Illite Crystallinity Index from the Mesoproterozoic Sedimentary Cover of the Kaladgi Basin, Southwestern India
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J. Earth Syst. Sci. (2019) 128:101 c Indian Academy of Sciences https://doi.org/10.1007/s12040-019-1124-7 Illite crystallinity index from the Mesoproterozoic sedimentary cover of the Kaladgi basin, southwestern India: Implications on crustal depths of subsidence and deformation Mrinal Kanti Mukherjee1,*, Kunal Modak1 and Jiten Ghosh2 1Department of Applied Geology, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, Jharkhand, India. 2Advanced Mechanical and Material Characterization Division, CSIR-Central Glass and Ceramics Research Institute, Jadavpur, Kolkata 700032, India. *Corresponding author. e-mail: mrinal [email protected] MS received 20 January 2018; revised 15 July 2018; accepted 18 October 2018 The grade of metamorphism and thermal maturity of the Mesoproterozoic Bagalkot Group in the Kaladgi basin of southwestern India has been determined using the illite crystallinity (IC) index. IC index was determined from the argillite samples of four stratigraphic levels viz., Ramdurg Formation (basal unit), Yargatti Formation (intermediate lower unit), Yadahalli Formation (intermediate upper unit) and Hoskatti formation (upper unit). IC index (Kubler equivalent) values range between 0.54◦Δ2θ and 0.24◦Δ2θ (in a set of 37 samples) and indicate a deep diagenetic to high anchizone metamorphic grade within a temperature range of ∼180 to 300◦C. The Mesoproterozoic sedimentary cover of the Kaladgi basin is deformed due to southerly directed gravity gliding of the cover over the basement. The general variation of the IC index along and across the basin as indicated by the distribution of IC index values and isocryst contour patterns is due to the combined effect of deformation and variable subsidence of the Mesoproterozoic cover of the basin. Considering an average Mesoproterozoic geothermal gradient of 35◦C/km, the crustal depth of deformation and/or subsidence of the Mesoproterozoic cover up to the sample point is estimated to vary between 5.14 and 8.57 km. Keywords. Kaladgi basin; illite crystallinity; deformation; subsidence. 1. Introduction layers separated by interlayer potassium cations (e.g., Moore and Reynolds 1997). The IC index The illite crystallinity (IC) index is a commonly is the numerical expression of the basal (001) used parameter for describing low-grade metamor- 10-A˚ illite X-ray diffraction (XRD) peak at one-half phism of pelitic rocks (e.g., K¨ubler 1967; Kisch of its height [known as field width at half max- 1983; Frey 1987; Merriman and Peacor 1999; ima (FWHM)] and is typically reported in units K¨ubler and Jaboyedoff 2000; Jaboyedoff et al. of ◦Δ2θ,whereΔ2θ is the XRD angle (K¨ubler 2001). Illite is a phyllosilicate consisting of ‘packet’ 1967; K¨ubler and Jaboyedoff 2000). The 10-A˚ of 10-A˚ thick sheets, which in turn are made peak of illite (001) in XRD gradually sharpens up of tetrahedral and octahedral aluminosilicate as metamorphic grade increases. The IC index 0123456789().: V,- 0123456789().,--: vol V vol 101 Page 2 of 15 J. Earth Syst. Sci. (2019) 128:101 was introduced by K¨ubler (1967) and is used to Peninsular Gneissic Complex (PGC) occurring to define diagenetic and low-grade metamorphic zones the south of the basin, granites (Closepet granite) in pelitic rocks. On the basis of IC index, these that occur to the north, central and southeastern zones are classified as diagenetic (IC >1◦Δ2θ), parts of the basin, and the Hungund schist belt anchizone (IC0.42◦Δ2θ−0.25◦Δ2θ) and epizone (HSB) that occupies the eastern part of the basin (IC <0.25◦Δ2θ). IC index is a commonly used (Jayaprakash et al. 1987; figures 1, 2). method to determine metamorphic zones and is The basin covers rock successions comprising also used as a common geothermometer because three sequences bounded by regional unconformi- of ease of measurements, despite arguments that ties (table 1). The lowermost Lokapur Subgroup IC cannot be used as an accurate geothermome- is the thickest sequence (∼3.1 km) that occurs ter (Essene and Peacor 1995). Abundance of illite throughout the basin. The Simikeri Subgroup is in most mudrocks in sedimentary basins makes confined to the east-central parts of the basin it possible to measure the thermal maturity in a and is separated from the underlying Lokapur basin-wide scale and to detect the potential heat Subgroup by a disconformity. The Lokapur and sources. IC is especially useful in reconstructing Simikeri Subgroups are of Mesoproterozoic age the structural and metamorphic history in very as determined from the stromatolite biostratigra- low- to low-grade metamorphic rocks (e.g., Frey phy (cf. Pillai 1997; Kulkarni and Borkar 1999) et al. 1980; Roberts and Merriman 1985; Frey 1988; and carbon/strontium isotope data from carbon- Gutierrez-Alonso and Nieto 1996; Roberts et al. ates and shales (cf. Padmakumari et al. 1998; 1996; Warr et al. 1996; Lee and Ko 1997; Mukher- Rao et al. 1999) and Late Palaeoproterozoic to jee 2003; Merriman 2006; Ruiz et al. 2008; Hara early Mesoproterozoic age by Sharma and Pandey and Kurihara 2010; Verdel et al. 2011; Fitz-D´ıaz (2012). Patil et al. (2018) generated 40Ar/39Ar et al. 2014; Fukuchi et al. 2014). In addition, the date of 1154 ± 4 Ma from a mafic dyke that is spatial variation in sediment thickness (e.g., Awan intrusive along an axial plane of a fold in the and Woodcock 1991) and the existence of concealed Bagalkot Group, indicating a Mesoproterozoic age plutons (e.g., Roberts et al. 1990; Lee and Lee of the deformation. The Lokapur and Simikeri Sub- 2001) can be examined on the basis of IC variation. groups constitute the Bagalkot Group (table 1). In this paper, the determination, distribution The younger Badami Group of Neoproterozoic age and variation of IC index are documented from the is separated from the underlying Simikeri Group deformed Mesoproterozoic sedimentary cover in an by an angular unconformity. The distribution of intracratonic basin in southwestern India. IC index the Badami Group of rocks is restricted to the values are further used to define low-grade meta- southern and western parts of the basin. In the morphic zones of the rocks and estimate the crustal southern parts of the basin, the Badami Group depth of subsidence and deformation. unconformably overlies the Lokapur Subgroup as well as the basement rocks of the Dharwar Cra- ton (figure 1). The general lithostratigraphy of the 2. Geological background basin is outlined in table 1. The Lokapur and Simikeri Subgroups, in gen- The east–west trending Kaladgi basin is located in eral, are composed of cherts and coarse siliciclastics the northern part of the Dharwar Craton, and it that include granular arkose, quartz arenites and is spread over an area of ∼8300 km2 (Jayaprakash lensoidal bands of polymictic conglomerates along et al. 1987, 2007). The basin has an irregular ellip- the basin margin that grades gradually through tical outline and extends over a length of 250 km argillites to carbonates towards the interior basin with a great part concealed beneath the vast spread (Kale and Phansalkar 1991; Pillai and Kale 2011). basaltic lava flows of the Deccan Traps of Upper The total thicknesses of the Mesoproterozoic sed- Cretaceous to Lower Eocene age (Wensink and iment cover of the Kaladgi basin is 4234 m. The Klootwijk 1970; Mani 1974) in the north and in Mesoproterozoic cover of the Kaladgi basin is the west (figure 1). In the north, the outcrops reported to have been deformed into elongated occur as inliers within the Deccan Traps near regional dome-and-basin patterns that are ori- Jamkhandi, and similar strata may extend fur- ented from WNW–ESE to E–W. Deformation is ther north beneath the Deccan Trap flows. The relatively intense in the south-central sectors of Archaean basement of the basin is a part of western the basin and mild to almost no deformation in Dharwar Craton and constitutes an assemblage of the northern margins (Jayaprakash et al. 1987; J. Earth Syst. Sci. (2019) 128:101 Page 3 of 15 101 Figure 1. (a) Inset shows the position of Kaladgi basin in peninsular India, (b) geological map of the Kaladgi basin (modified after Mukherjee et al. 2016). Study area is shown with a rectangular outline. The northern and south-central sectors are distinguished on the basis of association of structural elements, their distribution and interrelationships (see text and figure 2). Jayaprakash 2007) (figure 2). The deformational trending, both northerly and southerly verging, architecture of the basin and the structural details asymmetric-to-overturned, plane non-cylindrical, of the deformed Mesoproterozoic cover rocks have gently plunging folds with axial planar cleav- been comprehensively documented recently and ages, E–W trending thrusts and N–S trending explained in terms of basement–cover structural strike-slip faults, together define the contractional relationships and the origin of deformation of the domain. Distribution, variation and interrelation- cover (Mukherjee et al. 2016). On the basis of ship of structural elements in the cover rocks reveal type, geometry, distribution, association of struc- that the extensional and contractional domains tural elements, variation and relative chronology lie adjacently (figure 2), spatially linked and are of development, the deformation structures of the related to a single deformation event of the cover. Mesoproterozoic sedimentary cover rocks of the The orientation of the structural grain of the Meso- basin can be grouped to define an extensional proterozoic Bagalkot Group is E–W to WNW– domain in the northern sectors and a contrac- ESE. tional domain in the south-central sectors of the The basement cratonic assemblage consisting of basin (Mukherjee et al. 2016) (figure 2). The exten- the peninsular gneisses and the HSB have together sional domain is characterised by development undergone the multistage deformation with a struc- of a gently dipping (10◦–15◦ due south) homo- tural grain of NW–SE.