Manish A. Mamtani

Degree of magnetic anisotropy as a strain intensity gauge in ferromagnetic granites

Anisotropy of magnetic susceptibility data for a ferromagnetic granite (Godhra Granite, NW India) are presented and it is shown that the degree of magnetic anisotropy (P′) is not controlled by the mean susceptibility (Km). Analyses carried out across a high-strain zone lying between granite and adjacent gneiss show that P′ values are highest in samples that lie close to the contact and decrease away from it. Based on these results it is concluded that if P′ is not controlled by Km, then the former can be used to gauge strain-intensity variations in ferromagnetic granites.

Significance of AMS analysis in evaluating superposed folds in quartzites

Quartzites tend to be compositionally homogeneous, and because of this, deformation related fabric elements (foliations and lineations) are poorly developed in them. This makes structural analysis of deformed quartzites challenging. The measurement of anisotropy of magnetic susceptibility (AMS) is useful for recognizing structural imprints in rocks that lack mesoscopic fabrics and the present study is carried out with an aim to demonstrate the robustness of AMS in analysing such deformation imprints in quartzites. AMS data of samples from folded quartzites located in an approximately 10 km 2 area around Galudih (eastern India) are presented. Although on a regional scale, superposed deformation and ductile shearing are known from the area, the investigated quartzites do not preserve mesoscopic evidence of these large-scale features and have developed folds that plunge gently towards the SE with a vertical NW–SE-striking axial plane. The magnetic foliation recorded from AMS analysis is parallel to the axial plane, while the orientation of the magnetic lineation varies from SE through vertical to NW. This is similar to the large-scale fold axis variations recorded in various regional domains mapped over an area of about 200 km 2 . It is concluded that although the imprint of regional superposed deformation is not obvious on the mesoscopic scale in the quartzites around Galudih, this imprint can be detected from the magnetic fabric. The present study thus highlights the usefulness of AMS in analysing superposed folds in quartzites.

Fluid inclusions as metamorphic process indicators in the Southern Aravalli Mountain Belt (India)

Abstract Fluid inclusions from a biotite-garnet schist in the Southern Aravalli Mountain Belt (India) give information on both peak metamorphic conditions and post-peak metamorphic processes during uplift. A combination of careful petrography, microthermometry and Raman spectroscopy reveals the presence of at least five generations of enclosed fluids. Lower amphibolite-facies pressure-temperature conditions of the growth of garnet rims are reproduced by the highest fluid density of the relatively oldest inclusion type of CO2 (±N2)-rich compositions. A calculated fluid composition in the COH system, in equilibrium with the graphite buffer corresponds to a CO2-rich fluid at metamorphic conditions. However, the results of these calculations are very sensitive to small fluctuations in oxygen fugacity and the accuracy of thermodynamic properties of mineral equilibria. Re-equilibration, conceived by specific size-density distribution and the absence of an aqueous phase in inclusions that contain nahcolite crystals, is monitored in these inclusions as post-peak metamorphic processes, like partial decrepitation and preferential leakage. The other fluid types represent heterogeneous fluid trapping of coexisting aqueous NaCl-bearing solutions with CO2-CH4-rich vapour bubbles in healed cracks, and probably the introduction of external fluids containing high salinity aqueous CaCl2-rich solutions in nearly pure N2 vapour bubbles, at lower P-T conditions. This study illustrates that fluid inclusions remain a valuable database of peak metamorphic conditions, moreover, alterations of the entrapped fluids and surrounding crystals are illustrative for specific exhumation evolutions.

Microstructures in a banded iron formation (Gua mine, India)

This paper provides a detailed documentation of microstructures developed in the banded iron formation (BIF) of Gua mine, located in the Bonai Synclinorium (eastern India), where the rocks have been subjected to three deformations (D 1 to D 3 ). Folded iron ores, quartz strain fringes around rigid core objects and folded iron ore layers, and refracted quartz veins are described from samples taken from D 2 folds in the banded iron formation. Orientations of microstructures are compared with mesoscopic structures to interpret the generations of ore minerals, planar structures and the time relationship between deformation and development of different microstructures. The mechanism of D 2 folding is worked out and its bearing on microstructure development is discussed. The D 2 folds are inferred to have developed by a combination of tangential longitudinal strain in the competent layer, flexural flow in the incompetent layers and flexural slip at the interface between layers of differing competence. Homogeneous flattening strain superposed the earlier strain, which led to modification of the folds in the competent layer from class 1B to 1C. This strain is quantified and is found to be higher in the limb than the hinge of a fold. Diffusive mass transfer by solution and bulging dynamic recrystallization in quartz are inferred as the dominant deformation processes during folding. Moreover, based on comparison with published deformation microstructure maps, the microstructures of the present study are estimated to have developed between 300 and 350 °C temperatures at a strain rate of 10 −14 –10 −12 s −1 , which are geologically realistic conditions for naturally deformed rocks.

Are crenulation cleavage zones mylonites on the microscale

Abstract Mylonites commonly show characteristic structures such as S – C fabric and C ′ type shear bands. In the present paper, the presence of similar structures on the microscale is reported from the cleavage zones of differentiated crenulation cleavage in garnet biotite schists belonging to the Lunavada Group of Proterozoic metasedimentary rocks, India. These rocks have experienced three episodes of deformation. A differentiated crenulation cleavage ( S 2 ), characterized by alternating cleavage zones and microlithons developed during D 2 by microfolding of the S 1 foliation. Although the schists under investigation do not show any macroscopic- or mesoscopic-scale evidence of mylonitization, they show the presence of shear structures within the cleavage zones. The fabric resembling S – C and C ′ shear bands within these zones indicates shearing within them during D 2 deformation. A model incorporating shearing along the cleavage zones is proposed to explain the genesis of shear structures within them. Accordingly, it is invoked that solution transfer and grain rotation are important deformation mechanisms during the early stages of crenulation and this results in the migration of quartz from the limbs to the hinges of the microfolds. At the later stages of crenulation the phyllosilicates (micas) forming the limbs of the microfolds are at an oblique angle to the direction of shortening and most of the mobile material like quartz has already been removed from the limbs by solution transfer. Therefore, the stress conditions are ideal for shearing and intracrystalline crystal–plastic deformation to occur along the limbs during the later stages of crenulation. It is proposed that the fabric resembling S – C , embryonic C ′ type shear bands and well developed C ′ (in that order) develop with increasing strain and shearing within the cleavage zones. At still higher strains, the shear bands may rotate into parallelism with the domain boundary between the cleavage zones and the microlithons. Composition of muscovite constituting cleavage zones and microlithons is discussed and it is concluded that the deformation mechanisms that operate during the later stages of crenulation, especially under upper greenschist to lower amphibolite conditions, are similar to those during mylonitization.

Tectonic Evolution of the Southern Part of Aravalli Mountain Belt and its Environs: Possible Causes and Time Constraints

Abstract Structural studies on Proterozoic rocks belonging to the Lunavada Group, Southern Aravalli Mountain Belt (SAMB), India, have shown that they underwent three episodes of deformation which have led to the formation of various regional scale interference patterns. Whilst the northern parts of the SAMB underwent brittle-ductile deformation, the southern portion underwent more ductile deformation. On the basis of structural as well as metamorphic studies it has been established earlier that the region was subjected to uplift orogenesis during its evolutionary history. In the present paper an attempt is made to visualize the possible causes that led to deformation of the SAMB, the structural geology of which has been established by the authors, and to constraint the timing of these events on the basis of already available geochronological data. A “working-hypothesis” is proposed according to which it is suggested that deformation of the SAMB is a result of the accretion of the three protocontinents viz. Aravalli, Dharwar and Singhbhum during the Mesoproterozoic. It is envisaged that the accretion of Aravalli and Singhbhum Protocontinents occurred between 1600 and 1400 Ma along the NE-SW trending Son Suture and this event led to development of NE-SW trending structures in the SAMB. Suturing of Aravalli and Dharwar Protocontinents between 1400 and 935 Ma along the E-W Narmada Suture was responsible for the E-W to NW-SE trending D 3 structures of the SAMB. It is postulated that the Satpura orogeny which resulted in deformation of rocks in Satpura mountain range lying to the south of Narmada Suture was coeval with the accretion of Aravalli and Dharwar Protocontinents.

Serrated quartz grain boundaries, temperature and strain rate: testing fractal techniques in a syntectonic granite

Abstract In the past fractal (ruler) dimension (Dr) of quartz grain-boundary sutures and area–perimeter fractal dimension (Da) of quartz grains, respectively, have been shown to depend on temperature (T) and strain rate. However, the application of these methods to gauge temperature and strain rate in naturally deformed intrusive rocks has not yet been tested. In the present study Dr and Da are calculated in 12 thin sections from different parts of a syntectonic granite (Godhra Granite, India). Of these, six belong to the northern part, two to the central part and four to the southern part of the granite. Earlier work on the Godhra Granite showed both a strain and a temperature gradient, with high temperature in the north and high strain in the south. Microstructural studies reveal that the quartz grain-boundary sutures are less serrated in the northern samples compared to those from the remaining part of the granite. The northern samples contain abundant high-temperature solid-state deformation fabrics that formed between 675 and 725 °C (quartz chessboard pattern thermobarometry). Using a Dr v. T plot given by earlier workers, a Dr value of 1.05–1.14 is expected for the above T range. Dr calculations of quartz sutures from the northern samples give a median of 1.11 and most of the sutures have Dr <1.14. These data fit well with the expected temperature range in which the quartz chessboard pattern formed in the Godhra Granite. The central and southern parts of the granite are dominated by myrmekites (500–670 °C), recrystallized feldspars (450–600 °C), deformation twins in feldspar (400–500 °C) and kinked biotite (<300 °C). The expected Dr of quartz sutures under the above medium–low temperature ranges are 1.07–1.23, 1.11–1.25, 1.16–1.28 and <1.27, respectively. Dr calculations reveal that most of the quartz sutures from the central+southern part have Dr >1.14, and the median values are 1.18 (centre) and 1.17 (south). Using the Dr v. T plot, these Dr values indicate that most of the textures in the central+southern part of the Godhra Granite formed in the temperature range of 450–600 °C, which fits well with the temperature range required for the development of medium–low temperature fabrics that dominate this part of the granite. Thus, it is concluded that Dr of quartz sutures can be used as a geothermometer in syntectonic granites. Da for northern and southern samples is 1.10 and 1.14, respectively. Strain rates of the order of approximately 10−7 and 10−11 s−1, respectively, are obtained for high (675 °C) and low temperature (300 °C) using area-perimeter fractal dimension (Da) values. Although these are higher than geological strain rates that are known in nature (10−12–10−15 s−1), the calculated values for the lower-temperature range are similar to strain rates estimated for intrusions (10−10–10−12 s−1). The calculations indicate that the method to calculate strain rate using Da of quartz grains fails to give geologically reasonable strain rates for high temperature in a syntectonic granite. However, the method maybe useful in obtaining reasonable strain rate estimates for lower temperatures.

Time relationship between metamorphism and deformation in proterozoic rocks of the Lunavada region, Southern Aravalli Mountain Belt (India) — a microstructural study

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; 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.

Rocks, fabrics and magnetic anisotropy: an introduction to the issue in honour of František Hrouda

All rocks—igneous, sedimentary as well as metamorphic or tectonically deformed—develop a spatial and geometrical arrangement of their constituent minerals commonly referred to as fabric. The genesis of rock fabric, its orientation and intensity can be directly linked to certain geological processes or, on a larger scale, to tectonic phenomena. For this reason, fabric analysis plays a pivotal role in geological investigation. While in some rocks the fabric can be seen with the naked eye, in the others it can be recognized only with the help of instruments and special techniques. A very effective technique, among others, is the analysis of magnetic fabric based on the measurements of anisotropic magnetic properties. Due to its rapidity, accuracy, low cost and non-destructiveness, magnetic fabric analysis has been successfully applied in all branches of earth sciences for more than five decades. Because of his pioneering role in the study of magnetic properties of rocks, this issue is dedicated to Professor František Hrouda. It contains 19 papers dealing with magnetic anisotropy measurements in a variety of rocks— granite, gabbro, mica schist, quartzite, obsidian, monzodiorite, syenite, as well as tuffs, clays and Quaternary deposits—and in minerals, such as haematite. The first few papers are of a more fundamental nature, which are methodology-based and provide the reader with an insight into the variety of problems in rocks and minerals that can be handled using magnetic fabric. These are followed by several papers dealing specifically with the magnetic fabric analysis in igneous rocks. Finally, there are papers addressing problems related to alteration of rocks and highlighting the application of magnetic fabric analysis in understanding various processes in sediments and sedimentary rocks. The issue starts with a paper by Hirt and Almqvist, on the interference of dia-, paraand ferromagnetic subfabrics in polycrystalline rocks. The paper gives an overview on existing techniques for the isolation of magnetic subfabrics and shows examples for geological application. Martı́nHernández along with Guerrero Suarez presents two back-to-back papers dealing with magnetic anisotropy of haematite natural crystals. The papers underline the complex behaviour of natural haematite under the influence of different field amplitudes and illustrate the variation in principal directions, shape and degree of anisotropy of magnetic susceptibility (AMS) ellipsoids with changing field magnitude. Cañón-Tapia and Cárdenas report detailed AMS, hysteresis and thermomagnetic experiments on the magnetic properties of obsidian and their implications for measuring AMS. Particularly important is the indication of submicroscopic ferromagnetic mineral constituents, which control the AMS ellipsoid. This result suggests that similar submicroscopic fabrics may be present in further volcanic rocks, but have been overlooked as yet. Kontny et al. present AMS data from measurements taken in low field at room and liquid nitrogen temperatures, as well as in high field, along with Electron Back Scattered Diffraction (EBSD) data on magnetite bearing mylonitic garnet mica schists from the relatively HP-HT metamorphic Seve Nappe Complex (Scandinavian Caledonides). M. A. Mamtani (&) Indian Institute of Technology Kharagpur, Kharagpur, India e-mail: mamtani@gg.iitkgp.ernet.in

Evidence of Polyphase Deformation in Gneissic Rocks Around Devgadh Bariya: Implications for Evolution of Godhra Granite in the Southern Aravalli Region (India)

In this paper, field evidence documenting the polydeformed nature of banded gneiss that comprises a part of the Godhra Granite and Gneiss in the southern portion of Aravalli Mountain Belt (AMB), India, is presented. The structural geometry involving an episode of recumbent-reclined folding in the gneiss lying in the vicinity of Devgadh Bariya town is worked out. The banded gneiss occurs as enclaves in the granite. Therefore, it is suggested that the banded gneiss of the region is older than Godhra Granite. Microstructures preserved in the granitic rocks are documented and it is suggested that the granitic rocks underwent deformation and strain during their evolutionary history. Variation in the mesoscopic scale fabric of the gneiss and granite along a south-to-north traverse within the study area is documented, and different possibilities for evolution of Godhra Granite are discussed.