Metamorphic Events and Tectonic Evolution
Both the pre-Tertiary metamorphic basement and the Tertiary cover sequences have been subjected to
multi-stage deformation and recrystallization. Due to the tectonic complications and limited accessibility
in rugged mountainous terrain, a full elucidation of the metamorphic episodes and tectonic evolution of the
metamorphic basement has been quite difficult. However, through the strenuous efforts of many workers in
recent years, preliminary understanding of the evolutionary history of metamorphism and deformation of the
metamorphic rocks has been attained (Ernst and others, 1981; Liou, 1981 a and b; Ernst, 1983; Chao-Hsia Chen
and others, 1983). These studies have identified the major metamorphic mineral assemblages, determined
controlling factors of metamorphism, and mapped distribution of metamorphic fades. The principal metamorphic
events, their timing or sequence, and their significant characteristics have been more or less recognized
and delineated on the basis of stratigraphic relations, mineral chemistry, petrotectonic features and
radiometric age data. Because of repeated deformation and insufficient dating data, however, the discussion
of deformational and metamorphic events below is still quite generalized, and speculative in part. This
summary can serve as a basic reference useful for further extensive investigations, which will hopefully
provide more complete interpretations in the future.
An integrated investigation of the Phanerozoic metamorphism in Taiwan was recently made by Liou and Ernst
(1984). From geochronologic and other petrographic studies of mineral assemblages of the cover and basement
rocks, they have delineated three stages of metamorphic recrystallization to show the relationship of
metamorphism to tectonic evolution. These major episodes were then reviewed and interpreted in terms of a
speculative tectonic model. Although not necessarily synchronous, the three metamorphic episodes are: (I)
formation of the Late Mesozoic Tananao paired metamorphic belts of high T/P (Tailuko) and high P/T (Yuli)
types; (II) late Miocene (?) blueschist/greenschist metamorphic overprint on the Yuli rocks, possible
greenschist-fades recrystallization of the Tailuko rocks, and the two-stage ridge-crust and ocean-floor
metamorphism of the East Taiwan Ophiolite; and (III) Plio-Pleistocene collision-type progressive
metamorphism of both the basement and cover strata of the Asian plate.
MESOZOIC METAMORPHISM
This stage of metamorphism affected mainly the metamorphic complex of the Tananao Schist and can be further
divided into three deformational and recrystallization events.
| 1. Amphibolite fades metamorphism |
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At the northern tip of the basement complex in the Nanao-Suao area, exotic blocks of strongly foliated
amphibolites have been intruded by granitic rocks of about 87 m.y. apparent age. The amphibolites and
associated host rocks of marble and schists constitute the oldest metamorphic rocks in Taiwan. Geochemical
characteristics suggest that the protoliths of the amphibolites could be oceanic tholeiites.
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| 2. Late Mesozoic migmatization and high T/P arc metamorphism |
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The Tailuko metasedimentary rocks and amphibolites were intruded by calc-alkaline granitic rocks in Late
Mesozoic time. Effects of the granitoid emplacement include thermal metamorphism, localized migmatization,
and thermal progradation of amphibolites and gneisses to upper amphibolite fades conditions. This was
followed by Late Cretaceous (?) deformation and greenschist metamorphism of the granitic rocks and the
Tailuko metasedimentary rocks.
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| 3. Late Mesozoic subduction zone metamorphism |
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Concurrent with arc metamorphism in the Tailuko belt, rocks of the Yuli belt were undergoing subduction zone
metamorphism, resulting in high P/T metamorphic effects. The protolith of the Yuli rocks could have been a
melange deposit containing oceanic blocks in a pelitic matrix. Both the pelitic matrix and the exotic blocks
have been subjected to repeated deformation and re crystallization. About 80 m.y. ago, the blocks and matrix
were subjected to epidote-amphibolite fades metamorphism along a west- dipping subduction zone.
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PALEOGENE AND MIOCENE METAMORPHISM
Since the formation of a Late Mesozoic paired metamorphic belt in the Tananao Schist, the region has been
subjected to granitic intrusion and repeated greenschist fades recrystallization. This is shown by the
radiometric age dates that range, from 60 to 35 m.y. The earlier greenschist fades assemblages have been
heavily overprinted, however, by the Late Cenozoic collisional metamorphic event. Differentiation of the
various greenschist episodes is difficult.
A younger episode of subduction zone metamorphism caused major recystal-lization in the Yuli melange unit
approximately 8 to 14 m.y. ago. The most significant feature is the formation of glaucophane. This is the
main stage of blueschist/greenschist metamorphism mentioned previously.
PLIO-PLEISTOCENE COLLISION-TYPE METAMORPHISM
About 4 m.y. ago in Plio-Pleistocene time, collision between the Eurasian continent and the Luzon arc of the
Philippine Sea plate took place. Both the basement complex and the cover strata on the passive continental
margin of Taiwan have been significantly affected by the collision-type progressive metamorphism. The
metamorphic grade increases gradually eastward from the western foothills to the western Central Range and
then toward the basement complex on the eastern flank of the Central Range.
Table 2 summarizes the radiometric age data of the three important metamorphic stages in Taiwan given by
Liou and Ernst (1984).
| Taible 2. Main stages of metamorphism in Taiwan |
| Metamorphism |
Lithostratigraphic
Unit and Belt |
Age (m.y.) |
References |
Cretaceous(?)
Amphibolite Facies |
Suao-Nanao Amphibolite Tailuko Belt |
87
(K-Ar, Hb ?) |
(5) |
| Cretaceous migmatization High T/P metamorphism |
1. Suao-Nanao Pegmatite Tailuko Belt |
87 +/- 5
(K-Ar,muscovite)
72-86
(K-Ar,muscovite) |
(1)
(5) |
| 2. Suao-Nanao Hb Gneiss Biotite migmatite Tailuko Balt |
60
(K-Ar,biotite) |
(5) |
Cretaceous
Epidote amphibolits
High T/P arc metamorphism |
Juisui Tectonic block Yuli Belt |
76 +/- 7 |
(3) |
Oligocene(?)
High T/P arc metamorphism |
1. Suao-Nanao migmatized amphibolite Tailuko Belt |
35-40
(Rb-Sr, minerals) |
(4) |
| 2. Suao-Nanao orthogneiss |
35-37
(Rb-Sr, minerals) |
(4) |
| 3. Suao-Nanao Pagmatite |
40
(Rb-Sr, minerals) |
(4) |
| 4. Suao-Nanao orthogneiss Tailuko Belt |
39 +/- 0.8
(biotite) |
(2) |
| 5. Tungao-Nanao orthogneiss |
35
(K-Ar, biotite) |
(5) |
| 6. Nanao gneiss |
33 +/- 2
( biotite) |
(1) |
Miocene
Greenschist facies |
1. Tailuko Chipan paragneiss
Tailuko Belt |
9.7 +/- 2
(K-Ar, biotite) |
(2) |
2. Tailuko Chipan paragneiss
Tailuko Belt |
9
(K-Ar, biotite) |
(5) |
| 3. Meta-diabase (?) |
13
(K-Ar, biotite) |
(5) |
Miocenne
Blueschist / Greenschis facies |
1. Juisui tectonic block Yuli Belt |
8 - 14
(Rb-Sr, minerals) |
(3) |
| 2. Mica-amphibotite schis Yuli Belt |
6
(K-Ar, muscovite) |
(1) |
Pliocene-Holecene
Collision-type metamorphism |
1. Chipan oaragneiss Tailuko Belt |
2.6 - 6.5 |
(3) |
| 2. Juisui tectonic block Yuli Belt |
4.6 +/- 0.6 |
(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 |
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