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P ermian Peri‐glacial Deposits from Central M ongolia in C entral A sian O rogenic B elt: A Possible Indicator of the C apitanian Cooling Event

2012; Wiley; Volume: 62; Issue: 4 Linguagem: Inglês

10.1111/j.1751-3928.2012.00204.x

1751-3928

Tatsuya Fujimoto, Shigeru Otoh, Yuji Orihashi, Takafumi Hirata, Takaomi D. Yokoyama, Masanori Shimojo, Yoshikazu Kouchi, Hokuto Obara, Yasuo Ishizaki, Kazuhiro Tsukada, Toshiyuki Kurihara, Manchuk Nuramkhan, Sersmaa Gonchigdorj,

Geochemistry and Geologic Mapping

Abstract A dropstone‐bearing, M iddle P ermian to E arly T riassic peri‐glacial sedimentary unit was first discovered from the K hangai– K hentei B elt in M ongolia, C entral A sian O rogenic B elt. The unit, U rmegtei F ormation, is assumed to cover the early C arboniferous K hangai– K hentei accretionary complex, and is an upward‐fining sequence, consisting of conglomerates, sandstones, and varved sandstone and mudstone beds with granite dropstones in ascending order. The formation was cut by a felsic dike, and was deformed and metamorphosed together with the felsic dike. An undeformed porphyritic granite batholith finally cut all the deformed and metamorphosed rocks. LA‐ICP‐MS , U–Pb zircon dating has revealed the following 206 Pb/ 238 U weighted mean igneous ages: (i) a granite dropstone in the U rmegtei F ormation is 273 ± 5 M a ( K ungurian of E arly P ermian); (ii) the deformed felsic dike is 247 ± 4 M a ( O lenekian of E arly T riassic); and (iii) the undeformed granite batholith is 218 ± 9 M a ( C arnian of L ate T riassic). From these data, the age of sedimentation of the U rmegtei F ormation is constrained between the K ungurian and the O lenekian (273–247 M a), and the age of deformation and metamorphism is constrained between the O lenekian and the C arnian (247–218 M a). In P ermian and T riassic times, the global climate was in a warming trend from the S erpukhovian (early L ate C arboniferous) to the K ungurian long and severe cool mode (328–271 M a) to the R oadian to B ajocian ( M iddle J urassic) warm mode (271–168 M a), with an interruption with the C apitanian K amura cooling event (266–260 M a). The dropstone‐bearing strata of the U rmegtei F ormation, together with the glacier‐related deposits in the V erkhoyansk, K olyma, and O molon areas of northeastern S iberia (said to be of Middle to Late Permian age), must be products of the Capitanian cooling event. Although further study is needed, the dropstone‐bearing strata we found can be explained in two ways: (i) the U rmegtei F ormation is an autochthonous formation indicating a short‐term expansion of land glacier to the central part of S iberia in C apitanian age; or (ii) the U rmegtei F ormation was deposited in or around a limited ice‐covered continent in northeast S iberia in the C apitanian and was displaced to the present position by the C arnian.