Manish A. Mamtani a, *, S.S. Merh b, R.V. Karanth b, R.O. Greiling c. 1. Introduction

Transcription

1 Journal of Asian Earth Sciences 19 (2001) 195±205 Time relationship between metamorphism and deformation in Proterozoic rocks of the Lunavada region, Southern Aravalli Mountain Belt (India) Ð a microstructural study Manish A. Mamtani a, *, S.S. Merh b, R.V. Karanth b, R.O. Greiling c a Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur , West Bengal, India b Faculty of Science, M.S. University of Baroda, Vadodara , Gujarat, India c Geologisch-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 Mesoproterozoic. The Lunavada Group of rocks, which is an important constituent of the southern parts of AMB, reveals three episodes of deformation; D 1,D 2 and D 3. 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 (M 1 ) accompanied D 1 and was progressive. M 2-1 metamorphism associated with major part of D 2 was also progressive. However, M 2-2 that synchronized with the waning phases of D 2 and early-d 3 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-d 3 /post-d 3 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 km 2 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 * Corresponding author. address: mamtani@hotmail.com (M.A. Mamtani). porphyroblast±matrix relationships preserved in garnet± biotite schists of the region. 2. Geological setting and structural geology The Proterozoic rocks of the Lunavada region, Panchmahals district, Gujarat are assigned to the Lunavada Group which is the second youngest group of the Aravalli Supergroup (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-conglomerate, manganiferous phyllite and phosphatic algal meta-dolomite (Gupta et al., 1980, 1992, 1995). It occupies an area of 10,000 km 2 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 /01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S (00)

2 196 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 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.

3 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195± 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 southernmost 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 lowlying areas. According to Iqbaluddin (1989), the quartzite±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 D 2 folds, closely spaced axial plane fractures and joints. Shearing is observed to have occurred along these axial plane fractures during D 3 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

4 198 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 Fig. 4. (a) Photomicrograph of chlorite schist showing presence of S 0,S 1 and S 2 on the microscale. The bedding plane (S 0 ) is de ned by the contact between quartz-rich and quartz-poor layers. The schistosity S 1 is sub-parallel to S 0 and is marked by chlorite and muscovite. The schistosity S 2 is a discrete crenulation cleavage which has developed at high angles to S 0 and S 1. The occurrence of the discrete crenulation cleavage is restricted to the quartz-poor (phyllosilicaterich) layers. (b) Photomicrograph documenting drag effect along discrete crenulation cleavage (S 2 ) in chlorite schist. S 1 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 S 2. 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. D 1,D 2 and D 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 D 3 kinks and minor fold axis, there is no other mesoscopic evidence of D 3 folding. D 3 folds have developed on km-scale limbs of the D 1 ±D 2 folds. 5. The superposition of the three folds in various combinations 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 D 1 and D 2 folds while Type-I interference pattern has developed due to superposition of D 3 on D 1 ±D 2 folds. 6. The degree of overturning of D 2 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 development 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 Discrete crenulation cleavage This has developed in chlorite schists in the northern parts

5 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195± crenulation cleavages being planes of shear cannot be totally ruled out Differentiated crenulation cleavage Fig. 5. Differentiated crenulation cleavage (S 2 ) in garnet±biotite schist. Scale bar is 0.4 mm. Location: Anjavana area (southeast of Lunavada). of the study area. In these rocks, three planar surfaces are recognizable, viz. S 0 (bedding plane), S 1 ( rst schistosity) and S 2 (discrete crenulation cleavage) (Figs. 4 (a)±(c)). The rocks have preserved primary lithological layering (S 0 ) which is marked by alternating layers of quartz-rich and phyllosilicate rich layers. The rst schistosity (S 1 )issub-paralleltos 0 and comprises of chlorite, quartz and muscovite crystals aligned parallel to one another. The second schistosity is the discrete crenulation cleavage (S 2 ) which was formed on account of crenulation of S 1 foliation during D 2. The S 2 has developed almost perpendicular or at high angles to the S 1 andisobservedtohaveformedonlyinthe phyllosilicate rich layers. There is evidence of displacement along the S 2 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 recrystallized quartz aggregates and no metamorphic differentiation in the vicinity of the discrete crenulation cleavages along which the displacement occurred has been observed. Moreover, Fig. 4(b) shows some microscale dragging along the cleavages. Therefore the possibility of these discrete 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 (S 1 and S 2 ) are prominent microscopically. S 1 is made up of chlorite, muscovite and biotite crystals while new generation biotite and muscovite akes are developed parallel to S 2. The M-domains vary in thickness 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 microstructures 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, intracrystalline crystal plastic deformation becomes dominant during the later stages which results in the development of shear structures in cleavage zones Millipede microstructure This microstructure is characterized by oppositely concave microfolds (OCMs) and usually occurs as inclusion trails (S i ˆ 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 (S 1 ) within the biotite porphyroblast. S 1 is relatively straight in the core of the biotite and gradually curves towards the rims and continues to merge into the external schistosity (S 2 ). 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 (S 1 ) within the porphyroblast. (b) Explanatory line drawing of photomicrograph in (a). Scale bar is 0.1 mm. Location: Vankdi (south of Anjavana).

6 200 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 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 D Textures in quartzites Thin sections prepared from different localities of the area show that the quartzites comprise of two textural varieties based on grain boundaries Ð either the grain boundaries are irregular or they are straight. The irregular grain boundaries (Fig. 7(a)) are prevalent dominantly in the quartzite 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 recrystallization. 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). Dislocation creep has been recognized as an important deformation 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 characterized by straight to smoothly curved grain boundaries, 1208 triple points and sharp extinction (Fig. 7(c)). These microstructural characteristics clearly point to static recrystallization with Grain Boundary Area Reduction (GBAR) as the principal mechanism (Passchier and Trouw, 1996). The presence of 1208 triple points, referred to as foam microstructure by Vernon (1976), is indicative of heat outlasting deformation or annealing. Bons and Urai (1992) and Passchier and Trouw (1996) have stated that GBAR is especially pronounced at high temperatures after deformation ceases,

7 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195± 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 quartzites 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 development 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-deformational 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 characterized 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 deformational 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 microstructures 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 (S i ) within the porphyroblasts and the matrix foliation (S e ) 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 D 2 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-d 2 or late-d 2. Fig. 8(a) shows a porphyroblast of biotite with quartz inclusion trails (S i ˆ S 1 ) 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 D 2 deformation. A few porphyroblasts preserve relatively tight S 1 crenulations and also include the S 2 foliation at the rims (Fig. 8(b)). Such pophyroblasts are classi ed as late-d 2. Some garnet porphyroblasts preserve sigmoidal S 1 inclusion trails of quartz and feldspar which gradually curve into S 2 while the cleavage domain outside the porphyroblast has a single homogenized foliation S 2 (Fig. 8(c)). It is envisaged that the sigmoidally curving S 1 schistosity along with S 2 was included in the garnet porphyroblast during earlier stages of D 2. With continuing deformation, the matrix foliation further deformed and rotated into parallelism with the S 2 while the sigmoidal relation between S 1 and S 2 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-d 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 southwestern 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 [l n 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

8 202 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 Fig. 8. (a) Syn-D 2 biotite porphyroblasts in garnet±biotite schist. (b) Photomicrograph of biotite porphyroblast with microfolded quartz inclusion trails. The biotite is interpreted as late-syn-d 2. (c) Photomicrograph of garnet porphyroblast which has grown over a crenulation cleavage zone (S 2 ). Both S 1 and S 2 are present within the garnet and the inclusion trails of S 1 curve sigmoidally into S 2. However, the cleavage zone in the matrix (outside the garnet) is characterized by only a single schistosity (S 2 ). This implies that the garnet grew over the crenulation cleavage during D 2 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 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 Cashman and Ferry, 1988; Cashman and Marsh, 1988 and Mamtani and Karanth, 1996b for details of CSD plot interpretations). The heat for annealing was supplied by intrusion 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 prominent planar surfaces (S 0,S 1 and S 2 ). S 1 and S 2 developed during D 1 and D 2 respectively. Chlorite and muscovite crystals formed during D 1. These underwent rotation along S 2 and some recrystallization during D 2. No evidence of growth of new minerals cutting across D 2 is observed in the chlorite schists, thus implying that D 3 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 characterized by alternating Q and M domains. Garnet, biotite, chlorite, muscovite and quartz are the major minerals present. Chlorite and biotite crystals occur along foliations of different generations and are accordingly classi ed. Chlorite(1)

9 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195± 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 S 1 schistosity and are syn-d 1. The metamorphic event which accompanied D 1 is referred to as M 1. Biotite(2) crystals, which occur with their (001) planes parallel to S 2, have grown during D 2 deformation. Biotite(2) porphyroblasts with spiral (helictic) inclusion trails of quartz (e.g., Fig. 8(a) and (b)) are also syn-d 2. Similarly the garnet porphyroblasts with sigmoidal inclusion trails of quartz (e.g., Fig. 8(d)) are also syn-d 2. The metamorphic event which accompanied a major part of D 2 deformation is referred to as M 2-1. This was a progressive metamorphic event during which biotite(2) crystals grew along S 2. That these progressive events (M 1 and M 2-1 ) were followed by retrogressive metamorphism (M 2-2 ) during the waning phases of D 2 /early D 3 is evident by the presence

10 204 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 S 2 foliation, (b) chlorite around syn-d 2 garnet and (c) chlorite along fractures in garnet that penetrate from core to the rim. The last metamorphic event to affect the rocks was a late- D 3 /post-d 3 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 D 3. However, the eld evidence for granite and related pegmatites intruding the foliation in schists indicates that the intrusion continued even after D 3. This further supports the interpretation that the development of granoblastic 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 metamorphism 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 M 1 and M 2-1 accompanied D 1 and a major part of D 2 respectively. M 2-2 was a retrogressive event that accompanied the waning stages of D 2 or early D 3 deformation. 3. A thermal event related to late-d3/post D 3 Godhra Granite 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 carrying out CSD measurements in quartzites using a ªOmnimet Analyzerº. Comments by Jordi Carreras and an anonymous reviewer were found useful.

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