75-89

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.61, Jan. 2003, pp.75-89

0016-7622/2003-61-1-75/$ 1.00 © GEOL. SOC. INDIA

Reinterpretation of Stratigraphy and Structure of Sausar Group inRamtek-Mansar-Kandri Area, Maharashtra, Central India

ANUPAM CHATTOPADHYAY, A. S. KHAN, A. K. HUIN and B. K. BANDYOPADHYAY *Geological Survey of India, Seminary Hills, Nagpur 440 006, India

*Corresponding Author; Email: [email protected]

Abstract: Geological mapping and structural analysis of the manganese-bearing Sausar Group and associated rocks inRamtek-Mansar-Kandri area, Maharashtra, have led to reinterpretation of stratigraphy and structure. A local stratigraphicsuccession for the Sausar supracrustal rocks, giving due weightage to lithologic criteria and structural considerations,has been proposed which is different from the existing divergent lithostratigraphic successions suggested by differentworkers at different times. The lithologic unit interpreted as a conglomerate horizon by some earlier workers, near thecontact of the granite gneiss and the Sausar Group in Mansar-Kandri area, has been reinterpreted to be of tectonic origin.Sausar Group preserves evidence of four generations of folding (F

1 to F

4). The map pattern is controlled by F

2 and F

4

whereas F1 and F

3 have developed only on hand specimen to outcrop scale. Ramtek synform, the major structural

feature in the area, is a polyharmonic, second-generation fold with shallow southeasterly plunge and a steep to subvertical,WNW-ESE striking axial plane. It is not a large scale non-plane, non-cylindrical superposed fold (F

2 on F

1), as interpreted

earlier. Regional metamorphism of pelitic-psammopelitic rocks possibly reached amphibolite facies with peak metamorphiccondition post-dating D

1 deformation.

Keywords: Sausar Group, Structure, Stratigraphy, Folding, Metamorphism, Maharashtra, Central India.

INTRODUCTION

The Sausar Fold Belt (SSFB) is an importantMesoproterozoic (Sarkar et al. 1986; Lippolt and Hautman,1994) mobile belt in the central part of Indian Peninsularshield. It forms an arcuate belt covering an area of over7000 sq. km. in the states of Maharashtra and MadhyaPradesh, with a prominent southward convexity (Fig.1a).SSFB comprises two major litho-tectonic ensemblesviz. Tirodi Biotite Gneiss and migmatite (TBG), andmetasedimentary Sausar Group (SSG) to which TBG formsthe basement (Narayanaswami et al. 1963; Pal andBhowmik, 1998; Khan et al. 1998). SSG hosts the largestand richest manganese ore deposits of India, contained inmica schist (Mansar Formation) and in calcitic marble(Lohangi Formation). The present study area occurs in thesouthern part of the central portion of SSFB and forms partof the type area of manganese-bearing schistose rocks ofSausar Group. The famous Kandri-Mansar group of minesoccurs in the western part of the study area. Towards theeast, the dominant rock type is quartzite, forming high hillsand ridges around Ramtek.

The manganese-bearing Precambrian rocks of Ramtek-Mansar-Kandri area have been studied for a long time. The

first systematic study of the area was carried out byGeological Survey of India under the leadership of LewisFermor and a stratigraphic succession was proposed, in linewith the generalised stratigraphy of SSFB (Fermor, 1922;Pascoe, 1927; West, 1936). These workers designated theschistose rocks and gondite/manganese ore as Mansar‘Stage’, quartzites as Chorbaoli ‘Stage’ and dolomitic marbleas Bichua ‘Stage’, on the basis of strict lithologic criteria.Basu and Sarkar (1966) proposed a local stratigraphy forMahuli-Ramtek area, the implication of which is discussedin a later section. Later, Chakravarty (1973) undertook adetailed study of the Mansar-Kandri area, and mapped theSausar rocks with special reference to the geology ofmanganese ore (Fig. 1b). Mohanty (1988) and Mohantyet al. (2000) have also presented some new ideas on thestratigraphy and structure of the manganese-bearingPrecambrian rocks of this area, based on field observations.However, these interpretations by different workers oftencontradict each other when viewed in totality and leave manyproblems on stratigraphic and structural interpretationunresolved, the details of which are discussed in therespective sections. It has been already recognised(Basu, 1965) that in a polydeformed and high-grade

76 ANUPAM CHATTOPADHYAY AND OTHERS

JOUR.GEOL.SOC.INDIA, VOL.61, JAN. 2003

Fig.1. (a) Generalised geological map of Sausar fold belt (after Narayanaswami et al. 1963). Inset: location of the study area in India.(b) Geological map of Ramtek-Mansar area by Chakravarty (1973).

(a)

(b)

REINTERPRETATION OF STRATIGRAPHY AND STRUCTURE OF SAUSAR GROUP, MAHARASHTRA 77


metamorphosed Precambrian terrain like SSFB, nostratigraphic succession can be constructed without properlydeciphering the structural complexity. In this connection,the local stratigraphic succession of Ramtek-Mansar-Kandriarea, based on proper interpretation of the structural pattern,may be very important, as the area is occupied by metapeliticschist and quartzite, where deformation pattern of SausarGroup is better understood. The present paper is an accountof the reinterpretation of stratigraphic and structural aspectsof the Ramtek-Mansar-Kandri area, based on large-scale(1: 25,000) mapping and structural studies carried out bythe present authors in parts of SSFB.

GEOLOGICAL SETUP

The study area exposes the pelitic schists of SausarGroup with stratified manganese ore and gondite horizonsin Mansar-Kandri area. Thin bands of quartzite anddolomitic marble occur interlayered with the mica schists.A grey to pink coloured, foliated granitoid occurs alongthe southern fringe of the Mansar mine and continues alongthe southern slope of Mansar ridge to the west of KandriMines, showing a cross-cutting relationship with themetasedimentary units in the map (Fig.2). Towards east,near Ramtek, the dominant rock type is massive to flaggyquartzite with interbands of quartz-muscovite-stauroliteschist, defining a map-scale synformal structure with lowsoutheasterly plunge (Fig.3). The flat terrain south ofRamtek-Mansar road is mostly soil covered with isolatedexposures of mica schist or granite. The major structuralgrain of the study area is E-W and the map pattern iscontrolled by large scale folds in pelite-psammopelite-quartzite units. Following is a brief description of the majorrock types found in the area.

Biotite Gneiss: Very few exposures of typical biotitegneiss (Tirodi Biotite Gneiss or TBG) occur in the mappedarea. One such isolated exposure is found southwest ofParsoda (Fig. 2). The rock is compact and shows very welldeveloped gneissic banding defined by alternate layers ofmafic (biotite ± epidote) and felsic (quartz-plagiocalse-microcline) minerals. Occasionally, protomylonitic foliation,defined by grain size diminution of quartz and biotite byrecrystallisation, is observed . In the northern part of Sausarbelt, around Deolapar (Fig.1a), the rock contains quartz-fibrolite knots or ‘tabloids’ (West, 1931). West (1931)interpreted the development of sillimanite (fibrolite) in thisrock as a result of metamorphism under decreasingtemperature. Recent studies however showed that fibrolitedeveloped as a result of ‘base-leaching’ of feldspar grainsand fibrolite later transformed to coarse sillimanite blades

with increasing temperature (Pal and Bhowmik, 1998).Quartz-mica schist: Quartz-muscovite schist is

associated with manganese ore/ gondite horizons and is,therefore, included in Mansar Formation of SSG. The rockis strongly schistose and silvery white in hand specimen dueto great abundance of muscovite. Good exposure of thisrock is found in the road cutting on NH-7 north of Mansarand in the working pits of Mansar mine, northeast of Mansar.Petrographically, the rock is composed of quartz, muscovite(±biotite) with minor opaques and K-feldspar. Quartz andmica define the strong schistosity. Another variety of schistis quartz-muscovite-fibrolite (±sillimanite) schist, oftenstructurally underlying the manganese ore horizon e.g.northwest of Mansar village, and hence considered as partof Mansar Formation. This rock contains quartz, muscovite,biotite and fibrolite (sillimanite). Fibrolite generally occursas quartz-fibrolite ‘tabloids’ and is restricted within thequartz-mica rich bands (Q-M domains). These ‘tabloids’are morphologically similar to those described above. Paland Bhowmik (1998) reported transformation of fibroliteto sillimanite but argued that presence of sillimanite in peliticschists does not indicate sillimanite zone metamorphism. Athird variety of quartz-mica schist, interbanded with thequartzite and structurally overlying the manganese bearingschists, occurs in the core of Ramtek synform (Fig.2). Thismica schist unit is interpreted as part of Chorbaoli Formation(Table 1), and is composed of quartz, muscovite, biotite,garnet and staurolite. Garnet and staurolite show fine trailsof inclusion of quartz and chlorite, study of which are helpfulin correlation of deformation and metamorphism.

Quartzite: Near-continuous exposures of meta-morphosed micaceous and gritty quartzite are found nearRamtek (Fig 2), along with interbanded quartz-mica-garnet-staurolite schist described earlier. In hand specimen, thequartzite is yellow to buff coloured, massive to well-foliatedand fine to medium grained. It is composed of quartz,muscovite, biotite and opaques, in the order of abundance.Two distinctly different fabrics are observed in the rock –a coarse-grained quartz-muscovite fabric, cut across by afine-grained, more penetrative quartz-mica fabric. Detailsof these fabrics are discussed later. At some places e.g. nearKhindsi tank, the quartzite shows a protomylonitic fabricwith typical core-and-mantle structure of quartz observedin thin section, and a strong lineation defined by stretchedgrains of quartz. This unit has been shown to belong to theChorbaoli Formation of SSG (West, 1936).

Dolomitic marble: Of the two types of dolomitic marblefound in Sausar fold belt (West 1931), only the pure whitevariety occurs in the study area, intercalated with manganese-bearing mica schists of Mansar Formation. The rock is white



Fig

.2. G

eolo

gica

l map

of R

amte

k-M

ansa

r-K

andr

i are

a, N

agpu

r D

istr

ict,

Mah

aras

htra

(th

e pr

esen

t stu

dy a

rea)

.



in colour with granular texture in hand specimen and showstypical crocodile skin weathering features. It is composedmainly of dolomite, with subordinate diopside, tremolite andminor epidote.

Granite: A pink-coloured, foliated granitoid occurs inthe south of Mansar mine. Thick (2-3 cm) biotite rich bandsare found in the pink granite at places, imparting to it, agneissic character. Biotite occasionally defines a schistosefabric in the granite, subparallel to the gneissic bands. Boththese foliations in granite are nearly parallel to the schistosityobserved in the adjacent mica schists of Sausar Group. Ata few places e.g. near the weekly market place of Mansarvillage, the granite is dark grey coloured due to high biotitecontent. The granite is composed of quartz, K-feldspar(perthitic), biotite, muscovite and minor plagioclase. Foliatedgranite north of Kandri mine shows fragments of manganeseore and/or magnetite xenocrysts. Occasionally, the granitecontains large enclaves of basement rocks (TBG) anddolomitic marble (of SSG) as observed near Parsoda andWaitola respectively, indicating its late intrusive character.

PROBLEMS OF STRATIGRAPHY

Stratigraphic succession of Ramtek-Mansar-Kandri areahas been a matter of debate. Stratigraphic nomenclature onthe basis of strict lithologic criteria has led Fermor (1922)and West (1936) to put all quartzite units under Chorbaoli‘Stage’ and dolomitic marble units under Bichua ‘Stage’(Table 1). Later, Chakravarty (1973) partly upheld this viewfor the present study area. Basu (1965), on the other hand,strongly criticised the stratigraphic nomenclature of Fermor(1922) and West (1936) as they apparently assumed thequartzite and dolomitic marble units as regionally persistent‘lithotopes’, without any significant lateral variation.According to Basu (1965), this is unlikely in a Precambrianterrain like SSFB. Basu and Sarkar (1966) proposed a localstratigraphic succession of SSG from Mahuli-Ramtek areawhich is strikingly different from the earlier ones (West,1936) (Table 1). They also mapped a number of anticlinesand synclines in the area, prominent among them being theMahuli-Ramtek syncline. According to them, themanganese-bearing rocks of Kandri-Mansar area define ahorseshoe-shaped synclinal fold closure near Kandri mine,with a shallow to moderate plunge towards east and an E-Wtrending axial trace. As per their map, two faults haveaffected these rocks, displacing the dolomitic marble unitsand disrupting the continuity of the ore horizon east ofKandri. Towards east, quartzite-mica schist of Ramtekhill defines a map-scale, easterly plunging syncline withWNW-ESE trending axial trace. All these large-scale folds

were interpreted as first generation folds (F1) of Sausar

orogeny. On the basis of structural mapping, Basu and Sarkar(1966) placed the quartzite of ‘Ramtek Group’ (Table 1) atthe top of the stratigraphic column and proposed repeatedoccurrence of dolomite units at different levels within themica schist and both above and below the manganese orehorizon. Moreover, they argued that, the mica schist -dolomite - quartzite package exposed in the Kandri synformare structurally overlain by quartzite-mica schist units ofRamtek synform, as the Ramtek synform occurs along thesame axial trace as the Kandri synform and towards thegeneral plunge direction (east) of the large scale fold. Basedon his detailed mapping of the Kandri-Mansar area(Fig. 1b), Chakravarty (1973) included the manganese-bearing quartz-mica (±sillimanite) schists under ‘MansarFormation’ and advocated that the repeated occurrence ofdolomite (‘Bichua Formation’) at different levels is due totight folding.

On the basis of structural mapping and stratigraphiccorrelation in the present study area as well as in the adjacentparts of Sausar fold belt, we uphold the view of Basu andSarkar (1966) that quartzite and garnet-staurolite-mica schistof Ramtek area constitute the topmost part of the localstratigraphic succession. However, we correlate this quartziteand interbanded garnet-staurolite-quartz-mica schist toChorbaoli Formation as suggested earlier by West (1936).This is because the micaceous/gritty quartzite units alongwith the interbanded garnetiferous mica schists constitute amappable lithological unit which structurally overlie themanganese bearing schistose rocks of Mansar Formation,over a wide area of the Sausar fold belt. The dolomiticmarble, quartzite and pelitic and psammopelitic schists withmanganese ore and gondite layers are all clubbed under theMansar Formation (Table 1). Although we prefer to avoidstratigraphic nomenclature on the basis of strict lithologicalcriteria, occurrence of manganese/gondite horizons inschistose rock is accepted as a key criterion for identifying‘Mansar Formation’. The manganese horizons are thusconsidered as marker bands representing time-restrictedmetallogenic events – either in association with carbonates(Lohangi Formation) or with schistose rocks (MansarFormation) (cf. Narayanaswami et al. 1963). We alsoadvocate that the thin dolomitic marble units withinmanganese-bearing mica schists represent originalcarbonate intercalations within a predominantly peliticsedimentary facies. Lateral discontinuity of dolomitic marblebands is generally due to sedimentary pinching. Repetitionof dolomitic marble by folding, as suggested by Chakravarty(1973, p.31-32) cannot explain the occurrence of olderChorbaoli Formation (quartzite-mica schist) over younger



Fig

.3. (

a) S

truc

tura

l map

of

Ram

tek

synf

orm

; In

sets

: (b)

con

tour

dia

gram

of

pole

s to

S 0/ S

1, a

nd (c

) con

tour

dia

gram

of

L 3 (a

xes

of F 3

fold

s).

→



the northern Nagpur District (including Kandri-Mansar area)(Fig. 1b) does not show any outcrop of ‘orthogneiss’ east ofMansar, although it is clearly described from this area inthe adjoining text. (Chakravarty, 1973, p.30). That he (p.44)interpreted the ‘orthogneiss’ as a late intrusive into Sausarrocks, is clear from his proposed lithosuccession of the

Table 1. Stratigraphic succession of Sausar Group by different workers

West (1936) Narayanaswami et al. (1963) Basu and Sarkar (1966) Khan et al. (2000) and present authors

Intrusives: Massive potassic granite,aplite, pegmatite and quartz vein.Foliated granite1, locally rich inbiotite and/or fibrolite.

Ramtek Group: Quartzite etc.

Sitapar Stage: Hornblende Chargaon Group: Dolomiteschist and quartzite

Bichua Stage: Pure and Bichua Formation: Dolomitic Bichua Formation: Pure and impureimpure dolomitic marble, marble, calc silicate rocks etc. dolomitic marble with minor red,diopsidites etc. yellow and grey chert.

Junewani Stage: Muscovite Junewani Formation: Muscovite-biotite schist, autoclastic biotite schist; quartz-biotiteconglomerate. granulite etc.

Chorbaoli (=Ramtek?) Stage: Chorbaoli Formation: Quartzite Chorbaoli (Ramtek) Formation1:Quarzite and quartz- quartz-schist, quartz- Garnet-staurolite-quartz-muscovitemuscovite schist muscovite schist. schist. Micaceous and/or cherty

quartzite locally with garnet and/ormagnetite

Mansar Stage: Muscovite- Mansar Formation: Muscovite Mansar Group: Muscovite Mansar Formation1: Biotite-fibrolite-biotite-sillimanite schist schists etc. with Mn ore schist with Mn ore and quartz-muscovite schist, dolomiticwith Mn ore. gondite marble and quartzite with Mn ore

and gondite.

Ghuksi Group: Dolomite, micaschist, quartzite etc.

Mahuli Group: Dolomitic marblequartz muscovite gneiss,feldspathic gneiss etc.

Lohangi Stage: Calcitic Lohangi Formation: Kalapatha Group: Calc gneiss, Lohangi Formation: Calc gneiss, calcmarble, Mn ore etc. Lohangi Substage: Calcitic pink marble ± Mn ore etc. silicate rocks and calcitic marble

and dolomitic marble, often with or without Mn orewith Mn ore.

Utekata Stage: Utekata Substage: Calc silicate Parseoni Group: Quartzite,Banded calc-granulites rocks, calc granulite etc. Mn-ore and gondite

Kadbikheda Stage: Magnetite Kadbikheda Substage: Quartz-biotite granulite biotite granulite etc.

Sitasaongi Formation: Quartz-muscovite gneiss,conglomerate etc.

Tirodi Biotite Gneiss: Biotite Tirodi Biotite Gneiss: Biotite gneiss1,gneiss, amphibolite etc. migmatite, tonalite gneiss, cordierite

gneiss, amphibolite etc.

1Rock types observed in the Ramtek-Mansar-Kandri area.

SA

US

AR

GR

OU

P

(

SE

RI

ES

)T

BG

Bichua Formation (dolomitic marble) on regional scale(Fig.2).

There is another major contradiction regarding the statusof the granite gneiss, occurring south of Mansar mine.Chakravarty (1973) interpreted it as orthogneiss, intrusiveinto the Sausar metasediments. Unfortunately, his map of



Mansar mine area. The compiled map of Sausar Belt byNarayanaswami et al. (1963), however, clearly showspresence of ‘orthogneiss’ (younger to Sausar Group) inMansar-Kandri area (Fig.1a). Basu and Sarkar (1966) alsointerpreted this granite as part of intrusive ‘Ghuksi Granite’.Mohanty (1993), on the other hand, termed it as TirodiBiotite Gneiss, which formed the basement to the SausarGroup. His conclusion was mainly based on the reportedoccurrence of conglomerate near the contact of mica schistand granite gneiss. This conglomerate horizon was latercorrelated to Sitasaongi Formation of Sausar Group(Mohanty et al. 2000).

Regional structural mapping has led us to conclude thatthe biotite-granite is intrusive into Sausar Group, syntectonicwith the Sausar orogeny. This is because the granite exhibitsdeformational fabrics (gneissic as well as schistosefoliations) in Waitola area, which are similar to that inadjacent SSG and are identified as product of the Sausardeformation (D

2). Occurrence of thin granite veins along

the S1 schistosity in manganese-bearing mica schist (later

crenulated by F2) suggest that the granite intrusion took place

during first deformation (D1), as observed in Satak mine,

just outside the present study area towards south (Khanet al. 2002). Subsequent strong deformations codeformedthe granite and the associated supracrustal rocks, therebydeveloping a strong foliation and/or gneissic banding in thegranite and a strong crenulation cleavage in the mica schistsof the Sausar Group. The original discordant relationbetween granite and schists may have been partly modifiedduring deformation. However, the discordance in the mapscale is apparent in the area southeast of Waitola and also tothe west of Kandri mine (Fig. 2). The conglomerate reportedby Mohanty (1993), when revisited by the present authorsat the locations mentioned by him, appeared to be an‘autoclastic breccia’, produced by intense in situ brecciationof quartz veins and thin quartzite interbands within micaschist (Fig. 4). A similar observation has been recently madeby Bopche and Siddiqi (2000). The metapelitic rock withthin interbands of psammites often produce boudins in themore competent units under intense deformation. Verycommonly, such adjoining ripped-up boudins can beperfectly matched together to define the original competentinterbands. Moreover, lengths of the boudins developed inthese extremely flattened psammitic layers far exceed theircross-sectional dimensions. This indicates that they are partof a planar layer rather than individually transported pebbles/fragments.

Biotite gneiss (TBG) is rarely exposed in the study area,barring a single exposure south of Parsoda. This isolatedoutcrop of TBG is surrounded by discontinuous outcrops

of foliated granite (Fig. 2). Nearest basement (TBG) outcropto this is near Lohdongri, about 5 km to the south.Occurrence of this small outcrop of TBG within granite maybe explained as an enclave, as indicated already, althoughthe contact relation between the TBG and the granite at thisplace is not clear because of lack of continuous exposure ina dominantly soil covered terrain. Excellent exposures oftypical biotite gneiss, however, can be observed on NH-7about 10 km north of Mansar. In our view, Tirodi BiotiteGneiss is a basket term and comprises different gneissiccomponents viz. biotite gneiss, migmatite gneiss, tonalitegneiss and enclaves of older metasediments and metabasicigneous rocks, exposed in different parts of SSFB (see alsoPal and Bhowmik, 1998). Typical biotite gneiss is mainlycomposed of plagioclase and biotite with minor amounts ofopaques, garnet and occasionally fibrolite. The biotite-richfoliated granite found south of Mansar mine is rich in K-feldspar, muscovite and biotite and is not lithologicallysimilar to biotite gneiss (TBG), sensu stricto.

On the basis of mapping carried out in the present studyarea and adjoining parts of SSFB, we propose alithostratigraphic succession for the study area (Table 1). Itis to be noted that the original stratigraphic names (cf.Narayanaswami et al. 1963) have been mostly retained.However, structural relations between different lithounitshave been given more importance than strict lithologiccriteria for local as well as regional correlation. This isbecause, our observations in different parts of SSFB suggestthat the Sausar Group has been affected by intense thrusting,recumbent/reclined folding (only in outcrop scale) andtectonic slicing at a very early stage (D

1 deformation),

leading to tectonic interleaving of basement and supracrustalunits (Chattopadhyay et al. 2001). Subsequent folding hascomplicated the structural pattern even more. This has ledus to conclude that a generalised stratigraphy based onlithologic criteria alone, cannot hold good for the Sausarfold belt.

STRUCTURE

Based on structural studies in Ramtek-Mansar-Kandriarea, four generations of structures have been identified.Structures belonging to all the four generations are preservedin the quartzite-mica schist package of Ramtek area. Weshall, therefore, take Ramtek as a type area for structuraldescription. Towards west, in Mansar-Kandri area, at leastthree generations of structures are preserved in themanganese-bearing mica schist-quartzite-dolomite package.The detailed structural map of Ramtek Synform with theequal area plots has also been furnished (Fig. 3).



Fig

.4. ‘

Aut

ocla

stic

bre

ccia

’ pro

duce

d by

brit

tle d

efor

mat

ion,

frac

turin

g an

d bo

udin

age

of th

in q

uart

zite

ban

ds w

ithin

mic

a sc

hist

. Len

s ca

p di

amet

er 5

5 m

m.

Fig

.5. I

socl

inal

F1 f

old

inqu

artz

ite,

nort

hwes

t of

Ram

tek.

Len

s ca

p di

amet

er 5

5 m

m.

Fig

.6.(

a) I

nclu

sion

tra

il (S

i = S

1) o

f qu

artz

in g

arne

t po

rphy

robl

asts

. E

xter

nal s

chis

tosi

ty (

Se =

S2)

sw

erve

s ar

ound

garn

et. S

1 and

S 2 are

dis

cord

ant;

quar

tz-m

ica

schi

st fr

om K

awad

ak, 2

km

nor

thw

est o

f Ram

tek.

Sca

le b

ar: 0

.5 m

m. G

rt: G

arne

t. S

ectio

n pe

rpen

dicu

lar

to S

2 an

d L 2.

(b)

We

akl

yfo

lded

incl

usio

n tr

ail (

S 1) o

f qua

rtz

with

in g

arne

t at t

he c

rest

of a

F2 m

icro

fold

, qua

rtz-

mic

a sc

hist

nor

th o

f Ram

tek.

Sca

le b

ar: 0

.5 m

m. T

hin

sect

ion

perp

endi

cula

r to

sch

isto

sity

(S2)

and

cre

nula

tion

axis

(F

2).

Secretary

Placed Image



Fig

.7. C

ore-

and

man

tle te

xtur

e in

qua

rtzi

te p

roto

myl

onite

, sou

th o

f Khi

ndsi

lake

. Sca

le b

ar: 0

.5 m

m. Q

: qua

rtz.

Thi

n se

ctio

n pe

rpen

dic

ular

to th

e m

ylon

itic

folia

tion

(S 1) a

nd p

aral

lel

to t

he s

tret

chin

g lin

eatio

n (L 1)

. Fig

.8. O

utcr

op-s

cale

F 2 fo

ld in

qua

rtzi

te,

sout

hern

lim

b of

Ram

tek

synf

orm

. H

amm

er le

ngth

= 2

4 cm

. F

ig.9

. Fol

ded

L 2 lin

eatio

ns o

n a

F 3 fo

ldhi

nge,

sou

th o

f Am

bala

. Pen

leng

th =

13

cm.

Fig

.10.

Sta

urol

ite g

rain

poi

kilo

blas

tical

ly e

nclo

sed

by g

arne

t. A

cru

dely

def

ined

trai

l of

quar

tz in

clus

ions

(S

1) is

obs

erve

d w

ithin

garn

et a

s w

ell a

s st

auro

lite;

mic

a sc

hist

eas

t of A

mba

la ta

nk. S

cale

bar

: 0.5

mm

. Grt

: Gar

net,

Sta

u: S

taur

olite

, par

tially

cro

sse

d ni

col.

Sec

tion

perp

endi

cula

r to

S2/

L2.

Secretary

Placed Image



Structures of First Generation

These include first generation folds (F1) and associated

axial planar cleavage (S1) and lineations (L

1). F

1 folds in

Ramtek area are generally restricted to hand specimen andoutcrop scale. They are tight to isoclinal in nature (Fig.5)with spatially variable attitude, probably due to reorientationby large scale later folding. Thus, generally recumbent-reclined F

1 folds are found in the hinge zone of large scale

F2 fold in Ramtek area while gently to steeply inclined F

1

folds occur in the F2 limbs. No large-scale F

1 closure could

be mapped in the area. It is a general observation by thepresent authors from different parts of SSFB that first folding(F

1) has taken place only on mesoscopic scale, without

causing regional scale stratigraphic inversion in the SausarGroup. The stratigraphic succession of SSG is, however,disturbed by intense tectonic slicing caused by large scaleimbricate thrusting associated with D

1 deformation (Khan

et al. 2002). There is a strong axial planar cleavage (S1)

associated with F1. This occurs in the form of strong

schistosity in quartz-mica schist and as a weak quartz-micafabric in micaceous quartzite. Regionally, S

1 strikes WNW-

ESE and dips either northerly or southerly on the limbs ofthe large-scale later folds. However, local variations in theattitude of S

1 are very common, due to variable styles of

F1 folding. Fine quartz-chlorite inclusion trails trapped

within garnet porphyroblasts in micaceous quartzite andquartz-mica schist of Ramtek area represent S

1 in

microscopic scale (Fig. 6a,b). In mica schist of Mansar-Kandri area, S

1 can be distinguished in the microlithons of

the strongly differentiated crenulation cleavage (S2) and also

at the hinges of the second generation folds. Another firstgeneration planar structure is the protomylonitic foliationin quartzite, found near Khindsi lake and at the hinge of thelarge scale F

2 fold near Ram temple. Strong stretching of

quartz grains indicating crystal-plastic deformation of quartzand cataclastic fracturing of feldspar grains are observed inthis rock. Partial recovery-recrystallisation of quartz hasgiven rise to core-and-mantle texture, where large strainedgrains of quartz are surrounded by smaller recrystallised,strain-free quartz grains (Fig. 7). This protomylonite foliationis subparallel to the S

1 cleavage and both of them are parallel

to bedding (S0) in quartzite at many places. It appears that

mylonitisation in quartzite has taken place in local scale,due to strong flattening during F

1 folding. First generation

linear structures (L1) occur mainly in the form of hingelines

of small-scale F1 folds, stretching lineation defined by

stretched grains of quartz on the mylonite plane andsometimes as a mineral lineation defined by alignment offibrolite / sillimanite. All these lineations generally plunge

southeasterly as observed on the southern limb of Ramteksynform, but local variations in the attitudes are common.

Structures of Second Generation

Second generation folds (F2) occur both on mesoscopic

(outcrop) scale (Fig. 8) and on large scale. Folding ofbedding (S

0), cleavage/schistosity (S

1) and protomylonitic

foliation define a map-scale synformal F2 fold closure in

the western part of Ramtek hill. Axis of this large scale foldhas shallow plunge (20° towards 120°) (Fig. 3b). Outcropsof incongruous, higher order F

2 folds in the northern limb

of Ramtek synform show S-type asymmetry and those onthe southern limb show Z-type asymmetry, consistent withthe geometry of the large-scale fold. S- and Z-typeasymmetry are also observed on a larger scale from the mappattern of the quartzite band defining the large-scale F

2

fold (Fig. 3a). The map scale F2 fold at Ramtek is thus

polyharmonic in style. The map scale and outcrop scale F2

folds are associated with a strong, penetrative axial planarcrenulation cleavage (S

2) that strikes WNW-ESE and dips

steeply towards north (e.g. 100°/ 78° N), although dipreversal due to fanning is observed. S

2 is defined by

dimensional parallelism of fine quartz and muscovite grains.S

2 has almost totally transposed S

1 at most of the places. In

garnetiferous micaceous quartzite northwest of Ramtek, S1

is only preserved as fine quartz-chlorite (±biotite) inclusiontrails (S

i) within garnet porphyroblasts. S

2 cleavage fabric

often swerves round the garnet porphyroblasts (Fig. 6a).The large-scale synformal fold closure in mica schist-

dolomite-quartzite sequence near Kandri mine is a secondgeneration (F

2) fold, similar to the Ramtek fold, with low

ESE-plunging (20° towards 100°) fold axis (Fig. 2). Small,disharmonic F

2 folds also occur as crenulations on S

1 in

mica schist and as outcrop scale folds in quartzite anddolomitic marble. All these folds have a general shallowsoutheasterly plunge. The dominant schistosity in mica schist(S

2) is a crenulation cleavage. However S

1 can be identified

as relics in the microlithons of this differentiated S2 cleavage.

Second generation lineation (L2) is defined by

intersection of S0 / S

1 and S

2 planes, which is observed in

many parts of the Ramtek synform, especially on the limbsof the F

2 folds. Intersection of S

1 and S

2 in mica schist

also defines a weak L2 lineation in Mansar-Kandri area.

Hingelines of F2 crenulations often define a pucker

lineation in mica schist. L2 lineations are generally parallel

to F2 axes and plunge southeasterly (20° - 25° towards 100°)

(Fig. 3b).

Structures of Third Generation

The strong intersection lineation (L2) produced by



intersection of S2 and S

0 // S

1 associated with the Ramtek

synform is folded by a set of hand specimen to outcrop scalefolds, observed south of Ambala (Fig.9) and also west ofNagarjun temple. These folds are distinguished from thesmall-scale F

2 folds by the fact that L

2 lineations observed

in the field are parallel to the F2 fold axes whereas they are

bent/folded over the F3 fold hinges. These are therefore

interpreted as F3 folds. Axes of the F

3 folds trend E-W and

plunge easterly (12° → 100°) (Fig. 3c) and make an acuteangle with the general trend of the F

2 fold axes, on both

limbs of the Ramtek synfom. Strong axial planarmetamorphic fabric is not observed with F

3 folds in this

area. This is also in contrast to the strong S2 cleavage

necessarily associated with F2. If the F

3 folds are unrolled,

the folded L2 lineation defines almost a straight line. This

implies that F3 folding was dominantly flexural folding

without significant flattening. Third generation lineationoccurs only in the form of F

3 fold mullions.

Structures of Fourth Generation

Fourth generation structures occur in the form of broadwarps with north-south striking subvertical axial plane. Largescale warping on N-S axial plane is evident from the mappattern of Ramtek synform (Fig. 2). Smaller warps are alsoobserved at a few places in Ramtek area. There is no markedaxial planar fabric with the F

4 folds in the present study

area. However, the authors have observed close spacedfractures associated with F

4 in other parts of SSFB. Although

the effect of broad F4 warps can not be directly observed on

F3, their relative chronology can be worked out from the

overall warped pattern of the F2 limbs bearing the F

3 folds.

Late Recumbent Folds

In the quartz-mica schist occurring in the core of Ramteksynform near Ambala tank, a set of weak kink-type foldswith subhorizontal axial planes and shallow ESE plungingaxes, have developed on steeply dipping S

2 schistosity (cf.

Naha and Halyburton, 1974). These folds certainly postdateF

2 folding, but their geometry and structural attitude do not

match with either F3 or F

4 folds. A similar type of fold is

observed in dolomitic marble south of Kandri mine. Thepresent authors have recently observed this type of fold inmica schist in Nakadongri area in the north Bhandara Districtalso. Exact timing of this folding vis-a-vis F

3 and F

4 is yet

to be worked out due to lack of suitable exposures exhibitingunambiguous overprinting relationship. These weaklydeveloped recumbent kinks, which can never be correlatedwith the early recumbent folds of Sausar (F

1), may have

developed in the late phase of Sausar orogeny whensubvertical planar structures (schistosity, bedding etc.) got

weakly kinked due to gravity induced sagging underincreased vertical load of the thickened crust. Similarstructures caused by inhomogeneous gravity-induced flowwithin a thickened crust has been observed and explainedin recent experimental analyses (Chattopadhyay andMandal, in press).

Map Pattern of the Ramtek Area

Mohanty (1988) interpreted that the quartzite-mica schistunits around Ramtek preserve evidences of three generationsof superposed folding. The map-scale synformal closure onRamtek hill was identified by him as a second generationfold (F

2) with shallow easterly to southeasterly plunge. On

the basis of structural mapping of the Ramtek area, Mohanty(1988) interpreted that this large scale F

2 fold actually

refolded an earlier large scale fold (F1). Large scale

superposition of F2 on F

1 thus produced boomerang-shaped

outcrop pattern (Fig.2, Mohanty, 1988). In our opinion,Mohanty’s (1988) interpretation that Ramtek synform is alarge-scale non-plane, non-cylindrical superposed foldstructure is untenable because of the following reasons:(1) The southern limb of Ramtek synform does not

terminate south of Ambala, as shown by Mohanty(1988). It rather continues farther southeast, up toNawargaon, where a large-scale antiformal F

2 closure

is observed (Fig. 3a). Thus the southern quartzite bandcan not be joined with the northern limb, as shown inhis map.

(2) Map-scale F1 closures have been shown and described

by Mohanty (1988) from the eastern tip of the southernridge. However, outcrop scale folds at this locationare all F

2 folds as they have developed not only on

bedding but also on a cleavage and a protomylonitefabric (S

1 planes).

(3) The folded quartzite unit found in the core of Ramteksynform east of Ambala tank, is a stratigraphicallydifferent quartzite band occurring above the mainquartzite unit, and is cofolded with it. The quartziteunits are separated by garnet-staurolite-quartz-muscovite schist. The upper and lower quartzite unitscannot therefore be considered as repetition due totight F1 folding, as implied in Mohanty’s (1988) map.

(4) A large F1 closure has been shown in the northern

limb, near Nagarjun temple. There is however a largeS-shaped F

2 fold in this area and the quartzite band

continues after this fold eastward for several kilo-metres (Fig. 3a). This is also clearly seen in the aerialphotographs of the area (on 1:25,000 scale).

Based on the above evidences, collected through regionalmapping as well as study of aerial photographs, we



reinterpret the map scale synformal fold near Ramtek, as apolyharmonic, second generation (F

2) fold. Map pattern in

the study area (Figs. 2,3) is generally defined by the largescale F

2 folds. Map scale F

4 warps also control the map

pattern in Ramtek area. F1 and F

3 folds developed only in

outcrop and hand specimen scale and are not directlyidentifiable from the map pattern.

METAMORPHISM VIS-A-VIS DEFORMATION

Regional dynamothermal metamorphism in SSFBspatially varies from low to high grade. Earlier workers(Narayanaswami et al. 1963) pointed out that there is agradual increase in metamorphic grade from the eastern(chlorite-biotite zone) through central (kyanite-staurolitezone) to western part (sillimanite zone) of SSFB. It wasalso suggested that high grade metamorphism has led topartial melting of Sausar metasediments which generatedthe gneiss-migmatite rocks (TBG) (Phadke, 1990). Pal andBhowmik (1998), on the other hand, categoricallydiscounted the possibility of partial melting of Sausar Group.They also envisaged that grade of metamorphism in SSFBincreases both from east to west and from south to north.According to them, low grade metamorphism (uptogreenschsit facies / garnet-zone) is observed in the southern(Mansar-Kandri area) and eastern part (Ukwa area) of SSFB.The metamorphic grade increases towards north and west.It reaches amphibolite facies in the central part (aroundDeolapar) and upto upper amphibolite facies near thenorthwestern extremity of SSFB (see Fig 1 for locations).Although occurrence of sillimanite blades in pelitic schistsof Mansar area was reported by Pal and Bhowmik (1998),they argued that the sillimanite is derived from fibrolite withrising temperature within the realm of low to medium grademetamorphism (without crossing K-feldspar-sillimaniteisograd) and does not indicate high grade (sillimanite zone)metamorphism. Development of fibrolite in mica schist,according to them, is by ‘base leaching’ i.e. removal of Ca+

and K+ ions from feldspar grains by a hydrogenous fluid,through an intermediate stage of formation of muscovite(Pal and Bhowmik, 1998). Quartzite and associated quartz-mica schist exposed around Ramtek and the underlyingmanganese bearing mica schists exposed in Kandri-Mansararea exhibit similar metamorphic signatures as revealed bytheir metamorphic mineral assemblages. Detailed study onmetamorphic evolution was beyond the scope of the presentstudy. But an attempt is made to correlate deformation andmetamorphism through the study of S

i - S

e tectonites.

Micaceous quartzite from northwest of Ramtek hillshows development of large porphyroblasts of garnet

(almandine variety) along with finer grained quartz,muscovite and opaque minerals. The garnet grains preservefine inclusion trails of quartz, chlorite and/ or biotite asinternal schistosity (S

i). A strong external schistosity (S

e)

defined by quartz, muscovite and opaques swerves roundthese garnet porphyroblasts (Fig. 6a,b). S

i is sometimes

straight and planar and is discordant to Se (Fig. 6a)

(cf. Fig.8 of Bell and Rubenach, 1983). Macroscopically, Si

represents S1 and S

e represents S

2 of Sausar deformation.

This indicates that garnet formed at a time when S1

schistosity was already well developed, but the subsequentdeformation (crenulation of S

1) was yet to set in. Thus

formation of garnet, probably from an initial assemblage ofquartz, chlorite and/or biotite, post-dated S

1 (D

1 deformation)

but predated S2 development (D

2 deformation). Some garnet

grains have overgrown weakly crenulated S1 (Fig. 6b),

indicating that the garnet formed at a time when crenulation(F

2) of S

1 had already started (cf. Fig. 9 of Bell and

Rubenach, 1983). This suggests that garnet formation maybe, at places, synchronous with the early stage of D

2

deformation (Bell and Rubenach,1983).Quartz-mica schist associated with the quartzite of

Ramtek area shows development of porphyroblasticstaurolite in addition to almandine garnet. Garnet grains arelarge and poikiloblastically include staurolite grains(Fig. 10). A crudely defined trail of fine-grained quartz andopaque is observed within the garnet as well as the staurolitegrains, which represents S

1. Garnet and staurolite formation

is thus post-D1. Microstructural evidences are insufficient

to work out the relative timing of growth of garnet andstaurolite. Presence of staurolite and almandine garnetindicates that grade of regional dynamothermal meta-morphism of pelitic schists having Fe-rich bulk compositionin the study area may have reached ‘staurolite-in’ isogradi.e. onset of amphibolite facies of metamorphism (Winkler,1973), although more detailed study of metamorphic reactiontextures are needed before the grade of metamorphism isconcluded finally.

From the foregoing description, it emerges that thequartzite and quartz-mica schists of Mansar and ChorbaoliFormations exposed in the study area may have undergoneregional dynamothermal metamorphism of greenschist toamphibolite facies (upto ‘staurolite-in’ isograd). Peakmetamorphism (indicated by appearance of staurolite and/or garnet) certainly post-dated D

1 deformation and was

possibly pre- to syn-D2 deformation in these rocks.

CONCLUSIONS

From regional geological mapping, structural studies and



stratigraphic correlation of the Sausar Group of rocks inRamtek-Mansar-Kandri area, Maharashtra, the followingmajor conclusions can be drawn:(1) Stratigraphic succession of the Sausar Group should

not be built on strict lithologic criteria alone. Structuraldisposition of rocks should be taken into accountbefore building the local stratigraphic succession ofthe study area, as the rocks of Sausar Group in thisarea have been intensely folded. Thin bands ofdolomitic marble and quartzite within manganese -bearing mica schists of Mansar Formation areinterpreted as sedimentary intercalations rather thanfolded repetitions of stratigraphically higher Bichuaand Chorbaoli Formations as was interpreted earlier(cf. Chakravarty, 1973).

(2) Manganese ore and gondite horizons of Mansar andKandri mines, represent different parts of a largescale folded structure. Manganese-bearing quartz-sillimanite schists can be generally taken as a markerhorizon for regional stratigraphic correlation.

(3) The map-scale Ramtek synform is a polyharmonicsecond-generation fold (F

2) with southeasterly plunge

and E-W striking, steeply northerly dipping axial plane.Large-scale F

1 fold closures, as was interpreted by

Mohanty (1988), do not occur on the limbs of this F2

structure. Ramtek synform is therefore not a non-plane,non-cylindrical large scale superposed structure asinterpreted earlier (Mohanty 1988). Similar south-easterly plunging, large scale synformal fold closureis exposed near Kandri mine.

(4) At least four generations of folding are identified in

the Sausar Group. F1 occurs only in hand specimen to

outcrop scale; F2 occurs both as outcrop scale as well

as map scale fold; F3 occurs only locally in some parts

of Ramtek hill and is necessarily small scale fold thatshows bending of L

2 lineation. F

4 occurs as broad

warps with N-S axial plane. All the four generationsare preserved in Ramtek area. At least threegenerations of structures (F

1, F

2 and F

4) can be

observed in Mansar-Kandri area also.(5) Regional dynamothermal metamorphism of pelitic

schists and quartzite in the study area reached up toamphibolite facies (‘staurolite-in’isograd). Peakmetamorphism post-dated first deformation (D

1).

(6) The granite gneiss found south of Mansar mine, andcontinuing along the southern flank of Mansar ridgeupto the west of Kandri mine, is an intrusive granite,emplaced syntectonically(possibly syn-D

1) into the

Sausar Group and codeformed with it (cf. ‘orthogneiss’of Chakravarty, 1973). It does not represent truebasement (Tirodi Biotite Gneiss). The conglomeratehorizon described by Mohanty (1993) from Waitolaarea appears to be an ‘autoclastic breccia’ formed byintense brittle deformation of quartz veins and thinquartzite interbands within mica schist.

Acknowledgements: The work has been carried out as part ofa Specialised Thematic Mapping (STM) programme of the Sausarfold belt by Geological Survey of India (GSI). We thank the Dy.Director General, Central Region, and Dy. Director General, Op.Maharashtra, GSI, Nagpur for permitting us to publish these data.Technical discussions with Dr. Abhinaba Roy and colleagues ofPetrology Division have greatly benefited us during mapping.

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(Received: 14 March 2001; Revised form accepted: 10 April 2002)

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