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Comment on “New age constraints for the Proterozoic Aravalli–Delhi successions of India and their implications” by McKenzie et al. [Precambrian Res. 238 (2013) 120–128]
Precambrian Research 246 (2014) 319–320
Contents lists available at ScienceDirect
Precambrian Research journal homepage: www.elsevier.com/locate/precamres
Discussion
Comment on “New age constraints for the Proterozoic Aravalli–Delhi successions of India and their implications” by McKenzie et al. [Precambrian Res. 238 (2013) 120–128] Victor A. Melezhik a,∗ , Ritesh Purohit b , Dominic Papineau c,d a
Geological Survey of Norway, Postboks 6315 Sluppen, N-4791 Trondheim, Norway Department of Geology, Government P.G. College, Sirohi 307001, Rajasthan, India c London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, United Kingdom d Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, United Kingdom b
a r t i c l e
i n f o
Article history: Received 18 November 2013 Received in revised form 10 March 2014 Accepted 19 March 2014 Available online 29 March 2014
McKenzie et al. (2013) studied Proterozoic sedimentary successions of the southern Aravalli–Delhi Orogenic Belt and the Rajasthan Vindhyan area. They obtained robust detrital zircon age data that provided minimum depositional ages for several formations. The authors implied that the newly obtained ages gave constraints for correlations and demonstrated confidently that the northern extension of southern Alwar Group contains rocks considerably differing in age; hence the group does not represent a coherent stratigraphic unit. McKenzie et al. also studied sedimentary rocks of the Udaipur and Jhamarkotra ‘palaeobasins’, which contain commercial deposit of sedimentary phosphorites, and 13 C-rich (␦13 Ccarb up to +12‰) sedimentary carbonates. The 13 C-rich carbonates were assigned to the 2.2–2.06 Ga Lomagundi–Jatuli positive carbon isotope excursion based on principles of isotope chemostratigraphy, whereas the sedimentary phosphorites were considered to have been deposited somewhat later, hence representing the earliest known phosphate deposit in the Earth history (e.g., Banerjee et al., 1986; Maheshwari et al., 1999; Sreenivas et al., 2001; Purohit et al., 2010; Papineau et al., 2013). Hence, both 13 C-rich dolostones and phosphate-bearing dolostones with ␦13 Ccarb ≈ 0‰ (isotopically normal thereafter) have long been known in the Aravalli Supergroup but precise radiometric constraints on their depositional time has remained unresolved. McKenzie et al. extracted zircons from a sandstone situated stratigraphically below the Jhamarkotra phosphatic and
∗ Corresponding author. E-mail address: [email protected] (V.A. Melezhik). http://dx.doi.org/10.1016/j.precamres.2014.03.018 0301-9268/© 2014 Published by Elsevier B.V.
isotopically normal dolostone at the Jhamarkotra mine. These zircons yielded 1.9–1.7 Ga ages which have been used to constraining the deposition of the phosphorites and hosting dolostones. This provides the first radiometric age constraints on their deposition and demonstrates that phosphorites are much younger than they were thought before. Based on the same zircon ages obtained from the Jhamarkotra mine section, McKenzie et al. also redefined the depositional age of those Jhamarkotra dolostones that have markedly positive ␦13 C value (up to +12‰) and lack phosphorites, and declared that they discovered a new, 1.9–1.7 Ga ␦13 Ccarb anomaly within so-called boring Mezoproterozoic time. The extrapolation of the ‘Jhamarkotra-mine’ age constraints to the depositional time of the 13 C-rich dolostones has two shortcomings. Firstly, up to date no one was able to document that the Jhamarkotra mine dolostones with ␦13 C ≈ 0‰ have physical connection with 13 C-rich varieties. The sandstones sampled at the mine lie directly on Archaean basement and pass upward in to phosphatic dolostones with ␦13 Ccarb close to sub-zero values followed by black shales and phylites (e.g., Banerjee, 1971). Secondly, McKenzie’s et al. made unfortunate combination of the radiometric dates (obtained in one area) with the lithological correlation (assumption that 13 C-rich and isotopically normal dolostones deposited synchronously). Moreover, considering that isotopically normal dolostones have never been observed alternating vertically with or replacing laterally the 13 C-enriched varieties, common sense and chemostratigraphic principles would strongly advocate that they may represent carbonate rocks of two different age groups unless clear geological relationships or relevant age constraints suggest otherwise.
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V.A. Melezhik et al. / Precambrian Research 246 (2014) 319–320
Furthermore, McKenzie et al. introduced an unanswered geochemical puzzle by stating that both isotopically normal (0‰ at Jhamarkotra) and 13 C-enriched (up to +12‰) values represent synchronous (as opposed to contemporary, i.e., occurring in the same period of time, by Papineau et al. (2013)) seawater signals. What kind sedimentological, geochemical or biological processes do they have in mind, which might result in simultaneous marine deposition of a thick, laterally extensive carbonate formation differing in ␦13 C by more than 10‰? Our current knowledge of the carbon cycle does not offer any immediate meaningful solution. References Banerjee, D.M., 1971. Precambrian stromatolitic phosphorites of Udaipur, Rajasthan, India. Geol. Soc. Am. Bull. 82, 2319–2329.
Banerjee, D.M., Schidlowski, M., Arneth, J.D., 1986. Genesis of upper ProterozoicCambrian phosphorite deposits of India: isotopic inferences from carbonate fluorapatite, carbonate and organic carbon. Precambrian Res. 33, 239–253. Maheshwari, A., Sial, A.N., Chittora, V.K., 1999. High ␦13 C Palaeoproterozoic carbonates from the Aravalli Supergroup, Western India. Int. Geol. Rev. 41, 949–955. McKenzie, N.R., Hughes, N.C., Myrow, P.M., Banerjee, D.M., Deb, M., Planavsky, N.J., 2013. New age constraints for the Proterozoic Aravalli–Delhi successions of India and their implications. Precambrian Res., http://dx.doi.org/10.1016/j.precamres. 2013.10.006. Papineau, D., Purohit, R., Fogel, M.L., Shields-Zhou, G., 2013. High phosphate availability as a possible cause for massive cyanobacterial production of oxygen in the Paleoproterozoic atmosphere. Earth Planet. Sci. Lett. 362, 225–236. Purohit, R., Sanyal, P., Roy, A.B., Bhattacharya, S.K., 2010. 13 C enrichment in the Palaeoproterozoic carbonate rocks of the Aravalli Supergroup, northwest India: influence of depositional environment. Gondwana Res. 18, 538–546. Sreenivas, B., Das Sharma, S., Kumar, B., Patil, D.J., Roy, A.B., Srinivasan, R., 2001. Positive ␦13 C excursion in carbonate and organic fractions from the Paleoproterozoic Aravalli Supergroup, Northwestern India. Precambrian Res. 106, 277–290.
Contents lists available at ScienceDirect
Precambrian Research journal homepage: www.elsevier.com/locate/precamres
Discussion
Comment on “New age constraints for the Proterozoic Aravalli–Delhi successions of India and their implications” by McKenzie et al. [Precambrian Res. 238 (2013) 120–128] Victor A. Melezhik a,∗ , Ritesh Purohit b , Dominic Papineau c,d a
Geological Survey of Norway, Postboks 6315 Sluppen, N-4791 Trondheim, Norway Department of Geology, Government P.G. College, Sirohi 307001, Rajasthan, India c London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, United Kingdom d Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, United Kingdom b
a r t i c l e
i n f o
Article history: Received 18 November 2013 Received in revised form 10 March 2014 Accepted 19 March 2014 Available online 29 March 2014
McKenzie et al. (2013) studied Proterozoic sedimentary successions of the southern Aravalli–Delhi Orogenic Belt and the Rajasthan Vindhyan area. They obtained robust detrital zircon age data that provided minimum depositional ages for several formations. The authors implied that the newly obtained ages gave constraints for correlations and demonstrated confidently that the northern extension of southern Alwar Group contains rocks considerably differing in age; hence the group does not represent a coherent stratigraphic unit. McKenzie et al. also studied sedimentary rocks of the Udaipur and Jhamarkotra ‘palaeobasins’, which contain commercial deposit of sedimentary phosphorites, and 13 C-rich (␦13 Ccarb up to +12‰) sedimentary carbonates. The 13 C-rich carbonates were assigned to the 2.2–2.06 Ga Lomagundi–Jatuli positive carbon isotope excursion based on principles of isotope chemostratigraphy, whereas the sedimentary phosphorites were considered to have been deposited somewhat later, hence representing the earliest known phosphate deposit in the Earth history (e.g., Banerjee et al., 1986; Maheshwari et al., 1999; Sreenivas et al., 2001; Purohit et al., 2010; Papineau et al., 2013). Hence, both 13 C-rich dolostones and phosphate-bearing dolostones with ␦13 Ccarb ≈ 0‰ (isotopically normal thereafter) have long been known in the Aravalli Supergroup but precise radiometric constraints on their depositional time has remained unresolved. McKenzie et al. extracted zircons from a sandstone situated stratigraphically below the Jhamarkotra phosphatic and
∗ Corresponding author. E-mail address: [email protected] (V.A. Melezhik). http://dx.doi.org/10.1016/j.precamres.2014.03.018 0301-9268/© 2014 Published by Elsevier B.V.
isotopically normal dolostone at the Jhamarkotra mine. These zircons yielded 1.9–1.7 Ga ages which have been used to constraining the deposition of the phosphorites and hosting dolostones. This provides the first radiometric age constraints on their deposition and demonstrates that phosphorites are much younger than they were thought before. Based on the same zircon ages obtained from the Jhamarkotra mine section, McKenzie et al. also redefined the depositional age of those Jhamarkotra dolostones that have markedly positive ␦13 C value (up to +12‰) and lack phosphorites, and declared that they discovered a new, 1.9–1.7 Ga ␦13 Ccarb anomaly within so-called boring Mezoproterozoic time. The extrapolation of the ‘Jhamarkotra-mine’ age constraints to the depositional time of the 13 C-rich dolostones has two shortcomings. Firstly, up to date no one was able to document that the Jhamarkotra mine dolostones with ␦13 C ≈ 0‰ have physical connection with 13 C-rich varieties. The sandstones sampled at the mine lie directly on Archaean basement and pass upward in to phosphatic dolostones with ␦13 Ccarb close to sub-zero values followed by black shales and phylites (e.g., Banerjee, 1971). Secondly, McKenzie’s et al. made unfortunate combination of the radiometric dates (obtained in one area) with the lithological correlation (assumption that 13 C-rich and isotopically normal dolostones deposited synchronously). Moreover, considering that isotopically normal dolostones have never been observed alternating vertically with or replacing laterally the 13 C-enriched varieties, common sense and chemostratigraphic principles would strongly advocate that they may represent carbonate rocks of two different age groups unless clear geological relationships or relevant age constraints suggest otherwise.
320
V.A. Melezhik et al. / Precambrian Research 246 (2014) 319–320
Furthermore, McKenzie et al. introduced an unanswered geochemical puzzle by stating that both isotopically normal (0‰ at Jhamarkotra) and 13 C-enriched (up to +12‰) values represent synchronous (as opposed to contemporary, i.e., occurring in the same period of time, by Papineau et al. (2013)) seawater signals. What kind sedimentological, geochemical or biological processes do they have in mind, which might result in simultaneous marine deposition of a thick, laterally extensive carbonate formation differing in ␦13 C by more than 10‰? Our current knowledge of the carbon cycle does not offer any immediate meaningful solution. References Banerjee, D.M., 1971. Precambrian stromatolitic phosphorites of Udaipur, Rajasthan, India. Geol. Soc. Am. Bull. 82, 2319–2329.
Banerjee, D.M., Schidlowski, M., Arneth, J.D., 1986. Genesis of upper ProterozoicCambrian phosphorite deposits of India: isotopic inferences from carbonate fluorapatite, carbonate and organic carbon. Precambrian Res. 33, 239–253. Maheshwari, A., Sial, A.N., Chittora, V.K., 1999. High ␦13 C Palaeoproterozoic carbonates from the Aravalli Supergroup, Western India. Int. Geol. Rev. 41, 949–955. McKenzie, N.R., Hughes, N.C., Myrow, P.M., Banerjee, D.M., Deb, M., Planavsky, N.J., 2013. New age constraints for the Proterozoic Aravalli–Delhi successions of India and their implications. Precambrian Res., http://dx.doi.org/10.1016/j.precamres. 2013.10.006. Papineau, D., Purohit, R., Fogel, M.L., Shields-Zhou, G., 2013. High phosphate availability as a possible cause for massive cyanobacterial production of oxygen in the Paleoproterozoic atmosphere. Earth Planet. Sci. Lett. 362, 225–236. Purohit, R., Sanyal, P., Roy, A.B., Bhattacharya, S.K., 2010. 13 C enrichment in the Palaeoproterozoic carbonate rocks of the Aravalli Supergroup, northwest India: influence of depositional environment. Gondwana Res. 18, 538–546. Sreenivas, B., Das Sharma, S., Kumar, B., Patil, D.J., Roy, A.B., Srinivasan, R., 2001. Positive ␦13 C excursion in carbonate and organic fractions from the Paleoproterozoic Aravalli Supergroup, Northwestern India. Precambrian Res. 106, 277–290.