The Karatau Group (1050-650 Ma) occurs in outcrops within the Bashkirian Anticlinorium, the South Urals, and is the Upper Riphean stratotype. The Karatau, up to 2400-4700 m thick, comprises five lithostratigraphic units (from bottom upwards): the Zilmerdak, Katav, Inzer, Min'yar and Uk formations. All of these, but the terrigenous Zilmerdak Formation, are made up of carbonate or terrigenous-carbonate rocks.
Chemical treatment of the carbonate samples was used to remove loosely-bound chemical elements from the silicate component and partly eliminate any new-formed carbonate phases. The technique includes leaching with 1N ammonium acetate (AMA) and the dissolving of a residue in 10% acetic acid (ACA). The proportion of material which is dissolved in AMA under the experiment conditions averages 5.1% for limestones and 2.8% for dolostones. Rb content in the AMA-soluble phase of the limestones is 30-50 times greater than that in the remainder ACA-phase. Sr content in the AMA-phase of dolostones is 2.5 times that in the ACA-phase, whereas they are in rough agreement for limestones. The distinction of 87Sr/86Sr ratio between these two phases
averages 0.5*10-3, 1.2*10-3, and 4.8*10-3 for limestones of
the Inzer, Uk and Katav respectively, and 3.8*10-3 for dolostones.
The measured 87Sr/86Sr ratio in ACA-phase falls within the broad ranges of 0.706-0.712, 0.705-0.706, 0.705-0.708 and 0.705-0.712 for the Katav, Inzer, Min'yar and Uk formations, respectively. The variations of the 87Sr/86Sr are induced by a number of post-sedimentary transformations at different stages of carbonate lithogenesis, among which are:
1. Secondary dolomitization or calcitization which are accompanied by an enrichment of carbonate by Fe, Mn and external Sr. These processes are strictly manifested in a neighbourhood of the contacts with terrigenous strata or in the vicinity of tension fractures.
2. Exchange of Mn, Fe, Rb, and Sr between carbonate and silicate components during the burial of sediments. This closed- or semiclosed-system process is especially well-defined in carbonate rocks with a great silicate dopant.
3. Interaction between carbonate minerals with fresh water accompanied by the partial loss of the initial Sr and enrichment of carbonates by Mn and Fe. Such an interaction was evidenced in both the Inzer limestones and the Min'yar dolostones. The chemical and isotope data testify that post-sedimentary processes in the Karatau revealed themselves to a variable extent. The Inzer limestones are least altered, whereas the Uk limestones and the Min'yar dolostones have been subjected to pronounced changes. Clayey limestones of the Katav Formation are totally altered.
This work was supported by the RFBR Grant 93-05-09404 and the ISF-RFG Grant MLO-300.