- Email: [email protected]
The Rb-Sr isotopic record in Taiwan gneisses and its tectonic implication
Tectonophysics, 183 (1990) 129-143 Elsevier Science Publishers B.V.. Amsterdam
129
The Rb-Sr isotopic record in Taiwan gneisses and its tectonic implication * Ching-Ying
Lan, Typhoon
Institute of Earth Sciences, Academia
Lee ** and Cbibming
Sinica and Institute of Geology, National 10764 Taipei (Taiwan, China)
Wang Lee
Taiwan University, P.O. Box 23-59,
(Received February 10,1989; revised version accepted October 10, 1989)
ABSTRACT Ching-Ying Lan, Typhoon Lee and Chihming Wang Lee, 1990. The Rb-Sr isotopic record in Taiwan gneisses and its tectonic implication. In: J. Angelier (Editor), Geodynamic Evolution of the Eastern Eurasian Margin. Tectonophysics, 183: 129-143.
A Rb-Sr isotopic study of six Taiwan gneiss bodies has revealed three thermal events at ca. 90 Ma, ca. 40 Ma and < 12 Ma B.P. The 90 Ma event was identified by the Rb-Sr system for the first time. Applying the estimated closure temperatures for the Rb-Sr and other isotopic systems, we suggest that the 90 Ma event reached a temperature of higher than 500°C and therefore was probably the heating event that occurred under upper amphibolite facies condition, that produced the granitoids. Some whole-rock samples from areas several square kilometres in size provide isochrons of this age suggesting that the granitoids were produced by local scale melting. The latter two events produced greenschist facies metamorphism, with the 40 Ma event occurring at temperatures higher than those which occurred during the < 12 Ma event. Both events were more intense in the southern gneiss bodies. Thus either the southern bodies were buried deeper during the last 40 Ma or the tectonic activities were more intense in the south for both events. The second alternative is consistent with the fact that the on-going arc-continent collision responsible for the < 12 Ma event was from the south. The biotite cooling ages for the < 12 Ma event seem to be older in the west within two gneiss bodies. This fact could be explained by incomplete resetting by decreasing temperature toward the west, or by a real difference in cooling ages and hence slower uplift toward the west. When biotite cooling age data are combined with fission-track data, it may be demonstrated, in agreement with previous stratigraphic observations, that the rate of uplift due to the on-going collision has been accelerating during the past 2 m.y.
Introduction
major
Taiwan is situated at the junction between the Ryukyu and Luzon arcs on the eastern margin of the Eurasian continental plate bordering the Philippine Sea plate. It is a site of active collision between the Luzon Arc and Eurasia, which caused rapid uplift and erosion. This island is an example of continental growth via repeated accretion of exotic terranes (see the review by Ernst and Jahn (1987), for example). A potentially important recorder of these tectonic events is the gneisses which constitute a
* IES contribution 609 ** Also: Department of Physics, National Taiwan University. 0040-1951/90/$03.50
0 1990 - Elsevier Science Publishers B.V.
component
of the Taiwan
pre-Tertiary
metamorphic basement (Yen, 1960). Palaeontological evidence (Yen et al., 1951; Yen, 1953; Lee, 1984) and carbon (Yui, 1987) and Sr (Jahn et al., 1984) isotopic data on the marble sequence in the basement complex together with other geological data suggest that the crustal evolution of Taiwan probably began with elastic and carbonate deposition during Permo-Triassic times (255-200 Ma). Since then this basement has experienced several tectonothermal events, for which the chronological data are summarized in Table 1. The most recent event is the on-going arc-continent collision which is evident from the less than 2 Ma fission-track ages (Liu, 1982) and from the less than 10 Ma biotite ages determined using the Rb-Sr (Jahn et al., 1986; Lan et al., 1986a) and the K-Ar meth-
130
TABLE
C.-Y. LAN
ET At.
1
Summary
of radiometric
Rock type
ages (Ma) of gneisses Rb-Sr
a
and amphibolite
enclaves
from the basement “OAr_ 39Ar
K-Ar
C.7.8
complex U-Pb
of Taiwan
d.4
Kiyoku gneiss body (Ki) Gneiss
Amphibolite
enclave
kO.1
(B) 4
40.1
+2.0
(B) 4
gneiss body (F) 6.5 8.1 8.9 10.6 11.2 44.8 91.5 enclave 20.5
Tachoshui gneiss body (T) Gneiss
Chipan gneiss body (C) Gneiss
(B) 4 (B) 4 (B) (M)
39.7
Fanpaochiemhan Gneiss
Amphibolite
Fission es3 track
0.37 + 0.05 (A)
Yuanioushnn gneiss body (Y) Gneiss 35.0 k4.8 37.0 f3.6 42.0 +1.5 79.1 +4.1
Pegmatite
r.h
-0.04 2.1 7.3 8.4 8.4
2.5 3.5 3.9 6.4 10.3 11.6
+0.4 *0.4 kO.3 +0.4 +0.5 +1.8 +8.5 50.6
(B) (B) (B) (B) (B) (M) (M) (B)
30.5 33.0
+I.5 +2.0
(B) 5 (B) ’
33.1 33.5 35.4 39.0 57.8 63.9 78.0 62.0 74.4 76.7 76.8 85.2 86.0
*1.7 +1.8 +0.8 *2.8 +3.2 f3.5 +3.1 +3.6 *3.4 k3.8 f4.3 k55.0
(B) 4 (W) 5 (B) 5 (B) * (B) ’ (W) 5 (M) 6 (B) 5 (M) 5 (M) ’ (M) 5 (M) ’ (M) ’
82
(H)
92-135
(H)
(B)
38.0 82.5 lt4.1 83.8 +4.2 86.5 +4.3
(H) (W) (W) (W)
88.2
(H) 4
10.8 +0.5
(B) 6
12
f 1.8 (M) 6
46
39.0
4 5 5 5
1.9 (K) 20.8 +0.2(B) 30.4 i 0.2 (B) 32.7 f 0.1 (B) 78-85 (M)
’
85.8 + 1.4 (Z)
0.99 + 0.13 (S)
CM)
+0.01(B) +O.l (B) +0.3 (B) f0.3 (B) kO.4 (B)
+O.l *0.5 kO.3 +0.3 f0.4 +0.5
(B) 4 (B) 4 (B) (B) 4 (B) (B)
4.2 (B) 6.5 *0.3 (B) 9.0 dcO.5 (B) 9.65 + 1.05(B)
4 5 5 *
0.25 k 0.06 (A) 0.45 * 0.04 (A) 0.85 rt 0.10 (Z)
7.7 k 0.1 (B) 8.2 f 0.1 (B)
90.3 f 1.4 (Z)
0.29 0.47 0.48 1.31
f f * *
0.03 0.05 0.06 0.14
(A) (A) (A) (Z)
0.28 0.37 0.40 0.47 0.51 0.53 0.54 0.58
+ f f f + rt f f
0.02 0.05 0.03 0.05 0.06 0.05 0.05 0.07
(A) (A) (A) (A) (A) (A) (A) (A)
a Data from this work (Table 2) and other sources: r Yen and Rosenblum (1964); * Juan et al. (1972); 3 Liu (1982); 4 Jahn et al. (1986); 5 Juang and Bellon (1986); 6 Law (1988); ’ Lo et al. (1988); s Lo (1988). b Abbreviation: (A) = apatite; (B) = biotite; (H) = hornblende; (K) = K-feldspar; (M) = muscovite; (S) = sphene; (W) = whole rock; (Z) = zircon. ’ Integrated and plateau dates by the @Ar- 39Ar incremental heating technique. d Lower intercept of discordia. e Annealing age of 238U spontaneous fission. ’ Integrated age, stepwise heating analysis gave ages between 20 and 72 Ma without a plateau near 40 Ma. K-Ar, Ar-Ar and Rb-Sr were all performed on sample TCS37.
Rb-Sr
ISOTOPIC
RECORD
IN TAIWAN
GNEISSFS
AND
ITS TECTONIC
IMPLICATION
131 24”35’
24”30’
24’20’
0
I
I
I
,
5 I
Schist-slate Sample Rb-Sr
,
,
,
,
IOkn
Boundary Locality
Mineral
Age
in
Ma
Gnelss Marble
24’10’
Schist Village River Highway
24”05’ 121”50’ Fig. 1. Sample locality map showing the six gneiss bodies (stippled areas, modified from Ho, 1986) in the metamorphic basement complex of Taiwan. Rb-Sr mica ages in Ma are shown in the parentheses (biotite-no prefix; muscovite--M; biotite in amphiboiite enclave--A). Ki = Kiyoku; Y = Yuantoushan; F = Fanpaochienshan; T = Tachoshui; C = Chipan; Ka = Kanagan. Sample prefix “TCS’is omitted for bodies F and T. Sample localities of Jahn et al. (1986) are also included, with the prefix “J”. The four geological provinces of Taiwan are shown in the inset: IA = basement complex of the Central Range; ZB = Hsuehshan and Yushan of the Central Range; ZZ= Western Foothills; ZIZ = Coastal Range. Biotite ages in body Y are older than those in the bodies toward the south. The same is true for the muscovite ages. This suggests that the northernmost body was heated less severely in the most recent thermal event. In bodies T and C, the biotite ages are older in the west than in the east. 121’40’
132
C.-Y.
LAN ET AL.
ods (Yen and Rosenblum, 1964; Juan et al., 1972; Juang and BelIon, 1986; Law, 1988; Lo, 1988; Lo
these bodies. Ki is intercalated in the metapelites. Y and F are enclosed in the metapelites and
et al., 1988).
amphibolites,
covered earlier using
The
latter
two
a thermal
event
at around
event
around
the K-Ar
zircons recently,
the presence
dating
and by U-Pb (Jahn
Previous isotopic
un-
of marble
whereas with
T, C and Ka mainly
the marble.
Samples
lected from all six bodies at the localities
dating
Fig. 1. All gneisses
of
has suggested event
around
1988). studies of the Taiwan
at contacts
identified
et al., 1986). More
of a still more ancient
170 + 30 Ma (Jahn,
also
40 Ma. An
90 Ma has been
method
in the gneisses Pb-Pb
methods
composition,
was abundant gneisses,
consisting
two micas together amphibole, absent
have a similar
zircon,
of quartz,
with minor apatite
shown in
mineralogical plagioclase
amounts
and
occur
were col-
and
of garnet,
opaques.
Biotite
in Y, F, T and C, rare in Ka, and
in Ki. These
minerals
probably
formed
in
primarily those of Jahn et al. (1986), were conducted mainly on the two most accessible bodies:
at least two episodes, with an early high-temperature episode under amp~bolite facies conditions
Yuantoushan and Chipan. Furthermore, the RbSr method has been applied to mainly biotite separates and a limited set of whole-rock samples of rather narrow Rb/Sr ratios. We have expanded our study to all the major gneiss bodies with a
as indicated by the paragenesis of the assemblage plagioclase (An,,_,,)-biotite-muscovite-silhmanite-hornblende (brown) in all bodies except Ki (Lo and Wang Lee, 1981; Lan, 1982,1989; Wang Lee et al., 1982; Lan and Wang Lee, 1987), and a later greenschist facies metamorphism char-
wider range of composition (thus a greater range in Rb/Sr ratios). With the wide geographical
acterized by the assemblage rite-epidote-clinozoisite sphene in all bodies.
albite-phengite-chlo(Ps,,_,,)-actinolite-
coverage and the availability of minerals sensitive to different blocking temperatures (particularly muscovite), the purposes of our study were thus: (1) to establish the chronological orders of all tectonothermal events using the Rb-Sr dating
Procedure
method, (2) to better correlate these events with metamorphic episodes previously identified in petrographic studies, and (3) to determine the effect of these events on different isotopic systems for different minerals at distinct locations in order to
We generally follow the procedures described in Lan et al. (1986b). Whole-rock (WR) powder samples were prepared from fresh 1 kg pieces. Mineral samples from the 500-1000 pm fraction
understand the temporal-spatial distribution of peak temperatures and uplift rates. The preliminary results of this study have been presented by Lan et al. (1986a, 1988).
Samples
The granitic gneisses of Taiwan are widely distributed between Tungao and Sanchan on the eastern flank of the northern Central Range (Fig. 1). Six main bodies together with a number of small lenses have been mapped (Yen, 1954). From northwest to southeast, these bodies are Kiyoku (Ki), Yuantoushan (Y), F~pa~~ensh~ (F), Tachoshui (T), Chipan (C) and Kanagan (Ka) (Yen, 1954; Chen, 1963; Wang Lee, 1982). In this paper these abbreviations will be used to denote
for plagioclase, amphibole, biotite and muscovite grains were separated using a magnetic separator and by hand picking. Rb and Sr were separated by the usual cation exchange c~omato~aphy. The s7Rb and s4Sr spikes were separately added to the samples before digestion in HF and HNO, acids. For graphite-bearing gneisses, additional HClO, digestion and filtration against undigested carbonaceous particles was necessary. Isotopic ratios and concentrations of Rb and Sr were determined on our VG354 mass spectrometer. Typically, aliquots containing 300-500 ng of Sr and 30-50 ng of Rb were run. Total blanks were 0.5 ng for Rb and 0.25 ng for Sr. Analytical errors are l-5% for Rb, 0.5% for Sr and l-58 for 87Rb/86Sr. The s7Sr/s6Sr for the NBS987 Sr standard was 0.710226, with a long-term reproducibility of 0.000038 (95% confidence level} per analysis.
Rb-Sr
ISOTOPIC
RECORD
IN TAIWAN
GNEISSES
AND
ITS TECIQNIC
Results Mineral
displayed in isochron diagrams (Figs. 2a-f). No well-defined isochrons were observed, indicating a lack of complete equilibration among different constituent phases during the last metamorphic event(s). Tie lines are drawn connecting the high Rb/Sr phases (biotite and muscovite) with the low Rb/Sr material (plagioclase and whole rock).
ages
The Rb-Sr isotopic results are scald in Table 2. Data on mineral separates were obtained for four gneiss bodies (Y, F, T and C) and are
(a)
Yuantoushan
amiss
Body
(b) c
0.785
Gneiss
Body
0.800
.:I: 79.lf4.1
Fanpaochienshan
.
NA5
0.765
133
IMPLICATION
MB
2806
42.m1.5
k 8 \
0.745
0:
& 6
Ma
Whole
S “,
Rock
Ji 6
V: Biotite
0.725
0.750
A: Plagioclase l
0.705 ”
20
40
60
80
A: Amphibole
0.725
: Muscovite
100
120
140
0
zoo
100
a7Rb/8s Sr
(c)
Fanpaochienshan
0.750
Gneiaa
(d)
Body
/
400
44.8kl.8
I
500
Fanpaochienshan
&&se
0
Body
t I.210
0.740
300
e7Rb/Bg Sr
-Z TCti 94
Ma 1.110
-
L $m
0.730
\ 2 6
0.720
100 8Eb/
(e) 0.765
Tachoshui
150
200
0
50
’ 100
150
‘%r
200
250
300
350
400
450
87Rb/86 Sr
Cneim
(f)
Body
,
Chipan
Gneiss
Body
0.760
//I
~
0.750
.
0.740
-
0.730
-
CP3
f 2
/
CP4
11.6Z.tO.5
P -0.04f0.01
0.705
Ma
’ 0
0.720
I 100
200
300
“Rb/”
400
Sr
500
0.710 0
50
100
150
200
250
300
s7Rb/s6 Sr
Fig. 2. Rb-Sr isochron diagrams for coexisting minerals and/or whole rock in four gneiss bodies (Y, F, T and C) and an amphibolite enclave in F. The mineral ages for the arnp~~~~ enclave sample TCS32C from F (b) near the lower intercept are enlarged in the inset of(b). The ages for muscovite are always older than those for the coexisting biotites in the same body.
134 TABLE Rb-Sr
C.-Y.
LAN
2 data for gneiss and amphibolite
Sample No.
from the Tananao
W.R. or
Weight
Rb
Sr
mineral
(mg)
(ppm)
(ppm)
basement *‘Rb,‘%
complex,
Taiwan
s7sr/%r
*2om
Age
*‘Sr/s%r
(Ma)
(at 90 Ma)
Kiyoku gneiss body (Ki) P20 W.R.
66.3
57.4
213
0.779
0.70909
4
0.70809
P21
25.0
84.4
161
1.51
0.70853
4
0.70660
W.R.
Yuantourhan gnerss body (Y) NA9
W.R.
42.9
7.0
154
0.132
0.70531
4
0.70514
2806
W.R.
54.0
78.6
308
0.738
0.70821
4
0.70727
Biotite
72.8
0.78672
21
0.70802
4
Plagioclase NA5
Muscovite
3.0 278.2
Plagioclase
17.4
NA6A
W.R.
25.0
Na7A
W.R.
19.5
NA8
W.R.
18.0
YG23C
Plagioclase
Fanpaochienshnn gnerss TCS37
body
W.R. Muscovite Plagioclase W.R.
TCS49
3.8
12.5 315 59.8 114 95.8 143 31.9
121 e
71.5
512
142.1
395
3.2 19.2
Plagioclase
373
3.0
64.6 131 19.9
TCS53
W.R.
17.5
TCS90
W.R.
19.9
141
Biotite
70.3
487
TCS94
76.5
W.R.
51.1
148
Biotite
70.3
587
Muscovite
279.1
658
TCS103
W.R.
72.3
131
Biotite
70.5
504
TCSllO
W.R.
27.5
Biotite
70.7
f
99.5 464
W.R.
21.5
Biotite
65.5
Amphibole
67.4
8.3
Plagioclase
1.7
12.6
Tachoshut gnerss TCS14
8.2 612
132 0.059
42.0
+1.5
*
79.1
k4.1
h
0.75294
4
227
0.761
0.71174
4
247
1.33
0.71093
4
193
1.43
0.71146
5
0.70963
197
2.09
0.71179
4
0.70911
392
0.236
0.71150
4
24.4
37.4
0.70922
(F) 137.0
Biotite
TCS32C
32.1 380
155 e 1.5 48.9 559
2.25 198 23.4 0.334
0.71311
4
0.74321
4
11.2
kO.5 c
0.72641
4
44.8
fl.Sb
0.71172
4
249
1.52
0.71155
4
408
0.141
0.71141
4
132
1.67
0.71298
4
202
2.02
0.71328
6
0.80038
5
2.4 149 18.5 4.5 203 3.6 248 16.4 155 13.0 53.6 368
580 2.87 91.8 422 1.87 408 1.16 82.2 0.589 84.5
0.71022
0.70960 0.71084 0.71069 10.6
+0.4
a
0.71292
4
0.72115
10
6.5
,0.4
a
91.5
f8.5
d
8.9
f0.3
’
8.1
f0.4
*
20.5
f0.6
’
1.25833
4
0.71247
4
0.76377
4
0.71177
4
0.72104
4
0.70667
4
0.73026
4
0.446
0.70616
5
0.099
0.70578
4
0.70924
0.71008 0.71029
(T)
body
Biotite Plagioclase
72.6 3.6
470 28.3
2.7 4840
514
0.71114
4
0.017
0.71146
4
- 0.04 f 0.01 c
TCS71
W.R.
42.0
126
171
2.13
0.71189
4
TCS73C
W.R.
20.2
119
248
1.39
0.71106
4
0.70928
TCS88
W.R.
14.9
145
210
1.99
0.71179
4
0.70924
Biotite
70.9
536
W.R.
42.6
140 e
Biotite
70.6
604
HP4 HP5 HP8
Kanagan
ET AL
gneiss
W.R.
52.5
13Se
Biotite
70.2
513
W.R.
17.0
144’
Biotite
10.6
556
body
7.1 151 e 8.6 183 ’ 4.7 174’ 4.4
201 2.68 203 2.18 318 2.39 370
0.71762
4
0.71214
4
0.73611
4
0.71162
4
0.74450
4
0.71178
5
0.75556
9
0.70917
2.1
+0.1
d
8.4
+0.4
’
7.3
+0.3
a
8.4
kO.3 ’
0.70871 0.70883 0.70872
(Ka)
SC1
W.R.
33.6
161
112
4.13
0.71094
4
SC2
W.R.
66.0
113
153
2.15
0.71054
4
0.70779
HP1
W.R.
23.7
137
164
2.40
0.70964
4
0.70656
HP2
W.R.
17.9
2.41
0.70980
4
0.70671
HZ1
W.R.
22.0
151
127
3.43
0.71108
4
0.70669
HZ2
W.R.
21.3
131
190
1.99
0.70930
4
0.70674
80.4
96.2
0.70565
Rb-Sr
ISOTOPIC
TABLE
RECORD
IN TAIWAN
GNEISSES
AND
ITS TECTONIC
135
IMPLICATION
2 (continued)
Sample No.
87Rb/86Sr
W.R. or
Weight
Rb
Sr
mineral
(mg)
(ppm)
(ppm)
180
“Sr/“Sr
*2om
Age
87Sr/86Sr
(Ma)
(at 90 Ma)
Chipan gneiss bo& (C)
CP3 CP4 CL74-42-2
a Rb-Sr
mineral
W.R.
65.8
138
Biotite
10.2
510
W.R.
18.4
141
Biotite
10.6
553
Biotite
41.6
559
Plagioclasee
22.0
59
isochron
2.21
5.6
264
214
1.90
5.0
317
7.6
214
219
age taken from two points
0.620
0.71128
22
0.75448
4
0.71122
4
0.75724
4
0.12325
32
0.71150
10
on the biotite-whole-rock
0.70844 11.6
+0.5
a
10.3
+0.4
=
3.9 *0.3
c
0.70879
tie line, X(87Rb) = 1.42 X 10-r’
yr-‘.
b Same as “a”, but for muscovite-plagioclase. ’ Same as “a”, but for biotite-plagioclase. d Same as “a”, but for muscovite-whole e Concentration ’ Amphibolite
obtained
rock.
using X-ray fluorescence
method,
error
- 5-10%.
enclave in gneiss.
North-------------___South
Mineral ages were calculated from the slopes of these lines. The validity of this approach will be discussed later. The data in Table 2 and Fig. 2 clearly show that the ages for muscovites are always higher than those for the coexisting biotites in the same rock samples (e.g. TCS37 and TCS94 in F). This difference is also true for muscovite and biotite from different rocks in the same gneiss body (e.g. NA5 and 2806 in Y). Also note that in TCS32C, an amphibolite enclave sample from F, the amphibole yielded an age which was older than that for biotite. The temporal distribution of the eighteen mineral ages determined in this work together with seven from Jahn et al. (1986) are summarized in Table 1 and Fig. 3. Except for the enclave sample from F, these 24 ages form three clusters around O-12 Ma, 35-45 Ma and 79-92 Ma, these three clusters are most likely corresponding to distinct thermal episodes. The spatial variation of the mineral ages is shown in Figs. 1 and 3. In the northernmost body (Y) the biotite ages cluster around 40 Ma. In F, T and C they are < 12 Ma. Similarly, the biotite of the amphibolite enclave from Y yielded 40 Ma, which is consistent with the surrounding gneiss. This age is older than the 20.5 Ma biotite age for an enclave in F, which is somewhat older than the
Y
100
I I I
Pb
Sr
Ar
1
b
I
FT 1: r -i-
90
80 70 ‘;t;‘ z
6o 50
El 4
40 30 20 10 0
Fig. 3. Radiometric (Y, F, T and K-Ar
and Ar-Ar
arated
heating
be discerned
mineral
C). Methods
ages found used:
= Ar; fission
events around from the Rb-Sr
Rb-Sr
track = FT. Three clearly
79-92, isotopic
to F, the mica ages in the southern heating
in four gneiss bodies
U-Pb=Pb; 35-45
towards
sep-
and O-12 Ma can
system alone. Compared bodies
for the latter
events have been reset more severely, implying temperature
= Sr;
the south.
two
a higher
136
C.-Y. LAN
7-11 Ma ages for the neighbouring
the biotite the only
ages in the southern
< 12 Ma event
while
the 40 Ma event.
those
This
toward
the south.
in Y registered suggests
event (< 12 Ma)
This trend
may also
be true for the 40 Ma event. The muscovite yielded
a 79.1 Ma age, whereas
Thus,
registered
difference
that the effect of the last thermal increased
gneiss.
bodies
in Y
one of the musco-
vites in F showed a 91.5 Ma age and another
has a
44.8 Ma age. It appears
that the muscovite
Y reflects
of the 90 Ma event while
the influence
ET AL
age in
some of the muscovites in F have been reset by the 40 Ma event. Thus, the effect of the penultimate event (40 Ma) apparently also increased to the south.
Fig. 4. Rb-Sr
whole-rock
isochron
gneiss bodies indicating size (northern Fig.
1) yield
diagram
lens in Ka and southwestern tentative varying
of the F and Ka
that areas several square isochrons
kilometres
comer
of ca. 90 Ma, with
initial “Sr/s6Sr
in
in F, see widely
ratios.
A more subtle trend seems to exist within individual bodies. In T, the three ages for gneisses at the western end are older than the two about 12 km to the east. In C, the two ages in the western end are again older than the four to the east but
84 f 15 Ma (Fig. 4). The initial 87Sr/86Sr ratios for these two tentative isochrons are very different (0.7067 versus 0.7107). implying a lack of regional
the distance is only 3 km. Thus, among the < 12 Ma ages, closure ages in the west appear to be older than those in the east.
isotopic equilibration ture of the protoliths.
and
the heterogeneous
na-
Discussion Whole-rock
data
Twenty-eight whole-rock samples from the six gneiss bodies were measured (Table 2). These data, together with the five data f3r Y and C from Jahn et al. (1986) indicate restricted ranges of 87Rb/86Sr (0.1-4) and of 87Sr/86Sr (0.705-0.714). Each body seems to have distinct Rb/Sr and 87Sr/86Sr values. Regression lines fitted using the method of York (1969) with weighting factors inversely proportional to the errors gave “errorchron ages” between negative and 316 Ma, but none of these lines passes through all data from any single body. Thus, no significance regarding time can be attached to any of the whole-rock ages because no strict isochrons can be established. This implies that isotopic equilibration was not achieved for areas of the size of an entire gneiss body. However, over smaller areas within a single body there is some information regarding time available. All four samples from the northern lens of Ka form a linear array with an age of 90 + 10 Ma. The three samples (37, 103 and 110) from the southwestern corner of F fall on a straight line with an age of
Our study of the Taiwan gneisses has established the following: (1) Rb-Sr mineral ages for muscovite and amphibole are older than those for biotite, (2) three distinct events at < 12 Ma, ca. 40 Ma and ca. 90 Ma can be identified from the mineral ages, (3) the two younger events have affected the northernmost body less severely than the southern bodies, (4) the apparent ages that registered the effect of the last event become progressively younger from the west towards the east within two bodies, and (5) no whole-rock isochrons were found for entire bodies but some subunits several square kilometres in size yield whole-rock isochrons consistent with the 90 Ma event. The interpretation
of mineral ages
The mineral ages obtained in this work correspond to the time which has elapsed since the end of Sr isotopic equilibration (so called “closure”) of the high Rb/Sr (Sr-poor) mineral with the coexisting low Rb/Sr (Sr-rich) material. In a metamor-
Rb-Sr
ISOTOPIC
RECORD
phic terrane
IN TAIWAN
GNEISSES
this age is commonly
the time when the rock cooled acteristic isotopes
closure
through either that
mineral.
This
its closure
Rb/Sr
temperature.
(1) the equilibration the mineral
phase
was not
with the
cooling
composition
phase did not represent
of the high
as
the char-
below which the Sr
cease to exchange
of that
ITS TECTONIC
interpreted
through
not be valid if the isotopic
the low Rb/Sr ratio
temperature
in a mineral
surroundings would
AND
when
age of
it is obvious
only fresh samples, alteration petrographic positions
observation.
are those
The mineralogical
com-
expected greenschist to suspect
but
still
that one or more
of the above assumptions has been violated. A case in point is sample TCS14 from T. Compared
the closure
ferential from
exchange
marble)
because
temperatures.
for the assemblages facies
conditions.
Rb or Sr transport The effect of dif-
with other reservoirs
should
the biotite
not
or
during
under
below
we used
was noticed
equilibrated
so
is not easy to
However,
and no sign of weathering
at low temperature
We have no reason
retained a memory of a previous event, or (2) the two phases exchanged differentially with another reservoir (e.g. fluid). Also, the cooling age would be invalidated if the mineral did not remain closed to the transport of Rb and/or Sr after it cooled through the closure temperature (e.g. via low-temperature alteration). In some samples
lower than closure
it cooled
This may occur if reset
at temperatures
assess, and can be significant.
the initial
was not achieved, totally
137
IMPLICATION
(e.g. fluid
be significant.
This
has such a high Rb/Sr
is
ratio
(> 100) that its Sr is very radiogenic and the surrounding reservoirs (gneiss, schist and marble) generally have low 87Rb/86Sr ( < 4) and non-radiogenic 87Sr/86Sr ratios (about 0.707-0.711). For instance, the use of a tieline between whole-rock and mica instead of between plagioclase and mica would only change the inferred age by less than 0.7 Ma in all cases. Even if we were to use the
to plagioclase, the biotite of this rock has a Rb/Sr ratio 30,000 times higher, yet with a slightly lower
likely value (0.7075) for the marble as the initial Sr ratio for the micas the change in ages would still be only about 1-2 Ma. Therefore, we believe that
s7Sr/86Sr. The tie line between plagioclase and biotite has a negative slope and results in a meaningless age of - 0.04 Ma. The plagioclase has
this problem only causes an uncertainty of about 1 Ma. Within this uncertainty the problem of negative ages is easily removed, and none of the con-
an extremely high Sr content in excess of 4800 ppm and thus would require wholesale exchange
clusions in this paper will be substantially changed. The lack of full isotopic equilibration during low-grade metamorphism subsequent to gneiss formation is a distinct possibility. If the equilibration during the later event was not complete so
to alter its Sr isotopic composition, On the other hand, it is easy to contaminate the biotite, which contains only 2.65 ppm Sr. Therefore, a likely explanation for the negative age is the preferential isotopic exchange of the biotite with a less radiogenie Sr component. The marble surrounding this gneiss body (T) is rich in Sr of the composition 0.7073-0.7078 (Jabn et al., 1984). Infiltrating fluid originating from the marble would be an ideal candidate for the contaminating component. A similar phenomenon has been reported in the Aar massif in the central Swiss Alps by Dempster (1986). Cavazzini (1988) has also considered the problem of isochrons with negative age. It is difficult to know whether a similar fate has befallen other samples to lower their biotite ages. Although care must be exercised in the use of the cooling ages, we should not be prevented from using them provided the uncertainty involved can be estimated. The effect of open system behaviour
that the radiogenic S7Sr accumulated from the earlier event was not entirely lost to the surrounding low Rb/Sr reservoir through exchange and equilibration, an age intermediate between the earlier and later events would be obtained. major factor controlling the equilibration is ably temperature. Extensive investigations setting patterns in contact metamorphic (Hart, 1964) and in Alpine nappes (Jager, have established that the closure (resetting) peratures for the Rb-Sr system are 300 and 500 f 50°C for biotite and muscovite tively. There are additional secondary factors cooling rate, pressure, amount of infiltrating grain size, mineral chemistry and mineral that affect resetting (e.g. Dempster, 1986).
The probof rezones 1979) tem-
k 50°C respecsuch as fluid, texture In our
138
C.-Y.
I.AN
8’1‘Al..
data there are cases demonstrating the importance of secondary control. The two muscovite samples
local melting which produced the granitoids and equilibrated the Sr isotopes in areas several square
from F were from adjacent
areas: one was reset by
kilometres
the 40 Ma event whereas
the other
bodies other than C and Y is needed
the 90 Ma event addition,
and
hence
a range of about
in Y. Similar
were found in F, T and C for the that these spreads
reflect the difference
In
< 12 Ma event.
in time when various
the closure (resetting)
Alternatively,
these
spreads
by the differential Lo (1988) suggested
spreads
are real and truly
cooled through caused events.
reset.
7 Ma existed in biotite
ages for the 40 Ma event It is possible
still recorded
was not
could
samples
temperature. have
been
in size. Further
U-Pb
zircon
To gain insight into the protolith granitoids,
initial
Sr isotopic
work for
to check this. for the Taiwan
ratios were calculated
for all gneiss bodies at 90 Ma B.P. (Table 2). They are highly variable
and scatter
range
determined
from the two tentative
rock isochrons. its own
is essentially
between
0.711. This
0.705 and
the same as that 90 Ma whole-
Each gneiss body seems to posses
characteristic
initial
isotopic
ratios.
Ka
resetting of earlier that the difference
and Ki have low initial ratios of 0.706-0.708, C is intermediate around 0.708-0.709, while T and F
the two biotite ages of Jahn et al. (1986) for C was caused by such differential due to different degrees of chloritization.
have high values of 0.709-0.711. Y covers a particularly large range of 0.705-0.710. These characteristic initial ratios for the granitoids reflect the
We found that the most chloritized samples do show the youngest ages in T and C but not in F. However, in detail, the decrease in ages does not correlate well with either the degree of chloritization or the intensity of exsolution in each body.
heterogeneities in their protoliths and can be interpreted, when combined with additional data (Nd isotopes and REEs), as the result of mixing between juvenile mantle material and recycled continental crust (Lan et al., 1988; Lan, 1989).
We were unable to resolve the question ondary control of age resetting.
Tectonic implication
between reported resetting
The interpretation
of sec-
of whole-rock data
The timing and intensity Taiwan gneisses
The validity of whole-rock pends on whether the gneisses
isochron ages deof the same body
were
and
initially
homogenized,
whether
the
analyzed samples had remained closed throughout later metamorphic episodes and hence record the primary emplacement ages. Jahn et al. (1986) have attempted whole-rock dating on Taiwan granitoids. They noted that two samples from Y fell on a 90 Ma line while the tie line between these two data and two data from C corresponds to a 165 Ma age. However, the limited spread in Rb/Sr and the paucity of samples prevented any firm conclusions. Our more extensive data set does not substantiate their speculations. The only possibly meaningful Sr whole-rock ages we found were 90 and 84 Ma for subunits of Ka and F. Because this agrees with the ca. 90 Ma mineral ages found in F and Y, and particularly because the U-Pb analysis of zircon also established lower intercept ages of around 90 Ma for C and Y (Jahn et al., 1986), we interpret the 90 Ma heating event as causing
Our Rb-Sr
results
of events recorded in the
combined
with all previous
isotopic data are used to determine not only the timing but also the peak temperatures of the metamorphic events recorded in the Taiwan gneisses. In Fig. 3, we plot the chronological data (Table 1) for Y, F, T and C obtained using Pb, Sr and Ar isotopic methods and the fission-track technique. The U-Pb zircon lower intercept ages were around 90 Ma in both Y and C (Jahn et al., 1986). This event is also registered in Y for the K-Ar and Ar-Ar ages for muscovites and hornblende, and for the muscovite Rb-Sr age; for one muscovite Rb-Sr age in F; and for Rb-Sr whole-rock ages for the northern lens of the Ka body and possibly also for the western tip of the F body. The 90 Ma event has been recorded in four out of the six bodies, and hence must have been caused by a regional metamorphism. This event was responsible for most of the zircon crystallization. It has equilibrated the Sr in muscovite, whose resetting
Rb-Sr
ISOTOPIC
TABLE
RECORD
IN TAIWAN
GNEISSES
AND
ITS TECTONIC
IMPLICATION
139
3
Temperature Thermal
and pressure
event
Metamorphic
conditions
for the polymetamorphism
in the Y, F, T and C gneiss bodies
F
Y
T
condition inferred from Sr closure temperature
C
0 > 650°C
-9OMa
> 650°C
> 500°C
-40Ma
300-350°C
400-450°c
r > 300°C
t > 300°C
<12Ma
285-300°C
300-350°C
J
1
Metamorphic -9OMa
condition inferred from other methodr 680-720°C 550-700°C b, 5 kbar 310°C
-40Ma
330-550°C
e, 5 kbar
‘, 2-3.5
kbar
‘, 2-4 kbar
+ 630-725°C
d, 3.5-7.3
kbar -+
+ 420-480°C
d, 4.1-4.5
kbar +
and <12Ma
350-475’C
Closure
temperatures
Rb-Sr
muscovite
K-Ar
muscovite
Rb-Sr
biotite
K-Ar
biotite
i
b, 5 kbar
adopted
4Ot-430°c
300 f 50°C (Jager,
1979).
1979).
Mineral
chemistry
of white mica and Ca-amphibole
Mineral
chemistry
of phengite
Element
(Liou et al., 1981).
geothermometry partition
%,4 kbar
1979).
300 f 50°C (Hunziker,
isotopic
425 f 75°C
1979).
350 f 50°C (Hunziker,
assemblages
Carbon
>
were:
500 f 50°C (Jager,
Mineral
Pyrrhotite
f
thermometry between
and Ca-amphibole
and sphalerite
(Lan, 1989). (Wang
geobarometry
Lee et al., 1982). (Huang,
1979).
(Liu et al., 1987). coexisting
mica and chlorite
(Ernst,
temperature was 500°C. Thus, this event must have caused temperatures in excess of 500°C throughout the entire region, and the peak temperature apparently exceeded that for the greenschist facies but is consistent with that estimated for the amphibolite facies conditions of 550-700°C at 5 kbar (Liou et al., 1981) or 630-725°C at 3.5-7.3 kbar (Wang Lee et al., 1982) based on mineral parageneses in the Y and C bodies (Table 3). Therefore, the 90 Ma event is identified as the high-temperature regional metamorphic episode. The whole-rock data further imply that local partial melting occurred to form the granitoids during this event from heterogeneous protoliths, which probably comprised sediments and oceanic crust in various proportions. The 40 Ma event was registered in Y via the Ar and Sr ages for the biotite. In F it was recorded by the Ar in one muscovite, whereas the Sr age of one muscovite was reset by this event but the other still recorded the 90 Ma event. Muscovites are rare in T and C, and hence were not analyzed; it is not known if the 40 Ma event also affected T and C.
1983).
Ages near 40 Ma have been determined using the Rb-Sr (Jahn et al., 1986) and K-Ar (Juang and Bellon, 1986) methods only for the biotites in Y. The additional K-Ar biotite ages near 60 Ma and 20 Ma lead us to question whether there really was a 40 Ma event or whether these ages are fictitious and represent the incomplete resetting of the 90 Ma event by the most recent event. Our discovery of the 40 Ma Sr age in one muscovite from F together with the recently reported K-Ar ages for the same sample (Law, 1988) provide further evidence for the 40 Ma event. If this had been a partial resetting, the muscovite age should vary between 12 Ma and 90 Ma and there is no reason the Sr age should also be around 40 Ma. However, the Ar-Ar age spectrum of this same muscovite (Lo et al., 1988) does not have a 40 Ma plateau. Thus, the reality of the 40 Ma event outside the Y body still awaits further confirmation. In F, the temperature of this Paleogene event must have been higher than that required to completely reset the K-Ar system (350 + 50°C; Hunziker, 1979), but it barely reached that required to reset the
140
C-Y
LAN
ET AL
Rb-Sr in the muscovite (500 f 50°C). Therefore, 400-450°C may be a reasonable estimate. In Y,
(1987). The increase in 6’“C of the organic matter in the schist zone as compared with the slate zone
because
indicates
reset, 350°C
the Ar ages of the muscovites the temperature during
probably
this event.
that the 40 Ma heating of the greenschist The heating
We therefore
event reached
facies conditions
no more, at least around
were not
remained
zircon
but probably
bodies Y and F.
effect of the youngest and
conclude
the extreme
sphene
ages for
from all four bodies,
a temperature
schist, assuming similar Frey Carbon tween
in the Swiss Alps is valid
isotopic pair F and
Using sphalerite of Y (Huang,
400°C
and
in two schist T gave
by Hoefs
applicable
thermometry
using samples
a temperature
thermometry
for the
estimate
in a and
to Taiwan. the calcitelocated
be-
of 430°C.
in schists just north
1979). Liou et al. (1981) have esti-
and from the K-feldspar Ar age from Y (resetting temperature ca. 200°C; Hart, 1964). In addition, it
mated
has reset the biotite Sr and Ar ages for the three southern bodies while in Y they still reflect the 40 Ma event. Thus, the last event also affected the
Phase petrology and element partitioning in mica and chlorite have been used by Ernst (1983) to estimate the conditions of the greenschist facies metamorphism in C, and have yielded 425 f 75°C
entire region. Its peak temperature was greater than ca. 250°C throughout the entire region so that the tracks in zircon and sphene would be reset (Liu, 1982). have been below in F, T and C in not reset in the
The temperature of the last 300°C in Y but above this view of the fact that biotites former but that they were
event value were in the
latter. In F, this temperature would not have reached beyond 350°C thus avoiding resetting of the Ar in the muscovites. These constraints limit the temperature conditions to that of the mid-range of greenschist facies metamorphism. Of all the published mineral age data on the Taiwan gneiss, only three values fell outside the ranges of the above three events; all three values concern Ar ages for biotites from Y. Two of them lie intermediate between 40 and 90 Ma and the other between 40 Ma and the < 12 Ma event. We are inclined to interpret these three ages as fictitious owing to partial resetting. The last two events were both greenschist facies metamorphism events. The last event reached 250-300°C in Y and 300-350°C in F, while the 40 Ma event reached 300-350°C in Y and 400450°C in F. These temperatures depend on the closure temperature which was estimated elsewhere and which may not be strictly applicable to Taiwan. However, it is gratifying to see that they agree with the estimates based on other methods (Table 3). Carbon isotopic compositions of metasediments have been measured in a transect across the slate/schist boundary by Liu et al.
the
of around
that the temperature
profile (1976)
graphite
event can be
seen in the young (< 2 Ma) fission-track apatite,
below
metamorphism
conditions
of the
greenschist
in Y to be 350-475°C
facies
at 5 kbar.
at 4 kbar. Wang Lee et al. (1982) have derived similar values (420-48O”C, 4.1-4.5 kbar) for C and for T. From Table 3 it is clear that the conditions estimated by Liou et al. (1981) for the greenschist facies metamorphism in Y are applicable only to the Paleogene event and should not be confused with the most recent event. The carbon isotopic temperature is also for the Paleogene event, and was not re-equilibrated in the most recent event. Furthermore, since both events were capable of producing chlorites, it is not surprising that it is difficult to correlate the resetting of the biotite ages with chloritization. The tectonic significance
of the temperature
distribu-
tion Two types of gradient were observed for the youngest event. First, there is the temperature difference between Y and F. The more southerly body (F) seems to have been hotter (> 300°C) than the northern body (Y) (< 3OO’C). Secondly, we found that within bodies T and C the ages in the west are older than those in the east, indicating that the temperature was increasing towards the east. The temperature differences in these two directions may result from the tectonic activity caused by the ongoing arc-continent collision in eastern Taiwan. Such differences may also have resulted from different burial depthsthe overburden may increase towards the east or the south,
Rb-Sr
ISOTOPIC
RECORD
IN TAIWAN
GNEISSES
AND
ITS TECTONIC
which would cause the temperature to increase in these directions. The 40 Ma event seems to have been a more intense event than the < 12 Ma event because it reached higher temperatures. It also showed a temperature increase from Y to F, which is similar to that for the < 12 Ma event. Therefore, perhaps the Paleogene event was also caused by tectonic activity or a difference in burial depth. Jahn et al. (1986) have speculated that the 40 Ma event might have been related to the opening of the South China Sea. Note that the southward increase in temperature found in the gneiss contradicts that inferred from petrographic studies of the neighbouring schist. Chen et al. (1983) noticed that garnet was present in schists near Y, biotite occurred near C, and only chlorite occurred further to the south. They attribute this to a temperature decrease towards the south during greenschist facies metamorphism.
141
IMPLICATION
Uplift 0
(a)
G
Rate
60
T
Gneiss
v
Biotile
Body
e
12
10
8
6 Time
4
2
P*e,e”t
(Ma)
0
(b)
C
0 Gnei,ss
Body
60
360 , 12
I
L__~_I________L_____-~~-~
10
B
The uplift rate in the current mountain building
6 Time
Fig. 5. Uplift rate (mm/yr)
The fission-track ages of apatites have been determined by Liu (1982) for Y, T and C. In each of the three bodies they show an identical range from 0.25 to 0.6 Ma. This corresponds to uplift rates of between 7 and 13 mm/yr for the past 900,000 years. The Holocene rates have been estimated from dating of uplifted corals to be about 5 mm/yr (Peng et al., 1977), while the present rate has been measured with geodetic techniques to be up to 20 mm/yr (Liu, 1987). Using the Rb-Sr cooling ages of the biotites, we can compute the uplift rates in the Pliocene. The possible ranges are between 0.3 and 2.9 mm/yr for T and between 0.5 and 2.5 mm/yr for C (Fig. 5). Jahn et al. (1986) gave a rate of at least 3-4 mm/yr for C. These rates agree with those estimated from sedimentation rates in the Western Foothills of Taiwan dated using pollen, biostratigraphy and palaeomagnetism (Chen et al., 1977; Liew, 1986; Homg et al., 1988). It seems that different methods all suggest a gradual acceleration of uplift rates towards recent times. Note that the uplift rates estimated above assume a geothermal gradient of 30°C/km. However, in Barr and Dahlen’s recent thermal model of the Taiwan thrust belt (1989), the geothermal gradient in the
(mm/year)
r
4
2
Present
(Ma)
for the Central Range based on the
Rb-Sr biotite ages (this study) and fission-track ages of zircon and apatite (Liu, 1982) of the (a) T gneiss body and (b) C gneiss body assuming a geothermal gradient of 30°C/km
and
a surface temperature of 20°C. The adopted closure temperatures for biotite, zircon and apatite are 300*50, 135f20
235+50
and
“C, respectively. An acceleration in the uplift rates from Late Miocene towards Recent is apparent.
basement is expected to be much higher due to internal strain heating and frictional heating on the decollement. If so, the uplift rates based on the higher geothermal gradient would become lower. Furthermore, in these models the isotherms are distorted, so detailed particle trajectories must to be used in the computation of the uplift rates. Acknowledgements
We thank Messrs. W.K. Chau, D.C. Lee and H. Tsai for their very competent assistance and Miss H.F. Chiu and Mr. H.J. Yang for drafting the figures. The senior author is .indebted to Prof. Bor-ming Jahn and Mr. J. Comichet of the University of Remres for valuable discussion and laboratory tutoring during her visit. Comments from Y.N. Shieh (Purdue) were helpful. This paper
142
C.-Y
has benefitted
from the critical
views of B.M. Jahn (Rennes), and
C.H.
Lo (Princeton)
supported
and thorough
re-
J.G. Liou (Stanford) and
the research
from the National
was
Ho, C.S., 1986. An introduction explanatory
of the Repub-
J. and
Frey,
carbonaceous
Cosmochim.
C.S., Lee, T.Q., Chen,
Tsenwenchi
from the Chipan (C) gneiss body. 87Rb/86Sr and 87Sr/86Sr for muscovite and plagioclase separates were 48.3 k 1.0 and 0.76620 k 0.00008 and 5.5 k 0.1 and 0.71581 k 0.00005, respectively. The RbSr isochron age calculated from the slope of the tie line through these two minerals is 83.4 f 2.3 Ma. This age agrees with the ca. 90 Ma muscovite ages found in bodies F and Y, the whole-rock age for subunits in Ka and F, and the lower intercept age of the U-Pb analysis of zircon for C (Jahn et al., 1986). These new data strengthen the claim that the 90 Ma event was a metamorphic event for the entire region, including Chipan.
building.
2. Thermal
F.A., 1989. Brittle frictional structure
and heat budget.
mountain
ages from coexisting
between
pairs of Rb-Sr
metamorphic
micas.
C.H.,
Chu,
Explanatory Taiwan.
notes
Liou, for
the
area, Taiwan:
marine
W.G.,
facies
from
1983.
map
of
Shakatangchi
and
T.J., 1986. Isotope and exchange
Earth Planet. exposed
along
Taiwan.
J. Metamorph. and
metamorphism Philos. Trans.
of the Chishan 2: 169-190. of the chloritoid
Laohsichi
area,
Hualien.
10: 11-27. systematics during
in minerals:
biotite
metamorphism.
Tailuko Jahn,
parageneses Gorge,
Central
in Taiwan,
1987.
Mountain
Crustal
a post-Palaeozoic
R. Sot. London,
Taiwan:
(Abstr.).
an example
2: 47-56
of
Quater-
sulfide
ore
of a sphalerite
(in Chinese,
with En-
Springer, Jlger,
(Editors),
dating.
Lectures
in
In: E. JLger and Isotope
Geology.
Berlin, pp. 52-76.
E.. 1979. The
Hunziker
Rb-Sr
(Editors),
method.
Lectures
In: E. JIger
in Isotope
and
Geology.
J.C.
Springer,
Berlin. pp. 13-26. Jahn, B.M., 1988. Pb-Pb Nature,
dating
of young marbles
from Taiwan.
332(31): 429-432.
Jahn, B.M., Martineau,
F. and Cornichet,
cal significance
of Sr isotopic
line limestones
of the Central
J., 1984. Chronologi-
compositions Range,
in the crystal-
Taiwan.
Mem. Geol.
6: 295-301. F., Peucat,
J.J. and Comichet,
Geochronology
of the Tananao
and its regional
tectonic
Schist
significance.
J., 1986.
complex,
Taiwan,
Tectonophysics,
125:
103-124. V.C., Chow,
metamorphic
T.J. and Lo, H.J., 1972. K-Ar rocks
of Taiwan.
Acta
Geol.
ages of the Taiwan.,
15:
113-118. W.S. and Bellon, H., 1986. Potassium-argon
Tananao
Schist
in Taiwan.
Mem.
Geol.
ages of the Sot.
China,
7:
405-416. C.Y.,
1982. Mineral
Nanao
area,
I-Lan,
China,
25: 38-52.
chemistry
of Yuantoushan
northeastern
Lan, C.Y., 1989. Geochronology
Taiwan.
and rock chemistry
Ph.D. Thesis, Inst. Geol., National
gneiss,
Proc. Geol. Sot. of Taiwan
Taiwan
Univ..
211 pp. Lan, C.Y. and Wang
Lee, C., 1987. Fanpaochienshan
phy and mineral pp. 12-13
amphibolite: chemistry.
field occurrence,
Annu.
gneiss petrogra-
Meet. Geol. Sot. China,
(Abstr.).
Lan, C.Y., Lee, T., Shen, J.J. and Chau, W.K., 1986a. Isotopic in metamorphic
rocks Range,
Geol., 1: 305-329. B.M.,
Symp.
of an iron
J.C., 1979. Potassium-argon
and its associated
Alpine
the
sequences
Taiwan.
2nd (Taipei)
Ti-Chih,
Hunziker
gneisses.
Sci. Lett., 78: 355-367.
Ernst, W.G., 1983. Mineral
W.G.
J.C.
Lan,
biostratigraphy,
and petrology
Acta Geol. Taiwan.,
rejuvenation
sediments
stratigraphy,
Mem. Geol. Sot. China,
P.Y., 1963. Mineralogy
Taiwan.
Ernst,
Ernst,
T.C. and Tsai, L.P., 1977. A
paleomagnetic
and paleoclimate.
Dempster,
and
metamorphic
C.Y., Huang,
study of the late Neogene
rock
J.G.
Cent. Geol. Surv. Spec. Publ. 2, 32 pp.
Chen, P.H., Huang,
Chen,
Hunziker.
Juang, H.T.,
Plio-Pleistocene
Metamorphism
eastern
of
from
glish Abstr.).
Chem.
Geol., 72: 29-36. Chen,
composition profile
Acta, 40: 945-951.
southwestern
of Taiwan, 1979.
geothermometer.
Juan.
G., 1988. Linear correlation
isochron
W.L.,
body, Tungao,
J. Geophys.
Res., 94 (B4): 3923-3947. Cavazzini,
nary Geology Huang,
Jahn, B.M., Martineau,
Barr, T.D. and Dahlen,
163 pp. (in
M.P. and Chen, J.C., 1988. The
of
section,
Sot. China,
References
(2nd ed.).
Econ. Aff., Taiwan,
in a metamorphic
magnetostratigraphy
muscovites
J.
of Taiwan:
map of Taiwan
M.. 1976. The isotopic
matter
Swiss Alps. Geochim. Horng,
able to separate
age relaColorado.
Chinese). Hoefs,
Note
Range,
to the geology
text for the geologic
Cent. Geol. Surv., Minist.
lic of China.
We were recently
and isotopic-mineral
zone in the Front
ET AI
Geol., 72: 493-525.
and NSC77-0202-MOOl-11 Science Council
S.R., 1964. The petrology
tions of a contact
in part by grants NSC75-0202-MOOl-11,
NSC76-0202-MOOl-12
Hart,
LAN
records
of arc-continent
nita, 6(2): 204 (Abstr.
collision
in Taiwan.
Lan, C.Y., Shen, J.J. and Lee, T., 1986b. Rb-Sr accretion
and
mobile
belt.
Ser. A, 321: 129-161.
of andesites eruption
from Lu-Tao,
ages and isotopic
Sci.. Acad.
Terra
Cog-
P06).
Sin., 6: 211-226.
Lan-Hsu,
isotopic
study
and Hsiao-Lan-Hsu:
heterogeneity.
Bull. Inst. Earth
Rb-Sr ISOTOPIC
RECORD
IN TAIWAN
GNEISSES
AND
ITS TECTONIC
Lan, C.Y., Lee, T., Jahn, B.M. and Wang Lee, C.M., 1988. An isotopic
and geochemical
genesis
and tectonic
Evolution
study
of Taiwan
implication.
of the Eastern
gneisses:
Int. Symp.
Eurasian
Margin
p. 68
T.C. and Wang
sion
orogeny.
Symp.
revealed
by some inconsistant
Meet. Geol. Sot. China,
Publ.,
logs (Taipei),
Annu.
3: l-10
Taiwan.
Group
(in Chinese,
with
China,
in
Cent. Geol. Surv.
Wang
Arc-Continent
plications
of Pleistocene
Acta Geol. Taiwan., Liou, J.G., Ernst,
sediments
and climatic
in Western
W.G. and Moore,
rocks
in northeastern
ampbibolites
Taiwan.
Bull. Geol.
and
Liu, K.K.,
and
Chen,
C.H.
Technology
crustal
the Central
Range,
implication Taiwan.
and
Central
Range,
Geol.
1954.
bodies,
em Taiwan.
T.P.
(in Chinese).
and
Lo,
eastern
Taiwan.
Acta
C.H.,
1982.
Hoping-Cbipan
Geol.
Taiwan.,
21:
Sot.
China,
complex
of biotite in the granitic
The gneisses
for isotope
geochronol-
26: 291-315.
Taiwan.
micas
the Hoping-Chipan
Proc. Geol. Sot. China,
chemistry
in some
area, Hualien, 24: 40-55.
East-
fossils
Bull. Geol.
Surv.
Bull. Geol.
Surv. Schist
12: 53-66.
S., 1964. Potassium-argon
from the Tananao report.
study on the Tananao
Bull. Geol. Surv. Taiwan,
Yen, T.P. and Rosenbhun,
schist
terrane
Proc. Geol. Sot. China,
ages of
of Taiwan-a
pre-
7: 80-81.
Yen, T.P., Sheng, C.C. and Keng, W.P., 1951. The discovery fusuline York,
limestone
in the metamorphic
T.F.,
possible tion China,
errors. 1987.
Earth
Carbon
criterion
of
of Taiwan.
fitting
of a straight
line with
Planet.
Sci. Lett., 5: 320-324.
isotope
composition
of marble:
for geochronologic/stratigraphic
in the Tananao 8: 123-133.
complex
3: 23-25.
D., 1969. Least-squares
correlated Yui,
Lee, C., 1981. Mineral
of Taiwan.
Yen, T.P., 1960. A stratigraphical
25:
rocks of
of the late Paleozoic of Taiwan.
5: l-100.
liminary
of fission track ages from
Proc.
T.P.,
in northern
8: 95-112.
and its implications
ogy. Acta Geol. Taiwan., Lo, C.H. and Wang gneissose
Y., Yen,
Bull. Geol. Surv. Taiwan, Taiwan
for the Tananao
25: 5-12
4: 23-26.
Taiwan,
22-37. Lo, C.H., 1988. Chloritization eastern
Huahen,
Taiwan,
P.Y., 1987. Carbonaceous
Mem. Geol. Sot. China,
Liu, T.K., 1982. Tectonic
uplift rates
Mem. Geol. Sot.
in some gneiss bodies,
Sot.
deforma-
of Surveying
matter in some pelitic rocks of the northern Taiwan.
C., Wang,
in the metamorphic
(in Chinese).
and Chen,
Ana-
(Abstr.).
Yen, T.P., 1953. Gn the occurrence
Yen, sea level and vertical
Science
Lee,
and
Am., 92: 609-748.
6th, pp. 135-142
Oro-
and Ancient
2: 57-69.
area,
Taiwan.
D.E., 1981. Geology
associated
Symp.
Taiwan
colli-
and
122-139.
of some polymetamorphosed
Mapping,
Collision
island since the early Holocene.
Polymetamorphism
im-
24: 165-177.
petrology
Liu, C.C., 1987. Mean
pp. 36-37
some constraints
arc-continent
Lee, C., 1982. A new interpretation
Wang stratigraphy
in Eastern
Schist. Proc. Geol. Sot. China,
English
Abstr.). Liew, P.M., 1986. Palynological
of the young
Peng, T.H., Li, Y.H. and Wu, F.T., 1977. Tectonic of Taiwan
study of the Tananao
the region north of the Liwuchi, Spec.
dates.
p. 13 (Abstr.).
Lee, C.S., 1984. Stratigraphic Taiwan
K-Ar
Lee, C., 1988. “OAr/“Ar
rocks of Taiwan:
consequences
genie Sedimentation
Law, E.W., 1988. A revised model for the origin of the Tananao
tions.
Onstott,
studies on the basement on thermal
Geodynamic (Paris),
143
Lo, C.H.,
petro-
(Abstr.). Schist
IMPLICATION
Group,
Taiwan.
Mem.
a
correlaGeol.
Sot.
129
The Rb-Sr isotopic record in Taiwan gneisses and its tectonic implication * Ching-Ying
Lan, Typhoon
Institute of Earth Sciences, Academia
Lee ** and Cbibming
Sinica and Institute of Geology, National 10764 Taipei (Taiwan, China)
Wang Lee
Taiwan University, P.O. Box 23-59,
(Received February 10,1989; revised version accepted October 10, 1989)
ABSTRACT Ching-Ying Lan, Typhoon Lee and Chihming Wang Lee, 1990. The Rb-Sr isotopic record in Taiwan gneisses and its tectonic implication. In: J. Angelier (Editor), Geodynamic Evolution of the Eastern Eurasian Margin. Tectonophysics, 183: 129-143.
A Rb-Sr isotopic study of six Taiwan gneiss bodies has revealed three thermal events at ca. 90 Ma, ca. 40 Ma and < 12 Ma B.P. The 90 Ma event was identified by the Rb-Sr system for the first time. Applying the estimated closure temperatures for the Rb-Sr and other isotopic systems, we suggest that the 90 Ma event reached a temperature of higher than 500°C and therefore was probably the heating event that occurred under upper amphibolite facies condition, that produced the granitoids. Some whole-rock samples from areas several square kilometres in size provide isochrons of this age suggesting that the granitoids were produced by local scale melting. The latter two events produced greenschist facies metamorphism, with the 40 Ma event occurring at temperatures higher than those which occurred during the < 12 Ma event. Both events were more intense in the southern gneiss bodies. Thus either the southern bodies were buried deeper during the last 40 Ma or the tectonic activities were more intense in the south for both events. The second alternative is consistent with the fact that the on-going arc-continent collision responsible for the < 12 Ma event was from the south. The biotite cooling ages for the < 12 Ma event seem to be older in the west within two gneiss bodies. This fact could be explained by incomplete resetting by decreasing temperature toward the west, or by a real difference in cooling ages and hence slower uplift toward the west. When biotite cooling age data are combined with fission-track data, it may be demonstrated, in agreement with previous stratigraphic observations, that the rate of uplift due to the on-going collision has been accelerating during the past 2 m.y.
Introduction
major
Taiwan is situated at the junction between the Ryukyu and Luzon arcs on the eastern margin of the Eurasian continental plate bordering the Philippine Sea plate. It is a site of active collision between the Luzon Arc and Eurasia, which caused rapid uplift and erosion. This island is an example of continental growth via repeated accretion of exotic terranes (see the review by Ernst and Jahn (1987), for example). A potentially important recorder of these tectonic events is the gneisses which constitute a
* IES contribution 609 ** Also: Department of Physics, National Taiwan University. 0040-1951/90/$03.50
0 1990 - Elsevier Science Publishers B.V.
component
of the Taiwan
pre-Tertiary
metamorphic basement (Yen, 1960). Palaeontological evidence (Yen et al., 1951; Yen, 1953; Lee, 1984) and carbon (Yui, 1987) and Sr (Jahn et al., 1984) isotopic data on the marble sequence in the basement complex together with other geological data suggest that the crustal evolution of Taiwan probably began with elastic and carbonate deposition during Permo-Triassic times (255-200 Ma). Since then this basement has experienced several tectonothermal events, for which the chronological data are summarized in Table 1. The most recent event is the on-going arc-continent collision which is evident from the less than 2 Ma fission-track ages (Liu, 1982) and from the less than 10 Ma biotite ages determined using the Rb-Sr (Jahn et al., 1986; Lan et al., 1986a) and the K-Ar meth-
130
TABLE
C.-Y. LAN
ET At.
1
Summary
of radiometric
Rock type
ages (Ma) of gneisses Rb-Sr
a
and amphibolite
enclaves
from the basement “OAr_ 39Ar
K-Ar
C.7.8
complex U-Pb
of Taiwan
d.4
Kiyoku gneiss body (Ki) Gneiss
Amphibolite
enclave
kO.1
(B) 4
40.1
+2.0
(B) 4
gneiss body (F) 6.5 8.1 8.9 10.6 11.2 44.8 91.5 enclave 20.5
Tachoshui gneiss body (T) Gneiss
Chipan gneiss body (C) Gneiss
(B) 4 (B) 4 (B) (M)
39.7
Fanpaochiemhan Gneiss
Amphibolite
Fission es3 track
0.37 + 0.05 (A)
Yuanioushnn gneiss body (Y) Gneiss 35.0 k4.8 37.0 f3.6 42.0 +1.5 79.1 +4.1
Pegmatite
r.h
-0.04 2.1 7.3 8.4 8.4
2.5 3.5 3.9 6.4 10.3 11.6
+0.4 *0.4 kO.3 +0.4 +0.5 +1.8 +8.5 50.6
(B) (B) (B) (B) (B) (M) (M) (B)
30.5 33.0
+I.5 +2.0
(B) 5 (B) ’
33.1 33.5 35.4 39.0 57.8 63.9 78.0 62.0 74.4 76.7 76.8 85.2 86.0
*1.7 +1.8 +0.8 *2.8 +3.2 f3.5 +3.1 +3.6 *3.4 k3.8 f4.3 k55.0
(B) 4 (W) 5 (B) 5 (B) * (B) ’ (W) 5 (M) 6 (B) 5 (M) 5 (M) ’ (M) 5 (M) ’ (M) ’
82
(H)
92-135
(H)
(B)
38.0 82.5 lt4.1 83.8 +4.2 86.5 +4.3
(H) (W) (W) (W)
88.2
(H) 4
10.8 +0.5
(B) 6
12
f 1.8 (M) 6
46
39.0
4 5 5 5
1.9 (K) 20.8 +0.2(B) 30.4 i 0.2 (B) 32.7 f 0.1 (B) 78-85 (M)
’
85.8 + 1.4 (Z)
0.99 + 0.13 (S)
CM)
+0.01(B) +O.l (B) +0.3 (B) f0.3 (B) kO.4 (B)
+O.l *0.5 kO.3 +0.3 f0.4 +0.5
(B) 4 (B) 4 (B) (B) 4 (B) (B)
4.2 (B) 6.5 *0.3 (B) 9.0 dcO.5 (B) 9.65 + 1.05(B)
4 5 5 *
0.25 k 0.06 (A) 0.45 * 0.04 (A) 0.85 rt 0.10 (Z)
7.7 k 0.1 (B) 8.2 f 0.1 (B)
90.3 f 1.4 (Z)
0.29 0.47 0.48 1.31
f f * *
0.03 0.05 0.06 0.14
(A) (A) (A) (Z)
0.28 0.37 0.40 0.47 0.51 0.53 0.54 0.58
+ f f f + rt f f
0.02 0.05 0.03 0.05 0.06 0.05 0.05 0.07
(A) (A) (A) (A) (A) (A) (A) (A)
a Data from this work (Table 2) and other sources: r Yen and Rosenblum (1964); * Juan et al. (1972); 3 Liu (1982); 4 Jahn et al. (1986); 5 Juang and Bellon (1986); 6 Law (1988); ’ Lo et al. (1988); s Lo (1988). b Abbreviation: (A) = apatite; (B) = biotite; (H) = hornblende; (K) = K-feldspar; (M) = muscovite; (S) = sphene; (W) = whole rock; (Z) = zircon. ’ Integrated and plateau dates by the @Ar- 39Ar incremental heating technique. d Lower intercept of discordia. e Annealing age of 238U spontaneous fission. ’ Integrated age, stepwise heating analysis gave ages between 20 and 72 Ma without a plateau near 40 Ma. K-Ar, Ar-Ar and Rb-Sr were all performed on sample TCS37.
Rb-Sr
ISOTOPIC
RECORD
IN TAIWAN
GNEISSFS
AND
ITS TECTONIC
IMPLICATION
131 24”35’
24”30’
24’20’
0
I
I
I
,
5 I
Schist-slate Sample Rb-Sr
,
,
,
,
IOkn
Boundary Locality
Mineral
Age
in
Ma
Gnelss Marble
24’10’
Schist Village River Highway
24”05’ 121”50’ Fig. 1. Sample locality map showing the six gneiss bodies (stippled areas, modified from Ho, 1986) in the metamorphic basement complex of Taiwan. Rb-Sr mica ages in Ma are shown in the parentheses (biotite-no prefix; muscovite--M; biotite in amphiboiite enclave--A). Ki = Kiyoku; Y = Yuantoushan; F = Fanpaochienshan; T = Tachoshui; C = Chipan; Ka = Kanagan. Sample prefix “TCS’is omitted for bodies F and T. Sample localities of Jahn et al. (1986) are also included, with the prefix “J”. The four geological provinces of Taiwan are shown in the inset: IA = basement complex of the Central Range; ZB = Hsuehshan and Yushan of the Central Range; ZZ= Western Foothills; ZIZ = Coastal Range. Biotite ages in body Y are older than those in the bodies toward the south. The same is true for the muscovite ages. This suggests that the northernmost body was heated less severely in the most recent thermal event. In bodies T and C, the biotite ages are older in the west than in the east. 121’40’
132
C.-Y.
LAN ET AL.
ods (Yen and Rosenblum, 1964; Juan et al., 1972; Juang and BelIon, 1986; Law, 1988; Lo, 1988; Lo
these bodies. Ki is intercalated in the metapelites. Y and F are enclosed in the metapelites and
et al., 1988).
amphibolites,
covered earlier using
The
latter
two
a thermal
event
at around
event
around
the K-Ar
zircons recently,
the presence
dating
and by U-Pb (Jahn
Previous isotopic
un-
of marble
whereas with
T, C and Ka mainly
the marble.
Samples
lected from all six bodies at the localities
dating
Fig. 1. All gneisses
of
has suggested event
around
1988). studies of the Taiwan
at contacts
identified
et al., 1986). More
of a still more ancient
170 + 30 Ma (Jahn,
also
40 Ma. An
90 Ma has been
method
in the gneisses Pb-Pb
methods
composition,
was abundant gneisses,
consisting
two micas together amphibole, absent
have a similar
zircon,
of quartz,
with minor apatite
shown in
mineralogical plagioclase
amounts
and
occur
were col-
and
of garnet,
opaques.
Biotite
in Y, F, T and C, rare in Ka, and
in Ki. These
minerals
probably
formed
in
primarily those of Jahn et al. (1986), were conducted mainly on the two most accessible bodies:
at least two episodes, with an early high-temperature episode under amp~bolite facies conditions
Yuantoushan and Chipan. Furthermore, the RbSr method has been applied to mainly biotite separates and a limited set of whole-rock samples of rather narrow Rb/Sr ratios. We have expanded our study to all the major gneiss bodies with a
as indicated by the paragenesis of the assemblage plagioclase (An,,_,,)-biotite-muscovite-silhmanite-hornblende (brown) in all bodies except Ki (Lo and Wang Lee, 1981; Lan, 1982,1989; Wang Lee et al., 1982; Lan and Wang Lee, 1987), and a later greenschist facies metamorphism char-
wider range of composition (thus a greater range in Rb/Sr ratios). With the wide geographical
acterized by the assemblage rite-epidote-clinozoisite sphene in all bodies.
albite-phengite-chlo(Ps,,_,,)-actinolite-
coverage and the availability of minerals sensitive to different blocking temperatures (particularly muscovite), the purposes of our study were thus: (1) to establish the chronological orders of all tectonothermal events using the Rb-Sr dating
Procedure
method, (2) to better correlate these events with metamorphic episodes previously identified in petrographic studies, and (3) to determine the effect of these events on different isotopic systems for different minerals at distinct locations in order to
We generally follow the procedures described in Lan et al. (1986b). Whole-rock (WR) powder samples were prepared from fresh 1 kg pieces. Mineral samples from the 500-1000 pm fraction
understand the temporal-spatial distribution of peak temperatures and uplift rates. The preliminary results of this study have been presented by Lan et al. (1986a, 1988).
Samples
The granitic gneisses of Taiwan are widely distributed between Tungao and Sanchan on the eastern flank of the northern Central Range (Fig. 1). Six main bodies together with a number of small lenses have been mapped (Yen, 1954). From northwest to southeast, these bodies are Kiyoku (Ki), Yuantoushan (Y), F~pa~~ensh~ (F), Tachoshui (T), Chipan (C) and Kanagan (Ka) (Yen, 1954; Chen, 1963; Wang Lee, 1982). In this paper these abbreviations will be used to denote
for plagioclase, amphibole, biotite and muscovite grains were separated using a magnetic separator and by hand picking. Rb and Sr were separated by the usual cation exchange c~omato~aphy. The s7Rb and s4Sr spikes were separately added to the samples before digestion in HF and HNO, acids. For graphite-bearing gneisses, additional HClO, digestion and filtration against undigested carbonaceous particles was necessary. Isotopic ratios and concentrations of Rb and Sr were determined on our VG354 mass spectrometer. Typically, aliquots containing 300-500 ng of Sr and 30-50 ng of Rb were run. Total blanks were 0.5 ng for Rb and 0.25 ng for Sr. Analytical errors are l-5% for Rb, 0.5% for Sr and l-58 for 87Rb/86Sr. The s7Sr/s6Sr for the NBS987 Sr standard was 0.710226, with a long-term reproducibility of 0.000038 (95% confidence level} per analysis.
Rb-Sr
ISOTOPIC
RECORD
IN TAIWAN
GNEISSES
AND
ITS TECIQNIC
Results Mineral
displayed in isochron diagrams (Figs. 2a-f). No well-defined isochrons were observed, indicating a lack of complete equilibration among different constituent phases during the last metamorphic event(s). Tie lines are drawn connecting the high Rb/Sr phases (biotite and muscovite) with the low Rb/Sr material (plagioclase and whole rock).
ages
The Rb-Sr isotopic results are scald in Table 2. Data on mineral separates were obtained for four gneiss bodies (Y, F, T and C) and are
(a)
Yuantoushan
amiss
Body
(b) c
0.785
Gneiss
Body
0.800
.:I: 79.lf4.1
Fanpaochienshan
.
NA5
0.765
133
IMPLICATION
MB
2806
42.m1.5
k 8 \
0.745
0:
& 6
Ma
Whole
S “,
Rock
Ji 6
V: Biotite
0.725
0.750
A: Plagioclase l
0.705 ”
20
40
60
80
A: Amphibole
0.725
: Muscovite
100
120
140
0
zoo
100
a7Rb/8s Sr
(c)
Fanpaochienshan
0.750
Gneiaa
(d)
Body
/
400
44.8kl.8
I
500
Fanpaochienshan
&&se
0
Body
t I.210
0.740
300
e7Rb/Bg Sr
-Z TCti 94
Ma 1.110
-
L $m
0.730
\ 2 6
0.720
100 8Eb/
(e) 0.765
Tachoshui
150
200
0
50
’ 100
150
‘%r
200
250
300
350
400
450
87Rb/86 Sr
Cneim
(f)
Body
,
Chipan
Gneiss
Body
0.760
//I
~
0.750
.
0.740
-
0.730
-
CP3
f 2
/
CP4
11.6Z.tO.5
P -0.04f0.01
0.705
Ma
’ 0
0.720
I 100
200
300
“Rb/”
400
Sr
500
0.710 0
50
100
150
200
250
300
s7Rb/s6 Sr
Fig. 2. Rb-Sr isochron diagrams for coexisting minerals and/or whole rock in four gneiss bodies (Y, F, T and C) and an amphibolite enclave in F. The mineral ages for the arnp~~~~ enclave sample TCS32C from F (b) near the lower intercept are enlarged in the inset of(b). The ages for muscovite are always older than those for the coexisting biotites in the same body.
134 TABLE Rb-Sr
C.-Y.
LAN
2 data for gneiss and amphibolite
Sample No.
from the Tananao
W.R. or
Weight
Rb
Sr
mineral
(mg)
(ppm)
(ppm)
basement *‘Rb,‘%
complex,
Taiwan
s7sr/%r
*2om
Age
*‘Sr/s%r
(Ma)
(at 90 Ma)
Kiyoku gneiss body (Ki) P20 W.R.
66.3
57.4
213
0.779
0.70909
4
0.70809
P21
25.0
84.4
161
1.51
0.70853
4
0.70660
W.R.
Yuantourhan gnerss body (Y) NA9
W.R.
42.9
7.0
154
0.132
0.70531
4
0.70514
2806
W.R.
54.0
78.6
308
0.738
0.70821
4
0.70727
Biotite
72.8
0.78672
21
0.70802
4
Plagioclase NA5
Muscovite
3.0 278.2
Plagioclase
17.4
NA6A
W.R.
25.0
Na7A
W.R.
19.5
NA8
W.R.
18.0
YG23C
Plagioclase
Fanpaochienshnn gnerss TCS37
body
W.R. Muscovite Plagioclase W.R.
TCS49
3.8
12.5 315 59.8 114 95.8 143 31.9
121 e
71.5
512
142.1
395
3.2 19.2
Plagioclase
373
3.0
64.6 131 19.9
TCS53
W.R.
17.5
TCS90
W.R.
19.9
141
Biotite
70.3
487
TCS94
76.5
W.R.
51.1
148
Biotite
70.3
587
Muscovite
279.1
658
TCS103
W.R.
72.3
131
Biotite
70.5
504
TCSllO
W.R.
27.5
Biotite
70.7
f
99.5 464
W.R.
21.5
Biotite
65.5
Amphibole
67.4
8.3
Plagioclase
1.7
12.6
Tachoshut gnerss TCS14
8.2 612
132 0.059
42.0
+1.5
*
79.1
k4.1
h
0.75294
4
227
0.761
0.71174
4
247
1.33
0.71093
4
193
1.43
0.71146
5
0.70963
197
2.09
0.71179
4
0.70911
392
0.236
0.71150
4
24.4
37.4
0.70922
(F) 137.0
Biotite
TCS32C
32.1 380
155 e 1.5 48.9 559
2.25 198 23.4 0.334
0.71311
4
0.74321
4
11.2
kO.5 c
0.72641
4
44.8
fl.Sb
0.71172
4
249
1.52
0.71155
4
408
0.141
0.71141
4
132
1.67
0.71298
4
202
2.02
0.71328
6
0.80038
5
2.4 149 18.5 4.5 203 3.6 248 16.4 155 13.0 53.6 368
580 2.87 91.8 422 1.87 408 1.16 82.2 0.589 84.5
0.71022
0.70960 0.71084 0.71069 10.6
+0.4
a
0.71292
4
0.72115
10
6.5
,0.4
a
91.5
f8.5
d
8.9
f0.3
’
8.1
f0.4
*
20.5
f0.6
’
1.25833
4
0.71247
4
0.76377
4
0.71177
4
0.72104
4
0.70667
4
0.73026
4
0.446
0.70616
5
0.099
0.70578
4
0.70924
0.71008 0.71029
(T)
body
Biotite Plagioclase
72.6 3.6
470 28.3
2.7 4840
514
0.71114
4
0.017
0.71146
4
- 0.04 f 0.01 c
TCS71
W.R.
42.0
126
171
2.13
0.71189
4
TCS73C
W.R.
20.2
119
248
1.39
0.71106
4
0.70928
TCS88
W.R.
14.9
145
210
1.99
0.71179
4
0.70924
Biotite
70.9
536
W.R.
42.6
140 e
Biotite
70.6
604
HP4 HP5 HP8
Kanagan
ET AL
gneiss
W.R.
52.5
13Se
Biotite
70.2
513
W.R.
17.0
144’
Biotite
10.6
556
body
7.1 151 e 8.6 183 ’ 4.7 174’ 4.4
201 2.68 203 2.18 318 2.39 370
0.71762
4
0.71214
4
0.73611
4
0.71162
4
0.74450
4
0.71178
5
0.75556
9
0.70917
2.1
+0.1
d
8.4
+0.4
’
7.3
+0.3
a
8.4
kO.3 ’
0.70871 0.70883 0.70872
(Ka)
SC1
W.R.
33.6
161
112
4.13
0.71094
4
SC2
W.R.
66.0
113
153
2.15
0.71054
4
0.70779
HP1
W.R.
23.7
137
164
2.40
0.70964
4
0.70656
HP2
W.R.
17.9
2.41
0.70980
4
0.70671
HZ1
W.R.
22.0
151
127
3.43
0.71108
4
0.70669
HZ2
W.R.
21.3
131
190
1.99
0.70930
4
0.70674
80.4
96.2
0.70565
Rb-Sr
ISOTOPIC
TABLE
RECORD
IN TAIWAN
GNEISSES
AND
ITS TECTONIC
135
IMPLICATION
2 (continued)
Sample No.
87Rb/86Sr
W.R. or
Weight
Rb
Sr
mineral
(mg)
(ppm)
(ppm)
180
“Sr/“Sr
*2om
Age
87Sr/86Sr
(Ma)
(at 90 Ma)
Chipan gneiss bo& (C)
CP3 CP4 CL74-42-2
a Rb-Sr
mineral
W.R.
65.8
138
Biotite
10.2
510
W.R.
18.4
141
Biotite
10.6
553
Biotite
41.6
559
Plagioclasee
22.0
59
isochron
2.21
5.6
264
214
1.90
5.0
317
7.6
214
219
age taken from two points
0.620
0.71128
22
0.75448
4
0.71122
4
0.75724
4
0.12325
32
0.71150
10
on the biotite-whole-rock
0.70844 11.6
+0.5
a
10.3
+0.4
=
3.9 *0.3
c
0.70879
tie line, X(87Rb) = 1.42 X 10-r’
yr-‘.
b Same as “a”, but for muscovite-plagioclase. ’ Same as “a”, but for biotite-plagioclase. d Same as “a”, but for muscovite-whole e Concentration ’ Amphibolite
obtained
rock.
using X-ray fluorescence
method,
error
- 5-10%.
enclave in gneiss.
North-------------___South
Mineral ages were calculated from the slopes of these lines. The validity of this approach will be discussed later. The data in Table 2 and Fig. 2 clearly show that the ages for muscovites are always higher than those for the coexisting biotites in the same rock samples (e.g. TCS37 and TCS94 in F). This difference is also true for muscovite and biotite from different rocks in the same gneiss body (e.g. NA5 and 2806 in Y). Also note that in TCS32C, an amphibolite enclave sample from F, the amphibole yielded an age which was older than that for biotite. The temporal distribution of the eighteen mineral ages determined in this work together with seven from Jahn et al. (1986) are summarized in Table 1 and Fig. 3. Except for the enclave sample from F, these 24 ages form three clusters around O-12 Ma, 35-45 Ma and 79-92 Ma, these three clusters are most likely corresponding to distinct thermal episodes. The spatial variation of the mineral ages is shown in Figs. 1 and 3. In the northernmost body (Y) the biotite ages cluster around 40 Ma. In F, T and C they are < 12 Ma. Similarly, the biotite of the amphibolite enclave from Y yielded 40 Ma, which is consistent with the surrounding gneiss. This age is older than the 20.5 Ma biotite age for an enclave in F, which is somewhat older than the
Y
100
I I I
Pb
Sr
Ar
1
b
I
FT 1: r -i-
90
80 70 ‘;t;‘ z
6o 50
El 4
40 30 20 10 0
Fig. 3. Radiometric (Y, F, T and K-Ar
and Ar-Ar
arated
heating
be discerned
mineral
C). Methods
ages found used:
= Ar; fission
events around from the Rb-Sr
Rb-Sr
track = FT. Three clearly
79-92, isotopic
to F, the mica ages in the southern heating
in four gneiss bodies
U-Pb=Pb; 35-45
towards
sep-
and O-12 Ma can
system alone. Compared bodies
for the latter
events have been reset more severely, implying temperature
= Sr;
the south.
two
a higher
136
C.-Y. LAN
7-11 Ma ages for the neighbouring
the biotite the only
ages in the southern
< 12 Ma event
while
the 40 Ma event.
those
This
toward
the south.
in Y registered suggests
event (< 12 Ma)
This trend
may also
be true for the 40 Ma event. The muscovite yielded
a 79.1 Ma age, whereas
Thus,
registered
difference
that the effect of the last thermal increased
gneiss.
bodies
in Y
one of the musco-
vites in F showed a 91.5 Ma age and another
has a
44.8 Ma age. It appears
that the muscovite
Y reflects
of the 90 Ma event while
the influence
ET AL
age in
some of the muscovites in F have been reset by the 40 Ma event. Thus, the effect of the penultimate event (40 Ma) apparently also increased to the south.
Fig. 4. Rb-Sr
whole-rock
isochron
gneiss bodies indicating size (northern Fig.
1) yield
diagram
lens in Ka and southwestern tentative varying
of the F and Ka
that areas several square isochrons
kilometres
comer
of ca. 90 Ma, with
initial “Sr/s6Sr
in
in F, see widely
ratios.
A more subtle trend seems to exist within individual bodies. In T, the three ages for gneisses at the western end are older than the two about 12 km to the east. In C, the two ages in the western end are again older than the four to the east but
84 f 15 Ma (Fig. 4). The initial 87Sr/86Sr ratios for these two tentative isochrons are very different (0.7067 versus 0.7107). implying a lack of regional
the distance is only 3 km. Thus, among the < 12 Ma ages, closure ages in the west appear to be older than those in the east.
isotopic equilibration ture of the protoliths.
and
the heterogeneous
na-
Discussion Whole-rock
data
Twenty-eight whole-rock samples from the six gneiss bodies were measured (Table 2). These data, together with the five data f3r Y and C from Jahn et al. (1986) indicate restricted ranges of 87Rb/86Sr (0.1-4) and of 87Sr/86Sr (0.705-0.714). Each body seems to have distinct Rb/Sr and 87Sr/86Sr values. Regression lines fitted using the method of York (1969) with weighting factors inversely proportional to the errors gave “errorchron ages” between negative and 316 Ma, but none of these lines passes through all data from any single body. Thus, no significance regarding time can be attached to any of the whole-rock ages because no strict isochrons can be established. This implies that isotopic equilibration was not achieved for areas of the size of an entire gneiss body. However, over smaller areas within a single body there is some information regarding time available. All four samples from the northern lens of Ka form a linear array with an age of 90 + 10 Ma. The three samples (37, 103 and 110) from the southwestern corner of F fall on a straight line with an age of
Our study of the Taiwan gneisses has established the following: (1) Rb-Sr mineral ages for muscovite and amphibole are older than those for biotite, (2) three distinct events at < 12 Ma, ca. 40 Ma and ca. 90 Ma can be identified from the mineral ages, (3) the two younger events have affected the northernmost body less severely than the southern bodies, (4) the apparent ages that registered the effect of the last event become progressively younger from the west towards the east within two bodies, and (5) no whole-rock isochrons were found for entire bodies but some subunits several square kilometres in size yield whole-rock isochrons consistent with the 90 Ma event. The interpretation
of mineral ages
The mineral ages obtained in this work correspond to the time which has elapsed since the end of Sr isotopic equilibration (so called “closure”) of the high Rb/Sr (Sr-poor) mineral with the coexisting low Rb/Sr (Sr-rich) material. In a metamor-
Rb-Sr
ISOTOPIC
RECORD
phic terrane
IN TAIWAN
GNEISSES
this age is commonly
the time when the rock cooled acteristic isotopes
closure
through either that
mineral.
This
its closure
Rb/Sr
temperature.
(1) the equilibration the mineral
phase
was not
with the
cooling
composition
phase did not represent
of the high
as
the char-
below which the Sr
cease to exchange
of that
ITS TECTONIC
interpreted
through
not be valid if the isotopic
the low Rb/Sr ratio
temperature
in a mineral
surroundings would
AND
when
age of
it is obvious
only fresh samples, alteration petrographic positions
observation.
are those
The mineralogical
com-
expected greenschist to suspect
but
still
that one or more
of the above assumptions has been violated. A case in point is sample TCS14 from T. Compared
the closure
ferential from
exchange
marble)
because
temperatures.
for the assemblages facies
conditions.
Rb or Sr transport The effect of dif-
with other reservoirs
should
the biotite
not
or
during
under
below
we used
was noticed
equilibrated
so
is not easy to
However,
and no sign of weathering
at low temperature
We have no reason
retained a memory of a previous event, or (2) the two phases exchanged differentially with another reservoir (e.g. fluid). Also, the cooling age would be invalidated if the mineral did not remain closed to the transport of Rb and/or Sr after it cooled through the closure temperature (e.g. via low-temperature alteration). In some samples
lower than closure
it cooled
This may occur if reset
at temperatures
assess, and can be significant.
the initial
was not achieved, totally
137
IMPLICATION
(e.g. fluid
be significant.
This
has such a high Rb/Sr
is
ratio
(> 100) that its Sr is very radiogenic and the surrounding reservoirs (gneiss, schist and marble) generally have low 87Rb/86Sr ( < 4) and non-radiogenic 87Sr/86Sr ratios (about 0.707-0.711). For instance, the use of a tieline between whole-rock and mica instead of between plagioclase and mica would only change the inferred age by less than 0.7 Ma in all cases. Even if we were to use the
to plagioclase, the biotite of this rock has a Rb/Sr ratio 30,000 times higher, yet with a slightly lower
likely value (0.7075) for the marble as the initial Sr ratio for the micas the change in ages would still be only about 1-2 Ma. Therefore, we believe that
s7Sr/86Sr. The tie line between plagioclase and biotite has a negative slope and results in a meaningless age of - 0.04 Ma. The plagioclase has
this problem only causes an uncertainty of about 1 Ma. Within this uncertainty the problem of negative ages is easily removed, and none of the con-
an extremely high Sr content in excess of 4800 ppm and thus would require wholesale exchange
clusions in this paper will be substantially changed. The lack of full isotopic equilibration during low-grade metamorphism subsequent to gneiss formation is a distinct possibility. If the equilibration during the later event was not complete so
to alter its Sr isotopic composition, On the other hand, it is easy to contaminate the biotite, which contains only 2.65 ppm Sr. Therefore, a likely explanation for the negative age is the preferential isotopic exchange of the biotite with a less radiogenie Sr component. The marble surrounding this gneiss body (T) is rich in Sr of the composition 0.7073-0.7078 (Jabn et al., 1984). Infiltrating fluid originating from the marble would be an ideal candidate for the contaminating component. A similar phenomenon has been reported in the Aar massif in the central Swiss Alps by Dempster (1986). Cavazzini (1988) has also considered the problem of isochrons with negative age. It is difficult to know whether a similar fate has befallen other samples to lower their biotite ages. Although care must be exercised in the use of the cooling ages, we should not be prevented from using them provided the uncertainty involved can be estimated. The effect of open system behaviour
that the radiogenic S7Sr accumulated from the earlier event was not entirely lost to the surrounding low Rb/Sr reservoir through exchange and equilibration, an age intermediate between the earlier and later events would be obtained. major factor controlling the equilibration is ably temperature. Extensive investigations setting patterns in contact metamorphic (Hart, 1964) and in Alpine nappes (Jager, have established that the closure (resetting) peratures for the Rb-Sr system are 300 and 500 f 50°C for biotite and muscovite tively. There are additional secondary factors cooling rate, pressure, amount of infiltrating grain size, mineral chemistry and mineral that affect resetting (e.g. Dempster, 1986).
The probof rezones 1979) tem-
k 50°C respecsuch as fluid, texture In our
138
C.-Y.
I.AN
8’1‘Al..
data there are cases demonstrating the importance of secondary control. The two muscovite samples
local melting which produced the granitoids and equilibrated the Sr isotopes in areas several square
from F were from adjacent
areas: one was reset by
kilometres
the 40 Ma event whereas
the other
bodies other than C and Y is needed
the 90 Ma event addition,
and
hence
a range of about
in Y. Similar
were found in F, T and C for the that these spreads
reflect the difference
In
< 12 Ma event.
in time when various
the closure (resetting)
Alternatively,
these
spreads
by the differential Lo (1988) suggested
spreads
are real and truly
cooled through caused events.
reset.
7 Ma existed in biotite
ages for the 40 Ma event It is possible
still recorded
was not
could
samples
temperature. have
been
in size. Further
U-Pb
zircon
To gain insight into the protolith granitoids,
initial
Sr isotopic
work for
to check this. for the Taiwan
ratios were calculated
for all gneiss bodies at 90 Ma B.P. (Table 2). They are highly variable
and scatter
range
determined
from the two tentative
rock isochrons. its own
is essentially
between
0.711. This
0.705 and
the same as that 90 Ma whole-
Each gneiss body seems to posses
characteristic
initial
isotopic
ratios.
Ka
resetting of earlier that the difference
and Ki have low initial ratios of 0.706-0.708, C is intermediate around 0.708-0.709, while T and F
the two biotite ages of Jahn et al. (1986) for C was caused by such differential due to different degrees of chloritization.
have high values of 0.709-0.711. Y covers a particularly large range of 0.705-0.710. These characteristic initial ratios for the granitoids reflect the
We found that the most chloritized samples do show the youngest ages in T and C but not in F. However, in detail, the decrease in ages does not correlate well with either the degree of chloritization or the intensity of exsolution in each body.
heterogeneities in their protoliths and can be interpreted, when combined with additional data (Nd isotopes and REEs), as the result of mixing between juvenile mantle material and recycled continental crust (Lan et al., 1988; Lan, 1989).
We were unable to resolve the question ondary control of age resetting.
Tectonic implication
between reported resetting
The interpretation
of sec-
of whole-rock data
The timing and intensity Taiwan gneisses
The validity of whole-rock pends on whether the gneisses
isochron ages deof the same body
were
and
initially
homogenized,
whether
the
analyzed samples had remained closed throughout later metamorphic episodes and hence record the primary emplacement ages. Jahn et al. (1986) have attempted whole-rock dating on Taiwan granitoids. They noted that two samples from Y fell on a 90 Ma line while the tie line between these two data and two data from C corresponds to a 165 Ma age. However, the limited spread in Rb/Sr and the paucity of samples prevented any firm conclusions. Our more extensive data set does not substantiate their speculations. The only possibly meaningful Sr whole-rock ages we found were 90 and 84 Ma for subunits of Ka and F. Because this agrees with the ca. 90 Ma mineral ages found in F and Y, and particularly because the U-Pb analysis of zircon also established lower intercept ages of around 90 Ma for C and Y (Jahn et al., 1986), we interpret the 90 Ma heating event as causing
Our Rb-Sr
results
of events recorded in the
combined
with all previous
isotopic data are used to determine not only the timing but also the peak temperatures of the metamorphic events recorded in the Taiwan gneisses. In Fig. 3, we plot the chronological data (Table 1) for Y, F, T and C obtained using Pb, Sr and Ar isotopic methods and the fission-track technique. The U-Pb zircon lower intercept ages were around 90 Ma in both Y and C (Jahn et al., 1986). This event is also registered in Y for the K-Ar and Ar-Ar ages for muscovites and hornblende, and for the muscovite Rb-Sr age; for one muscovite Rb-Sr age in F; and for Rb-Sr whole-rock ages for the northern lens of the Ka body and possibly also for the western tip of the F body. The 90 Ma event has been recorded in four out of the six bodies, and hence must have been caused by a regional metamorphism. This event was responsible for most of the zircon crystallization. It has equilibrated the Sr in muscovite, whose resetting
Rb-Sr
ISOTOPIC
TABLE
RECORD
IN TAIWAN
GNEISSES
AND
ITS TECTONIC
IMPLICATION
139
3
Temperature Thermal
and pressure
event
Metamorphic
conditions
for the polymetamorphism
in the Y, F, T and C gneiss bodies
F
Y
T
condition inferred from Sr closure temperature
C
0 > 650°C
-9OMa
> 650°C
> 500°C
-40Ma
300-350°C
400-450°c
r > 300°C
t > 300°C
<12Ma
285-300°C
300-350°C
J
1
Metamorphic -9OMa
condition inferred from other methodr 680-720°C 550-700°C b, 5 kbar 310°C
-40Ma
330-550°C
e, 5 kbar
‘, 2-3.5
kbar
‘, 2-4 kbar
+ 630-725°C
d, 3.5-7.3
kbar -+
+ 420-480°C
d, 4.1-4.5
kbar +
and <12Ma
350-475’C
Closure
temperatures
Rb-Sr
muscovite
K-Ar
muscovite
Rb-Sr
biotite
K-Ar
biotite
i
b, 5 kbar
adopted
4Ot-430°c
300 f 50°C (Jager,
1979).
1979).
Mineral
chemistry
of white mica and Ca-amphibole
Mineral
chemistry
of phengite
Element
(Liou et al., 1981).
geothermometry partition
%,4 kbar
1979).
300 f 50°C (Hunziker,
isotopic
425 f 75°C
1979).
350 f 50°C (Hunziker,
assemblages
Carbon
>
were:
500 f 50°C (Jager,
Mineral
Pyrrhotite
f
thermometry between
and Ca-amphibole
and sphalerite
(Lan, 1989). (Wang
geobarometry
Lee et al., 1982). (Huang,
1979).
(Liu et al., 1987). coexisting
mica and chlorite
(Ernst,
temperature was 500°C. Thus, this event must have caused temperatures in excess of 500°C throughout the entire region, and the peak temperature apparently exceeded that for the greenschist facies but is consistent with that estimated for the amphibolite facies conditions of 550-700°C at 5 kbar (Liou et al., 1981) or 630-725°C at 3.5-7.3 kbar (Wang Lee et al., 1982) based on mineral parageneses in the Y and C bodies (Table 3). Therefore, the 90 Ma event is identified as the high-temperature regional metamorphic episode. The whole-rock data further imply that local partial melting occurred to form the granitoids during this event from heterogeneous protoliths, which probably comprised sediments and oceanic crust in various proportions. The 40 Ma event was registered in Y via the Ar and Sr ages for the biotite. In F it was recorded by the Ar in one muscovite, whereas the Sr age of one muscovite was reset by this event but the other still recorded the 90 Ma event. Muscovites are rare in T and C, and hence were not analyzed; it is not known if the 40 Ma event also affected T and C.
1983).
Ages near 40 Ma have been determined using the Rb-Sr (Jahn et al., 1986) and K-Ar (Juang and Bellon, 1986) methods only for the biotites in Y. The additional K-Ar biotite ages near 60 Ma and 20 Ma lead us to question whether there really was a 40 Ma event or whether these ages are fictitious and represent the incomplete resetting of the 90 Ma event by the most recent event. Our discovery of the 40 Ma Sr age in one muscovite from F together with the recently reported K-Ar ages for the same sample (Law, 1988) provide further evidence for the 40 Ma event. If this had been a partial resetting, the muscovite age should vary between 12 Ma and 90 Ma and there is no reason the Sr age should also be around 40 Ma. However, the Ar-Ar age spectrum of this same muscovite (Lo et al., 1988) does not have a 40 Ma plateau. Thus, the reality of the 40 Ma event outside the Y body still awaits further confirmation. In F, the temperature of this Paleogene event must have been higher than that required to completely reset the K-Ar system (350 + 50°C; Hunziker, 1979), but it barely reached that required to reset the
140
C-Y
LAN
ET AL
Rb-Sr in the muscovite (500 f 50°C). Therefore, 400-450°C may be a reasonable estimate. In Y,
(1987). The increase in 6’“C of the organic matter in the schist zone as compared with the slate zone
because
indicates
reset, 350°C
the Ar ages of the muscovites the temperature during
probably
this event.
that the 40 Ma heating of the greenschist The heating
We therefore
event reached
facies conditions
no more, at least around
were not
remained
zircon
but probably
bodies Y and F.
effect of the youngest and
conclude
the extreme
sphene
ages for
from all four bodies,
a temperature
schist, assuming similar Frey Carbon tween
in the Swiss Alps is valid
isotopic pair F and
Using sphalerite of Y (Huang,
400°C
and
in two schist T gave
by Hoefs
applicable
thermometry
using samples
a temperature
thermometry
for the
estimate
in a and
to Taiwan. the calcitelocated
be-
of 430°C.
in schists just north
1979). Liou et al. (1981) have esti-
and from the K-feldspar Ar age from Y (resetting temperature ca. 200°C; Hart, 1964). In addition, it
mated
has reset the biotite Sr and Ar ages for the three southern bodies while in Y they still reflect the 40 Ma event. Thus, the last event also affected the
Phase petrology and element partitioning in mica and chlorite have been used by Ernst (1983) to estimate the conditions of the greenschist facies metamorphism in C, and have yielded 425 f 75°C
entire region. Its peak temperature was greater than ca. 250°C throughout the entire region so that the tracks in zircon and sphene would be reset (Liu, 1982). have been below in F, T and C in not reset in the
The temperature of the last 300°C in Y but above this view of the fact that biotites former but that they were
event value were in the
latter. In F, this temperature would not have reached beyond 350°C thus avoiding resetting of the Ar in the muscovites. These constraints limit the temperature conditions to that of the mid-range of greenschist facies metamorphism. Of all the published mineral age data on the Taiwan gneiss, only three values fell outside the ranges of the above three events; all three values concern Ar ages for biotites from Y. Two of them lie intermediate between 40 and 90 Ma and the other between 40 Ma and the < 12 Ma event. We are inclined to interpret these three ages as fictitious owing to partial resetting. The last two events were both greenschist facies metamorphism events. The last event reached 250-300°C in Y and 300-350°C in F, while the 40 Ma event reached 300-350°C in Y and 400450°C in F. These temperatures depend on the closure temperature which was estimated elsewhere and which may not be strictly applicable to Taiwan. However, it is gratifying to see that they agree with the estimates based on other methods (Table 3). Carbon isotopic compositions of metasediments have been measured in a transect across the slate/schist boundary by Liu et al.
the
of around
that the temperature
profile (1976)
graphite
event can be
seen in the young (< 2 Ma) fission-track apatite,
below
metamorphism
conditions
of the
greenschist
in Y to be 350-475°C
facies
at 5 kbar.
at 4 kbar. Wang Lee et al. (1982) have derived similar values (420-48O”C, 4.1-4.5 kbar) for C and for T. From Table 3 it is clear that the conditions estimated by Liou et al. (1981) for the greenschist facies metamorphism in Y are applicable only to the Paleogene event and should not be confused with the most recent event. The carbon isotopic temperature is also for the Paleogene event, and was not re-equilibrated in the most recent event. Furthermore, since both events were capable of producing chlorites, it is not surprising that it is difficult to correlate the resetting of the biotite ages with chloritization. The tectonic significance
of the temperature
distribu-
tion Two types of gradient were observed for the youngest event. First, there is the temperature difference between Y and F. The more southerly body (F) seems to have been hotter (> 300°C) than the northern body (Y) (< 3OO’C). Secondly, we found that within bodies T and C the ages in the west are older than those in the east, indicating that the temperature was increasing towards the east. The temperature differences in these two directions may result from the tectonic activity caused by the ongoing arc-continent collision in eastern Taiwan. Such differences may also have resulted from different burial depthsthe overburden may increase towards the east or the south,
Rb-Sr
ISOTOPIC
RECORD
IN TAIWAN
GNEISSES
AND
ITS TECTONIC
which would cause the temperature to increase in these directions. The 40 Ma event seems to have been a more intense event than the < 12 Ma event because it reached higher temperatures. It also showed a temperature increase from Y to F, which is similar to that for the < 12 Ma event. Therefore, perhaps the Paleogene event was also caused by tectonic activity or a difference in burial depth. Jahn et al. (1986) have speculated that the 40 Ma event might have been related to the opening of the South China Sea. Note that the southward increase in temperature found in the gneiss contradicts that inferred from petrographic studies of the neighbouring schist. Chen et al. (1983) noticed that garnet was present in schists near Y, biotite occurred near C, and only chlorite occurred further to the south. They attribute this to a temperature decrease towards the south during greenschist facies metamorphism.
141
IMPLICATION
Uplift 0
(a)
G
Rate
60
T
Gneiss
v
Biotile
Body
e
12
10
8
6 Time
4
2
P*e,e”t
(Ma)
0
(b)
C
0 Gnei,ss
Body
60
360 , 12
I
L__~_I________L_____-~~-~
10
B
The uplift rate in the current mountain building
6 Time
Fig. 5. Uplift rate (mm/yr)
The fission-track ages of apatites have been determined by Liu (1982) for Y, T and C. In each of the three bodies they show an identical range from 0.25 to 0.6 Ma. This corresponds to uplift rates of between 7 and 13 mm/yr for the past 900,000 years. The Holocene rates have been estimated from dating of uplifted corals to be about 5 mm/yr (Peng et al., 1977), while the present rate has been measured with geodetic techniques to be up to 20 mm/yr (Liu, 1987). Using the Rb-Sr cooling ages of the biotites, we can compute the uplift rates in the Pliocene. The possible ranges are between 0.3 and 2.9 mm/yr for T and between 0.5 and 2.5 mm/yr for C (Fig. 5). Jahn et al. (1986) gave a rate of at least 3-4 mm/yr for C. These rates agree with those estimated from sedimentation rates in the Western Foothills of Taiwan dated using pollen, biostratigraphy and palaeomagnetism (Chen et al., 1977; Liew, 1986; Homg et al., 1988). It seems that different methods all suggest a gradual acceleration of uplift rates towards recent times. Note that the uplift rates estimated above assume a geothermal gradient of 30°C/km. However, in Barr and Dahlen’s recent thermal model of the Taiwan thrust belt (1989), the geothermal gradient in the
(mm/year)
r
4
2
Present
(Ma)
for the Central Range based on the
Rb-Sr biotite ages (this study) and fission-track ages of zircon and apatite (Liu, 1982) of the (a) T gneiss body and (b) C gneiss body assuming a geothermal gradient of 30°C/km
and
a surface temperature of 20°C. The adopted closure temperatures for biotite, zircon and apatite are 300*50, 135f20
235+50
and
“C, respectively. An acceleration in the uplift rates from Late Miocene towards Recent is apparent.
basement is expected to be much higher due to internal strain heating and frictional heating on the decollement. If so, the uplift rates based on the higher geothermal gradient would become lower. Furthermore, in these models the isotherms are distorted, so detailed particle trajectories must to be used in the computation of the uplift rates. Acknowledgements
We thank Messrs. W.K. Chau, D.C. Lee and H. Tsai for their very competent assistance and Miss H.F. Chiu and Mr. H.J. Yang for drafting the figures. The senior author is .indebted to Prof. Bor-ming Jahn and Mr. J. Comichet of the University of Remres for valuable discussion and laboratory tutoring during her visit. Comments from Y.N. Shieh (Purdue) were helpful. This paper
142
C.-Y
has benefitted
from the critical
views of B.M. Jahn (Rennes), and
C.H.
Lo (Princeton)
supported
and thorough
re-
J.G. Liou (Stanford) and
the research
from the National
was
Ho, C.S., 1986. An introduction explanatory
of the Repub-
J. and
Frey,
carbonaceous
Cosmochim.
C.S., Lee, T.Q., Chen,
Tsenwenchi
from the Chipan (C) gneiss body. 87Rb/86Sr and 87Sr/86Sr for muscovite and plagioclase separates were 48.3 k 1.0 and 0.76620 k 0.00008 and 5.5 k 0.1 and 0.71581 k 0.00005, respectively. The RbSr isochron age calculated from the slope of the tie line through these two minerals is 83.4 f 2.3 Ma. This age agrees with the ca. 90 Ma muscovite ages found in bodies F and Y, the whole-rock age for subunits in Ka and F, and the lower intercept age of the U-Pb analysis of zircon for C (Jahn et al., 1986). These new data strengthen the claim that the 90 Ma event was a metamorphic event for the entire region, including Chipan.
building.
2. Thermal
F.A., 1989. Brittle frictional structure
and heat budget.
mountain
ages from coexisting
between
pairs of Rb-Sr
metamorphic
micas.
C.H.,
Chu,
Explanatory Taiwan.
notes
Liou, for
the
area, Taiwan:
marine
W.G.,
facies
from
1983.
map
of
Shakatangchi
and
T.J., 1986. Isotope and exchange
Earth Planet. exposed
along
Taiwan.
J. Metamorph. and
metamorphism Philos. Trans.
of the Chishan 2: 169-190. of the chloritoid
Laohsichi
area,
Hualien.
10: 11-27. systematics during
in minerals:
biotite
metamorphism.
Tailuko Jahn,
parageneses Gorge,
Central
in Taiwan,
1987.
Mountain
Crustal
a post-Palaeozoic
R. Sot. London,
Taiwan:
(Abstr.).
an example
2: 47-56
of
Quater-
sulfide
ore
of a sphalerite
(in Chinese,
with En-
Springer, Jlger,
(Editors),
dating.
Lectures
in
In: E. JLger and Isotope
Geology.
Berlin, pp. 52-76.
E.. 1979. The
Hunziker
Rb-Sr
(Editors),
method.
Lectures
In: E. JIger
in Isotope
and
Geology.
J.C.
Springer,
Berlin. pp. 13-26. Jahn, B.M., 1988. Pb-Pb Nature,
dating
of young marbles
from Taiwan.
332(31): 429-432.
Jahn, B.M., Martineau,
F. and Cornichet,
cal significance
of Sr isotopic
line limestones
of the Central
J., 1984. Chronologi-
compositions Range,
in the crystal-
Taiwan.
Mem. Geol.
6: 295-301. F., Peucat,
J.J. and Comichet,
Geochronology
of the Tananao
and its regional
tectonic
Schist
significance.
J., 1986.
complex,
Taiwan,
Tectonophysics,
125:
103-124. V.C., Chow,
metamorphic
T.J. and Lo, H.J., 1972. K-Ar rocks
of Taiwan.
Acta
Geol.
ages of the Taiwan.,
15:
113-118. W.S. and Bellon, H., 1986. Potassium-argon
Tananao
Schist
in Taiwan.
Mem.
Geol.
ages of the Sot.
China,
7:
405-416. C.Y.,
1982. Mineral
Nanao
area,
I-Lan,
China,
25: 38-52.
chemistry
of Yuantoushan
northeastern
Lan, C.Y., 1989. Geochronology
Taiwan.
and rock chemistry
Ph.D. Thesis, Inst. Geol., National
gneiss,
Proc. Geol. Sot. of Taiwan
Taiwan
Univ..
211 pp. Lan, C.Y. and Wang
Lee, C., 1987. Fanpaochienshan
phy and mineral pp. 12-13
amphibolite: chemistry.
field occurrence,
Annu.
gneiss petrogra-
Meet. Geol. Sot. China,
(Abstr.).
Lan, C.Y., Lee, T., Shen, J.J. and Chau, W.K., 1986a. Isotopic in metamorphic
rocks Range,
Geol., 1: 305-329. B.M.,
Symp.
of an iron
J.C., 1979. Potassium-argon
and its associated
Alpine
the
sequences
Taiwan.
2nd (Taipei)
Ti-Chih,
Hunziker
gneisses.
Sci. Lett., 78: 355-367.
Ernst, W.G., 1983. Mineral
W.G.
J.C.
Lan,
biostratigraphy,
and petrology
Acta Geol. Taiwan.,
rejuvenation
sediments
stratigraphy,
Mem. Geol. Sot. China,
P.Y., 1963. Mineralogy
Taiwan.
Ernst,
Ernst,
T.C. and Tsai, L.P., 1977. A
paleomagnetic
and paleoclimate.
Dempster,
and
metamorphic
C.Y., Huang,
study of the late Neogene
rock
J.G.
Cent. Geol. Surv. Spec. Publ. 2, 32 pp.
Chen, P.H., Huang,
Chen,
Hunziker.
Juang, H.T.,
Plio-Pleistocene
Metamorphism
eastern
of
from
glish Abstr.).
Chem.
Geol., 72: 29-36. Chen,
composition profile
Acta, 40: 945-951.
southwestern
of Taiwan, 1979.
geothermometer.
Juan.
G., 1988. Linear correlation
isochron
W.L.,
body, Tungao,
J. Geophys.
Res., 94 (B4): 3923-3947. Cavazzini,
nary Geology Huang,
Jahn, B.M., Martineau,
Barr, T.D. and Dahlen,
163 pp. (in
M.P. and Chen, J.C., 1988. The
of
section,
Sot. China,
References
(2nd ed.).
Econ. Aff., Taiwan,
in a metamorphic
magnetostratigraphy
muscovites
J.
of Taiwan:
map of Taiwan
M.. 1976. The isotopic
matter
Swiss Alps. Geochim. Horng,
able to separate
age relaColorado.
Chinese). Hoefs,
Note
Range,
to the geology
text for the geologic
Cent. Geol. Surv., Minist.
lic of China.
We were recently
and isotopic-mineral
zone in the Front
ET AI
Geol., 72: 493-525.
and NSC77-0202-MOOl-11 Science Council
S.R., 1964. The petrology
tions of a contact
in part by grants NSC75-0202-MOOl-11,
NSC76-0202-MOOl-12
Hart,
LAN
records
of arc-continent
nita, 6(2): 204 (Abstr.
collision
in Taiwan.
Lan, C.Y., Shen, J.J. and Lee, T., 1986b. Rb-Sr accretion
and
mobile
belt.
Ser. A, 321: 129-161.
of andesites eruption
from Lu-Tao,
ages and isotopic
Sci.. Acad.
Terra
Cog-
P06).
Sin., 6: 211-226.
Lan-Hsu,
isotopic
study
and Hsiao-Lan-Hsu:
heterogeneity.
Bull. Inst. Earth
Rb-Sr ISOTOPIC
RECORD
IN TAIWAN
GNEISSES
AND
ITS TECTONIC
Lan, C.Y., Lee, T., Jahn, B.M. and Wang Lee, C.M., 1988. An isotopic
and geochemical
genesis
and tectonic
Evolution
study
of Taiwan
implication.
of the Eastern
gneisses:
Int. Symp.
Eurasian
Margin
p. 68
T.C. and Wang
sion
orogeny.
Symp.
revealed
by some inconsistant
Meet. Geol. Sot. China,
Publ.,
logs (Taipei),
Annu.
3: l-10
Taiwan.
Group
(in Chinese,
with
China,
in
Cent. Geol. Surv.
Wang
Arc-Continent
plications
of Pleistocene
Acta Geol. Taiwan., Liou, J.G., Ernst,
sediments
and climatic
in Western
W.G. and Moore,
rocks
in northeastern
ampbibolites
Taiwan.
Bull. Geol.
and
Liu, K.K.,
and
Chen,
C.H.
Technology
crustal
the Central
Range,
implication Taiwan.
and
Central
Range,
Geol.
1954.
bodies,
em Taiwan.
T.P.
(in Chinese).
and
Lo,
eastern
Taiwan.
Acta
C.H.,
1982.
Hoping-Cbipan
Geol.
Taiwan.,
21:
Sot.
China,
complex
of biotite in the granitic
The gneisses
for isotope
geochronol-
26: 291-315.
Taiwan.
micas
the Hoping-Chipan
Proc. Geol. Sot. China,
chemistry
in some
area, Hualien, 24: 40-55.
East-
fossils
Bull. Geol.
Surv.
Bull. Geol.
Surv. Schist
12: 53-66.
S., 1964. Potassium-argon
from the Tananao report.
study on the Tananao
Bull. Geol. Surv. Taiwan,
Yen, T.P. and Rosenbhun,
schist
terrane
Proc. Geol. Sot. China,
ages of
of Taiwan-a
pre-
7: 80-81.
Yen, T.P., Sheng, C.C. and Keng, W.P., 1951. The discovery fusuline York,
limestone
in the metamorphic
T.F.,
possible tion China,
errors. 1987.
Earth
Carbon
criterion
of
of Taiwan.
fitting
of a straight
line with
Planet.
Sci. Lett., 5: 320-324.
isotope
composition
of marble:
for geochronologic/stratigraphic
in the Tananao 8: 123-133.
complex
3: 23-25.
D., 1969. Least-squares
correlated Yui,
Lee, C., 1981. Mineral
of Taiwan.
Yen, T.P., 1960. A stratigraphical
25:
rocks of
of the late Paleozoic of Taiwan.
5: l-100.
liminary
of fission track ages from
Proc.
T.P.,
in northern
8: 95-112.
and its implications
ogy. Acta Geol. Taiwan., Lo, C.H. and Wang gneissose
Y., Yen,
Bull. Geol. Surv. Taiwan, Taiwan
for the Tananao
25: 5-12
4: 23-26.
Taiwan,
22-37. Lo, C.H., 1988. Chloritization eastern
Huahen,
Taiwan,
P.Y., 1987. Carbonaceous
Mem. Geol. Sot. China,
Liu, T.K., 1982. Tectonic
uplift rates
Mem. Geol. Sot.
in some gneiss bodies,
Sot.
deforma-
of Surveying
matter in some pelitic rocks of the northern Taiwan.
C., Wang,
in the metamorphic
(in Chinese).
and Chen,
Ana-
(Abstr.).
Yen, T.P., 1953. Gn the occurrence
Yen, sea level and vertical
Science
Lee,
and
Am., 92: 609-748.
6th, pp. 135-142
Oro-
and Ancient
2: 57-69.
area,
Taiwan.
D.E., 1981. Geology
associated
Symp.
Taiwan
colli-
and
122-139.
of some polymetamorphosed
Mapping,
Collision
island since the early Holocene.
Polymetamorphism
im-
24: 165-177.
petrology
Liu, C.C., 1987. Mean
pp. 36-37
some constraints
arc-continent
Lee, C., 1982. A new interpretation
Wang stratigraphy
in Eastern
Schist. Proc. Geol. Sot. China,
English
Abstr.). Liew, P.M., 1986. Palynological
of the young
Peng, T.H., Li, Y.H. and Wu, F.T., 1977. Tectonic of Taiwan
study of the Tananao
the region north of the Liwuchi, Spec.
dates.
p. 13 (Abstr.).
Lee, C.S., 1984. Stratigraphic Taiwan
K-Ar
Lee, C., 1988. “OAr/“Ar
rocks of Taiwan:
consequences
genie Sedimentation
Law, E.W., 1988. A revised model for the origin of the Tananao
tions.
Onstott,
studies on the basement on thermal
Geodynamic (Paris),
143
Lo, C.H.,
petro-
(Abstr.). Schist
IMPLICATION
Group,
Taiwan.
Mem.
a
correlaGeol.
Sot.