi, S. M., Divakara Rao, V. & C.A. Nat. Acad. Sd. Natl. Res. Council Pubin. 118[. Hussain, S. M. 1974: Geochemistry of Archaean Hess, H. H. & Otalora, G. 1964: Mineralow and amphlboHtes from Mysore. India. Chem. Gee. (Comchemical composition o~: the Mayaguez ser~ntinlte municated). cores. Pp. 152-168 In Burk, C. A. (Ed.), A Study Shapiro, L. & Brannock, W, W. 1962: Kapld analyses o/ Serpentlnltes. C.A. Nat. Acad. Sd. Natl. Kes. of silicate, carbonate and phosphate rocks. U.$.G.S. Council Publ. 1188. Bull. 1144-,4, 56. Karunakaran, C. 1970: Ultramaflc and related rocks Smales, A. A., Mapper, D. & Wood, A. J. 1957: The in the southern Penins~llar Indic. Prec. 2nd Syrup. determination by radioactivation, of small quantities U.M.P. Hydc~abad, India, 361-376. of nickel, cobalt and copper In rocks, marine sediKushiro, I., Syono, Y. & Aklmoto, S. 1968: Melting meats and meteorites. Analyst 82, 75-88. of a peridotite nodul, at high wessures and high Steuber, A. M. & Goles, G. G. 1967: Abundances of water pressures. 1. Geophys. Res. 73, 6023--6029. Ha, Mn, Cr, Sc and Co in ultramafic rocks. GeoMason, B. 1966: Composition of the earth. Nature 211, chim. Cosmochlm. Acta 31, 75-93. 616-618. Thayer, T. P, 1969: Gravity differentiation and Naqvi, S. M. & Hussain, S. M. 1972: Petrochemistry magmatlc reemplacement of podlform chromite of early Precambrian metasediments from the cendeposits. Econ. Geol. 4, 132-146. tral part of the Chitaldurg schist belt, Mysore, Thayeg, T. P. 1970: Chromite segregations as petroIndia. Chem. Geol. 10, 109-135. genetic indicators. Geol. $oc. South AJrica $p. Publ. Naqvi, S. M. & Hussain, S. M. 1973a: Geochemistry 1. 3t;0--390. of Dh~:r~var metavolcanics and the composition of Thayer, T. P. & Himmelberg, G. g. 1968: Rock succesthe primeval crust of Peninsular India. Geochim. sion in the alpine type mafic complex at Canyon Cosmochim. Acta 37, 159--164. Mountain, O,'egon, 23rd Int. Geol. Congr. Prague, Naqvi, S. M. & Hussain, S. M. 1973b: Relationship Section 1, 175-186. between trace and major element composition of Tureklan, K, K. & Cart', M. H. 1961: Chromium, the Chitaldrug metabasalts, Mysore, India and the cobalt and strontium In some Bureau of standard Archaean mantle, Chem. Geol. 11, 17-30. rock refe:ence s3mples. Oeochlm. Cosmochtm. Act, N~tqvi, S. M., Dlwkara RaG, V., Satyanarayana, K. & 24, 1-9. Hussain, S. M. 1974: Geochemistry of post-Dharwar Turekian, K. K. & Wedepohl, K. H. 1961: Distribution basic dikes and the crustal evolution of Peninsular of the elements in some major units of the earth's India. Geol. Mar, crust. Bull. Geol. Soc. Am. 72, 175-192. Narayanaswamy, S. 1970: Tectonic setting and maniVaradar~jan, S. 1970: Emplacement of chromite festation of the upper mantle in the Precambrian bearing ulLrat)asic rocks of Mysore State. Prec. 2nd rocks of south India. Prec. 2nd Syrup. U.M.P. Symp. U. M P., H~derabad, India, 441--454. Hyderabad, India, 377--404. Velensky, V. V., Pimus, G. V., Lesnov, F. P. & Nicholls, G. D. 1967: Geochemical stodle~ in the Vas[l'ye~r, Yu. R. 1970: Chemical characteristics of ocean as evidence for the composition of the xaantle ultra~af[c rocks beloni~ing to different magmatic Pp. 285-304 in Runcorn, ~ l~. (Ed.), Mutates o/ ¢.6e~ associations a~,d their petrogenetic, significance.
Geochemistry o f Dharwar ultrmnalics Doklady 191, 141-143. VflJoen, M. jr. & Viljoen, IL P. 1969: Evt/d~ce for tho e=imenco of mobUo ez~-uslve OeridoUte masma from KomaU foema~n of the Onver~Lcht Stoup. Upper mince Faject, Geal. Sac. $7 Al~ao Sp. Publ. 2, 87-112. Vinosradov, A. P. 1962: Average contents of chemical elements in the Irhtciple types of igneous rocks of the Earth's crust. Oeochemittry, 641-664. VJswanathan, S. & Sankaran, A. V. 1973: Discovery of a kamatite in the Precambrlan of Indla and its sisniflcan~ Jn tho nature of Archaean volcanism mad of the early crust in the .'ndJsn shield. CJwr. $c/. 42,266-269. Wedopohl, g . H. 1963: Die Nickel-und Chromzeh~to
91
yon b l u / t b c h m ~ und dmmt Oh~tn,tmuzn, den Blnsuldfamm. Neuet ]ahrb. Mineral Mcmat~. 237-242. White, L (3. 1967: Ultrsbasic rocks and tho composition of the uffper mantle. Earth & Planet. ~ Lett. 3, 11-18. Wyli~ P. J. 1967: Alpine type ultmmaflc sssocktiom~ P. 135 In Wylll~ P. J. (F.d.), ffltrama//a and R d a ~ d Rocks. John WHey & Sons. Wyilie. P. J. 1971. Role of water in m g n m 81meration and initiation of diapirlc u p r ~ In the mantle. I. Geophys. Res. 76, 1328-1338.
Acceptedfor publication November 1974 Printed April 1975
RAO, K. SATYANARAYANA, S. M. NAQVI & S. M. HUSSAIN
LITHOS
O
h o , V~ Sttmmmyam. L N~vi, & M. & Humim .~ M. l~S: of INmwzr dtmnafim and the Argheanmantle, Lithoz 8, 77-91. G e ~ and p e ~ studies on serpmtinised u l u ~ of r , ~ Mysore State, India, indicate that the Archaean nnntle beneath the In~aa Shield wa~ of ]m~Jotitic in nature. This p r o ~ e in the Archaean period was uruJffmm,tiated ~ ¢oaceatrathms of dderolddle and tramitional dements. The address of all authors is: National Geophys~d Research lmdtute, Hydrabad - 500007, ind~
Geochemical data, when interpreted in terms of modern concepts on the relationship between magmas, mantle and crust, provide a basis for investigation of early Precambr/an geotectonic pattern and the origin of shields (gingwood & Green 1966, Green & ginswood 1967). There are four approaches for estimating the composition a n d mineral fly of materhl within the earth, ~ y the mantle, (I) Use of data from extraterrestrial sources and formulation of physical and geochemical models for the origin of the Solar system including the earth; (2) study of ultramafic and basaltic rocks supposed to have been derived from the mantle; (3) compar/son of various physical properties such as density, elastic properties, etc. obtained by geophysical methods; and (4) working out the mineral assemblages and phase transitions in the interior from the above compositional and physical properties. Due to the strong fractionation of crustal rocks, emphasis cannot be laid on extraterrestrial evidence for bulk composition, but examination of igneous rocks derived from the mantle does place limits on the composition of •
6 - - Lithos 2/75
•
•
0
the upper mantle. In view of their importance in this regard, much work has been done on the geochemistry of ultramafic rocks such as peridotites and dunites, which are supposed to be the nearest to mantle composition (Hess 1964, ginBwood 1966). It is now clear that the major rock type of the upper w~ntle is peridofite with olivine and lesser orthopyroxene as its major mineral phases (Green 1972). Closer characterisation of the nature of the upper mantle is possible from the examination of the products of natut~ processes in the upper mantle. The composition of the prim/five CrtL~ and its formation along the continents have long constituted a fundamental problem in geology. The nature of the primitive undifferentiated mantle, its composition, mode, time and rate of its differentiation; all these queries are unanswered and investigations are in progress to
relatively high degrees of partial melting of l~ridotitic upper mantle at shallow depths
78
DivakaJ'a R a o er al. i f-
77 °
76 ° w
75 ° |
i
i I
" k.,.
/.'... : L'''" . . - :..',.:~
I
,HIMOGA;;'
( ~/
J
b'"~
,,J
I. °"~.
~.,° ~.
KOLAR MANGALORE
BANGALORE
//
/-'-" "-' "" ",~..~,°.~ ~,.I
('J
/
N
•
Ultrabasic Rocks Chromite Bearing Dharwar Schists
MF..RC~RA \ \ / \ ~ x ,~[~J
"\
\
Peninsular Gneisses
\,
Younger Granite ....
\
i I
I ~,/'
\ ~.,
x,. A . . ~ /
Northern L i m i t of Charnockites
f . . . . . "-- j ' ~ '
~,."yJ~,\ /
~,._
~U !
,~
i
0
~..~ A
Fig. 1.
,, i
I i t
~6°
, !
'
o
I i
40Mlles
I /
•
7s °
;,.-;1
-/7 °
I ' 20
,
I ' 40
8o I(m
I i
7s a
Geological map of Mysore, Peninsular India. Square in the diagram indicates area under study.
(Kushiro et al. 1968, Green ~1971). Concurrently with the geophysical revolution of the last decade, a resurgence of interest in Precambrian terrains has brought to light import.ant differences between the continental crust of present day and Archaean times (Goodwin 1968, Engel 1963, Anhaeusser et al. 1969, Glikson 1972, Naqvi & Hussain 1973a). From the viewpoint that similar studies in shield areas on Archaean ultramafics and younger mafic rocks may reveal the nature of Archaean mantle and its development through geological time, investigations have been carried out on the Archaean ultramafics of Peninsular India by the authors.
General geological setting and description of the rock types The zone of serpentinised ultramafic rocks associated with gneisses, granites, metasediments and metavolcanics of the Dharwar system (Precambrian), Peninsular India forms an 'en echelon' belt nearly 320 km long and extends from Sindhuvalli in the south to Shivani in the north (Fig. 1). The ultramafics includ(: peridotites, dunites (mostly or partially serpentinised), pyroxenites and amphibolites (Fig. 2). Peridotites are not identified in the field, but are classified by microscopic study, and hence not represented in Fig. 2. These are
Geochemistry of Dharwar ".,ltrameyics 79 |
i
?~,~so° .,'
,,
ss"
'
#~'
o
Ys" ~ ,
le
o
ChattQnahalli
-
45'
4o'
~
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~
U,
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,
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8J,
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0
O 7 6 ' 3o"
Fig. 2.
j
. .......
0
5'
0
O
IMile
Sampoe, Pyroxenite : Ounite Amphibolite
~ I ['::--~:~".1 i ~ , ~ _ :~.~'.:~.;1!
1
P,,i,,,lo, o,,i.,i' 4b' . . . .
@ o
Hemmanqlola
I' 7 ¢ 4~,' '~
Detailed 8eological map of the area under ~tudy, showing the distribution of different ultramaflcs.
Alpine type ultramafics (Varadarajan 1970, Karunakaran 1970), which occupy the axial zones of the Dharwar anticlinorium (Pichamuthu 1962). Individual lenses vary in size from a few metres to kilometres. Pyroxenite occurs as small patches, whereas amphibolites occur as small patches in dunite as well as individual lenses. This belt runs parallel to the structural axis in the region, steeply dipping and elongated in the trend of regional foliation, of approximate north--south to N N E SSW direction. Partial serpentinisation of dunite h ~ given rise to antigorite. Magnesite veins cut across dunite and are mined at many places (Dodkanya: Fig. 2). At ~he contact with gneisses the serpentinite is altered to talcschist. Peridotites occur to a lesser extent, distributed along the outer parts of dunite bodies, Chromite occurs in sizeable deposits in the dunites. Secondary alteration is responsible for the extensive deposits of magnesite and
talc. Asbestos mineralisation is also noticed, though less c~,mmon. Field relationships between the dunites and the Peninsular gheiss and the following field observations suggest that the dunites are probably older than the gneisses: (a) Veins of Peninsular gneiss cutting across dunite. (b) Enclaves of dunite in Peni_~ular gneiss. (c) Schistosity in the associated pyroxenites and dunites parallel to regional schistosity direction. (d) Alignment of many lenticular ultramafic lenses along the schistosity (Fig. 2). (e) Gravity surveys over the ultramafic lenses have revealed that they are connected to a large mass below under the cover of thin gneissic rock (Qureshy et al. 1970). (f) Lack of any high temperature metsmorphi.~n of the gneisses due to intrusion of the ultramaflcs.
It is possible that these ultramafi~ may belong to the missing greenstone sequence (Naqvi et al. 1974) older than ~ t i c activity around 2400 m.y. (Crawford 1969). The Kolar schist
80
Divakara Rao .et al,
Table 1. Major and trace element analysis of ultramsfl¢ rocks.
Wt%
1
2
3
4
5
6
SiO, TiO, A!tOº Fe, O,
39.33 0.18 1.59 7.29
39.00 0.07 1.32 6.48 2.12 1.57 47.28 0.48 0.02 0.12 0.33 640 590 1500 48 -
38.27 0.07 1.59 5.55 2.40 2.17 48.29 0.48 0.06 0.23 0.43 365 1050 1050 55 23
38.29 tr. 0.42 7.40 4.06 1.23 45.13 0.60 0.09 0.02 0.93 375 1500 1200 64 60
38.80 tr. 0.92 6.74 4.10 2.!}8 44.'18 1.i5 0.10 0.02 1.?1 2~'0 1200 11,':0 :;8 ~3
FeO
CaO MgO Na~O KsO MnO PsOs Co (ppm) Ni Cr Cu V
-
1.94 44.45 0.76 0.06 0.01 tr. 375 800 1000 64 18
7
8
9
4'.),44 I).19 1.28 1.25 3.00 2.26 49.24 0.94 0.02 0.01 0.27 440 600 900 59 -
42.34 0.14 0.95 1.96 3.94 1.08 48.39 0.62 0.02 0.03 0.30 550 2200 580 37 43
40.65 0.40 1.36 2.19 3.92 2.15 46.70 0.58 0.06 0.02 0.29 425 2600 2000 48 29
38.93 0.35 0.95 4.41 3.72 1.16 48.25 0.17 0.02 0.02 0.21 330 1400 1000 54 26
"
Table 1 (cont.).
Wt%
19
20
21
22
23
24
25
26
27
SiOs TiOs AlsOs Fe20 s FeO CaO MgO Na~O KgO MnO PgOs Co (ppm) Ni Cr Cu V Ga Y Zr
.~1.93 0.17 4.12 3.84 4.80 1.29 31.12 0.48 0.01 0.05 0.15 235 860 720 91 255
51.61 0.29 4.18 0.92 5.72 4.64 30.38 0.68 0.02 0.02 0.36 220 250 625 70 -
41.99 0.67 12.26 2.79 8.94 2.07 28.65 0.40 0.25 0.02 0.49 560 43 715 70 -
56.56 0.67 7.09 1.88 5.68 9.77 17.02 0.48 0.08 0.03 0.13 220 520 800 64 -
41.17 0.97 13.68 9.04 1.00 7.82 22.56 2.28 0.37 0.28 0.21 N.D. N.D. N.D. N.D. N.D.
47.91 0.35 8.53 3.65 7.30 2.72 27.29 1.50 0.10 0.02 0.71 50 450 435 35 220 8 20 45
50.99 1.07 9.42 2.38 9.94 8.17 12.18 2.70 0.25 0.02 0.61 44 410 430 74 230 15 14 60
47.68 0.32 10.30 1.92 1.56 3.60 29.51 2.44 1.73 0.19 0.22 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.
48.97 2.10 6.41 1.60 9.22 6.60 19.78 4.60 0.25 0.06 0.20 66 240 440 38 240 10 16 60
tr.=trace; ---not traceable; N.D.=not determined. 1 to 4=Peridotites; 5 to ll=Dunites; 12 to 23=Pyroxenites; 24 to 34=Amphibolites.
belt in the neighbouring area ~ a typical greenstone sequence and similar formations in this area are yet to be identified. These ultramafics thus m a y belong to the older greenstone sequence, which has been m e t a m o ~ h o s e d a n d gmnitised to form the vast surrounding gneissic terrain (Divakara R a o et al. 1974a). Pyro~enites alre fresh and mostly ~composed
of o r t h o p y r v x e n e (enstatite) 'with little clinopyroxene. H o r n b l e n d e and iron ore are the accessories. A p a t i t e and plagioclase occur sporadically. Amphibolites are m o s t l y hornblendites (mono-mineralic), occasionally containing a liittle plagioclase a n d magnetite. Peridotite is a harzburgite variety.
G e o c / ~ m b t ~ o f D h m ' u ~ u/wama/ic:r
10
11
12
13
14
15
38.17 1.23 1.76 6.57 2.44 2.29 43.75 1.08 0.44 0.02 0.15 N.D. N.D. N.D. N.D. N.D.
39.73 0.14 1.57 633 2.10 2.74 44.40 0.91 0.02 0.20 0.34 370 760 1300 70 130
46.26 0.03 4.98 5.23 4.10 2.93 34.11 1.20 0.10 0.01 0.98 320 710 480 48 255
50.16 0.55 6.49 6.88 4.50 10.80 16.92 1.40 0.10 0.04 0.31 170 250 1200 38 -
52.67 tr. 3.71 4.75 3.62 7.79 25.34 1.32 0.15 0.01 1.08 310 600 580 58 -
49.67 0.67 532 2.89 8.02 2.08 26.10 2.60 0.15 0.02 G.63 N.D. N J). N.D. N.D. N.D.
28
29
30
31
32
33
34
49.90 0.64 5.29 10.18 4.38 11.66 13.05 4.50 0.10 0.05 0.14 32 440 430 34 180 7 21 55
45.87 1.16 15.64 2.17 6.10 7.82 12.15 2.52 1.08 0.12 0.27 85 180 195 66 210 7 20 56
52.28 0.59 6.21 0.92 8.92 7.77 20.01 0.74 0.32 0.01 0.36 50 415 370 24 305 8 15 62
47.31 0.70 4.~.2 4.~.4 6.80 6.12 27.52 0.31 0.06 0.03 0.36 78 600 490 54 240 9 16 40
48.52 1.58 13.25 2.01 9.30 8.42 12.84 3.25 0.40 0.02 0.50 64 680 550 72 250 8 19 70
52.75 0.54 7.59 0.22 8~6 10.84 14.93 1.50 0.25 0.03 0.46 51 240 430 42 250 13 18 70
52.32 0~0 9.39 3.57 5.34 15.0~ 11.$9 1.30 0.17 0.04 0.08 62 330 480 34 185 9 14 52
Method of analysis and chemistry of the rocks Fresh ultramaf/cs, selected after microscopic examination of th~,n sections, have been ~_~_lysed for their major, minor and trace-elements. Major elements have been determined by the ra~d methods of Shapiro & Brannock (1962), alkalis by flame-photometry, and CaO and MgO by titration. Trace-elements have been determined by n~n$ Pd and In as internal standards
16 50.79 0.35 5.32 1.88 8.54 3.96 27.57 1.15 0.15 0.02 0.33 , N.D. N J). N.D. N.D. M.D.
81
17
18
44.80 0.03 2.89 4.16 4.48 12.17 28.64 0.68 0.02 033 0.15 250 1000 490 43 300
49.32 0.12 6.75 2.77 7.10 3.12 23.88 4.70 0.10 0.01 0.68 250 700 600 43 -
with a Hilser & Watts large quartz, Littrow spectro8raph. Base and standards ~ different concentrations for various elements were prepared in the iab6~t~,~,. and the procedure is reported elsmvhere (Nsqvi & Hussain 1972). lib and Cs have been determined in a few rumples by neutron-actlvation analysis (Gansadharam & lieddy 1969). Major aud ~ice-element concentrations in different samples are 8:iven in Table I. OHvine was separated from a few sawples with the help of Frantz's lsodynmni¢ .~.lmrat~r and analysed (Table 2).
82 Divakara Rao et at. Table 2. Chemical composition of olivine,. Wtc/~
~
2
3
4
5
6
7
8
9
10
11
SiO. TiO~ Al~O~ Fe:O~ FeO CaO MgO Na.O K ~O MnO i'~O~ .~Ag/Fe
38.68 tr. 0,76 3.50 3.80 0.19 51.85 0.16 tr. 0.01 0.15 5.80
39.43 0.15 0.52 4.32 2.32 0.24 52.04 0.18 tr. 0.06 tr. 6.51
40.44 tr. 0.52 6.88 0.36 0.18 51.20 0.19 t r.
40.80 0.30 0.20 3.84 2.16 0.2 ! 52.08 0.16 tr. O.01 O.13 ":.20
41.88 tr. 0.16 2.59 4.00 0.22 50.60 0.14 tr. 0.01 0.17 6.20
39A8 0,22 0.69 ::.41 ;.44 026 52.2~ ~, ; ~ tr. (.01 0.08 6.42
40,11 0.11 0A7 4A3 2.35 0.22 51.66 0.16 tr, 0.02 0.11 6.34
41.~;5 it. 0.0rl 0.36 8.22 0.04 49.76 0.03 0.04 0.08 . 4.52
41.70 0,01 0,23 5,98 0,.:9 $1,67 0.I7 0.¢ I 0.10 . . 6.7 !
40.75 0,18 0,68 10,05 0,00 48,01 N,D, N.D. 0.13 . 3.70
40A3 tr. 0,91 0.78 8,3 I 0.00 49,05 0.05 0.05 0.16
tr.
0.13 606
4.23
i to 6=analysis of authors; 7=Average of the authors: 8-composition of olivine from Mysore (Varadarajan 1970); 9=composition of olivine from u r , ~ r mantle (Carter 1970); 10=olivine compos!tic, n (Green ."964); l l=olivin¢ composition (G,lynethe & Challt'~ I%.Q.
]'he .<1 values [~] = 100 (FeO + Fe203)/FeO + + Fe20;;+ MgO] are plotted again.,.t different oxides and trace-elements in Figs. 3 & 4. Pyroxenites and amphibolites clearly indicate a differentiation trend which may suggest that ~he different marie rock types of this area are the products of differentiation from a single magma. This can also be correlated with the composition and proportions of different mir.era~ assemblages in the rock types. SiO 2, Al20 3, CaO, Na20. K20 and TiO 2 increase with increasing A, whereas MgO decreases, There is random variation from sample to sample in a given rock type for different oxides with iv.crease in A. FeO and FeTO3 show a|most the reverse trend with increase in A. Ferric iron is more abundant in dunites and peridotites and shows a decreasing trend in pyrosenites and amphibolites. Breakdown of olivine to magnetite due to serpentinisation ar.~ oxidation probably accounts for the high fe~'ric content in dunites z..nd peridotites, whereas the fresh py~oxenites and amphibolit,~s have much less ferric iron. Ferrous iron is lower in the former ar, d increases to 9% in pyroxenites and amphiboiites. MgO is fairly high (48%) in dunites and peridotites when compared to many reported values (Table 3). Random variation in different oxides frown s~mple to sample in a given rock type reflects the inhomogeneous nature of the pare~.t magma or in this case the mantle below. All these ultramafics are highly enriched in alkalis, especially K, Rb and Cs, this is attributed 1~
post- or (Divakara ever, the of alkalis out.
syntectonic crustal contamination Rao & Satyanarayana 1973), Howexistence of higher concentrations in parts of the mantle is not ruled
Trace-elements
Thoagh the concentration of major elemeats in the upper mantle appears to be relatively well established, larger uncertainties remain abo~,~t the concentration and distribution of trace-elements. Secondary alteration, which may appear insignificant in the petrographic sense., may cause serious contamination with respect to whole rock values for elements like kb, C~ and K (Wyllie 1971) Co, Ni, Cr and Cu decrease with increasing /I (Fig..4), V increaees with 3 in dunites and peridotites but decreases in amphibolites. Ni is low in these ultra~nafics when compared to ,:he reported value~ of 1700 ppm for dunites and 1100 ppm for pyroxenites (Edelshtein 1963); 4400 ppm in uitramafic inclusions (Wedepohl 1963); 2600 ppm in serpentinites (Hess & Of:alora 1964). It is within the range of 1000 to 4000 ppm given by Goles (1967), As the olivine lattice admits large amounts of Ni, there is a distinct tendency for this element to become enriched in the early products of differentiation, and a sympathetic variation of Ni with Mg can be expected, which is exhibited ~n the~e ultramafics (Fig. 5). Serpentinisation in these dunites should be taken into account when con-
Geochemistry of Dharwar ultrama[ics
.,.,
55
Q
e
.
e
~
®
•
•
.~,¢
v.,.....~,
&
&
®
&
A
&
_ .,i+,
-
,.p.,,,
35
rn
20 & A
i0
&
®0
0 i0 o
5 e
0 50
o x x
40 o =,
30
x PERIDOTITE o DUNITE
~
,~
•
e PYROXENITE & AMPHIBOLITE
• ~ , . , ~ e &® ~
20
&
&
•
" ~ ' ~ ' ~ ~
...~...~~
..,_
I0 15 o
I0
•
o 2:
0 5 4 :5 2
•
A
®o
o
I
0.4
&
j
&
0 0.6 o
i°
o •
.... ~
A
~.i~
.... . ~ ~ " l ,
& 1,7~
A
A I,Og
.
o
&
0.2 C
~v
O ~ J ~ , ~ e ~ ' ~ " O~ ~
-
0 2
•
k
&
& 0
o W-
I
0 •
0 I0 0
#.
OA ~
- -
m
&
® ~ " ~
-
J
O X0
5
~
.
•
0 5
Ft~o 3,
A
I0
|5
D[~am ~how|~.Bv a ~ n
20
25
:50
55
40
45
50
~5
of o-xid~ |n DL~rw~r ultmm~flc~ with diff~,~'~nt~tlon b~d~x LA),
83
84 Dlvakara Rao et al,
•
~00
A •
200 >
I00 0
0 I00
o
50
I
A
o
x,.o *nx
_
0
O
--X
•
.o *
B- •
•
a
•~
•
•
u
• •
•
2000
~. tOO0 13. '-~ G. O 3~.-,_, 0 0
~, zooo x
IOO0
o
0
,,,
•
o 600
-
X
t
•
~
~
, .
"
•
X PERIDOTITE • DUNITE • PYROXENITF • AMPHIBOUTE
O
u
400
(
~ _
200
~ •
"
0
•
5
Fig. 4.
,,L_,,,
I0
15
20
25
30
- - - - -
A
• 35
AI 40
"
t
45
50-
55
- - - - - -
Diagram showing variation of trace elements In Dharwar ultramaflcs with differentiation index (A).
Table 3. Comparison of Dharwar mantle composition with otheE compositions. Wt%
1
2
3
4
5
6
7
8
9
SiOs Tie s AlsOs Fe,O, Fe¢) CaO
38.72 0.08 1.23 6.68 2.14 1.72
39.86 0.35 1.25 4,16 3,31 1.96
48.74 0.41 6.40 3.92 5.54 5.70
49.87 0.84 8.74 2,97 7,01 8.07
44.29 0.42 4.40 4.43 4.50 4.34
38.68 0.03 0.91 3.06 5.35 0,30
40.49 0,04 1.49 2.58 5.05 0.99
43.60 0.04 2.40 1.00 7.80 2.50
45.16 0,71 3.54 0.48
MgO Na,O KjO
46.29 0.60 0,06
46.51 0.78 0.09
26.02 1.45 0.12
18.26 2.30 0,43
34.52 1.28 0.17
44.84 0.05 0.03
41.3J. 0.08 0.03
41.50 0.32 0.01
37.47 0.57 0.13
MnO P,O, CraO .
0.09 0.42 0.28
0.05 0.38 0.47
0.07 0.46 -
0.03 0.35 -
0.07 0.40 0.18
0.12 . 0.38
0.10 . -
0.14
0.11 .
. 0.33
1. Average perldotite (authors); 2. Average dunite (authors); 3. Average pyroxenite (authors); 4. Average amphibolite (authors); 5. Averages of 1, 2, 3, and 4; 6. Average composition of dunite rocks for Alpine association (Velensky et al. 1970); 7. Average composition of peridotito for Alpine association (ibid.); 8, Type 'C' pefldotlte (Hess 1964); 9. Pyrolite (IUngwood 1966); 10. Pyrolite (Green & Ringwood 1963); 11. Average of 27 peridotite nodules (Reay 1965); 12, Average of 3 garnet-peridotite nodules (ibid.); 13. Dominant ultramafic rock composition (White 1967); 14. Mantle composition (Mason 1966). i5. Estimated upper mantle composition from which volatiles have been lost without any basalt removal by partial fusion (Nicholls 1967); 16. E~timated upper mantle composition (Carter 1970); 17. Average peridotite komatiite - Barberton area South Africa (Viljoea & Vil]oen 1969); 18. ~)mposition of basaltic komatiite from Singhbhum, India CYiswanathan & Sankaran 1!)73); 19, Hornb!ende--enstatite--peridotite, IUchughuta, Bihar, Indh (Gho~h 1970).
8.04
3.08
-
Geochemistry of D ~
well w i t h t h e r e p o r t e d velum for upper mantle
I
2500
x Periclotite o Dunite • Pyroxenite
I I I I I
A Amphiboite
& Turekian 1961, Turekian & C a r t
i
1500
n_ n
rocks f r u r e t m & W e d e p ~ ~96~, vt- ,~ck,v 1962). Y, Ga and Cu show little variation but are within acceptable ranges (Bodse~o 1963}. Co is si~aificantly high comlmod to the values reported elsewhere (Smales et aL 1957, (3trr
l I I
2~0
x
•
~ 0!
Z
-
•
/
-
•
k
0 I0
• s.., "SS ~...,--
~-O b •
500 - .
l
1961,
Steuber & Goles 1967), but higher concentrations of this element are reported in these parts Of the Shield in amphg)olites and metabasalts (Naqvi & Hussain 1973a, Satyanarayana et al. 1974).
&
I000 "
ullramaflcz 85
I~
X 0
Ib
•
,
'5
a
,
20
30
40
50
Discussion The ch.aracteristics of distinctive ultramafic and basic m g m u s of Archaean 8reenstone belts are consistent with melting of a mantle compmieon close to that of pyrolite but
require conditions of melting different from
MgO%
FIE. $. Diagram showing the variation of Mg with NI in Dharwar ultramafics.
those operating to yield modern basalts (Green 1972). It can be suggested from the chemistry of the Mysore ultramafLs that a magma
similar to per/dmitk komatltte (FIs. 6) can give rise to these rocks, where the dunito.sidering the trace-element values (Faust 1963). The average V values of 25 ppm and 40 ppm in the peridotites and dunites of Mysore agree
i
peridottte-pyroxenite and the amphlbollte are the differentiation products of the same masma. The main perldotite-dunite zone was
U l
i
.
|i
10
11
12
13
14
15
16
17
18
19
42.71 0.47 3.30 1.57 6.51 2.11 41.41 0.49 0.18 0.13
43.40 0.08 1.80 2.70 6.70 1.80 42.40 0.21 0.01 0.16
45.70 0.13 2.70 1.60 5.70 2.00 41.50 0.22 0.03 0.12
48.00 0.13 3.00 13.00 2.30 31.00 1.10 0.13 0.43
0.06
0.04
0.02
0.33
0.35
45.10 0.50 4.10 ZOO 7.90 2.30 36.70 0.60 0.02 0.20 0.10 0.30
42.86 0.33 6.99 0.36 8.97 4.37 35.07 0.45 tr. 0.04 0.18
46.63 0,34 3~)2 1~0 9.63 4.79 3433 0.15 0.03 0.18 -
50.66 0.63 7.16 4.78 10.72 12.60 10.51 1.68 0.28 0.06 0.11 -
42.56 0,38 11.12 3,48 10.02 6.03 19.04
0.45
4430 2.50 8.20 2.23 41.70 035 0.01 0.17 -
0.34
0.55
3A7 0.53 0.17 0.40 -
86 Dlvakara R a o et al. MgO
r 12 -
•
:',,,.-" 4 - ,p
/
•
,< o8 o U
4
;Zi I
t
4
I
I
8
I
t
I
I
Jz 16 AI203 %
I
I
20
Ca0
AI 2 0 3
I. FIELD OF KNOWN PERIDOTITE CLASSES 2. F I E L D OF PERIDOTITE KOMATIITES 3 , 4 8 k 5 , FIELDS • BULK
Fig. 6.
OF BASALTIC KOMATIITES
CHEMICAL COMPOSITION OF DHARWAR
ULTRAMAFICS
(A) Diagram exhibiting the high Ca, low AI nature of duni~es and peridotites from Dharwar. (B) Ternary diagram for MgO-CaO-AI=Os indicating the peridotitic komatiite composition of average Mysore ultramafic magma.
the first unit to form, with much of the olivine having been derived from xenocrysts carried in the liquid peridotite magma from depth. With a decrease in temperature, olivine ceased to crystallise and was followed by orthopyroxene to give enstatite-rich pyroxenite. Clinopyroxene and plagioclase must have crystallised simultaneously with further decrease in temperature. Monomineralic amphibolite must have formed in the final stages with change in pressure,:, and temperature or the depth of the magma chamber. The intrusive relationships of amphibolite to dunite suggest late-stage crystallisation of amphibole. At present there are two major hypotheses for the origin of Alpine peridotites. One is that the peridotites originated by partial fusion, leaving over ultramafic residue after removal of more easily fusible components as mafic melts. This is suggested for the origin of basaltic magmas (Ringwood 1962, Green & Ringwood 1967, CarsweU 1968, Nicolas 1968, 1969). Dickey (1970) has recently presented evidence that lherzolite of the alpine type can yield grabbroic liquid by pa~ial fusion, leaving dunite and harzburgite as depleted residues, The second major hypothesis is that alpine peridotites originate from basic magmas by cumulus processes. Thayer (1969, 1970) in particular has advocated this origin for the
TOTAL F E M I C M O L E o/o RESIDUAL CRYSTALS
b,
X
60
,.o.
•
6s
( "-'o..
x
'~
O
.
~q~.
e,s,.,
DUNITES
/AOoOo~ " ' -I
:
,x .,,.
0
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Fig. 7. SiOR mol% vs total femlc mol% diagram ex-
hibiting the residual nature of Mysore dunlte from an ultramafic magma from which the basalt must have been removed.
harzburgite variety. The Mysore ultramafics thus fit into the first type (Fig. 7) where basaltic liquid must have been removed leaving a peridotite-dunite residue. The Mysore ultramafics are supposed to have been intruded in the solid state at cornparatively low temperature (Varadarajan 1970, Karunakaran 1970). It is generally accepted
Oeochem~y of Dh~rwar u l ~ ~ that peridotite material initially derived from the upper mantle could b e in one of several physical states OVy,ie 1967):
87
fall in the peridotitic komatiite field. Even in the mantle compositions suggested by different invest~tors, there are differences in Alz03, MgO, CaO and ~ Peridotitic komatiite • as a complete or nearly complete fusion melt of from Singhbhum Oriswanathan & Sankaran upper mantld, which could yield a mobile ultramafia 1973) and amphfl3el/te d/kes from Bihar magma; • as a more refractory crystalline residue composed (Ghosh 1970) also differ in chemical composilargely of olivine and pyroxene left behind after tion from these Mysore ultramafics; these partial fusion of the mantle, which yielded a basal- differences may be due to the degree of partial tic magma; • partial fusion of the upper mantle yielding a basal- melting and to movement of material through tic or picritlc liquid with an olivine crystal mush. different rock types. However, it is also posTransportation of thi~ mush and subsequent crystal. m~le that they represent to some extent chemIisatlon yielding ultranu~c rocks. This can be al- ical differences in the original magma and the most solid with little interstitial liquid; Archaean mantle of Penin.~mr India. The • upper mantle itself could be exposed. slightly lower silica and excess MgO in Mysore The Mysore ultramafics, from their lack of ultramafi~ compared to the Singhbhum rocks metamorphism, the alteration along their is reflected in the forsteritic ofivine in Mysore boundaries, and their strained olivine, appear and the more fayalite-rich variety in $inghto fit into the second type above. bhum. Partial fusion of peridotite or pyrolite According to Narayanaswamy (1970) a mass under different P - T conditions also may give of ultramafic peridotite liquid must have in- rise to these chemical differenceL truded first into the lower crust; during later A comparison of the composition of olivines orogeny this was intruded, cold, into the upper from fresh dunites and peridotites of Mysore crust. Gravity studies over these ultramafics, with those reported for other ultmmafics in as mentioned earlier, have indicated that these this area (Vm'adm'ajan 1970) and the upper lenses are connected below to a large mass, mantle olivine compositions of Green (1964), which supports the above view. Of primary Gwynethe & Challis (1965), and Carter (1970) importance in connection with the upper is given in Table 2. There is not much variamantle studies is the bulk composition of the tion from sample to sample and all oxides are parent magma which gives rise to differen- similar to the reported compositions in the tiated bodies. An important method of de- proposed mantle rocks. Total iron is almost termbJing this is the summation of the identical though ferric iron is higher in the chemical compositions of all the differen- Mysore olivines, due either to the level of tiated bodies combined in proportion. Another oxidation or the depth of generation of the method is to determine the chemical composi- magma from which these rocks have formed. tion of chilled borders of intrusions. A com- MgO is similar in all olivines, whereas CaO parison of various hypothetical and estimated is slightly higher in the Mysore olivines. The probable mantle compositions with those of Mg/Fe ratio is greater in the latter compared the average composition of Mysore ultramafic to reported values. rocks is given in Table 3. The interrelationships between different The average dunite and peridotite of Mysore trace-elements (Fig. 8) demonstrate the posihave less silica, and the pyroxenites and tive correlation between Cr and Co. In dunites amphibolites slightly more than any of the and peridotites their concentrations are higher reported mantle compositions. The average than in the amphibolite. Cr and Ni show compiled from the Mysore ultramafics (Table random variation though their concentrations 3, column 5) resembles in the concentrations are higher in dunite--peridotite, with similar of many oxides the pyrolite of Ringwood variation in Co-Ni. This distr£oution of trace0966) and Reay (1965), the peridotitic koma- elements suggests that their concentration is trite from Africa (Viljcen & Viljoen 1969), and higher in the parent magma, which was more the estimated mantle of Carter (1970). Perido- heterogeneous at the time of formation of the titic komatiite from Singhbhum (Viswanathan dunite-peridotite. A definite differentiation & Sankaran 1973) has !ess MgO, FeO and trend had set in when pyroxenites and amphibolites had formed. These dunites and peridom o r e S i O 2. These ultramafics belong to the high Ca, low AI group (Fig. 6) and distinctly tites may belong to the residual crystal mush
88 Divakara Rao et al, II+00
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of peridotitic komatiite magma from which probably basaltic liquid may have been separated (Fig. 7). Glikson (1971), on the basis of his studies of isocficmically metamorphosed Archaean igneous suites from Transvaal, Rhodesia, Western Australia and Canada, has suggested that the Archaean mantle had higher concentrations of siderophile and certain transitional elements than the pre~ent mantle. This also has been suggested in the case of Peninsular India (Naqvi & Hussain 1973b). Hart et al. (1970) suggested that the Archaean mantle was less depleted in Rb, a possibility also supported by data from Canada, Western Australia (Hallberg 1972) and the Mysore ultramafics (Divakara Rao & Satyanarayana 1973). Regional geochemical investigations in the Canadian Shield have revealed important variations between Archaean and Proterozoic terranes, mainly in relatively high Na, Ca, Mg, Ni and Cr and low K, Ti, U end Th for Archaean terranes (Fahr|g & Eade 1968). Work on the Archaean amphibolites of Mysore
State has also indicated higher concentrations of Co, Ni and Cr than in younger oceanic basalts (Satyanarayana et al. 1974) suggesting higher concentrations of these elements in the Archaean mantle. Table 4 gives a comparison of Permian Alpine type ultramafics from the Canyon Mountain Complex of northeastern Oregon (Thayer & Himmelberg 1968) and Tertiary amphibolites from Kumaon Himalayas (I)ivakara Rao et al. 1974b) with those of Archaean ultramafics of Mysore. If these rocks ere similar in origin, the differences in composition shown in ".'able 4 may reflect differences in mantle composition during these periods. A~chaean mantle would then appear to be less differentiated than the Permian mantle. The excess of SiOa, MgO, Na20, KzO and the abnormal concentrations of Rb and Cs in the Archaean ultramafics of Mysore indicate that the Archaean mantle may have contained more siderophile and transitional elements than at present.
Geochemis~y of D
~
ultram~i~
89
Table 4.
~SiOs "/'iO. Als~, ee~s CaO MSO Na.O K,6 MnO P,O,
Tertiary
Permian
Himalayan amphibolite
Dunitc
Peridotite
AmphiboHte Dmlite
Peridotite
AmphiboHte
47.13 0.57 13.69 2.31 8.62 8.63 14.05 1.19 0A9 0.14 0.37
33'85 0.01 1.58 4.59 2.79 0.00 41.69 0.00
41.53 0.03 1.73 2.29 6.48 1.36 41.01 0.02
52.80 1.90 14.70 0.06 10.40 7.80
38.72 0.08 1.23 6.68 2.14 1.72 46.29 0.60
49.87 0.84 8.74 2.97 7.01 8.07
4.40
39.86 0.35 1.25 4.16 3.31 1.96 46.51 0.78
0.00
0.00
0.90
0.09
0.06
0.43
0.I0 0.00
0.14 0.00
0.20 0.25
0.05 0.38
0.09 0.42
0.05 0.35
Ardmean
Conclusions Geochemical studies on A r c h a e a n ultramafics of Alpine-type from Mysore, Peninsular India indicate t h a t A r c h a e a n m a n t l e beneath t h e shield was peridotitic in n a t u r e and had h / s h e r concentrations of siderophile and transitional elements t h a n at present. Acknowledgements. - The authors are 8fateful to the Director of the National Geophysical Research Institute for his encouragament in work ,rod p m n l ~ o n to publish the data. The help rendered by Mr. V. Dlleap Kumar In the preparation of the manuscript is thankfully acimowledsed.
References Anhaeusser, C. R., Mason, R., VilJoen, M. J. & VilJoen, R. P. 1969: A reappraisal of some aspects of Precambrian 8eoloffy. Geol. See. Am. Bull. 80, 21752200. Borisenko, L. F. 1963: Some characteristics of the cllstrlbutlon of gallium In uitramaflc rocks. Geolchi. mya, 746-7$3. Cart, M. H. & Tureklan, K. K. 1961: The 8eochendstry of cobalt. Geochim. Cmmoehim. Aeta 23, 9-.60. Carswell, D. A. 1968: Possible primary upper mantle peridotlte in Norwe81an basal 8nel~ Lithos 1, 322355. Carter, N. L. 1970:Mlneralot7 and 8eochemistry of the upper mantle based on partial fusion, partial crystallis~ tlon model. Geol. See. Am. Bull. 81, 20212034. Crawford, A. R. 1969: Reconnaissance Rb-Sr datin8 of the Precambrlan rocks of southern Peninsular India. I. Geol. So¢. India 10, 117-166. Dickey, J. S. 1970: Partial fusion products in Alpinetype perldm:,es, Serranla de la Ronda and other examples. PiTth anniversary Sym. Min. See. Am. Sp. paper 3. Dlvakara Rao, V. & Satyanarayana, K. 1973: Anoma.
4.30
18.26
2.30
lously hI8h K, Rb and Cs concentrations In the serpentinised ultrmidlcs of Kadakola, Mysore and their significance. Chem. Geol. 12, 51--60. Dlvakara Rao, V., Naqvi, S. M., Satyanarayana, K. & Hussaln, S. M. 1974a: Geochemistry and oH8in of Penin-qular 8neiss, Mysore State, India. A Geul. &~c. India I$, 270-277. Dlvakara Rao, V., Naqvl, S. M., Satyanarayans, K. & Hussaln, S. M. 1974b. Geochemistry and petrosenesis of basic rocks from Bhowall, U.P. Geoi~y:. ReJ. Bull. 12, 63-74. Edelshtein, I. I. 1963: Petrolo8y and nickel content of ultrabatlc Intrudom In the Tobol-Bunyktal area of the southern Utah. Masmatlzm, Metamortlzm, Metallollertlya Urala. Akad. Nauk. $SSR Uralsk Filial, Petrolr. 319.-323. En8el, A. B. J. 1963: Geolo8ic evolution of North America, Science 140, 143~.152. Fahri8, W. F. & Fade, K. E. 1968: The chemical evolution of the Cenndlan Shield. Can. L £arth Sol. 5, 1247-1:52. Faust, G. T. 1963: Minor elements In serpentine additional data. GeocMm. Cmmochim. Acta 2?, 665-668. Gangadharam, B. V. & Reddy, O. R. 1969: Estimation of Potassium. rubidium and caesiem in standard and ultramafic recks ~ neutron-activation. Radio. cMm. A cta 11, 90..93. Gheah, N. C. 1970: Petrochemistry of the perldotito dykes from gichushuta, Blhar with special reference to the composition o f the upper mantle. Prec. 2rid Sym. U.M.P. Hyderabgul, India, 249-.360. Glikson, A. Y. 1971: Primitive Archaean element clistrlbution patterns: Chemical evidence and 8cotectonic sisniflcance. Earth & Plant. Sci. Lett. 12, 309--320. Gllkson, A. Y. 1972: Early Precambrlan evidence of a primitive oceanic crust and island nuclei of mclic granite. Geol. See. Am. Bull. 83, 3323--3344. Goles, G. C. 1967: Trace elements in ultramafie rocks. Pp. 352--362 In Wyllle, P. 1. (Ed.), Ultramafics and Related Rocks. John WHey & Sons, Goodwin, A. M. 1968: Archacan proto-continental growth and early crustal history of the Canadian shield. 23rd Int. Geol. C,ongr., Prague, Section I, 68-89.
90
Dlvakara R a t et al,
Earth and Terrestrial Planett. S l . Int, Sd. New Green, D, H, 1964: The petrogenests of the high York. temperature perldotite intrtaion in the Lizard area, Nicolas, A. 1968: Relations structurales entre le mauig Cornwall. L Petrol..5, 134--188, ultrabaxlque de Lanzo, seat satellites et la zone de Green, D. H. 1971: Compositions of basaltic magmas serla Lanzo. Bull. Seine. Min. Pdtrog. 48, 145as indicators of conditions of oils/n, application to 156, oceanic volcanism. Phil. Tran#. Roy. gee. l.ond. Nicolas, A. 1969: Ut~ rue unltaire concermmt l'odSino Set. A. 268, 707-725. de massifs ultrabaslques des Alpes occidenteles InGreen, D. H. 1972: Magmatic activity as the major ternes.,Acad. Sci. tempter Rendl~ 269, 1831-1834. process in the chemical evolution of the earth's crust Plchamuthu, C. S. 1962: Some obaervatlmm on the and mantle. Tectonophysics 13, 47-71. structure, metamorphim! and Ipmlogical evolution of Green, D. H. & gingwood, A. E. 1962: Mineral assemblages in a model mantle composition. I. Peninmlar India. I. Geol. See. India 3, 106-118. Qureshy, M. N., Bhatla, S. C. & Dlvakara Rao,~'V. Geophys. Rea. 68, 937-945. Green, D. H. & Ringwood, A. E. 1967: The g,~tblUty 1970: Preliminary Vavity studies over the ultrafields of aluminous pyroxene i~ddotite and garnet marie,s of gadakola, Mysore (abstrac0. 2rid Syrup. peridotite and their relavance in upper mantle U.M.P., Hyderabad, lndla, 130. structure. Earth & Planet. gel Left. 3, 151-160. Reay, A. 1965: Mantle composition and partial fusion Gwynethe, A. & EhaP~, G. A. 1965: The origin of of possible mantle material. Ph.D. Thesis, Univ. of Newzealand ultramaflc intrusives. I. Petrol. 6, 322Leeds, Leeds, England. 364. Rlngwood, A. E. 1962: A model for the upper mantle. Hallberg, J. A. 1972: Geochemistry of Archaean volI. Geophys. Reg. 67, 857-868. canic belts In the eastern Goldfield region of Ringwood, A. E. 1966: The chemical composition and Western AustraUa. I. Petrol. 13, 45-56. origin of the earth. Pp. 287-356 in Hurley, P. M. Hart, S. R., Brooks, C., Krogh, T. E., Davis, G. L. Mineralogy o/ the Mantle. M.I.T. Press. Cambridge, Mass. & Nova, D. 1970: Ancient and modern volcanic rocks; a trace.element model. Earth & Planet. $ci. Ringwood, A. E. & Green, D. H. 1966: An experiLett. 10, 17-28. mental investigation of the gabbro-eclogite transHess, H. H. 1964: The oceanic crust, the upper mantle formation and some geophysical consequences. Te¢. and the Mayaguez serpentinised perldotlte. Pp. 169tonophysl¢8 $, 383-427. 175 in Burk, C. A. (Ed.), A Study of Serpentinlte.,... Satyanarayana, K., Naqvi, S. M., Divakara Rao, V. & C.A. Nat. Acad. Sd. Natl. Res. Council Pubin. 118[. Hussain, S. M. 1974: Geochemistry of Archaean Hess, H. H. & Otalora, G. 1964: Mineralow and amphlboHtes from Mysore. India. Chem. Gee. (Comchemical composition o~: the Mayaguez ser~ntinlte municated). cores. Pp. 152-168 In Burk, C. A. (Ed.), A Study Shapiro, L. & Brannock, W, W. 1962: Kapld analyses o/ Serpentlnltes. C.A. Nat. Acad. Sd. Natl. Kes. of silicate, carbonate and phosphate rocks. U.$.G.S. Council Publ. 1188. Bull. 1144-,4, 56. Karunakaran, C. 1970: Ultramaflc and related rocks Smales, A. A., Mapper, D. & Wood, A. J. 1957: The in the southern Penins~llar Indic. Prec. 2nd Syrup. determination by radioactivation, of small quantities U.M.P. Hydc~abad, India, 361-376. of nickel, cobalt and copper In rocks, marine sediKushiro, I., Syono, Y. & Aklmoto, S. 1968: Melting meats and meteorites. Analyst 82, 75-88. of a peridotite nodul, at high wessures and high Steuber, A. M. & Goles, G. G. 1967: Abundances of water pressures. 1. Geophys. Res. 73, 6023--6029. Ha, Mn, Cr, Sc and Co in ultramafic rocks. GeoMason, B. 1966: Composition of the earth. Nature 211, chim. Cosmochlm. Acta 31, 75-93. 616-618. Thayer, T. P, 1969: Gravity differentiation and Naqvi, S. M. & Hussain, S. M. 1972: Petrochemistry magmatlc reemplacement of podlform chromite of early Precambrian metasediments from the cendeposits. Econ. Geol. 4, 132-146. tral part of the Chitaldurg schist belt, Mysore, Thayeg, T. P. 1970: Chromite segregations as petroIndia. Chem. Geol. 10, 109-135. genetic indicators. Geol. $oc. South AJrica $p. Publ. Naqvi, S. M. & Hussain, S. M. 1973a: Geochemistry 1. 3t;0--390. of Dh~:r~var metavolcanics and the composition of Thayer, T. P. & Himmelberg, G. g. 1968: Rock succesthe primeval crust of Peninsular India. Geochim. sion in the alpine type mafic complex at Canyon Cosmochim. Acta 37, 159--164. Mountain, O,'egon, 23rd Int. Geol. Congr. Prague, Naqvi, S. M. & Hussain, S. M. 1973b: Relationship Section 1, 175-186. between trace and major element composition of Tureklan, K, K. & Cart', M. H. 1961: Chromium, the Chitaldrug metabasalts, Mysore, India and the cobalt and strontium In some Bureau of standard Archaean mantle, Chem. Geol. 11, 17-30. rock refe:ence s3mples. Oeochlm. Cosmochtm. Act, N~tqvi, S. M., Dlwkara RaG, V., Satyanarayana, K. & 24, 1-9. Hussain, S. M. 1974: Geochemistry of post-Dharwar Turekian, K. K. & Wedepohl, K. H. 1961: Distribution basic dikes and the crustal evolution of Peninsular of the elements in some major units of the earth's India. Geol. Mar, crust. Bull. Geol. Soc. Am. 72, 175-192. Narayanaswamy, S. 1970: Tectonic setting and maniVaradar~jan, S. 1970: Emplacement of chromite festation of the upper mantle in the Precambrian bearing ulLrat)asic rocks of Mysore State. Prec. 2nd rocks of south India. Prec. 2nd Syrup. U.M.P. Symp. U. M P., H~derabad, India, 441--454. Hyderabad, India, 377--404. Velensky, V. V., Pimus, G. V., Lesnov, F. P. & Nicholls, G. D. 1967: Geochemical stodle~ in the Vas[l'ye~r, Yu. R. 1970: Chemical characteristics of ocean as evidence for the composition of the xaantle ultra~af[c rocks beloni~ing to different magmatic Pp. 285-304 in Runcorn, ~ l~. (Ed.), Mutates o/ ¢.6e~ associations a~,d their petrogenetic, significance.
Geochemistry o f Dharwar ultrmnalics Doklady 191, 141-143. VflJoen, M. jr. & Viljoen, IL P. 1969: Evt/d~ce for tho e=imenco of mobUo ez~-uslve OeridoUte masma from KomaU foema~n of the Onver~Lcht Stoup. Upper mince Faject, Geal. Sac. $7 Al~ao Sp. Publ. 2, 87-112. Vinosradov, A. P. 1962: Average contents of chemical elements in the Irhtciple types of igneous rocks of the Earth's crust. Oeochemittry, 641-664. VJswanathan, S. & Sankaran, A. V. 1973: Discovery of a kamatite in the Precambrlan of Indla and its sisniflcan~ Jn tho nature of Archaean volcanism mad of the early crust in the .'ndJsn shield. CJwr. $c/. 42,266-269. Wedopohl, g . H. 1963: Die Nickel-und Chromzeh~to
91
yon b l u / t b c h m ~ und dmmt Oh~tn,tmuzn, den Blnsuldfamm. Neuet ]ahrb. Mineral Mcmat~. 237-242. White, L (3. 1967: Ultrsbasic rocks and tho composition of the uffper mantle. Earth & Planet. ~ Lett. 3, 11-18. Wyli~ P. J. 1967: Alpine type ultmmaflc sssocktiom~ P. 135 In Wylll~ P. J. (F.d.), ffltrama//a and R d a ~ d Rocks. John WHey & Sons. Wyilie. P. J. 1971. Role of water in m g n m 81meration and initiation of diapirlc u p r ~ In the mantle. I. Geophys. Res. 76, 1328-1338.
Acceptedfor publication November 1974 Printed April 1975