In order to decipher the origin and tectonothermal history of the Kuncha nappe, we undertook a geological investigation in the Taplejung window in eastern Nepal, and carried out multichronological analyses of zircon, apatite, and mica of the Kuncha Formation and Taplejung granites. Three granite bodies that intrude into the Kuncha Formation show fission-track (FT) ages of 6.2 to 4.8 Ma for zircon and 2.9 to 2.

No abstract

Zircon OD-3 from the Paleogene Kawamoto Granodiorite (Mihara body) in Japan has been identified as a potential multi-grain secondary standard for U-Pb dating. We have carried out an inter-laboratory evaluation in order to evaluate possible heterogeneity amongst the OD-3 zircon grains. U-Pb ages were obtained using two analytical techniques (a sensitive high-resolution ion microprobe and a laser ablation-inductively coupled plasma-mass spectrometry) in eight laboratories. All the 238 U-206 Pb ages show good agreement, with an overall weighted average 238 U-206 Pb age of 33.0 Ϯ 0.1 Ma (2s). The U-Pb age results revealed no significant variation or heterogeneity in the U-Pb ages of the OD-3 grains. Twelve fission-track (FT) ages from three laboratories are also reported, and have a weighted average of 32.6 Ϯ 0.6 Ma (2s). Despite the different closure and annealing temperatures of the U-Pb and FT chronometers, respectively, the FT age is in good agreement with the U-Pb age. This suggests that the OD-3 zircon had a relatively fast cooling history and has not experienced later thermal annealing. The chronological dataset reported here clearly demonstrates that the OD-3 zircon could be a useful and reliable secondary standard for use during U-Pb dating studies of Cenozoic zircons.

The chronology of the World Heritage Site of Sangiran in Indonesia is crucial for the understanding of human dispersals and settlement in Asia in the Early Pleistocene (before 780,000 years ago). It has been controversial, however, especially regarding the timing of the earliest hominin migration into the Sangiran region. We use a method of combining fission-track and uranium-lead dating and present key ages to calibrate the lower (older) Sangiran hominin-bearing horizons. We conclude that the first appearance datum for the Sangiran hominins is most likely ~1.3 million years ago and less than 1.5 million years ago, which is markedly later than the dates that have been widely accepted for the past two decades.

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 40 Ar-39 Ar 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 87 Rb-86 Sr 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 .

A detailed paleomagnetic study conducted in the Sangiran area, Java, has provided a reliable age constraint on hominid fossilbearing formations. A reverse-to-normal polarity transition marks a 7-m thick section across the Upper Tuff in the Bapang Formation. The transition has three short reversal episodes and is overlain by a thick normal polarity magnetozone that was fission-track dated to the Brunhes chron. This pattern closely resembles another highresolution Matuyama-Brunhes (MB) transition record in an Osaka Bay marine core. In the Sangiran sediments, four successive transitional polarity fields lie just below the presumed main MB boundary. Their virtual geomagnetic poles cluster in the western South Pacific, partly overlapping the transitional virtual geomagnetic poles from Hawaiian and Canary Islands' lavas, which have a mean 40 Ar/ 39 Ar age of 776 ± 2 ka. Thus, the polarity transition is unambiguously the MB boundary. A revised correlation of tuff layers in the Bapang Formation reveals that the hominid last occurrence and the tektite level in the Sangiran area are nearly coincident, just below the Upper Middle Tuff, which underlies the MB transition. The stratigraphic relationship of the tektite level to the MB transition in the Sangiran area is consistent with deep-sea core data that show that the meteorite impact preceded the MB reversal by about 12 ka. The MB boundary currently defines the uppermost horizon yielding Homo erectus fossils in the Sangiran area.paleomagnetism | marine isotope stage 19 | magnetostratigraphy | geochronology

No abstract

No abstract