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Carbon isotope geochemistry and geochronological constraints of the Neoproterozoic Sirohi Group from northwest India

Abstract:

The early Neoproterozoic carbonates of the Sirohi Group, northwestern India, were studied for geochronology as well as for oxygen and carbon isotopes across three different sections to understand their relationship with contemporaneous global events. New single zircon Pb-Pb evaporation data constrain the age of the Sirohi Group between ca. 920 and ca. 820Ma, though actual span for deposition of the carbonates may be little shorter taking into allowance the time for peneplanation. Narrow range in variation of δ 18O values is generally observed in the carbonate which is consistent with generally well-preserved original compositions. Two sections (SA and SS) out of the three that were studied had similarly small variations in δ 18O (about 2‰), whereas the third section (SN) had a slightly wider range of δ 18O (around 5‰). The δ 18O values of the SN section varied between -11.9 and -17.6‰, which can be attributed to marginal modifications in primary composition caused by contact metamorphism which is evident by the occurrence of wollastonite in the carbonates.The δ 13C carb values of these carbonates varied between -4.1 and +4.7‰ in all sections. Positive and near-zero δ 13C carb values are from SA and SS sections which are least affected by the metamorphism. The δ 13C carb values of these two sections are interpreted as primary and can be correlated with contemporary sea-water compositions. The SN section showed negative δ 13C carb values which might be marginally affected by contact metamorphism, though we avoided sampling from the contact aureole. However, the metamorphic equilibration with 13C-depleted fluids leading to insignificant alteration in isotope compositions cannot be totally ruled out for the analyzed samples.Positive and near-zero carbon isotope values of Sirohi carbonates (SA and SS sections) closely match the global trends of carbonate dated between 900 and 850. Ma but the negative carbon isotope anomaly (SN section) shown by the Sirohi Group carbonates is unmatched. Since the SN section is stratigraphically lower to SA and SS sections we suggest that the negative carbon isotope values of the Sirohi Group as one of the oldest Neoproterozoic negative carbon anomaly possibly close to 900. Ma and not associated with glaciation. The carbon isotopic compositions of carbonates from the Sirohi Group suggest a variable degree of preservation of original compositions that represent yet another locality with moderately heavy values between 850 and 900. Ma and an unmatchable example of the associated negative values. This reflects a unique biogeochemical signature of changing carbon cycles prior to the onset of Snowball Earth Events in the Neoproterozoic. © 2012 Elsevier B.V.