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Geochronology of the Tananao Schist complex, Taiwan, and its regional tectonic significance
rectonophysics, 125 (1986) 103-124 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
103
GE~CHRONOL~Y OF THE TANANAU SCHIST COMPLEX, TAIWAN, ANT) ITS REGIONAL TECTONIC SIGNIFICANCE
B.M. JAHN, F. MARTfNEAU, 3.3. PEUCAT and J. CORNICHET C.A. E. S. S. $W. R. S.}, Universitk de Rennes, insiitut de Gkoiogie, Campus de Beaulieu, 35042 &nnes (France)
(Revised version received June 7, 1985; accepted September 17, 1985)
ABSTRACT
Jahn, KM., Martineau, F., Peucat, J.J. and Coruichet, J., 19%. Geochrondogy of the Tananao Schist complex, Taiwan, and its regional tectonic si~ficance, In: J. Angeher, R. Blanchet, C.S. Ho and X. Le Pichon (Editors), Geodynamics of the Eurasia-P~Iippine Sea Plate Boundary, ~ecro~o~hy~~c~, 125: 103-124. Isotopic analyses (Rb-Sr, U-Pb, Sm-Nd, K-Ar) on rocks and minerals of the Tananao Schist complex (the Taituko-Tienhsiang and the Nanao areas of eastern Taiwan) have yielded significant new age data corresponding to several important geologic events in the crustal evolution of Taiwan. The ages and corresponding events are summarized as Follows:
Crustal history O-IQ Ma: Arc-continent collision; regional rnet~o~~srn III (Pencil Orogeny). 35-40 Ma: Continental rifting and opening of the South China Sea; regional metamorphism fl. 80-W Ma: Granitic intrusions in Taiwan; regional metamo~~sm I (Nan90 Orogeny}. Overlapped with the most important world-wide, particularly circnm-Pacificz. thermal events of 90-110 Ma (Jahn, 1974; J&n et al., 1976). 200-240 Ma: Deposition of carbonates and elastic sediments, probably in a geosynclinal environment. Beginning of the crustal history of Taiwan. Pre-crustai history 500-650 Ma (or older): Separation of prototiths for the granitoids of Taiwan from a chondritic (or depleted mantle) reservoir. 1000-1700 Ma: Crystallization of zircons, of which some grains have survived aad been finally incorporated in the young (ca. 90 Ma) granitic magmas.
The youngest Rb-Sr mineral isochron ages of granite and paragneiss samples have been used to estimate an uplift rate of the Central Range (or more precisely, the Tauanao Schist complex). It is found that a very high uplift rate of at least 3-4 mm/yr, comparable to that of the Himalayas, has persisted since at least 3 Ma ago as a result of the collision of the Luzon Arc with the Eurasian plate.
~-19~~/8~/$03.50
0 X986 Elsetier Science Publishers B.V.
I04
22’
PHILIPPINE SEA
18’
14’
10’
6’
Fig. 1. Ceotectonic
setting
1981). Thin arrows
indicate
and tectonic
elements
in the Taiwan-Philippine
slip vectors of shallow earthquakes
region (after
Lin and Tsai.
along the plate boundaries
INTRODUCTION
The island of Taiwan
is situated
at the eastern
margin
of the Eurasian
continental
plate. The interaction of the continental lithosphere with the Philippine Sea plate is manifested by the mountain ranges and structural trends in the island. The lithospheric plate boundary here is defined by the Ryukyu trench to the northeast, the Lon~tudinal Valley on the island, and the Manila trench to the south (Fig. 1). The Coastal Range, on the east of the Valley, is an exotic island arc terrane (the Luzon arc) recently collided with and attached to the main part of Taiwan (e.g., Ho, 1982). Except for the Coastal Range, the entire island can be viewed as a thick pile of geosynclinal sediments which have been complexly folded, variably metamorphosed, rapidly uplifted and denuded, and in places, intruded by talc-alkaline and tholeiitic magmas. The eastern flank of the Central Range consists of a suite of metamorphic rocks including schists, marbles, amphibolites and granitic gneisses. These rocks together constitute the oldest crystalline basement of Taiwan, which is termed the Tananao Schist complex (Yen, 1954, 1960, 1967). Because the knowledge of crustal development of Taiwan hinges on a better understanding of the Tananao Schist complex, it
105
becomes imperative to engage in a systematic study of the nature and origin of their protoliths, as well as the chronological order of tectono-thermal events registered in rocks and minerals. In recent years, comprehensive studies of the metamorphic complex regarding petrography, mineral paragenesis, petrochemistry, mineral chemistry, structural analysis and tectonic models have been published (J.C. Chen, 1977a, b; C.H. Chen, 1979; Wang, 1979; Wang-Lee, 1979; Suppe, 1980; Weber-Diefenbach et al., 1980; Chu and Shieh, 1981; Ernst et al., 1981; Lan and Liou, 1981; Liou, 1981; Liou et al., 1981; Lo and Wang-Lee, 1981; Lu and Wang-Lee, 1981; Yang, 1981; Wang-Lee et al., 1982; Ernst, 1983; Ernst and Hamish, 1983). On the other hand, discussion of petrogenetic processes and crustal evolution using trace elements (p~ticularly, rare earth elements or REE) distribution and isotopic techniques has been very limited (Jahn and Liou, 1977; Chu and Shieh, 1981; Jahn et al., 1981). This article presents new results of multi-chronometric study using four isotopic systems (Rb-Sr, U-Pb, Sm-Nd, K-Ar) in rocks and minerals from the Tananao Schist complex. The basic principle of radiometric dating can be found in Faure (1977). The new data will be used together with the few published geochronological results to address the problem of crustal development in Taiwan and its regional tectonic significance.
GENERAL
GEOLOGY
OF THE SAMPLING
LOCALITIES
According to Yen (1954, 1960, 1967), the Tananao Schist complex of eastern Taiwan is divided into a western Tailuko belt characterized by open folds, and an eastern Yuli belt exhibiting folds overturned towards the east. The two juxtaposed lithologic assemblages were thought to represent paired metamorphic belts (Yen, 1963). Although glaucophane schists occur in the Yuli belt, it has been argued from geochemical and geochronological data that they represent exotic tectonic blocks that are not necessarily indicative of a high P/low T environment for the in-situ metamorphic rocks of the entire Yuli belt, hence the paired metamorphic concept of Yen (1963) may not be warranted (Jahn and Liou, 1977; Jahn et al., 1981). All analyzed samples of the present study come from two areas within the Tailuko belt: (1) the Nanao area, and (2) the Tailuko-Tienhsiang area (Fig. 2). The Tailuko belt generally consists of a variety of schists and marbles, together with minor amount of granitic gneisses (orthogneiss + paragneiss), amphibolites, metabasalts and serpentinites. Different rock types are commonly interlayered and only principal rock types are shown in Fig. 2. The general structural trend of the Tailuko belt is NNE-SSW. However, in the Nanao area, the structure is deflected to an E-W direction, probably as a result of the northerly subduction of the Philippine Sea plate. Based on recent seismic studies, an active convergent boundary exists about 50 km to the south and east of the Nanao area, which connects the kyukyu Trough with a probable transform fault (Tsai et al., 1977; Lin and Tsai, 1981; see Fig. l),
,43 c7
Fig. 2. a. Simplified from
Chu
82-postfix
and
omitted)
are the drainage b. Simplified central
geologic map of the Nanao
Shieh,
1981 and
Orthogneiss
1==I
Paragneiss
to area A of the index map (modified
area, corresponding
et al., 1981).
Sample
localities
are shown by solid dots. Heavy line represents
and
the Su-Hua
numbers
(BJ-prefix
Highway
and finer lines
and
system (rivers and tributaries).
geologic
map of the Tailuko-Tienhsiang
E-W cross island highway.
corresponds
Ernst
Schist.marble
L;;I1
Sample localities
area (after Ernst, and numbers
1983). Heavy line represents
the
are shown by solid dots. This map area
to area B of the index map (inset of Fig. 2a).
The Tananao Schist complex has undergone several periods of metamorphism (Wang, 1979; Wang-Lee, 1979; Ernst et al., 1981; Liou, 1981; Liou et al., 1981; Lo in the and Wang-Lee, 1981; Wang-Lee et al., 1982; Ernst, 1983). Moreover, Tailuko-Tienhsiang area, the grade of metamorphism increases towards the east; the crystalline limestone (marble) seems to be the oldest in stratigraphic sequence among the supracrustal rocks (Yen, 1960; C.H. Chen, 1979; Ernst, 1983). According to Liou (1981) and Ernst et al. (1981), the regional metamorphism of Taiwan probably began in Early Mesozoic time and is continuing episodically today (Table 1). Distinct
107
TABLE
1
Regional
metamorphic
episodes
of Taiwan
(based on Liou, 1981; Ernst et al., 1981)
Stage
Time
Grade
1
early Mesozoic
lower(?)
thick eugeosynchnal
amphibolites,
(?)
amphibolite
deposit
basic rocks transformed
mid- to late
upper amphibolite
Probable
Product
cause
from oceanic crust
facies 2
Mesozoic
metaultra-
(corresponding the Nanao
amphibolite
to
Orogeny)
facies
metasediments,
migmatites,
granite intrusions,
facies to anatexis +
retrograded
contact
basalt dike intrusions
and
rocks.
(Tertiary?)
retrograde metamorphism Plio-Pleistocene
3
greenschist
Collision
(blueschist)
Luzon Arc with the
of the
facies
Asian Plate
greenschist
facies rocks.
(blueschist
facies rocks).
( = Penlai Orogeny) 4
not specified
development
zeolite facies
of zeolite
facies minerals Note: The postulated
events mineral
times (periods)
for individual
stages are speculative.
(or episodes) have been recognized through studies of petrography chemistry, but the precise timing of individual events was not delineated.
ANALYTICAL
and
PROCEDURES
Sample preparation Whole-rock
(WR) powder
samples were prepared
blocks or pieces using a combination
of hydraulic
from a split of about press, jaw crusher
1 kg fresh
and agate ball
mill. All powdered samples are finer than 75 ~1. Mineral samples except zircons were extracted using mainly a Frantz “Isodynamic” magnetic separator and occasionally heavy liquids.
For zircon extraction,
about 5 kg of rock sample was crushed
to pass
size 60 mesh (250 p). Heavy minerals were first concentrated using a vibrating Wilfley table. Zircons were then separated from other heavy minerals by a Frantz separator and heavy liquids. The zircons were further divided into four or five size fractions and the final purification was achieved by hand-picking. Chemical separation For Rb-Sr analyses, spikes of Rb and Sr were added to the samples before digestion in HF + HClO, acids. Rb and Sr were separated by conventional ion exchange
chromatography
using Dowex AG 50X-8, 200400
mesh resin. The chem-
1ox
ical procedures (1973). Zircons isolated
for U and Pb isolation are basically those employed by Krogh were dissolved in HF i- HNO, in teflon bombs and U and Pb were
by ion exchange
procedures
in mini-columns.
can be found
isotope analysis,
in Peucat
separation
The
details
of the LJ-.Pb analytical
et al. (1981). For REE concentration
procedures
have been described
and Nd
in Jahn et al. (1980a. b).
Mass unulysis
Rb, Sr, U, Pb, and REE (Sm-Nd) concentrations were determined by the isotope dilution method. Mass analyses were performed using a Cameca THN-206 mass spectrometer
{nicknamed
Ag~anonix)
for Rb and
Sr, and
a Cameca
TSN-206
(Id&fix) for U, Pb, Sm and Nd. The Sr sample was loaded on a single oxidized Ta filament and the Rb was loaded on a single non-oxidized Ta filament. For UPb analysis,
the Pb was loaded
on a single Re filament
with silicagel
and phosphoric
acid, whereas the U was loaded on a single oxidized Ta filament and run as UO: species. For Sm-Nd concentrations, a double filament mode (Re central-Ta lateral) was employed, whereas a triple Re filament mode was used for Nd isotopic composition analysis. Total blank for Rb = 0.1 ng, Sr = 1 ng, Pb < 1 ng, U = 0.1 ng, Sm = 0.2 ng and Nd <( 1 ng. Analytical errors were 2% for X7Rb/8hSr, 0.2% for 207Pb/206Pb, less than 2% for U/Pb and 0.5% for ‘47Sm/‘44Nd ratios. The values of common Pb isotopic compositions used in the correction procedures were: 206Pb/2wPb
= 18. 207Pb,‘204Pb = 15.5, “‘Pb/’ 204Pb = 37. The decay constants used in the age computations are: s7Rb = 1.42 x 10-r’ yr-‘; ‘38U = 1.55125 X 10-I” yr ‘I; ztsU = 9.8485 x 10’ r0 yr GEOCHRONOMETRIC
K-Ar
‘.
RESULTS
results
The results of K-Ar
TABLE K-Ar
‘; ‘*‘Sm = 6.54 X 10. I7 yr
measurements
are presented
in Table 2.
2 age results of the Tanano
Sample no.: Analysis
no.
Mineral analyzed Weight (9) K (%)
Schist complex,
Taiwan
BJ-124-82
BJ-132-82
BJ-138-82
BJ-144-82
R8448
R8447
R8450
R8449
biotite 0.108732
biotite 0.100975
hornblende 0.110583
0.116812
hornblende
5.33
5.38
0.17
0.19
3.8925
31.139
1.1311
2.9795
Aratm (%)
75.38
1.81
87.63
73.03
4”Ar*/40K
0.0002447
0.001939
0.002229
0.005254
Age (Ma)
4.2
33.1
38.0
88.2
40Ar* (lO--”
mol/g)
Analyst:
Songshan
X(@K)=
5.543~10-‘~
Wang, yr-‘;
Institute
of Geology,
e = 0.581 x~O-”
Academia yr-‘;
Sinica,
‘%/K=1.167~10-~.
Beijing,
China.
Constants
used:
109
lib- Sr results
The Rb-Sr results of whole-rock samples and mineral separates are presented in Table 3. The results are further displayed in a series of isochron diagrams (Fig. 3). An isochron constructed from data of metamorphic minerals usually reflects the time when the minerals attained isotopic equilibration. This commonly corresponds to a metamorphic event and particularly reflects the waning stage when temperature dropped enough (blocking temperatures) to close the Rb-Sr isotopic systems in individual minerals. The blocking temperatures are variable, ranging from 200” to 4OO*C, and depending on mineral species, mineral chemistry, grain size, total pressure, and cooling rate (see Dodson, 1973; Jager, 1977). For three granitoid samples from the Tailuko area near Chipan (two granitic gneisses and one paragneiss), the mineral isochron ages are very young, varying from 6.4 to 2.5 Ma (Figs. 3a, b, c). This age range is similar to the Rb-Sr mineral isochron age of 4.6 Ma for an epidote amphibolite from the Yuli belt (Jalm et al., 1981). IS-Ar biotite ages of 9.7 and 9.0 Ma have been reported for paragneiss (Juan et al., 1972) and a granite (Juang and Bellon, 1984) from the Tailuko area. Moreover, new K-Ar biotite age of 4.2 Ma has been determined on granitic BE12482 which yields a Rb-Sr isochron age of 3.5 Ma (see Table 2 for the K-Ar results). In view of the unusual reverse discrepancy between the younger Rb-Sr and the older K-Ar ages for the same samples (Nos. 123 and 124), we suspect that there were small quantities of inherited excess Ar in these biotite samples. It is also recognized that a metamorphic episode may last for more than 10 Ma, hence the small variation in age as shown in Figs. 3a, b, and c needs not suggest different periods of metamorphism. Rather, it implies different responses of mineral isotopic systems (i.e., isotopic equilibration, blocking temperature, and uplift or cooling rate) to the same metamorphic episode. At any rate, we interpret the age of 7 to 3 Ma (and probably 10 Ma to the present time) as the latest metamorphic episode, presumably related to the collision of the Eurasian and the Luzon Arc of the Philippine Sea plate. This tectono-thermal event is also termed the Penlai Orogeny (Ho, 1982). On the other hand, the Rb-Sr mineral isotopic data for six samples from an abandoned granite quarry and its vicinity about 4 km northwest of Nanao (Fig. 2a) yield consistent isochron ages of 35-40 Ma, regardless of rock types. The outcrop relations suggest that the massive granite (of granodioritic composition) is intruded by basaltic (or diabasic) dikes, and both are in turn cut by pegmatite dikes. Thus geologically the pegmatite is the youngest among the trio. The isotopic results yield a somewhat reversed age patterns (Figs. 3d-h) as opposed to the field relationship. However, this situation should not cause any serious concern, because the measured ages do not indicate the intrusive order, but merely reflect various mineralogical responses to isotopic re-equilibration as a result of regional metamorphism. Figure 3i shows a mineral isochron for an amp~bolite enclave in granite. The occurrence of the enclave clearly suggests that the formation of amp~bolite and its
TABLE Rb-Sr
3 isotopic
Sample no.
data for rocks and minerals Rock type
Whole-rock (WR) or mineral
from the Tananao
Schist complex,
Rb
Sr
(ppm)
(ppm)
“Rb ~ &%r
Taluan K7Sr
+2 om
?r
fraction BJ-123-82
granite
WR
136
biotite
557
feldspar BJ-124-82
granite
WR
127
biotite
617
feldspar BJ-125-82
paragneiss
4.93
6.81
WR
139
biotite
591
feldspal
8.91
178 14.8 40.2 185 13.4 50.7 160 8.44 47.0
2.211
0.71167
9
0.72146
19
0.356
0.71162
Y
1.996
0.71115
21
0.71752
21
0.389
0.71087
6
2.506
0.71145
9
0.71867
109.0
133.8
202.8 0.549
0.71143
19 7
BJ-130-82
metabasic
rock
WR
18.5
504
0.106
0.70527
6
BJ-131-82
metabasic
rock
WR
19.0
333
0.1655
0.70527
I1
BJ-132-82
granite
87
318
WR biotite plagioclase
BJ-133-82
granite
WR
101 367
basalt
WR
(or diabase)
biotite plagioclase amphibole
BJ-135-82
2.44
biotite plagioclase BJ-134-82
364
basalt
WR
(or diabase)
biotite plagioclase
3.18 40.0 306 4.57 17.1 70 312 18.4 22.5
BJ-136-82
pegmatite
biotite feldspar
BJ-137-82
amphibolite
WR biotite
BJ-138-82
amphibolite
398 5.12 32 310
plagioclase
5.93
amphibole
6.27
5.33 332 288 9.74 311 264 7.95 535 21.45 209 6.11 569 22.2 8.57 542 169 9.55 195 36.8
0.791 199.7 0.0213
0.70838
8
0.81243
40
0.70764
12
0.70859
10
0.76209
50
0.0296
0.70776
6
0.438
0.70488
13
0.76600
22
1.010 109.59
112.15 0.0247
0.70488
2.305
0.70560
0.974
0.70655
7
0.78410
22
0.0937
0.70575
6
2.932
0.70636
5
148.6
0.78428
21
0.0273
0.70791
8
0.554
0.70757
12
0.76096
21
0.088
0.70701
18
0.4928
0.70723
19 7
135.4
94.55
WR
3.7
146
0.0737
0.70355
plagioclase
4.28
323
0.0383
0.70301
7
amphibole
1.80
0.434
0.70447
60
11.9
111
TABLE
3 (continued) Rock type
Sample no.
Whole-rock
Rb
Sr
(WR) or mineral
(ppm)
+2 urn
“Rb
“Sr
(ppm)
86Sr
s
241
0.043
0.70645
8
393
0.3507
0.70857
7
7.4
209
0.103
0.70469
5
24.9
205
0.350
0.70497
20
1.132
0.70430
19
158
0.260
0.70454
4
307
0.0744
0.70363
20
0.299
0.70421
13
fraction BJ-139-82
amphibolite
WR
BJ-142-82
amphibolite
WR
BJ-143-82
amphibolite
WR
3.6 48
plagioclase
3.78
amphibole BJ-144-82
amphibolite
WR
14.2
plagioclase
7.89
amphibole
1.84
9.65
17.8
basaltic protolith must predate the granitic intrusion. The Rb-Sr data yield an age of 40 Ma, indistinguishable from the ages of the trio mentioned above. Evidently the amphibolite
and the enclosing
granite
were metamorphosed
during
the same period.
From Figs. 3d-i, the consistent age patterns (35-40 Ma) obtained from various rock types of distinct intrusive order must be indicative of an important thermal episode (see also Table 2 for complementary K-Ar results). We interpret that the period of 35-40 Ma represents yet another metamorphic event which is unrelated to the most recent plate collision (the Penglai Orogeny). It is, however, quite likely that the thermal episode was related to the opening of the South China Basin during the late Eocene-early Oligocene (Ludwig et al., 1979; Taylor and Hayes, 1982). It should be noted that in all sinchron diagrams shown above, complete isotopic re-equilibrium has not been achieved during metamorphism either in the 35-40 Ma or in the < 10 Ma period. Since biotite is so much more radiogenic than other minerals
or WR samples,
different
data will make little difference vary considerably. among coexisting
choice of isochron
in the ages obtained
calculation
involving
but the calculated
biotite
errors
can
Nevertheless, we believe that incomplete isotopic re-equilibration mineral phases is probably a rather common phenomenon in
young and very active erogenic belts. Similar observations have been previously reported for glaucophane schists of the Yuli belt (Jahn and Liou, 1977; Jahn et al., 1981). Plagioclase and amphibole were separated from three amphibolite samples from the Nanao area. Due to the unfavorable range in Rb/Sr ratios and probable incomplete isotopic re-equilibration, no significant isochron ages have been obtained (Fig. 3j). However, K-Ar analysis of a hornblende sample of BJ-138-82 yielded an age of 38 Ma, whereas the result on another hornblende sample of BE144-82 gave an age of 88 Ma (Table 2). In addition,
three coherent
K-Ar
hornblende
ages of 83-87
II’
Ma have been reported
by Juang and Bellon (1984) for the amphibolite
samples
were also analyzed
in this work by the Rb-Sr
method.
K--Ar
(84 Ma) and
(40 Ma) ages for sample
deserves
hornblende attention.
This amphibolite
the Rb-Sr enclave
is entirely
The discrepancy embedded
that
between
the
BJ-137-82
in granite.
Alkali
113
0
0.2
0.4
0.6
0.0
1.0
(kl
1.2
B’Rb/=Sr 0.706 0
0.5
1.0
1.5
2.0
2.5
s7Rb/aaSr Fig. 3. a-i. Mineral
isochrons
in the boxes were calculated level. Biotite-WR P = plagioclase,
of various rock samples. using the method
ages are given in parentheses HB = hornblende,
All ages (T) and initial s7Srp6 Sr ratios (I) shown
of York (1966) and all regression on the isochrons.
Symbols:
errors are quoted
WR = whole-rock,
at 20
B = biotite,
F = feldspar.
j. Isotopic
compositions
of WR and mineral
k. Isotopic
compositions
of WR samples
fractions
of granitic
of three amphibolite rocks including
samples.
a paragneiss
(No. 125). Two lines of
165 Ma and 90 Ma are shown for reference.
metasomatism is shown by the presence of biotite, especially in the contact zone. This sample has been identified as a metamorphic product of a typical N-type MORB, in view of its REE pattern (Jahn, unpublished) and Nd isotopic data (see Table 5) but it has abnormally high contents of K,O (LlSW), Rb (32 ppm) and Ba (71 ppm) as compared to a typical N-type MORB. Because hornblende is the most resistent of the common minerals to Ar loss (Hart, is likely that the hornblende K-Ar age represents
1964; Hanson and Gast, 1967) it the time of a previous metamor-
phism
metamorphic
whose
age is not
reset
by the younger
episode of 35-40 Ma. Consequently, the coherent Ma) for all amphibolite samples probably represent in rocks and minerals later when dealing
of the Tananao
with the U-Pb
(and
metasomatic)
K-Ar hornblende ages (80-90 a third thermal event registered
Schist complex.
We will return
to this point
ages.
Data of granitoid WR samples including a paragneiss are shown in Fig. 3k. If all granitoids were cogenetic and the analyzed samples had remained closed through later metamorphic episodes, then the whole-rock data might define an isochron yielding their primary emplacement age. The data of the four granitoid samples are found to lie on or about a reference isochron of 165 Ma. Surprisingly, the age of 165 Ma is exactly the time of important granitic intrusions in SE China (Jahn, 1974; Jahn et al., 1976). Perhaps this is too coincidental and we do not believe at present that the age of 165 Ma represents the time of granitic magmatism in Taiwan for the following reasons: (1) the new U-Pb zircon ages to be presented below; and (2) these granites were probably produced by partial melting of sedimentary source(s) as
(ppm)
fractions
7.44
9.12
452
467
69-80
80-105
4.76
341
337
316
45-80
80-100
s 105
4.33
4.64
4.66
Sample BJ-132X? C 37 333
8.30
7.58
466
45-53
(ppm)
Pb
Sample BJ-123-82 +-z37 480
(r)
U
Size
U-Pb isotopic data
TABLE 4
791
1182
1436
640
103
338
163.8
0.06698
0.06093
0.05925
0.07184
0.08720
0.10378
0.14414
0.08846
*06Pb
‘“Pb
432
207Pb
‘Oi,Pb
Measured ratios
0.01609
0.1543 ~
0.1386
0.1353
0.1591
0.1360
0.1906
0.3072
0.01317
0.01383
0.01398
0.01410
0.02098
0.01862
0.01620
0.09208
0.09259
0.09462
0.09528
0.19344
0.15710
0.12429
0.12176
235 ”
0.1678
“‘Pb
__-
20bPb
--
*‘s Pb
-
________-__I__.... Calculated ratios
-.-
Pb
0.04849
0.04856
0.04907
0.04900
0.06686
0.06118
0.05562
0.05487
‘06
1°‘Pb
.x-- ~_
-_
88
x9
90
90
134
119
104
103
89
90
92
92
180
148
119
117
Apparent ages (Ma)
123
127
151
148
834
646
43x
407
115
0.3
0.2
20*Pt? 238 U
0.1
0
f
0
Fig. 4. U-Pb
concordia
2
diagram
3
4 5 207pb+U
for granite
sample
6
BJ-123-82
7
8
from Chipan,
Tailuko
area. The discordia
has a MSWD = 0.46.
evidenced from their geochemical characteristics (J.C. Chen, 1977b; Jahn, unpublished) and oxygen isotope compositions (Chu and Shieh, 19Sl), consequently, they are likely to have had heterogeneous initial *‘Sr/s’ Sr ratios. U-Pb
results
U-Pb zircon data for two granite samples, BJ-123-82 from Chipan, near Tailuko, and BE13282 from Nanao, are presented in Table 4, and are further displayed in concordia diagrams (Figs. 4 and 5). In all size fractions, U concentrations are rather homogeneous with about 450-480 ppm for one sample and 320-340 ppm for another. Pb contents vary from 7.44 to 9.72 ppm for BE123-82 and 4.3-4.8 ppm for BJ-132-82 (Table 4). The analyses clearly demonstrate that all data points are highly discordant but well aligned on a discordia. The discordia of BJ-123-82 from Chipan defines a precise lower intercept age of 90.3?::! Ma and an upper intercept age of about 1700 Ma. Similarly, the analytical data of the Nanao granite define again a precise lower intercept age of 85.8’::: Ma and an upper intercept age of about 1000 Ma (Fig. 5). In both cases, the high U/Pb* ratios (Table 4) of the zircon fractions suggest that most radiogenic Pb ( = Pb*) was produced quite recently. This leads us to interpret the ages of about 90 Ma as a period of granitic intrusions in Taiwan. In our opinion, it is quite improbable that the radiogenic Pb* composition has been modified significantly by recent diffusion, especially for the young zircons. We note that very similar zircon U-Pb isotopic discordancies and intercept ages were observed in the Kaniksu Batholith of southeast British Columbia (Archibald et al., 1984) and in some granitic batholiths of Malaysia (Liew and McCulloch, 1985). This important tectono-thermal event at 90-100 Ma (Late Cretaceous) is a world-wide phenomenon, especially in the circum-Pacific regions (Jahn, 1974; Jahn
BJ-132-82
I
0.2 206pb* 238
U 0.1
0 0
1
2
3
4
5
6
7
8
207Pb* 235~ /
Fig. 5. U-Pb
concordia
diagram
for granite
sample
BJ-13242 from Nanao
(MSWD
= 0.1 I )
et al., 1976). Evidently a supracrustal sequence including amphibolite protoliths could have been metamorphosed during this period along with the granitic intrusions in the Tananao Schist complex. In the coastal areas of southeast China, much of Jurassic
granitoids
(T=
765 Ma) have also been metamorphosed
(90-110 Ma), accompanied by new granitic and pegmatite intrusions. As shown in Table 4, the 707Pb/2”6Pb ages of individual fractions vary from 407 to 834 Ma. This demonstrates Pb component. age of ancient
the presence
in this period for BJ-123-82
of an ancient
radiogenic
The upper intercept age of about 1700 Ma may correspond zircons. It could equally have no precise geologic significance
to the if the
analyzed zircons were a mixture of zircon populations of different ages (older and younger than ca. 1700 Ma, see Peucat, 1983, for model interpretation). Nevertheless, the very old upper intercept age indicates clearly that ancient crustal materials have been recycled before the granitic intrusion at 90 Ma ago. Moreover. the provenance of these zircons is probably from mainland China where crust-forming events in the Proterozoic times have been abundantly recorded in crystalline rocks. On the other hand, the young *“Pb/ 206Pb ages for BJ-132-82 (Table 4) suggest that ‘ancient radiogenic
Pb component
is much reduced.
Sm- Nd results The results of Sm-Nd isotopic analyses are presented in Table 5. The biotite granite (No. 123) and the paragneiss (No. 125) have almost identical overall major element contents, REE patterns and isotopic (Sr and Nd) compositions, suggesting a genetic relation between them. The Nd isotopic data show that all fNd (0) values are negative (see the definition of eNd in the footnote of Table 5). This indicates a derivation of these granitoids from crustal sources characterized by light REE
5
2.44
1.68
granite
diabase
amphibolite
amphibotite
ampbibolite
BJ-132-82
BJ-134-82
BJ-137-82
BJ-138-82
BJ-143-82
similar
between
TSN 206.
were determined
1 and our Cameca
compositions
Mantle),
using the Minmass
(DM = Depleted
(t4’Sm/‘“Nd)crtUn I
Reservoir.
*
1 Spectrometer
at the University
= 0.1967;
0.513068
0.513213
0513150
0.512709
0.512402
0.512333
0.512390
‘43 Nd,?Nd
(‘47Sm/‘“Nd)CHUR
Uniform
0.2175
where:
= Chondritic
4.67
0.2201
0.2076
11.49 6.70
0.1425
0.1221
0.1204
0.1212
Taiwan
14.71
31.82
27.84
29.52
(‘43Nd/‘MNd)CHt,R
and CHUR
3.47
6.43
5.55
5.92
nM = 0.51315 (or end = + 10)
for 7&.,
(‘47Sm/‘“Nd),pte-
(‘43Nd/‘44Nd),,r,,e-
equation
1+
the Minmass
a Nd isotopic
(‘43Nd/‘~Nd)
and
T. ...+
Model ages:
= 0.51264,
are as follows:
enclave
where ( ‘43Nd/ ‘44Nd)cHUR
* Symbol definitions
3.95
paragneiss
dike
granite
BJ-125-82
@pm)
BE123-82
Nd
@pm)
Schist complex,
Sm
Rock type
data for rocks from the Tananao
Sample No.
Sm-Nd isotopic
TABLE
-195 6992 3699
+1.3 t 9.9 +11.2 +8.3
32 a 30 24 a 35 a
of Minnesota
X(‘47Sm)=
(July 1984). No instrumental
= 0.2137;
487
- 4.6
19
(14’Sm/ l”Nd)oM
614
3114
SOS
- 6.0
TCNVR
- 4.9
c,,@)
28
20
220,
TDM
Ga-‘;
bias was found
0.00654
1944
1244
1333
1251
(Ma)
enrichment (T,X,‘., protolithic
in their REE distribution
see the footnote materials
chondritic
mantle
patterns.
of Table 5). ranging
except
reservoir
recycled
The Sm-Nd
chondritic
model
from 487 to 614 Ma, signify
zircon
grains
have
been
at least 500 Ma ago, and probably
separated
ages
that the from
in Late Precambrian
times. However, depleted
in view of the increasing
mantle
as principal
sources
number
of isotopic
arguments
in favor of
for young
granitoids
(DePaolo,
1980. 1981;
Liew and McCulloch, 1985) depleted mantle model ages (T,,,, see footnote of Table 5) might be more significant than TC.r,rIK in terms of crustal formation ages. The calculated TDM ages for three granitoids. ranging from 1244 to 1333 Ma. are comparable with the upper intercept ages of zircons, and hence reinforce the probability of Precambrian ages for the protoliths of the granitoids. Two amphibolites
(Nos. 138 and 143) and a metasomatized
amphibolite
enclave
(No. 137) have highly positive The Sm-Nd interpretation
Ed,, values ranging from + 8.3 to + 11.2 (Table 5). isotopic data and unpublished REE patterns have confirmed the of ophiolitic origin of the amphibolites (Liou et al.. 1981). but the very
high %tJ R ages suggest that their original modified.
REE patterns
could have been slightly
DISCUSSION
Geologic events established by the isotopic data
Radiometric ages registered by various isotope clocks in rocks and minerals from the Tananao Schist complex are summarized in Fig. 6. The crustal history of Taiwan was probably initiated with geosynclinal deposition of carbonates and elastic sediments at the Asiatic continental margin. This early sedimentary deposition is believed to have taken place in Late Paleozoic-Early Mesozoic time, based on the occurrence
of fossil remains
supported
by the new Sr isotopic
Available Taiwan
from crystalline
isotopic data (U-Pb, were transported
data
Sm-Nd,
eastwards
limestone of marble
Rb-Sr)
(Yen et al., 1951) and further samples
(Jahn
et al., 1984).
suggest that the elastic sediments
of
from the Chinese mainland.
In Fig. 6 are shown several well recognized tectono-thermal events and metamorphic episodes recorded in the Tananao Schist complex. The polymetamorphic nature of the complex has earlier been delineated by petrological/mineralogical studies and structural analyses (see Table 1; Yen, 1967, 1976; Ernst et al., 1981; Liou, 1981; Lo and Wang-Lee, 1981; Wang-Lee et al., 1982; Ernst, 1983, among others). It appears that the three tectono-thermal events at T = O-10, 35-40, and 80-90 Ma are well established. It is possible that a fourth metamorphic episode in Early Mesozoic as depicted in Table 1 might have occurred, but it has yet to be demonstrated by future isotopic age studies.
119
EVENT
METHOD
AGE,Ma
Pb-Sr Min.,K-Ar Bio.
Regional Mets. III. Arc-continent collision: formation of glaucophane sch. (Yuli)
Granite, paragneiss -Tailuko
Rb-Sr
Min.,
K-A.r
Bio.
v,a, peg., amphibaIite enclave- Nanao
Rb-Sr Min., amph,-Yuli K-Ar WR-amphibolites -LJ-~b zircon Rb-Sr Min., granites SE Chi.na
m-sr
WR-granites SE China
Regional Hats. II. Continental rifting and formation of South China Sea.
Very important circum-Pacific thermal event! granitic intrusion into supracrustal sequence; metamorphism of basaltic basement. Regional Meta. I.
(Event to be found in Taiwan)
Sr isotopic ratios in carbonates
Beginning of crustal evolution in Taiwan, Carbonate and elastic sediments deposited on oceanic crust at continental margin.
Sm-Nd model
Separation of protoliths for granitoids from a chondritic mantle reservoir
ages
i TCK”R )
or from a depleted mantle reservoir.
J an-m3
model ages
CT
DM
1
Crystallisation
of
zircons.
isome
U-Pb
Fig. 6. Important
tectono-thermal
new mult~-~~ro~orn~t~c
age data.
zircon
-
)’
events for the Tananao
zircon grains have survived through cycles of geologic processes and finally incorporated in young granitic maomasl
Schist complex
of Taiwan
as revealed
by the
121)
The period of 3540 China
Basin
opening
(Fig.
Ma is tentatively
6). although
recent
of the basin was probably
initiated
Hays, 1982). In fact, the middle important India
period
and
Asia
Hawaiian-Emperor
of global and
correlated sea-floor
that
the and
to late Eocene
the change
in
data
of the South
slightly later at 32 Ma B.P. (Taylor
suggest
time has been recognized
plate reorganisations
hot spot (Taylor
with the opening magnetic
including
Pacific
plate
and Hays,
the collision
motion
with
to be an of greater
respect
to the
1982; Seno and Maruyama.
1984).
Furthermore, in the tectonic evolutionary scenario of the South China Basin as proposed by Taylor and Hayes (1982). the talc-alkaline magma intrusions of the SO-90 Ma period would strongly
suggest that the inferred
volcanic
to mid-Cretaceous
arc during
mid-Jurassic
pre-opening
Andean-type
period he positioned
to the east
of Taiwan. In so far as the pre-crustal history is concerned. two broad events are recognized for the first time in Taiwan. It is important to know that the protoliths of the young granites
of ca. 90 Ma probably
Precambrian times (Sm-Nd grains that were crystallized
had resided
in a crustal
environment
since
model ages) and they contained some inherited even earlier in Middle Proterozoic times.
It is extraordinary to observe minerals (biotites) of the granitoids
Late zircon
the very young Rb-Sr ages for metamorphic from the Tailuko area. The youngest ages of a
paragneiss and a granite (ca. 3 Ma) can be used to estimate the minimum rate of uplift since the minerals attained chemical and isotopic equilibrium. Through study of mineral chemistry and elemental partitioning between coexisting phases, Ernst (1983) estimated that physical conditions of metamorphic recrystallisation with were: T = 425 ?I 75OC, P = 4 kbar (ca. 13 km) for the moderate uHZo and ace Tailuko
area. Blocking
temperatures
for the Rb-Sr
system in biotite
from the Alps
has been estimated at about 300°C (Jager, 1979; Dodson, 1973). which would correspond to a depth of about 9 km. An uplift of 9 km within about 3 Ma yields an average net uplift rate of about 3-4 mm/yr. the compensation
of weathering
This rate is considered
and denudation
as a minimum
is taken into account.
if
At any rate,
this high uplift rate is comparable with that for the very recent period (1500-8500 yrs B.P.) as determined by the radiocarbon method on raised coral reefs and marine shells (Bonilla, 1977; Peng et al., 1977). The present average denudation rate of the Central Range of Taiwan has been estimated as at least 1.4 mg cm- * yr ‘, which is probably the highest known value in the world (Li, 1976). This high denudation rate is thought to be a direct consequence of the arc-continent collision. Most recently, Liu (1982) studied fission track ages on various minerals (apatite, zircon, sphene) and concluded that the Central Range has been even more rapidly uplifted at a rate of 8 to 10 mm/yr during the last 0.6 Ma. We propose here that uplift of at least 3-4 mm/yr,
which is comparable
to that
121
of the Himalayas (Chugh, 1974), probably began during the initial collision of Philippine Sea plate with the Eurasian continental lithosphere, that is, at least 3 B.P. The emplacement of glaucophane schists in the Yuli belt is consistent with high uplift rate of Taiwan over the past 10 Ma (Jahn and Liou, 1977; Jahn et 1981).
the Ma the al.,
CONCLUSIONS
(1) The polymetamorphic nature of the Tananao Schist complex has been confirmed by the new age data. Important tectono-thermal events registered in rocks and minerals of the complex are shown in Fig. 6. (2) The crustal history of Taiwan is believed to have begun with elastic and carbonate sedimental deposition, and the later emplacement of op~olitic materials, now represented by various schists, marbles and amphibolites of the Tananao Schist. The deposition probably took place about 200-240 Ma ago, based on the fossil record (Yen et al., 1951) and Sr isotopic compositions of crystalline limestones (Jahn et al., 1984). (3) Two to three pre-crustal geological events have been revealed for the first time by the Sm-Nd and zircon U-Pb isotopic analyses. The presence of inherited zircons in the young granites from the Tailuko and Nanao areas strongly suggests that ancient crustal components (T = 1700-1000 Ma) have been incorporated in the generation of young granitic magmas. These old zircons probably were transported eastwards from China’s ancient cratons. The Sm-Nd data also suggest that the bulk of granite protoliths was separated from a chondritic or depleted mantle reservoir in middle to late Precambrian times. (4) The very young Rb-Sr mineral isochron ages are used to estimate uplift rate of Taiwan, or more precisely, of the Tananao Schist complex. A very high uplift rate of at least 3-4 mm/yr, comparable to that of the Himalayas, has persisted since at least 3 Ma ago, as a result of the collision of the Luzon Arc with the Eurasian continental plate. ACKNOWLEDGEMENTS
The senior author is most grateful to Professor C.S. Ho for arranging the logistic support during his field trip in Taiwan. Very competent assistance of Ruey-chang Jeng (Central Geological Survey of Taiwan) and Chung-hua Chen (Academia Sinica, Taipei) in the sample collection in November, 1982, is deeply appreciated. We thank Songshan Wang (Academia Sinica, Beijing) for K-Ar age determinations, D. Hermitte for mineral separation and Nicole Morin for Rb-Sr and REE chemical separation. We are indebted to Char-yu Lu for his initial help in the sample preparation and scientific discussions. This article has been improied by Professor Gary Ernst of UCLA. The senior author dedicates this article to Professor C.S. Ho
122
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Hoping-Chipan Weber-Diefenbach, Central
characteristics
Range,
Wang.
eastern
Neues Jahrb.
rocks
m
on Taiwan.
Mem. Geol. Sot.
analysis
of metamorphic
rocks,
Loshao--Taiiuko
Lo, C.H.. Taiwan.
1982. Polymetamorphism Acta Geol. Taiwan.
in some
K.. 1980. The greenstones
Geol. Pal’aontol.
of the Tananao
gneiss
bodies.
21: 122- 139. of the Tananao
Schist Belt,
Abh.. 160: 258-285.
Schist at Juisui.
Hualien.
Ph.D. Thesis, National
Univ. (in Chinese). study
Bull. Geol. Surv. Taiwan. of the Tananao
Schist
5: l-100.
in northern
Taiwan.
Bull. Geol.
Surv.
12: 53-66.
Yen. T.P.. 1963. The metamorphic
belts within
the Tananao
Schist terrain
of Taiwan.
Proc. Geol. Sot.
6: 72-74.
Yen, T.P.. 1967. Structural
analysis
Yen, T.P., 1976. Geohistory
of the Tananao
of Taiwan.
Schist of Taiwan.
Proc. Geol. Sot. China,
Yen. T.P.. Sheng, C.C. and Keng, W.P.. 1951. The discovery complex
volcamc
3: 24992.52.
K., Cheng, Y.M. and Schmidt. Taiwan.
Yen, T.P.. 1954. The gneisses of Taiwan.
China,
of Cenozotc
basement
notes on the time relations
Y.. Yen, T.P.,
area. Hualien,
Yen, T.P.. 1960. A stratigraphic Taiwan,
dating
of the Tertiary
Mem. Geol. Sot. China,
Yang. C.N.. 1981. Petrofabric Taiwan
1983. K-Ar
3: 138-145.
area. eastern Wang-Lee,
D.Z.,
areas. Sci. Geol. Sin.. 3: 2055216.
of Taiwan,
Bull. Geol. Surv. Taiwan.
York. D.. 1966. Least squares
fitting of a straight
Bull. Geol. Surv. Taiwan.
18: l-110.
19: 52258. of fusuline
limestone
3: 23-26. line. Can. J. Phys., 44: 23-26.
in the metamorphic
103
GE~CHRONOL~Y OF THE TANANAU SCHIST COMPLEX, TAIWAN, ANT) ITS REGIONAL TECTONIC SIGNIFICANCE
B.M. JAHN, F. MARTfNEAU, 3.3. PEUCAT and J. CORNICHET C.A. E. S. S. $W. R. S.}, Universitk de Rennes, insiitut de Gkoiogie, Campus de Beaulieu, 35042 &nnes (France)
(Revised version received June 7, 1985; accepted September 17, 1985)
ABSTRACT
Jahn, KM., Martineau, F., Peucat, J.J. and Coruichet, J., 19%. Geochrondogy of the Tananao Schist complex, Taiwan, and its regional tectonic si~ficance, In: J. Angeher, R. Blanchet, C.S. Ho and X. Le Pichon (Editors), Geodynamics of the Eurasia-P~Iippine Sea Plate Boundary, ~ecro~o~hy~~c~, 125: 103-124. Isotopic analyses (Rb-Sr, U-Pb, Sm-Nd, K-Ar) on rocks and minerals of the Tananao Schist complex (the Taituko-Tienhsiang and the Nanao areas of eastern Taiwan) have yielded significant new age data corresponding to several important geologic events in the crustal evolution of Taiwan. The ages and corresponding events are summarized as Follows:
Crustal history O-IQ Ma: Arc-continent collision; regional rnet~o~~srn III (Pencil Orogeny). 35-40 Ma: Continental rifting and opening of the South China Sea; regional metamorphism fl. 80-W Ma: Granitic intrusions in Taiwan; regional metamo~~sm I (Nan90 Orogeny}. Overlapped with the most important world-wide, particularly circnm-Pacificz. thermal events of 90-110 Ma (Jahn, 1974; J&n et al., 1976). 200-240 Ma: Deposition of carbonates and elastic sediments, probably in a geosynclinal environment. Beginning of the crustal history of Taiwan. Pre-crustai history 500-650 Ma (or older): Separation of prototiths for the granitoids of Taiwan from a chondritic (or depleted mantle) reservoir. 1000-1700 Ma: Crystallization of zircons, of which some grains have survived aad been finally incorporated in the young (ca. 90 Ma) granitic magmas.
The youngest Rb-Sr mineral isochron ages of granite and paragneiss samples have been used to estimate an uplift rate of the Central Range (or more precisely, the Tauanao Schist complex). It is found that a very high uplift rate of at least 3-4 mm/yr, comparable to that of the Himalayas, has persisted since at least 3 Ma ago as a result of the collision of the Luzon Arc with the Eurasian plate.
~-19~~/8~/$03.50
0 X986 Elsetier Science Publishers B.V.
I04
22’
PHILIPPINE SEA
18’
14’
10’
6’
Fig. 1. Ceotectonic
setting
1981). Thin arrows
indicate
and tectonic
elements
in the Taiwan-Philippine
slip vectors of shallow earthquakes
region (after
Lin and Tsai.
along the plate boundaries
INTRODUCTION
The island of Taiwan
is situated
at the eastern
margin
of the Eurasian
continental
plate. The interaction of the continental lithosphere with the Philippine Sea plate is manifested by the mountain ranges and structural trends in the island. The lithospheric plate boundary here is defined by the Ryukyu trench to the northeast, the Lon~tudinal Valley on the island, and the Manila trench to the south (Fig. 1). The Coastal Range, on the east of the Valley, is an exotic island arc terrane (the Luzon arc) recently collided with and attached to the main part of Taiwan (e.g., Ho, 1982). Except for the Coastal Range, the entire island can be viewed as a thick pile of geosynclinal sediments which have been complexly folded, variably metamorphosed, rapidly uplifted and denuded, and in places, intruded by talc-alkaline and tholeiitic magmas. The eastern flank of the Central Range consists of a suite of metamorphic rocks including schists, marbles, amphibolites and granitic gneisses. These rocks together constitute the oldest crystalline basement of Taiwan, which is termed the Tananao Schist complex (Yen, 1954, 1960, 1967). Because the knowledge of crustal development of Taiwan hinges on a better understanding of the Tananao Schist complex, it
105
becomes imperative to engage in a systematic study of the nature and origin of their protoliths, as well as the chronological order of tectono-thermal events registered in rocks and minerals. In recent years, comprehensive studies of the metamorphic complex regarding petrography, mineral paragenesis, petrochemistry, mineral chemistry, structural analysis and tectonic models have been published (J.C. Chen, 1977a, b; C.H. Chen, 1979; Wang, 1979; Wang-Lee, 1979; Suppe, 1980; Weber-Diefenbach et al., 1980; Chu and Shieh, 1981; Ernst et al., 1981; Lan and Liou, 1981; Liou, 1981; Liou et al., 1981; Lo and Wang-Lee, 1981; Lu and Wang-Lee, 1981; Yang, 1981; Wang-Lee et al., 1982; Ernst, 1983; Ernst and Hamish, 1983). On the other hand, discussion of petrogenetic processes and crustal evolution using trace elements (p~ticularly, rare earth elements or REE) distribution and isotopic techniques has been very limited (Jahn and Liou, 1977; Chu and Shieh, 1981; Jahn et al., 1981). This article presents new results of multi-chronometric study using four isotopic systems (Rb-Sr, U-Pb, Sm-Nd, K-Ar) in rocks and minerals from the Tananao Schist complex. The basic principle of radiometric dating can be found in Faure (1977). The new data will be used together with the few published geochronological results to address the problem of crustal development in Taiwan and its regional tectonic significance.
GENERAL
GEOLOGY
OF THE SAMPLING
LOCALITIES
According to Yen (1954, 1960, 1967), the Tananao Schist complex of eastern Taiwan is divided into a western Tailuko belt characterized by open folds, and an eastern Yuli belt exhibiting folds overturned towards the east. The two juxtaposed lithologic assemblages were thought to represent paired metamorphic belts (Yen, 1963). Although glaucophane schists occur in the Yuli belt, it has been argued from geochemical and geochronological data that they represent exotic tectonic blocks that are not necessarily indicative of a high P/low T environment for the in-situ metamorphic rocks of the entire Yuli belt, hence the paired metamorphic concept of Yen (1963) may not be warranted (Jahn and Liou, 1977; Jahn et al., 1981). All analyzed samples of the present study come from two areas within the Tailuko belt: (1) the Nanao area, and (2) the Tailuko-Tienhsiang area (Fig. 2). The Tailuko belt generally consists of a variety of schists and marbles, together with minor amount of granitic gneisses (orthogneiss + paragneiss), amphibolites, metabasalts and serpentinites. Different rock types are commonly interlayered and only principal rock types are shown in Fig. 2. The general structural trend of the Tailuko belt is NNE-SSW. However, in the Nanao area, the structure is deflected to an E-W direction, probably as a result of the northerly subduction of the Philippine Sea plate. Based on recent seismic studies, an active convergent boundary exists about 50 km to the south and east of the Nanao area, which connects the kyukyu Trough with a probable transform fault (Tsai et al., 1977; Lin and Tsai, 1981; see Fig. l),
,43 c7
Fig. 2. a. Simplified from
Chu
82-postfix
and
omitted)
are the drainage b. Simplified central
geologic map of the Nanao
Shieh,
1981 and
Orthogneiss
1==I
Paragneiss
to area A of the index map (modified
area, corresponding
et al., 1981).
Sample
localities
are shown by solid dots. Heavy line represents
and
the Su-Hua
numbers
(BJ-prefix
Highway
and finer lines
and
system (rivers and tributaries).
geologic
map of the Tailuko-Tienhsiang
E-W cross island highway.
corresponds
Ernst
Schist.marble
L;;I1
Sample localities
area (after Ernst, and numbers
1983). Heavy line represents
the
are shown by solid dots. This map area
to area B of the index map (inset of Fig. 2a).
The Tananao Schist complex has undergone several periods of metamorphism (Wang, 1979; Wang-Lee, 1979; Ernst et al., 1981; Liou, 1981; Liou et al., 1981; Lo in the and Wang-Lee, 1981; Wang-Lee et al., 1982; Ernst, 1983). Moreover, Tailuko-Tienhsiang area, the grade of metamorphism increases towards the east; the crystalline limestone (marble) seems to be the oldest in stratigraphic sequence among the supracrustal rocks (Yen, 1960; C.H. Chen, 1979; Ernst, 1983). According to Liou (1981) and Ernst et al. (1981), the regional metamorphism of Taiwan probably began in Early Mesozoic time and is continuing episodically today (Table 1). Distinct
107
TABLE
1
Regional
metamorphic
episodes
of Taiwan
(based on Liou, 1981; Ernst et al., 1981)
Stage
Time
Grade
1
early Mesozoic
lower(?)
thick eugeosynchnal
amphibolites,
(?)
amphibolite
deposit
basic rocks transformed
mid- to late
upper amphibolite
Probable
Product
cause
from oceanic crust
facies 2
Mesozoic
metaultra-
(corresponding the Nanao
amphibolite
to
Orogeny)
facies
metasediments,
migmatites,
granite intrusions,
facies to anatexis +
retrograded
contact
basalt dike intrusions
and
rocks.
(Tertiary?)
retrograde metamorphism Plio-Pleistocene
3
greenschist
Collision
(blueschist)
Luzon Arc with the
of the
facies
Asian Plate
greenschist
facies rocks.
(blueschist
facies rocks).
( = Penlai Orogeny) 4
not specified
development
zeolite facies
of zeolite
facies minerals Note: The postulated
events mineral
times (periods)
for individual
stages are speculative.
(or episodes) have been recognized through studies of petrography chemistry, but the precise timing of individual events was not delineated.
ANALYTICAL
and
PROCEDURES
Sample preparation Whole-rock
(WR) powder
samples were prepared
blocks or pieces using a combination
of hydraulic
from a split of about press, jaw crusher
1 kg fresh
and agate ball
mill. All powdered samples are finer than 75 ~1. Mineral samples except zircons were extracted using mainly a Frantz “Isodynamic” magnetic separator and occasionally heavy liquids.
For zircon extraction,
about 5 kg of rock sample was crushed
to pass
size 60 mesh (250 p). Heavy minerals were first concentrated using a vibrating Wilfley table. Zircons were then separated from other heavy minerals by a Frantz separator and heavy liquids. The zircons were further divided into four or five size fractions and the final purification was achieved by hand-picking. Chemical separation For Rb-Sr analyses, spikes of Rb and Sr were added to the samples before digestion in HF + HClO, acids. Rb and Sr were separated by conventional ion exchange
chromatography
using Dowex AG 50X-8, 200400
mesh resin. The chem-
1ox
ical procedures (1973). Zircons isolated
for U and Pb isolation are basically those employed by Krogh were dissolved in HF i- HNO, in teflon bombs and U and Pb were
by ion exchange
procedures
in mini-columns.
can be found
isotope analysis,
in Peucat
separation
The
details
of the LJ-.Pb analytical
et al. (1981). For REE concentration
procedures
have been described
and Nd
in Jahn et al. (1980a. b).
Mass unulysis
Rb, Sr, U, Pb, and REE (Sm-Nd) concentrations were determined by the isotope dilution method. Mass analyses were performed using a Cameca THN-206 mass spectrometer
{nicknamed
Ag~anonix)
for Rb and
Sr, and
a Cameca
TSN-206
(Id&fix) for U, Pb, Sm and Nd. The Sr sample was loaded on a single oxidized Ta filament and the Rb was loaded on a single non-oxidized Ta filament. For UPb analysis,
the Pb was loaded
on a single Re filament
with silicagel
and phosphoric
acid, whereas the U was loaded on a single oxidized Ta filament and run as UO: species. For Sm-Nd concentrations, a double filament mode (Re central-Ta lateral) was employed, whereas a triple Re filament mode was used for Nd isotopic composition analysis. Total blank for Rb = 0.1 ng, Sr = 1 ng, Pb < 1 ng, U = 0.1 ng, Sm = 0.2 ng and Nd <( 1 ng. Analytical errors were 2% for X7Rb/8hSr, 0.2% for 207Pb/206Pb, less than 2% for U/Pb and 0.5% for ‘47Sm/‘44Nd ratios. The values of common Pb isotopic compositions used in the correction procedures were: 206Pb/2wPb
= 18. 207Pb,‘204Pb = 15.5, “‘Pb/’ 204Pb = 37. The decay constants used in the age computations are: s7Rb = 1.42 x 10-r’ yr-‘; ‘38U = 1.55125 X 10-I” yr ‘I; ztsU = 9.8485 x 10’ r0 yr GEOCHRONOMETRIC
K-Ar
‘.
RESULTS
results
The results of K-Ar
TABLE K-Ar
‘; ‘*‘Sm = 6.54 X 10. I7 yr
measurements
are presented
in Table 2.
2 age results of the Tanano
Sample no.: Analysis
no.
Mineral analyzed Weight (9) K (%)
Schist complex,
Taiwan
BJ-124-82
BJ-132-82
BJ-138-82
BJ-144-82
R8448
R8447
R8450
R8449
biotite 0.108732
biotite 0.100975
hornblende 0.110583
0.116812
hornblende
5.33
5.38
0.17
0.19
3.8925
31.139
1.1311
2.9795
Aratm (%)
75.38
1.81
87.63
73.03
4”Ar*/40K
0.0002447
0.001939
0.002229
0.005254
Age (Ma)
4.2
33.1
38.0
88.2
40Ar* (lO--”
mol/g)
Analyst:
Songshan
X(@K)=
5.543~10-‘~
Wang, yr-‘;
Institute
of Geology,
e = 0.581 x~O-”
Academia yr-‘;
Sinica,
‘%/K=1.167~10-~.
Beijing,
China.
Constants
used:
109
lib- Sr results
The Rb-Sr results of whole-rock samples and mineral separates are presented in Table 3. The results are further displayed in a series of isochron diagrams (Fig. 3). An isochron constructed from data of metamorphic minerals usually reflects the time when the minerals attained isotopic equilibration. This commonly corresponds to a metamorphic event and particularly reflects the waning stage when temperature dropped enough (blocking temperatures) to close the Rb-Sr isotopic systems in individual minerals. The blocking temperatures are variable, ranging from 200” to 4OO*C, and depending on mineral species, mineral chemistry, grain size, total pressure, and cooling rate (see Dodson, 1973; Jager, 1977). For three granitoid samples from the Tailuko area near Chipan (two granitic gneisses and one paragneiss), the mineral isochron ages are very young, varying from 6.4 to 2.5 Ma (Figs. 3a, b, c). This age range is similar to the Rb-Sr mineral isochron age of 4.6 Ma for an epidote amphibolite from the Yuli belt (Jalm et al., 1981). IS-Ar biotite ages of 9.7 and 9.0 Ma have been reported for paragneiss (Juan et al., 1972) and a granite (Juang and Bellon, 1984) from the Tailuko area. Moreover, new K-Ar biotite age of 4.2 Ma has been determined on granitic BE12482 which yields a Rb-Sr isochron age of 3.5 Ma (see Table 2 for the K-Ar results). In view of the unusual reverse discrepancy between the younger Rb-Sr and the older K-Ar ages for the same samples (Nos. 123 and 124), we suspect that there were small quantities of inherited excess Ar in these biotite samples. It is also recognized that a metamorphic episode may last for more than 10 Ma, hence the small variation in age as shown in Figs. 3a, b, and c needs not suggest different periods of metamorphism. Rather, it implies different responses of mineral isotopic systems (i.e., isotopic equilibration, blocking temperature, and uplift or cooling rate) to the same metamorphic episode. At any rate, we interpret the age of 7 to 3 Ma (and probably 10 Ma to the present time) as the latest metamorphic episode, presumably related to the collision of the Eurasian and the Luzon Arc of the Philippine Sea plate. This tectono-thermal event is also termed the Penlai Orogeny (Ho, 1982). On the other hand, the Rb-Sr mineral isotopic data for six samples from an abandoned granite quarry and its vicinity about 4 km northwest of Nanao (Fig. 2a) yield consistent isochron ages of 35-40 Ma, regardless of rock types. The outcrop relations suggest that the massive granite (of granodioritic composition) is intruded by basaltic (or diabasic) dikes, and both are in turn cut by pegmatite dikes. Thus geologically the pegmatite is the youngest among the trio. The isotopic results yield a somewhat reversed age patterns (Figs. 3d-h) as opposed to the field relationship. However, this situation should not cause any serious concern, because the measured ages do not indicate the intrusive order, but merely reflect various mineralogical responses to isotopic re-equilibration as a result of regional metamorphism. Figure 3i shows a mineral isochron for an amp~bolite enclave in granite. The occurrence of the enclave clearly suggests that the formation of amp~bolite and its
TABLE Rb-Sr
3 isotopic
Sample no.
data for rocks and minerals Rock type
Whole-rock (WR) or mineral
from the Tananao
Schist complex,
Rb
Sr
(ppm)
(ppm)
“Rb ~ &%r
Taluan K7Sr
+2 om
?r
fraction BJ-123-82
granite
WR
136
biotite
557
feldspar BJ-124-82
granite
WR
127
biotite
617
feldspar BJ-125-82
paragneiss
4.93
6.81
WR
139
biotite
591
feldspal
8.91
178 14.8 40.2 185 13.4 50.7 160 8.44 47.0
2.211
0.71167
9
0.72146
19
0.356
0.71162
Y
1.996
0.71115
21
0.71752
21
0.389
0.71087
6
2.506
0.71145
9
0.71867
109.0
133.8
202.8 0.549
0.71143
19 7
BJ-130-82
metabasic
rock
WR
18.5
504
0.106
0.70527
6
BJ-131-82
metabasic
rock
WR
19.0
333
0.1655
0.70527
I1
BJ-132-82
granite
87
318
WR biotite plagioclase
BJ-133-82
granite
WR
101 367
basalt
WR
(or diabase)
biotite plagioclase amphibole
BJ-135-82
2.44
biotite plagioclase BJ-134-82
364
basalt
WR
(or diabase)
biotite plagioclase
3.18 40.0 306 4.57 17.1 70 312 18.4 22.5
BJ-136-82
pegmatite
biotite feldspar
BJ-137-82
amphibolite
WR biotite
BJ-138-82
amphibolite
398 5.12 32 310
plagioclase
5.93
amphibole
6.27
5.33 332 288 9.74 311 264 7.95 535 21.45 209 6.11 569 22.2 8.57 542 169 9.55 195 36.8
0.791 199.7 0.0213
0.70838
8
0.81243
40
0.70764
12
0.70859
10
0.76209
50
0.0296
0.70776
6
0.438
0.70488
13
0.76600
22
1.010 109.59
112.15 0.0247
0.70488
2.305
0.70560
0.974
0.70655
7
0.78410
22
0.0937
0.70575
6
2.932
0.70636
5
148.6
0.78428
21
0.0273
0.70791
8
0.554
0.70757
12
0.76096
21
0.088
0.70701
18
0.4928
0.70723
19 7
135.4
94.55
WR
3.7
146
0.0737
0.70355
plagioclase
4.28
323
0.0383
0.70301
7
amphibole
1.80
0.434
0.70447
60
11.9
111
TABLE
3 (continued) Rock type
Sample no.
Whole-rock
Rb
Sr
(WR) or mineral
(ppm)
+2 urn
“Rb
“Sr
(ppm)
86Sr
s
241
0.043
0.70645
8
393
0.3507
0.70857
7
7.4
209
0.103
0.70469
5
24.9
205
0.350
0.70497
20
1.132
0.70430
19
158
0.260
0.70454
4
307
0.0744
0.70363
20
0.299
0.70421
13
fraction BJ-139-82
amphibolite
WR
BJ-142-82
amphibolite
WR
BJ-143-82
amphibolite
WR
3.6 48
plagioclase
3.78
amphibole BJ-144-82
amphibolite
WR
14.2
plagioclase
7.89
amphibole
1.84
9.65
17.8
basaltic protolith must predate the granitic intrusion. The Rb-Sr data yield an age of 40 Ma, indistinguishable from the ages of the trio mentioned above. Evidently the amphibolite
and the enclosing
granite
were metamorphosed
during
the same period.
From Figs. 3d-i, the consistent age patterns (35-40 Ma) obtained from various rock types of distinct intrusive order must be indicative of an important thermal episode (see also Table 2 for complementary K-Ar results). We interpret that the period of 35-40 Ma represents yet another metamorphic event which is unrelated to the most recent plate collision (the Penglai Orogeny). It is, however, quite likely that the thermal episode was related to the opening of the South China Basin during the late Eocene-early Oligocene (Ludwig et al., 1979; Taylor and Hayes, 1982). It should be noted that in all sinchron diagrams shown above, complete isotopic re-equilibrium has not been achieved during metamorphism either in the 35-40 Ma or in the < 10 Ma period. Since biotite is so much more radiogenic than other minerals
or WR samples,
different
data will make little difference vary considerably. among coexisting
choice of isochron
in the ages obtained
calculation
involving
but the calculated
biotite
errors
can
Nevertheless, we believe that incomplete isotopic re-equilibration mineral phases is probably a rather common phenomenon in
young and very active erogenic belts. Similar observations have been previously reported for glaucophane schists of the Yuli belt (Jahn and Liou, 1977; Jahn et al., 1981). Plagioclase and amphibole were separated from three amphibolite samples from the Nanao area. Due to the unfavorable range in Rb/Sr ratios and probable incomplete isotopic re-equilibration, no significant isochron ages have been obtained (Fig. 3j). However, K-Ar analysis of a hornblende sample of BJ-138-82 yielded an age of 38 Ma, whereas the result on another hornblende sample of BE144-82 gave an age of 88 Ma (Table 2). In addition,
three coherent
K-Ar
hornblende
ages of 83-87
II’
Ma have been reported
by Juang and Bellon (1984) for the amphibolite
samples
were also analyzed
in this work by the Rb-Sr
method.
K--Ar
(84 Ma) and
(40 Ma) ages for sample
deserves
hornblende attention.
This amphibolite
the Rb-Sr enclave
is entirely
The discrepancy embedded
that
between
the
BJ-137-82
in granite.
Alkali
113
0
0.2
0.4
0.6
0.0
1.0
(kl
1.2
B’Rb/=Sr 0.706 0
0.5
1.0
1.5
2.0
2.5
s7Rb/aaSr Fig. 3. a-i. Mineral
isochrons
in the boxes were calculated level. Biotite-WR P = plagioclase,
of various rock samples. using the method
ages are given in parentheses HB = hornblende,
All ages (T) and initial s7Srp6 Sr ratios (I) shown
of York (1966) and all regression on the isochrons.
Symbols:
errors are quoted
WR = whole-rock,
at 20
B = biotite,
F = feldspar.
j. Isotopic
compositions
of WR and mineral
k. Isotopic
compositions
of WR samples
fractions
of granitic
of three amphibolite rocks including
samples.
a paragneiss
(No. 125). Two lines of
165 Ma and 90 Ma are shown for reference.
metasomatism is shown by the presence of biotite, especially in the contact zone. This sample has been identified as a metamorphic product of a typical N-type MORB, in view of its REE pattern (Jahn, unpublished) and Nd isotopic data (see Table 5) but it has abnormally high contents of K,O (LlSW), Rb (32 ppm) and Ba (71 ppm) as compared to a typical N-type MORB. Because hornblende is the most resistent of the common minerals to Ar loss (Hart, is likely that the hornblende K-Ar age represents
1964; Hanson and Gast, 1967) it the time of a previous metamor-
phism
metamorphic
whose
age is not
reset
by the younger
episode of 35-40 Ma. Consequently, the coherent Ma) for all amphibolite samples probably represent in rocks and minerals later when dealing
of the Tananao
with the U-Pb
(and
metasomatic)
K-Ar hornblende ages (80-90 a third thermal event registered
Schist complex.
We will return
to this point
ages.
Data of granitoid WR samples including a paragneiss are shown in Fig. 3k. If all granitoids were cogenetic and the analyzed samples had remained closed through later metamorphic episodes, then the whole-rock data might define an isochron yielding their primary emplacement age. The data of the four granitoid samples are found to lie on or about a reference isochron of 165 Ma. Surprisingly, the age of 165 Ma is exactly the time of important granitic intrusions in SE China (Jahn, 1974; Jahn et al., 1976). Perhaps this is too coincidental and we do not believe at present that the age of 165 Ma represents the time of granitic magmatism in Taiwan for the following reasons: (1) the new U-Pb zircon ages to be presented below; and (2) these granites were probably produced by partial melting of sedimentary source(s) as
(ppm)
fractions
7.44
9.12
452
467
69-80
80-105
4.76
341
337
316
45-80
80-100
s 105
4.33
4.64
4.66
Sample BJ-132X? C 37 333
8.30
7.58
466
45-53
(ppm)
Pb
Sample BJ-123-82 +-z37 480
(r)
U
Size
U-Pb isotopic data
TABLE 4
791
1182
1436
640
103
338
163.8
0.06698
0.06093
0.05925
0.07184
0.08720
0.10378
0.14414
0.08846
*06Pb
‘“Pb
432
207Pb
‘Oi,Pb
Measured ratios
0.01609
0.1543 ~
0.1386
0.1353
0.1591
0.1360
0.1906
0.3072
0.01317
0.01383
0.01398
0.01410
0.02098
0.01862
0.01620
0.09208
0.09259
0.09462
0.09528
0.19344
0.15710
0.12429
0.12176
235 ”
0.1678
“‘Pb
__-
20bPb
--
*‘s Pb
-
________-__I__.... Calculated ratios
-.-
Pb
0.04849
0.04856
0.04907
0.04900
0.06686
0.06118
0.05562
0.05487
‘06
1°‘Pb
.x-- ~_
-_
88
x9
90
90
134
119
104
103
89
90
92
92
180
148
119
117
Apparent ages (Ma)
123
127
151
148
834
646
43x
407
115
0.3
0.2
20*Pt? 238 U
0.1
0
f
0
Fig. 4. U-Pb
concordia
2
diagram
3
4 5 207pb+U
for granite
sample
6
BJ-123-82
7
8
from Chipan,
Tailuko
area. The discordia
has a MSWD = 0.46.
evidenced from their geochemical characteristics (J.C. Chen, 1977b; Jahn, unpublished) and oxygen isotope compositions (Chu and Shieh, 19Sl), consequently, they are likely to have had heterogeneous initial *‘Sr/s’ Sr ratios. U-Pb
results
U-Pb zircon data for two granite samples, BJ-123-82 from Chipan, near Tailuko, and BE13282 from Nanao, are presented in Table 4, and are further displayed in concordia diagrams (Figs. 4 and 5). In all size fractions, U concentrations are rather homogeneous with about 450-480 ppm for one sample and 320-340 ppm for another. Pb contents vary from 7.44 to 9.72 ppm for BE123-82 and 4.3-4.8 ppm for BJ-132-82 (Table 4). The analyses clearly demonstrate that all data points are highly discordant but well aligned on a discordia. The discordia of BJ-123-82 from Chipan defines a precise lower intercept age of 90.3?::! Ma and an upper intercept age of about 1700 Ma. Similarly, the analytical data of the Nanao granite define again a precise lower intercept age of 85.8’::: Ma and an upper intercept age of about 1000 Ma (Fig. 5). In both cases, the high U/Pb* ratios (Table 4) of the zircon fractions suggest that most radiogenic Pb ( = Pb*) was produced quite recently. This leads us to interpret the ages of about 90 Ma as a period of granitic intrusions in Taiwan. In our opinion, it is quite improbable that the radiogenic Pb* composition has been modified significantly by recent diffusion, especially for the young zircons. We note that very similar zircon U-Pb isotopic discordancies and intercept ages were observed in the Kaniksu Batholith of southeast British Columbia (Archibald et al., 1984) and in some granitic batholiths of Malaysia (Liew and McCulloch, 1985). This important tectono-thermal event at 90-100 Ma (Late Cretaceous) is a world-wide phenomenon, especially in the circum-Pacific regions (Jahn, 1974; Jahn
BJ-132-82
I
0.2 206pb* 238
U 0.1
0 0
1
2
3
4
5
6
7
8
207Pb* 235~ /
Fig. 5. U-Pb
concordia
diagram
for granite
sample
BJ-13242 from Nanao
(MSWD
= 0.1 I )
et al., 1976). Evidently a supracrustal sequence including amphibolite protoliths could have been metamorphosed during this period along with the granitic intrusions in the Tananao Schist complex. In the coastal areas of southeast China, much of Jurassic
granitoids
(T=
765 Ma) have also been metamorphosed
(90-110 Ma), accompanied by new granitic and pegmatite intrusions. As shown in Table 4, the 707Pb/2”6Pb ages of individual fractions vary from 407 to 834 Ma. This demonstrates Pb component. age of ancient
the presence
in this period for BJ-123-82
of an ancient
radiogenic
The upper intercept age of about 1700 Ma may correspond zircons. It could equally have no precise geologic significance
to the if the
analyzed zircons were a mixture of zircon populations of different ages (older and younger than ca. 1700 Ma, see Peucat, 1983, for model interpretation). Nevertheless, the very old upper intercept age indicates clearly that ancient crustal materials have been recycled before the granitic intrusion at 90 Ma ago. Moreover. the provenance of these zircons is probably from mainland China where crust-forming events in the Proterozoic times have been abundantly recorded in crystalline rocks. On the other hand, the young *“Pb/ 206Pb ages for BJ-132-82 (Table 4) suggest that ‘ancient radiogenic
Pb component
is much reduced.
Sm- Nd results The results of Sm-Nd isotopic analyses are presented in Table 5. The biotite granite (No. 123) and the paragneiss (No. 125) have almost identical overall major element contents, REE patterns and isotopic (Sr and Nd) compositions, suggesting a genetic relation between them. The Nd isotopic data show that all fNd (0) values are negative (see the definition of eNd in the footnote of Table 5). This indicates a derivation of these granitoids from crustal sources characterized by light REE
5
2.44
1.68
granite
diabase
amphibolite
amphibotite
ampbibolite
BJ-132-82
BJ-134-82
BJ-137-82
BJ-138-82
BJ-143-82
similar
between
TSN 206.
were determined
1 and our Cameca
compositions
Mantle),
using the Minmass
(DM = Depleted
(t4’Sm/‘“Nd)crtUn I
Reservoir.
*
1 Spectrometer
at the University
= 0.1967;
0.513068
0.513213
0513150
0.512709
0.512402
0.512333
0.512390
‘43 Nd,?Nd
(‘47Sm/‘“Nd)CHUR
Uniform
0.2175
where:
= Chondritic
4.67
0.2201
0.2076
11.49 6.70
0.1425
0.1221
0.1204
0.1212
Taiwan
14.71
31.82
27.84
29.52
(‘43Nd/‘MNd)CHt,R
and CHUR
3.47
6.43
5.55
5.92
nM = 0.51315 (or end = + 10)
for 7&.,
(‘47Sm/‘“Nd),pte-
(‘43Nd/‘44Nd),,r,,e-
equation
1+
the Minmass
a Nd isotopic
(‘43Nd/‘~Nd)
and
T. ...+
Model ages:
= 0.51264,
are as follows:
enclave
where ( ‘43Nd/ ‘44Nd)cHUR
* Symbol definitions
3.95
paragneiss
dike
granite
BJ-125-82
@pm)
BE123-82
Nd
@pm)
Schist complex,
Sm
Rock type
data for rocks from the Tananao
Sample No.
Sm-Nd isotopic
TABLE
-195 6992 3699
+1.3 t 9.9 +11.2 +8.3
32 a 30 24 a 35 a
of Minnesota
X(‘47Sm)=
(July 1984). No instrumental
= 0.2137;
487
- 4.6
19
(14’Sm/ l”Nd)oM
614
3114
SOS
- 6.0
TCNVR
- 4.9
c,,@)
28
20
220,
TDM
Ga-‘;
bias was found
0.00654
1944
1244
1333
1251
(Ma)
enrichment (T,X,‘., protolithic
in their REE distribution
see the footnote materials
chondritic
mantle
patterns.
of Table 5). ranging
except
reservoir
recycled
The Sm-Nd
chondritic
model
from 487 to 614 Ma, signify
zircon
grains
have
been
at least 500 Ma ago, and probably
separated
ages
that the from
in Late Precambrian
times. However, depleted
in view of the increasing
mantle
as principal
sources
number
of isotopic
arguments
in favor of
for young
granitoids
(DePaolo,
1980. 1981;
Liew and McCulloch, 1985) depleted mantle model ages (T,,,, see footnote of Table 5) might be more significant than TC.r,rIK in terms of crustal formation ages. The calculated TDM ages for three granitoids. ranging from 1244 to 1333 Ma. are comparable with the upper intercept ages of zircons, and hence reinforce the probability of Precambrian ages for the protoliths of the granitoids. Two amphibolites
(Nos. 138 and 143) and a metasomatized
amphibolite
enclave
(No. 137) have highly positive The Sm-Nd interpretation
Ed,, values ranging from + 8.3 to + 11.2 (Table 5). isotopic data and unpublished REE patterns have confirmed the of ophiolitic origin of the amphibolites (Liou et al.. 1981). but the very
high %tJ R ages suggest that their original modified.
REE patterns
could have been slightly
DISCUSSION
Geologic events established by the isotopic data
Radiometric ages registered by various isotope clocks in rocks and minerals from the Tananao Schist complex are summarized in Fig. 6. The crustal history of Taiwan was probably initiated with geosynclinal deposition of carbonates and elastic sediments at the Asiatic continental margin. This early sedimentary deposition is believed to have taken place in Late Paleozoic-Early Mesozoic time, based on the occurrence
of fossil remains
supported
by the new Sr isotopic
Available Taiwan
from crystalline
isotopic data (U-Pb, were transported
data
Sm-Nd,
eastwards
limestone of marble
Rb-Sr)
(Yen et al., 1951) and further samples
(Jahn
et al., 1984).
suggest that the elastic sediments
of
from the Chinese mainland.
In Fig. 6 are shown several well recognized tectono-thermal events and metamorphic episodes recorded in the Tananao Schist complex. The polymetamorphic nature of the complex has earlier been delineated by petrological/mineralogical studies and structural analyses (see Table 1; Yen, 1967, 1976; Ernst et al., 1981; Liou, 1981; Lo and Wang-Lee, 1981; Wang-Lee et al., 1982; Ernst, 1983, among others). It appears that the three tectono-thermal events at T = O-10, 35-40, and 80-90 Ma are well established. It is possible that a fourth metamorphic episode in Early Mesozoic as depicted in Table 1 might have occurred, but it has yet to be demonstrated by future isotopic age studies.
119
EVENT
METHOD
AGE,Ma
Pb-Sr Min.,K-Ar Bio.
Regional Mets. III. Arc-continent collision: formation of glaucophane sch. (Yuli)
Granite, paragneiss -Tailuko
Rb-Sr
Min.,
K-A.r
Bio.
v,a, peg., amphibaIite enclave- Nanao
Rb-Sr Min., amph,-Yuli K-Ar WR-amphibolites -LJ-~b zircon Rb-Sr Min., granites SE Chi.na
m-sr
WR-granites SE China
Regional Hats. II. Continental rifting and formation of South China Sea.
Very important circum-Pacific thermal event! granitic intrusion into supracrustal sequence; metamorphism of basaltic basement. Regional Meta. I.
(Event to be found in Taiwan)
Sr isotopic ratios in carbonates
Beginning of crustal evolution in Taiwan, Carbonate and elastic sediments deposited on oceanic crust at continental margin.
Sm-Nd model
Separation of protoliths for granitoids from a chondritic mantle reservoir
ages
i TCK”R )
or from a depleted mantle reservoir.
J an-m3
model ages
CT
DM
1
Crystallisation
of
zircons.
isome
U-Pb
Fig. 6. Important
tectono-thermal
new mult~-~~ro~orn~t~c
age data.
zircon
-
)’
events for the Tananao
zircon grains have survived through cycles of geologic processes and finally incorporated in young granitic maomasl
Schist complex
of Taiwan
as revealed
by the
121)
The period of 3540 China
Basin
opening
(Fig.
Ma is tentatively
6). although
recent
of the basin was probably
initiated
Hays, 1982). In fact, the middle important India
period
and
Asia
Hawaiian-Emperor
of global and
correlated sea-floor
that
the and
to late Eocene
the change
in
data
of the South
slightly later at 32 Ma B.P. (Taylor
suggest
time has been recognized
plate reorganisations
hot spot (Taylor
with the opening magnetic
including
Pacific
plate
and Hays,
the collision
motion
with
to be an of greater
respect
to the
1982; Seno and Maruyama.
1984).
Furthermore, in the tectonic evolutionary scenario of the South China Basin as proposed by Taylor and Hayes (1982). the talc-alkaline magma intrusions of the SO-90 Ma period would strongly
suggest that the inferred
volcanic
to mid-Cretaceous
arc during
mid-Jurassic
pre-opening
Andean-type
period he positioned
to the east
of Taiwan. In so far as the pre-crustal history is concerned. two broad events are recognized for the first time in Taiwan. It is important to know that the protoliths of the young granites
of ca. 90 Ma probably
Precambrian times (Sm-Nd grains that were crystallized
had resided
in a crustal
environment
since
model ages) and they contained some inherited even earlier in Middle Proterozoic times.
It is extraordinary to observe minerals (biotites) of the granitoids
Late zircon
the very young Rb-Sr ages for metamorphic from the Tailuko area. The youngest ages of a
paragneiss and a granite (ca. 3 Ma) can be used to estimate the minimum rate of uplift since the minerals attained chemical and isotopic equilibrium. Through study of mineral chemistry and elemental partitioning between coexisting phases, Ernst (1983) estimated that physical conditions of metamorphic recrystallisation with were: T = 425 ?I 75OC, P = 4 kbar (ca. 13 km) for the moderate uHZo and ace Tailuko
area. Blocking
temperatures
for the Rb-Sr
system in biotite
from the Alps
has been estimated at about 300°C (Jager, 1979; Dodson, 1973). which would correspond to a depth of about 9 km. An uplift of 9 km within about 3 Ma yields an average net uplift rate of about 3-4 mm/yr. the compensation
of weathering
This rate is considered
and denudation
as a minimum
is taken into account.
if
At any rate,
this high uplift rate is comparable with that for the very recent period (1500-8500 yrs B.P.) as determined by the radiocarbon method on raised coral reefs and marine shells (Bonilla, 1977; Peng et al., 1977). The present average denudation rate of the Central Range of Taiwan has been estimated as at least 1.4 mg cm- * yr ‘, which is probably the highest known value in the world (Li, 1976). This high denudation rate is thought to be a direct consequence of the arc-continent collision. Most recently, Liu (1982) studied fission track ages on various minerals (apatite, zircon, sphene) and concluded that the Central Range has been even more rapidly uplifted at a rate of 8 to 10 mm/yr during the last 0.6 Ma. We propose here that uplift of at least 3-4 mm/yr,
which is comparable
to that
121
of the Himalayas (Chugh, 1974), probably began during the initial collision of Philippine Sea plate with the Eurasian continental lithosphere, that is, at least 3 B.P. The emplacement of glaucophane schists in the Yuli belt is consistent with high uplift rate of Taiwan over the past 10 Ma (Jahn and Liou, 1977; Jahn et 1981).
the Ma the al.,
CONCLUSIONS
(1) The polymetamorphic nature of the Tananao Schist complex has been confirmed by the new age data. Important tectono-thermal events registered in rocks and minerals of the complex are shown in Fig. 6. (2) The crustal history of Taiwan is believed to have begun with elastic and carbonate sedimental deposition, and the later emplacement of op~olitic materials, now represented by various schists, marbles and amphibolites of the Tananao Schist. The deposition probably took place about 200-240 Ma ago, based on the fossil record (Yen et al., 1951) and Sr isotopic compositions of crystalline limestones (Jahn et al., 1984). (3) Two to three pre-crustal geological events have been revealed for the first time by the Sm-Nd and zircon U-Pb isotopic analyses. The presence of inherited zircons in the young granites from the Tailuko and Nanao areas strongly suggests that ancient crustal components (T = 1700-1000 Ma) have been incorporated in the generation of young granitic magmas. These old zircons probably were transported eastwards from China’s ancient cratons. The Sm-Nd data also suggest that the bulk of granite protoliths was separated from a chondritic or depleted mantle reservoir in middle to late Precambrian times. (4) The very young Rb-Sr mineral isochron ages are used to estimate uplift rate of Taiwan, or more precisely, of the Tananao Schist complex. A very high uplift rate of at least 3-4 mm/yr, comparable to that of the Himalayas, has persisted since at least 3 Ma ago, as a result of the collision of the Luzon Arc with the Eurasian continental plate. ACKNOWLEDGEMENTS
The senior author is most grateful to Professor C.S. Ho for arranging the logistic support during his field trip in Taiwan. Very competent assistance of Ruey-chang Jeng (Central Geological Survey of Taiwan) and Chung-hua Chen (Academia Sinica, Taipei) in the sample collection in November, 1982, is deeply appreciated. We thank Songshan Wang (Academia Sinica, Beijing) for K-Ar age determinations, D. Hermitte for mineral separation and Nicole Morin for Rb-Sr and REE chemical separation. We are indebted to Char-yu Lu for his initial help in the sample preparation and scientific discussions. This article has been improied by Professor Gary Ernst of UCLA. The senior author dedicates this article to Professor C.S. Ho
122
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