Paleoproterozoic age of the protoliths of metaterrigenous rocks in the east of the Irkut Granulite-Gneiss block (Sharyzhalgai salient,Siberian Craton) |
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| Authors: | O M Turkina L N Urmantseva N G Berezhnaya S L Presnyakov |
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| Institution: | 1.Institute of Geology and Mineralogy, Siberian Branch,Russian Academy of Sciences,Novosibirsk,Russia;2.Novosibirsk State University,Novosibirsk,Russia;3.Center of lsotopic Research,A.P. Karpinsky Russian Geological Research Institute (VSEGEI),St. Petersburg,Russia |
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| Abstract: | The Early Precambrian granulite-gneiss complex of the Irkut Block (Sharyzhalgai salient of the Siberian Craton basement) with
the protoliths represented by a wide range of magmatic and sedimentary rocks, has a long-term history including several magmatic
and metamorphic stages. To estimate the age of sedimentation and metamorphism of the terrigenous deposits, the composition
of the garnet-biotite, hyper-sthene-biotite, and cordierite-bearing gneisses has been studied; their isotopic Sm-Nd values
have been revealed; and the U-Pb zircon dating has been performed using the SHRIMP II ion microprobe. The protoliths of the
terrigenous sediments metamorphosed under conditions of the granulite facies correspond to a rock series from siltstones and
graywackes to pelites. The Nd model ages of paragneisses range from 2.4 to 3.1 Ga. Zircons of the cordierite-bearing and hypersthene—biotite
gneisses show the presence of cores and rims. The clastic, smoothed, and irregular shape of the cores indicates their detrital
character and relicts of oscillatory zoning suggest the magmatic origin of zircon. The rim’s metamorphic genesis is indicated
by the lack of zoning and by the lower Th/U ratio compared to that of the cores. The age of the detrital cores (≥2.7, ~2.3,
and 1.95—2.0 Ga) and metamorphic rims (1.85–1.86 Ga) defines the time of sedimentation at 1.85–1.95 Ga ago. Potential sources
for the Archean detrital zircons were metamagmatic rocks of the granulite—gneiss complexes in the southwestern margin of the
Siberian Craton. The age of the dominant detrital cores at 1.95–2.0 Ga ago, together with the minimal TNd(DM) values, indicates the contribution of the juvenile Paleoproterozoic crust to the formation of sediments. The juvenile
Paleoproterozoic crust was likely represented by magmatic complexes similar to the volcanic and granitoid associations of
the Aldan shield, which were formed 1.99–2.0 Ga ago and showthe model age of 2.0—2.4 Ga. The isotopic Sm-Nd data show that
the Late Paleoproterozoic metasedimentary rocks occur not only in the Sharyzhalgai salient but in the Aldan and Anabar shields
of the Siberian Craton as well. |
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