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.