| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 4715469 | Lithos | 2016 | 13 Pages |
•εNd data refute Dharwar craton as a main source of solutes to the Papaghni sub-basin.•A magmatic arc was the likely major source of solute and its contribution increased with time.•δ13C/δ18O of U-mineralised dolomites alludes to a redox-stratified anoxic basin.•Decay of organics and consumption of dissolved oxygen lead to a bottom water anoxia.•Papaghni sub-basin opened as a back-arc extensional basin at ~ 2 Ga.
The Cuddapah basin (CB) is one of a series of Proterozoic basins that overlie the Archaean cratons of India, and contains a unique stratiform carbonate-hosted uranium mineralisation. In the present work, we discuss stable (C, O) and radiogenic (Nd, Sr) isotope systematics of carbonates of the Papaghni sub-basin in order to understand uranium ore forming processes and geodynamic evolution of the CB. Uranium mineralised dolomites (UMDs) of the basal Vempalle Formation show a significantly lighter (~ 1.5‰) C-isotope signature compared to that of open-marine stromatolitic sub-tidal facies, suggesting input of isotopically lighter carbon through in situ remineralisation of organic matter (OM). This implies deposition in a hydrologically-restricted, redox-stratified lagoonal basin wherein exchange with open oceanic dissolved inorganic carbon (DIC) was limited. Persistent bottom water anoxia was created and maintained through consumption of dissolved oxygen (DO) by decaying OM produced in oxidised surface water zone. Significantly more radiogenic εNd(t) of UMD (− 6.31 ± 0.54) compared to that of Dharwar upper crust (− 8.64 ± 3.11) indicates that dissolved constituents did not originate from the Dharwar craton, rather were derived from more juvenile exotic sources — possibly from a continental arc. Dissolved uranyl ions (U+ 6) were introduced to the basin through fluvial run-off and were reduced to immobile uranous ions (U+ 4) at the redox interface resulting in precipitation of pitchblende and coffinite. Carbonate horizons of upper Vempalle Formation and Tadpatri Formation show progressively more radiogenic Nd isotope compositions signifying increased juvenile arc contribution to the Papaghni sub-basin through time, which is also corroborated by the presence of younger zircons (1923 ± 22 Ma) in Pulivendla quartzites. We propose that the Papaghni sub-basin opened as a back-arc extensional basin at ~ 2 Ga as a result of westerly-directed subduction of oceanic crust beneath the eastern Indian continental margin. The ‘Papaghni’ back-arc basin eventually evolved to ‘upper Cuddapah’ foreland basin with collision of the Dharwar craton and Napier block of Antarctica at ~ 1.6 Ga, possibly, during the final stage of amalgamation of the Columbia supercontinent.
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