R. V. Karanth

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.

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.

Petrography of ancient Indian lime plaster

Abstract Samples of ancient Indian lime plaster have not previously been analysed. Excavation of a seventh century AD baked brick structure provided the necessary samples for a detailed laboratory examination of plaster. The plaster samples were studied by chemical and petrographic methods. The probable sources of lime and sand were indicated by the mineralogy.