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Comments on: ‘Petrogenesis of Proterozoic Lamproites and Kimberlites from the Cuddapah Basin and Dharwar Craton, Southern India’
NATIONAL GEOPHYSICAL RESEARCH INSTITUTE, UPPAL ROAD, HYDERABAD 500 007, INDIA
Kimberlites intrude the late Archaean eastern Dharwar Craton in two spatially separate fields (
150 km apart, near Mahbubnagar in the north and Anantapur in the south) to the west of the Proterozoic Cuddapah Basin in southern India. Two lamproite bodies intrude the thick Cuddapah Basin sediments near Chelima and Zangamarajupalle, and a third occurs just outside the present NE margin of the basin near Ramannapeta. Chalapathi Rao et al. (2004)
have studied this spatially, and possibly temporally, close association of kimberlites and lamproites to provide insights into their genesis and genetic relationship. Although the main emphasis of their paper is the petrochemical characterization of these two suites of rocks, they also report Sr–Nd isotope data for a small subset of samples, which they use to provide constraints on the nature of the mantle source regions.
Of some 65 samples analysed for their whole-rock composition and mineral chemistry, only eight are from the three lamproite bodies. Possibly as a result of their intrusion into the thick Cuddapah Basin sediments, the Chelima and Zangamarajupalle lamproites have high LOI (8–16%) and high values of the contamination index (2·4–3·5), and contain extensive secondary carbonate. This clearly compromises diagnostic petrochemical characterization of the lamproites based on the data reported in this paper. Until fresh lamproite samples can be analysed (conceivably from the new lamproite bodies discovered very recently in this area), any petrogenetic constraints or modelling based on the very limited data presented by Chalapathi Rao et al. (2004) can only be regarded as indicative, not definitive.
Our comments on their work are, however, primarily directed towards the Nd–Sr isotope data and the interpretations thereof in terms of distinct mantle sources for the kimberlites and lamproites. We note that the Rb–Sr and Sm–Nd data reported in table 5 of Chalapathi Rao et al. (2004) are exactly the same as those reported by the same researchers 6 years previously (Chalapathi Rao et al., 1998a) and are not new data as apparently implied. The key issue is the appropriate age correction of these data, aspects of which have been previously debated by Gopalan et al. (1999) and Chalapathi Rao et al. (1998b, 1999b). Chalapathi Rao et al. (2004) provide a brief summary of the available age constraints (including references to our own studies—Anil Kumar et al., 1993, 2001) for the emplacement age of the lamproites and kimberlites (p. 913). We disagree strongly with their choice of ages (1400 Ma and 1090 Ma, respectively, for the Mahbubnagar and Anantapur kimberlites and 1418 Ma for the Cuddapah Basin lamproites) used to correct the measured Nd–Sr isotope compositions to initial ratios.
Conventional K–Ar ages of Precambrian rocks, even those based on multiple sample analyses, are not reliable for precise age correction of measured Sr and Nd isotope ratios. Chalapathi Rao et al. (1996), nevertheless, relied on preliminary K–Ar ages for just one sample from each of the two kimberlite fields [Kotakonda (Mahbubnagar) and Mulgiripalli (Anantapur)] and two lamproites (Chelima and Ramannapeta) to claim that the kimberlites in the Mahbubnagar cluster and all the three lamproite bodies (including the undated Zangamarajupalle lamproite) were emplaced contemporaneously at 1400 Ma—more than 300 Myr earlier than the Anantapur kimberlites [previously dated based on more reliable Rb–Sr phlogopite ages close to 1100 Ma by Anil Kumar et al. (1993)]. Chalapathi Rao et al. (1999a) subsequently reported 40Ar/39Ar plateau ages for groundmass phlogopites separates from the Kotakonda kimberlite and Chelima lamproite of 1401 ± 5 Ma and 1418 ± 8 Ma, respectively. However, because of alteration of the groundmass phlogopite by extensive secondary carbonation, as seen in the acid leaching studies of the Chelima sample (Chalapathi Rao et al., 1998a, 2004), both its K–Ar and Ar–Ar ages are likely to be suspect, despite their similarity. Hegner et al. (1995) have discussed the problems with Ar–Ar dating of kimberlites; the reader is referred to that work for further discussion.
To verify Chalapathi Rao et al.'s (1996, 1999a) contention of contemporaneity of the Mahbubnagar kimberlites and all three lamproite bodies, but not the Anantapur kimberlites, Anil Kumar et al. (2001) determined Rb–Sr isochron ages [following the approach of Brown et al. (1989)] for mildly acid leached phlogopite separates from two Mahbubnagar kimberlites (Kotakonda and Mudalbid) and all three lamproites. Their results indicated that the two kimberlite groups are contemporaneous at 1100 Ma, but not with the lamproites from Ramannapeta (1224 ± 14 Ma) or Chelima (1354 ± 17 Ma). Only the Zangamarajupalle lamproite gave a similar age to the kimberlites (1070 ± 22 Ma). The Chelima and Zangamarajupalle ages are, however, only provisional because of the possible presence of residual secondary carbonate with anomalously high 87Sr/86Sr in the acid leached phlogopite grains. In the light of these Rb–Sr isochron ages, particularly for the two kimberlites from the Mahbubnagar field, we argue that there is no firm evidence that the Kotakonda kimberlite is contemporaneous with the Chelima lamproite but not with the Ananthapur kimberlites.
Calculation of initial Nd–Sr isotope ratios, particularly for partially altered rocks, is preferably done on physically separated pristine mineral phases (with low parent/daughter ratios). The analytical procedures followed by Chalapathi Rao et al. (2004) are, therefore, subject to large and/or unconstrained errors in each of the following steps.
- Analysis of the composite residue after strong (6N) and warm HCl leaching of whole-rock samples. The real danger of this leaching procedure is differential leaching of Rb and Sr, even from acid-resistant phases—an extreme case being the total loss of Rb from the Chelima and Zangamarajupalle samples.
- The Sm/Nd ratio in the residue is not actually measured but is assumed to be the same as in the corresponding unleached whole-rock sample [determined by inductively coupled plasma mass spectrometry (ICP-MS)]. It is extremely unlikely that the main rare earth element host phases—perovskite, HCl soluble apatite and extraneous carbonate (abundant in lamproites)—will have indistinguishable Sm/Nd ratios.
- The poor precision of about ±10% for both Sm and Nd in the ICP-MS analyses of the whole rocks. This error (±15% in Sm/Nd ratio) alone will translate into an error of as large as ±2 epsilon units in the calculated initial Nd isotope ratios—which is not explicitly quoted in table 5 of Chalapathi Rao et al. (2004). Propagated errors in calculated initial Sr and Nd isotope ratios based on the above uncertainties, and also including the uncertainties in the age of emplacement, could be high.
We also note that the 
Sr values reported in table 5 and fig. 12 of Chalapathi Rao et al. (2004) are all too low by a factor of 10 due to a calculation error—for example, Sr for Chelima C1-C is not 51·5 but 515 and for Ramannapeta not 3·2 but 33·3. In their earlier paper (Chalapathi Rao et al., 1998a), they correctly plotted the same dataset in terms of initial 87Sr/86Sr ratio rather than Sr. If the correct values are plotted in fig. 12, the three lamproites are widely separated, and their respective positions difficult to reconcile with the relatively restricted and distinct fields of the well-documented kimberlite/lamproite provinces shown.
Chalapathi Rao et al. have invoked two isotopically distinct sources to explain the apparent difference in the Nd values of the two kimberlite groups. We suggest that this difference may be an artefact of assuming different ages for the two kimberlite clusters.
Based on the apparently uniform negative Nd (–7·0) values for the lamproites, Chalapathi Rao et al. (2004) argue that their mantle source was enriched in incompatible elements long before the generation of the lamproite melts. Close scrutiny of this limited dataset, however, reveals significant weaknesses in this claim. First, and most importantly, it is premature to infer mantle signatures from only three samples given that two of them are far from pristine. The apparent uniformity of Nd values may also be an artefact because of the minimum error of ±2 epsilon units in the data, and the uncertainty in the age of the lamproites. It is probable that the range of Nd–Sr isotopic compositions of the lamproites do not reflect their mantle source characteristics at all, but only the effects of variable degrees of crustal contamination and/or secondary alteration.
* Corresponding author. E-mail: ganilkumar{at}rediffmail.com
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  N. V. CHALAPATHI RAO, S. A. GIBSON, D. M. PYLE, A. P. DICKIN, and J. DAY Petrogenesis of Proterozoic Lamproites and Kimberlites from the Cuddapah Basin and Dharwar Craton, Southern India: a Reply J. Petrology, June 1, 2005; 46(6): 1081 - 1084. [Full Text] [PDF]
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