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Natalia Berezhnaya, A.P. Karpinsky Russian Geological Research Institute (VSEGEI) (Russian Federation)
O. Turkina, Institute of Geology and Mineralogy, SB RAN (Russian Federation)
S. Skublov, Institute of Precambrian Geology and Geochronology RAN (Russian Federation)
Elena Lepekhina, A.P. Karpinsky Russian Geological Research Institute (VSEGEI) (Russian Federation)
Ilya Paderin, A.P. Karpinsky Russian Geological Research Institute (VSEGEI) (Russian Federation)
Tatiana Saltykova, A.P. Karpinsky Russian Geological Research Institute (VSEGEI) (Russian Federation)
Sergey Sergeev, A.P. Karpinsky Russian Geological Research Institute (VSEGEI) (Russian Federation)
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Four crustal blocks (Bulunsky, Onotsky, Kitoysky and Irkutny) are composing the Sharyzhylgay terrane, Siberian craton, Russia. The southerly Irkutny block consists of granite and charnokite domes surrounded by biotite-amphibole-hypersthene orthogneiss (BAHG) plus amphibole-pyroxene crystal schist, overlain by garnet-biotite paragneiss, amphibolites and high-titanium crystal schist (TCS). Archaean and Proterozoic granulite facies metamorphic episodes within Irkutny block were found (Salnikova et al., 2007) by U-Pb ID-TIMS method- as old as 2649±6 Ma (for metabasalts), 2.56 Ga (anatexite granites) and 1.87 Ga (anatexite granites and charnokites)
Our high-resolution U-Pb SHRIMP zircon dating allowed defining the cluster of individual ages, corresponding to specific part of grains (zircon cores, rims and edges). For BAHG there are 3347±7, 3158±15, 3026±17 and 1850±12 Ma-aged material, whereas for TCS - 2663±18, 2570±13 and 1857±40 Ma.
For meaningful interpretation of obtained geochronological results, we have studied REE patterns in dated zircon grains. Zircons of metamorphic, magmatic and metasomatic origin show different geochemical characteristics (Rubatto, 2002; Hoskin,2005).
Local measurements revealed distinct types of REE patterns for specific zircon material from both BAHG and TCS rocks. U-Pb age and REE characteristics are shown below.
BAHG cores (magmatic, mantle origin): 3347±7 Ma; (Sm/La)n=17-193; (Lu/Sm)n=32-56; (Ce/Ce*)=15-64; (Yb/Gd)n=11-16; (Yb/Sm)n=32-56;
BAHG cores (magmatic, crustal origin): 3158±15 Ma; (Sm/La)n=17-58; (Lu/Sm)n=59-246; (Ce/Ce*)=15-81; (Yb/Gd)n=31-48; (Yb/Sm)n=51-260;
BAHG red rims (metamorphic, granulite facies): 3026±17 Ma; (Sm/La)n=20-71; (Lu/Sm)n=6-16; (Ce/Ce*)=9-15; (Yb/Gd)n=1,2-8,3; (Yb/Sm)n=4,3-28;
BAHG edges (metamorphic, eclogite? facies): 1850±12 Ma; (Sm/La)n=0,7-3,2; (Lu/Sm)n=117-175; (Ce/Ce*)=3-4; (Yb/Gd)n=7-15; Y=48-63 ppm; Yb=5-10 ppm;
TCS cores (magmatic): 2663±18 Ma; (Sm/La)n=26-145; (Lu/Sm)n=34-143; (Ce/Ce*)=18-187; (Yb/Gd)n=12-28; (Yb/Sm)n=31-131;
TCS red rims (metamorphic, granulite facies): 2570±13 Ma; (Sm/La)n=27-30; (Lu/Sm)n=16-19; (Ce/Ce*)=5-7; (Yb/Gd)n=4-5; (Yb/Sm)n=16-20;
TCS pink grains (metamorphic, eclogite, granulite facies): 1857±40 Ma; (Sm/La)n=9-31; (Lu/Sm)n=10-41; (Ce/Ce*)=7-24; (Yb/Gd)n=4-10; Y=71-123 ppm; Yb=14-33 ppm;
We conclude that combined local U-Pb and REE SIMS measurements from the same zircon material allow us to offer reliable geological interpretation of the complicated metamorphic history. At the first time we prove Mesoarchaean (3026±17 Ma) granulite facies metamorphic event within Irkutny block, Sharyzhylgay terrane and different P-T conditions for Meso- and Neoarchaean (2570±13 Ma) granulite metamorphism.
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