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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
Occurrence and Lithology of Metamorphic Rocks
Distribution of metamorphic belts and roks of oceanic affinities in the pre-Tertiary metamorphic complex
Figure3. Distribution of metamorphic belts and roks of oceanic affinities
in the pre-Tertiary metamorphic complex
The Tananao Schist represents the oldest metamporphic complex in Taiwan. The total thickness of this metamorphic series cannot be measured with certainty but must be on the order of several thousand meters. Based on lithology, five cartographic or mappable units can be identified in the Tananao Schist. All the different types of metamorphic rocks in these five units will be described in this section. Lithologically and tectonically all the metamorphic rocks are divided into two metamorphic belts (Fig. 3): the western Tailuko belt and the eastern Yuli belt. Both of these belts will be discussed more fully in the later section of this chapter.

BLACK SCHISTS

The black schists or pelitic schists are the dominant schistose rocks exposed in the eastern part of the metamorphic complex. These dark schistose rocks crop out in an elongate belt from Chian, south of the city of Hualien, to the west of the Chihpen hot springs near the city of Taitung. This belt is nearly 150 kilometers long and the average width is 7-8 kilometers, reaching a maximum of 15 kilometers in the middle of the belt. The black schists occur also as interlayers of varying thicknesses in other schistose rocks in other parts of this metamorphic complex.
The belt of black schists is marked by the symbol PM5 on the geologic map. It is characterized by a monotonous lithology, composed mainly of black schists with thin to thick interlayers of greenschists. Locally there are also distinct sandy or marble layers in the black schists, each layer varying in thickness from scores of centimeters to several meters. Numerous small quartz veins penetrate the black schists in many localities. Cherty rods and lenses are also found in the black schists. The black schist is medium-to fine-grained, deep gray to blackish gray, and well foliated. The main constituent minerals include quartz, micas, chlorite, albite, sphene, and graphite, with minor amounts of epidote and pyrite. The original sediments that formed the black schists could have been shale and siltstone comparatively rich in carbonaceous materials. Some pelitic schists are characterized by the presence of abundant porphyroblasts of albite. This type of rock is especially common in the Juisui area. The black schist has been named graphitic schist by some workers, but the carbon content is generally less than 2 percent.

GREENSCHISTS

The greenschists constitute the other important member of the metamorphic complex and are exposed extensively throughout the whole length of the metamorphic belt. Although the greenschists are more abundant in the western part of the metamporphic belt, interlayers of green rocks are also common in the black schists exposed in the southeastern part. The greenschists occur chiefly as thick beds, individual layers, or lenticular bodies interbedded with other kinds of metamorphic rocks. Black schist, chert and mamorized limestone are the most common associated rocks. The greenschists are derived largely from mafic volcanic flows or pyroclastic rocks. Stretched pillows have been recognized in some greenschist outcrops. Chlorite schist is most predominant in the green-schists and is either fine-grained or medium-grained. It is deep green or dark green, exhibiting well-marked foliation. Along with abundant chlorite, it is composed of epidote, quartz, calcite, biotite, albite, and actinolite.
The thickness of individual chlorite schist layers varies from several centimenters to scores of meters, and averages 20 to 30 meters. The greenschists are important because they host massive cupriferous sulphide deposits in eastern Taiwan. In a number of places, cupriferous pyrite ores occur in association with greenschists along planes of schistosity as lenticular or bedded ore bodies.

SILICEOUS SCHISTS

The siliceous schists include quartzite, quartzite schist, meta-chert, and schistose sandstone. They are derived mainly from various kinds of sandstones and cherty rocks. The siliceous schists are gray and weather to brownish gray. Foliation is well-developed in some rocks but obscure in others. The siliceous schists are associated with greenschists and black schists in the Tailuko metamorphic belt. Siliceous schists and black schists are commonly intimately interlayered in the metamorphic terrain, and intercalations of black schists are common in the principal exposures of siliceous schists.

METAMORPHOSED LIMESTONE

Metamorphosed limestone (marble) forms a conspicuous belt in the northeastern and western parts of the metamorphic complex. The metamorphosed limestone is indicated by the symbol PM3 and is confined exclusively to the Tailuko metamorphic belt. The main limestone belt begins at Kufeng on the Suao-Hualien highway, north of the Hopingchi stream, and extends southward for nearly 150 kilometers to a point west of Kuanshan. Limestone beds and lenses extend still north of Kufeng, intercalated in the schistose rocks. The main marble outcrops there are distributed to the south of Nanao and south of Suao, where limestone quarries important for cement and fertilizer industries are abundant. There are also small streaks and lenses of metamorphosed limestone in the schists south of Kuanshan down to the upper course of the Chihpenchi stream, south of the city of Taitung. Disseminated marmorized limestone beds and lentils are also exposed in the schistose rocks west of the main limestone belt, reaching almost to the western edge of the metamorphic complex. The distribution of isolated limestone beds and lenses in the metamorphic complex is probably much more widespread than can be shown on the present geologic map. More detailed field mapping is required to locate all the scattered limestone outcrops hidden in the remote areas of the Central Range.
The best development of the metamorphosed limestone in the main limestone belt is located in the middle part of the Suao-Hualien highway between the Hopingchi stream and the city of Hualien. The maximum width of the limestone belt in this area is nearly 10 kilometers. Southward the limestone belt tapers and branches off in diminishing thickness. The limestone is usually interbedded with various kinds of schists. Individual limestone exposures vary in thickness from a few meters to several hundred meters. The metamorphosed limestone is massive or thick-bedded, ranging from fine- to coarse-grained. Three color varieties are known. Light gray to dark gray limestone is by far the most predominant type. White and black limestones are the other two types, less abundant but of higher value as dimension stone or ornamental stone. In places the limestone displays striking mottled color fabrics, bands, or streaks due to the presence of serpentinite, carbonaceous material, or other impurities. Limestone affected by igneous intrusion is strongly recrystallized.
Deformed fossil remains, including fusulinids and corals, have been found at several places in the metamorphosed limestone (Yen and others, 1951). Only a few of the organisms still show well-preserved internal structures and can be identified. The recognizable fusuline fossils were discovered in the limestone exposed at Nantzu and were identified as Schwagerina (?), Parafusulina (?), and Neoschwagreina (?). One coral specimen collected in the Mataianchi stream was identified as Waagenophyl-lum. Ail these fossils indicate a Permian age or more generally a Late Paleozoic age. These fossils yield the only age clue of the protolith of the Tananao Schist, which is, considered broadly as pre-Tertiary.
Dolomite or dolostone is often associated with the metamorphosed limestone in eastern Taiwan. The dolomite was formed by molecular replacement of calcium by magnesium during recrystallization of the limestone. All gradations between limestone and dolostone are known. The dolomite occurs as lenticular bodies, thick beds, or irregular pockets intercalated in the main limestone belt. The thickness of dolomite beds ranges from a few meters to scores of meters, and these beds vary greatly in thickness and in extension. Important dolomite deposits are found in a number of places in the metamorphosed limestone belt, but the principal economic deposits are in Chingchangshan, Hopingchi, and Mukuashan.

GNEISSES

The gneisses are exposed mostly in the northern part of the metamorphic complex, and are confined to the Tailuko metamorphic belt. They are scattered in different places between Suao and Hualien near the eastern coast. Six main gneiss bodies and a number of small gneiss lenses were first found by Yen (1954a). Several other small gneiss bodies have been discovered in later mapping. Each varies in shape, length, and width from the others. The maximum length of the largest gneiss body reaches 16 kilometers and the maximum width recorded is about 3 kilometers. The "host" lithology in which gneiss bodies are found varies widely. The two large gneiss bodies in the north are enclosed in schist and amphibolite. The other gneiss bodies in the south are mainly in contact with metamorphosed limestone. Inclusions of country rock are abundant in some large gneiss masses but scarce to absent in others. Some of the gneisses have been cut or intruded by numerous pegmatite dikes, diabase dikes, or quartz veins.
The gneisses can be differentiated into quartzofeldspathic paragneisses and metagranitic orthogneisses (Ernst et al., 1981). The metasedimentary gneisses are coarse-grained, with relict clastic textures. These rocks are characterized by the presence of abundant quartz, biotite, and albitic plagioclase. They also contain a small amount of muscovite, chlorite, rare epidote, sphene and garnet. Protoliths for the paragneisses may have included both metasandstone and metashale.
The orthogneisses are light gray with relict granitic texture and are in irregular contact with the county rock. The chemical composition indicates the magma was most likely granodiorite (Fuh, 1962). Characteristic intrusive features at the contacts include lit-per-lit injection, migmatization, and inclusions of amphibolite, schist, and paragneiss fragments. In places the psammitic gneisses grade into injection gneisses along the margins and are converted into typical migmatites. The main constituent minerals of the orthogneiss are sodic plagioclase, quartz, biotite, and muscovite, with minor chlorite, garnet, epidote-clinozoisite, sphene, and iron oxides. After the intrusion of this granite mass, pervasive greenschist fades metamorphism took place in all the rocks of the Tananao Schist. The age of intrusion of the granite will be discussed in later section.

PEGMATITES

Many irregular and lenticular pegmatite dikes have been found in the largest gneiss body, the Yuantoushan gneiss, which is located between Tungao and Suao on the eastern coast. Some pegmatites have also been found in the schists south of the main gneiss body. A minor amount of the pegmatites are located in the amphibolite north of the gneiss belt. These pegmatite dikes usually, but not invariably, cut the gneisses and schists concordantly, and are sub-parallel to the foliation of the enclosing rocks. Many pegmatites show an easterly strike, but the strike varies between outcrops. The pegmatites usually dip at rather high angles. These dikes vary in width from 10 centimeters to a maximum of 10 meters with an average of 0.5 to 1 meter. They range in length from 3 to 50 meters. The pegmatites have apparently been deformed during and after their formation, because strained minerals occur in the dikes. Most of the pegmatites originated from partial melting of metasedtments at depth. Some pegmatites in the orthogneisses could be the products of metamorphic differentiation (Chi-Chieu Chen, 1981). A total of 160 pegmatite dikes were discovered in an early exploration program (Mineral Survey Team, 1958), and more probably will be discovered by further detailed work in the potential areas. Intrusion of pegmatites and accompanying quartz veins may represent the final stage of magmatic activity related to emplacement of the granodiorite. Some pegmatites are of economic value for the production of micas and feldspars. Uranium minerals were also discovered in the pegmatites but are not of economic value.

AMPHIBOLITES

The amphibolites are distributed mainly in the northern part of the Tananao Schist, generally as large, elongate lenticular bodies. Most of these could be fault-bounded tectonic blocks. Yen (1954b) ascribed the amphibolites to one type of the greenschists. Three major amphibolite bodies were reported by him (Fig. 3). The amphibolites are medium- to coarse-grained, massive and well-foliated. The foliation plane is generally parallel to that of the surrounding schist. Major constituent minerals in the amphibolites include green hornblende, plagioclase, zoisite, epidote, and minor amounts of sphene, titanite, and rutile. The protoliths may have been basaltic flows and tuffs. The amphibolites are locally associated with serpentinite pods. This suggests that some of the amphibolites were derived from gabbro or diabase. The amphibolites may have formed during the Mesozoic (?) amphibolite fades metamorphism of the Tananao Schist.
Two lenticular bodies of amphibolite in the Suao-Nanao area have been petrographically studied by Liou and others (1981a) and Ernst and others (1981). These two bodies are bounded on both sides by high-angle faults and are in sharp contact with the marble and graphite-bearing pelitic schists. In the northern amphibolite body, pegmatite dikes and granitic layers were not found and quartz veins are relatively rare. This amphibolite body shows no apparent thermal effects of granitic intrusion.
However, the southern part of the southern amphibolite body, adjacent to an orthogneiss, exhibits intrusive features such as amphibolite interleaved with injected granitic layers. Quartz veins and pegmatitic dikes are common. Evidently strongly foliated and tightly folded amphibolitic units were intruded by granitic magma; migmatization and high-temperature alternation of amphibolites must have accompanied this intrusion. In other places, individual rotated amphibolite fragments of diverse sizes occur as xenoliths in the granitic rocks.

DIABASE DIKES

Many mafic dikes are found in the metamorphic rocks near Nanao and Suao in northeastern Taiwan. These dikes are generally a few centimeters to more than 10 meters thick and occur within amphibolites, meta-granites and pelitic schists. Attitudes vary from place to place; some are parallel to the foliation, whereas others crosscut the schistosity of the country rocks. These dikes were described as lamprophyre by Yen (1954a) but were identified as diabasic dikes by Liou (1981a) and Ernst and others (1981). The diabasic dikes are massive and lack visible foliation. Chilled margins are locally developed in the thicker dikes. Because the diabase is not foliated, the dikes must have been formed after the major orogenic deformation of the metamorphic rocks. They have been subjected to late phase greenschist-fades recrystallization, however, because they contain the greenschist-fades assemblage: actinolite, chlorite, albite, epidote, sphene, and others. Ernst and others (1981) believe that at least some of the diabasic dikes appear to represent the feeders for the basaltic rocks of the Tertiary cover formations and have thus been subjected only to the latest phase of metamorphism.

MAFIC AND ULTRAMAFIC IGNEOUS ROCKS

This litho-unit includes all the mafic and ultramafic igneous rocks of possible oceanic affinities and their metamorphic derivatives. These rocks are widely scattered in the metamorphic complex, mostly in the eastern part. The known localities are, from north to south: (1) Suao-Nanao area, (2) Fengtien area (3) Wanyung area, (4) Juisui area, (5) Yuli area, and (6) north of Litao in Taitung (see Fig. 3). Amphibolites with associated small pods of serpentinite are exposed in the Suao-Nanao area (1) in northeastern Taiwan and are perhaps the oldest rocks in Taiwan. These rocks have been discussed in the previous paragraph on "amphibolites" and will not be described here. The last area (6) is remote and geologic data are very scarce.

(1) Suao-Nanao area
According to the petrotectonic study of Liou (198 la) and Lan and Liou (1981), the ultramafites in the other four areas (2-5) are mainly tectonic fragments of oceanic affinities and are composed of ultramafites, gabbros, pillow basalts, and associated pelagic sediments. They have discordant (i.e. fault) contacts with the autochthonous country rocks (mainly pelitic schists), and these contacts have been folded during later deformation. These rocks have been extensively recrystallized and belong to six distinctive metamorphic types; greenschist, amphibolite, epidote amphibolite, glaucophane schist, metagabbro, and serpentinite.
These mafic-ultramafic rocks contain important economic resources of serpentinite, jade, talc, and asbestos; therefore the occurrence, distribution, and genesis of these rocks have been the target of great attention and active exploration programs. Better exposure and accessibility in the mining areas have encouraged extensive geologic study. Probably more mafic oceanic fragments of this category could be discovered in the remote parts of the Central Range by extensive investigations.
The origin of these tectonic oceanic blocks has been hypothesized by Liou (198 la) to be comparable to that of a metamorphosed melange complex. The oceanic fragments are interpreted to be tectonically dismembered constituents of an ophiolite suite. The pelitic schists are the metamorphosed equivalent of poorly stratified muddy matrix enclosing the exotic blocks. Under repeated deformation and metamorphism, the resultant metamorphic terrain would contain a large expanse of pelitic schistose matrix enclosing abundant metamorphosed ophiolitic fragments as we see it today. The melange may have been emplaced by tectonic processes during an ancient continent-arc collision, at a time not yet well constrained but most likely in the Late Mesozoic. These metamorphosed oceanic blocks have been subjected to polystage recrystallization and deformation. High-pressure metamorphism may have affected these blocks, as shown by the presence of glaucophane schist. All the major mafic to ultramafic igneous bodies in the Tananao Schist, except the locality (1) described earlier, are described in more detail in the following.

(2) Fengtien, Hualien
This is a well-known asbestos-talc mining area in eastern Taiwan. The deposit is about 5 kilometers west of Fengtien village and is located on a high and precipitous mountain slope. Asbestos and talc are distributed in the fissures and cracks of serpentinite layers enclosed in pelitic schist. The serpentinite layers are generally conformable to the foliation of the pelitic schist, which strikes nearly east-west and dips to the north at 20° to 30°. Seven serpentinite layers have been found in the Fengtien mining area, but only one is found in the Chingchangchi stream to the southeast (Tan et al., 1975). The main and longest layer is exposed between the Paipaochi and the Chingchangchi streams over a length of nearly 4 kilometers. All the other layers are shorter, ranging from 0.5 to 2 kilometers long. Relic gabbroid texture observed in some serpentinite suggests that it may be an altered metagabbro. Asbestos and talc generally fill cracks in the serpentinite or are found at the contact between serpentinite and schist. They are often associated with nephrite, garnet, and other minerals. The Fengtien mine is the leading producer of asbestos, talc, and nephrite in eastern Taiwan. The serpentinite belt in Fengtien extends discontinuously southward in the precipitous mountains of the Central Range, but no detailed information is available at present.

(3) Wanjung, Hualien
In the Changhanshan range, mafic rocks are exposed about 1.5 kilometers northwest of Wanjung station on the Hualien-Taitung railway (F. C. Lin, 1974). The area lies between the Mataianchi and the Wanlichiaochi streams. Alternating layers of black and green schists are the predominant schistose rocks in which several irregular bodies of serpentinite and metagabbro are emplaced. Th largest serpentinite body is 350 meters long and 20 meters wide. Another exposure is irregular, about one to six thousand square meters in areal extent. Probably more serpentinite will be discovered in the high mountains to the west, but detailed information is not available. The contact between the serpentinite and the country rocks is usually strongly sheared and fractured. Low-grade talc minerals are commonly localized in the serpentinite as well as in the chlorite schists. Good exposures of the black schists to the west have a foliation striking northeast and dipping northwest.

(4) Juisui, Hualien
Mafic to ultramafic tectonic blocks are distributed mainly in the pelitic schists exposed to the west of the village of Juisui. Recent studies of these rocks have been reported by Yang (1981) and T. P. Hsu (1983). Two large mafic blocks have been found together with many small fragments of varying sizes scattered in the schists of the Yuli belt. The largest ultramafite block, named the Tamayenshan block, is exposed northwest of Juisui, and covers an area of 20 square kilometers. The other smaller block, named the Tsenghuanshan block, has an areal extent of 3 square kilometers and is exposed about 10 kilometers west of Juisui. The general thickness of all the other small ultramafite fragments is one to five meters and the length is scores of meters. These mafic blocks consist mainly of epidote-amphibolite, amphibolite, glaucophane schist, metagabbro serpentinite and greenschist.

(5) Chingshuichi, Yuli, Hualien
Serpentinite is exposed 14 kilometers southwest of Yuli in the middle course of the Chingshuichi stream. The area is underlain mostly by pelitic schists whose foliation trends northeast and dips northwest. Green rocks enclosed in the black schists include diabase, gabbro, and peridotite; many of these rocks have been altered to serpentinite. The serpentinite is very compact, showing many different colors and textures that signify different origins. Brecciated fault features are found at the contact of the serpentinite and country rocks. The serpentinite exhibits a brilliant luster after polishing. Cracks and black spots are scarce in the rock so that the serpentinite in Yuli is known as the best building stone produced in Taiwan. The serpentinite in places is foliated, and is locally steatitized.

(6) North of Litao, Taitung
The largest mass of mafic to ultramafic rocks in Taiwan extends from Litao on the southern cross-island highway northward to the east of Hsiukuluanshan in the Central Range. As shown on the present geologic map, it forms an elongate belt, about 25 kilometers long and 1-1.5 kilometers wide, that closely follows the contact between the Eocene slate and the pre-Tertiary metamorphic complex. Due to inaccessibility and inadequate mapping, little is known about this belt of mafic rocks, and there is some doubt about whether it is as extensive or continuous as shown on the present map. Petrographic descriptions of this belt are not available. More field mapping is needed to study the distribution, field relations, and nature of this belt of mafic rocks in the Central Range.

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