NAGARJUNASAGAR AREA SHOWING MAFlC DYKE SWARMS ...
Transcript of NAGARJUNASAGAR AREA SHOWING MAFlC DYKE SWARMS ...
The Dhawar craton forms a part of the Southern Indian Precambrian
shield. The lithostratigraphic components are grouped under Gorur
Gneisses, Sargur schist belts (Ancient supracrustals), Peninsular Gneissic
Complex (PGC), Dhawar greenstonelschist belts, younger granitoids and
unconfomably overlying Proterozoic sedimentary basins from older to
younger successions. The eastern part of the craton is juxtaposed with the
Eastern Ghat Mobile belt (EGMB). The Godavari rift valley seperates Bastar
craton from Dhatwar craton and marked by NW-SE trending Karimnagar and
Bhopalpatnam granulite belts bounding the rift valley.
The Dharwar craton is divided in to eastern and western blocks and
the dividing line is located at the eastern margin of Chitradurga Schist Belt
(CSB). The division was made based on differences in characters of
greenstone belts, tectono-magmatic environs, magmatism (granitoid
characters) and metallogeny (Swaminath et at, 1981 ).The northern part of
craton is covered by Deccan Basalts of Cretaceous-Palaeocene age (Plate-
la). The tectonic domains of Southern Peninsular shield is shown in Plate-
12(after Drury et a1.,1984)
The following compilation on geological set-up of granite-greenstone
belts of A.P. was based on the works of the following-
Sarvothaman,l996,1998,2001 ;Sarvothaman et a1,1998,1999,Sarvothaman
and Lelanandam,l 992 ;Gopal Reddy and Suresh, 1993,1994,1998,2004;
Suresh et al,-1996,1998,20013,2007, Suresh,2007. Eastern Dharwar Block
(EDC) is dominantly made up of vast stretches of gneisses, migmatites and
granitoids with minor greenstonel schist belts. which are preserved in
synclinal keels. Although systematic geological mapping by GSI on scale
1:50,000 is completed, major part of the granite-gneiss country still remains
unclassifii. On regional scale, it has been grouped under the grand term
"Peninsular Gneissic Complex " (PGC). The PGC is vaguely depicted as a
stratigraphic entity either as oMer in its entirety to the greenstom sequences
or shown to consist of two phase's viz, the older and the younger. All the
migmatites of various ages are grouped under "Migmatite Group"
irrespective of compositions. The upper amphibolite group of rocks is
grouped under older metamorphics or under Sargur Group which is well
constrained or holds good to localized situations. Some of the coarse
grained rocks are put under granulites. However recent mapping by GSI
since late 1980's in different parts of the state of A.P has shown that the
PGC consists of a variety of granitoids, gneisses and migmatites of diverse
composition and ages(Sarvothaman 1994,96,98,20Ol,Sarvothaman and
LeeJanandam,l992, Sarvothaman et al, ,1998 ,98,99; Gopal Reddy
etal, 1989-93, Suresh etal.191 to 94 and 2000-2007). The granitic rocks have
been grouped into different suites based on the field relationship, type of
enclaves, modal and chemical composition, structural and textural
characteristics and the relation with the regional tectonics. Each of the suites
is well defined and form mappable units. It has been found in general that
the granitoids in the vicinity of and within-the schist belts are generally
different from those occurring farther away. The former are part of a
compositionally expanded suite of granitoids of mixed origin and probable
products of crust-mantle interaction. Some of these granitoids carry the
shear zone hosted gold mineralisation particularly in close proximity with the
schist belts.
Peninsular gneisses show transition to charnockites and schists'
grade into granulites in southern part of EDC.Charnockite formation and
Closepet Granite formation are co-eval around 2500 Ma. Similarly the
Karimnagar granulite belt formed in a similar fashion and now represents an
exhumed lower crustal assemblage bounding the Godavari rift valley.
Hanumanthu and Babaiah (1996) gave origin of granitoids occurring
adjacent to Ramagiri. Schist Belt, Anantapur district. A.P. The tonalite-
granodiorite suite is magmatic, metaluminous and calcalkaline and attributed
for mixed origin. Various lines of evidences show that this suite is a partial
melt of the amphibolite of the Ramagiri Schist Belt which was recycled at the
crust-mantle levels,
GREENSTONE (SCHIST) BELTS OF EASTERN DHARWAR BLOCK
(EDC)
A number of linear NNW to NW trending narrow greanstone belts and
intervening gneiss-granitoid terranes constitutes EDC. Important greenstone
belts from west to east are Sandur-Nelamangala. Kuniga1,Ramagiri - Penakacherla - Hungundu - Kusthigi, Kolar - Kadiri llulakalva - Jonnagiri - Hutti, Gadwal - Raichur, Veligallu. Tsundupalli -Penumuru,Peddavura,
Ghanpur, Sitanagar(Yerraballi) and Khammam-Nellore belts.(Plate-3)
Majority of the belts are bimodal type with basic and acid volcanics
and minor ultamafics, pyroclastics-tuffs and BIF. Metasediment components
increase from south to north and also from west to eastern parts of EDC.
The metamorphism also increases from north to south and from west to east
i.e from greenschist facies to upper amphibolite facies. Overall the high
temperature-low pressure regional metamorphic characters are
characteristic. Invariably the eastern contacts of the belt are sheared and
some of them form possible suture zones such as eastern boundary of
Chitradurga schist belt, Kolar-Kadiri-Hutti line, Shernawala line, Rudraram
line, eastern boundary of Nallamalai fold belt (NFB) and contact between
NSB and EGMB(Drury, 1984, swaminath et at, 1981 ;Rajamani etal, 1989, etc).
Regarding evolutionary models of schist belts, the belts of EDC form "inner-
arc beltsn while the belts of WDC are considered as outer- arc belts. As per
Chadwick et al., (1997) the schist belts along with associated intrusive
granitoids represent a major batmlith called the 'Dharwar batholith' and
accretionary model has been proposed Chadwick et al.;.Majorfty of the
workers suggest horizontal tectonics model i.e. crustal shortening by
subduction folbwed by collision for the evolution of granitegreenstone belts.
Some others believe vertical tectonic model i.e.plume model.
The southern part of schist belts towards craton margins are
constituted by more volcanic dominated sequence while towards north they
mdst of more of metasediments and pymdastks. Similarly the same
feature has been noticed from west to east. Another characteristic feature of
greenstone belts is the presence of 'volcanic conglomerate' as seen in Kolar,
Veligallu, Kadiri, Julakalva and Gadwal.
Regarding mineralisation, the high- Mg basalts contain rich lodes of
gold. Both syngenetic (stratifom type) and epigenetic (shear controlled)
hydrothermal type - gold mineralisation with minor W & Mo has been
recorded. In Jonnagiri and Julakalva schist belts, new extensions wrapping
around granitoid diapirs have been identified. Here sheared granodiorite
contain gold mineralisation. Layered mafic-uttramafic sequence with or
without chromite and metagabbro intrusions with football anorthosites have
been recognised in schist belts.
NNW-SSE trending ductile to ductile-brittle sinistral shears (5 gold,
Cu-Pb-Zn-Mo sulfides etc.) in association with schist belts are found.
Important are Bhadrampalli, Jonnagiri, and Peddavura etc. Concentration of
gold is attributed to shearing due to supply of fluids by the emplacement of
granitic rocks. Some of the workers have postulated transpressional model
for the origin of gold. Role of calcalkaline migmatisrn, diaprisrn and ductile
shearing of contact zones of schist belts and granitoids and concentration of
gold mineralisation had been brought out.
PENINSULAR GNEISSIC COMPLEX (PGC) AND YOUNGER
GRANITOIDS
Important findings are:- Reexamination of F e m r line has resulted in
identi-flcation and characterisatiin of the transitional area between DC and
EGMB, i.e., marginal zone (MZ).
a) Study of granitoids indicated granulite - granite tink in the formation of
granitoids and evolution of crust.
b) The event of anorogenk granite magmatism (A- type granite) i n d i e s
cratonisatbn and the onset of rHt tectdcs.
SEGMENT W E IMPORTANT FiNMIGS
HYDERABAD - NALGONOA - KHAMMAM SEGMENT:
It is dominantly consisting of peraluminous K-rich granite and a suite
of metaluminous to miMly peraluminous gmnodiorite - rnonzogmnita - granite (GMG). The soda rich tonalite - tmndhjemile suite occurs as
enclaveslbands. Besides, the hypersthene bearing 'enderbits and
mangerite' occur as diatexitic enclaves in GMG suite, which indicate
granulite - granite link in the origin of granites and evolution of the
crust(Sarvotharnan, 7994.7992). They show Island Arc Granite (IAG)
characteristics. Different phases of GMG suita occur in batholithic
proportions. In contrast, the peralurninous granite suite exhibits continental
collision granite (CG) and the metaluminous to mildly peraluminous
granodiorite - monzogranite - granite suita reveal continental arc granite
(CAG) affinities. The origin of the granitoids of this segment has Men
referred to crustal axatexis by underplating of lower crustal assemblages and
due to fractional crystallisation of mantlederived magmas. A- Type granites(
Sarvothaman,l996), derived from lower crustal assembleges(enderbite-
mangerite suite of rocks/ tonalite-granodiorite), are also found confined to
major lineaments.
In the marginal tone (MZ) area, the gneissic assemblages and their
granitic protoliths form major lithologias with protoliths of granodiorlte and
rnonzogranite and were intruded by plutons of granite (ss). Enclaves of
intermediate chamockii and bask granultte are found within gametifarous
gngiases and their protolib. The gneisses localy show arrested
c h m i t e s . In the chamoddte suite of rocks, three main types of gmff)tk:
rock are observed, viz, bwcutomlite, ~ ~ e r o u s granite, monzogranHe
~~.
MARGINAL ZONE (MZ) BETWEEN DHARWAR CRATON (DC) AND
EASTERN GHATS MOBILE BELT (EGMB)
Ferrnor (1832) has broadly divided the EGMB from Dharwar craton
by separating them as charnockitic region and non-chamockitic region
respectively.. The revision mapping has brought to light the existence of
transitional zone between DC and EGMB. A 15 to 60 km wide and N-S
trending marginal zone (MZ) has been identified between Dharwar craton
(DC) and Eastern Ghats Mobile Belt (EGMB). It is characterised by presence
of distinct geological traits in the EGMB and DC. The dominant lithotypes
are granitoid gneiss and their granodioritic, adamellitic and granitic protoliths
(2 garnet) all of which enclose xenoliths of chamockite and two pyroxene
granulites. The 'Kannegiri granulite' represents an uplifted lower crustal
assemblage forming horst with the complimentary graben filled by
Gondwanas. Five phases of folding have been identified (Sarvothaman et at,
1994-95). NW-SE and NE-SW trending faults intersect the horst structure
along which carbonatite intruded as seen at near Tallapenta and
Chintapalle. Gametiferous gneisses show development of hypersthene in
oily-looking patches (arrested charnockites). Over all prograde metamorphic
characters are noticed from the margin of DC to EGMB. Marginal zone is
characterized by the presence of transcratonic Kharnmam - Nellore schist
belt. At the junction of EGMB and MZ gneisses, a number of alkaline rocks-
nepheline syenlte, syenite, alkali granites, larnprophyres, carbonatites and
layered gebbro plutons ernplsced. The Nellore Schist Belt (NSB), between
Darsi and Krtshnra River, is very attenuated and occur as rafts of amphibolite
within host MZ granites of different compositions and ages. 8ioUt8 granite is
dominant followed by alkali granite. Trondhjernite plutons aswxiaed with
albitites are recorded at Singareyakonda and Ramudupalem. A-type granites
intrude into the chamkites et Phirangipuram. The contad between
Nallamalai FoM Belt and the marginal zone gnelsses is a thnrsted one.
MAHBUBNAGAR-NALGONDA MSTRICTS AND . RAYALASEEMA
SEGMENT
The unclassifkd Peninsular Gneissic Complex (PGC) has been
classified into various suites like, (i) tonalite - trandhpmite gneiss suite
(TTG). (ii) synkinematic tonalite - granodiorite - monmgmnite (TGM), (lii)
late kinematic monzogranite-syenogmnite (MS) suite and (iv) post-arogenic
granite -- alkali feldspar granite (GAF). The occurrence off youngest alkali
granite (A-typeXzakaulla et a1.1998) - alkali syenite suite and also the GAF
suite emplaced into brittle- ductile shears and faults has bean recognized
(Gopal Reddy and, Suresh. 1998 and 2004).
MAFlC DYKE SWARMS (M0S):-Mafic dyke swarms of Dharwar
craton have attracted the attention of many earth scientists before mSd-
nineties while studying the dykes thraugh out Southern India. Events of
mafic magmatism in Andhra Pradesh are shown in Table-I. PIchamuthu
(1959) observed that clouded feldspar and pyroxene bearing dykes are not
deformed and may signify their emplacement into the deeper sections of
earth's crust before the emergence of Eastern Ghats Mobile Belt. Post-
Dhawar dyke activity and the geochemistry of the dykes of Dharwar craton
were accounted by Naqvi et at. (1974). Palaeomagnetism and geochemistry
of Chittoor district and Anantapur district was accounted by Anjanappa
(1975) and Anil Kumar and Bhalla (1983) respectively. Halls (1982)
documented the distribution and trends of mafic dyke swarms in pans of the
Indian *ieM and found that the dykes are > 1 6 0 Ma as they do nat intrude
CvWspah basin. Dwly (1984) suggested preCuddapah arching about an
easterly axis with initially the N-S crustel extension for the emplacement of
spectacular E-W trending ma& dykes to the south af Cuddapah basin after
thet main Archwan defamation and before th Cuddapgh sedif'nmtatbn. He
suggested northerty ampmsskn k# thew fmmatbn of NNW and NNE
mdykeirwannsandmthatthsadMtytookpraceduetohotmt
a d M t y b t a f w e t h e ~ O h a f s ~ . ~ c n n d J ~ . ~ n g a n d
subsidence due to thermal activity was attributed as causative factor e for
the thinning of crust and formation of Cuddapah basin . Y.G.K. MurthY et
(1987) and N.G.K. Murthy et al. (1987) gave regional account on distribution
and geochemistry of different dyke swarms of Peninsular India.
Palaeomagnetic and geochemical studies of dyke swarms of Karirnnagar
district by Rao et a1.,*(1990) indicated that the fractures initiated in NE to
ENE -WSW direction during the early period of rifting of the NW-SE trending
Proterozoic Pakhal basin led to emplacement of multiple phases of dyke
swarms in which the quartz normative tholeiites are older than olivine
tholeiites. Sarma (1988 to 1991 ) and Reddy (1 991) indicated the presence of
dominant quartz normative tholeiites in dyke swarms in parts of Chittoor,
Anantapur and Mahbubnagar districts. Padmakumari and Dayal(1987) gave
K-Ar ages of 1480 to 590 +50 Ma for the dykes of Nalgonda district and
attributed youngest dyke activity (norites & granophyres) to the westward
thrust of Eastern Ghats Mobile Belt. In the present thesis, the author
describes the geology and geochemistry of mafic dyke swarms of Nalgonda
district, a study that was tied up to the IGCP - 257 Programme.
GEOLOGICAL SET UP OF NALGONDA DISTRICT, A. P
Of the many Precambrian maf i~ dyke swarrns(MDS) in Andhra
Pradesh.the dyke swarms of Nalgonda district including present areas of
study i.e., Narainpur Dyke cluster(N0C) and Peddavura -Narainpur Mafic
Dyke swarrn(PNSMDS) which are located to the north of intracratonic
Mesoproterozoic Cuddapah basin, stands out prominently, even in Landsat ". . imagery interpretations (Plates 5, 6.7 and 7a). It forms an important crustal
feature in the eastern block of Dhannrar craton and was emplaced into the
Archaean Peninsular ' Gneissic Complex, late-Archaean intrusive granitoid
suites and Peddavura greenstone be8 during Palaeo to Mesopmterozoffi
period .Geological map of mafic dyke swarms of Nalgonda district is shown
in plate-8. Syn to post-kinematic granitoid suites assdated with Dharwar
supracrustals of late-Archaean age and palaeopmtemmic M E are
unconfdrmabiy overbin by Mesopmterozoic sediments (Cudbapah & Palnad
basins) (Plate 7 and 9XSrinivasan and Krishnappa, 1989 & 1991).The
enderbite- mangerite suite of rocks occur in GAG suite in the northern part
around Hyderabad and Narayanpur. The granitoids and schist belts were
affected by NNW-SSE trending near coaxial 01 and Dz deformations and E-
W trending D3 deformation. N-S, E-W, NW-SE and N€-SW trending joints
are prominent. N 30°E - S30°W trending joints are shear joints. Shear zones
and faults are found in the NNW-SSE. E-W and NNE-SSW direction. Mafic
dyke swarms are intrusive into the late-Archaean greenstone-granite terrain
but not into the Mesoproterozoic Cuddapah Supergroup. They occur in N-S,
E-W, and NW-SE to WNW-ESE and NE-SW to ENE-WSW directions
(Plate. 9).
FIELD CHARACTERS
MORPHOLOGY OF PROTEROZOIC MDS OF A.P
Mafic dykes occur as positive ridges extending discontinuously over
distances of 10 to 60 km with widths of 25 to 100 m. The longest and dense
swarm can be seen in Chittoor district. An individual dyke is made up of
lenticular segments, each 2 to 4 km long. The segments show both dextral
and sinistral, enechelon shift along their trends. Their width ranges from less
than 5 m to 20 m with maximum widths up to 40 m recorded in Chittoor
swarm. The dyke trend and segment trend are always at an acute angle.
Features like 'bridge', 'bayonet', 'fork', 'step' and 'end' types are commonly
observed morphologies. The widths of the dykes vary as they pass through
different host lithologies. Apophyses join in opposite direction on either
margin of the dyke and tend to regain parallelism with the main dyke trend.
They make acute angles with the main body. Important feature of apophyses
is that regardless of their width they are entZrely chilled, In contrast to the
main dyke which exhibits only narrow discontinuous selved~es of chilling. A
broad correlation exists between joints and dyke trends suggesting magma
emplacement alang fractures that had trends close to the orientation of dyke
P r O P a P ~ .
Dykes show rounded bouldery (spheroidal weathering) and blocky
features at surface. Typically dykes show two sets of joints - one parallel to
the dyke margins and the other orthogonal to it. Dyke margins are never
straight as to be expected but are curvilinear on a regional scale. They are
generally sharp. Interaction with the country rocks is notable at places. The
dykes cut across Dhamvarian trend at an angle varying from acute to
orthogonal. Dyke incidence vanes from region to region as shown in the
Table-11.
Some of the regional scale dykes emplaced along major crustal
faults/fractures occurring in the form of regional scale lineaments.
Reactivation of faults caused shearing and ductile deformation locally along
contacts thereby producing schistose/foliated varieties. At places some
older dykes were seen off-set by later faults and displaced either sinistrally
(a) dextrally. Still at some places dykes ate seen traversing silicified quartz
reefs occupying fault zones. At some places dykes occur in wavy fashion
and at places as radiating type.
Most of the dykes are simple but in places a few autolith bearing and
multiple dykes are found. Some of the dykes contain crustal xenoliths of
angular to rounded (fluidisation) type measuring upto few tens of
centimetres. In general, dykes show grain size variation from fine at the
margins to medium to coarse grained at the centres. Chilled margins of few
mm to a cm are aphanitic to microcrystalline. At some dykes contacts,
phenocrysts of feldsparstpyroxenes of a few mm to cm size represent
primary magmatic foliation.
Some wider dykes impart pink colouration at the contact with most
rocks due to iron-oxide leaching. Common contact effects noticed are
epidotisation, silicification etc.
The colour of the dykes varies from greenish-black in the central part
to deep black at the margins. In some dykes leucocratic and mela-
bands .impart banding on weathered surfaces (they impart ribbed or
corrugated appearance). The olivine bearing dykes upan weathering show
pined nature. Some WNW-ESE trending dykes found at the basement for
northern margin of Cuddapah basin are unusualty vesiculated and
amygdulated containing quartz, calcite, epidote, etc. They represent near
surface intrusions and exhibit pitted or spotted surfaces cumulus textures are
rare in Proterozoic MDS but they are found in Archaertn layered mafic-
ultramafic complexes. Cumulative minerals are olivine or pyroxene or
magnetite-chromite.
Although majority of the dykes are aphyric some are porphyritic with
phenocrysts of epidotised feldspar. Distribution of phenocrysts varies from
sparse to moderate with neither preferred orientation nor preference to any
spatial location in the dyke. Rarely seen confinement of phenocrysts at one
hatf of the dyke. Crude banding of a few cm wide and few meters long are
noticed in some dykes. Banding is due to arrangement of segregations of
mafics & felsics or concentration of olivine rich & olivine poor zones. In some
dyke interiors zones with caught up fragments of feldspar, vein quartz and
grey quartz are found. in places the dyke rock has a speckled appearance
owing to the near uniform distribution of felsic and mafic minerals. Some of
the dykes contain pegmatoidal patcheslveins (or) fine-grained veinlets and
dykes (in- dyke veinlets).
Dyke trends for different swarms are shown in Table-ll. Dykes occur
as individual ones, clusters and swarms emplaced as near vertical bodles
showing pinch-swell nature, bifurcations, branching and swings in their
trends. Most of the dykes are simple, undifferentiated and exposed
intermittently parallel to sub-parallel and in some places as enedrelon type
bodies with or without shifts. Multiple intrusions are sparse. The dyke system
resembles a typical 'diabase dyke swarm' exposed in other parts of
Peninsular India. Different varieties of dykes occur in parallel trends
depending upon order of emplacement sequence. Pyrite is ubqultous as
traces in all the dykes. Fine grained older group of dykes are best suited for
decorativefpolished stones. Younger group dykes at places contain calcite
vainlets with sufphkles
MORPHOLOGY AND STRUCTURE OF NDC AND PNSMDS
MORPHOLOGY
Two blocks viz, Narianpur dyke cluster (NDC) located away from the
Cuddapah Basin and Peddavuru Nagarjunasagar mafic dyke swarm
(PNSMDS) located within the northern vicinity of Cuddapah Basin have been
selected for the present study. The dykes determine largely the ~eomorphic
pattern in parts of toposheets 56 W16 and 56 PI1, 2. 5 and 6. The
geological maps of the mafic dykes of NDC and PNSMDS respectively are
shown in plate 10 and platell respectively. The major trends of dykes are
plotted in rose diagrams (plate 13 a & b)
The MDS shows an average density of 1 dykelkm, with an assymitric
distribution. The dyke swarms are classified as older and younger groups
based on their mutual field relationships and petrographic characters and
both the groups occur in the said blocks but the incidence of older group in
NDC block is relatively greater. Subalkaline quartz gabbros and sparsely
distributed younger alkaline dykes have been confined in PNSMDS only.
There is no specific directional and compositional distribution of the dykes of
these two groups except that of WNW-ESE trending quartz gabbros and N-S
trending alkaline dykes. The greater azimuth of the dyke distribution is in E-
W and NNW-SSE / NW-SE directions. They occupy the major fracture zones
and sometimes emplaced along fault planes.Detailed geological maps and
photos are included for the evidence.
Dominantly the dykes are linear, tabular and vertical-to subvertical.
which occur in clusters. pinch and swell, bifurcate and reunite along strike
and swing in their trends along strike. The length of the dykes varies
between few meters to a few kilometers and the average width of the dyke is
around 25m with a maximum width of 200m. Chilled margins of a few mm to
cm wide are commonly seen in younger group (Photo.3). OIivine gabbros
show pitted nature (Photo-l) A majority of the dykes ere simple,
undifferentiated and only rarely show muttiple nature (Phato.5). "In these
characters, they resemble shalbw level diabase dyke swarms in India and
elsewhere. The average dyke density in NDC varies from one dyke I I to 2
sq-km for E-W trend and one dyke I four sq.km for N-S trending set. The
asymmetrically distributed dykes in PNSMDS have the dyke density of one
dyke / one sq.km for E-W set and 1 dyke / 4 sq.km for N-S trending set. In
PNSMDS, the dykes of younger group mostly occur as enechelon type
(sinistral); The variants of the older group are pyroxenites, gabbraic
pyroxenites and gabbros ( some are sanukitoid like). The younger group is
with gabbro, dolerite and quartz gabbro. Contact granophyres and hybrid
diorite veinlets are not uncommon in some older dykes. Pegmatitic patches
are most frequent in older group while fine-grained 'indyke' veinlets are
common in younger group. The dyke swarms and even an indivMual dyke of
both blocks show either along or across without much textural and mineral
variation. Typical layering and cumulate textures are absent, with the
exception of pyroxenites which show locally crude layering comprising
olivine rich (peridotitic) and olivine poor layers. They are simple and
monotonous. In some dykes, the plagioclase feldspar phenocrysts are
concentrated at the margins and in places gradually decrease towards
center due to flowage differentiation. Some dykes contain autoliths.
(Photo. 4)
CONTACT RELATIONS
Screens of country of rocks are entrapped in multiply connected dyke
systems, occurring as large thin slabs or with varying shapes and
dimensions. Small oval patches of country rocks at places can be seen .In
many cases, presence of earlier xenoliths is indicated by cavity and pothole
like features, indicating removal due to differential erosions.
Contacts between the dykes and the country rocks are generally
sharp, (Photo. 8) but in most cases k either highly weathered or under cover
ofdetwis. T h e c o n t a d m a y w e n h a v e ~ k e ~ n a s a n s l a ,
W i , for instance msrks the contad of the major dykes wlth granite, for
most part. Chilled zones, where observed, are only a few mrn wide. There is
no apparent grain size variation across the dyke body. The fractures in
country rocks here are at a high angle to dyke contact, decrease in
abundance farther away and show fitling by tachylite or secondary epidote,
quartz-epidote or epidote-calcite veins. Dyke in contact with gneiss has sent
offshoots into the country rock at a high angle to the contact. Sometimes the
dyke contacts are sheared and silicified / chloritised / epidotised.
STRUCTURES
PRIMARY IGNEOUS FEATURES
In this section, non-diastrophic primary igneous features (Turner and
Weiss. 1963) of the MDS are described. In this regard, the form, type of
intrusion and such details have already been described.
Most of the dykes appear to be simple in nature that is an individual
dyke appears to represent crystallization of a single pulse of magma
(Hughes, 1982). Possible multiple nature is locally seen as small dykelets
occurring within major dyke as veinlets confined to that dyke only.
Composite dykes have not been observed.
There are no ideally fractionated dykes in the area studied. However,
imperfect features including evidences for flowage differentiation and mineral
segregation (layering) (Wager, 1967) have been observed. These are welt
seen in the older pyroxenite and gabbros. In all these instances, the
segregation (layering) had been found as ribbed surface due to differential
erosion and represents relatively pyroxene and plagioclase rich zones.
Individual segregates (layers) are utmost a cm in width and show highty
discontinuous and often irregular strike and dip patterns in the same
occurrence. These occur both at marginal and central parts of a dyke body
and can be occasbnally observed along the strike-The impersistent layering
in some dykes indicate development of layering before full dilation of the
igneous body during emplacement and this continuing dilation probaMyalso
subsequently minimiskd or destroyed such layering.
Glomeroporphyritic and porphyritic types with crystal segregation are
more commonly observed. Plagioclase phenocrysts rarely exceed 5% of the
mode as seen at the outcrop. The tendency for phenoctyst concentration in
central parts of dyke is seen in some of the above examples. The
phenocrysts range in size from 1 - 3 crns, most are around 2 crns. They are
commonly irregularly distributed, but could also be glomeroporphyritic. A
gabbroic dyke with megacrysts/cumulates of plagioclase (anorthosite
composition) with dimensions of length 500m and width 20 m, occurs near
Chinthalapalem is noticed (photo 8b).
SECONDARY STRUCTURES
These are post consolidation features due to effects of later, mainly
non-penetrative deformations (Turner & Weiss, 1963). The mineralogical
changes due to (deuteric) alteration are treated in the petrography chapter.
Folding in the MDS has not been observed as seen from their characters on
the geological map or as observed from aerial photographs. The
deformations observed include shearing, faults and joints.
Tensional fractures as well compression related conjugate pairs of
shear fractures played a major role in the evolution of MDS. MDS in most
cases cut across Dhawarian trends (foliation, shear zones) and found
orthogonal to joint systems. However, they emplaced along both N-S and E-
W trending ex-tensional zones and conjugate sets of strike slip fractures.
The enechelon shifts (mostly dextral in PNSMDS) in MDS indicate semi-
brittle conditions of the crust and while emplacement of MDS if there is a
change in the structure in the crust at an angle to magma propagation from
below then the enechelon nature of MDS is obvious. At places the same
dyke take the advantage of both the conjugate fractures and interconnecting
them without any crosscutting relations. Brldges, bayonets, forks, swerves,
branching etc are the features noted.The effects of shearing are well seen in
both the country rock and ppxenfte dyke located to the west of Narainpur.
In both, the shearing is not extensive, extending for a few cm from the
Wntact in the dyke, and for a few tens of cms in the granite country rock. In
this instance, the shearing has hastened secondary alteration and
weathering affecting the dyke and the dyke has become chlorite schist. In
the granite, brittleductile type of marginal recovery and recrystallisation is
noticed in quartz grains and microclines, It is not certain if the granite had
been sheared prior to dyke emplacement or during such process, but the
shearing in the dyke itself indicates its continuation even after emplacement
of the dyke. Spatial continuation of this shearing could not be established.
A number of faults, offsetting the trend of dykes have been noticed
and can be noticed even in aerial photographs. The displacement across
fault planes is never large and can be measured in tens of meters (Photo 6).
Vertical component could not be estimated. Apart from this, a large number
of dextral and sinistral shifts are noticed in enechelon continuation of dykes
in NDC and PNSMDS area. These-again show less than tens of meters of
displacement across enechelon trends of dykes and again, the vertical
component is not measured. It is possible that these features could have
formed prior to emplacement of dykes.
The dykes show well developed jointing. However, at many places, as
has been stated earlier, it is difficult to know the orientation of joint planes '
due to rubble and bouldery weathering of the outcrops. The measurement of
joints indicates that there are no distinct maxima in the pattern. Most joints
are subvertical to vertical, but paucity of low dipping planes may be due to
above reasons plus lack of good vertical sections from most parts where
measurements have been made so horizontal and low dipping planes are
necessarily poorly represented.
REGONAL TECTONtCS AND STRUCTURE
SIGNATURES OF EXTENSIONAL TECTONISM.
The extensional tectonism was commenced, prior to the
emplacement of MDS, right from late Archaean times after calcalkaline arc
magmatism in the form of NW-SE trending major syenite bodies (episodic
alkaline .magmatism) such as Koppal (Chadwick et a1,2001.) and
Pulikonda(Suresh, 2007.) emplaced at the craton margins (accretionary
boundaries) at an angle to NNW trending sinistral shear zones (Plate-4 and
12). Minor alkaline magmatism took place at the end stages of early
phase of MDS and finally reappeared during post-Cuddapah times
(Mesoproterozoic) confining to the NFB of Cuddapah basin and at the
junction of EGMB and Dharwar craton.
Pre-MDS alkaline magmatism occurred in two different tectonic
environs in different periods i.e 1 )early episode of ate Archaean alkali
magmatism(syenites followed by alkali granites- A-type) at the craton
margins near accretionary boundaries along post-subduction rift zones after
arc magmatism and 2) in the craton interiors along tensional zones after the
formation of collision related peraluminous granites and postomgenic alkali
feldspar granites of bimodal suite. The main stage of MDS formation
confining to the basement is locally interspersed by the emplacement of
minor quartz-syenite dykes and alkali gabbro dyke lets (Suresh,2007)The
Neo-proterozoic youngest alkaline magmatism occurred at the southern
transitional part of craton around Kanakapura and Sdrangapatna
area(0evaraju eta!, 1995).
EVENTS AND EMPLACEMENT MECHANISM OF MAFlC DYKE SWARMS
The present area of investjgation lies in between plume affected
Cuddapah basin basement area and NW-SE trending Pakhal rift(half
graben. AU the dykes emplacsd in EDC might not be generated by a single
hot-spot activfty as there are several epi80des of dyke activity reported which
are distributing all over the EDC. Regional characters and associations of
MDS in conjunction with the present study have been taken into account for
arriving at the evolutionary history of the MDS.
Two stages of Palaeoproterozoic pre-Cuddapah mafic dyke activity
(abortive rift)and one stage of synsedimentary Cuddapah mafic extrusive
and intrusive activity have been recognized(re1ated to syn-Cuddapah
passive rifting).
The oldest cycle represents clouded feldspar bearing dykes
characterized by presence of fractionates and cumulates (olivine bearing
pyroxenites and anorthosite dykes). The second stage younger group of
dykes characterized by presence of olivine gabbros and differentiates of
quartz-gabbro and ganophyres which were emplaced dominantly around the
Cuddapah basin. This cycle was end.ed with the emplacement of alkaline
rocks such as alkali gabbro and syenites which do not cross-cut the
Cuddapah Basin. The only reported sole dyke cross-cutting Cuddapah
basin to the NE of Veldurthi (western margin of the basin) as reported by
Vijayam (1968) is GSI ruled out as intrusive by GSI(Cuddapah basin map of
GS1,1981)
Westerly thrusting of EDC over WDC caused E-W compression
resulting into development of NNW trending regional scale major ductile-
brittle sinistral shear zones at the contact zones of Dharwar supracrustab
and granitoids. The E-W compression resulted into ENE-WSW and WNW-
ESE conjugate pairs of fractures. Pre-Cuddapah warping of regional
Archaean structural trends initially about an E-W axis is itsetf a reflection of
doming and N-S compression during the initiation of the precursors of
Cuddapah basin.(Drury,1984).Dykes were emplaced along or controlled
minor fautt zones. The N W and NNE trending dykes represent conjugate
fractures with strike-slip movements and EN€ dykes represent numerous
extensional fractures with normal strike slip movements. In the southern side
ENE to E-W, .trending dyke swarms are abundant while to the north of
Cuddapah basin the WMN-ESE trending MDS are abundant. This could be
change in the stress domain in the wustal levels and also the propagation of
magma direction from subsurface magma chamber due to disturbances in
the crust-mantle interface.
Later- on the N-S compression resulted into NNW and NNE trending
conjugate sets of fracture planes. The emplacement of dykes took place
along these fracture planes .These planes at places are sheared affecting
the dykes also. Pre-MDS faults and fractures played a major role in the
propagation of MDS. The EN€ and WNW trending and NNW and NNE
trending conjugate sets of fractures are strike slip in nature.
The boundary between WDC and EDC is marked by a thrust (shear
zone) zone along which linear easterly dipping Closepet Granite batholith
(400kmx50km) emplaced. Almost during same time the eastern margin of
EDC was accreted on to the EDC by transcratonic Khammam-Nellore schist
bett(KNSB) thus formed major parallel zones such as Rudraram and
Velikonda lines( These zones were later reactivated and responsible for the
formation of Nallamalai fold bett(NFB)). Towards NE margin of EDC the
Karimnagar granulite belt was formed simultaneously along with granite.
This was later uplifted as a NW-SE trending belt lying across the NE margin
of EDC representing accretionary boundary of KNSB or NE trending Eastern
Ghat Mobile Belt. Similar tectonism played a major role in the adjacent
Bastar craton where a number of NW-SE trending linear granulite belts such
as Bhopalpatnam, Kondagaon, Narainpur belts etc were formed and
uplifted(Ramachandra et a1,1995 and 2000). Later on, the subsidence of
Karimnagar granulie belt was responsible for the formation of Pakhal rlft
valley.
in the EDC ,at 45 O angb to the NWSE trending boundaries ; N-S
and E-W trending dilatational zones were formed and earlisst phase of
mafk dyke actMty(oMer grarp) took place and major dykes such as N-S
trending great dykes emplaced at the boundary d Clawpet Granite and €0
W trending major dykes crosg cutting Closepet Granite and ED C.
The initiation of upliftment of EGMB on to the accreted KNSB was
probably responsible for the hot-spot (mantle plume/mantle upwelling) as a
subsurface lopolith (funnel shaped mafic magma cupola) body emplaced at
present below south-westen margin of Cuddapah basin. This was
responsible for the profuse emplacement of MDS along tensional zones
(younger group) there by inducing crustal heating and doming. The varied
stress regimes controlled the direction of MDS around this cupola .After
thermal relaxation, the crust was subsided along NW and NE trending
crustal weak zones and started initiation of formation of proto- Cuddapah
basin. The dykes were emplaced around lopolith in shallow crustal levels.
The early phase older Group of dykes might have emplaced before the
formation of lopolith .They are characterized by clouded feldspars which
indicates lower crust or heated crust. The doming caused tilting of the
terrane. During same time the NW-SE rift valley was formed along
Karimnagar granulite belt. The pre-Cuddapah MDS emplaced along
extensional zones but could not succeed in the formation of rift volcanics in
Pakhal basin.(Failed arm tectonics) .NE-SW trending cross rift tensional
tones were emplaced by dykes besides emplacement along tensional zones
parallel to rift. The fractures at 45" angle to the main NW-trending rift valley
are also occupied by MDS. The NE trending faults are formed across rift
while the much younger E-W trending fault zones are strike slip in nature.
The reactivation of faults during post MDS and post-Cuddapah times is also
evident making complex situation. The dilatational zones across and parallel
to Pakhal rift valley caused formation of NW and NE trending fractures, later
occupied by MDS.
The local abundance of particular MDS could be local stress regime
resulted by crustal doming /arching as visualized in the central and southern
part of EDC or vertical gravity didurbances(uplift and subsidence) of cmst
as noticed in the northern part of EDC. While static crust above plume
prompted rifting and emplacement of volcanics in the Cuddapah basin.
PLATE 4 - BOUQUER GRAVITY ANOMALY MAP OF ANDHRA PRADESH (After OSI and NORI, 1975)
PUTE 5 -IMAGERY OF PART OF NAlWOA WTWT, A W M PRAOU)II
RATE 8 -(8) 0eou)OtcAc MAP of MAW Dm WARMS OF ' N A U K N Y O A O I ~ , A?.
(Sourn 08l)
GENERALISED GEOLOGICAL MAP OF PEDDAVURA- NAGARJUNASAGAR AREA SHOWING MAFlC DYKE SWARMS, NALGONO DISTRICT,A,P, (Parla of topoaheet no. 56 PI1 and B).
FIG138 ROSE DIAGRAM SHOWING MAJOR TRENDS OF DYKES OF A) rNDC and B) =PNSMDS.