Geochemistry of the Paleoproterozoic metaterrigenous rocks of the Biryusa Block,southwestern Siberian Craton |
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| Authors: | N V Dmitrieva A D Nozhkin |
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| Institution: | 1.Institute of Geology and Mineralogy, Siberian Branch,Russian Academy of Sciences,Novosibirsk,Russia |
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| Abstract: | Major and trace element compositions of the Paleoproterozoic metaterrigenous rocks (Neroi Group) formed in a large sedimentation
basin in the southwestern Siberian Craton (Biryusa Block) were determined to reconstruct the protoliths of metasediments,
degree of their recycling, and maturity of source rocks. Primary rocks from the lower part of the sequence (Alkhadyr Formation)
are represented by both petrogenic (“first cycle”) and recycled sediments of the graywacke to siltstone and aluminous pelite
series. Protoliths of the micaceous and carbonaceous schists from the upper part of the sequence (Tumanshet Formation) correspond
to silty pelites and pelites. As the micaceous schists of the Alkhadyr Formation, these rocks have K2O/Al2O3 < 0.3 and elevated Th concentrations, indicating the contribution of recycling in the formation of the fine-grained rocks.
Distribution of trace and rare earth elements (REE) in metaterrigenous rocks of the Neroi Group testifies to the predominance
of felsic rocks in the source area, while the prominent Eu minimum indicates the presence of granitoids—the products of crustal
melting. Rocks of the Alkhadyr Formation also show elevated contents of Cr, Co, Ni, Sc, and Fe, indicating the development
of mafic rocks in the source area. Comparison of the trace element contents and their ratios in rocks of the Neroi Group with
those in the Archean (3.5–2.5 Ga) and Paleoproterozoic (2.5–1.6 Ga) upper continental crust made it possible to establish
that metasedimentary rocks of the Neroi Group were formed by the erosion of sufficiently mature (geochemically differentiated)
protoliths, which are similar to the Paleoproterozic crust. Judging from the Sm-Nd isotope data, one of the components of
source areas for the terrigenous rocks of the Neroi Group were Archean rocks similar to basement rocks of the Biryusa block
with the Nd model ages within 2.8–2.6 Ga. The second component in the source area could be juvenile Paleoproterozoic crust
(Nd model age ∼1.9 Ga), which was probably represented by the metavolcanic associations of grabens surrounding the Biryusa
block. The minimum Nd model ages for metaterrigenous rocks of the Neroi Group define the lowermost sedimentation boundary
at 1.9 Ga. |
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