Vol. 43 No. 4
SCIENCE IN CHINA (Series D)
August 2000
Isotopic chronology and geological events of Precambrian
complex in Taihangshan region
LIU Shuwen (刘树文)1, LIANG Haihua (梁海华)1, ZHAO Guochun (赵国春)2
HUA Yonggang (华永刚)1 & JIAN Anhua (简安华)1
1. Department of Geology, Peking University, Beijing 100871, China;
2. Apply Technology School, Curtin University, West Australia
Correspondence should be addressed to Liu Shuwen (email: [email protected])
Received May 17, 1999
Abstract
There are five major geological events in Precambrian complex, Taihangshan region
determined by researching into geology and isotopic chronology of the complex. Basaltic magma
erupted and quartz-dioritic to tonalitic magma intruded in earlier neo-Archaean, which formed hornblende-plagiogneiss of Fuping gneiss complex and metamorphic mafic rock enclaves in TTG
gneiss complex. Granulite facies metamorphism and emplacement of biotite-plagiogneiss occurred
in late neo-Archaean. Extension and uplifting from the end of neo-Archaean to Paleoproterozoic era
formed Chengnanzhuang large extensional deformation zones and metamorphic mafic veins emplaced into the deformation zones. Remobilization of Precambrian complex and tectonic uplifting in
late Paleoproterozoic era formed Longquanguan ductile shear zone and emplacement of Nanying
gneiss. Occurrence of regional granite pegmatite at the end of Paleoproterozoic era means the end
of the Lüliang movement.
Keywords:
Taihangshan region, Precambrian complex, isotopic chronology, geological events.
1 Geological setting
Research in recent years shows that Precambrian complex, Taihangshan region is made up of
three geological units[1,2]. The first unit, whose lithology is TTG gneiss, is Fuping Gneiss complex
distributed mainly in foothill belt around Fuping County. The unit contains diverse enclaves such
as mafic granulites, garnet-clinopyroxene-amphibolite (some samples contain less quartz), pyroxene-hypersthene-magnetite-quarzite, garnet-biotite-plagiogneiss and garnet-hypersthene-plagiogneiss and so on. Lithologies of TTG gneisses are mainly hornblende-plagiogneisses and biotiteplagiogneisses. Dark minerals in the former are mainly hornblende with less biotite that come up
to 15% in the rocks. Dark minerals in the latter are made up of biotite with less hornblende which
are no more than 10% in the rocks. Two types of TTG gneisses cannot be distinguished in the map
on the present research extent. The second unit is called Wanzi metamorphic layered rock series. It
consists of metamorphic layered rock series distributed in the southeast and west of Fuping gneiss
complex, which are metamorphic supracrustal rocks in amphibolite facies. Lithology of the unit is
diverse marbles, calc-silicate rocks and Al-rich gneisses. The third unit is called Nanying gneiss. It
emplaced in between Fuping gneisses and Wanzi metamorphic layered rock series above in
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ISOTOPIC CHRONOLOGY OF PRECAMBRIAN COMPLEX IN TAIHANGSHAN REGION
387
space[1,2]. Lithology of the unit is mainly granodioritic gneiss to adamellitic gneisses (granite),
some Sill-Qz-sphere granites and some amphibolites. This unit is an intrusion emplaced along
extension-deformation zones between Wanzi metamorphic layered rock series and Fuping gneiss
complex. They are distributed in peak belts. Based on a lot of research in field geology, we have
carried out some Sm-Nd isotopic analysis for some representative samples of biotite plagiogneisses of Fuping complex and Nanying gneiss, some amphibolites as metamorphic veins and Rb-Sr
isotopic analysis for some representative adamellite of Nanying gneiss (some samples with SillQz-sphere) near Mengjiazhuang. Referring to precedent isotopic data and results of geological
chronological research, we determine the geological events of the Precambrian complex in Taihangshan in this paper. Three geological units and the distribution of samples analyzed in isotope
are drawn on the geological sketch map (fig. 1).
Fig. 1. The sketch map of early Precambrian complex in Taihangshan region. 1, Mesozoic granite; 2,
granite in Fuping age; 3, granite in Wutai age; 4, Nanying gneiss (third geological unit); 5, Wanzi metamorphic layered rock series (second geological unit); 6, Fuping gneiss complex; 7, Wutai group; 8, Hutuo
group; 9, Paleozoic group; 10, middle-upper protozoic group; 11, geological borderline; 12, fault; black
diamond showing the location gathered samples of amphibolite dikes in table 1, samples: f103-1
f108-1;
black rhombus presenting the location of samples I9725-1, 9758-1, 9711-1 in table 1; black-star presenting location of samples: 9737-1, 9711-1-1, 9735-1 in table 1 and black hexagon presenting the location of
samples in table 2.
2 Isotopic chronology of Fuping gneiss complex
Isotopic chronology studies of Fuping gnesiss complex in recent years have made great advances. Meanwhile the advances enhance research on the area. Previous researchers got many
[3]
isotopic ages of the complex. For example, clastic zircon concordant U-Pb age of 2.8 +−00..23
in
15 Ga
the gneiss, Sm-Nd isochron age of (2.79±0.171) Ga[4] in granulite and gneisses, single zircon
207
Pb-206Pb peak age of (2.69±0.06) Ga[5] in hornblende-plagiogneiss through LP-ICPMS method
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SCIENCE IN CHINA (Series D)
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and single zircon U-Pb age of (2.535±0.000 8) Ga[6] in biotite-plagiogneiss through SHRIMP
method. The first two ages were determined according to the acquaintanceship that Fuping gneiss
complex is stratigraphic system. So Sm-Nd age is determined by connecting isochron between
gneisses and metamorphic mafic rocks[4]. Research in recent years confirms that TTG gneisses of
Fuping gneiss complex are plutonic intrusion, enclaves of mafic granulites and clinopyroxeneamphibolite that are metamorphic supracrustal rock[1, 2, 7
9]
. They belong to different isotope sys-
tems. We get isochron age of (2.679±0.069) Ga, M. S. W. D.=1.59, ε Nd (t) = +3.5, using six samples of granulites and clinopyroxene-amphibolites of previous researchers’ isotopic data[4, 8] and
average depleted mantle model age of 2.767 Ga (table 1, fig. 2(a)). Some gneiss samples and
Table 1
Sample
−6
Sm-Nd isotopic characteristics of Precambrian complex in Taihangshan
Sm(×10 )
Nd(×10−6)
2.757
2.717
3.694
5.558
2.789
1.940
9.847
10.05
15.782
29.94
11.956
7.146
147
Sm/144Nd
143
Nd/144Nd
2σ
ε Nd (0)
ε Nd (t)
f Sm/Nd
2.82
2.80
2.72
2.73
2.79
2.75
−6.125 2
−8.173 4
−15.215 4
−25.768 7
−16.054 2
−7.510 2
3.309 9
3.3653
3.808 1
3.458 2
3.445 1
3.720 6
−0.14
−0.17
−0.28
−0.43
−0.28
−0.17
2.92
3.04
2.98
2.87
2.86
−25.144 45
−19.272 86
−22.842 63
−29.767 59
−32.205 96
−0.255 6 c)
−0.498 9 c)
−0.507 4 c)
−0.224 4 c)
−0.413 9 c)
−0.39
−0.29
−0.35
−0.46
−0.50
2.47
2.43
2.37
2.35
2.42
2.45
2.51
−33.122 79
−4.740 187
−1.658 09
−3.764 84
−3.686 812
−5.188 847
−21.925 8
5.471 0
5.110 3
5.309 9
5.570 2
5.126 9
5.017 5
4.674 0
−0.60
−0.15
−0.11
−0.15
−0.14
−0.16
−0.42
t DM/Ga
Amphibolite and granulite enclaves in Fuping gneiss complex
8098-6a)
f4-3b)
8055a)
f1-3b)
8256a)
Ag04a)
0.169 4
0.163 3
0.141 6
0.112 0
0.141 1
0.164 2
0.512 324
0.512 219
0.511 858
0.511 317
0.511 815
0.512 253
0.000 010
0.000 012
0.000 008
0.000 008
0.000 039
0.000 009
Fuping gneiss complex
I9725-1
9758-1
9711-1
8252a)
H8506a)
4.081
4.310
4.083
3.767
2.894
20.505
18.736
19.264
21.487
17.652
0.120 4
0.139 1
0.128 2
0.106 1
0.099 2
fl03-1
fl04-2
fl06-1
fl07-1
fl07-2
fl08-1
fl03-2
2.077
2.218
1.758
1.771
2.694
2.242
7.576
16.179
8.063
6.071
6.375
9.605
8.202
39.934
0.077 7
0.166 4
0.175 3
0.168 0
0.169 6
0.165 3
0.114 7
0.511 349
0.511 650
0.511 467
0.511 112
0.510 987
0.000 009
0.000 007
0.000 008
0.000 02
0.000 009
Amphibolite veins
0.510 940
0.512 395
0.512 553
0.512 445
0.512 449
0.512 372
0.511 514
0.000 020
0.000 007
0.000 015
0.000 009
0.000 007
0.000 008
0.000 007
Nanying gneiss
a)
78111-1
1.517
9.493
0.096 7
0.511 101
78115-1a)
1.107
6.174
0.108 5
0.511 260
f06-1b)
2.665
27.02
0.059 5
0.510 586
9737-1
3.900
22.795
0.103 5
0.511 181
f06-4b)
1.962
18.94
0.062 5
0.510 604
9711-1-1
5.990
38.550
0.093 9
0.511 035
9735-3
6.521
37.585
0.104 9
0.511 165
a) b) Sample numbers cited from refs.[4,8], other samples are
0.000 007
0.000 015
0.000 012
0.000 007
0.000 016
0.000 006
0.000 007
analyzed in
2.65 −29.982 17 −2.723 6 −0.51
2.72 −26.880 57 −2.840 9 −0.43
2.52 −40.028 25 −2.623 6 −0.70
2.71 −28.421 62 −3.011 0 −0.47
2.55 −39.677 12 −3.093 1 −0.68
2.68 −31.269 63 −3.250 3 −0.52
2.76 −28.733 73 −3.716 9 −0.47
this paper. Analyses of all samples in this
paper are made by VG354 in the Institute of Geology, Chinese Academy of Sciences. Analyst: Qiao Guangsheng. Analysis
flow was given in ref. [13],
143
Nd/144Nd=0.512 635±8 in BCR-1a and
143
143
Nd/144Nd=0.512 644±6 in BCR-1b. All ratios of
Nd/144Nd=0.712; ε Nd is calculated by CHUR reference. Today’s CHUR are
143
Nd/144Nd in table 1 are standardized by
143
Nd/144Nd=0.512 638, 147Sm/143Nd=0.196 7. c) Data in εNd(t) are calculated by 2.539 Ga, depleted mantle MORB parameter
values are (143Nd/144Nd)r=0.513 153 and (147Sm/143Nd)r=0.213 7.
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ISOTOPIC CHRONOLOGY OF PRECAMBRIAN COMPLEX IN TAIHANGSHAN REGION
389
Fig. 2. Sm-Nd isochron of Precambrian complex in Taihangshan. (a) Sm-Nd isochron of granulite-clinopyroxeneamphibolite enclaves in Fuping gneiss complex; (b) Sm-Nd isochron of biotite-plagiogneiss in Fuping gneiss
complex; (c) Sm-Nd isochron of late amphibolite veins in Fuping gneiss complex; (d) Sm-Nd isochron of leucogneisses in Nanying gneiss.
the samples (8255-1 and 8266) [4, 8] that have abnormally high depleted mantle model age in
original isochron are excluded. The isochron age should reflect lithogenetic age of granulites
and clinopyroxene-amphibolites. These isochron ages are in accordance with 207Pb-206Pb peak
age of single zircon in hornblende-plagiogneiss in the error bound of the isochron. It reflects
that there was a period of intensive magma activity whose characteristics are emplacement of
amphibolite plagiogneiss and volcanic activity of basaltic magma.
Two biotite-plagiogneiss samples of Fuping gneiss complex were obtained near Daliushu[4],
three samples (I9725-1, 9758-1 and 9711-1) were supplied in the same geological unit east of
Guanganqiao, Fuping County (the location of supplied samples is shown on the sketch map, black
rhombus (fig. 1), three samples are all biotite-plagiogneiss). In the light of Sm-Nd isotope analytical results of the three samples and that of two samples (8252 and H8506, table 1) by Zhang Z. Q.
(1991), we have obtained five samples’ isochron age, (2.502±0.050) Ga (fig. 2(b)). The average
depleted mantle model age is 2.93 Ga (table 1), M. S. W. D.=1.40 and ε Nd ( t ) = −0.7. The average depleted mantle model age is older than 2.9 Ga reflecting that middle-Archean crust may exist.
That ε Nd ( t ) is –0.7 showing that the source was mixed by crustal material. SHRIMP single zircon U-Pb age of 2.535 Ga[6] is in the error range of the isochron age. It is suitable to believe that
lithogenetic age of biotite-plagiogneiss is 2.535 Ga, which reflects that there was an event of a
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SCIENCE IN CHINA (Series D)
Vol. 43
great deal of TTG magma activity in the late Archean in this area. Researching in petrology and
metamorphic fluid confirms that biotite-plagiogneiss did not undergo granulite facies metamorphism[1]. The compositions of metamorphic fluid of biotite-plagiogneiss are similar to that of retrometamorphic stage of granulite during the period decompression and uplifting. These explain
why the emplacement and crystallization of biotite-plagiogneisses were later than the peak metamorphism of granulite facies and formed in earlier uplifting process of Archaean crust. The metamorphic peak of granulite facies was before crystallization of biotite-plagiogneisses. However,
metamorphism of granulite facies and magma activity of biotite-plagiogneiss belong to the same
period of tectonomagmatic event in neo-Archean era.
3 Metamorphic mafic veins
The samples of metamorphic mafic veins were obtained from the northern slope (f103-2) of
the mountain, boundary between Fuping County and Laiyuan County, to Dongzhuang on the foot
of the mountain (f108-1) along Fuping-Laiyuan road. The sampling area, about 1 km long, is an
outcrop area of Chengnanzhuang low-angle extensional deformation zone. Amphibolites appear to
be low angle veins whose width is about 0.3 2 m. Their boundary intersects gneissosity of biotite-plagiogneiss at a low angle. Isotopic data of 7 samples are shown in table 1. Sm-Nd isochron
age is (2.519±0.044) Ga (fig. 2(c)), M.S.W.D. is 1.46, ε Nd (t)=+5.2. ε Nd (t) value is above MORB
depleted mantle line. So MORB depleted mantle model age not only is less than isochron age but
also has a wider range of distribution (2.34 2.51 Ga, table 1). The average model age is 2.42 Ga.
The reasons why ε
Nd
(t) value is above MORB depleted mantle line and why MORB depleted
mantle model age is dispersed are as follows: (i) Isochron age may be real age and magma came
from the mantle which was more depleted than MORB depleted mantle. (ii) Samples connecting
isochron are not the same source. (iii) Late magma was hybridized by older crust during the period
of emplacement and crystallization. Study of metamorphic mafic volcanic rocks in Taihang-Wutai
area showed that they underwent contamination of crust material[9, 10]. So isochron age of
(2.519±0.044) Ga has obscure geological significance and needs to be researched further.
However in the field, because Nanying gneiss emplaced and intersected the metamorphic mafic
dikes, the metamorphic mafic dikes were formed prior to Nanying gneiss from the end of Archean
to earlier period of Paleoproterozoic era. They can be formed in the extension event.
4 Nanying gneiss
Leuco-granitoid gneisses in Nanying gneiss are mainly biotite-plagiogneiss, adamellitic
gneiss and K-feldspar-gneiss (granites), whose obvious characteristics are few dark minerals and
heterogeneous gneissosity. They emplaced along gneissosity of biotite-plagiogneiss and hornblende-plagiogneiss (Shihu) in Fuping gneiss complex or were intersecting gneissosity of Fuping
gneiss complex at a low angle (Lishuyuan and Xicaokou). Depleted mantle model age of seven
samples at Daliushu and east of Guanganqiao is 2.52 2.76 Ga (table 1). Average value is 2.66 Ga.
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ISOTOPIC CHRONOLOGY OF PRECAMBRIAN COMPLEX IN TAIHANGSHAN REGION
Isochron age is (2.123±0.055) Ga (fig. 1(d)). ε
Nd
391
(t) is –2.62, M.S.W.D.=1.22. The average de-
pleted mantle model age is approximate to isochron age of fig. 1(a). This isochron age of 2.123 Ga
may have two different geological senses as follows: (i) If the source rock formed in about 2.70
Ga, it was remelted in later metamorphism-magma event, which did not come up to Sm-Nd isotopic homogenization, isochron age is the age of crystallization. (ii) If late magma was hybridized
by older crust minerals, the isochron age has no geological sense.
Another type of rock in Nanying gneiss, which was called leucoleptynite because of fewer
dark minerals, is adamellite containing Sill-Qz-sphere in some parts of it. Their geological distribution is very broad. The main rocks are pink adamellite and K-felspar granite containing a tiny
amount of dark minerals, made up of biotite, a little muscovite, magnetite and Sill-Qz-sphere.
Samples that were used to measure Rb-Sr isotope in this paper were pink adamellites and Kfelspar granites (some contain Sill-Qz-sphere) near Mengjiazhuang (fig. 1). Such rocks emplaced
in supracrustal rocks and Fuping gneisses complex. Contacting relationship between these rocks
and country rock does not mean unconformity. These rocks have few dark minerals, unclear
gneissosity and an orient distribution of Sill-Qz-sphere. Rb-Sr isotopic data obtained from 6 samples is given in table 2, which shows that isochron age is (1.933±0.08) Ga (fig. 3). According
Table 2
Rb-Sr isotopic characteristics of adamellites in Nanying gneiss
Sample
Name of rocks
Rb(×10−6)
Sr(×10−6)
87
97165-1
fine-grain adamellite
155.354 0
33.655 0
13.826 60
1.089 680
0.000 038
97156-3
magnetite adamellite
102.783 0
40.650 0
7.455 80
0.924 589
0.000 020
97158-2
coarse-grain adamellite
186.601 0
75.978 0
7.235 88
0.915 776
0.000 028
97129-1
gneissic fine-grain adamellite
163.327 0
140.541 0
3.386 68
0.802 414
0.000 016
97156-1
fine-grain biotite adamellite
143.798 0
152.954 0
2.736 06
0.788 452
0.000 021
97145-2
two micas adamellite
159.112 0
193.005 0
2.397 26
0.780 115
0.000 023
Rb/86Sr
87
Sr/86Sr
2σ
Isotopic analysis of all samples in this table was conducted by Qiao G.S. et al. using VG354, Institute of Geology, Chinese
Academy of Sciences. NBS607’s 87Sr/86Sr=1.200 000 0±30.
to the existence of granite texture and unclear gneissosity in the rocks, it is obvious that late thermal disturbance and metamorphism-deformation seldom affect the rocks. The fact that the
Rb-Sr isochron is highly concordant with
M.S.W.D. being 1.27 shows that the activity of
Rb-Sr isotope in the late period was not obvious
and so the Rb-Sr isochron age should be lithogenetic age. They should come from crust according to ISr=0.712. The results that Rb-Sr
isochron age is lower than Sm-Nd isochron age
of leuco-granitoid gneisses in Nanying gneiss
Fig. 3. Rb-Sr isochrone of adamellites.
show that the formation of leuco-gneiss may be
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SCIENCE IN CHINA (Series D)
earlier than pink adamellites. There are many isotopic age values[4, 5, 10
Vol. 43
12]
of Lüliang period in
Taihangshan region, which reflects that Taihangshan region is an abnormally active area during
the period of Lüliang movement. The characteristics were tectonic uplifting and formation of
Longquanguan ductile shear zone. Middle-crust remelted and formed over-aluminum adamellites
(similar to S-type granite in components) and leuco-granitoid gneiss.
5 Formation age of granite pegmatite
There are a lot of granite pegmatites in Taihangshan Precambrian complex, distributed in all
the area and concentrated from Fuping to Fangli. They appear to be distinctive scale veins whose
color is from gray to pink. They are irregular veins intersecting gneissosities of Fuping gneiss
complex. Eleven zircons of granite pegmatites in Fuping gneiss complex were determined with an
evaporating method. We have obtained 207Pb/206Pb data of 107. These data show the age of
(1.799±7) Ga which is concordant with SHRIMP single zircon U-Pb age of (1.790±8) Ga[6] well.
Two ages show that there was an important geological fluid event in about 1.8 Ga, which can reflect the end of remobilization events of Precambrian complex in Taihangshan area during the
period of Lüliang.
6 Conclusion
Integrating the above geological and chronological characteristics, we summarize geological
events in Precambrian complex in Taihangshan region as follows.
1) Basaltic magma erupted and quartz-dioritic to tonalitic magma emplaced in 2.7 Ga, which
formed hornblende-plagiogneisses, original rocks of enclaves of granulite and clinopyroxeneamphibolites in hornblende-plagiogneiss and biotite-plagiogneiss.
2) Peak of granulite facies metamorphism is about 2.55 Ga. Biotite-plagiogneiss distributed
in all the area were formed in 2.50 2.55 Ga and predominated the Fuping gneiss complex.
3) Taihangshan Precambrian complex extended and uplifted from the end of Archean to earlier period of Paleoproterozoic era. Metamorphic mafic dikes that were formed along extensional
deformation zone show the end of tectonothermal event in Archaean era.
4) The region reactivated and the Longquanguan ductile shear zone formed in about 2.0 Ga
(Lüliang Period). The area continued to uplift and the middle-crust melted. Leuco-granitoid gneiss
and pink K-feldspar-granite (containing Sill-Qz sphere) were formed and consist predominantly of
Nanying gneiss.
5) Much granite pegmatite was formed in about 1.8 Ga and showed the end of Lüliang tectonic thermal event.
Acknowledgements
We thank Qiao G. S. for his assistance in sampling and measurement.
We are also grateful
to Prof. Han B. F. for his valuable comments and assistance in writing the paper. This work was supported by the National Natural Science Foundation of China (Grant Nos. 49572153 and 49832030).
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