Chapter -IV
The last phase of igneous activity in the study area is marked by mafic dykes. The
'dyke rocks have significance as they mark recurring of crustal extension during which
enormous quantitj of niafc material fiom the mantle was tmnstkmd to the crust. 'l'heir
study is expected to throw light on the crustal evolution of Peninsular India. '(bey cut
across all other formations. They are conspicuously exposed on mounds, hillocks and ulso
in plains along with the other rocks. In recent years. rnafic dyke rocks have attilined
cconomic importance becnusc of tlreir ornamcntnl quality.
FIELD STIIDIES:
The contact between the lnafic dykes and granites is rather sharp and marked by
chil!ed lnargins (Plate VI, Fig, -3). Ilykes arc bouldary or hlock!,
111
nature and cxhihi~
spheroidal weathering. The dykes generally arc unaltered and standout as ridgcs. I'he
widti1 of the dykes varies tiom 10-25 meters, but some me as wide us 50-601n. Some
d>l\es show zig-zag pattern and sonic timcs porphyritic nature with plugiocluse
phcnocrysts.
Kaju et al. (1070) have noticed threc prominent dyke trcnds
--- NW-Sli,
\J1NW-
I:SI' to L W . and NI:-SII in parts 01' Anantapurdistrict. 'l'hc), observed on ;~criul
pliortrgraphs that the f-:-\I\' trcnding dykcs c u ~across NW-SII oncs and both ~ h cd) kt:.;
cut
:tt.c
I-! the NE-SW dqkcs.
Rased on landsat imagery map. I)rurq (1084) has delir~catcdthree distinct tr~ilds
NUJ-Sf:, NNE-SSE, and ENE-WSW in so~rthernpart of the ('uddapnh basin. ('tlrtly
(ILIXO) has noticed three prominent dyke:; trending E-W, NW-SIC and N-S in porls of
Al~~!;lpur,
Kurnool and C'uddapah districts. 1 lc ulso observed t l ~ ali-W
~ trend is prominent
in 11w southern part, N W-SE in the central part and N-S in the northern part.
he dykes of the area exhibit threc distinct trends NW-St.;. NE-SW and I<-W; with
a scarce N-S trend. These dyke trends corre:;pond with the trends of the joint patlerns and
nmior faults in the study area. It is nhsen~edthat the mafic dykes have emplaced along the
joint planes and other structural weaknesses of the earlier formations.
Gcochronology: There is no unanimity of opinion regarding thc age of the dykes. Sumc
considered their age as pre Cuddapah (Bruce foote, 1863; King. 1872; Muthy. 1904,
1987 1. some others as C'uddapah (Crawforti. 1969) and a few olhers as post-Cudclapdl
aye (Pascoe. 1950). Kcliahle intbrmation on the chronc~logictlldata on dykes is very
scanty. The data available in the literature art. hole-rock ages. which are nor reliable due
to argon loss due to superposed thermal ac~i\~ity.
In continuation of the efbrts to decipher thc chrc\nologj*ol'important d!kc events
in the Dharwir craton. C'raw1bl.d iltld C'on~pslon( 1973) ga\e Rb-Sr i1gl.s 01' 1 i8-7-t-147Ma
for basic flows from lower C'udliapahs atid indicatcti that the age oi' thc hascmcnt might
bc as old as 1700Ma. Hhattuc1iar:ji (1087) ticscribcd thc cpisociic dykc i n ~ n ~ s i v nins 1l.1~
Ilhawar hasetncnt that took place o\.cr a period between (350 and 2300
Mi1
oil thc basis of
K-Ar dcterminntions.
.
Ihc aiailable peo-chri3nc)logical data on thc n~;tlicdykcs around thc ('uddapah
basin and castcrn Ilhanvar craton suggest at least three m;!ior episodes ul' tlt kc iu!rusive
activity durir~g 1700-1900 Ma.
1300- 1500 Ma and 640-1 200 Ma v;spcctively
(Padmakun-lari and Dayal. 1087: Malliknrjuna Kao el nl. 1005: Snrkar ancf hlullik, 10')S 1.
Most of thc dl hc xharms around cuddapah basin havc more than 1600Ma c\ccpt alkaline
dykes, which g ~ v younger
e
ages (Murthy et al. 1087).
'I'hc I:-&' trending niatic d ~ l , ccutting across I<;ln~agiriSchist belt
ill
i\nantapur
district. Antlra I'radesh yielded SIII-Nc1 niineral apes ol' 7400 + 50 M:l (/;lcharia
1995). 'l'hc Sin-NJ and lib-Sr mincral ages of 1:-W trending gobbro dyhc
01
et
al.
R4;ltldur in
Mahaboobnagat district gave 200 Ma age (Murthy & Sarma, 1992).'1Ill., rnJicatcs that
the E-W trending dykes are older rhan the two trends.
All these references put togcether lead to the conclusion that the dyke\ ot'the stucly
area are obviously pre-Cuddap:ihs and the Cuddapah sedimentation st;~rtcdthere after,
limited chronological data limits t l ~ cscope of further discussion regarding thclr age.
PETROGRAPHY:
The matic dykes indicate that they are melanocratic in colour, uph;\nitic ta subphaneritic in texture. with grain sizes ranging from 0.2 to 1.56rnm. Most of' then1 display
ophiticl sub-ophitic, intergrsu~ularlintersertal, and vitrophyric textures (l'lale X, Fig.
2&3). Variations in the grain size are observed across the dykes; central portions are
coarse, intermediate portions medium and marginal portions fine to glassy. The ophitic
WXtuw is present in the cmtrat portion. intcrgranular texturc in thc marginal portinn and
i
v h p h y r i c texture in chilled portions. l'hc dykes consist of plapioclitse. clinopyrcviene.
.afivfne.micro-pegmatite. quartz anti iron o ~ c Microscopic
.
study shows that niost ol'the
samples are fresh. whilc a ti.& 01' them show deuteric iillcfi~tion eftkcts such us
cazrlitisation of the p\,ro:icnes ~uid saussur~tisntiolor thc pli~gioclnse giving rise to
ohlorite. epidole and calcite. Hascd on mincralog! thc? arc divicicti into thrcr varieties.
1 . Olivine-orthop!,roscnt' doleritc
2 . Dolerite and
3 . Ortho pyrosc~ic-rnicropegn~utitc
dolcritt'.
Olivine-orthopyroxene dolerite: It is a c~:i~~sc'-graitiC'd
rock tvitli t\p!lifi;'
LO
sub- phit tic
tcjxhue !Plate X, Fig. 2 ) representing the central portions. 'fhc mineral a ~ ~ ~ ~ i l h l i t g e ~
exhibited by dykes are olivinc, clinopyrosc~tc,plagiclclasc. orthopyroscnc und iron ore ;IS
the ma.ior constituents tvith chlorite anti cpidotc scen as altcrcd procli~cts.Olivinc ui:h
2V= (-) 75" to 80" and
NY-Ns-0.036-0.0.10. is cuhcdral with numcrous cracks lined by
dusty iron ore. Olivine gritins are sccn poikilitically cncloscd in clynopyro~cn~
;tntl
plagioclnse, Both olivillc allti l'yroxencs arc subjected to altcratioti. ('linopyroxerie
\L ill1
2V= 46"-56": %"c=3OU-45":Nz-Nx=0.023--0.03. cxhibits a single cleavage, and most ,)I
the grains are twin~~eci.
( )rthopyroxcnc (2'v'
(
-)
60"- 65"; N/.-N\: 0.0 13-0.14) rc1,laccs
and sometimes cokplcccl> pscudomorphs tllc clinopyroxcnc. I1lagioclasc feldspar (Ail ,,,,
so) is lath-shaped with ucah to strong normal foning, and occurs as pltenoclys!~as well
:IS
in groundmass. It displays C'arlsbad. multiple and pericline twinning. Opaque minerals
mainly magnetite, ilnienite, hematite and p),rite. 7'he modal compositions of doleritc
d'ykes are given in 'Sable - XIIT.
Dolerite: Ir is a medium-grained-rock exhibiting intergranular texture with pyroxenes
occupyir~gthe interstices berween plagioclasc laths. Plagioclasc (Anss.h8) laths show
twining of Albite, Carlsbad and Albite-Oarlsbad laws. Clinopyru~cneis augite with 2 V
4
'
-48"; ZAc=40" biz-Nx-0.023- 0.024.
-
The anhedral quartz grains arc found occupylny
?,he interstices plagioclase and pyroxene grains. Magnetite and ilrnenite constitu~etnc
m u e s . Modal compcsitions of these rocks arc given in Table -XIII.
~hopyroxene-micropegmatite dolerite: It
*r\crtal
IS
a medium to
to vitrophyric. <tnd pilotaxitic textures representing
.I 1
dyke. In some. pyroxene and plagioclase clustered cshibiting glomrrc~porphyritictextun.
'i
in a glassy groundmass. C'linopyrosene. orthop~.rcwt.ne, pingiocliise. yuartz.
rr~icmpegmatite and opaque constitt~te the niinerillogy of' the mck. 'I'hc pliipioclase
phenocryst is twinned (Alhite. C'nrlsbnd and Albite-Carlshad) and has composition (Anso.
60).
Clinopyrosene is identified
ils
augite with 7V-42"-55".
%Ac-42"-47"nnci Nz-
N~=0.07-7-0.078:while suh-calcic nugitc. gr;lins are unt\\inrd and hnve 2V ,.34''. %"c-38".
Orthopyroxene shows yello\r
to
pi~thplt.ochrr\ism rtnct h;ts 2V-.
(-) 65"
ttr70" ;tnd N I -
Nx=O.Olh. Quartz and n1icropcg1113titc ; ~ r r Soi~nd o c c i ~ p y i ~the
~ g intcrsticcs b~t\vccn
plagioclasc and pyroxene grains. 1:pitlote and chlorite ;~rcti~und rrt thc cspcnsc of'
p\,rosenc and plagioclase. Modal compositions of' these rocks are pivcn in 'l'ahlc -XIV.
Older group of dykcs are char:~ctt.rized by intergranular tcxure with the presence
of clouded. ~ o n e dand twinnec! fkldspars with strain shadows. While youngtsr group of
dykes by the presencc of sericitiscti kldspars and frcsh pyroxenes.
Rascd on the tield relationships and petrographic characters, eight rrprcscntiilive
dyke samples have bcen chemicallq annly~cdfor mi~ior-ancl trace clcn~cnts.'l'hc chemical
analysis togotl~crwith ('IPW nornl. Niggli values and otlicr computed valucs ; ~ r cgiven in
the Table - X V .
'The SiO, in the mafic dykcs c,anges from 46.20 to 55.35 wt%; 'l'i02 Lkon~0.65 to
1.25%; AI2C)3 liom 1 1.70 to 14.83'%1;total iron as FeO horn 8.7 to 15.0 %I;
'
MnO is low
and varies from 0.10 to 0.20%; MgO varies from 4.36 lo 8.68%. Mg numbcr varies from
37 to 56, the highest being in the carly formed ones and thc least being in thc last formed
ones and thus suggesting the evol\ui nature of matic magma (ilmson alld Lagmuir,
1978; Vijaya Kumar and Ratnakai, 1001 and Surdas Singh and Sanlosh Kumar, 2010).
CaO varies from 9.77 to 14.84%; and this range in the dolerites of the area is within the
limits of chenlical screen (5 to 15
')/;I)
for basaltic and gabbroic rocks (hga:~son,1967).
NazO shows a narrow range of varion from 1.69 to 2.85'Yo, KzO from 0.45 to 1.40% and
P205from 0.08 to 0.58.
The mafic dykes of the study area compare in major and trace elements to the
continental thnleiites (Condie, 197h). According to Muller and Rogers (1973) the
continental tholeiitets are characteri~edby their .412C)>
(-
12°/~),low K/Kb ratio ( 8 W o
i
<:350)and high Rb/Sr ratio (average 0.17). In the study weo. the n\.Crage mafic dykcs
shot\ AI:Ol= 13.37. KiRh--2 1 J and Rb/Sr=0.28. I'hesr values correspond ta those of
continental tholciites.
I'he various chemical classification cliagranls suggest the parctlt rnt~gnia is
tholeiitic in nature. 'I'hc matic dyke rcrcks i11.c either olivine- or qtrcirtz- nom~ati\~c
indicating them to he tholc~itcs(1.odcr and I'illc!. 1902). 'I'hc studictl mnlic dykes cnn be
classified as basalt and hasultic andesite i~ccordingto the total alkali-silica diagram (I'ip.
36; LC 13as et al. (1986). I'hc dykc rocks plot in tholciitc licld in SiO?-(Na?OJ-li?O)
diagrarn (I:ig. 37) and Si02-T:cO,/MgOdiagram (I:ig. 3 8 ) . AFM (Nil?O+K20-l:cC),-MpO!
diagram (Fig. 39) has shown tholriitic trend of niafjc dykes whcrcas tcin:lry rZ120-i(FcO,t'l'i0:)-MgO contents of rnalic dykes exhihit a fairly unifhrtn coliipc~sitior~s
ol' highI:e tholeiitic type (Fig. 40). In the KrO - 7'iOl -P20! trilinear digri1111(I:ig. 41 : Peclrcc el
al. Ic)75). the dykc rocks Sail in the continental licld.
PETKOG ENISIS:
llasaltic volcanisn~ is a common mlunilkstation of exlcnsion;~l tectonics ;~nd
intrapliitc magmatism. Malic dykes represent
basaltic magma from rn:ilitlc
lo
{tic
most imporla~~t
a\LLntlcsfbr transfirr o f
upper crust (ili~lls.1082; Ilarnst
; L I I ~ Il~~chan.
1007).
;LIICI
oAcn intcrprcted as fecderb to tlood basalts. particularly in contincn~;~l
;Ireas (I'earce c! ;I!.
107s). T)ykcs have been rcponcd in varied tectonic setting ranging I'rorn passive to activu
continental margins (C'allol and Geoffrey, 2004; I'eng et al. 2 0 0 8 ) . '1'11c numerous dykcs
ant1 dyke swarms in South lndian shield indicate periodic crustal clil;;tion during earlyProtero~oicperiod (Murthj . I 995).
'l'hc dense swarms of dorerite dykes in and around the ('i~ddspah basin wcrc
studied earlier by Narayanaswami (1 966), Hhattacharji ( 1986). llrury ( 1984). Chetty
(1989) and Mallikarjuna Kao et al. (1995). A rccent geochemical and petrogenetic study
of the djkes around the western margins of the (uddapah basin brought out two distinct
groups of dykes, one enriched in high field strength (HFTP) and relative to the other
(LI:'J-P) suggesting independunt magma sources. 'I'he LFTP dyke pclpulation is chemically
unlike any plume or island magmatism and differs fiom continental-rift- related within
plate thaleiites (Rao and Puffer, 1996; Ramarn and Murthy, 1997). The mafic dykes ot'
the study area are a part of dyke suwnls mound C'uddupnh basin. The origin of' the
present dykes can also be explained hy the sanle mechmisn~.
j.,
The pe~rographic tkaturc.5 re\.c;ll thni olivine cr~,stnliizcdtirst folic~\~edh?
orthopyrosenc. clinopyroxent. and plagic)clase. The grain size variutio~ilion1 the nlrtlpin
to central portion niay he due to dllfkrent vcloci:ies in the zones of thc dykes nnd the
tcstural variation by different rates ot' cooling in diif;.rcnt zoncb (%'inkla.. 1'94'):
Sadashivaiah and Ikramuddin, 1972; I)cvaraju and Si1d;tsivaiiih. 1900: I,at>'pov ct at.
'7007). The presence of' quartz in most nt'thc dolcrites indicates that the purcnt nltrgma is
thuleiite which on diif'erentiatio~l gave rise to various members of dolcrites. 'l'he
occurrence of' nratic dykes in grnnitc t)nsement indicates uplift of the bilscinc~~t
to thc
level at which brittle fractures coultl clc\elop and dykes he intruded (Senpuptu. 1')9.i 1.
-.
I he basement rocks were rigicl and yielded to ir:lcturcs for deep-sentcd li~eament
which werc connected to the magma source that provided dircct passage to uppcr cruslal
Icvcls. through uhicli the mag111ii hild emplaced as dyke swarms. 14upert :rnd Sparks
(1985) designated a rclevant theorctic~~l
triode1 in which tins~lticmagmas lractionatc: to
mrying degrccs in deep crustal m:lgrna chambers and thcy itre enlitled pc~rioriically
through fractures to the surface. Sucl~:I process may h ; ~ t ehccn opcrative d u r i ~ ~cpibodic
i:
dyke etnplaccmcrit in and around thC ('~iddapahbasin. 'l'hc sudden conpcaii~~g
;rtrci rapid
cooling at the margin of the cooler colln:ry rocks resullcd
it1
the developme111of chilled
and glassy phascs. l'he centre portions are coarse-gminecl as the rate of'cool~r~g
is slow.
'I'his has set up a telnperature gradient from the central Lo marginal portiolis resulting in
the movement of' magma similar
10
rhermodiffusion and cc~nvection mechanism as
observed by Wahl (1946), Barth (ICi52). Sadashivaiah and Ikramuddin (1077,) and
Latypov et al. (3007).
MAGMA GENKSIS AND CRUSTAI, THICKNESS:
The depth of the magma generation is important factor controlling the
composition of the magma. The high content of Ni and C'r in dykes indicates that they
might have been derived from the mantle source. According to Kuno (1996). different
basaltic magmas are produced independently at deferent depths. According to Yoder and
Tilley (1962) oli:,ine-tholeiites, which is slightly poarer in silica than normal tholeiites is
produced by partial melting of garnet- peridotites of the mantle at greater depth. 1f the
magma tiactionales at shallocv depth. it evolves into oversaturated tholciitc: utld if the
same magma undergoes fractional crystallization at great depth. it gives rise to olkaliolivine bil.';c?lts. In both the hypotheses. the dcpth of magma getier:ttion of frr~ctinni~tion
is
an important factor in controlling the compositiori of the magma. 'I'herctbrt.. the chemistry
of the basaltic rocks recentl~,has hec11~used to estimiite the depth e f t h e mugrn:\ genesis.
I<ccent experimental studies suggest that ttlc depth of nlngma generation is one 01'
thc important factors in controlling the composition of'lhc 11liig11ius.Supimura ( 1968) has
used the 'I'heto" (8) value to estimate the depth of' the magnra pcnesis. and this is
follow-ed by nimy investigators (Condie et al.. I W ) ; Naqvi and Hussain. 1073: Naqvi ct
al. 1974: Armbrustmacher. 1977: Satyanarayuna el al. 1080: tiimumantu ct al. 2008). 'l'hc
average theta value of -33 (Icss than 36) Ibr the prescnt dykes is charncreristic (*I'
continental tholeiites (('ondie st al. 19h9). This indicates that the thick~icssofcnlst was of'
34 - 40 km at the time of the emplacement ol'these dykes. 'l'he thcr:~vnluc 29 (Sinha ant1
Krishna Klro. 1968) of basic intrusions in lowcr C'uddapahs si~ggcslthat the thickness ol'
thc crust was more at thc tirnc of thcir emplaccmcnt. 'l'his suggcsts that thc present
thickness ofthis part of the crust might have hccn obtained bcfibrc the intrusion of' tl-~c
basic rocks of lower C'uddapahs of middle ProtCro/oic age and crustal thickening migllt
h a w hccn occured during thc cmplaccment o!'tlic*scdykes illso. 1:rom this it is clear that
the prcsenl dykes are not o f satnu ape of basic i ~ l ~ r l ~ s iof
v e slower ('~~dclapahs
and sonic
time gap is present in bet~vccnthcir emplaccrnc.nt. 'I'his supports I'ichamuthu's ( 1 C ) Y O )
contention that thc dykes in gnuisses are post-Archaean but not ncccssurily C'uddapah
age. l'hc high theta value is in dl-kes somc timcs may be due to crustal contarnination.
'1 hc petrological and geochemical intcrprctations led Lo inf'cr that the partial
melting of high Mg rich parental magma frorn deeper levels of' mantle seems to be
responsible fbr the generation of mafic magmas. I'he episodic release of mafic magmas
was responsible fbr the generation of younger group of mafic dyke swarms.
TABLE
Coostituents
aXIV:
I;l1(J21129
MODAL COMPOSITION OF MAFlC DYKES (VOLUME Yo),
31145 4jI18 5/25 6iW 71150 81156 91135 101123 llIll1 121121
-
-
-
---
-
-
Clinopyroxene 48.5 16.0 38.5 30.1 10.0 12.0 38.5 446 464 354 %.I 264
1.2
Orthopyroxene 24 1.2 3 0
-.
.. ".
=-
--
.. ..
..
2
2,5
1,8
10
..
..
..
..
Olivine
82 5,5 10,5 6J
Plagiaelase
390 454 1 16J 58.0 55.0 53.6 / 54.1 48.6 31.6 1 51.8 ' 416 200
Q u ~
1
/
.. ". ..
."
- 0.6
..
*.
I
L
- 1 0.1 2.9
26
3.5
I
Mitropeematite
-=a
I
.-
.. ..
2,5
-*
2.6
!
3.2
16 1
I
1
Epidote
Chlorite
0.1 0.Z
O.!
0.i
0.3 0.1 1.5
Opaque minerls 1.5 4
Ground mass
1
-- ..
1 0 I.?
2.5
I.!
Id 1 1 . 5
1.5
10 , I 0 2.5 1I 1 . 8
1
4
I
1.Z 1 2 . 5
1.5
1.8 1 0 1 6 1.5 10 1 4.8
..
..
:
i
I.14: Olivine~rthopyroxenedolerites.
!. i-I: Dolerites.
3.9-12: Olop!rorene-micropegmatite dolerites.
~
I
I
.-
1
,
I.?
.. I
-*
j
1
.. ..
me
-- l
I
1.5 3.2
3.5
--
47.50
l
1
I
Table-XV: CHEMICAL ANALYSIS OF MAFlC DYKES
CIPW NORM AND NICCLI VALUES
Fig. 36. Total alkali vs Silica (TAS) diagram for Mafic dyke rocks
(after Le Bas et al. 1 986).
X Mafic dykes
1P
0
f 83
Y
0
2
+
6ALKALI BASALT
0
2
z
6-
240
k5
50
55
Fig. 37. SiOz- Na20+ K 2 0 diagram (after Macdonald and Katsura,! 964).
X - Mafic dykes
Fig. 38.Si02- FeO,/ MgO diagram ( after Miyashiro, 1974).
X - Mafic dykes
Fig. 39. A-F-M plot for mafic dyke rocks
(after lrvine and Barger, 197 1 ).
X - Mafic dykes
Fig. 40. AI2O3- (FeOt+Ti02)- MgO diagram of mafic dykes showing
discrimination between high~Mg(HMT) and High-Fe( HFT) tholeiites
series and name of other fields is given in Jensen, (1976).
X - Mafic dykes
Fig. 41. K 2 0 - TiO? - P 2 0 5diagram for mafic dykes
(after Pearce et al. 1 975).
X - Mafic dykes