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
caDepartment 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, D2and 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 D1and was progressive. M2-1metamorphism associated with major
part of D2was also progressive. However, M2-2that synchronized with the waning phases of D2and early-D3deformation 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.q2001 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 km2extending 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 km2in 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
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* Corresponding author.
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 196
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.
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
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 D3kinks and minor fold
axis, there is no other mesoscopic evidence of D3folding. D3folds 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 D3on D1±D2folds.
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 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205
198
Fig. 4. (a) Photomicrograph of chlorite schist showing presence of S0, S1and S2on the microscale. The bedding plane (S0) is de®ned by the contact between
quartz-rich and quartz-poor layers. The schistosity S1is sub-parallel to S0and is marked by chlorite and muscovite. The schistosity S2is a discrete crenulation
cleavage which has developed at high angles to S0and 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. S1foliation de®ned by muscovite and
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 S0and 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 S1foliation during D2. The S2has developed almost perpendicular or at high angles to the S1and is observed to have formed only in the phyllosilicate rich layers. There is evidence of displacement along the S2surface (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 (S1and S2) are prominent micro-scopically. S1is 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 M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205
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
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 condideforma-tions 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 withGrain 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, M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205
200
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
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 D2deformation (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 (SiS1) 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 S2foliation at the rims (Fig. 8(b)). Such pophyroblasts are classi®ed as late-D2. Some garnet porphyroblasts preserve sigmoidal S1inclusion trails of quartz and feldspar which gradually curve into S2while 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 S2while 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 (Lmm) 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.
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; CashCash-man 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 D1and D2respectively. 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 chlorite1muscovite1quartz 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) M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205
202
Fig. 8. (a) Syn-D2biotite 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 S1and S2are
present within the garnet and the inclusion trails of S1curve 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 D2deformation. Scale bar is 0.4 mm in (a), 0.2 mm in
and biotite(1) occur along the S1schistosity and are syn-D1. The metamorphic event which accompanied D1is 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 D3is evident by the presence M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205
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-D3thermal 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 M1and M2-1 accom-panied D1and a major part of D2respectively. M2-2was a retrogressive event that accompanied the waning stages of D2or early D3deformation.
3. A thermal event related to late-D3/post D3Godhra 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 uscarry-ing a ªOmni-met Analyzerº. Comments by Jordi Carreras and an anonymous reviewer were found useful.
M.A. Mamtani et al. / Journal of Asian Earth Sciences 19 (2001) 195±205 204
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