diorites from kuderu and atmakuru areas, anantapur district, andhra pradesh

Prec. Indian Acad. Sei., Vol. 82 B~ No. 4, 1975, pp. 155-166

Dior i tes f r o m K u d e r u a n d A t m a k u r u areas , A n a n t a p u r Dis t r i c t , A n d h r a P r a d e s h

G. LAKSHMI REDDY AND M. S. MURTY

Department of Geology, S.V. University, Tirupati 517502

MS received 24 February 1975; after revision 30 August 1975

ABSTRACT

Diorites are mainly of two types--hornblende diorites and biotite diorites having transitions between them. The former possesses panidio- morphic and the latter allotrimorphic texture. Hornblende diorite is considered to be a magmatic rock and the biotite diorite is formed by assimilation of the hornblende diorite by granite magma.

1. INTRODUCTION

DURING the course of geological mapping of about 230 sq. km. around Kuderu and Atmakuru in Anantapur District, Andhra Pradesh (14 ° 35'- 14 ° 45' N; 77 ° 20' 27"-77 ° 29' 33" E) four main groups of rocks, namely, schistose formations, diorites, granites and dolerites are encountered (figure 1). The present paper reports results of investigations made on the dioritic rocks.

2. GEOLOGICAL SETTING

The schistose formations occupy about one-fourth of the area mapped and constitute chlorite schists, quartzites, ferruginous quartzites, granite gneisses and amphibolites; granitic rocks with their members granodiorites, adamellites, tonalites and granites occupy the rest of the area. Dolerites are intrusive into both schistose formations and granitic rocks. Diorites occur in isolated outrops in the form of xenoliths in granite; the contact between them is sharp and diffused at times. No mafic (amphibolite) inclusions are observed in diorite in the area mapped, but such inclusions may be found on a regional scale. In Kolar Gold Field area, the granite, however, is seen to be basified due to the interaction and intermingling with the hornblende schists and has attained the mineral composition of a dio- rite. The dioritic xenoliths are hornblende-rich in the central and biotite- rich in the peripheral portions.

155

156 G. LAKSHMI R.EDDY AND M. S. MURTY

3. PETROGRAPHY

Diorites are hard, massive, compact rocks with a melanocratic look. plagioclase, quartz, potash felspar, hornblende and biotite are the main constituents, and muscovite, iron ore, epidote, etc., are the minor ones in these rocks. Based on mineral composition the diorites are divided into two main units--hornblende diorites and biotite diorites. Transitional rocks like biotite-bearing hornblende diorites, hornblende-bearing biotite diorites are present in between them.

Hornblende diorite

Most of the rocks examined are coarse grained and exhibit panidio- morphic texture (plate I, figure 1). Plagioclase is fresh, twinned on Albite and Carlsbad laws and has composition An~ to An4s. Quartz is po!kiliti- tally enclosed in hornblende and has perfectly straight contact with ~ plagio- clase (plate I, figure 2). Potash felspar is minor, altered and occupies the interstitial spaces between plagioclase and hornblende. Hornblende is in

7 0 -

6 0 -

5 0 -

4 0 -

30-

2 0 -

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I CLASE

HORNBLENDE BIOTITE GRANITE DIORITE DIORITE

Figure 1. Modal mineral variation diagram of averages of hornblende diorite, biotite atad granite.

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G e o l o g i c a l m a p o f t h e K u d e r u a n d A t m a k L ~ u A r e a s , (facing page 156)

G. Lakshmi Reddy and M. S. Murty

Plate I

Proc. Indian Acad. Sci., Vol. 82.~1, No. 4, 1975, pp. 155-166

Figure 1. Panidiomorphic texture in hornblende diorite--Crossed Nicols, 65 ×. Figure 2. Sharp contact between quartz and plagiuoclase in hornblende diorite--Crossed Nicols, 65 ×.

Figure 3. Developmen~ of epidote at the expense of plagioclase in biotite diorite--Crossed Nicols, 65 ×.

Figure 4. Highly elongated grain of apatite enclosed in anhedral and highly undulant quartz grain--Crossed Nicols, 65 X.

(facing page 156)

Diorite rocks 157

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158 G. LAKSHMI REDDY AND M. S. MURTY

coarse, subhedral to euhedral grains with light green to dark green pleochroism and has 2V = -- 66 ° to -- 70 °, Z A c = 14 ° to 18 °, Nz = 1.680 and Nz - - N z = 0.022. Chlorite is secondary to hornblende. The modal compositions of hornblende diorites are given in columns 1 to 6 of table 1.

Hornblende from hornblende diorite is separated with the help of Frantz isodynamic separator and analysed chemically; the analysis, structural formula and Niggli values are given in table 2.

The structural formula reveals the abundance of aluminium with four- fold co-ordination, substituting for si in Z-group, and remaining in Y group

Table 2. Chemical analysis, distribution of metal atoms, Niggli values, etc. of hornblende

Weight Molecular No. of Constituents percentage proportions oxygen Metal Basis

atoms atoms 24 (0)

SiO~

TiO~

AlzO3

Fe2Oz

FeO

MnO

MgO

CaO

Na~O

K20

P205

H,,O

40" 02 0" 667 1' 334 0" 667 6' 05

0.55 0.008 0 '016 0 0 0 8 0 '07

11" 60 0" 114 0' 342 0.228 2 '06

1 "58 0"010 0"030 0 '020 0'18

13' 68 0" 190 0' 190 0' 190 1- 73

0 '82 0'0~1 0.011 0"011 0.10

13" 82 0" 345 0" 345 0 ' 345 3" 13

14" 14 0"252 0"252 0-252 2' 29

1-01 0' 116 0.016 0"032 0"29

0' 52 0" 005 0' 005 0' 010 0' 09

0.01 . . . . . . . .

1" 86 0-103 0' 103 0-206 1" 87

F = 24/2. 644 = 9"076.

Structural formula: (OH)1.87 (Na, K, Ca) 2. 67 (Mg, Fe 11, FO 11, Ti, ML,, A1)~.a~ (Si, A1) 8 Ozz. Niggh values :

Si 69.98 al 11.96 fin 59.39 c 26- 44 alk 2" 21 Q 14.89 L 22- 97 M 62' 14

D~riW rocks 159

indicating that the hornblende is formed at high temperatures (Thompson 1947, Buerger 1948; and Kushiro 1960), probably at magmatic temperatures.

The Niggli values of the hornblende are in the range of values of common hornblende. The AI +3 + Fe +3, Na +1 + K +a and Ca +2 values classify the hornblende as pargasite according to Sundius (1946). The Q, L and M values have a position in the common hornblende field of eruptive rocks. Thus, the chemical interpretation of the hornblende in diorite lends support to its magmatic lineage.

Biotite-bearing hornblende diorite

The hornblende diorite transforms into biotite-bearing variety as a consequence of part of the hornblende being converted to biotite. The panidiomorphic texture of the hornblende diorite is still retained in the biotite-bearing variety in spite of the change. The modes of two rocks are given in columns 7 and 8, table 1.

Hornblende-bearing biotite diorite

The idomorphic nature of plagioclase and hornblende is disturbed and the rock exhibits aUotrimorphic texture in contrast to panidiomorphic texture of the hornblende diorite. Biotite is increased and hornblende has become subordinate. The plagioclase has changed its composition to An40. The modes of two rocks are given in columns 9 and 10 of table 1.

Biotite diorite

The rock exhibits aUotriomorphic texture. The constituent minerals-- plagioclase, quartz, potash felspar, biotite and epidote are anhedral. Plagioelase is altered, twinned and has composition An4o. Quartz occurs as coarse discrete grains and also as inclusions in biotite. Potash felspar is highly altered and occurs as small interstitial grains between plagioclase grains. Biotite is in coarse laths having pleochroism from light yellow to dark brown and has birefringence ( N z - Nz)=0.050. Epidote crystals (plate I, figure 3) are developed at the expense of plagioclase. Highly elongated grains of apatite (plate I, figure 4) are observed enclosed in an- hedral and highly undulose quartz grains. The modes of the rocks are given in columns 11 to 17 of table 1.

Amphibolite

The rock exhibits schistose texture and is composed of hornblende (80"7~o), plagioclase (15.6~), quartz (1.7~), sphene (1 .7~ )and iron ore (0-3~). Hornblende is in stout grains and has green colour, and light green to yellow pleochroism; it has 2 V = - - 7 2 °, Z/ \c = 1 7 ° and Nz- -N:c=

B 3--Oct . 75

160 G. LAKSHMI REDDY AND M. S. MURTY

Table 3. Chemical analyses of diorites--C.I.P.W. Norm and Niggli values

CoJastituents 1 2 3 4 5 6 7 8 9

SiO s

T ies

A!:O a

F e t e a

F e e

MnO

MgO CaO

Na20

K,O P~O6 H20

Total

C.I.P.W. norm

Quartz

Orthoclase

Aibite

Anorthite

Corundum

Diopside

Hypersthene

Magnetite

Ilme~aite

Apatite

Niggli values

d

fm

alk

54-85 51.52 53.19 59.59 66.80 65.20 63.86 72.27 44.95

0-23 0.25 0.24 0.19 0.66 0.25 0.36 0.14 0.95

22.22 19.60 20.91 19-56 16.59 17.70 17.95 14.50 14.02

2-79 4-70 3-75 3-70 0.67 1-21 1.86 0.39 7.62

4.76 6.83 5.80 5.05 1.75 1.46 2.75 1.67 10.40

0.06 0.06 0.06 0.02 .. 0 '06 0.03 0.04 0"08

2.68 4.23 3.45 1.63 4.06 4.23 3.31 1.20 7.04

6.77 7.88 7.32 4.39 4.29 4.00 4.23 2.43 12.70

3"86 3.51 3.69 4"08 3.18 3.45 3.56 3.70 0.82

0.92 0.72 0-82 0-64 2-09 2-45 1-73 3-55 0-02

0"05 0.05 0"05 0"06 0.12 . . 0.06 0.14 0.21

0.96 0.51 0.73 0.62 0.45 0-06 0 '56 0.58 1.51

100.15 99.86 100.01 99.53 100.66 100.61 100.20 100.61 100.32

7-56 2-82 4.62 19.56 24"48 19.92 21.54 29.20 1.86

5"56 3-89 5.00 3.34 12.23 15.01 10.01 21.13 . .

33.01 29.34 31.44 34.58 27.25 29.34 30.39 31.44 6.81

32.80 35.86 35.31 2.13 20.57 19.74 20.02 11.12 34.47

2.75 . . 0-92 4.49 1.53 2.04 2.86 0"41 . .

. . 2 .04 . . . . . . . . . . . . 22.88

12"90 17-69 15-86 9-68 11-78 11-92 11-20 5.54 21-50

4"18 6.73 5.57 5.34 0"93 1.86 2.78 0.70 11-14

0-46 0.61 0"46 0"46 1.37 0.61 0.76 0.15 1.98

0 '34 0.34 0.34 0"34 0"34 o. 0.34 0.34 0"34

156-80 131"10 142.90 119.10 248.20 230.80 225.00 352.30 94.19

37.40 29.32 33.14 38'50 36.34 36.88 37.20 41.55 17.48

29"33 39.52 34.80 31.23 29.98 33.47 30.87 17.20 52.37

20.75 21.54 20"94 15"84 17.17 15.05 15.86 12.58 28.52

12.52 9-62 11.12 14.43 16-51 17.60 16.07 28.67 1.63

1 and 2 Honlblende diorites; 3 average hornblende diorite; 4 to 6 Biotite diorites; 7 average biotite diorite; 8 average granite; 9 amphibolite.

Diorite rocks 161

0"021. Plagioclase (An~) occupies the interstitial spaces of the hornblende grains. Quartz shows undulose extinction and is intimately associated with plagioclase. Sphene is brown in colour with spindle shape.

4. CHEMICAL PETROLOGY

Two hornblende diorites and three biotite diorite ° are chemically analysed and the analysis together with their C.I.P.W. norms and Niggli values are given in table 3. As per Gulson et al's (1972) classification, all the diorites appear to be normal and agree with the limits set by them for SiO2 and K~O. In the norm, quartz and orthoclase are less, and Colour Index (Z' mafic minerals) is more in hornblende diorites than in biotite diorites. The biotite in biotite diorites is partly responsible for high quantities of K20 (and consequently high quantity of orthoclase).

90

8O

70

6O

%0

d

4O

3O

.20

tO

/ O

Q.UAR T Z An-CONTENT

POTASH FELSPAR

~ . o M A F ICS

! I AMPHIBOL ITE HORNBLENDE BIOTITE GRANITE

DIORITE OIORI T £

Figure 2. Modal mineral variation diagram of amphib01ite, average hornblende diorite, aT~'ase biotite diorite and average granite,

162 G. LAKSHMI REDDY AND M. S. MURTY

5. D i s c u s s i o n

The petrographic description makes it clear that the hornblende diorite and biotite diorite are the main types having biotite-bearing hornblende diorite and hornblende-bearing biotite diorite as tran, itions between them. From this association, it is visualised that the hornblende transforms itself succesqvely into biotite, g;ving rise to the two trandtional varieties and ultimately to biotite diorite. This transformation of hornblende into biotite that requires potash is supplied by the adjacent granites.

In order to ascertain the probable role played by the amphibolite in the evolution of biodte rocks, a diagram is drawn (figure 2) for the modal

2

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2 I o

15

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5

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J i p . . . . . J ~

----_____

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DIORITE DIORITE

K20

No20

CoO

M 9 0

Fe O

F¢203

Ai203 51O 2

Figure 3. Variation diagram of the chc~mical constituents of amphibo!ite, average hornblvndq diorite, average biotite diorite and average granite,

Diorite rocks 163

minerals of amphibolite, the average hornblende diorite (6), average bio- tite diorite (7), and average granite (129) which indicates the decrease of colour index and increase of quartz and potash felspar from amphibolite to granite with intermediate values for hornblende diorite and biotite diorite. Plagioclase has shown a fall from hornblende diorite to granite through biotite diorite on the one hand, and to amphibolite on the other. The An- content also exhibits a similar relationship.

The variation diagram (figure 3), based on chemical constituents for amphibolite, average hornblende diorite, average biotite diorite and average granite shows increase of SiO2 and K20; and decrease of CaO, MgO, FeO and Fe~O3; from amphibolite to granite with intermediate values

20--

lO--

O--

30- -

2 0 ~

iO--

O--

6 0 - -

4 0 - -

2 0 ~

O--

40--

20--

O--

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3 0 0 -

200 - -

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DIORITE DIORITE

0.t~,

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eLI

Figure 4. Variation diagram of the Niggli values o f amphibolite, average hornblende diorite, iaverag¢ bigtite diorite and average granite,

164 G. LAKSHMI REDDY AND M. S. MURTY

for hornblende diorite and biotite diorite. Na~O has shown an unchanged position from hornblende diorite to granite, but shows a decrease to amphi- bolite. AlcOa has shown a fall from hornblende diorite to granite on one side and to amphibolite on the other.

In the modal (figure 2) and chemical (figure 3) variation diagrams, if amphibolite is omitted in the plots, the constituents either show an increase or decrease with no fluctuations indicating a genetic link of granite with hornblende diorite and biotite diorite but not with amphibolite.

A variation diagram based on Niggli values (figure 4) for amphibolite, average hornblende diorite, average biotite diorite and the granite shows a gradual increase of si, al and alk and decrease of c and fm from amphi- bolite to granite. This plot, in contrast to the other two (figures 2 and 3), shows an evolution of diorites through assimilation of amphibolite by granite.

In general, the variation diagrams clearly show the systematic behaviour of the curves with either rise or fall from hornblende diorite to granite through biotite diorite; but deviates from that behaviour when amphi- bolite is also included.

The absence of amphibolite inclusion in diorite, the non-systematic behaviour of the modal and chemical constituents on variation diagrams when amphibolite is included in plots, the higher An-content plagioclase in hornblende diorite in comparison to amphibolite and the schistose tex- ture of amphibolite in contrast to the panidiomorphic texture of hornblende diorite are the features that indicate the non-participation of amphibolite in the evolution of diorite,~. Furthermore, the textural evidence and chemical mineralogy support a magmatic origin of hornblende diorite. Biotite diorite has an intermediate position between hornblende diorite and granite, its origin being linked with them.

Since the mineralogical and chemical products of reaction between crystalline basic rocks and acid liquids may be the same as those produced by conventional crystallisation differentiation, only textural evidence remains as a guide to the origin of associated intermediate rocks. The section on petrography shows that hornblende diorite has panidiomorphic texture with perfect euhedral crystals and biotite diorite has only allotriomorphic texture with anhedral minerals. Had the biotite diorite been magmatic like hornblende diorite, it also would have possessed panidiomorphic texture. As it does not possess panidiomorphic texture, it is reasonable to consider that biotite diorite is a hybrid product between hornblende diorite and granite. The following mineralogical features confirm hybrid origin for the biotite diorite: (i) transformation of hornblende to biotite, (ii) acidi,

Diorite rocks 165

fication and development of sodium-rich felspars and (iii) big crystals of apatite. Nockolds (1932) described abundant apatite in the felspars of hybrid rocks and concluded that the whole of the felspar must have recrystal- lised.

As a process, hybridisation involves " . . . . . . . . . . . . the action of an independent magma upon an already consolidated rock or the comming- ling of two independent magmas of different chemical composition . . . . . . " (Thomas and Bailey 1924). The concept of an 'independent magma ' acting upon an earlier rock distinguishes hybridisation from crystal-liquid reaction and solid-solution during normal fractional crystallisation. However, the independent magma may only suggest the period of time between the emplacement and crystallisation of one rock and its reaction with a later magma with which it may be comagmatic.

In this area, assimilation probably occurred through the mechanism of reciprocal reaction as suggested by Nockolds (1933). In accordance with this mechanism, the granite magma was contaminated because of an exchange of chemical components between the magma and the hornblende diorite inclusions. This exchange was facilitated by the presence of volatiles, largely water, supplied by the intruding magma. Biotite prevailing in dio- rite near the contact is an evidence for the availability of volatiles.

As a whole, the data support the conclusion that the peripheral biotite diorite is a hybrid product resulting from the assimilation of hornblende diorite by the intruding granite magma.

ACKNOWLEDGEMENT

The authors thank Prof. M. G. Chakrapani Naidu for providing facilities to carry out this work. GLR acknowledges financial assistance from C.S.I.R.

REFERENCES

Thompsoa, J. B., Role of aluminium in rock-forming silicates, Geol. Soc. Amer. Bull. 58 1232 (1947).

Buerger, M. T., The role oI temperature in mineralogy, Amer. Min. 33 101 (1948).

Kushiro, I., Si-AI relation in clinopyroxenes from igneous rocks, Amer. J. Sci. 258 548-554 (1960).

Suadius, N., The classification of the hornblendes and the solid solution relations in the amphi- bole groups, Sveriges, Geol. Undersokning Ser. Arsbok 40 1-31 (1946).

166 G. LAKSHMI REDDV AND M. S. MURTY

Gulson, B. L., Lovering, J. R., Taylor, S. R. and White, A. J. R., High Kdiorites, their place in the calcalkaline association and relationship to andesites, Lithos 5 269-279 (1972).

Noekolds, S. R., The contaminated granite of Bibette head, Aldarmey, Geol. Mag. 69 433--452 (1932).

Thomas, H. H. and Bailey, E. B., Tertiary and Post-Tertiary Geology of Mull, Lock Aline and Oban, Mern. GeoL Surv. U.K. (1924).

Nockolds, S. R., Some theoretical aspects of cc, ntamination of acid magmas, Jour. Geol. 41 561-589 (1933).