March 2010 LIP of the Month

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Mafic dykes of Deccan age in the Chhattisgarh (Mesoproterozoic) Basin, Central India: implications for the origin and original spatial extent of the Deccan Large Igneous Province

N. V. Chalapathi Rao1*, B. Lehmann2, R. Burgess3, S. K. Pande4 and K. R. Hari5

1Centre of Advanced Study in Geology, Banaras Hindu University, Varanasi-221005, India (*Corresponding author: E-mail:
2Mineral Resources, Technical University of Clausthal, Clausthal Zellerfeld, 38678 Germany,
3School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, M13 9 PL, UK
4School of Studies in Geology and Water Resources Management, Pandit Ravishankar Shukla University, Raipur-492010, India
5Department of Geology, Government Y.V.T.Post graduate College, G.E. Road, Durg – 49100, India

(see also abstract presented at the 6th International Dyke Conference held at the Banaras Hindu University, Varanasi 4-7 February, 2010 [])



The Deccan Traps, straddling the Cretaceous-Tertiary boundary, and covering more than 510,000 km2, constitute an important  Large Igneous Province (LIP). Their spatial extent, geochemistry, geochronology, origin and relationship to the mass extinction event(s) at the K-T boundary are the subject of intense scientific debate. The Deccan LIP also encompasses early- and late-phases of alkaline magmatism, represented by nepheline syenite complexes, lamprophyres and carbonatites, in the Kutchh and Kathiawar regions in NW India (e.g., Basu et al. 1993; Simonetti et al. 1998; Woolley and Kjarsgaard 2008; Ernst and Bell 2010; Fig. 1A). In addition, the mafic intrusive component, represented by widespread dykes, sills and plugs (e.g. Bhattacharji et al. 1996; Bondre et al. 2006; Ray et al. 2007) constitutes an integral part of Deccan-related igneous activity.

Figure 1 (A) Outline of the Deccan flood basalts and location of the associated carbonatites, lamprophyres, alkaline rocks and kimberlites. Also shown are the locations of various isolated occurrences of mafic dykes and lavas of Deccan age located away from the presently exposed Deccan Traps. The Majhgawan diamondiferous pipe of 1100 Ma is also shown. Dashed circle is the postulated Deccan plume (after White and McKenzie, 1989; Cox, 1989). Dashed lines represent the Narmada-Son rift and Cambay rift (labelled). Abbreviations for various alkaline complexes, carbonatites and lamprophyres (locations are from Ernst and Bell, 2010) are given below: MJ= Murud Jhanjira; BT = Bassein and Trombay; N = Netrang; SD = Siriwasan Dughda; AD = Amba Dongar; PM = Phenai Mata; PK = Panwad-Kanwat; B = Barwaha; BP = Bakhatgarh-Phulmahal; G = Mount Girnar; K = Kadi; CK = Central Kutchh; M = Mundwara; SAD= Sarnu-Dandali; DLM = Danta-Langera-Mahabar.

In this communication, we report a Deccan age for sub-surface mafic dykes intruding the Mesoproterozoic Chhattisgarh sedimentary basin at Janjgiri (near Raipur; Fig. 1B) in the Bastar craton. On the basis of their geochemical similarity to Deccan flows and associated dykes, we can expand by 85,000 sq. km the known extent of the Deccan event. We also attempt to address two of the most contentious aspects of the Deccan LIP viz., (i) the role of various transport mechanisms to explain the presence of mafic dykes and basaltic flows in domains far away from the presently exposed Trap regions and (ii) whether or not a mantle plume was involved in the generation of the Deccan LIP. This study also considers the significance of the geophysical arguments for an abnormally thin present-day Indian lithosphere (e.g., Pandey and Agrawal, 1999; Kumar et al. 2007; Rychert and Shearer, 2009) compared to its counterparts elsewhere in Gondwanaland.

Figure 1 (B) A larger scale map of the Chhattisgarh basin and the Bastar craton showing the location of the Mainpur kimberlite (65 Ma) field, sub-surface mafic dykes of this study and the Deccan Traps.

Impetus for this discovery

A major impetus for this discovery came from our recent identification, on the basis of combined 40Ar/39Ar (whole rock) and U-Pb (perovskite) dating methods, of a previously unknown young end-Cretaceous/ Palaeocene kimberlite event in the Bastar craton (central India) which is synchronous with the Deccan flood basalts (Lehmann et al., 2010; Fig. 2). This has prompted us to turn our attention to a report by Subba Rao et al. (2007) on the occurrence of dolerite dykes with continental flood basalt (Deccan Traps) affinity from the Mesoproterozoic Chhattisgarh basin. As the geochemistry of the Deccan basalts is more or less unique amongst known basaltic rocks from the Indian cratons, we have decided to carry out a reconnaissance geochronological study (K-Ar whole-rock) of a sample from one such sub-surface dyke from Janjgiri (near Raipur city), Chhattisgarh basin, Bastar craton; we indeed obtained a Deccan age of 66.1± 1.4 Ma. This also incidentally constitutes the first report of mafic dykes of Phanerozoic age from the Bastar craton.

Figure 2 Summary of  40Ar/39Ar whole-rock ages and 238U/207Pb perovskite ages (2 σ) for the Behradih and Kodomali kimberlite pipes, and frequency plot of 40Ar/39Ar ages for Deccan Trap flood basalts as compiled by Hofmann et al. (2000). Grey column marks the limits of the Cretaceous-Tertiary Boundary (KTB). Adapted from Lehmann et al. (2010).

Geology, petrography and geochemistry of the Chhattisgarh sub-surface mafic dykes of Deccan age

The sub-surface mafic dykes of the Chhattisgarh basin were reached at depths of 100-200 m during drilling for groundwater by the Central Ground Water Board (CGWB) as well as by the general public. Gravity and scaling power spectral analysis studies show that the basin has a sediment thickness of up to 4 km (Bansal and Dimri, 2001) thereby demonstrating that the mafic dykes have penetrated through most of the sedimentary sequence. Magnetic studies reveal that the basic bodies are widespread in the Chhattisgarh basin (Srinivas et al., 1999). Petrographic studies reveal that the dykes are remarkably fresh and display ophitic and interstitial textures typical of diabases (Fig. 3). Calcic plagioclase, augite, Ti-magnetite, and ilmenite constitute the mineralogy. Their incompatible trace element ratios (Fig. 4) such as Rb/Y (0.31-0.48), Nb/Y (0.33-0.38), Ba/Nb (6.97-10.56) and La/Nb (1.02-1.46) are remarkably similar to those of the lower western Deccan basalt formations of  Poladpur and  Ambenali (Rb/Y =0.11-0.42 , Nb/Y =0.20-0.38 , Ba/Nb =6.2-12.22 and La/Nb =1.1- 2.0) (Peng et al. 1994).

Figure 3 (A) and (B) Petrographic studies reveal that the Chhattisgarh sub-surface mafic dykes of this study are very fresh and display ophitic to sub-ophitic texture typical diabases (crossed polars). Cp= Clinopyroxene; Fsp= plagioclase and O= opaques.

Figure 4 Rb/Y vs Nb/Y plot for the Chhattisgarh basin sub-surface dykes. The other fields are taken from Peng et al. (1994) and Sheth et al. (2009).

Significance of the finding

The age obtained for the Janjgiri dyke (66.1± 1.4 Ma) overlaps with that of the Deccan Traps (whose hitherto nearest known exposures are at a distance of ~150 km) and the diamondiferous kimberlites of the Mainpur kimberlite field (located at a distance of ~170 km from the Traps). Our recent findings identify Deccan Trap related mafic dykes and Deccan age kimberlites for the first time from the Chhattisgarh Basin and the Bastar craton and considerably enlarge the original spatial extent of the Deccan event by 85,000 sq km, almost to the Bastar Craton - Eastern Ghats mobile belt boundary.

A temporal link between (i) the sub-surface mafic dykes within the Chhattisgarh basin and the kimberlites in the Bastar craton, (ii) the Deccan feeder dykes at Goa, near the West coast of India (Widdowson et al. 2000), (iii) the Salma dyke in the Eastern Indian craton (Kent et al. 2002), (iv) the Rajahmundry Traps off the eastern coast of southern India (Knight et al. 2003), (v) the tholeiitic dykes of Deccan-age and -composition from the Seychelles (Dewey and Stephens, 1991) and (v) the Deccan basalts – all support their common geotectonic control vis-à-vis a large mantle plume head (of the order of 2000-2500 km across), as envisaged from geophysical [White and McKenzie (1989)], geomorphological [Cox (1989)] and geochemical and petrogenetic [White and McKenzie (1995) ]arguments and impose important constraints on the various transport mechanisms for dykes and flows of Deccan age in domains far away from the presently exposed trap regions.

Thermal influx from such a large-plume head can not only account for the strikingly contemporaneous Deccan-related ages reflected in disparate magma types emplaced over great distances but also can explain the large-scale mafic magmatic underplating discovered from seismic and gravity studies in parts of Mahanadi basin in the central eastern coastal region of the Indian shield (e.g. Misra et al. 1999; Behera et al. 2004).

The presence of macrodiamonds in Mainpur kimberlites (Newlay and Pashine, 1993) imply the presence of thick and cold lithospheric roots (at least 150 km thick) at the time of their emplacement compared to the present-day thinned Indian lithosphere of only 80 km thickness (Kumar et al. 2007; Rychert and Shearer, 2009). It is well acknowledged that carbonatites and other alkaline magmas are characteristic of thinned (extension-related) lithosphere and are typically, but not exclusively, associated with rift zones (e.g., Ernst and Bell, 2010). On the contrary, diamondiferous kimberlites are not known to be related to normal rift systems, and their emplacement is controlled by deep-seated faults and fractures in an abnormally thick lithosphere (e.g., Moore et al., 2008). The location of the Mainpur kimberlites at the core of the Bastar craton and their remoteness from the major Indian rift systems together with their emplacement synchronous with the Deccan event at 65 Ma demonstrates a common geotectonic control, which we infer to be a mantle plume.

The plume-model (but not non-plume models involving passive rifting (Sheth, 2005, 2007)), can also better account for the geophysical models of present-day thinned lithosphere of the entire Indian shield that had a contrastingly different and thickened (150 km), lithosphere at least until the end of the Cretaceous (see Lehmann et al. 2010). An incubating large mantle plume head of 2500 Km across beneath an Indian lithosphere of variable thickness is also capable of explaining (i) the geographic distribution of the kimberlite, carbonatite and alkaline rock spectrum in the Deccan LIP (Fig. 5; Chalapathi Rao and Lehmann, 2010; Chalapathi Rao et al. 2010) and (ii) the occurrence of mafic dykes and basaltic flows (in eastern, southeastern and southwestern India) in domains far away from the presently exposed Trap regions. We strongly believe that our findings can provide new impetus to the ongoing research on the Deccan LIP.

Figure 5 Proposed model for the generation of disparate magma types in the Deccan LIP (Chalapathi Rao and Lehmann, 2010).


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