time relationship between metamorphism and deformation in ... filethe southern margin of the...

Time relationship between metamorphism and deformation in Proterozoicrocks of the Lunavada region, Southern Aravalli Mountain Belt (India) Ð

a microstructural study

Manish A. Mamtania,*, S.S. Merhb, R.V. Karanthb, R.O. Greilingc

aDepartment of Geology & Geophysics, Indian Institute of Technology, Kharagpur-721302, West Bengal, IndiabFaculty of Science, M.S. University of Baroda, Vadodara-390002, Gujarat, India

cGeologisch-PalaÈontologisches Institut, Ruprecht-Karls-UniversitaÈt Heidelberg, INF-234, D-69120, Heidelberg, Germany

Accepted 2 May 2000

Abstract

The southern margin of the Aravalli Mountain Belt (AMB) is known to have undergone polyphase deformation during the Mesoproter-

ozoic. The Lunavada Group of rocks, which is an important constituent of the southern parts of AMB, reveals three episodes of deformation;

D1, D2 and D3. In this paper, interpretations based on petrographic studies of schists and quartzites of the region are presented and the

relationship between metamorphic and deformational events is discussed. It is established that from north to south, there is a marked zonation

from chlorite to garnet±biotite schists. Metamorphism (M1) accompanied D1 and was progressive. M2-1 metamorphism associated with major

part of D2 was also progressive. However, M2-2 that synchronized with the waning phases of D2 and early-D3 deformation was retrogressive.

Porphyroblast±matrix relationships in the garnet±biotite schists of the region have been useful in establishing these facts. The metamorphic

rocks studied were intruded by Godhra Granite during the late-D3/post-D3 event. The heat supplied by this granite resulted in static

recrystallization and formation of annealing microstructures in rocks close to the granite. It is established that Grain Boundary Migration

Recrystallization associated with dislocation creep and Grain Boundary Area Reduction were the two deformation mechanisms dominant in

rocks lying far and close from the Godhra Granite, respectively. q 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction

The Southern Aravalli Mountain Belt (SAMB) forms the

southernmost tip of the Aravalli Mountain Belt (AMB)

which is a major Proterozoic orogenic belt in northwestern

India (Fig. 1). The SAMB occupies an area of more than

30,000 km2 extending from southern parts of Rajasthan into

northeastern Gujarat and comprises metasedimentary and

granitic rocks. The metasediments belong to the Lunavada

and Champaner Groups of the Aravalli Supergroup (Gupta

et al., 1992, 1995). Mamtani (1998) and Mamtani et al.

(1999a, 2000) have worked out the structural geology of

the area around Lunavada. In the present paper, various

microstructures observed in schists and quartzites of the

Lunavada region are described. These microstructures

have been used to understand microscale deformation

mechanisms. Moreover, a correlation is established between

metamorphic and deformation events on the basis of

porphyroblast±matrix relationships preserved in garnet±

biotite schists of the region.

2. Geological setting and structural geology

The Proterozoic rocks of the Lunavada region, Panchma-

hals district, Gujarat are assigned to the Lunavada Group

which is the second youngest group of the Aravalli Super-

group (Gupta et al., 1980, 1992, 1995). The Lunavada

Group comprises phyllite, mica schist, calc-silicate,

quartz±chlorite schist, meta-subgreywacke, meta-siltstone,

meta-semipelite, meta-protoquartzite with minor layers and

thin sheets of dolomitic marble, petromict meta-conglomer-

ate, manganiferous phyllite and phosphatic algal meta-dolo-

mite (Gupta et al., 1980, 1992, 1995). It occupies an area of

10,000 km2 in the SAMB and is ¯anked in the northeast and

northwest by the Udaipur and Jharol Groups of the Aravalli

Supergroup (Fig. 2). To its west and south lie the Godhra

granite and gneisses. The Godhra granite has been dated as

955 ^ 20 Ma by Rb/Sr method (Gopalan et al., 1979). These

granitic rocks have an intrusive relationship with the

Journal of Asian Earth Sciences 19 (2001) 195±205

1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.

PII: S1367-9120(00)00029-8

www.elsevier.nl/locate/jseaes

* Corresponding author.

E-mail address: [email protected] (M.A. Mamtani).

M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205196

Fig. 1. Generalized geological map of the AMB. Box in the southern parts marks the area of Fig. 2. Arrow points to the SAMB. Map is after published maps of

Geological Survey of India.

Fig. 2. Lithostratigraphic map of southern parts of AMB (after Gupta et al., 1995). A, B and C marked by asterisk are locations of schist samples for which

CSD studies were done. Q1, Q2, Q3 and Q4 marked by asterisk in circle are locations of quartzite samples which were subjected to CSD measurements. Inset:

L is Lunavada and G is Godhra.

surrounding metasedimentary rocks. The rocks of the south-

ernmost part of SAMB belong to the Champaner Group

which comprises of low grade phyllites and quartzites.

The present investigation was carried out around the

towns of Lunavada, Santrampur and Kadana where the

rocks encountered are quartzites alternating with schists

along with some calc-silicate bands. The quartzites form

long ridges whilst the schistose rocks occur in the low-

lying areas. According to Iqbaluddin (1989), the quart-

zite±schist layers belong to the Kadana Formation of the

Lunavada Group.

Field and satellite imagery studies have shown that

the quartzite ridges have a complex regional scale

outcrop pattern which is characteristic of a history

involving polyphase folding (Fig. 3). The northern part

of the study area shows tight D2 folds, closely spaced

axial plane fractures and joints. Shearing is observed to

have occurred along these axial plane fractures during

D3 deformation (Mamtani et al., 1999a). The southern

part of the study area (around Lunavada, Santrampur

and further south in Fig. 3) is characterized by regional

scale folds. Mamtani (1998); Mamtani et al. (1998,

1999a, 2000) have worked out the structural history of

the region which is summarized below:

1. The Proterozoic rocks of the Lunavada region have

M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 197

Fig. 3. Geological map of the study area. Schists of different metamorphic grades are shown by different symbols. Inset: Arrow points to study area.

undergone three episodes of deformation, viz. D1, D2

and D3.

2. The ®rst two deformational events were coaxial and

resulted in NE±SW trending folds.

3. The third episode of deformation resulted in open folds

with trends varying between E±W and NW±SE.

4. Except for the presence of a few D3 kinks and minor fold

axis, there is no other mesoscopic evidence of D3 folding.

D3 folds have developed on km-scale limbs of the D1±D2

folds.

5. The superposition of the three folds in various combina-

tions has resulted in the development of different types of

large scale interference patterns. Type-III interference

pattern (Ramsay and Huber, 1987) has developed on

account of superposition of D1 and D2 folds while

Type-I interference pattern has developed due to super-

position of D3 on D1±D2 folds.

6. The degree of overturning of D2 folds increases from

north to south. The folds are upright in the northernmost

part of the area (around Ditwas). In the south, they get

overturned with a southeasterly vergence.

3. Microstructures and mechanisms of deformation

Petrographic study of schists from the study area has

revealed that the regional metamorphism progressed up to

lower amphibolite facies. This has resulted in the develop-

ment of porphyroblasts of garnet and biotite. From north to

south, a zonation from chlorite to garnet±biotite schist

through biotite schist is recorded (Fig. 3). In this section,

the various microstructures observed in quartzites and

different types of schists are described and have been used

to decipher deformational mechanisms.

3.1. Discrete crenulation cleavage

This has developed in chlorite schists in the northern parts


Fig. 4. (a) Photomicrograph of chlorite schist showing presence of S0, S1 and S2 on the microscale. The bedding plane (S0) is de®ned by the contact between

quartz-rich and quartz-poor layers. The schistosity S1 is sub-parallel to S0 and is marked by chlorite and muscovite. The schistosity S2 is a discrete crenulation

cleavage which has developed at high angles to S0 and S1. The occurrence of the discrete crenulation cleavage is restricted to the quartz-poor (phyllosilicate-

rich) layers. (b) Photomicrograph documenting drag effect along discrete crenulation cleavage (S2) in chlorite schist. S1 foliation de®ned by muscovite and

chlorite is observed to have dragged due to movement along the cleavage. (c) Photomicrograph of chlorite schist in PPL showing microscale displacement

along S2. Scale bar is 0.4 mm in (a) and (c), and 0.1 mm in (b). Location: Ditwas (north of Kadana).

of the study area. In these rocks, three planar surfaces

are recognizable, viz. S0 (bedding plane), S1 (®rst

schistosity) and S2 (discrete crenulation cleavage)

(Figs. 4 (a)±(c)). The rocks have preserved primary

lithological layering (S0) which is marked by alternating

layers of quartz-rich and phyllosilicate rich layers. The ®rst

schistosity (S1) is sub-parallel to S0 and comprises of chlorite,

quartz and muscovite crystals aligned parallel to one another.

The second schistosity is the discrete crenulation cleavage (S2)

which was formed on account of crenulation of S1 foliation

during D2. The S2 has developed almost perpendicular or at

high angles to the S1 and is observed to have formed only in the

phyllosilicate rich layers. There is evidence of displacement

along the S2 surface (Fig. 4(c)). Similar evidence has been

linked by Gray (1979) to pressure solution. However, at the

present scale of observation, no signi®cant evidence of recrys-

tallized quartz aggregates and no metamorphic differentiation

in the vicinity of the discrete crenulation cleavages along

which the displacement occurred has been observed. More-

over, Fig. 4(b) shows some microscale dragging along the

cleavages. Therefore the possibility of these discrete

crenulation cleavages being planes of shear cannot be

totally ruled out.

3.2. Differentiated crenulation cleavage

This has developed in the higher grade schists of the

region and is particularly well developed in the garnet±

biotite schists to the south of Lunavada and Santrampur. It

is made up of alternating quartz (Q) and mica (M) domains

(Fig. 5). Two schistosities (S1 and S2) are prominent micro-

scopically. S1 is made up of chlorite, muscovite and biotite

crystals while new generation biotite and muscovite ¯akes

are developed parallel to S2. The M-domains vary in thick-

ness from 0.1 to 0.5 mm. A few of these zones also preserve

a shear band cleavage that lies at a low angle (,458) to the

domain boundary between M and Q domains (Mamtani and

Karanth, 1996a; Mamtani et al., 1999b). All these micro-

structures in the cleavage zones have been used to interpret

the mechanisms of deformation during origin of crenulation

cleavages (Mamtani et al., 1999b). Accordingly it has been

argued that pressure solution is an important deformational

mechanism during the early stages of crenulation and this

imparts the domainal fabric to the rock. However, intracrys-

talline crystal plastic deformation becomes dominant during

the later stages which results in the development of shear

structures in cleavage zones.

3.3. Millipede microstructure

This microstructure is characterized by oppositely

concave microfolds (OCMs) and usually occurs as inclusion

trails (Si � internal foliation) within porphyroblasts in

schists (Bell and Rubenach, 1980). Millipede microstructure

is preserved in some biotite porphyroblasts in garnet±biotite

schists of the study area (Figs. 6(a) and (b)). It is de®ned by

oppositely curving quartz inclusion trails (S1) within the

biotite porphyroblast. S1 is relatively straight in the core

of the biotite and gradually curves towards the rims and

continues to merge into the external schistosity (S2). Similar


Fig. 5. Differentiated crenulation cleavage (S2) in garnet±biotite schist.

Scale bar is 0.4 mm. Location: Anjavana area (southeast of Lunavada).

Fig. 6. (a). Photomicrograph of biotite porphyroblast in garnet±biotite schist showing presence of millipede microstructure characterized by oppositely

concave microfolds (OCMs) of quartz±feldspar inclusion trails (S1) within the porphyroblast. (b) Explanatory line drawing of photomicrograph in (a). Scale

bar is 0.1 mm. Location: Vankdi (south of Anjavana).

structures are known to develop around rotating rigid

objects at low strains in laboratory experiments (Ghosh,

1975, 1977; Ghosh and Ramberg, 1976). Johnson and

Moore (1996) and Johnson and Bell (1996) have stated

that the presence of millipedes indicates a state of low strain

during their genesis. Since the microfolds that make up the

millipedes within the biotite are open compared with those

in the matrix, the biotite porphyroblast is interpreted to have

grown under a low strain state during D2.

3.4. Textures in quartzites

Thin sections prepared from different localities of the

area show that the quartzites comprise of two textural vari-

eties based on grain boundaries Ð either the grain bound-

aries are irregular or they are straight. The irregular grain

boundaries (Fig. 7(a)) are prevalent dominantly in the quart-

zite occurrences that are distant from the Godhra Granite.

According to Urai et al. (1986) and Passchier and Trouw

(1996), the presence of irregular grain boundaries indicates

intracrystalline deformation as the rock underwent dynamic

recrystallization by Grain Boundary Migration (GBM).

Some of the quartz crystals show subgrains (Fig. 7(b)), a

textural feature pointing to recovery during dynamic recrys-

tallization. This also indicates that during deformation,

recrystallization-accommodated dislocation creep was

important (Nicolas and Poirier, 1976; Tullis and Yund,

1985; Tullis et al., 1990; Passchier and Trouw, 1996). Dislo-

cation creep has been recognized as an important deforma-

tion mechanism for quartz aggregates under conditions of

greenschist facies or higher (White, 1976; Mitra, 1978;

Hirth and Tullis, 1992).

Thin sections of quartzites occurring closer to the margin

of the Godhra Granite show a granoblastic texture charac-

terized by straight to smoothly curved grain boundaries,

1208 triple points and sharp extinction (Fig. 7(c)). These

microstructural characteristics clearly point to static recrys-

tallization with Grain Boundary Area Reduction (GBAR) as

the principal mechanism (Passchier and Trouw, 1996). The

presence of 1208 triple points, referred to as foam micro-

structure by Vernon (1976), is indicative of heat outlasting

deformation or annealing. Bons and Urai (1992) and Passch-

ier and Trouw (1996) have stated that GBAR is especially

pronounced at high temperatures after deformation ceases,


Fig. 7. (a) Photomicrograph of quartzite showing irregular grain boundaries between quartz crystals implying dynamic recrystallization or GBMR (Grain

Boundary Migration Recrystallization). (b) Photomicrograph of quartzite showing subgrain microstructure in quartz crystals which points to recovery during

dynamic recrystallization. (c) Photomicrograph of quartz crystals in quartzite showing granoblastic texture characterized by straight grain boundaries and 1208

triple points. The quartz crystals have sharp extinction. These microstructures are interpreted to indicate that the rock underwent static recrystallization. See

text for detailed discussion. Location: (a) and (b) Anjavana; (c) Boriya. Scale bar is 0.2 mm in (a) and (c), and 0.1 mm in (b).

i.e., in a static environment. In the present case, the high

temperature for static recrystallization was supplied by the

Godhra Granite that intruded the region. Fig. 7(a) and (c) are

photomicrographs (taken at same magni®cation) of quart-

zites occurring far and close to the granite margin. It is quite

clear that the former has ®ner crystals while the latter has

coarser crystals. This indicates that the heat supplied by the

granite played an important role in microstructure develop-

ment of the latter. Further corroboration of this fact has also

come from Crystal Size Distribution (CSD) study of quartz

crystals in schists and quartzites. Moreover, post-deforma-

tional changes in microstructure are known to occur at the

end of an orogeny when deformation has essentially ceased

and the rocks are at high temperatures (.3008C) or when

deformed rocks are subjected to sustained heating from

post-tectonic plutons (Knipe, 1989) and also in laboratory

experiments with octachloropropane (Ree and Park, 1997).

It is envisaged that prior to the intrusion of granite, the

quartz crystals were in a higher strain condition character-

ized by irregular grain boundaries. Such a microstructure is

thermodynamically unstable and would have a tendency to

proceed to a lower energy state. The late to post deforma-

tional granitic intrusion provided the necessary heat energy

required for release of internal strain and achievement of a

thermodynamically stable microstructure. As a result, a

stable granoblastic microstructure developed which is

more pronounced in the rocks close to the granite margin.

It can be argued that a granoblastic fabric can also form

syntectonically by dynamic recrystallization (Means and

Ree, 1988) or in high grade gneisses (Passchier et al.,

1990). However, in the present study, it is clearly seen

that the quartzites close to the granite show a granoblastic

texture, sharp extinction and coarser grain size. Quartzites

farther from the granite show irregular grain boundaries,

sub-grains, a ®ner grain size and absence of a granoblastic

texture. It is therefore logical to assume that the microstruc-

tures in quartzites close to the granite are a result of static

recrystallization by GBAR related to heat supplied by the

granite. This is in accordance with Bons and Urai (1992)

and Passchier and Trouw (1996) who have suggested that

GBAR is pronounced only after the deformation ceases.

4. Porphyroblast±matrix relationships

The mica schists around Lunavada and Santrampur are

characterized by foliations of different generations and

porphyroblastic minerals such as garnet and biotite which

contain foliations as quartz inclusion trails. The relationship

between the internal foliation (Si) within the porphyroblasts

and the matrix foliation (Se) outside the porphyroblast was

used to determine the relative timing of growth of minerals

with reference to foliation of a particular generation. This is

in accordance with the criteria described by Zwart (1962),

Spry (1969), Vernon (1976), Ghosh (1993), and Passchier

and Trouw (1996).

Most of the garnet and biotite porphyroblasts preserving

the microfolded or sigmoidal inclusion trails are identi®ed

as syntectonic with D2 deformation (Figs. 8(a) and (b); also

Fig. 6). The intensity/tightness of folding of the inclusions

with respect to those in the matrix has been further useful in

classifying the porphyroblasts as early-D2 or late-D2. Fig.

8(a) shows a porphyroblast of biotite with quartz inclusion

trails (Si � S1) which show open microfolds. In contrast, the

microfolds outside the porphyroblast are tightly crenulated.

This indicates that the biotite porphyroblast grew during the

early stages of D2 deformation. A few porphyroblasts

preserve relatively tight S1 crenulations and also include

the S2 foliation at the rims (Fig. 8(b)). Such pophyroblasts

are classi®ed as late-D2. Some garnet porphyroblasts

preserve sigmoidal S1 inclusion trails of quartz and feldspar

which gradually curve into S2 while the cleavage domain

outside the porphyroblast has a single homogenized folia-

tion S2 (Fig. 8(c)). It is envisaged that the sigmoidally

curving S1 schistosity along with S2 was included in the

garnet porphyroblast during earlier stages of D2. With conti-

nuing deformation, the matrix foliation further deformed

and rotated into parallelism with the S2 while the sigmoidal

relation between S1 and S2 within the porphyroblast

remained frozen in the same stage at which it was included,

thus remaining unaffected by later deformation (Mamtani

and Karanth, 1997). Such porphyroblasts of garnet are also

classi®ed as syn-D2.

5. Thermal metamorphism

Regional metamorphism in the Lunavada±Santrampur

region was followed by thermal metamorphism related to

the intrusion of the Godhra Granite. The effects of heat

supplied by the Godhra granite are signi®cant in the south-

western part of the study area, i.e., to the south of Lunavada.

Since the granite does not lie in the immediate vicinity of

the study area, common high-temperature metamorphic

minerals like andalusite and sillimanite are not observed.

Nevertheless, the effect of the thermal event is quite obvious

from the CSD studies on rocks of the region. The method of

measuring CSDs using thin sections of rocks has been

described by Marsh (1988) and Mamtani and Karanth

(1996b). CSD studies provide statistical data for crystals

(of a particular mineral) of different sizes within a unit

area of a thin section. Based on this data, CSD plots such

as size (L mm) vs. normal log of population density [ln�n�]can be prepared. The shape of a CSD plot represents the

extent to which a rock underwent annealing.

In the present investigation, CSD of quartz crystals were

calculated in thin sections of three schist and four quartzite

samples collected at varying distance from the boundary of

the Godhra Granite. Fig. 2 shows the location of the schist

and quartzite samples. Figs. 9(a) and 9(b) are CSD plots for

schist and quartzite samples, respectively. Both rock types

indicate that, in comparison with samples away from the


Godhra Granite boundary, samples close to the granite

possess (i) quartz crystals which have crystallized over a

wider size range, (ii) CSD plots with a lower slope, and

(iii) fewer quartz crystals within a unit area. Moreover,

the CSD plots for schists lying close to the granite have a

bell shape (A and B in Fig. 9(a)) while the plot for sample

away from the granite is near linear (C in Fig. 9(a)). This

indicates that all the rocks initially underwent continuous

nucleation and growth. Subsequently, rocks closer to the

granite underwent annealing such that smaller crystals

were resorped at the expense of larger crystals (see Cash-

man and Ferry, 1988; Cashman and Marsh, 1988 and

Mamtani and Karanth, 1996b for details of CSD plot inter-

pretations). The heat for annealing was supplied by intru-

sion of the Godhra Granite.

6. Discussion

On the basis of the present petrographic study, the time

relationship between deformation and metamorphism can

be established. The metamorphic history of chlorite schists

occurring in the northern parts of the study area is rather

simple. As mentioned earlier, these rocks show three promi-

nent planar surfaces (S0, S1 and S2). S1 and S2 developed

during D1 and D2 respectively. Chlorite and muscovite crys-

tals formed during D1. These underwent rotation along S2

and some recrystallization during D2. No evidence of

growth of new minerals cutting across D2 is observed in

the chlorite schists, thus implying that D3 was generally

devoid of any metamorphism. The chlorite schists therefore

only record a single metamorphic event. The paragenesis

observed is chlorite 1 muscovite 1 quartz which is typical

of a chlorite zone within the greenschist facies (Yardley,

1989; Spear, 1993). The garnet±biotite schists of the region

are most important in determining the various metamorphic

events that accompanied different deformation episodes.

These possess differentiated crenulation cleavage character-

ized by alternating Q and M domains. Garnet, biotite, chlor-

ite, muscovite and quartz are the major minerals present.

Chlorite and biotite crystals occur along foliations of differ-

ent generations and are accordingly classi®ed. Chlorite(1)


Fig. 8. (a) Syn-D2 biotite porphyroblasts in garnet±biotite schist. (b) Photomicrograph of biotite porphyroblast with microfolded quartz inclusion trails. The

biotite is interpreted as late-syn-D2. (c) Photomicrograph of garnet porphyroblast which has grown over a crenulation cleavage zone (S2). Both S1 and S2 are

present within the garnet and the inclusion trails of S1 curve sigmoidally into S2. However, the cleavage zone in the matrix (outside the garnet) is characterized

by only a single schistosity (S2). This implies that the garnet grew over the crenulation cleavage during D2 deformation. Scale bar is 0.4 mm in (a), 0.2 mm in

(b), and 0.1 mm in (c). Location: (a), (b) and (c) Vankdi (south of Anjavana).

and biotite(1) occur along the S1 schistosity and are syn-D1.

The metamorphic event which accompanied D1 is referred

to as M1. Biotite(2) crystals, which occur with their (001)

planes parallel to S2, have grown during D2 deformation.

Biotite(2) porphyroblasts with spiral (helictic) inclusion

trails of quartz (e.g., Fig. 8(a) and (b)) are also syn-D2.

Similarly the garnet porphyroblasts with sigmoidal inclu-

sion trails of quartz (e.g., Fig. 8(d)) are also syn-D2. The

metamorphic event which accompanied a major part of D2

deformation is referred to as M2-1. This was a progressive

metamorphic event during which biotite(2) crystals grew

along S2. That these progressive events (M1 and M2-1)

were followed by retrogressive metamorphism (M2-2) during

the waning phases of D2/early D3 is evident by the presence


Fig. 9. (a) CSD plot for schist samples, viz. Sample A, B and C collected at 3, 4 and 22 km distance from margin of Godhra Granite. (b) CSD plot for quartzite

samples, viz Q1, Q2, Q3 and Q4 collected at 4, 10, 22 and 30 km distance from margin of Godhra Granite. Location of each sample with reference to contact of

Godhra Granite is shown in Fig. 2.

of (a) chlorite(2) crystals that overgrow S2 foliation, (b)

chlorite around syn-D2 garnet and (c) chlorite along frac-

tures in garnet that penetrate from core to the rim.

The last metamorphic event to affect the rocks was a late-

D3/post-D3 thermal metamorphism. In rocks that lie close to

the granite, this event resulted in annealing, coarser crystals

and the development of granoblastic microstructure in

quartzites. It is concluded that the thermal event led to static

recrystallization of quartz on the microscale due to the heat

supplied by intrusion of the Godhra Granite. Emplacement

of the granite may have been initiated during the waning

phases of D3. However, the ®eld evidence for granite and

related pegmatites intruding the foliation in schists indicates

that the intrusion continued even after D3. This further

supports the interpretation that the development of grano-

blastic texture, annealing and grain growth in quartzite

occurred due to static recrystallization on the microscale.

It is also observed that muscovite crystals in garnet±biotite

schists lying close to the granite are large and free from the

effects of intracrystalline deformation such as undulose

extinction. This indicates that the thermal event also

resulted in recrystallization and grain growth of muscovite.

Fig. 10 summarizes the time relationship between

crystallization and deformation of various minerals in

garnet±mica schists.

7. Conclusions

The present study has provided considerable insight into

the metamorphic history and deformation mechanisms of

the Proterozoic rocks around Lunavada, SAMB (India).

The following conclusions are evident:

1. The rocks of the Lunavada region have undergone meta-

morphism up to lower amphibolite facies. There is a

progression from chlorite grade in the northern parts to

garnet grade in the southern parts.

2. Progressive regional metamorphism M1 and M2-1 accom-

panied D1 and a major part of D2 respectively. M2-2 was a

retrogressive event that accompanied the waning stages

of D2 or early D3 deformation.

3. A thermal event related to late-D3/post D3 Godhra Gran-

ite intrusion followed regional metamorphism. This led

to static recrystallization on the microscale and grain

growth in rocks close to the granite.

4. GBM associated with dislocation creep is suggested to

have been an important deformation mechanism in

quartzites lying far from the granite margin.

5. Annealing by GBAR has been discerned in quartzites

close to the granite.

Acknowledgements

The present paper is an outcome of the doctoral

research on Precambrian rocks of Lunavada region

(India) carried out by M.A.M. Financial support to

M.A.M during various stages of the study was provided

by M.S. University Research Scholarship, ®eldwork grant

from the Association of Geoscientists for International

Development (Brazil), Senior Research Fellowship of

the Council of Scienti®c and Industrial Research, New

Delhi (No. 9/114/(92)/96/EMR-I) and DAAD-Fellowship

of the German Academic Exchange Service, Bonn (No.

A/97/00792). We are grateful to Bruce Marsh and

Michael Zeig (Johns Hopkins University, USA) for carry-

ing out CSD measurements in quartzites using a ªOmni-

met Analyzerº. Comments by Jordi Carreras and an

anonymous reviewer were found useful.


Fig. 10. Diagram showing the time relationship between crystallization and deformation in garnet±mica schists of the study area. (a) shows the various

minerals that crystallized during the different deformation events, and (b) shows the correlation between metamorphic and deformation events.

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