Forming topography in granulite terrains: Evaluating the role of chemical weathering

Abstract

Granulite terrains have gently undulating topography, with charnockites and khondalites forming hillocks within low-lying areas comprising quartzofeldspathic gneisses (QFG). Petrographic, XRD and spectroscopic studies reveal that QFGs and charnockites show minimal clay mineral formation, indicating their resistance to chemical weathering. In contrast, khondalites weather progressively to form a variety of clay minerals, the proportion of which increases with elevation, ultimately stabilizing bauxite on hill-tops. Geochemical modelling indicates that this weathering pattern in khondalites can develop under open system conditions prevailing on hill tops and slopes, as rainwater is not retained within the system. This implies that the khondalite hills existed before bauxite formation. Since khondalite hills occur within more resistant but low-lying QFG, the present granulite terrain topography was not shaped by chemical weathering. Rather, mechanical weathering or neo-tectonic activity may be responsible for topography formation in stable granulite terrains.

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Acknowledgements

The main funding for this study was from the RTAC-BRNS project scheme, India (Sanction No. 35/14/12/2016-BRNS/35049) to SG. AM thanks IIT Kharagpur and the Ministry of Human Resource Development (MHRD), Department of Higher Education, New Delhi, India for providing the regular student assistantship. AM conveys her sincere gratitude to the Department of Geology and Geophysics, IIT Kharagpur for facilitating this study, and Prof. Kumar Biradha of the Department of Chemistry, IIT Kharagpur, for facilitating the XRD analysis. Additional support and discussion with Kaushik Mitra and Himela Moitra, Department of Geology & Geophysics, IIT Kharagpur, were also extremely useful. The paper has benefited greatly from the comments of the handling editor and two anonymous referees.

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Correspondence to Saibal Gupta.

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Communicated by N V Chalapathi Rao

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Maitra, A., Chatterjee, A., Keesari, T. et al. Forming topography in granulite terrains: Evaluating the role of chemical weathering. J Earth Syst Sci 129, 17 (2020). https://doi.org/10.1007/s12040-019-1293-4

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Keywords

  • Granulite terrain
  • chemical weathering
  • monadnock
  • bauxitization
  • topography