Geochronology of detrital zircons from schist in Kyrgyz South Tianshan: evidence for the provenance of metamorphic sediments in the accretionary complex
Chen Bo1,2,Wang Bin2
1Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MLR, Xi’an 710054, Shaanxi China, chenbo-geo@pku.edu.cn
2Xi’an Center of Geological Survey, CGS, Xi’an 710054, Shaanxi China
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The Tian Shan of Central Asia is located in the southwestern part of Central Asian Orogenic Belt. The tectonic evolution of the South Tian Shan suture is subject to hot debate. In the Kyrgyzstan, South Tian Shan mainly consists of ophiolitic mélange, volcanic, metamorphic rocks and sedimentary rocks ranging in age from Silurian to Late Carboniferous,. Metamorphic rocks are exposed along the Atbashi Range. Eclogite from the higher-grade schist yielded a Sm-Nd isochron age of 319±4Ma for the HP metamorphism[1]. This work focuses on the depositional age of the metamorphic sedimentary by detrital zircons and simply discusses the geological implications.
Large scale interbeded muscovite-quartz-schist and marble occur to 20 km southwest of the well-known Atbashi eclogite. Zircons from muscovite-quartz-schist mainly have irregular shape. These zircons have chaotic internal structures. Some zircons are characterized by subhedral to euhedral shape with zonation, and some show metamorphic texture. The metamict crystals are high-U with very low-luminescent CL (cathode luminescence). Some zircons have narrow rims with high luminescent CL, which might be related to the metamorphism. These zircons show wide age variations from 406 ± 4Ma (206Pb/238U) – 2856 ± 36Ma (207Pb/206Pb). Muscovite-quartz schist was considered as a part of Precambrian metamorphic terrane (Mesoproterozoic Atbashi Formation). The youngest age (406Ma) of detrital zircon implies the sedimentary age of protolith should be no earlier than the Early Devonian. The number of 22 zircon ages for the muscovite-quartz schist sample is far too low for a detailed provenance analysis and only provides a sketch of the age spectrum of the source. The old zircons are similar to that reported from the Tarim craton and the North and Middle Tian Shan. Ages ranging from 1.5Ga to 900Ma and from 850Ma to 750Ma are also known in the Tarim craton and the North and Middle Tian Shan [2-3]. The four youngest grains have 206Pb/238U ages of 436 – 406Ma are absent in the Tarim craton. Such ages are typical of granitoids in North Tian Shan in Krygyzstan [4], and these grains therefor could have been derived from those granitoids. Tectonic reconstructions for the Paleozoic South Tian Shan basin of Kyrgyzstan suggest that clastic sediments on the northern side of the South Tian Shan could have been deposited on the continental slope of the Kazakhstan continent [1]. Accepting this hypothesis, the provenance of the metamorphic sediments was probably derived from the North and Middle Tian Shan, which was to the north of the oceanic basin.
References:
[1] Hegner E et al. (2010). Mineralogy ages and P–T conditions of late Paleozoic high-pressure eclogite and provenance of mélange sediments in the South Tianshan Orogen of Kyrgyzstan. American Journal of Science 310: 916–950
[2] Lu S et al. (2008) Geological and geochronological evidence for the Precambrian evolution of the Tarim craton and surrounding continental fragments . Precambrian Research 160: 94–107
[3] Kröner A et al. (2013) Mesoproterozoic (Grenvilleage) terranes in the Kyrgyz North Tianshan: zircon ages and Nd–Hf isotopic constraints on the origin and evolution of basement blocks in the southern Central Asian Orogen. Gondwana Research 23: 272–295
[4] Konopelko et al. (2008) Deciphering Caledonian events: Timing and geochemistry of the Caledonian magmatic arc in the Kyrgyz Tien Shan . Journal of Asian Earth Sciences 32: 131–141