Time relationship between metamorphism and deformation in Proterozoic rocks of the Lunavada region, Southern Aravalli Mountain Belt (India) — 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-721302, West Bengal, India b Faculty of Science, M.S. University of Baroda, Vadodara-390002, Gujarat, India c Geologisch-Pala ¨ontologisches Institut, Ruprecht-Karls-Universita ¨t 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; 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 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 km 2 in the SAMB and is flanked 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).
Time relationship between metamorphism and deformation in Proterozoic rocks of the Lunavada region, Southern Aravalli Mountain Belt (India) � a microstructural study
Jun 23, 2023
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