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

V. Srinivasan ¹, Amit Sahay ², Deepak C. Srivastava ²

Affiliations
1 Geological Survey of India. Op: TNPK, Chennai, IN
2 Earth Sciences, IIT Roorkee, IN

Abstract

No Abstract.

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Deepak C. Srivastava ¹, Amit Sahay ¹, Devendra Kumar ¹, Pulok Mukherjee ²

Affiliations
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667 (UA), IN
2 Wadia Institute of Himalayan Geology, 33, Gen Mahadev Singh Road, Dehra Dun - 248 001 (UA), IN

Abstract

Two distinct types of enclaves, namely, gneissic enclaves and microgranitoid enclaves occur abundantly in the granitoid rocks belonging to the granulite-Granitoid terrane of the Banded Gneissic Complex Field relationships, and petrographic and microstructural studies indicate that the gneissic enclaves are the xenoliths of the older rocks but the microgranitoid enclaves are formed by the process of magma mixing and magma mingling.

Both microgranitoid enclaves and the host granitoids are cut by numerous normal fault type and strike-Slip type of ductile shear zones. Stress analysis by the fault-Slip method and, strain analysis using the feldspar megacrysts as strain markers, reveal that the normal fault type of shear zones are the extensional structures formed by non coaxial and flattening type of deformation. A critical comparison of the concentrations of major and trace elements, in the unsheared and the sheared microgranitoid enclaves, implies the addition of SiO₂ Al₂O₃, Na₂O, K₂O, Pb, Rb and Sr, and loss of Cr and Cu during the ductile shearing. The slopes of isocon lines, passing through the plots of the immobile elements, such as Y and TiO₂ indicate that 18-64% gain in volume has occurred during the ductile shearing of the microgranitoid enclaves.

Keywords

Microgranitoid Enclaves, Magma Mixing and Mingling, Stress and Strain Analyses, Ductile Shear Zone, Volume Change, Isocon, Devariya Bandanwara Area, Central Rajasthan.

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Amit Sahay ¹, Deepak C. Srivastava ¹

Affiliations
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN

Abstract

The Nimbahera shale beds show an hourglass map pattern due to juxtaposition of an anticline and a syncline along a common NNE striking, upright axial plane. Several possible hypotheses, such as superposed folding, sheath folding, variation in shortening, variation in orientation of pre-Existing S-Surfaces and coalescence of fold waves are tested for explaining the development of the hourglass structure. Evidence from geometrical characteristics of the folds, associated mesoscopic scale structures and the published experimental results suggest that the hourglass structure has developed due to the linkage of the two out-Of-Phase fold waves that propagated towards each other during the folding of the Nimbahera shale beds by buckling.

Keywords

Hourglass Structure, Buckle Folding, Nimbahera Shale, Chittaurgarh, Rajasthan.

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Amit Sahay ¹, A. Rai Choudhuri ², D. B. Guha ²

Affiliations
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN
2 Geological Survey of India, Kolkata - 700 009, IN

Abstract

No Abstract.

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