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GEODYNAMICS & TECTONOPHYSICS
PUBLISHED BY THE INSTITUTE OF THE EARTH'S CRUST SIBERIAN BRANCH OF RUSSIAN ACADEMY OF SCIENCES
2017 VOLUME 8 ISSUE 3 PAGES 565-568 ISSN 2078-502X
https://doi.org/10.5800/GT-2017-8-3-0292
Proceedings of the Second Russia-China International Meeting on the Central Asian Orogenic Belt (September 6-12, 2017, Irkutsk, Russia)
Geochemistry, zircon U-Pb geochronology, Nd-Hf isotopic
characteristics and tectonic implications of the south
Muya block metasediments (Northeastern Central Asian Orogenic Belt)
S. Yu. Skuzovatov1, Yu. V. Noskova1, S. I. Dril1, K.-L. Wang2, Y. Iizuka2
1 A.P. Vinogradov Institute of Geochemistry, Siberian Branch of RAS, Irkutsk, Russia
2 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
For citation: Skuzovatov S.Yu., Noskova Yu.V., Dril S.I., Wang K.-L., Iizuka Y., 2017. Geochemistry, zircon U-Pb geochronology, Nd-Hf isotopic characteristics and tectonic implications of the South Muya block metasediments (Northeastern Central Asian Orogenic Belt). Geodynamics & Tectonophysics 8 (3), 565-568. doi:10.5800/GT-2017-8-3-0292.
The Neoproterozoic to Cenozoic collage of the Central Asian Orogenic Belt is well-known to include Pre-cambrian continental blocks and microcontinents traditionally attributed to rifting of Siberia or Gondwana prior to CAOB assembly that significantly contributed into the geochemical and isotopic composition of younger subduction- and accretion-related crustal litholo-gies via processes of crust-mantle interaction and crus-tal recycling. In the Neoproteorozoic Baikal-Muya ter-rane, the Anamakit-Muya zone has been found to resemble variable to profound isotopic signatures of the ancient crust recently confirmed by whole-rock Nd and in-situ zircon studies of the high-grade Muya block
rocks [Shatskii et al., 2014; Shatsky et al., 2015; Skuzovatov et al., 2016a, 2016b, 2017]. More specifically, the ancient crust might serve as a basement for a Neo-proterozoic continental arc possibly related to a single Tuva-Mongol-Muya continental domain [Skuzovatov et al., 2016b]. Here we report the results of whole-rock trace-element (ICP-MS) and Nd isotope geochemistry (TIMS) coupled with (MC)-LA-ICP-MS U-Pb geochronology and Hf isotope composition of detrital zircons from high-grade metasedimentary sequences of the Kindikan series, South Muya block, which has been yet considered as possible Early Precambrian basement for the Muya subduction-related complexes.
S.Yu. Skuzovatov et al.: Geochemistry, zircon U-Pb geochronology, Nd-Hf isotopic characteristics...
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Results and discussion. Within the area of the current study (Serebryakovsky and Dlinny tributaries of Tuluya river, the South Muya range), Kindikan (Tuluya) metamorphic sequence is composed mainly of highgrade schists (mostly within the amphibolite and gra-nulite facies, 600-900 °C, 8-15 kbar) and gneisses in-terbedded with paraamphibolites, marbles and calc-si-licate rocks, quartzites. The studied rocks are mostly layered quart-albite-biotite-garnet±amphibole schists and quartz-ablite-K-feldspar-biotite gneisses, with some of the schists showing notable amount of Mg-Fe-minerals, and minor calc-silicate rocks. Based on major and trace element chemistry, the sedimentary protolith for most of the schists had a nature of poorly sorted, immature and weakly weathered greywackes with Na2O/K2O>1, with silty matrix and resembling geo-chemical signatures of mixed (felsic + mafic) oceanic island arc or active continental margin provenance (Fig. 1). The Qtz-Ab-Bt-Grt schists have LREE- and LILE-enriched trace-element patterns with Eu/Eu*<1 similar to the upper crust, enriched with LREE and with Nb-Ta and Ti minima, whereas the rocks with significant amount of mafic minerals (especially, amphibole and garnet) and increased total MgO+Fe2O3* and K are relatively depleted in Th-U, show lower La/Ybn,
and some have none or positive Eu-anomaly. Nonetheless, all rocks show positive Pb and Sr anomalies typical for subduction-related rocks.
Six of the studied samples were analyzed for whole-rock Nd isotopes. Three schists with 147Sm/144Nd close to the average crustal value (0.1160-0.1223) exhibit clearly negative sNd(800) of -4.7..-7.0 and Paleoproterozoic two-stage Nd model ages of 1.88-2.06 Ga. One sample with similar 147Sm/144Nd (0.1123) has a less radiogenic Nd isotope composition leading to notably "younger" sNd(800) of -1.9 and model age (1.65 Ga). Two remaining samples with evidently higher *47Sm/i44Nd (0.1303-0.1401) lead to sNd(800) -0.2.-1.5 and Mesoproteorozoic model ages of 1.5-1.61 Ga.
For U-Pb studies, ~240 detrital zircon grains have been separated from five samples. Two Mg-Fe-poor and Mg-Fe-rich samples have bimodal age distribution dominated by the Cryogenian igneous and partially re-crystallized/reset zircons related to Early Neoprote-orozoic suprasubduction magmatism (720-800, 730780, 700-820 Ma), with minor to notable amount of Ediacarian metamorphic grains (630-650, 600-620, 560-640 Ma), while in one sample two discordant grains of clearly Early Precambrian affinity were
Geodynamics & Tectonophysics 2017 Volume 8 Issue 3 Pages 565-568
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detected. The other two Mg-Fe-rich samples have magmatic zircons exhibit more uniform distribution with the dominate age range of 680-800 and 710-780 Ma, respectively, with one of the samples showing a notable amount of discordant Early Precambrian zircons. As revealed by in-situ Hf isotopes, most of zircons have slightly positive to positive to strongly negative eHf(T) signatures reflecting the (a) reworking by the Early Precambrian crust (evident from the Paleoprote-rozoic and Archean £Hf(T) and Tdm-2st values of both discordant Early Precambrian and Cryogenian grains), and (b) metamorphic recrystallization with Pb removal but without any input of juvenile radiogenic Hf component due to Late Ediacarian (probably 550-560 Ma)
juxtaposition of the Baikal-Muya composite terrane to oceanic island-arc structure in the southern Siberia (Fig. 2). Considering the whole-rock geochemistry and Nd isotopes, we may suggest that the sediment chemistry could be controlled by processes of mechanical mixing and subsequent reworking of subduction-related mafic (zircon-poor) volcanogenic substrate with the Early Precambrian terrigenous sediments, as well as -vice versa - zircon derivation from subduction-related felsic rocks mixed with strongly reworked Early Pre-cambrian sediments as a rock matrix.
Acknowledgements. This ongoing study is supported by the Russian Federation President Foundation (Grant No. MK-4852.2016.5).
References
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S.Yu. Skuzovatov et al.: Geochemistry, zircon U-Pb geochronology, Nd-Hf isotopic characteristics.
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Skuzovatov S.Yu., Sklyarov E.V., Shatsky V.S., Wang K.-L., Kulikova K.V., Zarubina O.V., 2016a. Granulites of the South-Muya block (Baikal-Muya Foldbelt): age of metamorphism and nature of protolith. Russian Geology and Geophysics 57 (3), 451-463. https://doi.org/10.1016Zj.rgg.2016.03.007.
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Skuzovatov S.Yu., Shatsky V.S., Dril S.I., 2017. High-pressure mafic granulites of the South Muya block (Central Asian Orogenic Belt). Doklady Earth Sciences 473 (2), 423-426. https://doi.org/10.1134/S1028334X17040067.
