Petrogenesis of Proterozoic Lamproites and Kimberlites from the Cuddapah Basin and Dharwar Craton, Southern India: a Reply
1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF CAMBRIDGE, DOWNING STREET, CAMBRIDGE CB2 3EQ, UK
2 DEPARTMENT OF GEOLOGY, McMASTER UNIVERSITY, HAMILTON L8S 4M1, ONTARIO, CANADA
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INTRODUCTION |
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Mid-Proterozoic kimberlites and lamproites from southern India are some of the world's oldest known, small-fraction, volatile-rich, silicate melts that, until recently, have remained poorly documented in the international literature. The main aim of the work by Chalapathi Rao et al. (2004)
was to understand the tectono-magmatic processes operating beneath the Dharwar Craton and Cuddapah Basin during the Mid-Proterozoic. To constrain (1) the melt generation processes and (2) the nature of the source regions involved in the genesis of the small-fraction mantle melts it was necessary for us to undertake detailed petrographic descriptions and high-precision analyses of the mineral and bulk-rock chemistry of Dharwar craton kimberlites and Cuddapah Basin lamproites. The main emphasis of the paper by Chalapathi Rao et al. (2004) was not to classify rocks as kimberlites or lamproites, as Anil Kumar & Gopalan (2005) suggest, although this was made possible by results from our detailed mineralogical and geochemical studies.
Anil Kumar & Gopalan (2005) have questioned the number of bulk-rock analyses that Chalapathi Rao et al. (2004) have published for each lamproite. Bulk-rock analyses, consisting of 42 elements, were published for about three samples from each of three lamproite intrusions and an average of about four samples from each of 15 kimberlite pipes. The number of samples that we have analysed from each intrusion reflects our decision to focus on sample quality rather than quantity. We note that this number of analysed samples per intrusion is similar to or greater than that published in recent studies of other worldwide kimberlite and lamproite provinces (e.g. Gibson et al., 1995; Beard et al., 2000; Mahotkin et al., 2000; Schmidberger & Francis, 2001). We acknowledge that our interpretations of small-fraction melt generation in and around the Cuddapah Basin are based on the results of samples from only three lamproite intrusions and in no way do we imply that our scientific interpretations are definitive; we hope that our paper will be a building block for future research on Indian lamproites. We are pleased to hear that since the completion of our fieldwork ‘pristine lamproites’ have been discovered in the Cuddapah Basin and look forward to the publication of high-precision analyses of their mineral and bulk-rock chemistry.
Given their Proterozoic age, the kimberlites and lamproites from the Dharwar Craton and Cuddapah Basin are remarkably fresh [see photomicrographs in Chalapathi Rao et al. (2004, Supplementary Data, Appendix 3)]. High loss-on-ignition values for small-fraction volatile-rich melts are not unusual; in the case of the Cuddapah Basin lamproites, these are due to secondary carbonation. For mica-rich rocks, the contamination index (C.I.) may also be misleading (see Mitchell, 1986) and the high C.I. of the Cuddapah Basin lamproites is probably due to their micaceous nature [see petrography and geochemistry sections of Chalapathi Rao et al. (2004)]. The classification of the Chelima and Zangamarajupalle rocks as lamproites by Chalapathi Rao et al. (2004) was based on the chemistry of mica, which is petrographically fresh and unaffected by secondary carbonation.
Anil Kumar & Gopalan (2005) expressed concern over the origin of the Sr and Nd isotopic data in table 5 of Chalapathi Rao et al. (2004). We certainly had no intention of implying that the data were new and referred to Chalapathi Rao et al. (1998) in the text. To avoid any ambiguity, however, we have also included a reference to Chalapathi Rao et al. (1998) in the revised caption to Table 5. We thank Anil Kumar & Gopalan (2005) for pointing out the calculation error in the 
Sr values shown in Table 5; we have corrected this in the revised version of the table and also in a revised version of Fig. 12 of Chalapathi Rao et al. (2004). This revision does not affect our comparisons of isotopic ratios with other worldwide occurrences, as these are based only on Nd values.
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The comments relating to the analytical procedures used in the determination of Sr isotopes are almost identical to those raised by Gopalan & Anil Kumar (1999)
and we refer the reader to Chalapathi Rao et al. (1999a) for our responses. We state clearly in Chalapathi Rao et al. (2004) that ‘we are cautious about interpreting the petrogenetic significance of these (Sr isotope) ratios’ and our interpretations of melt generation processes and mantle source characteristics are based on rare-earth elements (REE) and Nd isotopic ratios, which are less susceptible to alteration. We have reanalysed leachates from the kimberlites and lamproites to test the claim that there are ‘large and unconstrained errors’ in the analytical procedures involved in the determination of Nd-isotopic ratios as suggested by Anil Kumar & Gopalan (2005). Our ICP-MS analyses (revised Table 5) show that leaching of the kimberlites with 6 M HCl has very little effect on the concentrations of Nd and Sm in the rock residue. Recalculated Ndi values for the kimberlites, using the Nd and Sm concentrations in the leached residue, range from 0·7 to 5·2 (compare with 0–4 using unleached kimberlite Nd and Sm concentrations; Chalapathi Rao et al., 2004). The effect of leaching on Nd and Sm concentrations is slightly greater for the lamproites. However, the change in Sm/Nd ratios during leaching is much smaller than the change in REE concentrations. Thus, recalculated Ndi values, based on Nd and Sm concentrations in the leached residue, vary from –6·0 to –7·7 (comprared with –6·2 to –7·2 using unleached lamproite Nd and Sm concentrations; Chalapathi Rao et al., 2004). We do not believe, therefore, that analytical techniques are responsible for the large variation in Nd values displayed by the kimberlites and lamproites and the recalculated Ndi values do not change the interpretations of Chalapathi Rao et al. (2004) regarding their melt source regions.
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Chalapathi Rao et al. (2004)
proposed that the emplacement of Proterozoic kimberlite and lamproites in and around the Cuddapah Basin was essentially non-contemporaneous, which is in agreement with Anil Kumar et al. (2001) but not Anil Kumar et al. (1993). Nevertheless, Anil Kumar & Gopalan (2005) disagree with the ages that we have used to correct Sr and Nd isotopic ratios from measured to initial values. They state that Rb–Sr ages published for the lamproites by Anil Kumar et al. (2001) are ‘only provisional’ and we therefore prefer to use our previously published age determinations (Chalapathi Rao et al., 1996, 1999b) as we are familiar with the petrography of the dated samples and are able to assess the quality of the data. The phlogopites in the groundmass of our samples from the Chelima lamproite have not undergone extensive secondary carbonation and we did not [as implied by Anil Kumar & Gopalan (2005)] undertake acid leaching on this phase prior to Ar/Ar age dating. For further discussion of K/Ar and Ar/Ar age determinations for the kimberlites and lamproites we refer the reader to Chalapathi Rao et al. (1999c) and Gopalan et al. (1999).
Clearly the Nd values are influenced by the age used to correct the measured 143Nd/144Nd ratios. However, the Nd-isotope ratios of the kimberlites and lamproites developed along sub-parallel but widely spaced evolution lines. Therefore, changing the ages of the lamproites from 1418 Ma to the ‘tentative’ and ‘provisional’ ages suggested by Anil Kumar & Gopalan (2005) still gives a large difference between the Nd values of the Dharwar Craton kimberlites and Cuddapah Basin lamproites. Our interpretation that metasomatic enrichment of the lamproite melt source region occurred a long time prior to final melt generation is, therefore, still valid. We dismiss the proposal that the Nd isotopic characteristics of the lamproites are due to crustal contamination. It has been widely demonstrated that the high concentrations of Sm and Nd of small-fraction melts, and corresponding low concentrations of these elements in crustal melts, ensure that the Nd isotopic ratios of kimberlites and lamproites are relatively unaffected by crustal contamination (e.g. Fraser et al., 1985; Gibson et al., 1996).
Anil Kumar & Gopalan (2005) suggest that we have proposed two isotopically distinct sources to explain the origin of the two petrographically and spatially distinct groups of kimberlites that intrude the Dharwar Craton. This is incorrect. The Nd values of the diamondiferous kimberlites from the Anantapur cluster appear to be lower than those of the non-diamondiferous Mahbubnagar kimberlites but incompatible trace-element ratios suggest that both groups of kimberlites were predominantly derived from metasomatically veined lithospheric mantle. Our interpretation that the mantle xenocryst-rich Anantapur kimberlites have lower Nd values because they contain more entrained material from the cratonic lithospheric mantle than the Mahbubnagar kimberlites seems highly plausible. There is undoubtedly much further work to be done on the Dharwar Craton kimberlites and Cuddapah Basin lamproites. We hope that our work will encourage this, especially a high-precision (U/Pb?) age dating study.
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SUPPLEMENTARY DATA |
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TOPINTRODUCTIONSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available at Journal of Petrology online.
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FOOTNOTES |
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* Present address: Ore Dressing Division, Indian Bureau of Mines, Hingna Road, Nagpur 440016, India.
Corresponding author. E-mail: sally{at}esc.cam.ac.uk
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REFERENCES |
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TOPINTRODUCTIONSUPPLEMENTARY DATAREFERENCES |
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