Abstract
Geochronological research on the Deccan traps, one of the larger continental flood basalt provinces, has been underway for over 50 years. Initial attempts by K–Ar dating, yielding scattered dates, was superseded by 40Ar/39Ar step-heating studies; however, several of these later attempts failed to follow the guidelines for what defines (statistically valid) plateau/isochron ages. It has been shown that working with altered samples can significantly hamper success; an alteration index, recovered from the argon isotopic data, uncovers which samples are altered. Over the last decade, high-precision argon ages narrowed the range of crystallisation ages of the bulk of tholeiitic material in this province to <1 Myr, straddling the Cretaceous–Palaeogene boundary. A group of researchers continue to insist that the duration of volcanism forming most of the composite Western Ghats section (CWGS) is 3–4 Myr. This dichotomous position is cleared up by critical assessment of all reports, including high-precision 40Ar/39Ar ages on plagioclase separates from flows, as well as U–Pb dating of single crystals of zircon, following chemical abrasion, from ash fall deposits between the lava flows, or high silica aggregations within the flows. It is shown that the duration of volcanism for the CWGS was limited to ~800 ka, starting at ~66.4 Ma. Lingering problems with the presentation and use of these high-precision ages hamper working out full details of the timing and duration of lavas, both within and between the formations of the CWGS. Lavas from the Mandla lobe, the Malwa plateau and the Rajahmundry traps, were also formed within this time period. Reports of argon ages of ~69 Ma for silicic volcanism in the Saurashtra area are shown to be incorrect; much of the silicic–alkaline volcanism in the Deccan is known to be coeval with the main phase of tholeiitic volcanism, based on both argon and U–Pb research. The bulk of volcanism in other continental flood basalt provinces (the Siberian traps, the Parana province and Columbia river basalt group) are now known to have been each confined to <1 Myr.
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(modified from Shrivastava et al. 2015). The CWGS, the main focus of this effort, reaches a total thickness of >3 km of tholeiitic lavas. Other areas for which geochronological data are assessed are: the Mandla lobe, the Malwa plateau, the Rajahmundry traps and some alkali/felsic complexes in the northern Deccan (Narmada lineament area) and Saurashtra.
(modified from Schoene et al. 2015). The most recent geochronological data suggest that the KPgB lies close to the Poladpur to Ambenali formation boundary (Schoene et al. 2019). Almost the whole section was formed in Chron 29R, with only the oldest and youngest lavas in the Jawhar and Mahabaleshwar formations, respectively, showing normal magnetic polarity. The immense amount of tholeiitic material seen in the CWGS was formed in ~800 ka (~66.3–65.5 Ma) – figures 3 and 5. The lavas of the Mandla lobe, the Malwa plateau and the Rajahmundry traps (figure 1), were also primarily formed within this time interval (figures 6–8, and text).
taken from Schoene et al. 2019). Formation boundaries and names are shown in black. Entries in blue are the ages obtained using the Bayesian technique, those in red are the youngest single zircon age with small error estimate, for each horizon (Schoene et al. 2019); two ages (for horizons RBAY and RBAN in the (crowded) middle Ambenali), omitted for the purposes of clarity. The age difference between the two techniques is generally small, in all cases, the two sets overlap at the 95% confidence value test. However, doubts persist regarding the use of (only) the Bayesian values. Firstly, in most instances, the ‘youngest’ measured ages are younger (albeit marginally) than the Bayesian ages; secondly, the Bayesian errors are considerably smaller than those for the measured youngest ages (see text for further details). The use of such unrealistically small errors – often 2–4 times smaller than that resulting from the mass spectrometric measurements, appears unrealistic. Both in figure 5 and text, I utilised only the youngest single zircon age with small (measured) errors.
taken from Shrivastava et al. 2015). A summary of the findings is listed in table 4. Samples are shown in order from oldest (1) to youngest (37). Ages adjusted to the calibrations of Renne et al. (2011) and errors shown at the 1σ level, internal precision (i.e., σJ = 0.0%). T: age, IR: initial 40Ar/36Ar ratio, F: goodness of fit, p: probability of occurrence. (a and b) Results for the lowest flow. The plateau (a) appears statistically acceptable but shows increasing step ages to the right, indicative of 40Ar* loss. The isochron plot shows an IR lower than the atmospheric value (295.5), confirming partial 40Ar* loss. The freshness (AI) test (figure 7b) suggests the sample is unaltered. The isochron age may be used with caution – but not the plateau age. (c) Age spectra for two runs on samples from lava 4. Both yield acceptable plateau ages that average ~64.6 Ma. The AI test (figure 7b) suggests that the split shown in green in (a) is quite altered; the other split appears to be fresher and its age can be used tentatively as a crystallisation value (see table 4). Two other samples from near the top of the Mandla section yield acceptable plateau ages (d, e, table 4), but are clearly altered (figure 7b). Hence their ages cannot be used as estimates of the time of crystallisation.
taken from Shrivastava et al. 2015). A summary of the findings is listed in table 4. Samples shown in order from oldest (11) to youngest (27). Ages adjusted to the calibrations of Renne et al. (2011) and errors shown at the 1σ level, internal precision (i.e., σJ = 0.0%). T: age, F: goodness of fit, p: probability of occurrence. (a) For flow 11, a marginal plateau (<50% of the total 39Ar released) is seen. The AI test (figure 7a) shows the sample is altered and the age is rejected. (b) For flow 23, a plateau age of ~61 Ma is recovered, but the AI test (figure 7a) shows the sample is quite altered; the age is rejected. (c) For flow 27, a plateau age of ~48 Ma is obtained. This age appears too young to belong to the Deccan, and the AI test shows it is severely altered (figure 7a); the age is rejected. None of the three whole-rock samples gave accurate estimates of the crystallisation age. In conjunction with the results for the plagioclase separates (figure 8 and table 4), it is apparent that plagioclase within these rocks is less altered than the whole-rock splits.
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Acknowledgements
This paper is dedicated to the memory of my maternal grandmother, Prafullabala Lahiri, who passed away in 1972. A tip of the hat to Felix Sanchez who so honoured his Abuela, with his victory in the London Olympic Games (2012); the late N Krishna Brahmam introduced me to the problems associated with the Rajahmundry traps. Much of the early (late 1980s) efforts on 40Ar/39Ar dating of the Deccan traps was driven by the generosity of the (late) Peter Hooper, who supplied samples and his knowledge of the CWGS to all interested parties. The official reviewers and the editor offered useful comments on an earlier draft of this manuscript. I thank Romila Raman and Nola Yun Mi for helpful discussions. Long-term support from the Basanti Bhattacharyya Foundation is gratefully acknowledged.
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Communicated by N V Chalapathi Rao
This article is part of the Topical Collection: Deccan Traps and other Flood Basalt Provinces – Recent Research Trends.
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Baksi, A.K. Critical assessment of the geochronological data on the Deccan traps, India: Emphasis on the timing and duration of volcanism in sections of tholeiitic basalts. J Earth Syst Sci 131, 135 (2022). https://doi.org/10.1007/s12040-022-01842-z
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DOI: https://doi.org/10.1007/s12040-022-01842-z