Forming Topography in Granulite Terrains: Evaluating the Role of Chemical Weathering
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J. Earth Syst. Sci. (2020) 129 17 Ó Indian Academy of Sciences https://doi.org/10.1007/s12040-019-1293-4 (0123456789().,-volV)(0123456789().,-volV) Forming topography in granulite terrains: Evaluating the role of chemical weathering 1 1 2 ASMITA MAITRA ,ADITI CHATTERJEE ,TIRUMALESH KEESARI and 1, SAIBAL GUPTA * 1 Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India. 2 Isotope and Radiation Application Division, Bhaba Atomic Research Centre (BARC), Mumbai 400 085, India. *Corresponding author. e-mail: [email protected] MS received 28 May 2019; revised 20 August 2019; accepted 30 August 2019 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. Keywords. Granulite terrain; chemical weathering; monadnock; bauxitization; topography. 1. Introduction earlier stage of ‘youth’ (Holmes 1978). Such ‘mature’ or ‘old’ landscapes, characteristic of most In tectonically active regions on continents, shield areas, have faint relief with occasional iso- prominent topographic features result from colli- lated mounds or hills; these hills are commonly sional orogenesis, volcanic arc formation and rift- considered to represent residual topography or ing; this Brst order ‘young’ topography is ‘monadnocks’ (Holmes 1978) or ‘inselbergs’ (Skin- subsequently modiBed by weathering and erosion. ner and Porter 1995), deBned by rock types that Erosion rates in such areas of tectonically driven are resistant to weathering and erosion in com- rock uplift are now believed to be controlled by parison to associated lithologies. This is the increased rates of mechanical weathering processes topography commonly observed in granulite ter- such as landsliding (e.g., Montgomery and Bran- rains in the Indian shield, such as the Eastern don 2002). On the other hand, continental shield Ghats Belt (EGB), that are considered to represent areas that are at present tectonically inactive sites of ancient collisional orogenesis (e.g., Gupta typically preserve low elevation topography, tac- 2012; Dasgupta et al. 2013). Since these old, itly assumed to result from long term degradation granulite terrains are not at present undergoing of earlier landscapes that have passed through an tectonically driven uplift, and the subdued 17 Page 2 of 13 J. Earth Syst. Sci. (2020) 129 17 differential elevation is expected to limit mechan- gneisses are all present within the CMZ. The ical weathering, chemical weathering may be con- presence of all these lithologies in close proximity sidered to play a significant role in shaping their in this unit of the EGB makes it easier to system- landscapes. Since this implies a switch in the rela- atically study the differences in topographic ele- tive importance of mechanical and chemical vation in relation to lithology, and to compare the weathering processes as an orogen is eroded, an weathering pattern in these different rock types evaluation of the relative roles of mechanical or both with respect to lithology and elevation. chemical weathering in creating granulite land- scapes, that represent old orogens, can be of con- siderable interest. 3. Methodology In this study, we address this problem in an ‘old’ landscape within the EGB, a granulite terrane that Since the CMZ contains all three major rock types borders the eastern fringe of the Bastar and of the EGB, a suitable area within this unit was Dharwar cratons in peninsular India (Bgure 1a). selected for geological mapping. The selected area The northern part of the belt, where this study was was almost completely accessible in all sectors, carried out, is characterized by generally Cat, low- contains all three major lithologies, and has the lying topography interspersed with occasional jag- characteristic undulating topography as indicated ged as well as rounded hillocks and mounds in GoogleEarth imagery as well as in the Beld (Bgure 1d). We have conducted geological mapping (Bgure 1b, c). Comparison of the geological map across a representative part of this area, and prepared in this study (Bgure 1c) with the Goo- through systematic petrographic, clay mineral and gleEarth image of the study area (Bgure 1b) allows geochemical modelling studies, we suggest that evaluation of the correlation, if any, between even in such areas of modest topography, chemical lithology and elevation. The nature of weathering weathering is unlikely to play a major role in cre- of khondalite, charnockite and quartzofeldspathic ating relief. The implication is that mechanical gneiss can be traced from the petrographic and weathering may be more important than chemical spectroscopic analysis, and therefore, rock samples weathering even in shaping ‘mature’ and ‘old’ were collected from all rock types in the area, landscapes. including weathered varieties. Khondalites showed a distinct difference in appearance and degree of weathering with elevation; thus, khondalite sam- 2. Geology of the study area ples were collected both from topographically high (Sample no. 3B at an elevation of 255 m) and low The Eastern Ghats Belt (EGB) mainly comprises regions (Sample no. 2 at 112 m and Sample no. 16A granulite facies rocks, with migmatitic quartzo- from 52 m) (Bgure 1c). Charnockite and quart- feldspathic gneiss (QFG), charnockite and khon- zofeldspathic gneiss showed no such variation with dalite as the main lithologies. These three major elevation. For characterization, representative lithologies vary in proportion within distinct litho- samples of the charnockite (Sample A11R) and tectonic units that are oriented parallel to the QFG (Sample A18R) were selected for analysis trend of the belt (Ramakrishnan et al. 1998). The (Bgure 1c). Petrographic studies of thin sections northern part of the EGB is dominated by quart- prepared from the rock samples were conducted zofeldspathic gneisses and is referred to as the using a LEICA DM 4500 P microscope, and the Charnockite Migmatite Zone (CMZ of Ramakr- captured images were processed using the Lei- ishnan et al. 1998; Bgure 1a). Further south, the caQwin software. To identify clay minerals that Eastern and Western Khondalite Zones (EKZ and cannot be detected using the petrological micro- WKZ, respectively), and the Western Charnockite scope, FTIR analysis was conducted. For this, the Zone (WCZ) generally show higher elevations, rock samples were initially powdered and mixed deBning much of the ‘Eastern Ghats’ hill ranges in with KBr powder, ground to a grain size less than this region. Thus, the litho-tectonic units domi- 5 mm in diameter and pressed into a transparent nated by charnockites and khondalites (i.e., the pellet. This pellet was then analysed in a NICO- WCZ, WKZ and EKZ) are elevated compared to LET 6700 model Fourier Transform Infra-Red the unit dominated by migmatitic quartzofelds- (FTIR) spectrometer at the Central Research pathic gneisses (i.e., the CMZ). Charnockites, Facility (CRF), IIT Kharagpur, with a spectral khondalites and migmatitic quartzofeldspathic resolution of 4 cmÀ1 in the Mid IR range J. Earth Syst. Sci. (2020) 129 17 Page 3 of 13 17 GoogleEarth Image of (b) ). Red box shows the exact position of the study area; 1998 ( et al. Isolated hills of khondalite overlying low-lying terrain of QFG. (d) Geological map of the area prepared in this study. (c) ) Geological map of the Eastern Ghat Belt, redrawn after Ramakrishnan a the study area; Figure 1. ( 17 Page 4 of 13 J. Earth Syst. Sci. (2020) 129 17 (4000–400 cmÀ1). For further conBrmation of the weathering. The QFGs are characterized by leuco- major and clay mineral species, powder X-ray cratic and melanocratic segregation layering. Bio- diAraction analyses of the whole samples were also tite-poor leucocratic layers consist of quartz and conducted using a Bruker AXS D8 DiAractometer plagioclase feldspar, whereas the melanocratic lay- hosted in the Department of Chemistry, Indian ers are predominantly composed of biotite and gar- Institute of Technology Kharagpur. Each sample net. Plagioclase and alkali feldspar grains are was analyzed within the two-theta range 10°–90° deformed and recrystallized. All phases, including using Cu–ka excitation with a 50 mA current and a feldspars, have perfectly preserved grain boundaries power of 40 kV. Peaks obtained from XRD analy- and are mostly unaltered (Bgure 2a). Charnockites ses were compared with the available online data- show weathering patterns similar to the QFGs. base using the X’pert HighScore Plus software. Fresh grains of quartz, plagioclase feldspar, biotite, Finally, geochemical modelling