Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

R. K. Verma ¹, D. Ghosh ¹, S. K. Roy ¹, A. Ghosh ¹

Affiliations
1 Indian School of Mines, Dhanbad, IN

Abstract

Results of Gravity surveys conducted over Bankura anorthosite complex lying between latitude 23°28.5' to 23°33'N and longitude 86°50' to 87°15'E are discussed. The area is characterised by a gravity high of the order of 20 mgals in the eastern part and decreases to 4-5 mgals in the western part. The gravity high is found to be unrelated to the anorthosite and extends beyond the outcrop of this exposure. It is attributed to the presence of a gabbroic body underlying the anorthosite and its adjacent areas. The thickness of anorthosite layer is found to be less than 200 meters. Assuming a density contrast of +0.23 gm/cc between the underlying gabbro and the surrounding Bengal gneisses, the maximum thickness of the gabbro is found to be 5.0 km in the eastern part near Rajarampur (23°29.7'N, 87°11'E). The causative body is found to be elongated along the strike of the anorthosite exposure. It appears to be faulted underneath the Damodar river along the northern as well as the northeastern sides.

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L. K. Das ¹, D. C. Mishra ², D. Ghosh ¹, B. Banerjee ¹

Affiliations
1 Central Geophysics Division, Geological Survey of India; P-49; C II Road, Beliaghata, Calcutta-700 016, IN
2 National Geophysical Research Institute, Hyderabad-500 007, AP, IN

Abstract

Ground magnetic survey of part of central India covering the Vindhyan Basin of the upper Son valley region has delineated some subsurface vsolcanogenic Proterozoic rifts below the Vindhyan cover and several faults which have controlled the tectonics of the region. The magnetic map in general shows four distinctly different domains with magnetic anomalies differing both in amplitude and their nature. In general, the Vindhyan basin is characterized by low amplitude broad anomalies while the Mahakoshal (lower Proterozoic) Group depicts large amplitude sharp anomalies due to the presence of high susceptibility (2-19 × 10^-3 CGS units) ferruginous rocks in them. The domain boundaries are either faults or indicate formational contacts between the different types of rocks with contrasting susceptibility values.

The subsurface basement relief map of the Pre-Vindhyan topography determined by 2-dimensional Harmonic Inversion of the magnetic data reveals an asymmetric graben structure with the southern boundary fault between the Vindhyans and the Mahakoshals as the most prominent linear structure.

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D. Ghosh ¹

Affiliations
1 AMSE Wing, Geological Survey of India, Bangalore - 560 078, IN

Abstract

The aeromagnetic signatures of Agucha-Malpura-Jaipur block, Rajasthan, have been interpreted in terms of lithology and associated grade of regional metamorphism for the pre-Aravalli Bhilwara Supergroup (BSG) assemblage. High-grade metamorphic terrains of the BSG rocks are dominantly identified by several elongated blocks showing moderate to strong linear, continuous, high frequency magnetic contour patterns with preferential NE-SW orientation. The magnetic response also suggests that the high-grade BSG litho-unit consists of a heterogeneous assemblage of alternating acidic and basic members, substantially more basic in overall composition, The magnetite-bearing basic Iitho-assemblages, which occupy luuch of the high-grade terrain, show consistently high apparent susceptibility contrast and normal remanent magnetization. These zones are characterized by numerous sub-parallel brittle/ductile shear zones. - Low amplitude moderate frequency complex magnetic anomalies, oriented in variable directions, are observed over greenstone-lower amphibolite facies rocks of the BSG. This metamorphic asscmblage is essentially acidic in composition and deficient in magnetic minerals. Four zones have been identified over low to moderate-grade metamorphic terrain, which require critical examination in the light of ore-environmeant and mineralisation potential.

Keywords

Aeromagnetic Signatures, Bhilwara Supergroup, Ore-Potential, Agucha-Malpura-Jaipur block, Rajasthan.

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D. Ghosh ¹

Affiliations
1 AMSE Wing, Geological Survey of India, Bangalore - 560 078, IN

Abstract

Aeromagnetic data over Riamal-Deogarh-Lodhajhan area (Degree sheet 73C), Deogarh district, Orissa has been interpreted in terms of geology and structure around the contact zone of the Eastern Ghat Mobile (EGM) belt and North Onssa-Singhbhum (NOS) craton Several major longitudinal and later phased oblique faults have been identified fragmenting the crustal block contact zone between the granulite facies rocks of EGM belt and the greenschist-lower amphibolite facies supracrusta! rocks of NOS craton Both northern and southern boundaries of the high-grade Rengah charnockite assemblage are outlined with strong linear magnetic anomalies with dominant normal remanent magnetization Basic/ultrabasic members of charnockite pluton or highly magnetic Ductile Shear Zones (DSZ) appear to define the boundary of the Rengah charnockite belt The northern limit of the Rengali charnockite belt may be considered as the northern Fermor line of the EGM belt Major magnetic longitudinal and oblique faults are suggested for detailed ground check up for possible location of alkaline plutomc rocks and chromiferous ultrabasic rocks, if any.

Keywords

Eastern Ghat Mobile Belt, Aeromagnetic Response, Deogarh District, Orissa.

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D. Ghosh ¹, D. G. Ramesh Acharya ¹

Affiliations
1 AMSE Wing, Geological Survey of India, Bangalore - 560 078, IN

Abstract

A comprehensive suite of gradient enhancement techniques have been employed to the International Geomagnetic Reference Field (IGRF) corrected reduced to pole (RTP) aeromagnetic data obtained over parts of the highly fractured Bundelkhand Granite Massif (BGM) around Jhansi, Uttar Pradesh. This study helps in delineation of magnetic units within the supracrustals, tensional fracture-Shear systems and associated acid/basic intrusives.

High degree of correlation between the magnetic pattern and the surface geology indicates that the magnetic anomalies in the area are derived primarily from relatively shallow features. The various magnetic lithological units within the Archaean enclaves show characteristic short-Wavelength and high-Amplitude aeromagnetic signatures, by which, they were mapped. Two main regional structural patterns are evident in magnetic image and contour maps. The linear NE-SW striking magnetic anomalies are explained with the tensional fractures emplaced with demagnetized materials (quartz reef, fine-Grained granite etc) at the fault zone. The NW-SE trending strong magnetic linear features are correlated with the basic dykes. Magnetic aureole observed in the area between Basai-Talbehat-Purakalan points to possible tin mineralization.

Keywords

Aeromagnetic Survey, Bundelkhand Granite Massif, Jhansi, Uttar Pradesh.

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