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
||82.5 - 86.5
|Muscovite in Pegmatite
||74.5 - 86
|Biotite in Migmatite
||58 - 64
|Migmatite, Gneiss, pegmatite
||39 - 40
|Bioite in Gneiss
||30 - 39
|Actinolite in Schist
||32.1 - 33.9
|Actinolite in Metamafics
||24 - 29
|Biotite in Gneiss
||6.5 - 9.6
|Granitic Metamorphics, Gneiss
||2.6 - 6.5
||8 - 14
||6 - 10
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.
|(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|
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.
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.
|80-90 Ma||Granitic intrusion in Taiwan: regional metamorphism I (Nanao Orogeny)|
|35-40 Ma||Continental rifting and opening of South China Sea: regional metamorphism II|
|0-10 Ma||Arc-continent collision: regional metamorphism III (Penglai Orogeny)|