The Provenance and Weathering Conditions of Riphean (Mesoproterozoic and Neoproterozoic) Shales, Siltstones, and Sandstones with Time, Southeastern Russia (Siberia)

Abstract

The mineralogy, major-element, and trace-element composition of shales, siltstones, and sandstones of the Uchur Group (Lower Riphean) and Aimchan and Kerpyl groups (Middle Riphean) in southeastern Russia were determined. The shales and siltstones are mostly illite to muscovite and quartz, with lesser K-feldspar, plagioclase, chlorite, carbonate, hematite, magnetite, and smectite. The sandstones are quartz-rich and range from quartz arenites and wackes to feldspathic arenites and wackes. They contain minor rock fragments (mostly shale or schist). Estimated CIA values (chemical index of alteration) of most shales and siltstones are fairly high (70–91) and ICV values (index of compositional variability) are less than one, suggesting fairly intense weathering in the source and no significant input of firstcycle material. A number of shales and siltstones of the Kerpyl Group have ICVs greater than one, suggesting that some first-cycle material was incorporated. The Eu/Eu* and (La/Lu)cn ratios of the shales–siltstones tend to decrease (0.75 to 0.65 and 13.8 to 7.8, respectively) and the Th/Sc ratios increase (0.76 to 0.95) from the Uchur to Aimchan to Kerpyl groups. These results suggest that the average source of the shales and siltstones changed from a more tonalite-rich source in the Uchur Group to a more granodiorite-rich source in the Kerpyl Group. The higher K-feldspar relative to plagioclase in the sandstones from the Uchur Group is thus anomalous, and it may have been a result of removal of the plagioclase due to intense weathering or diagenesis. Previous studies of the younger Neoproterozoic Lakhanda and Ui groups indicate that shales and siltstones have lower Eu/Eu* values than those of the Kerpyl Group. Thus, during the Mesoproterozoic the sources changed from a high abundance of tonalite in the Uchur Group to gradually increasing amounts of granodiorite in the Aimchan Group to more granite in the younger Lakhanda and Ui groups. Mesoproterozoic and Neoproterozoic (Riphean) shales in southeastern Russia have compositional trends in time similar to those from the southeastern USA and western Urals due to similar recycling (up to 70–75% of the rock volume) of original granitoid sources coupled with periodic input of first-cycle sediment. The Nd isotopes support mainly Paleoproterozoic (TNdDM 1.9–2.4 Ga) sources for most groups, with Archean block erosional input (TNdDM up to 2.8 Ga with moderately negative values of eNd) only for the Aimchan rocks. Younger Nd model ages (1500–1350 Ma) are observed in Early Upper Riphean (1025–1005 Ma) Lakhanda Group and Vendian Yudoma Group (< 560? –542 Ma) shales and correspond to juvenile input during late Grenvillian events (1000–950 Ma rift-related mafic magmatism along the eastern margin of the Siberia platform) and post-Rodinian (Vendian) destruction of the eastern periphery of the Siberian craton before opening of the Paleozoic ocean. The new data support the hypothesis of the existence of the supercontinent Rodinia (< 1050–600 Ma for the Siberian craton) but do not give direct evidence of Siberian juxtaposition to Laurentia during the Lower and Middle Riphean. Proterozoic Geology of Western North America and Siberia SEPM Special Publication No. 86, Copyright © 2007 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-126-1, p. 227–253. INTRODUCTION The Uchur–Maya region (Figs. 1, 2) contains the most complete thick and unmetamorphosed Riphean and Vendian (1600– 540 Ma) sedimentary successions in Siberia (Semikhatov, 1991; Khudoley et al., 2001). The stratigraphy and sedimentology of this intracratonic succession has been studied previously (Chumakov and Semikhatov, 1981; Semikhatov and Serebryakov 1983). Little attention, however, was paid to the sedimentary provenance, age, and tectonic nature of contained units as a tool for paleo-continent reconstruction. The reconstruction of sedimentology and provenance of the Riphean and Vendian sedimentary rocks of southeastern Siberia provides constraints on the possible reconstruction of Laurentia and Siberia within the supercontinent Rodinia (e.g., Sears and Price, 1978, 2000; Hoffman, 1991; Condie and Rosen, 1994; Frost et al., 1998; Rainbird et al., 1998; Khudoley et al., 2001). Both lithostratigraphic (Aitken et al., 1978; Rainbird et al., 1996; Rainbird et al., 1998; Khudoley and Guriev, 2003; Khudoley et al., 2001; Sears et al., 2003) and chemostratigraphic (Bartley et. al. 2001, Podkovyrov, 2001; Semikhatov et al., 2002) comparisons yield similarities in the juxtaposed Mesoproterozoic and Neoproterozoic successions of western Laurentia–Siberia. These supracrustal successions can be compared using the techniques of lithochemical and Ndisotope reconstruction of the composition and provenance of sedimentary rocks (Sochava et al., 1994; Cullers et al., 1997; Cullers and Podkovyrov, 2000, 2002). The provenance of sandstones can be determined from their contained mineralogy and the ratios of quartz to feldspar to lithic fragments (QFL diagrams) (Dickinson and Suczek, 1979; Ingersoll et al., 1984; Dickinson, 1985) or the compositions or ratios of certain accessory minerals (Basu and Molinaroli, 1991). The minV.N. PODKOVYROV, R.L. CULLERS, AND VICTOR P. KOVACH 228 low (a) and upper (b) Trekhgorka Fm. Lakhanda Gp. section location and number strike and dip faults main unconformities