Rates of cooling and denudation of the Early Penglai Orogeny, Taiwan, as assessed by fission-track constraints

As an attempt to define the timing of the peak temperature of the Penglai Orogeny and estimate the early cooling and denudation rates of the Taiwan mountain belt, fission-track dating of zircon and apatite was conducted on several key metamorphic rock samples. The zircon fission-track ages for the metasandstone clasts, collected from the central and northern parts of the Coastal Range, were determined as 4.0+/-0.5 and 3.6+/-0.3 Ma, respectively. Both of the clasts were identified as from the ancient Central Range which was metamorphosed at temperatures high enough to cause a complete reset of the zircon fission-track system. During 1-2 Ma, they were exhumed and deposited in the Coastal Range basin due to the later Penglai Orogeny of Taiwan. Obviously, they have not further been annealed since their deposition in the Coastal Range. The difference between the above fission-track ages and the stratigraphical age of the host sedimentary formation represent the ancient cooling ages when they were exposed on the early Central Range. These ancient cooling ages are comparable with the zircon fission-track age of a present-day outcrop of the Tananao Schist, 1.8+/-0.2 Ma. This accordance implies that at ca. 5 Ma the northern and central parts of the Central Range achieved the peak temperature of the Penglai Orogeny and then they began to emerge above sea level. Accepting this scenario, we calculate the rates of denudation and cooling of the Central Range to be ca. 2.5-4.6 mm yr(-1) and similar to 120 degrees C m.y.(-1), respectively, for the last 4 Ma. (C) 2000 Elsevier Science B.V. All rights reserved.