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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 1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(00)00029-8 196 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. 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 super- position 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).

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