Sr Isotope Chemostratigraphy And Pb-Pb Age Of The Riphean Carbonate Deposits Of The Kharaulakh Uplift (Northeastern Margin Of The Siberian Platform)

The least altered limestones of the Ukta and Eselekh formations in the Precambrian section of the Kharaulakh uplift have a minimum Sr-87/Sr-86 ratio of 0.70673-0.70715. The lowest Sr-87/Sr-86 ratio of the overlying Neleger and Sietachan formations is 0.70791-0.70817. Based on these data, along with the earlier obtained positive delta C-13. values (up to 8 parts per thousand) for the Kharaulakh section, we have estimated the age of the Ukta and Eselekh formations at 800-670 Ma and the age of the Neleger and Sietachan formations at similar to 640-580 Ma. The Pb-Pb isochron age of the least altered limestones of the Eselekh Formation calculated from eight samples is 720 +/- 30 Ma. This age permits us to define the lower part of the Kharaulakh section of the Ukta and Eselekh formations to be the late Tonian of the International Chronostratigraphic Chart or to the Upper Riphean of the General Stratigraphic Scale of Russia. The presence of reliably dated Upper Riphean sediments in the Kharaulakh uplift indicates a more complex structure of the Precambrian sedimentary cover on the Arctic margin of the Siberian Platform than assumed earlier.