Rb-Sr geochronology of middle Riphean shales, the Yusmastakh Formation of the Anabar Massif, northern Siberia

Isotopic dates for upper horizons in one of the Riphean reference sections of northern Eurasia are calculated on the basis of the Rb-Sr systematics of fine size-variable subfractions (SF) of clay minerals. The SFs with particle size ranges <0.1, 0.1-0.2, 0.2-0.3, 0.3-0.6, 0.6-1, 1-2, and 2-5 mum are separated out of two shale samples characterizing the lower and upper Yusmastakh subformations. Clay mineral in all the SFs is the low-temperature illite; feldspar and quartz admixture was detected in the coarsest SFs only. The established crystallinity index standard (CIS) of illite corresponds to that characterizing the diagenesis zone. Leaching in 1N NH4OAc solution and subsequent Rb-Sr analysis of untreated SFs, leachates and residues offered an opportunity to plot the "inner Rb-Sr isochron" for each SF and to determine its apparent age and initial Sr-87/Sr-86 ratio. In series of SFs arranged according to their size range decrease from 2-5 to < 0.1 mum, the following parallel changes of various parameters are established: (1) CIS values acid I-002/I-001 ratios increase; (2) Rb and Sr concentrations and Sr-87/Sr-86 ratios grow; (3) Rb-87/Sr-86 ratios in leachates go up; (4) the apparent Rb-Sr age decreases from 1272-1284 down to 1004-1048 Ma. For each sample, data points characterizing leachates and residues of size-variable SFs plot along mixing lines in the Rb-87/Sr-86-Sr-87/Sr-86 graphs. The XRD analysis and isotopic data show that SFs include three generations of authigenic illite. The considered paleontological, chemostratigraphic, and isotopic-geochronological data clarify age of the Yusmastakh Formation and show that the burial diagenesis affected the corresponding deposits 1280-1270 Ma ago (age of the first illite generation). The subsequent uprising and renewed interstitial fluids initiated two stages of regressive catagenesis 1090-1130 and 1000-1060 Ma ago, when new generations of authigenic illite originated in the Yusmastakh deposits.