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Some characteristics of the Precambrian crust in the northern part of central India


, : Some characteristics of the Precambrian crust in the northern part of central India. Special Publication Series - Geological Survey of India 55(2): 181-204

The northern part of the central Indian Shield displays some distinctive geological features in Archaean-Proterozoic and even in younger groups of rocks examined along a NW-SE section from Gwalior to Malanjkhand. Cummulative outcrop widths of various stratigraphic units along this 500 km long section are as indicated in parentheses: - Gwalior Group (8 km), Bundelkhand Granite massif (180 km), Bijawar Group (20 km), Vindhyan Supergroup (100 km), Mahakoshal Group (40 km), Deccan Trap cover (80 km), Baihar Gneiss (40 km), Sausar Group (Bharweli-Ukwa Group along the section) to the east of Balaghat (10 km), Amgaon Gneiss (10 km), Chilpi/Khairagarh Group at Malanjkhand (15 km) ending in Malanjkhand Granite. These groups of rocks show the following salient features: i) All the above groups occur in ENE-WSW trending long linear belts with moderate to steep dips mainly to the north. Even the far-flung BMQ and other metasedimentary enclaves within the Bundelkhand Granite massif occur as bands with lengths 500 m to 60 km with the same trend, ii) The schistosity in the metasediments and the foliation in the Bundelkhand granitic rocks also show the same trend. iii) in the supracrutals, volcanic rocks are more abundant in the eastern parts and carbonate rocks are better developed in the middle and western parts, iv) Most of the belts are narrow rift basins and, at least, the Bijawar basin is not very deep in many places; yet basic volcanic rocks are common and the swarms of basic dykes in the Bundelkhand granite massif are related to Bijawar rifting. Dolerite swarms are localised near the Bijawar margin with a 20 km gap in between. The peridotites at Angor and the coarsely crystalline gabbros and pyroxenites at Madaura, Kakarwao and Dhori Sagar in the Bundelkhand massif on the northern fringe of the Bijawar basin are intrusions in solid state and are possibly related to the evolution of the Bijawar basin. The BMQ bodies have a maximum width of 150 m and are invariably underlain by and, at places, intercalated with basaltic rocks with relict igneous texture, indicating greenstone elements. Therefore, the contention of Condie (1983) that the Archaean crust was equal to or more than the thickness of the present crust (28 to 46 km) need reexamination; alternately, the mantle convections were very strong. v) Acid volcanics are, totally absent in the present area except in the porcellanites in the Lower Vindhyans, vi) Metamorphism and granitisation resulting in the formation of Tirodi Gneisses in the Sausar belt and in granites in the Jabalpur-Narsimhapur sector of the Mahakoshals are more intense in the western parts, vii) All major structural trends including shears and photogeological lineaments such as the south Bundelkhand lineament, the Panna lineament, the Son-Narmada lineaments, Tapti-Seoni lineament, a number of shears within the Sausar Group (the southern one from Balaghat westwards), the Central India Shear and its continuation as the northern boundary fault of the Sakoli Group are all ENE-WSW to NE-SW trending. This trend in the block is known as the Satpura trend. Bouguer gravity anomaly contour patterns and the aero-magnetic anomaly zones in the Panna area have the same trend. Two diamond-bearing pipes on or near the Panna lineament, ultrabasic volcanism (Jungel) and alkaline magmatism (Bari) and the younger Ambadongar carbonatite, the alkali basalt plugs in northern Kuchchh and basic and ultrabasic plugs in the Harda area indicate deep lithospheric connection of this crustal block. vii) The Son-Narmada lineament could be a semi-global feature across India, Africa, the Atlantic and possibly also South America. Considering the antiquity of the lineament, these continents behaved as a mega-block and moved as a front. ix) Folding is not intense and their axes are parallel to the trend of the belts. Vertical tectonism is not decipherable in the older rocks but it is clear in the Gondwana rocks. x) Bouguer gravity anomaly data and the Moho depth data largely corroborate the geological section prepared and xi) Proterozoic crust shows spatial control over the spread of the Deccan trap. Thus, it would be seen that a large part of the Central Indian Shield evolved in a steady non-rotational set-up through a very long segment of the geological history, that is, from pre-2500 m. y. for the enclaves in the Bundelkhand massif to Late Proterozoic for the Upper Vindhyans. The Son-Narmada lineament has been active in post-Deccan Trap period also and is seismically active even at the present day. A mechanism for the generation and operation of the lineament has been suggested. The compressive action related to the operation of the lineament has ensured the steadiness in structural trend in the Bundelkhand block and adjoining areas and show overwhelming dominance of sinistral movement in structures of Late Archaean Bundelkhand elements, the Son-Narmada lineament and the Central India Shear. The localisation of epigenetic copper m ineralisation at Malanjkhand has been given a new structural interpretation based on which at least two other localities including the northeast corner of the Sakoli triangular basin have been identified for search of ore deposits. Find of molybdenite mineralisation and identification of the host rock granite in Bundelkhand have opened up a potentially huge store-house of the mineral. Detailed information on the Bundelkhand block presented here would facilitate construction of its evolutionary model by future researchers.

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