Time Relationship Between Metamorphism and Deformation in Proterozoic Rocks of the Lunavada Region, Southern Aravalli Mountain Belt (India) Ð a Microstructural Study

Journal of Asian Earth Sciences 19 (2001) 195±205 www.elsevier.nl/locate/jseaes

Time relationship between metamorphism and deformation in Proterozoic rocks 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, India bFaculty 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 porphyroblast±matrix relationships preserved in garnet± biotite schists of the region. 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 2. Geological setting and structural geology India (Fig. 1). The SAMB occupies an area of more than 30,000 km2 extending from southern parts of Rajasthan into The Proterozoic rocks of the Lunavada region, Panchma- northeastern Gujarat and comprises metasedimentary and hals district, Gujarat are assigned to the Lunavada Group granitic rocks. The metasediments belong to the Lunavada which is the second youngest group of the Aravalli Super- and Champaner Groups of the Aravalli Supergroup (Gupta group (Gupta et al., 1980, 1992, 1995). The Lunavada et al., 1992, 1995). Mamtani (1998) and Mamtani et al. Group comprises phyllite, mica schist, calc-silicate, (1999a, 2000) have worked out the structural geology of quartz±chlorite schist, meta-subgreywacke, meta-siltstone, the area around Lunavada. In the present paper, various meta-semipelite, meta-protoquartzite with minor layers and microstructures observed in schists and quartzites of the thin sheets of dolomitic marble, petromict meta-conglomer- Lunavada region are described. These microstructures ate, manganiferous phyllite and phosphatic algal meta-dolo- have been used to understand microscale deformation mite (Gupta et al., 1980, 1992, 1995). It occupies an area of mechanisms. Moreover, a correlation is established between 10,000 km2 in the SAMB and is ¯anked in the northeast and metamorphic and deformation events on the basis of 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 * Corresponding author. 955 ^ 20 Ma by Rb/Sr method (Gopalan et al., 1979). These E-mail address: [email protected] (M.A. Mamtani). granitic rocks have an intrusive relationship with the

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. 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. surrounding metasedimentary rocks. The rocks of the south- outcrop pattern which is characteristic of a history ernmost part of SAMB belong to the Champaner Group involving polyphase folding (Fig. 3). The northern part which comprises of low grade phyllites and quartzites. of the study area shows tight D2 folds, closely spaced The present investigation was carried out around the axial plane fractures and joints. Shearing is observed to towns of Lunavada, Santrampur and Kadana where the have occurred along these axial plane fractures during rocks encountered are quartzites alternating with schists D3 deformation (Mamtani et al., 1999a). The southern along with some calc-silicate bands. The quartzites form part of the study area (around Lunavada, Santrampur long ridges whilst the schistose rocks occur in the low- and further south in Fig. 3) is characterized by regional lying areas. According to Iqbaluddin (1989), the quart- scale folds. Mamtani (1998); Mamtani et al. (1998, zite±schist layers belong to the Kadana Formation of the 1999a, 2000) have worked out the structural history of Lunavada Group. the region which is summarized below: Field and satellite imagery studies have shown that the quartzite ridges have a complex regional scale 1. The Proterozoic rocks of the Lunavada region have 198 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205

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).

undergone three episodes of deformation, viz. D1,D2 part of the area (around Ditwas). In the south, they get and D3. overturned with a southeasterly vergence. 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 3. Microstructures and mechanisms of deformation with trends varying between E±W and NW±SE. 4. Except for the presence of a few D3 kinks and minor fold Petrographic study of schists from the study area has axis, there is no other mesoscopic evidence of D3 folding. revealed that the regional metamorphism progressed up to D3 folds have developed on km-scale limbs of the D1±D2 lower amphibolite facies. This has resulted in the develop- folds. ment of porphyroblasts of garnet and biotite. From north to 5. The superposition of the three folds in various combina- south, a zonation from chlorite to garnet±biotite schist tions has resulted in the development of different types of through biotite schist is recorded (Fig. 3). In this section, large scale interference patterns. Type-III interference the various microstructures observed in quartzites and pattern (Ramsay and Huber, 1987) has developed on different types of schists are described and have been used account of superposition of D1 and D2 folds while to decipher deformational mechanisms. Type-I interference pattern has developed due to superposition of D3 on D1±D2 folds. 3.1. Discrete crenulation cleavage 6. The degree of overturning of D2 folds increases from north to south. The folds are upright in the northernmost This has developed in chlorite schists in the northern parts M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 199 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

Fig. 5. Differentiated crenulation cleavage (S2) in garnet±biotite schist. a shear band cleavage that lies at a low angle (,458) to the Scale bar is 0.4 mm. Location: Anjavana area (southeast of Lunavada). domain boundary between M and Q domains (Mamtani and Karanth, 1996a; Mamtani et al., 1999b). All these micro- of the study area. In these rocks, three planar surfaces structures in the cleavage zones have been used to interpret are recognizable, viz. S0 (bedding plane), S1 (®rst the mechanisms of deformation during origin of crenulation schistosity) and S2 (discrete crenulation cleavage) cleavages (Mamtani et al., 1999b). Accordingly it has been (Figs. 4 (a)±(c)). The rocks have preserved primary argued that pressure solution is an important deformational lithological layering (S0) which is marked by alternating mechanism during the early stages of crenulation and this layers of quartz-rich and phyllosilicate rich layers. The ®rst imparts the domainal fabric to the rock. However, intracrystalline crystal plastic deformation becomes dominant during schistosity (S1)issub-paralleltoS0 and comprises of chlorite, quartz and muscovite crystals aligned parallel to one another. the later stages which results in the development of shear

The second schistosity is the discrete crenulation cleavage (S2) structures in cleavage zones. which was formed on account of crenulation of S1 foliation during D2. The S2 has developed almost perpendicular or at 3.3. Millipede microstructure high angles to the S1 andisobservedtohaveformedonlyinthe phyllosilicate rich layers. There is evidence of displacement This microstructure is characterized by oppositely concave microfolds (OCMs) and usually occurs as inclusion along the S2 surface (Fig. 4(c)). Similar evidence has been linked by Gray (1979) to pressure solution. However, at the trails (Si internal foliation) within porphyroblasts in present scale of observation, no signi®cant evidence of recrys- schists (Bell and Rubenach, 1980). Millipede microstructure tallized quartz aggregates and no metamorphic differentiation is preserved in some biotite porphyroblasts in garnet±biotite in the vicinity of the discrete crenulation cleavages along schists of the study area (Figs. 6(a) and (b)). It is de®ned by which the displacement occurred has been observed. More- oppositely curving quartz inclusion trails (S1) within the over, Fig. 4(b) shows some microscale dragging along the biotite porphyroblast. S1 is relatively straight in the core cleavages. Therefore the possibility of these discrete of the biotite and gradually curves towards the rims and continues to merge into the external schistosity (S2). Similar

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

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). structures are known to develop around rotating rigid Some of the quartz crystals show subgrains (Fig. 7(b)), a objects at low strains in laboratory experiments (Ghosh, textural feature pointing to recovery during dynamic recrys- 1975, 1977; Ghosh and Ramberg, 1976). Johnson and tallization. This also indicates that during deformation, Moore (1996) and Johnson and Bell (1996) have stated recrystallization-accommodated dislocation creep was that the presence of millipedes indicates a state of low strain important (Nicolas and Poirier, 1976; Tullis and Yund, during their genesis. Since the microfolds that make up the 1985; Tullis et al., 1990; Passchier and Trouw, 1996). Dislo- millipedes within the biotite are open compared with those cation creep has been recognized as an important deforma- in the matrix, the biotite porphyroblast is interpreted to have tion mechanism for quartz aggregates under conditions of grown under a low strain state during D2. greenschist facies or higher (White, 1976; Mitra, 1978; Hirth and Tullis, 1992). 3.4. Textures in quartzites Thin sections of quartzites occurring closer to the margin of the Godhra Granite show a granoblastic texture charac- Thin sections prepared from different localities of the terized by straight to smoothly curved grain boundaries, area show that the quartzites comprise of two textural vari- 1208 triple points and sharp extinction (Fig. 7(c)). These eties based on grain boundaries Ð either the grain bound- microstructural characteristics clearly point to static recrys- aries are irregular or they are straight. The irregular grain tallization with Grain Boundary Area Reduction (GBAR) as boundaries (Fig. 7(a)) are prevalent dominantly in the quart- the principal mechanism (Passchier and Trouw, 1996). The zite occurrences that are distant from the Godhra Granite. presence of 1208 triple points, referred to as foam micro- According to Urai et al. (1986) and Passchier and Trouw structure by Vernon (1976), is indicative of heat outlasting (1996), the presence of irregular grain boundaries indicates deformation or annealing. Bons and Urai (1992) and Passch- intracrystalline deformation as the rock underwent dynamic ier and Trouw (1996) have stated that GBAR is especially recrystallization by Grain Boundary Migration (GBM). pronounced at high temperatures after deformation ceases, M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 201 i.e., in a static environment. In the present case, the high Most of the garnet and biotite porphyroblasts preserving temperature for static recrystallization was supplied by the the microfolded or sigmoidal inclusion trails are identi®ed

Godhra Granite that intruded the region. Fig. 7(a) and (c) are as syntectonic with D2 deformation (Figs. 8(a) and (b); also photomicrographs (taken at same magni®cation) of quart- Fig. 6). The intensity/tightness of folding of the inclusions zites occurring far and close to the granite margin. It is quite with respect to those in the matrix has been further useful in clear that the former has ®ner crystals while the latter has classifying the porphyroblasts as early-D2 or late-D2. Fig. coarser crystals. This indicates that the heat supplied by the 8(a) shows a porphyroblast of biotite with quartz inclusion granite played an important role in microstructure develop- trails (Si S1) which show open microfolds. In contrast, the ment of the latter. Further corroboration of this fact has also microfolds outside the porphyroblast are tightly crenulated. come from Crystal Size Distribution (CSD) study of quartz This indicates that the biotite porphyroblast grew during the crystals in schists and quartzites. Moreover, post-deforma- early stages of D2 deformation. A few porphyroblasts tional changes in microstructure are known to occur at the preserve relatively tight S1 crenulations and also include end of an orogeny when deformation has essentially ceased the S2 foliation at the rims (Fig. 8(b)). Such pophyroblasts and the rocks are at high temperatures (.3008C) or when are classi®ed as late-D2. Some garnet porphyroblasts deformed rocks are subjected to sustained heating from preserve sigmoidal S1 inclusion trails of quartz and feldspar post-tectonic plutons (Knipe, 1989) and also in laboratory which gradually curve into S2 while the cleavage domain experiments with octachloropropane (Ree and Park, 1997). outside the porphyroblast has a single homogenized folia-

It is envisaged that prior to the intrusion of granite, the tion S2 (Fig. 8(c)). It is envisaged that the sigmoidally quartz crystals were in a higher strain condition character- curving S1 schistosity along with S2 was included in the ized by irregular grain boundaries. Such a microstructure is garnet porphyroblast during earlier stages of D2. With conti- thermodynamically unstable and would have a tendency to nuing deformation, the matrix foliation further deformed proceed to a lower energy state. The late to post deforma- and rotated into parallelism with the S2 while the sigmoidal tional granitic intrusion provided the necessary heat energy relation between S1 and S2 within the porphyroblast required for release of internal strain and achievement of a remained frozen in the same stage at which it was included, thermodynamically stable microstructure. As a result, a thus remaining unaffected by later deformation (Mamtani stable granoblastic microstructure developed which is and Karanth, 1997). Such porphyroblasts of garnet are also more pronounced in the rocks close to the granite margin. classi®ed as syn-D2. 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., 5. Thermal metamorphism 1990). However, in the present study, it is clearly seen that the quartzites close to the granite show a granoblastic Regional metamorphism in the Lunavada±Santrampur texture, sharp extinction and coarser grain size. Quartzites region was followed by thermal metamorphism related to farther from the granite show irregular grain boundaries, the intrusion of the Godhra Granite. The effects of heat sub-grains, a ®ner grain size and absence of a granoblastic supplied by the Godhra granite are signi®cant in the south- texture. It is therefore logical to assume that the microstruc- western part of the study area, i.e., to the south of Lunavada. tures in quartzites close to the granite are a result of static Since the granite does not lie in the immediate vicinity of recrystallization by GBAR related to heat supplied by the the study area, common high-temperature metamorphic granite. This is in accordance with Bons and Urai (1992) minerals like andalusite and sillimanite are not observed. and Passchier and Trouw (1996) who have suggested that Nevertheless, the effect of the thermal event is quite obvious GBAR is pronounced only after the deformation ceases. 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 4. Porphyroblast±matrix relationships (1996b). CSD studies provide statistical data for crystals (of a particular mineral) of different sizes within a unit The mica schists around Lunavada and Santrampur are area of a thin section. Based on this data, CSD plots such characterized by foliations of different generations and as size (L mm) vs. normal log of population density [ln n] porphyroblastic minerals such as garnet and biotite which can be prepared. The shape of a CSD plot represents the contain foliations as quartz inclusion trails. The relationship extent to which a rock underwent annealing. between the internal foliation (Si) within the porphyroblasts In the present investigation, CSD of quartz crystals were and the matrix foliation (Se) outside the porphyroblast was calculated in thin sections of three schist and four quartzite used to determine the relative timing of growth of minerals samples collected at varying distance from the boundary of with reference to foliation of a particular generation. This is the Godhra Granite. Fig. 2 shows the location of the schist in accordance with the criteria described by Zwart (1962), and quartzite samples. Figs. 9(a) and 9(b) are CSD plots for Spry (1969), Vernon (1976), Ghosh (1993), and Passchier schist and quartzite samples, respectively. Both rock types and Trouw (1996). indicate that, in comparison with samples away from the 202 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205

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).

Godhra Granite boundary, samples close to the granite be established. The metamorphic history of chlorite schists possess (i) quartz crystals which have crystallized over a occurring in the northern parts of the study area is rather wider size range, (ii) CSD plots with a lower slope, and simple. As mentioned earlier, these rocks show three promi-

(iii) fewer quartz crystals within a unit area. Moreover, nent planar surfaces (S0,S1 and S2). S1 and S2 developed the CSD plots for schists lying close to the granite have a during D1 and D2 respectively. Chlorite and muscovite crys- bell shape (A and B in Fig. 9(a)) while the plot for sample tals formed during D1. These underwent rotation along S2 away from the granite is near linear (C in Fig. 9(a)). This and some recrystallization during D2. No evidence of indicates that all the rocks initially underwent continuous growth of new minerals cutting across D2 is observed in nucleation and growth. Subsequently, rocks closer to the the chlorite schists, thus implying that D3 was generally granite underwent annealing such that smaller crystals devoid of any metamorphism. The chlorite schists therefore were resorped at the expense of larger crystals (see Cash- only record a single metamorphic event. The paragenesis man and Ferry, 1988; Cashman and Marsh, 1988 and observed is chlorite 1 muscovite 1 quartz which is typical Mamtani and Karanth, 1996b for details of CSD plot inter- of a chlorite zone within the greenschist facies (Yardley, pretations). The heat for annealing was supplied by intru- 1989; Spear, 1993). The garnet±biotite schists of the region sion of the Godhra Granite. are most important in determining the various metamorphic events that accompanied different deformation episodes. These possess differentiated crenulation cleavage character- 6. Discussion ized by alternating Q and M domains. Garnet, biotite, chlorite, muscovite and quartz are the major minerals present. On the basis of the present petrographic study, the time Chlorite and biotite crystals occur along foliations of differ- relationship between deformation and metamorphism can ent generations and are accordingly classi®ed. Chlorite(1) M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 203

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.

and biotite(1) occur along the S1 schistosity and are syn-D1. sion trails of quartz (e.g., Fig. 8(d)) are also syn-D2. The The metamorphic event which accompanied D1 is referred metamorphic event which accompanied a major part of D2 to as M1. Biotite(2) crystals, which occur with their (001) deformation is referred to as M2-1. This was a progressive planes parallel to S2, have grown during D2 deformation. metamorphic event during which biotite(2) crystals grew Biotite(2) porphyroblasts with spiral (helictic) inclusion along S2. That these progressive events (M1 and M2-1) trails of quartz (e.g., Fig. 8(a) and (b)) are also syn-D2. were followed by retrogressive metamorphism (M2-2) during Similarly the garnet porphyroblasts with sigmoidal inclu- the waning phases of D2/early D3 is evident by the presence 204 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205

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.

of (a) chlorite(2) crystals that overgrow S2 foliation, (b) progression from chlorite grade in the northern parts to chlorite around syn-D2 garnet and (c) chlorite along frac- garnet grade in the southern parts. tures in garnet that penetrate from core to the rim. 2. Progressive regional metamorphism M1 and M2-1 accom- The last metamorphic event to affect the rocks was a late- panied D1 and a major part of D2 respectively. M2-2 was a D3/post-D3 thermal metamorphism. In rocks that lie close to retrogressive event that accompanied the waning stages the granite, this event resulted in annealing, coarser crystals of D2 or early D3 deformation. and the development of granoblastic microstructure in 3. A thermal event related to late-D3/post D3 Godhra Gran- quartzites. It is concluded that the thermal event led to static ite intrusion followed regional metamorphism. This led recrystallization of quartz on the microscale due to the heat to static recrystallization on the microscale and grain supplied by intrusion of the Godhra Granite. Emplacement growth in rocks close to the granite. of the granite may have been initiated during the waning 4. GBM associated with dislocation creep is suggested to phases of D3. However, the ®eld evidence for granite and have been an important deformation mechanism in related pegmatites intruding the foliation in schists indicates quartzites lying far from the granite margin. that the intrusion continued even after D3. This further 5. Annealing by GBAR has been discerned in quartzites supports the interpretation that the development of grano- close to the granite. 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 Acknowledgements extinction. This indicates that the thermal event also resulted in recrystallization and grain growth of muscovite. The present paper is an outcome of the doctoral Fig. 10 summarizes the time relationship between research on Precambrian rocks of Lunavada region crystallization and deformation of various minerals in (India) carried out by M.A.M. Financial support to garnet±mica schists. 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 7. Conclusions Development (Brazil), Senior Research Fellowship of the Council of Scienti®c and Industrial Research, New The present study has provided considerable insight into Delhi (No. 9/114/(92)/96/EMR-I) and DAAD-Fellowship the metamorphic history and deformation mechanisms of of the German Academic Exchange Service, Bonn (No. the Proterozoic rocks around Lunavada, SAMB (India). A/97/00792). We are grateful to Bruce Marsh and The following conclusions are evident: Michael Zeig (Johns Hopkins University, USA) for carry- ing out CSD measurements in quartzites using a ªOmni- 1. The rocks of the Lunavada region have undergone meta- met Analyzerº. Comments by Jordi Carreras and an morphism up to lower amphibolite facies. There is a anonymous reviewer were found useful. M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 205

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