Stratigraphy and Geologic Ages
In addition to the difficult terrain and inaccessibility, this metamorphic belt presents all the problems of
stratigraphic correlation and terminology characteristic of a metamorphic complex. It is difficult to
subdivide the rocks stratigraphically because they lack diagnostic fossils for correlation and radiometric
age dates are scarce. In addition, the original sequence of the various protoliths cannot be adequately
determined. Several stratigraphic names of the Tananao Schist were proposed earlier by Yen (1960) based on
interpreted protoliths. He defined four "protolith" formations in the Tananao Schist as follows:
| Yuli Formation: |
coarse-grained sandstone containing little shale, basic tuff and serpentinite |
| Tailuko Formation: |
divided into two facies
Tungao facies -- sandstone, shale, basic lava tuffs, limestone, chert and serpentinite
Tachinshui facies -- mainly limestone containing a little tuff and chert
|
| Kanagan Formation: |
arkose and coarse-grained sandstone with intercalated fine-grained sandstone, shale, and limestone |
| Sanchui Formation: |
divided into two fades
Gong facies -- sandstone, shale-tuff-limestone
Raushi facies -- mainly limestone containing a little sandstone, shale and basic tuffs
|
The basis of the above classification is lithologic rather than stratigraphic, because the stratigraphic
relations among these units cannot be accurately established. No type section or stratotype of the units has
been given and thus the thickness of the units is uncertain and has been only vaguely estimated. The upper
and lower mapping boundary of each formation cannot be dearly defined so that these units cannot be
adequately used in field mapping or map compilation. Stratigraphic classification such as this naturally
involves some assumptions regarding the age, structure, and sequence of the rocks and a great deal of
uncertainty and guesswork cannot be avoided. The age of each formation was determined largely by lithologic
comparison with the metamorphic rocks of known ages in the Sanbagawa Group exposed in Japan. Further
evidence is needed to verify the validity of this correlation because Taiwan and Japan are widely separated
geographically and markedly different in geologic background.
Recent study of the metamorphic complex in eastern Taiwan by the students and professors of the Geology
Department of the University of Taiwan proposed a new stratigraphic scheme (Chiming Wang-Lee, 1982) of the
Tananao Schist. This stratigraphic classification is based on the metamorphic rocks in the basement complex
rather than on the interpreted protoliths as in the scheme of Yen and others (1960). The stratigraphic names
and localities were derived mostly from the central cross-island highway in the Tailuko area. They are as
follows:
| Yuli Formation: |
mainly quartz-mica schist and mica schist with intercalated greenschist and serpentinite, locally one layer of spotted schist. |
| Changchun Formation: |
greenschist, thin-bedded limestone, metachert and amphibolite |
| Chiuchu Formation: |
massive to thick-bedded marble |
| Kanagan Formation: |
migmatite and gneiss |
| Tienhsiang Formation: |
quartz-mica schist, phyllite and metasandstone |
All the rock units in the above stratigraphic system are ascribed to Permian or Late Paleozoic age. This is
contrary to the general belief that Mesozoic rocks are also involved in the Tananao Schist. Due to the lack
of fossil evidence, the age of the metamorphic rocks has to be determined rather subjectively and is often
based mainly on personal judgments. The Changchun Formation and the Chiuchu Formation in Wang-Lee's scheme
are reported to be correlated with the Tailuko Formation in Yen's scheme; the Tienhsiang Formation of
Wang-Lee may be partly correlative to the Sanchui Formation of Yen. An unconformity reported by P. Y. Chen
and others (1979) between the Changchun Formation and the Chiuchu Formation has been based on the
interpretation of a chloritoid schist layer as a fossil soil horizon.
The two stratigraphic schemes of the Tananao Schist suggested by Yen (1960) and Wang-Lee (1982) are based
mainly on the sections observed in the drainage area of the Tailuko stream in northern Taiwan, where thick
and well exposed marble units and gneiss bodies serve as good lithologic markers. To the south, these marble
and gneiss units thin out or disappear, however, and the application of these stratigraphic names becomes
quite uncertain. In addition to the rapid north-south fades change, problems in correlation are increased by
the lack of fossil evidence, the uncertainty in stratigraphic sequence, and the complicated structural
features. For these reasons, some local ion-national names of the Tananao Schist have been proposed by
Stanley and others (1981) during their field study along the southern cross-island highway. The correlation
of these units with those proposed by Yen and Wang-Lee in the north is inevitably conjectural. As this case
shows, it is extremely difficult to establish a well-documented stratigraphic classification that can be
applied unequivocally throughout the entire metamorphic complex.
Because there is no single stratigraphic system that is readily applicable to the whole metamorphic belt,
the revised map (1986) shows the Tananao Schist divided into four lithologic units following the older maps
in principle. The subdivision is based mainly on difference in lithology and not on interpreted ages. The
scheme of subdivision follows that shown on the 1953 map of Chang, but the distribution and subdivision of
the units have been modified, on the basis of more recent mapping data from many sources. The schists
constitute the dominant rock type in the Tananao Schist. They are divided into two units with different map
patterns. One unit (PM4) is composed mainly of pelitic schist with interbeds of greenschist and minor
amounts of siliceous schist. The other unit (PM3) is made up of interlayed pelitic schist, greenschist, and
siliceous schist. These two groups of rocks represent two different metamorphic belts, and the distinction
between the two may be of regional tectonic significance in the interpretation of the metamorphic rocks of
eastern Taiwan. A third unit consists of the metamorphosed limestone (PM2). Two remaining units in the 1974
map are composed of gneisses which are divided into migmatite and gneisses, but they are combined into one
unit (PM1) on the revised map. Mafic and ultramafic rocks in the metamorphic complex are shown as an igneous
rocks map unit.
Over most areas the exact age of these metamorphic rocks cannot be determined because fossils are poorly
preserved or are nearly absent. The deformed Permian fossils mentioned above indicate that at least a part
of the protolith of the Tananao Schist is of Late Paleozoic age (Yen and others, 1951). This is corroborated
by Sr isotope analyses of the marble by Jahn and others (1984), suggesting that carbonate deposition must
have taken place prior to 190 Ma and most likely in the Late Paleozoic to Early Mesozoic. The Tananao Schist
is thus inferred to be of Late Paleozoic to Mesozoic age or, more generally, pre-Teriary. Although some
attempts were made on the earlier maps to assign different ages to the different rocks in this metamorphic
complex (see 1953 map), these ages were assigned largely by inference. Because of the lack of convincing
evidence, no attempt is made on the present map to differentiate the ages of the various metamorphic rocks.
|
