Permian peri-glacial deposits from central Mongolia in Central Asian Orogenic Belt: A Possible Indicator of the Capitanian Cooling Event

A dropstone-bearing, Middle Permian to Early Triassic peri-glacial sedimentary unit was first discovered from the Khangai-Khentei Belt in Mongolia, Central Asian Orogenic Belt. The unit, Urmegtei Formation, is assumed to cover the early Carboniferous Khangai-Khentei 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 ²⁰⁶Pb/²³⁸U weighted mean igneous ages: (i) a granite dropstone in the Urmegtei Formation is 273 ± 5Ma (Kungurian of Early Permian); (ii) the deformed felsic dike is 247 ± 4Ma (Olenekian of Early Triassic); and (iii) the undeformed granite batholith is 218 ± 9Ma (Carnian of Late Triassic). From these data, the age of sedimentation of the Urmegtei Formation is constrained between the Kungurian and the Olenekian (273-247Ma), and the age of deformation and metamorphism is constrained between the Olenekian and the Carnian (247-218Ma). In Permian and Triassic times, the global climate was in a warming trend from the Serpukhovian (early Late Carboniferous) to the Kungurian long and severe cool mode (328-271Ma) to the Roadian to Bajocian (Middle Jurassic) warm mode (271-168 Ma), with an interruption with the Capitanian Kamura cooling event (266-260 Ma). The dropstone-bearing strata of the Urmegtei Formation, together with the glacier-related deposits in the Verkhoyansk, Kolyma, and Omolon areas of northeastern Siberia (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 Urmegtei Formation is an autochthonous formation indicating a short-term expansion of land glacier to the central part of Siberia in Capitanian age; or (ii) the Urmegtei Formation was deposited in or around a limited ice-covered continent in northeast Siberia in the Capitanian and was displaced to the present position by the Carnian.