40ar/39ar thermochronology constraints on jurassic tectonothermal event of nyainrong microcontinent

Journal of Earth Science, Vol. 25, No. 1, p. 98–108, February 2014 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-014-0403-0

Xie, C. M., Li, C., Wu, Y. W., et al., 2014. 40Ar/39Ar Thermochronology Constraints on Jurassic Tectonothermal Event of Nyainrong Microcontinent. Journal of Earth Science, 25(1): 98–108, doi:10.1007/s12583-014-0403-0

40Ar/39Ar Thermochronology Constraints on Jurassic Tectonothermal Event of Nyainrong Microcontinent

Chaoming Xie*, Cai Li, Yanwang Wu, Ming Wang, Peiyuan Hu

College of Earth Science, Jilin University, Changchun 130061, China

ABSTRACT: To reveal the Jurassic tectonothermal event occurring to the Nyainrong microcontinent which is gripped among the Bangong-Nujiang suture zone, 40Ar/39Ar dating was carried out on the basement orthogneiss and Jurassic granitc gneiss in the microcontinent. In the heating stage, four sam-ples exhibited a flat plateau age, with the value Tp concentrated in the range of 166–176 Ma; isochron age Ti was concentrated in the range of 165–175 Ma, and their corresponding ages were the consistent within allowable range. The ages should be representative of the era of the final deformation of the Amdo gneiss and cooling emplacement of the magmatic rock in the Jurassic. The geochronological studies have shown that the final deformation of microcontinent crystalline basement and the cooling of the Mesozoic large-scale tectonothermal events occurred in late Middle Jurassic. In Middle Jurassic, Nyainrong microcontinent experienced strong tectonic movement. Combining with the geochronologi-cal with isotope geochemistry for the microcontinent, the cause of the tectonothermal event should be attributed to the collision between the Nyainrong microcontinent and South Qiangtang Block following the northward subduction of Bangong-Nujiang oceanic crust. KEY WORDS: Tibet Plateau, Nyainrong microcontinent, Jurassic, tectonothermal event, 40Ar/39Ar thermochronology.

1 INTRODUCTION Nyainrong microcontinent, located in the juncture of

South Qiangtang Block and Gangdise Block, is the only tec-tonic block with the outcropping of a wide range of crystalline basements. Nyainrong microcontinent is the window for under-standing and studying the early formation and evolution of the Qinghai-Tibet Plateau, and therefore is of high tectonic signifi-cance. Currently, Jurassic tectonism and magmatism is rarely studied and the viewpoints diverge concerning Jurassic tectonic-magmatism (Guynn et al., 2012, 2006; Xie et al., 2010; Li et al., 2008; Xu Z Q et al., 2005; Xu R H et al., 1985). Xu et al. (1985) obtained U-Pb age (171±6 Ma) of gneiss samples in the south of Amdo, and considered the Early–Mid Jurassic magmatism to be related to the collision between the Lhasa and the Qiangtang blocks; Guynn et al. (2012, 2006) believed the granitoid magmatism and metamorphism to occur in the same period as that in the Amdo region and related to the northward subduction of Bangong-Nujiang oceanic crust. Combining with the structural and stratigraphic studies, Zhu et al. (2008a, b) suggested that Mesozoic magmatism in the Gangdise belt may be explained by the bidirectional scissors-style subduction model (the scissors open toward the west), with the Bangong- Nujiang oceanic crust subducting southward and the *Corresponding author: [email protected] © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2014 Manuscript received November 12, 2012. Manuscript accepted February 28, 2013.

neo-Tethys oceanic crust subducting northward. On the basis of the petrology, whole-rock geochemistry, U-Pb zircon dating and Lu-Hf isotopic data on zircon of host granites and dioritic enclaves from the Nyainrong pluton, Liu et al. (2011, 2010) proposed that the Early–Mid Jurassic ~175 Ma magmatism in the Nyainrong pluton could be related to the collision between the Nyainrong microcontinent and Qiangtang terrane following the northward subduction of the Bangong-Nujiang oceanic crust. The high-pressure granulite newly found is hosted in the Nyainrong microcontinent. Metamorphism study indicated that the high-pressure granulite underwent fast tectonic uplift through the isothermal decompression (Zhang X R et al., 2010; Zhang X Z et al., 2010), and this new result is of great signifi-cance for further discussion.

The existing studies mainly deal with the geochronology and geochemistry of Jurassic magmatism in the microcontinent, but the relationship between the formation age of the basement gneiss and the age of Mesozoic magmatic rock deformation is rarely discussed, which restricts the understanding of the Juras-sic tectonothermal event and the tectonic evolution of micro-continent.

In this article, we focus on 40Ar/39Ar thermogeochronol-ogy of the basement gneiss and Jurassic magmatic rocks, and select the strongly deformed basement gneisses and Jurassic granitic gneiss of Nyainrong microcontinent for the dating of biotite. The purpose of this article is to relate the basement gneiss with the large-scale Jurassic magmatic activity, and to discuss the relationship between the basement gneiss and other deformed rocks of microcontinent (Jurassic magmatic rocks). A precise age constraint is provided for the in-depth understand-


99

ing the Jurassic tectonic movement of Nyainrong microconti-nent. 2 GEOLOGICAL SETTING

Nyainrong microcontinent, located in the middle of Bangong-Nujiang suture zone is bounded by the Ganong- Nimaqu-Xiaqiuka fault to the south and the Bangong-Nujiang suture zone to the north (Fig. 1). It is lens-like and east-west trending. It is predominately composed of Nyainrong Group, the Paleozoic strata and Mesozoic granitoids.

The Nyainrong Group, occurring in the Cuonaco-Nimaqu area, is the oldest unit in the Nyainrong microcontinent and composed of strongly foliated orthogneisses (two-feldspar gneiss, plagiogneiss) that contain small but abundant mafic amphibolite lenses (Guynn et al., 2012, 2006; Zhang X Z et al., 2010; Bai et al., 2005; Nima et al., 2005; Coward et al., 1988; Xu et al., 1985). The gneisses were intruded by widespread,

generally undeformed granitoids of variable composition dur-ing Mesozoic (Guynn et al., 2012, 2006; Zhang X R et al., 2010; Bai et al., 2005; Nima et al., 2005; Coward et al., 1988; Harris et al., 1988; Kidd et al., 1988; Xu et al., 1985). Metasedimen-tary rocks are exposed to the south of the Nyainrong microcon-tinent, and contain marble, quartz-mica schist, quartzite, and garnet-kyanite schist. Most radiometric ages for the gneisses from the Nyainrong Group are in the range of 490–530 Ma, corresponding to the Pan-African metamorphic event.

Mesozoic granitoids are extensively exposed to the north of the Nyainrong microcontinent. The granitoids are composed of tonalite, granodiorite, hornblende monzogranite, mon-zogranite and syenogranite which intrude into Nyainrong Group with the attitude of abyssolith or stocklike (Fig. 2a). The fractional granitoids were also deformed by metamorphic- deformational event.

MZ

Amdo

Naqu

QiazeXiaqiuka

Nyainrong

Zigetang CoCuona Co

Nimaqu

Ganong

Lhasa

NaquAmdo

Shuanghu Yushu

Qamdo

Medog

Northern Qiangtang-Qamdo Plate

Basu

Southern Qiangtang Plate

Gangdise Plate

C Z

AnЄ

MZ

PZ

oph

MZ

MZ

MZ

MZ

MZ

MZ

MZ

MZ

MZ MZ

PZ

PZ

PZ

PZ

PZ

CZ

CZ

CZ

CZ

C Z

CZ

CZ

CZ

CZ

oph

oph

oph

oph

ophoph

AnЄ

PZ

oph

PZ

PZoph

0 40 km

1 2 3

4 5 6

90º40' 91º30' 92º30' 93º32'E

32

º47

'N3

1º1

3'

Ma

Ma175 Ma

172 Ma

180 Ma

179 Ma

178 Ma175 Ma

176 Ma

185

910 Ma

838 Ma

493 Ma

447 Ma

AD03

AD04

AD05b1AD05b2

JSS

LSLSBNS

YYS

AD03

AnЄ

AnЄ

N

AnЄ

492 Ma

0 300 km

Figure 1. Tectonic diagram of the Amdo region in northern Tibet showing the distribution of the Nyainrong Group (accord-ing to 1 : 1 500 000 Metamorphic Geology Map of Himalaya-Gangdise; 1 : 250 000 Geology Map of Amdo Town, Naqu Town and Zigetang Co). 1. Bangong-Nujiang north main fault; 2. Bangong-Nujiang south main fault; 3. main fault; 4. Mesozoic granitoids; 5. sampling position; 6. sample number; JJS. Xijinwulan-Jinshajiang suture zone; LSLS. Lungmu Co-Shuanghu-Lancangjiang suture zone; BNS. Bangong-Nujiang suture zone; YYS. Indus Yarlung Zangbo suture zone; AnЄ. Precambrian Nyainrong Group; Pz. Paleozoic; Mz. Mesozoic; Cz. Cenozoic; oph. ophiolite; age data after Guynn et al., 2012, 2006; Liu et al., 2010; Bai et al., 2005; Xu et al., 1985. 3 PETROGRAPHY

The sampling position is 30 km south of Amdo County, with lithologies of biotite two-feldspar gneiss, granite gneiss and two-mica plagiogneiss, well-developed gneissosity. The trending of gneissic layering is east-northeast. The detailed characteristics of the samples are shown in Table 1.

Mineral compositions of the samples are quartz+ plagioclase+biotite±perthite±accessory mineral (zircon, allanite, apatite). Some mineral shave ductile deformation including

deformation lamella of quartz, kink bands and mica fish of biotite, and mechanical twinning of plagioclase（Fig. 2g). Bio-tite is partially replaced by chlorite. The temperature of the mineral deformation is in the range of 300–500 ºC (Passchier and Trouw, 1996; Etheridge, 1971).

4 40Ar/39Ar DATING 4.1 Analytical Method

40Ar/39Ar dating has become one of the most important

Chaoming Xie, Cai Li, Yanwang Wu, Ming Wang and Peiyuan Hu

100

methods of isotope geochronology. Unique step heating tech-nique and Ar isotope correlation diagram of internal compo-nents can not only obtain high-precision age but also reveal multi-stage geological evolution of the sample (Chen et al., 2011). As described in the former section, biotite is replaced by chlorite in some samples. In order to obtain reliable 40Ar/39Ar age of biotite, we selected the sample whose biotite grains were idiomorphic and without chloritization.

The two samples of biotite two feldspar gneiss (AD03 and AD05b1) were selected: one was two-mica plagiogneiss (AD05b2) and the other was granite gneiss (AD04). They were selected for 40Ar/39Ar dating. The biotite flakes were purified using magnetic separation, and further cleaned in an ultrasonic bath with ethanol. The purity was higher than 99%. The mica sample was wrapped in aluminum foil and loaded into a tube with Al foil, and then sealed into a quartz bottle. The bottle was irradiated for 65 h in a nuclear reactor. The reactor delivered a neutron flux of ~6×1012 n·cm-2·s-1 and the integrated neutron flux was about 1.16×1018 n·cm-2. Ar isotope analysis was car-ried out on an MM-1200B mass spectrometer at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing. The measured isotopic ratios were corrected for mass discrimi-nation, atmospheric Ar component, procedural blanks and mass interference induced by irradiation. The blanks of m/e of 40Ar, 39Ar, 37Ar and 36Ar were less than 6×10-15, 4×10-16, 8×10-17 and 2×10-17 mol, respectively. The correction factors for interfering isotopes produced during the irradiation were determined by the analysis of irradiated K2SO4 and CaF4 pure salts. Their values are (36Ar/37Aro)Ca=0.000 238 9, (40Ar/39Ar)K=0.004 782 and (39Ar/37Aro)Ca=0.000 806, respectively. All 37Ar values were corrected for radiogenic decay (half-life 35.1 days). The 40K decay constant used was 5.543×10-10 a-1 (Steiger and Jager, 1977). The standard sample was ZBH-25 which had an age of

132.7±1.2 Ma and a potassium content of 7.6% (Chen et al., 2002). The errors on the apparent ages for each step were quoted at the 1σ level, but those of weighted mean plateau ages and isochron ages were given at the 2σ level. A detailed de-scription of 40Ar-39Ar dating has been published elsewhere (Zhai et al., 2011; Chen et al., 2006; Zhang et al., 2006; Sang et al., 1992; Zeitler et al., 1985).

4.2 Results

The results of 40Ar/39Ar dating are shown in Table 2. Two samples (AD03 and AD05b1) yielded well-defined plateau ages during the hearting phase, with the width of plateau reaching 95%. The 40Ar/36Ar intercept values were comparable with the atmosphere value (295.5) within the allowable range of error for both samples. The initial 40Ar/36Ar of the sample AD04 was 233, which was lower than the modern air ratio of 295.5. The sample might lose Ar in the magmatism process. The initial 40Ar/36Ar of sample AD05b2 was 392, which was higher than the modern air ratio of 295.5. The sample might have excess 40Ar, which came from the degasification of potash minerals, and entered into the biotite by microcrack channel in the de-formation process.

The ages were all close to the Jurassic magmation. The plateau ages of four samples were in the range of 166–176 Ma, and the isochron ages were in the range of 165–175 Ma. The agreement between the plateau and isochron ages, the atmos-pheric composition of trapped 40Ar/36Ar values argue against a significant excess argon component in the biotite of this study. These ages represent the time when the temperature of biotite was cooled to the closure temperature of 250–350 ºC. We con-clude that the obtained ages are represent the time of ultimate deformation of granitoids and cooling in granite magma.

Table 1 General features of the dated samples of granite gneiss in Amdo region of Tibet

Sample AD03 AD04 AD05b1 AD05b2

Lithology Greyish black Bi two-feldspar

gneiss

Greyish white granite gneiss

Flesh pink Bi two-feldspar gneiss

Greyish white two-mica plagiogneiss

Location Fig. 1 Fig. 1 Fig. 1 Fig. 1

Attitude Gneissosity NEE

Gneissosity 115º∠60º



Structure Granoblastic texture;

gneissose

Granoblastic texture; gneissose (Fig. 2d)

Granoblastic texture; gneissose

Granoblastic texture; gneissose

Mineral component

Quartz (35%)+ perthite (25%)+

plagioclase (30%)+ biotite (10%)

Quartz (~55%)+ plagioclase (35%)+

K-feldspar (~5%)+ bio-tite (~5%)

Quartz (~50%)+ K-feldspar (25%)+plagioclase (20%)+

biotite (~5%)+(zircon, allanite, apatite)

Quartz (~45%)+plagioclase (~20%)+biotite (~30%)+ muscovite (5%)+(zircon,

apatite)

Biotite characteristic

Dark brown, Schistose,

directionality, mica fish (Fig. 2e)

Dark brown, schistose, directionality,

biotite was partial replaced by chlorite

(Fig. 2f)

Tawny, schistose, biotite was partial

replaced by chlorite, kink bands (Fig. 2h)

Bronzing, schistose, biotitewas partial replaced by

chlorite, kink bands (Fig. 2e)


101

Figure 2. Outcrop and micrographs of granitoids in the Amdo region, northern Tibet. (a) Outcrop of biotite plagiogneiss and deuteric intrusion (lens towards south-west); (b) outcrop of Neoproterozoic and Cambrian gneiss (lens towards north); (c) outcrop of biotite two-feldspar gneiss; (d) close shot of granite gneiss; (e) gneissosity and mica fish of two-mica plagiogneiss; (f) photomicrograph of granite gneiss; (g) deformation lamella of quartz of biotite two-feldspar gneiss; (h) biotite was partial replaced by chlorite and kink bands of biotite; Q. quartz; Pl. plagioclase; Pe. perthite; Bi. biotite; Ms. muscovite; Ch. chlorite; Ap. apatite; Zrn. zircon.

Tab

le 2

R

esu

lts

for

40A

r/39

Ar

step

wis

e h

eati

ng

dat

ing

for

bio

tite

s in

gra

nit

oid

s fr

om A

md

o

T (

C)

(40A

r/39

Ar)

m

(36A

r/39

Ar)

m(37

Ar/

39A

r)m

(38A

r/39

Ar)

m40

Ar

(%)

F

39A

r (1

0-14 m

ol)

39A

r/C

um (

%)

Age

(M

a)±1

(Ma)

AD

03 b

ioti

te W

=30

.78

mg,

J=

0.00

8 61

7, to

tal a

ge=

171.

8 M

a, T

p=17

1.1±

1.3

Ma,

Ti=

168.

6±3.

8 M

a

700

32.8

09 1

0.

089

5 0.

036

0 0.

033

3 19

.35

6.34

8 1

0.45

0.

85

96.1

2.

0

800

13.7

79 6

0.

007

5 0.

008

8 0.

016

3 83

.83

11.5

51 8

8.

65

17.0

7 17

1.2

1.6

840

11.9

42 5

0.

000

8 0.

005

3 0.

014

8 98

.06

11.7

11 3

9.

84

35.5

3 17

3.4

1.7

880

11.9

06 1

0.

000

7 0.

000

0 0.

014

8 98

.11

11.6

80 8

5.

40

45.6

5 17

3.0

1.7

930

11.8

68 9

0.

001

1 0.

001

9 0.

015

0 97

.21

11.5

38 2

3.

18

51.6

2 17

1.0

1.7

980

11.9

01 8

0.

001

4 0.

003

0 0.

014

9 96

.36

11.4

68 8

2.

55

56.4

0 17

0.0

1.6

1 03

011

.646

2

0.00

1 3

0.00

8 7

0.01

5 3

96.5

9 11

.249

5

3.94

63

.79

166.

9 1.

6

1 07

011

.864

3

0.00

0 6

0.00

6 6

0.01

4 8

98.5

9 11

.696

5

5.94

74

.94

173.

2 1.

7

1 11

011

.946

8

0.00

0 6

0.00

1 6

0.01

4 7

98.5

0 11

.768

1

7.22

88

.47

174.

2 1.

7

1 19

011

.973

6

0.00

0 4

0.00

0 0

0.01

4 6

98.9

5 11

.847

3

5.59

98

.95

175.

4 1.

7

1 30

012

.248

1

0.00

1 5

0.00

0 0

0.01

5 6

96.3

1 11

.796

6

0.47

99

.83

174.

6 3.

2

1 40

014

.881

0

0.01

4 6

0.72

0 1

0.02

0 0

71.3

3 10

.620

7

0.09

10

0.00

15

8 12

AD

04 b

ioti

te W

=30

.03

mg,

J=

0.00

8 77

0, to

tal a

ge=

160.

5 M

a, T

p=16

5.8±

1.2

Ma,

Ti=

168.

1±2.

6 M

a

700

18.0

69 1

0.

045

4 0.

052

9 0.

023

9 25

.83

4.66

6 7

2.26

5.

01

72.3

6 0.

77

760

13.8

90 1

0.

011

4 0.

017

9 0.

016

2 75

.67

10.5

10 1

4.

59

15.1

8 15

9.1

1.5

800

11.6

77 1

0.

002

2 0.

007

4 0.

014

3 94

.42

11.0

25 9

6.

69

30.0

1 16

6.5

1.6

840

11.5

62 6

0.

001

5 0.

002

5 0.

014

1 96

.14

11.1

16 0

5.

86

42.9

9 16

7.8

1.6

880

11.9

81 9

0.

003

1 0.

000

0 0.

014

3 92

.36

11.0

66 3

2.

92

49.4

6 16

7.1

1.6

940

12.1

83 8

0.

004

4 0.

003

8 0.

014

7 89

.41

10.8

93 0

2.

97

56.0

6 16

4.6

1.6

1 00

012

.620

7

0.00

6 4

0.02

0 4

0.01

5 1

85.1

0 10

.740

4

6.21

69

.82

162.

4 1.

6

1 05

011

.669

4

0.00

2 6

0.01

3 5

0.01

4 3

93.5

0 10

.910

6

7.50

86

.44

164.

9 1.

6

1 10

011

.574

2

0.00

1 6

0.00

6 7

0.01

4 1

95.8

3 11

.091

6

4.37

96

.13

167.

5 1.

6

1 18

011

.784

5

0.00

1 9

0.04

3 4

0.01

4 2

95.2

6 11

.225

8

1.42

99

.28

169.

4 1.

7

1 30

012

.369

7

0.00

3 9

0.00

0 0

0.01

5 1

90.5

6 11

.202

1

0.30

99

.96

169.

1 4.

0

1 40

029

.086

4

0.06

7 1

1.17

1 4

0.02

9 9

32.1

1 9.

347

6 0.

02

100.

00

142

44

AD

05b1

bio

tite

, W=

30.8

9 m

g, J

=0.

009

095,

tota

l age

=17

4.7

Ma,

Tp=

175.

6±1.

1 M

a, T

i=17

5.3±

1.8

Ma

700

14.8

62 6

0.

027

4 0.

045

6 0.

021

2 45

.42

6.75

0 6

0.72

1.

33

107.

5 1.

6

750

12.5

96 4

0.

005

0 0.

030

8 0.

016

2 88

.16

11.1

05 4

2.

15

5.28

17

3.6

1.7

800

11.8

67 1

0.

002

2 0.

000

0 0.

015

6 94

.51

11.2

15 7

4.

83

14.1

6 17

5.2

1.7

850

11.3

71 5

0.

000

7 0.

001

7 0.

015

3 98

.24

11.1

71 1

10

.24

32.9

9 17

4.6

1.7

Con

tin

ued

T (

C)

(40A

r/39

Ar)

m

(36A

r/39

Ar)

m(37

Ar/

39A

r)m

(38A

r/39

Ar)

m40

Ar

(%)

F

39A

r (1

0-14 m

ol)

39A

r/C

um (

%)

Age

(M

a)±1

(Ma)

AD

03 b

ioti

te W

=30

.78

mg,

J=

0.00

8 61

7, to

tal a

ge=

171.

8 M

a, T

p=17

1.1±

1.3

Ma,

Ti=

168.

6±3.

8 M

a

900

11.4

16 1

0.

000

6 0.

001

5 0.

015

3 98

.46

11.2

40 8

6.

00

44.0

2 17

5.6

1.7

950

11.5

21 8

0.

000

8 0.

010

8 0.

015

3 97

.88

11.2

77 5

3.

15

49.8

1 17

6.1

1.7

1 00

011

.797

4

0.00

1 2

0.00

0 0

0.01

5 4

96.9

1 11

.432

6

3.04

55

.40

178.

5 1.

7

1 05

011

.373

8

0.00

0 5

0.00

9 0

0.01

5 2

98.6

2 11

.217

1

5.91

66

.27

175.

2 1.

7

1 10

011

.307

6

0.00

0 3

0.00

8 9

0.01

5 2

99.2

3 11

.221

2

11.5

4 87

.49

175.

3 1.

7

1 15

011

.375

1

0.00

0 4

0.01

6 6

0.01

5 3

99.0

0 11

.261

7

5.18

97

.01

175.

9 1.

7

1 25

011

.587

9

0.00

0 3

0.00

0 0

0.01

4 9

99.2

6 11

.501

6

1.57

99

.89

179.

5 1.

7

1 40

015

.995

7

0.00

6 9

0.00

0 0

0.01

2 6

87.3

1 13

.966

6

0.06

10

0.00

21

6 13

AD

05b2

bio

tite

, W=

30.8

3 m

g, J

=0.

009

095,

tota

l age

=17

5.1

Ma,

Tp=

176.

1±1.

2 M

a, T

i=17

4.2±

2.3

Ma

600

119.

683

7 0.

396

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000

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096

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700

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6.94

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175.

7 1.

7

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.510

6

0.00

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Err

ors

are

1; F

=40

Ar*

/39A

r; C

um. c

umul

ativ

e.


104

180

160

140

120

100

800 20 40 60 80 100

Ag

e (M

a) 800-1 070 ºC

Plateau age=171.1±1.3 Ma (2 )�

Includes 74.1% of the Ar39

800-1 300 ºC WMPA=172.5±1.7 Ma

0.000 6

0.000 4

0.000 2

0.000 00.068 0.072 0.076 0.080 0.084 0.088

800-1 300 ºCAge=168.6±3.8 Ma

Initial =322±4840 36Ar/ Ar

MSWD=101

BiotiteBiotite

Biotite Biotite

Biotite Biotite

Biotite

Biotite

190

170

150

130

110

90

0 20 40 60 80 100

Ag

e (M

a)

70

50

800-1 100 ºC



Tp=165.8±1.2 Ma

0.000 6

0.000 5

0.000 4

0.000 3

0.000 2

0.077 0.081 0.085 0.089

0.000 1

0.000 0

800-1 100 ºCAge=168.1±2.6 Ma


MSWD=49

240

200

160

120

800 20 40 60 80 100

Ag

e (M

a)

750-1 150 ºC



Tp=175.6±1.1 Ma

0.000 5

0.000 4

0.000 3

0.000 1

0.000 00.077 0.085

0.000 2

0.081 0.089

750-1 150 ºC

Age=175.3±1.8 Ma

MSWD=26

200

160

120

80

40

00 20 40 60 80 100

Cumulative Ar percent39

Ag

e (M

a)

780-1 400 ºC



Tp=176.1±1.2 Ma0.002 0

0.001 6

0.001 2

0.000 8

0.000 00.05 0.06 0.07 0.08 0.09

0.000 4

780-1 400 ºCAge=174.2±2.3 Ma

MSWD=53

AD03

AD04

AD05b1

AD05b2

(a) (b)

(c) (d)

(e) (f)

(g) (h)

AD03

AD04

AD05b1

AD05b2

36

40

Ar/

Ar

36

40

Ar/

Ar

36

40

Ar/

Ar


36

40

Ar/

Ar

39 40Ar/ Ar


Figure 3. 40Ar/39Ar age spectra and 40Ar/39Ar isochron diagram of biotites from the granite gneiss in Amdo region. 5 DISCUSSION 5.1 Age Records of Magmatic Event of Nyainrong Micro-contient 5.1.1 Crystalline basement of microcontient

More and more studies indicate that there are Neo-Proterozoic–Early Paleozoic crystalline basement rocks in

Nyainrong microcontient. The orthogneisses ages of basement rocks are concentrated in 467–910 Ma and the U-Pb zircon dating results are shown in Table 3. Xu et al. (1985) carried out U-Pb zircon dating on Amdo gneiss and the discordia upper and lower intercept ages are 531±13 and 171±16 Ma (MSWD=0.91), which indicates that Nyainrong microcontient


105

underwent Early Paleozoic magmatic events and the basement rocks were metamorphosed at Middle–Late Jurassic. The 1 : 250 000 Regional Geological Survey of Amdo County showed that the ages of the tonalite felsic gneiss in Nyainrong Group are 491±2 and 492±1 Ma and the ages of the gneissic adamel-lite are 814±18 and 515±14 Ma (Bai et al., 2005). We also car-ried out U-Pb dating on the zircons of orthogneiss in Nyainrong Group and the results are in the range of 488–505 Ma. Guynn et al. (2012) obtained many age records of Pan-African Early Paleozoic in this area. The study results above show that Nyainrong microcontient underwent Neo-Proterozoic magmatic events and Early Paleozoic metamorphic events, and the crys-talline basement was formed in Late Cambrian.

5.1.2 Jurassic magmatic event

In this article, LA-ICP-MS U-Pb dating was carried on the zircons of granitic gneiss (sample AD04) in Nyainrong Group, and the detailed principle and process of the experiment fol-lowed those by Song et al. (2002). CL images of zircons (Fig. 4) showed that most zircon particles were recrystallized with clear oscillatory zoning, with the particle size ranging from 150 to 200 μm and the length/width ratio ranging from 3 : 2 to 2 : 1.17 analyses gave Th/U=0.31–1.44, which indicated a magmatic origin. Their 206Pb/238U weighted mean age was 178±2 Ma, which was the best estimate of the emplacement age of granitic gneiss.

Jurassic magmatic rocks are widely developed in Nyain-rong microcontient and many chronologic data have been re-ported (Table 4, Fig. 1). Guynn et al. (2006) carried out U-Pb zircon dating on Nyainrong and the results were concentrated in 171–183 Ma; Bai et al. (2005) obtained similar results (171–185 Ma). Liu et al. (2011, 2010) reported that the ages of monzodiorite, granodiorite and alkali granite in Nyainrong Group were all 175 Ma, while the dating results of a diorite inclusion in Nyainrong Group and its host granite were 184±1 and 185±2 Ma, respectively. They suggested that the basement of Nyainrong microcontient was remelted at Early Jurassic, and subsequently the melt was mixed with magma derived from EM-type lithospheric mantle and influenced by the surrounding rocks. These Jurassic magmatic rocks might be formed by the

188184180176172168

164

=17, MSWD=1.7, 95% conf.n

Mean=177.9±2.0 Ma

0.0140.10 0.14 0.18 0.260.22

207 235Pb U/

100

140

160

180

200

120

220

240

0.018

0.022

0.026

0.030

0.034

0.038

3

1220

62

38

Pb

U/

20

62

38

Pb

U/

Figure 4. U-Pb concordia diagram of zircon from granitoids in Amdo.

convergence of Nyainrong microcontient and Qiangnan Block following the northern subduction of Bangong-Nujiang oceanic crust. 5.2 Jurassic Tectonothermal Event of Nyainrong Micro-contient and Its Tectonic Significance

The primary rocks of sample AD05b2 were formed at Neo-Proterozoic, while the ages of sample AD03 and AD05b1 were concentrated in Cambrian. These sample were parts of the basement of the microcontient, and the results of 40Ar/39Ar isotopic dating are in the range of 166–176 Ma. Sample AD04, which intruded in the Precambrian Nyainrong Group, had a zircon age of 178±2 Ma and a biotite 40Ar/39Ar age of 166±1 Ma, so it was formed at Early Jurassic and metamorphosed at Middle Jurassic, because biotite had a lower blocking tempera-ture (250–350 ºC) than zircon (Chen et al., 2011).

In this article, we also collected the reported isotope data of Jurassic in microcontient, which was a strong evidence of this tectonic event. Xu et al. (1985) got a lower intercept ages of 171±16 Ma by zircon U-Pb dating of Amdo gneiss and sug-gested that the basement of Nyainrong microcontient was reac-tivated at Middle Jurassic and there was a huge tectonic event in northern Lhasa terrane in Jurassic. Nyainrong microcontinent was missing Triassic–Middle Jurassic deposition. The meta-morphic ages of several granitic gneisses from the basement rocks are in the range of 176–181 Ma (Guynn et al., 2012). Recently, the high-pressure granulite was discovered in Nyain-rong microcontient (Zhang X R et al., 2010; Zhang X Z et al., 2010), and the U-Pb isotope age of zircon cores from the high-pressure granulit lens, 20 km south to Amdo County, was (169±1 Ma) (Zhang X R et al., 2010). In this study, the U-Pb dating of zircons from the high-pressure granulite in Nyainrong microcontient indicated a metamorphic age of 179±2 Ma. Ac-cording to isotopic chronological data, both the granulite and its surrounding rocks underwent Pan-African movement, while the granulite also underwent a tectonic event of Late Proterozoic– Early Paleozoic and the peak high-pressure metamorphism at Early–Middle Jurassic. During the 1 : 250 000 regional geo-logical survey of Amdo County, SHRIMP zircon U-Pb and 40Ar/39Ar isotope dating were carried out on porphyry-like granodiorites. The results indicated that the zircon U-Pb ages were 1854 and 1783 Ma, the biotite’s 40Ar/39Ar plateau age was 1711 Ma and isochron age was 1723 Ma, the amphi-bole’s isochron age was 17419 Ma. Moreover, the survey also obtained the SHRIMP zircon U-Pb age of 1716 Ma and bio-tite’s 40Ar/39Ar plateau age of 1881 Ma for the large- porphyritic amphibole monzonitic granite (Bai et al., 2005).

Guoqu rocks deposited in Late Jurassic–Early Cretaceous, which was a combination of continental shelf, contained am-monoid, and had an angular unconformity with the Jiayuqiao Formation. This was also the strong evidence for Jurassic tec-tonic event.

All the studies above indicate that both the old basement and Mesozoic granitic rocks underwent strong tectonic activity. The Early–Middle Jurassic rocks were metamorphosed at 166–176 Ma about 10 Ma after their intrusion when the Ar isotope system of biotite was reseted. Moreover, this Ar isotope


106

Table 3 Isotopic ages of magmatic rocks in the Amdo region in Tibet

Other age (zircon) Sample Lithology Crystallize age (Ma)

Biotite Ar-Ar plateau age

(Ma) Type Age (Ma)

Reference

AD03 Bi+two-feldspar gneiss

488±4 171±1 Xie et al., 2010

AD04 Granite gneiss 178±2 166±1 This article AD05b1 Bi two-feldspar

gneiss 505±4 176±1 Xie et al., 2013

AD05b2 Two-mica plagiogneiss

820±5 176±1 Wang et al., 2012

AD10 High pressure granulite

541–834 (inheritance) Metamorphism 179±2 Xie et al., 2013

XGS-63 Gneiss Higher intercept 531±13 Lower intercept 171±6 Xu et al., 1985 JG062004-4 Porphyritic quartz

syenite 171±3 Guynn et al., 2006

AP062104-A Alkali-feldspar gran-ite

172±4 Guynn et al., 2006

AP061504-B Porphyritic Bi granite 175±4 Guynn et al., 2006AP061604-B Bi granite 174±4 Guynn et al., 2006JG062204-1 Bi+Hbl granodiorite 177±3 Guynn et al., 2006AP052904-A Porphyritic quartz

syenite 178±5 Guynn et al., 2006

AP060604-A Porphyritic quartz syenite

179±3 Guynn et al., 2006

JG061504-7 Bi+Hbl granodiorite 183±3 Guynn et al., 2006PK97-6-4-3A Orthogneiss Higher intercept 852±18 Lower intercept 167±68 Guynn et al., 2006not quite clear High pressure

granulite Metamorphism 169±1 Zhang X R et al.,

2010 NR13-1 Diorite (enciave) 175±1 Liu et al., 2010 NR13-3 Granodiorite (host

plant) 175±1 Liu et al., 2010

NR17-1 Alkali granite 175±0.4 Liu et al., 2010 Not quite clear Corcovadite 185±4 171±0.8 Bai et al., 2005 Not quite clear Corcovadite 172±3 172±3 Amphibole

Ar-Ar 174±19 Bai et al., 2005

Not quite clear Hornblende adamellite

171±6 188±1 Bai et al., 2005

NR04-1 Granite 185±2 Liu et al., 2011 NR06-2 Diorite 184±1 Liu et al., 2011 JG053104-1 Granite gneiss 910±16 Guynn et al., 2012JG061504-2 Granodiorite gneiss 878±15 Guynn et al., 2012JG060504-2 Granodiorite gneiss 838±23 Metamorphism 176±6 Guynn et al., 2012JG061604-1 Granite gneiss 468±53 Metamorphism 181±4 Guynn et al., 2012JG061504-1 Granite gneiss 532±7 Guynn et al., 2012PK970604-1A Granodiorite gneiss 483±13 Guynn et al., 2012PK970604-1B Granite gneiss 498±11 Guynn et al., 2012JG053104-2 Granite gneiss 487±16 Inherited ~880 Guynn et al., 2012JG061504-4 Paragneiss Max.

depositional 800–700 Guynn et al., 2012

JG061504-4 Paragneiss Metamorphism ~180 Guynn et al., 2012JG062505-3 Quartzite Max.

depositional 493±64 Guynn et al., 2012

AP061304-A Quartzite Max. depositional

447±30 Guynn et al., 2012


107

Table 4 Zircon LA-ICP-MS U-Pb dating of granitoids from Amdo

No. Th (10-6) U (10-6) Th/U 207Pb/206Pb ±1σ 207Pb/235U ±1σ 206Pb/238U ±1σ 206Pb/238U

age (Ma)

±1σ

AD04-03 217.63 376.92 0.58 0.049 6 0.001 8 0.197 7 0.007 5 0.028 9 0.000 4 184 3

AD04-05 257.11 353.14 0.73 0.048 1 0.001 9 0.185 5 0.007 4 0.028 0 0.000 4 178 3

AD04-06 278.57 302.41 0.92 0.048 0 0.002 0 0.186 1 0.007 8 0.028 1 0.000 4 179 3

AD04-07 241.16 789.67 0.31 0.051 0 0.001 6 0.194 0 0.006 3 0.027 6 0.000 4 175 3

AD04-09 360.69 866.75 0.42 0.050 3 0.001 6 0.196 1 0.006 3 0.028 3 0.000 4 180 3

AD04-10 192.87 248.41 0.78 0.048 9 0.002 3 0.193 6 0.009 0 0.028 7 0.000 5 183 3

AD04-12 155.25 218.41 0.71 0.051 1 0.002 9 0.197 0 0.011 0 0.027 9 0.000 5 178 3

AD04-16 211.85 437.62 0.48 0.050 8 0.001 9 0.199 2 0.007 6 0.028 5 0.000 4 181 3

AD04-17 221.60 302.17 0.73 0.051 9 0.002 9 0.191 6 0.010 5 0.026 8 0.000 5 170 3

AD04-18 206.82 222.90 0.93 0.046 5 0.002 7 0.182 3 0.010 3 0.028 5 0.000 5 181 3

AD04-19 333.83 335.73 0.99 0.050 1 0.002 1 0.197 2 0.008 3 0.028 5 0.000 4 181 3

AD04-20 204.90 292.44 0.70 0.050 3 0.002 3 0.194 1 0.008 7 0.028 0 0.000 4 178 3

AD04-21 124.65 90.23 1.38 0.053 1 0.004 4 0.198 4 0.016 3 0.027 1 0.000 6 172 3

AD04-22 250.29 458.23 0.55 0.049 3 0.001 9 0.184 4 0.007 1 0.027 1 0.000 4 173 3

AD04-23 379.11 262.40 1.44 0.050 2 0.002 3 0.189 9 0.008 6 0.027 5 0.000 4 175 3

AD04-24 264.76 400.00 0.66 0.047 4 0.002 2 0.184 7 0.008 6 0.028 3 0.000 5 180 3

AD04-25 312.92 410.52 0.76 0.048 8 0.001 9 0.187 6 0.007 4 0.027 9 0.000 4 177 3

system was closed at 166 Ma and the later tectonic activities were not strong enough to reset it again. This strong tectonic activity may be correlated to the convergence of Nyainrong microcontient and Qiangnan Block which reactivated the basement rocks of microcontient, and reseted the Ar isotope system and formed the gneiss. 6 CONCLUSIONS

The ages of the final deformation of crystalline basement and the cooling of the huge Mesozoic tectonothermal are con-strained by 40Ar/39Ar dating on biotite (166–176 Ma). In the Middle Jurassic, Nyainrong microcontinent experienced strong tectonic movement. Combining with the geochronological study with isotope geochemistry of this microcontinent, the cause of the tectonothermal could be related to the collision between the Nyainrong microcontinent and South Qiangtang Block following the northward subduction of Bangong-Nujiang oceanic crust.

ACKNOWLEDGMENTS

We thank Yaowu Xie from the Bureau of Geology of Tibet Autonomous Region for his assistance with the field work. Prof. Wen Chen and his research group in Chinese Academy of Geological Sciences provided laboratory assistance. We also thank Prof. Li Su from China University of Geosciences for assistance with the LA-ICP-MS U-Pb age dating. This study was supported by the National Natural Science Foundation of China (Nos. 41272240 and 41072166), and China Geological Survey (Nos. 1212011121248 and 1212011221093).

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40ar/39ar thermochronology constraints on jurassic tectonothermal event of nyainrong microcontinent

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