Chitradurga Schist Belt Research Articles

Archaean stromatolites from the Chitradurga schist belt, Dharwar Craton, South India

Abstract Stromatolites have been found in the cherty dolomite member of the Vanivilas Formation of the Chitradurga Group belonging to > 2600 Ma old Archaean Dharwar Supergroup, South India. They occur in the Bhimasamudra, Marikanve and Dodguni areas of the Chitradurga schist belt. The stromatolites of the Bhimasamudra and Marikanve areas are described here for the first time. The stromatolites of Bhimasamudra consist of cylindrical and terete columns, as well as pseudocolumns with concave tops developed on a wavy base. Coalescence of columns and γ-type branching have been noticed. The Marikanve stromatolites are stratiform, often grading into pseudocolumnar and dome-shaped types. Naked stromatolite columns showing β-type and markedly divergent branching are found in the Dodguni area. The Bhimasamudra and Dodguni stromatolites probably developed in an intertidal environment and the Marikanve ones in a subtidal environment.

Geochemistry of the archaean greywackes from the Northwestern part of Chitradurga schist belt, dharwar craton, South India — Evidence for granitoid upper crut in the Archaean

Geochemistry of the archaean greywackes from the Northwestern part of Chitradurga schist belt, dharwar craton, South India — Evidence for granitoid upper crut in the Archaean

Geology, geochemistry and tectonic setting of Archaean greywackes from Karnataka nucleus, India

More than 80% of the area covered by the Chitradurga and Shimoga schist belts (3.0-2.6 Ga) of Karnataka nucleus, India, is made up of greywackes. Feldspathic, lithic and quartzose greywackes are deposited, respectively, along the western, northern and the eastern margin of the Chitradurga schist belt. Greywackes of the Shimoga schist belt show a large compositional scatter between feldspathic and lithic types. Compositional differences indicate variable provenance for the different localities. Tonalite-trondhjemite gneisses (TTG), mafic-ultramafic schists, basic-intermediate-acid volcanic rocks, sedimentary rocks like banded iron-formation, quartzites and cherts, and K-granites in minor quantity, were exposed in the source area. Volcanic debris was also contributed from contemporaneous volcanism in the basin. Greywackes in this region are invariably interbedded with fine-grained, biotite-rich phyllites and schists. Marked compositional differences are observed between the greywackes and the interbedded phyllites. The greywackes have K 2O/Na 2 O < 1, whereas phyllites show large variation in K 2O/Na 2O ratios ranging from 1.5 to 15.0. High Cr-Ni content of the greywackes and phyllites of the Chitradurga belt are indicative of an important contribution from ultramafic rocks. Light REE enrichment and negative Eu anomalies are particularly pronounced in the mudpacks of the greywackes represented by phyllite. The REE and other chemical data, such as La/Th, Ti/Zr and K 2/MgO ratios, indicate that these greywackes were supplied from a juvenile continental nucleus and were laid down on a simatic and converging active continental margin. An Archaean miniplate model with low-angle subduction and sedimentary basins of moderate size is proposed to explain the geochemical and tectonic features of the schist belts.

Precambrian Tectonics of the South Indian Shield Inferred from Geophysical Data

Geophysical parameters such as MAGSAT, LANDSAT, gravity, deep seismic sounding, seismicity, and heat flow data have identified cratonic and mobile units in the Precambrian continental crust of the South Indian shield. LANDSAT imagery and gravity field data suggest that the shield is divided into four geophysical domains from north to south, namely: (1) the Deccan trap-covered gneissic basement and the gneiss terrain around Hyderabad, (2) the Dharwar craton, subdivided into its eastern and western blocks, (3) the granulite terrains of Kerala, Karnataka, and Tamil Nadu, and (4) the Eastern Ghats granulite belt. The smooth transition in the gravity field from the Dharwar greenstone terrain to the Deccan trap-covered area to the north indicates the northward continuity of the granite-gneiss basement. However, the trap-covered region lacks any significant short-wavelength anomalies of the type associated with the Dharwar greenstone belts, indicating the probable absence of any such large-scale high-density belts underneath the Deccan traps. The Chitradurga schist belt, a 450 km-long linear feature of Archean-Early Proterozoic age, is technically one of the most important features of the shield. Geophysical studies suggest that the eastern margin of the Chitradurga belt (CDB) is the site of crustal shortening during Late Archean-Early Proterozoic times, a result of E-W compressional forces operating at the time of the evolution of the Early Proterozoic mobile belt (EPMB) in the eastern block of the Dharwar craton. The CDB probably represents a paleo-rift on the Archean crust where early tensional forces created a rift opening followed by later closure under compressional forces. In its northern part, the southern granulite terrain is geophysically continuous with the Dharwar craton, but in more southern portions it is segmented by fault-bounded blocks of different degrees of uplift. Gravity and seismic crustal investigations suggest that the Eastern Ghats belt (EGB) is a continental collision zone produced by some kind of plate convergence. Suggestions that the Eastern Ghats trends are geophysically continuous into the southern granulite terrain are disproven by the existence of an elongate gravity high in the southern part of the cratonal eastern block, which truncates gravity-map trends.

Gas from heated metal: G. Belloc. Comptes rendus, cxlix, 672

The ∼1.2 km long and ∼250 m wide Chikkasiddavanahalli (C.S. Halli) hill range consists of mixed sulphidic-oxide banded iron formations (BIFs) and Fe-rich phyllites (±carbonaceous), which overlie carbonated schistose and massive meta volcanics. In stratigraphic succession, the lithologies represent the Ingaldhal Formation and are an integral part of the Chitradurga schist belt in the Western Dharwar Craton. The general strike at C.S. Halli varies from N–S to 340° with vertical to steep dips towards east and west. The sulphides, oxides and silicates exhibit intergrowth replacement textures developed during regional greenschist- and amphibolites- facies metamorphism. The BIFs show mesobands of recrystallised cherts and iron sulphides such as pyrite, arsenopyrite, and silicates such as subordinate grunerite, hornblende, chlorite, muscovite, actinolite and minor carbonates such as ankerite, calcite and magnesian siderite. Chemical data indicate depletion in Ti, Mn, Co, Cu, Cr and Ni in these iron formations. Most chondrite normalized REE patterns of the iron formation show moderate LREE and HREE enrichment coupled with strong positive Eu anomaly; the mineralized portions exhibit characteristic negative Ce and Eu anomalies (Eu/Eu∗ 0.21 to 3.00). The total REE abundance varies, correlates well with the iron contents of the BIFs, and similar to those exhibited by hydrothermal plumes [Chown, E.H., Dah, E.N., Mueller, W.G., 2000. The relation between iron formation and low temperature alteration in a Archean volcanic environment. Precambrian Research 101, 263–275]. Trace and REE data suggest that primary mantle-derived hydrothermal solutions were added to the pore fluids of sediments of the Chitradurga basin and supplied chemical constituents such as FeO, SiO2 and REE. Oxidation of FeO to Fe2O3 was caused by the photosynthesis of primitive stromatolite-building cyanobacteria. Geochemical data suggest a model involving epigenetic gold mineralisation in close association with shear zone deformation, quartz-calcite vein activity and sulphidation in the mixed sulphide oxide facies BIF and associated iron phyllites in the C.S. Halli area, Western Dharwar Craton, India.