Lithogeochemical features of Riphean fine-grained terrigenous rocks in the Kama-Belaya aulacogen and their formation conditions |
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| Authors: | A V Maslov M V Isherskaya M T Krupenin V G Petrishcheva T Ya Gulyaeva N P Gorbunova |
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| Institution: | 1.Institute of Geology and Geochemistry, Uralian Division,Russian Academy of Sciences,Yekaterinburg,Russia;2.Institute of Geology, Ufa Scientific Center,Russian Academy of Sciences,Ufa,Russia |
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| Abstract: | Lithogeochemical features of Riphean fine-grained terrigenous rocks of the Kama-Belaya aulacogen are discussed. It is shown
that aluminosiliciclastic material delivered to the aulacogen during the Riphean was characterized by a low maturity degree.
The successively increasing K2O/Al2O3 values in the Riphean summary section correlate negatively with the CIA index values, indicating a gradually strengthening
tendency for climate aridization in erosion zones. Data on some indicator ratios of trace elements and REE systematics in
Riphean silty mudstones and shales of the Kama-Belaya aulacogen imply the involvement of mafic and ultramafic rocks, in addition
to acid igneous and metamorphic varieties, in erosion during accumulation of the Nadezhdino, Tukaevo, Ol’khovka, Usinsk, and
Priyutovo formations. Comparison of data on the composition of rocks in provenances based on the mineralogical-petrographic
study of sandstones and investigation of geochemical features of silty mudstones and shales revealed their sufficiently high
similarity. The geochemical data made it possible to specify the composition of rocks in provenances. Low Ce/Cr values in
the fine-grained terrigenous rocks of the Lower Riphean Kyrpy Group suggest their formation with a significant contribution
of erosion products of the Archean substrate, which is atypical for higher levels of the section. Thus, the Early-Middle Riphean
transition period was likely marked by substantial changes in the mineral composition of material delivered to the Kama-Belaya
aulacogen. The lack of exhalative components in the examined specimens of silty mudstones and shales points to a relatively
low permeability of the Earth’s crust in the eastern East European Platform through the entire Riphean. |
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