Fission-Track Ages of Apatite from Tachosui gneiss and their Geologic Implications

• Main Authors: Cheng-Yin Wu, 吳承穎
• Other Authors: Tsung-Kwei Liu
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/28090862141975998846

碩士 === 國立臺灣大學 === 地質科學研究所 === 97 === Taiwan is located in a remarkable tectonic setting at the convergence between the Luzon arc and Eurasian plate. In order to understand the thermal history of the northern Tananao Schist of East. Taiwan, since the Penglai Orogeny, a total of nine bedrock samples were collected from the Tachosui Gneiss along the Hopin forest trail as a vertical transect within an area of few km2. The samples were analyzed by fission-track dating technique. The results and the localities of the sub-vertical section allowed us to discuss the relationship between elevation and apatite fission-track ages. The apatite fission-track ages of the nine samples range from 0.6±0.1Ma to 0.3±0.1Ma and display unimodal distributions, implying these bedrock samples were totally annealed during the Penglai Orogny. It suggests that the northern Tananao Schist had been subjected to a higher temperature and then cooled down below the closure temperature of the apatite fission-track system which probably initiated at ~0.6 Ma. The positive correlation between apatite fission-track ages and sampling elevations implied that the bedrock of the studied area was close to exhume steady state. In this study, two different uplift rates of the studied area based on different approaches can be obtained. The denudation rate of 2.2mm/yr was estimated by the difference in elevation divided by the difference in apatite fission-track ages between two samples; the other estimation involves assumption of geothermal gradient and closure temperature which gives uplift rates in the range of 4-9 mm/yr. Combined with the previous zircon and apatite fission-track studies, these data demonstrate that the cooling rates have continuously increased since the Penglai Orogeny. Key word: Tananao schist, Tachosui gneiss, fission track, apatite, denudation rate