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:::
Introduction
Earlier Geologic Maps of Taiwan
Geographic Setting
General Geology And Geologic Provinces Of Taiwan
Explanation Of Legend And Representation Of Geologic Data
Eastern Central Range
Western Central Range Backbone Ridges
Western Foothills
Eastern Coastal Range
Geology Of The Hengchun Peninsula
Major Geologic Features Of Taiwan
Plate Tectonic Setting
References


:::Eastern Central Range
Introduction Occurrence and Lithology of Metamorphic Rocks Stratigraphy and Geologic Ages Radiometric Ages Metamorphic Belts Structural Features and Orogenic Events Metamorphic Events and Tectonic Evolution
Radiometric Ages
Because the metamorphic complex is poorly dated paleontologically, isotopic dating is the principal means to study the magmatic activities, metamorphic episodes, and crustal movements in the metamorphic rocks. As radiometric dating laboratories were not operating in this country when this text was prepared, most geo chronologic analyses have been produced in the laboratories in other countries. Significant progress on radiometric dating of the metamorphic rocks have been made in recent years with the cooperation of U.S.A. and French scientists (Yen and Rosenblum, 1964; Juan, Chow and Lo, 1972; Jahn and Liou, 1977; Jahn, Liou and Nagasawara, 1981,; Jahn and others, 1986; Juang and Bellon, 1986). These isotopic studies have produced a seemingly complex span of dates ranging from 90 to 5 m.y. Most of the samples dated thus far, however, have been collected in limited areas restricted to localities of easy accessibility. The present data do not adequately cover the entire extent of the metamorphic terrain, or all significant rock types. Additional radiometric dates of critical rock types and careful stratigraphic analysis will no doubt clarify and modify our present knowledge of the geologic development of the metamorphic complex to a considerable degree.
On the other hand, the known radiometric dates do suggest ages for the major episodes of metamorphism and deformation of the metamorphic complex, although somewhat hypothetical. A brief discussion is presented in the last section of this chapter, "Metamorphic events and tectonic evolution." Table 1 is a summary of the published isotopic ages of the metamorphic rocks in the basement complex. This was prepared by Juang and Bellon (1986) with slight modification for publication in this text.
Taible 1. Radiometric ages of metamorphic rocks of Taiwan
Lithotectonic Ragion rock/Mineral Sample Age (m.y.) References
Tungao-Nanao
(Tailuko Belt)
Amphibolite 82.5 - 86.5 (5)
Muscovite in Pegmatite 74.5 - 86 (1)(5)
Biotite in Migmatite 58 - 64 (5)
Migmatite, Gneiss, pegmatite 39 - 40 (4)
Bioite in Gneiss 30 - 39 (1)(2)(5)
Actinolite in Schist 35 (5)
Diabase 32.1 - 33.9 (5)
Metadiabase 13 (5)
Chipan-Tailuko
(Tailuko Belt)
Actinolite in Metamafics 24 - 29 (5)
Biotite in Gneiss 6.5 - 9.6 (5)
Granitic Metamorphics, Gneiss 2.6 - 6.5 (4)
Juisui-Yuli
(Yuli Belt)
Epidote Amphibolite 79 (3)
Glaucophane Schist 8 - 14 (3)
Mica Schist 6 - 10 (5)
Epidote Amphibolite 4.6 (3)
(1) Yen & Rosenblum (1964), K-Ar dating
(2) Juan et al. (1972), K-Ar dating
(3) Jahn & Liou (1977), Jahn et al. (1981), Rb-Sr dating
(4) Jahn et al. (1986), Rb-Sr dating
(5) Juang & Bellon (1986), K-Ar dating
The radiometric ages can be correlated with the important events of igneous intrusion, metamorphism and orogenic movement in the metamorphic complex. These events generally took place over long spans of geologic time, however, which is difficult to appreciate from the isotopic geochronology.
On the basis of the isotopic data summarized above and other information, Jahn and others (1986) postulated three major geologic events in the crustal evolution of Taiwan.
80-90 MaGranitic intrusion in Taiwan: regional metamorphism I (Nanao Orogeny)
35-40 MaContinental rifting and opening of South China Sea: regional metamorphism II
0-10 MaArc-continent collision: regional metamorphism III (Penglai Orogeny)
In addition, they suggested that during 240-200 Ma, carbonates and clastic sediments of the Tananao Schist were deposited on oceanic crust. This may have been the beginning of crustal history of Taiwan.


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