Geology of the summit limestone of Mount Qomolangma (Everest) and cooling history of the Yellow Band under the Qomolangma detachment

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Abstract:

Abstract 

Newly discovered peloidal limestone from the summit of Mount Qomolangma (Mount Everest) contains skeletal fragments of trilobites, ostracods and crinoids. They are small pebble-sized debris interbedded in micritic bedded limestone of the Qomolangma Formation, and are interpreted to have been derived from a bank margin and redeposited in peri-platform environments. An exposure of the Qomolangma detachment at the base of the first step (8520 m), on the northern slope of Mount Qomolangma was also found. Non-metamorphosed, strongly fractured Ordovician limestone is separated from underlying metamorphosed Yellow Band by a sharp fault with a breccia zone. The 40Ar–39Ar ages of muscovite from the Yellow Band show two-phase metamorphic events of approximately 33.3 and 24.5 Ma. The older age represents the peak of a Barrovian-type Eo-Himalayan metamorphic event and the younger age records a decompressional high-temperature Neo-Himalayan metamorphic event. A muscovite whole-rock 87Rb–86Sr isochron of the Yellow Band yielded 40.06 ± 0.81 Ma, which suggests a Pre-Himalayan metamorphism, probably caused by tectonic stacking of the Tibetan Tethys sediments in the leading margin of the Indian subcontinent. Zircon and apatite grains, separated from the Yellow Band, gave pooled fission-track ages of 14.4 ± 0.9 and 14.4 ± 1.4 Ma, respectively. These new chronologic data indicate rapid cooling of the hanging wall of the Qomolangma detachment from approximately 350°C to 130°C during a short period (15.5–14.4 Ma).

Keywords: 40Ar–39Ar age; 87Rb–86Sr isochron age; Mount Everest; Qomolangma detachment; Yellow Band; fission-track age

Document Type: Research Article

DOI: http://dx.doi.org/10.1111/j.1440-1738.2005.00499.x

Affiliations: 1: Department of Earth Sciences, Kyushu University, Ropponmatsu, Fukuoka 810-8560, Japan ( ), Email: hsake@scs.kyushu-u.ac.jp 2: Kanto Gakuen University, Ohta-shi, Gunma 373-8515, Japan, 3: Earthquake Research Institute, University of Tokyo, Yayoi, Tokyo 113-0032, Japan, 4: Kyoto Fission-Track Co. Ltd, Ohmiyaminami Tajiri, Kyoto 603-8832, Japan, 5: Institute of Geology, Chinese Academy of Science, Dewai Qiliahouzi, Beijing 100029, China and 6: Nanjing Institute of Geology and Paleontology, Academia Sinica, 39 East Beijing Road, Nanjing 210008, China

Publication date: December 1, 2005

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