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39Ar dating of the Rajahmundry Traps, eastern India and their relationship to the Deccan Traps” by Knight et al. [Earth Planet Sci. Lett. 208 (2003) 85–99]
Earth and Planetary Science Letters 239 (2005) 368 – 373 www.elsevier.com/locate/epsl
Discussion
Comment on b 40Ar / 39Ar dating of the Rajahmundry Traps, eastern India and their relationship to the Deccan TrapsQ by Knight et al. [Earth Planet Sci. Lett. 208 (2003) 85–99] Ajoy K. Baksi Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA Received 24 May 2004; received in revised form 15 February 2005; accepted 11 March 2005 Available online 23 September 2005 Editor: E. Boyle
Abstract Knight et al. presented age and chemical data on two (sets of) lava flows from the Rajahmundry area, on either bank of the Godavari River. The age and petrogenesis of these flows and their possible link to sections of the main Deccan Province are of importance to the understanding of many aspects of flood basalt volcanism. I comment on (a) the use of geochemical fingerprints for lava identification/correlation at Rajahmundry, superceding (apparent) field relations, (b) their 40Ar / 39Ar data and its refinement based on statistical tests and the alteration state of the samples (c) correlation of age data and the magnetic polarity of the lavas to the geomagnetic polarity time scale and (d) the possibility that both lavas at Rajahmundry were formed by intracanyon flows derived from ~1000 km away. D 2005 Elsevier B.V. All rights reserved.
1. Field relations and geochemical fingerprints Earlier work [2,3] suggested lavas on the west bank of the Godavari River show both normal and reverse magnetic polarity, whereas on the east bank only normal polarity lavas are found. On the western bank, lavas (showing opposite magnetic polarities), are separated by a calcareous sedimentary (intertrappean) layer [4 and references therein]. The upper (normal polarity) lava on this bank shows the same chemical composition as that on the eastern bank [5] and this finding is supported by Knight et al. [1]. Their data [1], for immobile elements on samples RA99.06/23 (western bank) match that of RA99.14
E-mail address: [email protected]. 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.03.029
(eastern bank), and the nine samples from the upper lava in an earlier report [4]. This is best seen on a primitive mantle normalized (PMN) plot for trace elements (Fig. 1a). The lower (reversed polarity) flow on the west bank shows distinctly different chemical composition [5]. Knight et al. [1] report considerable chemical variability for this lower flow. Thus, sample RA99.12 shows more depleted composition than the upper flow (Fig. 1b). Geochemically, two samples from the lower flow (RA99.1B/02) closely resemble the upper flow (see Fig. 1b). They comment b(our) six analyses . . .. shows two statistically separate groups of lavas within the Rajahmundry Traps which do not (their italics) reflect the division of Upper Traps and Lower TrapsQ [1, p. 86]. Chemical fingerprints on immobile elements (and magnetic polarity), can clearly distinguish between the two lava
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lower flow. Samples RA99.1B/02, collected from just below the intertrappean sedimentary layer [1, Fig. 1c], were formed in this way, and belong to the upper (younger) flow; chemical fingerprints for these two samples match that for the upper flow (see Fig. 1b). A similar situation is noted for the Grouse Creek interbed for the Wenaha Flow in the Columbia River Basalt [8]. Using immobile element concentrations for samplesRA99.1B/02—lower lava flow of Knight et al. [1]—a search through the GEOROC data base (http://georoc.mpch-mainz.gwdg.de/georoc), reveals very few matches. The most notable are the Rajahmundry upper flow of Baksi [4] and Knight et al. [1]. 2. Constraints on the geochronological data
Fig. 1. Primitive mantle normalized plot for elements on whole-rock basalts from Rajahmundry. Data taken from Knight et al. and Baksi [1,4]. (a) Upper Flow, RA99.06/23/14 of Knight et al. [1] and B01, average of nine analyses of Baksi [4]. Samples show uniform composition for immobile elements. (b) U, the average of three samples (RA99.06/23/14) from the upper flow (see (a) above); RA99.12/1B/ 02, thought to be from the lower flow [1]. RA99.1B/02 show patterns identical to the upper flow, whereas RA99.12 shows a different pattern. Sample RA99.12 (only) belongs to the lower flow; RA99.1B/02 belong to the upper flow (see text).
flows. The field location of sample sites with respect to the sedimentary intertrappean layer, is not an unequivocal guide in separating the older from the younger flow. This unusual assertion arises because of the burrowing action of a subaerial sheet flow when it encounters unconsolidated sediment in shallow water. Distal facies of flood basalt flows are often characterized by complex relations with sedimentary interbeds [6]. The upper flow at Rajahmundry is thought to have formed by an intracanyon flow coming down from the main Deccan province, to be deposited in shallow marine conditions [4,7]. On the western bank, material in the upper flow, worked its way beneath the sedimentary layer at the time of emplacement, and now appears to be part of the
I reexamine the 40Ar / 39Ar data on plagioclase separates [1]—(http://www1.elsevier.com/pub/14/17/ 34/S0012821X02011548/data.pdf). The SEM on weighted average ages was multiplied by (MSWD)1 / 2, when MSWD was N 1 (cf. [9]). Errors in all ages are reported at the 2r level. Firstly, the two samples dated by Baksi [4] showed normal magnetic polarity [10] and therefore belong to the upper flow. They were not btwo whole rock samples taken from above and below the sedimentary interlayerQ [1, p. 88]. The weighted mean age of Baksi [4] for the upper flow was 64.12 F 0.34 Ma; this includes a term for r J = 0.4% introduced quadratically (see J values in Baksi [4, Table 2]). To convert this age to the calibrations preferred by the Berkeley group, it has to increased by 1.0% [11]. The recalculated age of Baksi [4] for the upper (normal polarity) flow relative to FC Sanidine at 28.02 Ma, is 64.76 F 0.34 Ma. This compares with the age of 64.5 F 0.8 Ma on plagioclase separates [1]. Knight et al. [1] noted that the whole-rock age spectra [4], indicate partial loss of 40Ar* and/or recoil of 39Ar. In light of these potential problems with the whole-rock work, the agreement in ages between the two groups of workers is excellent. For the lower (reversed polarity) lava, an average age of 65.0 F 1.1 Ma on plagioclase separates was obtained [1]. Step-heating experiments on two wholerock samples from this flow (Baksi, unpubl. data—see Fig. 2) yielded a weighted plateau age—relative to the Berkeley calibrations—of 65.6 F 1.2 Ma. Prior to attempting detailed comparison of the ages obtained in these studies, the (alteration) state and identification of sampling sites for the study of Knight et al. [1] needs to be critically examined.
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clearly rejected on statistical grounds. Critical examination shows that the plateau steps of RA99.02 show high levels of 36Ar. For reasons outlined below, I prefer to use its isochron age of 65.07 F 0.98 Ma. Based on chemical fingerprints, samples RA99.1B/02 belong to the upper flow (see Fig. 1b). Chemical data for samples RA99.1A/11 were not listed [1]; in their absence, there is no unequivocal proof as to which of the two flows they belong to, and their ages are not considered further. Final ages for the plagioclase separates are as follows: Lower flow—64.70F 0.57 Ma—average of two plateauages on RA99.12. The MSWD value is 0.08, and the probability of occurrence is 0.78. Upper flow—64.57 F 0.19 Ma—average of plateau ages on RA99.1B/06/14/23, and the isochron age on RA99.02. The MSWD value is 1.58, and the probability of occurrence is 0.18.
Fig. 2. Age spectra for 40Ar / 39Ar incremental heating analyses on four low K (~0.07%) whole-rock samples from the lower (reversed polarity) flow at Rajahmundry (Baksi, unpubl. data); errors shown at the 1r level. Plateau ages (in Ma) and 2r errors are (% 39Ar in parentheses): 65.8 F 2.4 (100.0), 66.7 F 2.8 (100.0), 64.3 F 2.4 (100.0), 65.8 F 2.2 (75.4). The weighted average age is 65.6 F 1.2 Ma (2r error), MSWD = 0.62.
2.1. Upper flow The following plateau ages (in Ma) were obtained on plagioclase separates [1]: 64.6 F 1.9 and 63.5 F 1.1 (RA99.06), 64.26 F 0.30 (RA99.14) and 64.66 F 0.18 (RA99.23). It is critical to note that the correct error estimates for RA99.14/23 are lower than the values of F 0.4 my listed by Knight et al. [1]. The weighted mean age (one age per sample) is 64.54 F 0.30 Ma. The corresponding MSWD value is 3.91, and for two degrees of freedom, the probability of occurrence from Chi Square Tables is ~0.02. This leads to rejection of this average age at the 95% confidence level. I suggest the age of 63.5 F 1.1 Ma on RA99.06 should be rejected as the plateau steps show high amounts of 36 Ar, reflecting alteration (see below). The three remaining ages on the upper flow average 64.55 F 0.25 Ma, with an MSWD value of 2.62, and a probability of occurrence of ~0.07. This average age is statistically (more) acceptable. 2.2. Lower flow For this flow, the following plateau ages (in Ma) were reported for plagioclase separates [1]: 66.0 F 2.9 (RA99.1A), 64.58 F 1.38 (RA99.1B—weighted average of three experiments), 65.80 F 0.50 (RA99.02), 63.70 F 0.80 (RA99.11) and 64.70 F 0.57 (RA99.12— weighted average of two determinations). The weighted average of these five measurements is 65.02 F 0.78 Ma; the corresponding MSWD value is 5.69, probability of occurrence ~0.0001. This average age is
Whole-rock age measurements (see above) are 64.74 F 0.34 (upper flow) and 65.6 F 1.2 Ma (lower flow). For both the upper and lower flows, the agreement between the ages obtained on different material, at the two laboratories, is good. 3.
36
Ar content—an indicator of alteration state
This parameter has been used both in K–Ar dating and 40Ar / 39Ar studies, to recognize altered samples [9,11–13]. In 40Ar / 39Ar dating experiments, the component of 36Ar derived from the reaction 40Ca (n, na), must be removed from the total amount of 36Ar measured mass spectrometrically, as it is generated by neutron irradiation in the reactor. The plateau steps of the first (125–425 Am) and second (63–125 Am) runs on RA99.06 display ~2–5 and ~20–35 10 18 moles of 36Ar, respectively. The second run displays almost an order of magnitude more 36Ar in each step; plateau steps contain argon drawn from altered sites within the plagioclase that had suffered minor loss of 40Ar*. Sample RA99.06’s (63–125 Am) age should be rejected, in spite of the fact that it yields a statistically acceptable plateau age [cf. 13]. Plateau steps on the other plagioclase samples display 2–5 10 18 moles of 36 Ar, with one other exception; sample RA99.02, shows ~70–150 10 18 moles per plateau step. Sample RA99.02 appears altered and its age should be lower than the crystallization value, ~ 64.7 Ma. However, its plateau age is measurably older at 65.8 F 0.5 Ma suggesting the atmospheric argon correction for this sample should be carried out using (40Ar / 36Ar)initial N
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4. Comparison to the geomagnetic polarity time scale (GPTS)
Fig. 3. The age and magnetic polarity of the two lavas at Rajahmundry plotted against the geomagnetic polarity time scale [15] at ~65 Ma. The K–T boundary is placed at 65. 58 Ma [1], and chrons are numbered. The age range in each case is at the F 2r level. Ages on plagioclase separates [1], as revised herein, shown as K04; ages on whole-rock basalts [4, Baksi, unpubl. data] shown as B01 and B04, respectively. Based on the whole-rock ages (only), the polarity transition (R to N) in the Rajahmundry Traps, clearly spans C29 r–C29 n (see text).
295.5. This is reflected in the recalculated isochron age (65.07 F 0.98 Ma) and (40Ar / 36Ar)initial = 296.8 F 1.4, MSWD = 0.64. (I note that there are some errors in the step ages and errors reported by Knight et al. [1] for sample RA99.02. My ages and errors were calculated using the formulae of Dalrymple et al. [14]).
I utilize the GPTS of Cande and Kent [15], placing the K–T boundary at 65.58 Ma [1], to examine the ages and magnetic polarity of the two sets of lava flows (see Fig. 3). It has been suggested [2] the polarity change seen in the lavas at Rajahmundry spans the C29 r-C29 n boundary. The (revised) age of the plagioclase separates [1] and whole-rock material [4] on the upper (normal polarity) flow, place it unequivocally within chron 29 n. The (revised) plagioclase separate age on the lower (reversed polarity) flow, suggests it may belong to chron 28 r rather than 29 r; this is at odds with its stratigraphic location, which indicates it is older than the upper flow. However, the low precision whole-rock age on the lower flow suggests it was formed during chron 29 r, in keeping with its stratigraphic position with respect to the upper flow. 5. Intracanyon flows seen at Rajahmundry? Earlier [4,7] it was suggested that the upper (normal) polarity lava at Rajahmundry was derived from a source near Kolhapur in the western Deccan (see Fig. 4). The
Fig. 4. Map of a section of Peninsular India, showing the drainage patterns and the location of the Rajahmundry (RT), Kolhapur (K) and Bavda (B) sections (modified from Baksi [4]). Dotted lines denote the outer limits of the Deccan Traps. Lava flows at Rajahmundry were formed by intracanyon flows from the western Deccan; the lower one from a source at Bavda, the upper one from a source at Kolhapur (see text and Fig. 5).
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comparison of chemical data from these two areas was problematical making use of analyses carried out by XRF [16] and ICP-MS [4]. A specimen of sample BAS235 from a flow in the Kolhapur area was obtained and analyzed by ICP-MS. The direct comparison of REE and trace element data (bapples to applesQ) is shown in Fig. 5. The upper flow at Rajahmundry has an age of ~64.6 Ma; BAS235 (Mahabaleshwar Formation—[17]), was collected from an elevation of 823 m, above the level (~600 m) where the reversed to normal polarity transition (C29 r-C29 n) occurs [2,16,17]. The lava from which BAS235 was collected belongs to chron 29 n. BAS235 and the normal polarity lava flow at Rajahmundry appear to be equivalent in age and chemistry; they can be (genetically) linked, if the latter was formed by an intracanyon flow [4,7]. The petrogenesis of the reversed polarity flow at Rajahmundry was sought in a similar manner. Scrutiny of all available published and unpublished chemical data
on Deccan basalts, led to examination of a flow near Bavda (see Fig. 4), for which only a limited amount of unpublished chemical data was available. A specimen of this sample (BAS365—Ambenali Formation [17]) was obtained and analyzed by ICP-MS; the results are presented in Fig. 5, and compared to those of sample RA99.12—lower flow at Rajahmundry [1]. The match of patterns is striking. Specimen RA99.12 was taken from a reversed polarity lava flow and formed during chron 29r (see Fig. 3). Sample BAS365 was collected from a flow in the Bavda section at an elevation of 113 m [C.J. Hawkesworth, pers. comm, 2000], below the ~400 m level where the reverse to normal polarity transition (C29 r-C29 n) occurs in this area [2]. The lava from which BAS365 was collected, belongs to chron C29 r. bEquivalenceQ of age and chemistry suggests the lower lava flow at Rajahmundry was (also) formed by an intracanyon flow, derived from ~1000 km away (Fig. 4). The possible role of overland flow of Deccan Trap lavas towards deposition on the east coast of India (as the Rajahmundry Traps) was considered earlier [1]. These authors quoted the work of Halkett (Ph.D. thesis, 2002) arguing for the existence of SE draining (paleo Godavari and Krishna River?) basins before the eruption of the Deccan Traps. They also noted the general lack of evidence of individual flows ~100 km long in peninsular India [1]. The existence of remnant volcanism within the Godavari and Krishna River channels [1]—clear proof of intracanyon flows at ~65 Ma—can only be documented by careful field work. A promising start in this direction is the work of Misra [18]; lava channels (unroofed rivers of lava that frequently develop surface crusts [19]) more than 300 m wide, are found in many areas of the Deccan Traps. These dimensions suggest a very high effusion rate, and allied to very low viscosity, permitted individual lavas to flow long distances. I conclude by suggesting that all lavas found in the Rajahmundry area (both onshore and offshore), may have resulted from intracanyon flows derived from the main Deccan province. Acknowledgments
Fig. 5. (a) Chondrite normalized REE plot and (b) primitive mantle normalized plot for selected elements on whole-rock basalts. B01 = average of Rajahmundry upper flow [4], K = BAS235 from the Kolhapur Unit, RA99.12 = lower flow at Rajahmundry [1], B = BAS365 from the Bavda Unit. Close match of patterns on both plots indicate formation of the upper and lower flows at Rajahmundry by intracanyon flows derived from the Kolhapur and Bavda Units, respectively (see text).
I thank K.V. Subbarao for supplying samples of the lower lava flow (western bank) at Rajahmundry for dating work and Mike Widdowson for locating and providing splits of BAS235 and BAS365 from the Deccan Traps. This study was considerably aided by access to the wealth of (un)published geochemical data on the Deccan Traps provided by Chris Hawkesworth, and the unpublished Ph.D. thesis made available by Peter Lightfoot. Bob Fleck and an anonymous reviewer
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made numerous helpful comments during formal review of this manuscript. References [1] K.B. Knight, P.R. Renne, A. Halkett, N. White, 40Ar / 39Ar dating of the Rajahmundry Traps, eastern India and their relationship to the Deccan Traps, Earth Planet. Sci. Lett. 208 (2003) 85 – 99. [2] D. Vandamme, V. Courtillot, Paleomagnetic constraints on the structure of the Deccan Traps, Phys. Earth Planet. Inter. 74 (1992) 241 – 261. [3] K.V. Subbarao, S. Pathak, Reversely magnetized flows, Rajahmundry, Andhra Pradesh, J. Geol. Soc. India 41 (1993) 71 – 72. [4] A.K. Baksi, The Rajahmundry Traps, Andhra Pradesh: evaluation of their petrogenesis relative to the Deccan Traps, Proc. Indian Acad. Sci. 110 (2001) 397 – 407. [5] K.V. Subbarao, J.N. Walsh, A.M. Dayal, J. Zachariah, K. Gopalan, Enriched mantle at Rajahmundry, east coast of India [abstr.], Proc. Conf. Isotopes in the Solar System, PRL, Ahmedabad, India, vol. 1, 1997, p. 23. [6] G.R. Byerly, D.A. Swanson, Invasive Columbia River basalt flows along the northwestern margin of the Columbia River plateau, north-central Washington, Abstr. Programs - Geol. Soc. Am. 10 (1978) 10. [7] A.K. Baksi, G.R. Byerly, L.-H. Chan, E. Farrar, Intracanyon flows in the Deccan province, India? Case history of the Rajahmundry Traps, Geology 22 (1994) 605 – 608. [8] M.E. Ross, Stratigraphic relationships of subaerial, invasive and intracanyon flows of Saddle Mountain Basalt in the Troy Basin, Oregon and Washington, Spec. Pap. - Geol. Soc. Am. 239 (1989) 131 – 142. [9] J.C. Roddick, The application of isochron diagrams in 40 Ar–39Ar dating: a discussion, Earth Planet. Sci. Lett. 41 (1978) 233 – 244.
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[10] J. Singh, M.S. Bhalla, Preliminary paleomagnetic studies on igneous rocks of U.P., Andhra Pradesh and Mysore, Curr. Sci. 41 (1972) 92 – 94. [11] A.K. Baksi, Critical evaluation of the age of the Deccan Traps, India: implications for flood-basalt volcanism and faunal extinctions, Geology 15 (1987) 147 – 150. [12] A.K. Baksi, Reevaluation of the timing and duration of extrusion of the Imnaha, Picture Gorge and Grande Ronde basalts, Columbia River Group, Spec. Pap. - Geol. Soc. Am. 239 (1989) 105 – 111. [13] A.K. Baksi, Critical evaluation of 40Ar / 39Ar ages for the central Atlantic magmatic province: timing, duration and possible migration of eruption centers, Amer. Geophys. Union Monogr. 136 (2003) 77 – 90. [14] G.B. Dalrymple, E.C. Alexander, M.A. Lanphere, G.P. Kraker, Irradiation of samples for 40Ar / 39Ar dating using the Geological Survey Triga Reactor, U. S. Geol. Surv. Prof. Pap. 1176 (1981) (55 pp.). [15] S.C. Cande, D.V. Kent, A new geomagnetic polarity time scale, J. Geophys. Res. 97 (1992) 13917 – 13951. [16] P.C. Lightfoot, C.J. Hawkesworth, C.W. Devey, N.W. Rogers, P.W.C. van Calsteren, Source and differentiation of Deccan Trap lavas; implications of geochemical and mineral chemical variations, J. Petrol. 31 (1990) 1165 – 1200. [17] P.C. Lightfoot, Isotope and trace element geochemistry of the south Deccan lavas, India, Unpubl. Ph.D. thesis, The Open University, Milton Keyes, UK (1985) 589 pp. [18] K.S. Misra, Arterial system of lava tubes and channels within Deccan volcanics of western India, J. Geol. Soc. India 59 (2002) 115 – 124. [19] R. Greeley, The role of lava tubes in Hawaiian volcanoes, U. S. Geol. Surv. Prof. Pap. 1350 (1987) 1589 – 1602.
Discussion
Comment on b 40Ar / 39Ar dating of the Rajahmundry Traps, eastern India and their relationship to the Deccan TrapsQ by Knight et al. [Earth Planet Sci. Lett. 208 (2003) 85–99] Ajoy K. Baksi Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA Received 24 May 2004; received in revised form 15 February 2005; accepted 11 March 2005 Available online 23 September 2005 Editor: E. Boyle
Abstract Knight et al. presented age and chemical data on two (sets of) lava flows from the Rajahmundry area, on either bank of the Godavari River. The age and petrogenesis of these flows and their possible link to sections of the main Deccan Province are of importance to the understanding of many aspects of flood basalt volcanism. I comment on (a) the use of geochemical fingerprints for lava identification/correlation at Rajahmundry, superceding (apparent) field relations, (b) their 40Ar / 39Ar data and its refinement based on statistical tests and the alteration state of the samples (c) correlation of age data and the magnetic polarity of the lavas to the geomagnetic polarity time scale and (d) the possibility that both lavas at Rajahmundry were formed by intracanyon flows derived from ~1000 km away. D 2005 Elsevier B.V. All rights reserved.
1. Field relations and geochemical fingerprints Earlier work [2,3] suggested lavas on the west bank of the Godavari River show both normal and reverse magnetic polarity, whereas on the east bank only normal polarity lavas are found. On the western bank, lavas (showing opposite magnetic polarities), are separated by a calcareous sedimentary (intertrappean) layer [4 and references therein]. The upper (normal polarity) lava on this bank shows the same chemical composition as that on the eastern bank [5] and this finding is supported by Knight et al. [1]. Their data [1], for immobile elements on samples RA99.06/23 (western bank) match that of RA99.14
E-mail address: [email protected]. 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.03.029
(eastern bank), and the nine samples from the upper lava in an earlier report [4]. This is best seen on a primitive mantle normalized (PMN) plot for trace elements (Fig. 1a). The lower (reversed polarity) flow on the west bank shows distinctly different chemical composition [5]. Knight et al. [1] report considerable chemical variability for this lower flow. Thus, sample RA99.12 shows more depleted composition than the upper flow (Fig. 1b). Geochemically, two samples from the lower flow (RA99.1B/02) closely resemble the upper flow (see Fig. 1b). They comment b(our) six analyses . . .. shows two statistically separate groups of lavas within the Rajahmundry Traps which do not (their italics) reflect the division of Upper Traps and Lower TrapsQ [1, p. 86]. Chemical fingerprints on immobile elements (and magnetic polarity), can clearly distinguish between the two lava
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lower flow. Samples RA99.1B/02, collected from just below the intertrappean sedimentary layer [1, Fig. 1c], were formed in this way, and belong to the upper (younger) flow; chemical fingerprints for these two samples match that for the upper flow (see Fig. 1b). A similar situation is noted for the Grouse Creek interbed for the Wenaha Flow in the Columbia River Basalt [8]. Using immobile element concentrations for samplesRA99.1B/02—lower lava flow of Knight et al. [1]—a search through the GEOROC data base (http://georoc.mpch-mainz.gwdg.de/georoc), reveals very few matches. The most notable are the Rajahmundry upper flow of Baksi [4] and Knight et al. [1]. 2. Constraints on the geochronological data
Fig. 1. Primitive mantle normalized plot for elements on whole-rock basalts from Rajahmundry. Data taken from Knight et al. and Baksi [1,4]. (a) Upper Flow, RA99.06/23/14 of Knight et al. [1] and B01, average of nine analyses of Baksi [4]. Samples show uniform composition for immobile elements. (b) U, the average of three samples (RA99.06/23/14) from the upper flow (see (a) above); RA99.12/1B/ 02, thought to be from the lower flow [1]. RA99.1B/02 show patterns identical to the upper flow, whereas RA99.12 shows a different pattern. Sample RA99.12 (only) belongs to the lower flow; RA99.1B/02 belong to the upper flow (see text).
flows. The field location of sample sites with respect to the sedimentary intertrappean layer, is not an unequivocal guide in separating the older from the younger flow. This unusual assertion arises because of the burrowing action of a subaerial sheet flow when it encounters unconsolidated sediment in shallow water. Distal facies of flood basalt flows are often characterized by complex relations with sedimentary interbeds [6]. The upper flow at Rajahmundry is thought to have formed by an intracanyon flow coming down from the main Deccan province, to be deposited in shallow marine conditions [4,7]. On the western bank, material in the upper flow, worked its way beneath the sedimentary layer at the time of emplacement, and now appears to be part of the
I reexamine the 40Ar / 39Ar data on plagioclase separates [1]—(http://www1.elsevier.com/pub/14/17/ 34/S0012821X02011548/data.pdf). The SEM on weighted average ages was multiplied by (MSWD)1 / 2, when MSWD was N 1 (cf. [9]). Errors in all ages are reported at the 2r level. Firstly, the two samples dated by Baksi [4] showed normal magnetic polarity [10] and therefore belong to the upper flow. They were not btwo whole rock samples taken from above and below the sedimentary interlayerQ [1, p. 88]. The weighted mean age of Baksi [4] for the upper flow was 64.12 F 0.34 Ma; this includes a term for r J = 0.4% introduced quadratically (see J values in Baksi [4, Table 2]). To convert this age to the calibrations preferred by the Berkeley group, it has to increased by 1.0% [11]. The recalculated age of Baksi [4] for the upper (normal polarity) flow relative to FC Sanidine at 28.02 Ma, is 64.76 F 0.34 Ma. This compares with the age of 64.5 F 0.8 Ma on plagioclase separates [1]. Knight et al. [1] noted that the whole-rock age spectra [4], indicate partial loss of 40Ar* and/or recoil of 39Ar. In light of these potential problems with the whole-rock work, the agreement in ages between the two groups of workers is excellent. For the lower (reversed polarity) lava, an average age of 65.0 F 1.1 Ma on plagioclase separates was obtained [1]. Step-heating experiments on two wholerock samples from this flow (Baksi, unpubl. data—see Fig. 2) yielded a weighted plateau age—relative to the Berkeley calibrations—of 65.6 F 1.2 Ma. Prior to attempting detailed comparison of the ages obtained in these studies, the (alteration) state and identification of sampling sites for the study of Knight et al. [1] needs to be critically examined.
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clearly rejected on statistical grounds. Critical examination shows that the plateau steps of RA99.02 show high levels of 36Ar. For reasons outlined below, I prefer to use its isochron age of 65.07 F 0.98 Ma. Based on chemical fingerprints, samples RA99.1B/02 belong to the upper flow (see Fig. 1b). Chemical data for samples RA99.1A/11 were not listed [1]; in their absence, there is no unequivocal proof as to which of the two flows they belong to, and their ages are not considered further. Final ages for the plagioclase separates are as follows: Lower flow—64.70F 0.57 Ma—average of two plateauages on RA99.12. The MSWD value is 0.08, and the probability of occurrence is 0.78. Upper flow—64.57 F 0.19 Ma—average of plateau ages on RA99.1B/06/14/23, and the isochron age on RA99.02. The MSWD value is 1.58, and the probability of occurrence is 0.18.
Fig. 2. Age spectra for 40Ar / 39Ar incremental heating analyses on four low K (~0.07%) whole-rock samples from the lower (reversed polarity) flow at Rajahmundry (Baksi, unpubl. data); errors shown at the 1r level. Plateau ages (in Ma) and 2r errors are (% 39Ar in parentheses): 65.8 F 2.4 (100.0), 66.7 F 2.8 (100.0), 64.3 F 2.4 (100.0), 65.8 F 2.2 (75.4). The weighted average age is 65.6 F 1.2 Ma (2r error), MSWD = 0.62.
2.1. Upper flow The following plateau ages (in Ma) were obtained on plagioclase separates [1]: 64.6 F 1.9 and 63.5 F 1.1 (RA99.06), 64.26 F 0.30 (RA99.14) and 64.66 F 0.18 (RA99.23). It is critical to note that the correct error estimates for RA99.14/23 are lower than the values of F 0.4 my listed by Knight et al. [1]. The weighted mean age (one age per sample) is 64.54 F 0.30 Ma. The corresponding MSWD value is 3.91, and for two degrees of freedom, the probability of occurrence from Chi Square Tables is ~0.02. This leads to rejection of this average age at the 95% confidence level. I suggest the age of 63.5 F 1.1 Ma on RA99.06 should be rejected as the plateau steps show high amounts of 36 Ar, reflecting alteration (see below). The three remaining ages on the upper flow average 64.55 F 0.25 Ma, with an MSWD value of 2.62, and a probability of occurrence of ~0.07. This average age is statistically (more) acceptable. 2.2. Lower flow For this flow, the following plateau ages (in Ma) were reported for plagioclase separates [1]: 66.0 F 2.9 (RA99.1A), 64.58 F 1.38 (RA99.1B—weighted average of three experiments), 65.80 F 0.50 (RA99.02), 63.70 F 0.80 (RA99.11) and 64.70 F 0.57 (RA99.12— weighted average of two determinations). The weighted average of these five measurements is 65.02 F 0.78 Ma; the corresponding MSWD value is 5.69, probability of occurrence ~0.0001. This average age is
Whole-rock age measurements (see above) are 64.74 F 0.34 (upper flow) and 65.6 F 1.2 Ma (lower flow). For both the upper and lower flows, the agreement between the ages obtained on different material, at the two laboratories, is good. 3.
36
Ar content—an indicator of alteration state
This parameter has been used both in K–Ar dating and 40Ar / 39Ar studies, to recognize altered samples [9,11–13]. In 40Ar / 39Ar dating experiments, the component of 36Ar derived from the reaction 40Ca (n, na), must be removed from the total amount of 36Ar measured mass spectrometrically, as it is generated by neutron irradiation in the reactor. The plateau steps of the first (125–425 Am) and second (63–125 Am) runs on RA99.06 display ~2–5 and ~20–35 10 18 moles of 36Ar, respectively. The second run displays almost an order of magnitude more 36Ar in each step; plateau steps contain argon drawn from altered sites within the plagioclase that had suffered minor loss of 40Ar*. Sample RA99.06’s (63–125 Am) age should be rejected, in spite of the fact that it yields a statistically acceptable plateau age [cf. 13]. Plateau steps on the other plagioclase samples display 2–5 10 18 moles of 36 Ar, with one other exception; sample RA99.02, shows ~70–150 10 18 moles per plateau step. Sample RA99.02 appears altered and its age should be lower than the crystallization value, ~ 64.7 Ma. However, its plateau age is measurably older at 65.8 F 0.5 Ma suggesting the atmospheric argon correction for this sample should be carried out using (40Ar / 36Ar)initial N
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4. Comparison to the geomagnetic polarity time scale (GPTS)
Fig. 3. The age and magnetic polarity of the two lavas at Rajahmundry plotted against the geomagnetic polarity time scale [15] at ~65 Ma. The K–T boundary is placed at 65. 58 Ma [1], and chrons are numbered. The age range in each case is at the F 2r level. Ages on plagioclase separates [1], as revised herein, shown as K04; ages on whole-rock basalts [4, Baksi, unpubl. data] shown as B01 and B04, respectively. Based on the whole-rock ages (only), the polarity transition (R to N) in the Rajahmundry Traps, clearly spans C29 r–C29 n (see text).
295.5. This is reflected in the recalculated isochron age (65.07 F 0.98 Ma) and (40Ar / 36Ar)initial = 296.8 F 1.4, MSWD = 0.64. (I note that there are some errors in the step ages and errors reported by Knight et al. [1] for sample RA99.02. My ages and errors were calculated using the formulae of Dalrymple et al. [14]).
I utilize the GPTS of Cande and Kent [15], placing the K–T boundary at 65.58 Ma [1], to examine the ages and magnetic polarity of the two sets of lava flows (see Fig. 3). It has been suggested [2] the polarity change seen in the lavas at Rajahmundry spans the C29 r-C29 n boundary. The (revised) age of the plagioclase separates [1] and whole-rock material [4] on the upper (normal polarity) flow, place it unequivocally within chron 29 n. The (revised) plagioclase separate age on the lower (reversed polarity) flow, suggests it may belong to chron 28 r rather than 29 r; this is at odds with its stratigraphic location, which indicates it is older than the upper flow. However, the low precision whole-rock age on the lower flow suggests it was formed during chron 29 r, in keeping with its stratigraphic position with respect to the upper flow. 5. Intracanyon flows seen at Rajahmundry? Earlier [4,7] it was suggested that the upper (normal) polarity lava at Rajahmundry was derived from a source near Kolhapur in the western Deccan (see Fig. 4). The
Fig. 4. Map of a section of Peninsular India, showing the drainage patterns and the location of the Rajahmundry (RT), Kolhapur (K) and Bavda (B) sections (modified from Baksi [4]). Dotted lines denote the outer limits of the Deccan Traps. Lava flows at Rajahmundry were formed by intracanyon flows from the western Deccan; the lower one from a source at Bavda, the upper one from a source at Kolhapur (see text and Fig. 5).
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comparison of chemical data from these two areas was problematical making use of analyses carried out by XRF [16] and ICP-MS [4]. A specimen of sample BAS235 from a flow in the Kolhapur area was obtained and analyzed by ICP-MS. The direct comparison of REE and trace element data (bapples to applesQ) is shown in Fig. 5. The upper flow at Rajahmundry has an age of ~64.6 Ma; BAS235 (Mahabaleshwar Formation—[17]), was collected from an elevation of 823 m, above the level (~600 m) where the reversed to normal polarity transition (C29 r-C29 n) occurs [2,16,17]. The lava from which BAS235 was collected belongs to chron 29 n. BAS235 and the normal polarity lava flow at Rajahmundry appear to be equivalent in age and chemistry; they can be (genetically) linked, if the latter was formed by an intracanyon flow [4,7]. The petrogenesis of the reversed polarity flow at Rajahmundry was sought in a similar manner. Scrutiny of all available published and unpublished chemical data
on Deccan basalts, led to examination of a flow near Bavda (see Fig. 4), for which only a limited amount of unpublished chemical data was available. A specimen of this sample (BAS365—Ambenali Formation [17]) was obtained and analyzed by ICP-MS; the results are presented in Fig. 5, and compared to those of sample RA99.12—lower flow at Rajahmundry [1]. The match of patterns is striking. Specimen RA99.12 was taken from a reversed polarity lava flow and formed during chron 29r (see Fig. 3). Sample BAS365 was collected from a flow in the Bavda section at an elevation of 113 m [C.J. Hawkesworth, pers. comm, 2000], below the ~400 m level where the reverse to normal polarity transition (C29 r-C29 n) occurs in this area [2]. The lava from which BAS365 was collected, belongs to chron C29 r. bEquivalenceQ of age and chemistry suggests the lower lava flow at Rajahmundry was (also) formed by an intracanyon flow, derived from ~1000 km away (Fig. 4). The possible role of overland flow of Deccan Trap lavas towards deposition on the east coast of India (as the Rajahmundry Traps) was considered earlier [1]. These authors quoted the work of Halkett (Ph.D. thesis, 2002) arguing for the existence of SE draining (paleo Godavari and Krishna River?) basins before the eruption of the Deccan Traps. They also noted the general lack of evidence of individual flows ~100 km long in peninsular India [1]. The existence of remnant volcanism within the Godavari and Krishna River channels [1]—clear proof of intracanyon flows at ~65 Ma—can only be documented by careful field work. A promising start in this direction is the work of Misra [18]; lava channels (unroofed rivers of lava that frequently develop surface crusts [19]) more than 300 m wide, are found in many areas of the Deccan Traps. These dimensions suggest a very high effusion rate, and allied to very low viscosity, permitted individual lavas to flow long distances. I conclude by suggesting that all lavas found in the Rajahmundry area (both onshore and offshore), may have resulted from intracanyon flows derived from the main Deccan province. Acknowledgments
Fig. 5. (a) Chondrite normalized REE plot and (b) primitive mantle normalized plot for selected elements on whole-rock basalts. B01 = average of Rajahmundry upper flow [4], K = BAS235 from the Kolhapur Unit, RA99.12 = lower flow at Rajahmundry [1], B = BAS365 from the Bavda Unit. Close match of patterns on both plots indicate formation of the upper and lower flows at Rajahmundry by intracanyon flows derived from the Kolhapur and Bavda Units, respectively (see text).
I thank K.V. Subbarao for supplying samples of the lower lava flow (western bank) at Rajahmundry for dating work and Mike Widdowson for locating and providing splits of BAS235 and BAS365 from the Deccan Traps. This study was considerably aided by access to the wealth of (un)published geochemical data on the Deccan Traps provided by Chris Hawkesworth, and the unpublished Ph.D. thesis made available by Peter Lightfoot. Bob Fleck and an anonymous reviewer
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