Wang_Tectono-magmatism Tianshan (China): Carboniferous Subduction, Permian Strike-slip_International...

8/3/2019 Wang_Tectono-magmatism Tianshan (China): Carboniferous Subduction, Permian Strike-slip_International Journal of

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ORIGINAL PAPER

Evolution of calc-alkaline to alkaline magmatism throughCarboniferous convergence to Permian transcurrenttectonics, western Chinese Tianshan

Bo Wang Dominique Cluzel Liangshu Shu Michel Faure Jacques Charvet Yan Chen Sebastien Meffre Koen de Jong

Received: 29 November 2007 / Accepted: 15 December 2008 / Published online: 8 January 2009 Springer-Verlag 2009

Abstract Continuous magmatic activity occurred in thewestern Chinese Tianshan, Central Asia, from the Car-boniferous to the Permian, i.e. before and after the LateCarboniferous amalgamation of Junggar and the YiliBlocks. Zircon UPb LA-ICPMS and ArAr data reveal acoincidence in time between regional wrench faulting andgranitoid emplacement. Permian post-collisional granitoidscrop out within or at the margins of large-scale dextralstrike-slip shear zones, some of them show synkinematicfabrics. The whole rock geochemical features of the Early-Middle Permian granitoids indicate an evolution fromhigh-K calc-alkaline towards alkaline series. In other pla-ces of the North Tianshan, alkaline magmatism occurredtogether with deep marine sedimentation within elongatedtroughs controlled by wrench faults. Therefore, in contrastwith previous interpretations that forwarded continental riftor mantle plume hypotheses, the coexistence of diversemagmatic sources during the same tectonic episode sug-gests that post-collisional lithosphere-scale transcurrent

shearing tightly controlled the magmatic activity during thetransition from convergent margin to intraplate anorogenicprocesses.

Keywords Granitoids Zircon UPb geochronology Geochemistry Transcurrent shear zones Late Palaeozoic Chinese Tianshan

Introduction

The tectonic assemblage of the Central Asian OrogenicBelt was suggested to occur during Palaeozoic time by theclosure of Paleoasian Ocean and multi-phase subductionaccretion of various micro-continents, ancient island arcsand fragments of oceanic island (Coleman 1989 ; Dobretsovet al. 1995 ; Jahn et al. 2000 , 2004 ; Jahn 2004 ; Xiao et al.2004 ; Windley et al. 2007 ).

The Chinese Tianshan Belt which is a part of this oro-genic belt, separates the Tarim Basin to the south from theJunggar Basin to the north (Fig. 1). Classically, it is con-sidered that the western Chinese Tianshan (west of Meridian88 E; Fig. 1) was built by two collisions, during the EarlyPalaeozoic between Tarim and Central Tianshan blocks, andduring the Late Palaeozoic between Central Tianshan andNorth Tianshan (Windley et al. 1990 ; Allen et al. 1993 ; Gaoet al. 1998 ; Chen et al. 1999 ; Carroll et al. 2001 ; Zhou et al.2001 ), but there is no agreement about the end of conver-gence. Generally, the convergent orogeny is considered tobe terminated before the Early Permian (Wang et al. 1994 ;Liu et al. 1996 ; Xiaoet al. 2004 , 2006 ; Gao and Klemd 2003 ;Klemd et al. 2005 ; Gao et al. 2006 ; Li et al. 2006a ; Charvetet al. 2007 ), and this phase of amalgamation was followedby regional-scale Late Palaeozoic (Permian) wrench faulting(Allen et al. 1995 ; Allen and Vincent 1997 ; Laurent-Charvet

B. Wang ( & ) L. ShuState Key Laboratory for Mineral Deposits Research,Department of Earth Sciences, Nanjing University,210093 Nanjing, Chinae-mail: [email protected]; [email protected]

B. Wang D. Cluzel M. Faure J. Charvet Y. Chen K. de JongISTO UMR 6113, University of Orle ans,45067 Orleans, Cedex 2, France

B. WangInstitute of Earth Sciences, Academia Sinica,Taipei 11529, Taiwan, ROC

S. MeffreSchool of Earth Sciences, University of Tasmania,Hobart, Australia

123

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et al. 2002 , 2003 ). Some authors argued that continentamalgamation was accomplished in the pre-Carboniferousand that a post-collisional continental rifting occurred duringthe Carboniferous to Permian (Che et al. 1996 ; Xia et al.2004b ). Others suggest that the collision in southwesternTianshan occurred as late as Late Permian-Triassic (Li et al.2002 , 2005 ; Zhang et al. 2007 ).

The magmatic afnities and emplacement ages providekeys for understanding the geodynamic evolution of theorogen, but in Chinese Tianshan, the coexistence of Car-boniferous to Permian calc-alkaline and alkaline magmaticrocks has inferred contrasting interpretations. It has beenwidely accepted that the Carboniferous magmatism issubduction-related and/or syncollisional, and the Permianone is post-collisional. However, Carboniferous-Permianvolcanic rocks were also interpreted as a result of conti-nental rifting (Che et al. 1996 ; Xia et al. 2004b ); or parts of a Large Igneous Province (Xia et al. 2004a , 2006 ) althoughthese views are not supported by compelling tectonic orstratigraphic evidences.

Recent studies suggest that the genetic links may existbetween Permian wrench faulting and post-collision mag-matism, and therefore could be a clue for understanding theevolution of the Chinese Tianshan during this period. Thisarticle presents our eld observations, new zircon UPbLA-ICPMS dating and geochemical data from Permianigneous rocks of the western Chinese Tianshan. Combiningwith previous data, we discuss the petrogenesis of Permianigneous rocks and suggest that most petrologic featureswere controlled by the location of wrench faults thatallowed local asthenosphere uplift and transition fromcalc-alkaline to alkaline magmas to occur.

Geologic setting

The Late Palaeozoic evolution of the Chinese Tianshan ischaracterized by polyphase deformation and transitionfrom continental active margin to intraplate tectonics. Thepre-Carboniferous subduction nally induced a continental

Urumqi

Korla

0 100km

HP/LT metamorphic complex

ophiolitic rocks

Carboniferous sedimentary& magmatic rocks

Early-Middle Palaeozoicand Precambrian rocks

Permian continental sediments& magmatic rocks

Post-Permian strata

indistinct fault

suposed fault

T A R I M

J U N G G A R

Permianstrike-slip fault

1

23

4

Bogda

Kuluketage

Al a t a w

Wusun

Nalati

Baluntai

A w u l a l e

B o r o h o r o Y I L I

T U - H A

Fig. 3

Fig. 5

Fig. 6

C - S T S

YILI N TS

W J G A l t a y

?

Tarim Basin

Junggar

basin

T u -H a B a s in

45N 45N

40N

85E 90E SutureThrustStrike-slip fault

China

Fig. 1 Simplied sketch map of the western Chinese Tianshan Belt(modied from XBGMR 1993 ; insets after Zhang et al. 1993 , Allenand Vincent 1997 ), showing the occurrences of Carboniferous toPermian igneous rocks and the main wrench faults. Numbers 14

correspond to the main faults, 1 North Tianshan Fault (NTF), 2 MainTianshan Shear Zone (MTSZ), 3 Qingbulak-Nalati Fault (QNF), 4Sangshuyuanzi Fault (SF), W JG Western Junggar, N TS NorthTianshan, C-S TS Central-South Tianshan

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collision between the Yili Block and Tarim in Early Car-boniferous ( * 345 Ma) along the HP/LT metamorphiczone in SW of the Tianshan Belt (Fig. 1; Allen et al. 1993 ;Gao et al. 1998 ; Gao and Klemd 2003 ; Wang et al. 2007c ).During the Carboniferous, massive limestone, peliticlimestone and sandstone were deposited (XBGMR 1993 ).Meanwhile, in the NW part, extensive volcanic rocks(mainly andesite and rhyolite) erupted indicating a long-term magmatic activity (Figs. 1, 2). Between the latestCarboniferous and Early-Middle(?) Permian, in the NorthTianshan (Fig. 1), the formation of an ophiolitic me langeincluding blocks of Late Carboniferous oceanic crustalrocks and turbidites suggests that the amalgamation of continental blocks came to the end (Windley et al. 1990 ,2007 ; Allen et al. 1993 ; Wang et al. 2006a ).

During the Permian, large-scale dextral ductile shearingoccurred along the North Tianshan Fault (Allen and Vincent1997 ; or Dzhungarian Fault by Zhang et al. 1993 ) and MainTianshan Shear Zone (Laurent-Charvet et al. 2003 ), andalong the Qingbulak-Nalati Fault (Windley et al. 1990 )and Sangshuyuanzi Fault (Yin and Nie 1996 ) (faults 1, 2, 3and 4 in Fig. 1, respectively). They are developed in a LP-HTthermal regime as testied by the widespread developmentof synkinematic andalusite (Wang et al. 2006a ). The age of ductile deformation is constrained at ca. 290245 Ma by40 Ar/ 39 Ar dating of synkinematic feldspar (Zhou et al.2001 ) and biotite (Yin and Nie 1996 ; Laurent-Charvet et al.2002 , 2003 ; de Jong et al. 2008 , this volume). In CentralAsia as a whole, a Permian transcurrent regime followed theearlier NS-directed convergence (Bazhenov et al. 1999 ,

limestonepelitic limestone

felsitedacite porphyry

pelitic siltstonepelitic sandstone

siltic limestonesandy limestone

andesiteandesite porphyry

siltstone

biogenic limestonedolomite

basaltbasitic andesite

sandstonefine grain sandstone

tuffaceous sandyconglomerate

sandy conglomerateconglomerate

detritus tuff breccia tuff

rhyoliterhyolitic porphyry

tuffaceous brecciavolcanic rocks unconformity

1000

0m

Nalati

C1q

S

Borohoro(Altaw)

P1w

C1aq

C1a

C1d

Mz

D

C2d

C2n

C2o

P3t

C1a

C1d

C2t

P3t

P2h

P2x

P1w

C2d

C1a

C1m

Mz

S

P3t

C2y

C1a

Mz

Pz1

Awulale

Wusun

C1d

P2a

Mz

P3t

C2y

Fig. 2 Simplied stratigraphiccolumns of Carboniferous-Permian volcanic andsedimentary rocks innorthwestern Chinese Tianshan(modied from 1:200,000geological maps of Xinjiangregion). Abbreviations of formations: C 1 d lowerCarboniferous DahalajunshanFm., C 1 m Meiluokahe Fm., C 1 qQiergustao Fm., C 1 a AkeshakeFm., C 1 aq Aqialehe Fm., C 2 yupper Carboniferous YishijilikeFm., C 2 d Dongtujin Fm., C 2 nNaogaitu Fm., C 2 o Oyiman Fm.,C 2 t Tuergong Fm., P 1 w lowerPermian Wulang Fm., P 2 xXiaoshansayi Fm., P 2 h HamistFm., P 2 a Aikendaban Fm., P 3 t Tielimuke Fm, D Devonian, S Silurian, Pz 1 Early Palaeozoicstrata, Mz Mesozoic strata

Int J Earth Sci (Geol Rundsch) (2009) 98:12751298 1277

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2003 ; Allen et al. 2006 ; Van der Voo et al. 2006 ; Wanget al. 2007b ). During this period, a major paleogeographicchange occurred in the Chinese Tianshan, and terrestrialsediments associated with sub aerial volcanic rocksdeposited unconformably or disconformably over Carbon-iferous sedimentary and volcanic rocks. In contrast, inrestricted areas such as the south of Bogda Mountain(Fig. 1), deep water sediments accumulated in elongatedtroughs where they overlie collapse breccia (olistoliths) andpillow basalt (Shu et al. 2005 ).

A synthesis of previous data

Carboniferous and Permian igneous rocks are widespreadin the Chinese Tianshan Belt (Fig. 1). In order to get aunied understanding on the geochemistry and chronologyof these magmatic rocks, the following paragraphs sum-marise the previous data, except for undated or poorlytime-constrained rocks that will not be considered.According to their geochemical features, three types of magmatic rocks are dened, namely calc-alkaline (CA),alkaline (A) and transitional series (TR, calc-alkaline rockswith prominently high REE contents) (see Table 1).

Carboniferous magmatism

Carboniferous volcanic rocks consist of mac, intermediateand felsic rocks (XBGMR 1993 ) that are exposed in theBorohoro, Awulale, Nalati and Wusun mountains (Figs. 1,2). The Lower Carboniferous Dahalajunshan Formation(C 1 d) is well known for its extensive distribution andremarkable thickness. It is mainly composed of andesite,rhyolite, felsite, tuff breccia and minor basalt, associatedwith limestone and sandstone. The volcanic rocks showsignicant depletion in Nb and Ta, moderate depletion of Hf and Zr, and prominent enrichment in Rb and Th(Li et al. 1998 ; Ma and Wang 2000 ; Yang et al. 2003 ;Zhang and Li 2006 ; A et al. 2006 ; Guo and Zhu 2006 ;Li et al. 2006b ; Shao et al. 2006 ). The intermediate toacidic volcanic rocks of the Lower CarboniferousAkeshake Fm. (C 1 a) and Upper Carboniferous TuergongFm. (C 2 t) in the Nalati range have similar geochemicalcompositions (Wang et al. 2007a ), they show positiveeNd (t ) values ( ? 0.32 to ? 4.90), and variable

87 Sr/ 86 Sr(i)ratios (0.70150.7068) (Zhu et al. 2005 , 2006a ; Guo andZhu 2006 ; Qian et al. 2006 ). Recent SHRIMP zirconU-Pb dating provided consistent Carboniferous ages (363313 Ma, Zhu et al. 2005 , 2006b ; Zhai et al. 2006 ). Thesevolcanic rocks are dened as CA series in Table 1.

Carboniferous adakites were recognized in Alataw,Borohoro and Baluntai areas (Fig. 1) and show high Sr, Eucompositions, eNd (t ) values ( ? 1.5 to ? 10.0), and low Y,

Yb contents and 37 Sr/ 86 Sr(i) ratios ( \ 0.7070) (Wang et al.2003 , 2006b ; Zhao et al. 2003b , 2006 ). The volcanic rocksof the Upper Carboniferous Yishijilike Fm. (C 2 y) in theAwulale range (Figs. 1, 2) display enrichment in incom-patible elements and a moderate Ta and Nb negativeanomaly (Liu et al. 2006 ). The Lower Carboniferousandesite and rhyolite of Borohoro and Wusun mountainsshow high Ti/Y ( [ 500), Ce/Y ( [ 3) ratios, and relativelylow total Fe (5.87.8 ppm) (Che et al. 1996 ; Xia et al.2004b ); the Carboniferous basalt, andesite and rhyolite inBaluntai and Bogda areas have lower Ti/Y ( \ 500), Ce/Y(\ 3), high Total Fe (6.411 ppm) (Xia et al. 2004b ). Theserift-related volcanic rocks are characterized with posi-tive eNd (t ) values ( ? 4.1 to ? 9.7) and consistent

87 Sr/ 86 Sr(i)ratios (0.70340.7059) suggesting origin from a mantlesource and weak crustal contamination (Xia et al. 2004b ).These rocks are not directly dated, and therefore are notlisted in Table 1.

Carboniferous plutonic rocks in Wusun, Nalati and westof Borohoro mountains (Fig. 1) were formed during 352308 Ma interval (zircon UPb TIMS by Xu et al. 2006 andLA-ICP-MS by Wang et al. 2006a ). Geochemistry indi-cates that they are mainly I-type granites associated withminor S-type granites (Wang et al. 1993 ; Li et al. 1995 ),they display enrichment in Rb, Th and depletion in Nb, Ta,Zr and Hf (Xu et al. 2006 ). In the east of Alataw area,I-type granodiorite and K-granite at ca. 307290 Ma(40 Ar/ 39 Ar) display a weak LREE enrichment and strongenrichment in K, Rb; the eNd (t ) values (0 to ? 7), and87 Sr/ 86 Sr(i) ratios (0.70200.7110) (Chen et al. 1994 , 2000 ;Zhou et al. 1994 , 1995 , 1996 ) are characteristic of the calc-alkaline series and therefore they are correlated to type CA(Table 1).

Permian magmatism

The Permian of the western Chinese Tianshan is charac-terized by plant-bearing conglomeratic red-beds thatunconformably overlie older strata (XBGMR 1993 ). Therelatively small-sized granitoids that are generally referredto as Late Hercynian plutons on 1/200,000 maps (e.g.XBGMR 1973 ) are distributed widely in the study area(Fig. 1). On the basis of recent UPb zircon dating, theypredominantly belong to Permian with only a few LateCarboniferous rocks. In the southern Tianshan, diorite andporphyritic granite were formed during ca. 298284 Ma(UPb dating on zircon), they are rich in K, Rb, Th, anddepleted in Ta, Nb and Zr; alkali feldspar granite wasemplaced during 265260 Ma, and shows enrichment inNb, Y, LREE, and lower Ti, Sr and Ba contents (Jiang et al.1999 ). In Alataw area, the S-type K-granite and monzonitewere formed during 298271 Ma ( 40 Ar/ 39 Ar and zirconUPb La-ICPMS), they have at REE distribution patterns,


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T a

b l e 1

A c o m p i l a t i o n o f g e o c h r o n o l o g i c a l a n d g e o c h e m i c a l d a t a o f C a r b o n i f e r o u s t o P e r m i a n i g n e o u s r o c k s i n w e s t e r n C h i n e s e T i a n s h a n

R o c k s

L o c a l i t i e s

A g e

M a i n g e o c h e m i c a l

R e f e r e n c e s

T y p e s

M a

M e t h o d s

( L a / Y b )

N

e N d (

T )

A n d e s i t e

B o r o h o r o

3 6 3 . 2 5 . 7

Z i r c o n U P b S H R I M P

4 . 9 1 2

. 6

Z h a i e t a l . (

2 0 0 6 ) a n d W a n g e t a l . (

2 0 0 7 a )

C A

B a s a l t

N a l a t i

3 5 3 . 7 4 . 5


4 . 1 5 . 8

? 0 . 3 t o

? 3 . 1

Z h u e t a l . (

2 0 0 6 a )

C A

G r a n o d i o r i t e

W B o r o h o r o

3 5 2 2

Z i r c o n U P b T I M S

3 . 0 2

X u e t a l . (

2 0 0 6 )

C A


W u s u n

3 4 8 1


2 . 2 1

X u e t a l . (

2 0 0 6 )

C A

T r a c h y t e a n d e s i t e

N a l a t i

3 1 2 . 8 4 . 2


3 . 5 5 . 9

? 2 . 7 t o

? 4 . 9

Z h u e t a l . (

2 0 0 5 ) a n d G u o a n d

Z h u ( 2 0 0 6 )

C A

G r a n o d o r i t e

N a l a t i

3 0 8 1


0 . 8 9

X u e t a l . (

2 0 0 6 )

C A


E A l a t a w

3 0 7 3

A r / A r p l a t e a u

2 . 2 4 . 5

? 0 . 1 t o

? 6 . 9

C h e n e t a l . (

1 9 9 4

, 2 0 0 0 )

C A

P o r p h y r i t i c g r a n i t e

N A l a t a w

2 9 8 . 4 5 . 7

Z i r c o n U P b

3 . 1 5 5 . 9

6

? 3 . 5 t o

? 3 . 6

L i u e t a l . (

2 0 0 5 )

C A

D i o r i t e

S T i a n s h a n

2 9 8

Z i r c o n U P b

3 4

- 6 . 5

J i a n g e t a l . (

1 9 9 9 )

A

B i o t i t e g r a n i t e ( B 1 0 1 )

B o r o h o r o

2 9 4 7

Z i r c o n U P b

7 . 2

T h i s s t u d y

T R

K - g r a n i t e

S A l a t a w

2 9 2 . 4 4 . 9

Z i r c o n U P b

2 . 7 5 3 . 5

? 5

L i u e t a l . (

2 0 0 5 )

C A

K - g r a n i t e

W A l a t a w

2 9 0 5


2 . 6 6 . 2

? 0 . 9 t o

? 3 . 2

L i u e t a l . (

2 0 0 5 )

T R

D i a b a s e p o r p h y r e

B o g d a

2 8 8 . 9 4 . 7

Z i r c o n U P b

4 . 5 5 . 9

S h u e t a l . (

2 0 0 5 )

A

B i o t i t e g r a n i t e ( B 9 4 )

B o r o h o r o

2 8 5 . 3 7 . 3

Z i r c o n U P b

1 6 . 8

T h i s s t u d y

A

G r a n i t i c d y k e

A l a t a w

2 8 5 1 3

Z i r c o n U P b

& 1

? 2 . 1 t o

? 2 . 5

L i u e t a l . (

2 0 0 5 )

C A

P o r p h y r i t i c g r a n i t e

S T i a n s h a n

2 8 4 . 4 1 . 5

Z i r c o n U P b

1 1 2

5

- 4 . 1 t o

- 6 . 3


1 9 9 9 )

A

K - g r a n i t e ( B 1 0 2 )

B o r o h o r o

2 8 0 5

Z i r c o n U P b

5 . 7

T h i s s t u d y

T R

T w o - m i c a g r a n i t e

S T i a n s h a n

2 8 0 2 7 0

Z i r c o n U P b

2 3

- 4 . 4 t o

- 4 . 7


1 9 9 9 )

C A

R h y o l i t e

A l a t a w

2 7 0 . 7 6 . 5

Z i r c o n U P b

4 5

? 3 t o ? 5 . 2

L i u e t a l . (

2 0 0 5 )

T R

K - g r a n i t e ( B 9 5 )

B o r o h o r o

2 6 6 6

Z i r c o n U P b

2 0 . 9

T h i s s t u d y

A

A l k a l i - g r a n i t e

S T i a n s h a n

2 6 5 2 6 0

Z i r c o n U P b

1 . 1 6 . 8

- 4 . 9 t o

- 6 . 2


1 9 9 9 )

T R

Q u a r t z a l b i t o p h r e

A l a t a w

2 6 0 5


1 . 9 4 3 . 6

? 1 . 6 t o

? 3 . 3

Z h a o e t a l . (

2 0 0 3 b )

C A

T r a c h y t i c b a s a l t

N a l a t i

2 7 1 2 6 1

W h o l e r o c k K A r

2 . 9 4 . 9

C h e n e t a l . (

2 0 0 4 a , b )

T R

T r a c h y t e a n d e s i t e

N a l a t i

2 7 1 2 6 1

W h o l e r o c k K A r

9 . 1 1 6

C h e n e t a l . (

2 0 0 4 a , b )

A

C A C a l c - a l k a l i n e s e r i e s , A

a l k a l i n e

s e r i e s ,

T R t r a n s i t i o n a l s e r i e s


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depletion in Ba, Nb, Sr and Ti, enrichment in Rb, K, U andTh, and positive eNd (t ) values ( ? 2.1 to ? 5.2) (Chen et al.1994 , 2000 ; Zhou et al. 1994 , 1995 , 1996 ; Liu et al. 2005 ).

Lower-Middle Permian volcanic rocks are locally dis-tributed in Awulale, Nalati and Alataw areas (XBGMR1993 ; Figs. 1, 2). The volcanic and hypabyssal rocks of Awulale and Nalati areas were dated at 296260 Ma(40 Ar/ 39 Ar; Zhao et al. 2003b ). The Wulang Fm. (P 1 w) thatdevelops in the Awulale and Altaw areas is composed of tuffaceous breccia, crystal-bearing tuff, amygdaloidalbasalt, porphyritic augite-andesite, rhyolitic porphyrite anddacitic porphyrite (Fig. 2). In Awulale range, MiddlePermian Hamist Fm. (P 2 h) consists of basalt, rhyolite,sodic dacite, albitophyre and tuffaceous mudstone thatbears sh fossils (XBGMR 1993 ) (Fig. 2). The albitophyre(260 5 Ma, 40 Ar/ 39 Ar plateau age) show high Sr, lowYb and Y contents, less variable Nd isotopic ratios(143 Nd/ 144 Nd(i) = 0.51240.5125; eNd( t ) = ? 1.57 to? 3.26), and low Sr isotopic ratios ( 87 Sr/ 86 Sr(i) = 0.70510.7054) (Xiong et al. 2001 ). To the Northeast of Nalatirange, volcanic rocks develop within terrestrial red beds of Aikendaban Fm. (P 2 a) (Fig. 2), felsite and trachyte yieldwhole rock KAr age of ca. 270260 Ma, associatedtrachytic basalt show slight LREE enrichment depletion inZr, Hf, Nb, Ta and Ba; while trachyte andesite is signi-cantly rich in LREE and depleted in Sr (Chen et al. 2004a ,b). To the south of Bogda range, where collapse brecciaand deep-water sediments overlie alkaline pillow basalt,a mac feeding dyke yields Early Permian zircons(289 5 Ma, UPb) (Shu et al. 2005 ).

The above-mentioned magmatic rocks are diverselycorrelated to CA, A and TR types (Table 1). Thus, theycould neither be simply related to an extensional intraplatesetting, nor to a subduction regime. It appears that thecomplex geochemical features present in the study area cannot unambiguously constrain the geodynamic setting at agiven period. The Carboniferous magmatic rocks aremostly of CA type, whereas diverse suites (CA, A and TR)coexist during the Permian. Therefore, it is necessary totake into account every eld and structural features beforeproposing an evolutionary model. In order to get moreconstraints on the chronology and petrogenesis of thePermian magmatism, several key sections across the mainwrench faults were investigated in the following sections.

The Borohoro plutons

The Borohoro plutons develop to the south of the NorthTianshan Fault (NTF) in a poorly accessible area. Theyshow a spindle shape extending parallel to the NTF(Figs. 1, 3). They were previously correlated with theCarboniferous on 1:1,500,000 and 1:200,000 geologicalmaps (XBGMR 1973 , 1975 , 1993 ), but remained undatedand their geochemical compositions were unknown.

Petrology

TheBorohoroplutons aremainlycomposed of greyish-whitebiotite granite and pink K-granite; both are characterized by

Bayingou

0 10 20km

1 7

38

45 1026 9 11

B102

B101

B94B95

N T F

75

80

6080

75

60 6540

80

Fig. 3 Structural sketch maps of Borohoro area (modied fromXBGMR 1973 , 1975 ), showing the spatial relationship of graniticplutons with ductile dextral North Tianshan Fault. For location, seeFig. 1. The sampling localities are marked with open pentaclesfollowed by sample numbers, and their GPS coordinates are listed inTable 2. 1 fault, 2 North Tianshan Fault zone, 3 foliation/bedding and

dip angle, 4 Meso-Cenozoic, 5 Permian pink granite, 6 Permiandark granite, 7 Early Carboniferous volcaniclastic rocks, 8 weaklydeformed Carboniferous turbitite, 9 mylonitic and metamorphicturbitite, 10 blocks of ultra-mac rocks, 11 Silurian-Devonianundeformed sedimentary rocks


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coarse grain texture, and typically consist of plagioclase,K-feldspar, quartz, biotite and hornblende; Biotite oftenoccurs as euhedral grains and rarely as amphibole pseud-omorphs. In K-granite, pyroxene relics are preserved inpoikilitic hornblende. Accessory minerals include apatite,zircon and ilmenite.

Internal structure

A deformation fabric can be recognized in most rocks of the Borohoro plutons, it comprises shallow linear and steepplanar textures. The lineation is dened by aligned biotite,hornblende, and feldspar grains and elongated quartzaggregates, the planar fabric is marked by ribbons of macminerals (Fig. 4a). Micro-texture does not show signicantinternal deformation in euhedral grains of plagioclase,biotite, hornblende and interstitial ovoid quartz, althoughlocally quartz domains display minor sutured boundariesand weak ondulose extinction, and occasional fracturing

and disaggregation of the grain corners (Fig. 4b). These aretypical high-temperature fabrics developed in condition of pre-critical melt percentage (e.g. Tribe and DLemos 1996 )suggesting a still magmatic state deformation. Although nosystematic investigation has been undertaken, the planarfabric strikes N120 N130 , slightly oblique to the strikeof the NTF, but parallel to the intrusion margin.

Relationships with country rocks and the NTF

The Silurian-Devonian (?) and Early Carboniferous hostrocks of the Borohoro plutons (Fig. 3) display an unusualkind of contact metamorphism. Spotted slate often devel-ops within the contact zones while hornfels is rare. Thehost rock is characterized by steeply dipping foliation thatbears a shallow dipping stretching lineation with andalusiteporphyroblasts. This foliation represents the effect of syn-tectonic thermal metamorphism due to synkinematicgranite intrusion associated with a ductile shear zone

Fig. 4 a Photograph of hand specimen of Borohoro biotite K-graniteshowing the linear fabric dened by alignment of biotite, hornblende,feldspar and elongated quartz, and the planar fabric marked byribbons of mac minerals; b microphotograph of Borohoro K-granite,pale grains are mostly quartz, feldspar and brown minerals are biotiteand minor hornblende, dark grains are mainly extinct quartz and

feldspar; c, d eld photographs in Kekesu section showing boudin-aged or folded K-granite dykes (directed by white arrows ) intrudingthe mylonitic gabbro, black arrow in d shows the location of ahammer for scale; e mylonitized K-granite in Gangou section, shallowdipping lineation included in sub-vertical SE extending foliation


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(Wang et al. 2006a ). Along the northern margin of theplutons, metre- to decimetre-scale sheets of pink K-granitedykes intrude Carboniferous turbidite. Although some arerandomly oriented, most of granitic dykes have an east-west trend, sub-parallel to the NTF, suggesting a structuralcontrol on magma emplacement as well.

Kekesu and Gangou sections

In order to make a comparison, two sections were alsoinvestigated in the Qingbulak-Nalati and Sangshuyuanzishear zones, which are briey described below. In theKekesu section (Fig. 5), massive pink granites are com-posed of K-feldspar, plagioclase, quartz and minor macminerals that are often chloritized and/or epidotized, theyintrude cataclastic or mylonitized gabbro of Devonian toCarboniferous age and probable Proterozoic orthogneiss.Locally, K-granite dykes are boudinaged or folded, show-ing axial plane parallel to the SWNE trending gabbrofoliation (Fig. 4c, d). Within this shear zone, Carboniferousgranite (313 4 Ma, UPb age on zircon) underwentductile deformation and yield Middle Permian biotite(263.4 0.6 Ma, 40 Ar/ 39 Ar plateau age) related to Ar loss

during mylonitisation (Wang et al. 2007c ). The boundariesof the K-granite are generally parallel to the Qingbulak-Nalati fault zone (Fig. 5), indicating a close relationshipbetween the emplacement of granite and the shear zone.

In Gangou area, the Sangshuyuanzi Fault extends par-allel to the SENW Main Tianshan Shear Zone (MTSZ)which is connected westwards with the NTF (Figs. 1, 6).K-granite and synchronous dykes showing a SENWelongated shape intrude Proterozoic orthogneiss and Silu-rian-Carboniferous strata. Both K-granite and its countryrock display a well developed mylonitic foliation thatstrikes N130 E (Fig. 4e), and typically bears a low dipstretching lineation (15 20 SE). Sigmoid K-feldspar andasymmetrical biotite pressure shadow reveal a dextralductile deformation (Allen et al. 1995 ; Allen and Vin-cent 1997 ; Laurent-Charvet et al. 2002 ). Although thegranite underwent signicant mylonitisation, a coarse tomedium grain texture can be recognized, K-feldspar,quartz, plagioclase, minor biotite and sometimes amphi-bole are the main minerals.

New geochronological and geochemical data

Two biotite granite and two K-granite samples were col-lected from the Borohoro plutons, one K-granite and onebiotite K-granite samples were collected from the Kekesuand the Gangou sections, respectively. Sampling locationsare shown in Figs. 3, 5, 6 and Table 2.

Analytical techniques

The selected samples were crushed and milled into rock powder, zircon crystals are enriched using heavy liquidsand magnetic separator, and nally selected by handpick-ing under binocular microscope. Euhedral and colorlesszircon grains were selected for laser ablation UPb datingthat was carried out at the University of Tasmania, Aus-tralia, using a Hewlett Packard HP 4500 quadrupoleInductively Coupled Plasma Mass Spectrometer (ICP-MS)coupled with a 213 nm NewWave Merchantek UP213Nd-YAG Laser. Preserved rock powder was used for wholerock geochemical analyses by ICP-AES for major elementsand by ICPMS for incompatible and rare earth elements atCentre de Recherches Pe trographiques et Ge ochimiques(CRPG-CNRS, Nancy, France). The limit of determinationis less than 0.07% for major elements, and less than0.5 ppm for most of trace elements, and up to 1.56 ppmfor Ba, Cr, Sr, Zn and Zr. Analytical uncertainties are givenas 2% for major elements, and 5 or 10% for trace elementconcentrations around 20 ppm, and the precision for REEis estimated at 5 or 10% depending on the chondrite-nor-malised concentrations are [ 10 ppm or lower, respectively.

Q N F

80

702560

85

20

2 km

15

K e k e s u

R i v e r 1

2

34

5

6

78

KKS5

6020

Q N F

Fig. 5 Structural sketch map of Kekesu River section (modied fromXBGMR 1979 ). For location, see Fig. 1. The sampling locality ismarked with open pentacles followed by sample numbers, andreference Table 2 for the GPS coordinate. 1 Qingbulake-Nalati faultzone, 2 foliation with dip angle and lineation with pitch angle, 3Permian A-type K-granite, 4 Carboniferous undeformed I-typegranite, 5 Devonian?-Carboniferous strongly deformed granite andgabbro, 6 mylonite, 7 greenschist facies meta-sedimentary rocks, 8Proterozoic gneiss


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LA-ICPMS dating

Six granitoids samples were dated, and twelve zirconsingle grains were analysed for each sample. The resultsare shown in Table 2 and plotted on reversed Concordiadiagrams (Fig. 7). Although different in U and Th abso-lute compositions, all zircons show consistently accordantTh/U ratios of [ 0.2, indicating a magmatic origin (Vavraet al. 1996 ). Four granitoids from Borohoro Mountainwere dated. The formation time of the granodiorite sam-ple B101 can be constrained by nine zircons yielding aconcordant age of 294 7 Ma (Fig. 7a), while the restthree zircons provide older ages of 312589 Ma thatprobably represent inherited zircon derived from Palaeo-zoic country rocks. Zircons of granodiorite sample B94can be divided into two groups, wherein the uraniumcontents of high U zircons are ten times more than thoseof low U ones (Table 2). The rst low U zircon wasrejected due to lots of common Pb, ve low U zirconsand one high U zircon yield consistent apparent agesdening a concordant age at 272.8 6 Ma, and the last

low U zircon together with the other four high U zirconsyield a concordant age at 293 0.5 Ma (Fig. 7b); Thelatter one is consistent with the age of the sample B101 atthe uncertainty level, but the former one is signicantlyyounger, indicating that the granodorite was emplacedduring two distinct episodes. All zircons together give anaverage age at 285.3 7.3 Ma.

For the K-granite sample B95, the rst zircon displayhigh common Pb content throughout the analysis inducingyoung age and large analytical uncertainty (225 10 Ma),the second one also yield a rather younger age of 247 4 Ma but shows no analytical anomaly, this isprobably due to statistical outlier. The last zircon yieldingan age of 340 3 Ma is likely an inherited one mixed witholder core. The rest nine zircons provide accordant agesranging from 257277 Ma that are used to calculate aconcordant age at 266 6 Ma, indicative of the formationtime of the K-granite (Fig. 7c). On the Concordia diagram(Fig. 7d) of the K-granite B102, except one point that wasrejected due to discordant apparent age, the other elevenpoints were used in the calculation of concordant age at280 5 Ma.

Seven zircons from the Kekesu K-granite KKS5 yieldconcordant apparent ages that are used to calculate aweighted average UPb age at 277 3 Ma. Older ages of 312375 Ma are also obtained from two zircons that wereprobably derived from foliated/mylonitized country rocks.The other three points are discordant because of either Pbloss in high U zircon (the rst point), or high proportion of common Pb (the last one) (Fig. 7e) and consequentlyprovide geologically meaningless ages.

From the Gangou K-granite KMX13, eight points con-cordantly dene a UPb age at 252 4 Ma (Fig. 7f), andcan be interpreted as the intrusion time of the K-granite.The last four zircons yield signicantly older ages rangingfrom 312 to 357 Ma (Table 2; Fig. 7f), which may reectthe incorporation of zircon xenocrysts derived fromCarboniferous country rocks. The occurrence of Devonian-Carboniferous strata and granitic rocks around theK-granite (Fig. 6; XBGMR 1993 ; Xu et al. 2006 ) conrmsthis interpretation.

Geochemical composition

Only four granitic rocks from Borohoro Range were ana-lysed for whole rock geochemistry. Table 3 comprises thealready published geochemical data and our new results.Borohoro granites are signicantly rich in alkali (79.7wt%) with relatively high content of K 2 O with respect toNa 2 O. Relative low aluminum contents (13.814.1 wt%;Aluminum Saturation Index & 1) indicate that they belongto metaluminous or slightly aluminous I-type granitoidseries (Fig. 8).

Kumux10 km

6080

M T S Z

G a n g o u

S F

KMX13

80

85

60

7075

12

3

4

5

12

13

14

9

10

11

6

7

8

8 0

Fig. 6 Structural sketch map of Gangou section (modied fromXBGMR 1959 ). For location, see Fig. 1. The sampling locality ismarked with open pentacle followed by sample number, and the GPScoordinate can be found in Table 2. 1 Early Palaeozoic thrust,2 strike-slip faults, where MTSZ refers to the Mains Tianshan ShearZone, and SF represents the Sangshuyuanzi Fault, 3 unconformity,4 foliation/bedding and dip angle, 5 Cenozoic, 6 Permian molassicdeposits, 7 Permian foliated K-granite, 8 Carboniferous sedimentaryrocks, 9 Carboniferous volcaniclastic rocks, 10 Early Palaeozoicundeformed granite, 11 Early Palaeozoic foliated granite, 12Ordovician-Devonian rocks, 13 ophiolitic melange, 14 Proterozoicorthogneiss


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T a

b l e 2

Z i r c o n U P

b L A - I

C P M S a n a l y s i s r e s u l t s o f t h e g r a n i t o i d s f r o m t h e w e s t e r n C h i n e s e T i a n s h a n

N o

C o m p o s i t i o n s

A t o m i c r a t i o s

A p p a r e n t a g e s ( M a )

D i s c ( % )

T h ( p p m )

U ( p p m )

T h / U

2 0 7 P b /

2 3 5 U

1 r

2 0 6 P b /

2 3 8 U

1 r

2 0 7 P b /

2 0 6 P b

1 r

2 0 7 P b /

2 3 5 U

1 r

2 0 6 P b /

2 3 8 U

1 r

2 0 7 P b /

2 0 6 P b

1 r

B o r o h o r o g r a n o d i o r i t e B 9 4 ( G P S : N 4 3 5 0

. 3 0 0

, E 8 3 3 1

. 5 5 0 )

1

2 6 2

4 2 4

0 . 6 2

0 . 3 2

0 . 0 6

0 . 0 3 9

0 . 0 0 2 0

0 . 0 6 8

0 . 0 1 2

2 8 3

4 4

2 4 9

1 3

8 8 0

3 6 7

1 3

2

1 3 8

2 3 2

0 . 6 0

0 . 3 0

0 . 0 2

0 . 0 4 3

0 . 0 0 0 6

0 . 0 5 3

0 . 0 0 3

2 6 6

1 4

2 6 8

4

3 1 7

1 3 6

4

3

2 0 7

4 0 2

0 . 5 2

0 . 3 1

0 . 0 2

0 . 0 4 3

0 . 0 0 0 6

0 . 0 5 4

0 . 0 0 3

2 7 4

1 2

2 7 0

4

3 8 9

1 1 3

4

4

1 2 0

3 2 0

0 . 3 8

0 . 2 7

0 . 0 2

0 . 0 4 3

0 . 0 0 0 7

0 . 0 4 9

0 . 0 0 4

2 4 7

1 4

2 6 8

4

1 5 6

1 7 5

4

5

7 3

1 6 3

0 . 4 5

0 . 3 0

0 . 0 2

0 . 0 4 4

0 . 0 0 0 6

0 . 0 5 0

0 . 0 0 3

2 6 3

1 5

2 7 5

4

1 9 5

1 5 4

4

6

7 2

1 5 0

0 . 4 8

0 . 3 3

0 . 0 2

0 . 0 4 5

0 . 0 0 0 8

0 . 0 5 6

0 . 0 0 5

2 8 6

1 9

2 8 2

5

4 5 3

1 8 2

5

7

6 2

1 2 6

0 . 4 9

0 . 3 3

0 . 0 3

0 . 0 4 7

0 . 0 0 0 9

0 . 0 5 6

0 . 0 0 5

2 9 2

2 2

2 9 8

6

4 4 9

2 0 7

6

8

2 , 6 7 1

5 , 0 1 3

0 . 5 3

0 . 3 0

0 . 0 1

0 . 0 4 4

0 . 0 0 0 3

0 . 0 5 1

0 . 0 0 1

2 7 0

6

2 7 6

2

2 4 8

5 8

2

9

1 , 5 0 4

3 , 3 3 9

0 . 4 5

0 . 3 2

0 . 0 1

0 . 0 4 6

0 . 0 0 0 3

0 . 0 5 2

0 . 0 0 1

2 8 3

5

2 9 1

2

2 8 0

5 1

2

1 0

2 , 4 8 0

5 , 8 4 4

0 . 4 2

0 . 3 2

0 . 0 1

0 . 0 4 6

0 . 0 0 0 4

0 . 0 5 2

0 . 0 0 1

2 8 4

5

2 9 3

2

2 6 7

4 4

3

1 1

3 , 4 9 8

6 , 4 1 8

0 . 5 5

0 . 3 3

0 . 0 1

0 . 0 4 7

0 . 0 0 0 4

0 . 0 5 3

0 . 0 0 1

2 9 3

5

2 9 6

2

3 2 3

4 4

2

1 2

3 , 0 3 5

5 , 2 1 5

0 . 5 8

0 . 3 5

0 . 0 1

0 . 0 4 7

0 . 0 0 0 7

0 . 0 5 5

0 . 0 0 2

3 0 3

1 1

2 9 7

4

4 1 9

9 7

4

B o r o h o r o K - g r a n i t e B 9 5 ( G P S : N 4 3 5 0

. 3 0 0

, E 8 3 3 1

. 5 5 0 )

1

1 3 8

1 3 7

1 . 0 1

2 . 8 2

0 . 1 6

0 . 0 5 7

0 . 0 0 1 7

0 . 3 5 2

0 . 0 1 4

1 , 3 6 1

4 5

3 5 6

1 0

3 , 7 1 4

6 2

1 0

2

7 9

1 7 8

0 . 4 5

0 . 3 3

0 . 0 2

0 . 0 4 0

0 . 0 0 0 7

0 . 0 6 3

0 . 0 0 4

2 9 3

1 5

2 5 0

4

6 9 5

1 2 7

4

3

3 , 0 9 8

5 , 9 7 6

0 . 5 2

0 . 4 6

0 . 0 1

0 . 0 4 2

0 . 0 0 0 3

0 . 0 8 0

0 . 0 0 2

3 8 7

6

2 6 6

2

1 , 1 8 6

3 7

2

4

1 , 6 9 1

3 , 3 2 4

0 . 5 1

0 . 4 3

0 . 0 1

0 . 0 4 2

0 . 0 0 0 5

0 . 0 7 6

0 . 0 0 3

3 6 5

1 1

2 6 6

3

1 , 0 8 7

6 7

3

5

9 7

1 8 2

0 . 5 3

0 . 3 4

0 . 0 2

0 . 0 4 2

0 . 0 0 0 6

0 . 0 6 1

0 . 0 0 4

2 9 6

1 7

2 6 4

4

6 3 4

1 5 3

4

6

1 2 4

2 7 9

0 . 4 4

0 . 3 4

0 . 0 2

0 . 0 4 2

0 . 0 0 0 6

0 . 0 5 8

0 . 0 0 4

2 9 6

1 7

2 6 4

3

5 2 2

1 3 8

4

7

1 , 2 7 0

2 , 7 1 6

0 . 4 7

0 . 3 1

0 . 0 1

0 . 0 4 2

0 . 0 0 0 3

0 . 0 5 4

0 . 0 0 1

2 7 4

6

2 6 6

2

3 5 4

5 2

2

8

1 1 8

1 8 1

0 . 6 5

0 . 3 1

0 . 0 3

0 . 0 4 2

0 . 0 0 1 3

0 . 0 5 8

0 . 0 0 7

2 7 7

2 6

2 6 7

8

5 1 4

2 5 2

8

9

6 9 8

1 , 9 9 8

0 . 3 5

0 . 3 5

0 . 0 1

0 . 0 4 3

0 . 0 0 0 4

0 . 0 6 0

0 . 0 0 2

3 0 8

7

2 7 1

3

6 2 1

6 7

3

1 0

8 , 2 7 1

1 0 , 4

7 9

0 . 7 9

0 . 3 2

0 . 0 1

0 . 0 4 3

0 . 0 0 0 2

0 . 0 5 4

0 . 0 0 1

2 8 2

5

2 7 0

2

3 8 1

4 2

2

1 1

1 , 4 4 5

3 , 7 9 8

0 . 3 8

0 . 3 2

0 . 0 1

0 . 0 4 4

0 . 0 0 0 3

0 . 0 5 3

0 . 0 0 1

2 8 1

5

2 7 7

2

3 3 1

5 0

2

1 2

2 0 3

5 2 7

0 . 3 8

0 . 3 9

0 . 0 1

0 . 0 5 4

0 . 0 0 0 5

0 . 0 5 2

0 . 0 0 2

3 3 1

1 1

3 3 9

3

2 7 6

9 5

3

B o r o h o r o g r a n o d i o r i t e B 1 0 1 ( G P S : N 4 3 4 1

. 3 4 0

, E 8 4 2 4

. 8 6 0 )

1

9 9

2 3 1

0 . 4 3

0 . 2 6

0 . 0 2

0 . 0 4 3

0 . 0 0 1

0 . 0 4 4

0 . 0 0 4

2 3 4

1 7

2 7 4

6

- 8 6

2 1 8

6

2

3 1 7

7 1 7

0 . 4 4

0 . 4 0

0 . 0 2

0 . 0 4 6

0 . 0 0 1

0 . 0 6 1

0 . 0 0 3

3 4 3

1 6

2 8 9

4

6 4 4

1 1 8

4

3

1 8 4

3 4 9

0 . 5 3

0 . 3 6

0 . 0 2

0 . 0 4 6

0 . 0 0 1

0 . 0 5 6

0 . 0 0 4

3 0 8

1 9

2 8 9

5

4 5 1

1 7 1

6

4

1 4 0

2 7 0

0 . 5 2

0 . 3 7

0 . 0 3

0 . 0 4 6

0 . 0 0 1

0 . 0 5 9

0 . 0 0 4

3 2 3

2 0

2 9 0

6

5 7 5

1 6 4

6

5

1 5 6

5 6 2

0 . 2 8

0 . 3 4

0 . 0 2

0 . 0 4 6

0 . 0 0 1

0 . 0 5 4

0 . 0 0 4

2 9 4

1 8

2 8 9

5

3 6 3

1 6 5

5

6

6 0

1 1 2

0 . 5 4

0 . 3 4

0 . 0 4

0 . 0 4 7

0 . 0 0 1

0 . 0 5 4

0 . 0 0 7

2 9 6

3 0

2 9 8

9

3 8 8

2 8 9

9


123


11/24

T a

b l e 2

c o n t i n u e d

N o




D i s c ( % )

T h ( p p m )

U ( p p m )

T h / U

2 0 7 P b /

2 3 5 U

1 r

2 0 6 P b /

2 3 8 U

1 r

2 0 7 P b /

2 0 6 P b

1 r

2 0 7 P b /

2 3 5 U

1 r

2 0 6 P b /

2 3 8 U

1 r

2 0 7 P b /

2 0 6 P b

1 r

7

2 0 8

4 6 7

0 . 4 5

0 . 3 5

0 . 0 2

0 . 0 4 8

0 . 0 0 1

0 . 0 5 2

0 . 0 0 3

3 0 7

1 5

3 0 1

5

2 7 4

1 3 6

5

8

3 1 9

1 , 0 7 0

0 . 3 0

0 . 3 7

0 . 0 1

0 . 0 4 8

0 . 0 0 1

0 . 0 5 5

0 . 0 0 2

3 2 2

1 1

3 0 3

3

4 1 6

9 1

3

9

3 3 2

1 , 5 6 7

0 . 2 1

0 . 6 1

0 . 0 2

0 . 0 5 0

0 . 0 0 1

0 . 0 8 6

0 . 0 0 3

4 8 6

1 3

3 1 7

5

1 , 3 3 6

6 7

5

1 0

1 6 9

3 4 7

0 . 4 9

0 . 3 8

0 . 0 2

0 . 0 4 9

0 . 0 0 1

0 . 0 5 5

0 . 0 0 4

3 2 6

1 8

3 1 1

5

3 9 2

1 4 9

5

1 1

2 8 4

5 6 4

0 . 5 0

0 . 3 8

0 . 0 2

0 . 0 5 0

0 . 0 0 1

0 . 0 5 4

0 . 0 0 3

3 2 5

1 6

3 1 5

5

3 7 6

1 3 7

5

1 2

3 9 0

1 , 1 1 1

0 . 3 5

0 . 8 1

0 . 0 3

0 . 0 9 6

0 . 0 0 1

0 . 0 6 0

0 . 0 0 2

6 0 5

1 6

5 9 0

7

5 9 4

8 0

7

B o r o h o r o K - g r a n i t e B 1 0 2 ( G P S : N 4 3 4 6

. 1 0 0

, E 8 4 2 6

. 2 0

)

1

5 7

9 4

0 . 6 0

0 . 6 1

0 . 0 4

0 . 0 4 4

0 . 0 0 1

0 . 1 1 2

0 . 0 0 8

4 8 3

2 7

2 7 9

6

1 , 8 2 7

1 2 9

6

2

3 6

8 9

0 . 4 1

0 . 2 8

0 . 0 2

0 . 0 4 3

0 . 0 0 1

0 . 0 5 4

0 . 0 0 5

2 4 9

2 0

2 7 0

6

3 7 7

2 1 0

6

3

4 2

8 1

0 . 5 2

0 . 3 5

0 . 0 3

0 . 0 4 4

0 . 0 0 1

0 . 0 6 5

0 . 0 0 6

3 0 2

2 4

2 7 8

6

7 8 6

1 8 8

6

4

5 4

9 5

0 . 5 7

0 . 2 7

0 . 0 3

0 . 0 4 4

0 . 0 0 1

0 . 0 4 9

0 . 0 0 5

2 4 5

2 3

2 7 5

6

1 6 6

2 4 5

6

5

3 8

7 2

0 . 5 3

0 . 3 3

0 . 0 4

0 . 0 4 5

0 . 0 0 1

0 . 0 6 2

0 . 0 0 7

2 9 2

2 8

2 8 1

7

6 5 9

2 3 1

8

6

5 0

8 7

0 . 5 8

0 . 2 5

0 . 0 3

0 . 0 4 4

0 . 0 0 1

0 . 0 5 2

0 . 0 0 6

2 2 3

2 5

2 7 9

6

3 0 7

2 7 5

6

7

4 4

7 2

0 . 6 1

0 . 2 8

0 . 0 3

0 . 0 4 5

0 . 0 0 1

0 . 0 5 2

0 . 0 0 6

2 4 9

2 6

2 8 1

7

2 7 2

2 7 5

7

8

6 4

9 2

0 . 7 0

0 . 2 4

0 . 0 3

0 . 0 4 4

0 . 0 0 1

0 . 0 4 3

0 . 0 0 6

2 2 0

2 7

2 8 0

7

- 1 5 3

3 4 2

7

9

9 0

1 2 8

0 . 7 0

0 . 3 0

0 . 0 3

0 . 0 4 5

0 . 0 0 1

0 . 0 5 4

0 . 0 0 5

2 6 3

2 3

2 8 4

6

3 6 6

2 2 0

6

1 0

3 1

6 7

0 . 4 7

0 . 3 0

0 . 0 4

0 . 0 4 5

0 . 0 0 1

0 . 0 5 2

0 . 0 0 7

2 6 7

2 9

2 8 5

7

3 0 5

2 9 5

7

1 1

5 8

9 5

0 . 6 1

0 . 3 4

0 . 0 4

0 . 0 4 6

0 . 0 0 1

0 . 0 6 1

0 . 0 0 7

2 9 4

2 8

2 9 2

6

6 4 7

2 3 6

7

1 2

2 9

6 6

0 . 4 4

0 . 3 9

0 . 0 4

0 . 0 4 8

0 . 0 0 1

0 . 0 6 8

0 . 0 0 6

3 3 5

2 6

3 0 1

7

8 5 7

1 9 4

8

K e k e s u K - g r a n i t e K K S 5 ( G P S : N 4 2 4 5

. 9 8 0

, E 8 1 5 5

. 4 4 0 )

1

1 , 6 9 4

3 , 2 5 5

0 . 5 2

0 . 3 2

0 . 0 1

0 . 0 3 7

0 . 0 0 0 3

0 . 0 6 5

0 . 0 0 2

2 7 9

6

2 3 5

2

7 7 8

5 0

2

2

1 , 1 3 1

8 8 9

1 . 2 7

0 . 5 4

0 . 0 2

0 . 0 4 4

0 . 0 0 0 5

0 . 0 9 3

0 . 0 0 3

4 4 1

1 3

2 8 0

3

1 , 4 8 7

6 8

3

3

3 3 3

8 0 9

0 . 4 1

0 . 3 6

0 . 0 1

0 . 0 4 4

0 . 0 0 0 4

0 . 0 6 2

0 . 0 0 3

3 1 1

1 0

2 7 8

2

6 7 4

8 7

3

4

1 , 7 5 4

9 9 8

1 . 7 6

0 . 3 1

0 . 0 1

0 . 0 4 4

0 . 0 0 0 4

0 . 0 5 3

0 . 0 0 2

2 7 1

9

2 7 5

2

3 4 1

8 3

2

5

5 3 6

7 1 8

0 . 7 5

0 . 4 6

0 . 0 3

0 . 0 4 5

0 . 0 0 1 1

0 . 0 8 0

0 . 0 0 6

3 8 3

2 5

2 8 6

7

1 , 2 0 9

1 4 5

7

6

7 4 6

6 9 1

1 . 0 8

0 . 4 3

0 . 0 2

0 . 0 4 5

0 . 0 0 0 6

0 . 0 7 2

0 . 0 0 3

3 6 1

1 5

2 8 5

4

9 7 7

9 9

4

7

7 5 2

8 3 2

0 . 9 0

0 . 3 3

0 . 0 1

0 . 0 4 4

0 . 0 0 0 4

0 . 0 5 6

0 . 0 0 2

2 9 0

1 1

2 8 0

3

4 6 8

9 4

3

8

6 1 6

1 , 4 6 4

0 . 4 2

0 . 3 6

0 . 0 1

0 . 0 4 5

0 . 0 0 0 4

0 . 0 6 1

0 . 0 0 2

3 1 1

7

2 8 3

2

6 4 9

5 6

2

9

4 5 8

7 5 8

0 . 6 0

0 . 4 5

0 . 0 2

0 . 0 4 6

0 . 0 0 0 5

0 . 0 7 4

0 . 0 0 4

3 7 8

1 5

2 8 8

3

1 , 0 4 8

9 5

3

1 0

5 0 3

4 3 4

1 . 1 6

0 . 3 7

0 . 0 3

0 . 0 5 0

0 . 0 0 1 1

0 . 0 5 9

0 . 0 0 4

3 2 3

2 0

3 1 5

7

5 5 8

1 5 2

7

1 1

1 8 2

6 4 8

0 . 2 8

0 . 4 2

0 . 0 2

0 . 0 6 0

0 . 0 0 0 6

0 . 0 5 4

0 . 0 0 2

3 5 8

1 1

3 7 5

4

3 7 2

8 1

4

1 2

7 4 6

5 0 1

1 . 4 9

3 . 4 9

0 . 3 3

0 . 0 7 4

0 . 0 0 3 4

0 . 3 6 2

0 . 0 2 2

1 , 5 2 6

7 6

4 5 8

2 0

3 , 7 6 0

9 2

1 9


123


12/24

The samples B101 and B102 are characterized byenrichment in Rb, Th and K compared to REE, and lowcontents of Nb and Ta (Table 3), indicative of calc-alkalineseries. However, the samples B94 and B95 show lowerBa (125134 ppm) and Sr (2040 ppm), characteristic of transitional type as dened above. A similar feature, e.g.co-existence of both calc-alkaline and transitional series,occurs for Alataw and southern Tianshan granitoids (Jianget al. 1999 ; Liu et al. 2005 ). This feature can be observedon the expanded REE and trace elements spider diagramnormalized to the primitive mantle (Sun and McDonough1989 ) (Fig. 9a), in which, all the granitic rocks of Boro-horo, Alataw and southern Tianshan as well as Nalativolcanitic rocks exhibit a regular decrease with increasingcompatibility for HFS elements. In addition, depletions of Nb, Ta, Y and Yb can be observed for all these magmaticrocks. Otherwise, the samples B94 and B95 exceptionallyshow enrichments of Ce, Zr and Hf.

The samples B101 and B102 display total REE contentof 90188 ppm, slight LREE enrichment (La N /Yb N = 5.77.2), weak negative Eu anomaly ( d Eu = 0.60.88). Theyhave low Ce/Pb (1.73.7) and Nb/La ( & 0.4) ratios, andTh/La ratio of 0.40.7 (Table 3). These features, consistentwith calc-alkaline granites, are similar to those of Alatawgranitoids, southern Tianshan two-mica granite andpotassium granite (Table 3). In contrast, the samples B94and B95 have higher REE contents (231236 ppm), higherLa N /YbN (16.820.9) and Ce/Pb (7.76.9) ratios, and lowerNb/La ( & 0.16) and Th/La ( & 0.15) ratios (Table 3). Thesefeatures are those of alkaline or transitional granites, andare comparable to the characteristic of the trachyticandesite from Nalati and granodiorite from southernTianshan (Table 3). On the Chondrite-normalized REEpatterns (Fig. 9b; Pearce 1982 ), samples B101 and B102 of Borohoro plutons, and Alataw granitoids show lesser REEfractionation by comparison with the samples B94 and B95as well as the volcanic rocks of the Nalati and Bogda areas.

Discussion

Persistence and evolution of magmatism throughCarboniferous to Permian

On the basis of our study of the Borohoro plutons, the high-K calc-alkaline granites formed during 294280 Ma,whereas the transitional plutonism occurred between 285and 266 Ma, and continued until 250 Ma in other places.Combining the previous results synthesized above, themagmatic activity in western Chinese Tianshan was apermanent process that started as early as the beginning of Carboniferous and lasted up to the end of Permian. Geo-chemical compositions suggest that the Carboniferous T

a b l e 2

c o n t i n u e d

N o




D i s c ( % )

T h ( p p m )

U ( p p m )

T h / U

2 0 7 P b /

2 3 5 U

1 r

2 0 6 P b /

2 3 8 U

1 r

2 0 7 P b /

2 0 6 P b

1 r

2 0 7 P b /

2 3 5 U

1 r

2 0 6 P b /

2 3 8 U

1 r

2 0 7 P b /

2 0 6 P b

1 r

G a n g o u b i o t i t e K - g r a n i t e K M X 1 3 ( G P S : N : 4 2 3 1

. 2 2 0

, E : 8 8 3 1

. 0 8 0 )

1

8 3 7

6 2 7

1 . 3 4

0 . 2 7

0 . 0 1

0 . 0 3 9

0 . 0 0 1

0 . 0 5 2

0 . 0 0 2

2 4 4

1 0

2 4 8

3

3 0 6

1 0 0

3

2

2 4 2

2 4 0

1 . 0 1

0 . 3 0

0 . 0 2

0 . 0 4 0

0 . 0 0 1

0 . 0 5 6

0 . 0 0 4

2 6 4

1 5

2 5 1

4

4 5 8

1 4 1

4

3

6 5 8

6 9 8

0 . 9 4

0 . 3 2

0 . 0 2

0 . 0 4 0

0 . 0 0 1

0 . 0 6 1

0 . 0 0 3

2 8 2

1 3

2 5 4

5

6 5 4

1 1 9

5

4

2 7 7

5 7 7

0 . 4 8

0 . 3 2

0 . 0 2

0 . 0 4 0

0 . 0 0 1

0 . 0 6 3

0 . 0 0 4

2 8 4

1 6

2 5 5

5

7 0 9

1 2 8

5

5

6 1 7

4 5 1

1 . 3 7

0 . 3 2

0 . 0 1

0 . 0 4 1

0 . 0 0 1

0 . 0 6 0

0 . 0 0 3

2 8 1

1 1

2 5 8

4

6 0 3

9 7

5

6

1 2 0

2 2 7

0 . 5 3

0 . 3 0

0 . 0 3

0 . 0 4 1

0 . 0 0 1

0 . 0 5 7

0 . 0 0 6

2 6 7

2 5

2 5 8

6

4 9 9

2 3 3

6

7

5 8

1 2 2

0 . 4 8

0 . 2 9

0 . 0 4

0 . 0 4 1

0 . 0 0 1

0 . 0 5 4

0 . 0 0 7

2 6 1

3 2

2 5 9

8

3 6 1

3 1 4

8

8

3 7 0

3 1 7

1 . 1 7

0 . 3 3

0 . 0 3

0 . 0 4 2

0 . 0 0 1

0 . 0 6 1

0 . 0 0 5

2 8 7

2 1

2 6 2

7

6 5 6

1 7 3

7

9

1 2 3

2 1 4

0 . 5 8

0 . 3 3

0 . 0 3

0 . 0 4 9

0 . 0 0 1

0 . 0 5 0

0 . 0 0 5

2 8 8

2 3

3 1 1

6

2 0 4

2 1 7

6

1 0

2 0 7

3 2 5

0 . 6 3

0 . 3 6

0 . 0 2

0 . 0 5 4

0 . 0 0 1

0 . 0 5 1

0 . 0 0 3

3 1 4

1 7

3 3 7

6

2 5 9

1 4 9

6

1 1

8 5 2

8 7 4

0 . 9 8

0 . 4 0

0 . 0 2

0 . 0 5 7

0 . 0 0 1

0 . 0 5 3

0 . 0 0 2

3 3 8

1 2

3 5 6

6

3 1 7

9 0

6

1 2

7 5 7

5 5 4

1 . 3 7

0 . 4 3

0 . 0 2

0 . 0 5 7

0 . 0 0 1

0 . 0 5 7

0 . 0 0 3

3 6 2

1 7

3 5 8

7

4 7 4

1 2 1

7


123


13/24

magmatic rocks are mainly calc-alkaline although a fewtransitional series may occasionally appear (Table 1); incontrast, the Permian magmatic rocks are characterised bythe coexistence of calc-alkaline, alkaline and transitionalseries, indicating a transition of magma chemistry fromcalc-alkaline to alkaline.

Petrogenesis of Carboniferous-Permian magmatic rocksand geodynamic implications

Some authors favor a continental rift (Che et al. 1996 ; Xiaet al. 2004b ) or an intra-plate environment (Liu et al. 2006 )for the Carboniferous volcanic rocks, and a post-collisionsetting for the Carboniferous granites (Xu et al. 2006 ).Nevertheless, their geochemical and isotopic featuresindicate that subduction play a prominent role, such assuggested by the occurrence of adakite, high-Mg andesite,and Nb-poor andesitic basalt (Wang et al. 2003 , 2006b ;

Zhao et al. 2003b , 2006 ). Moreover, stratigraphic featuresof Carboniferous volcanic rocks and the shallow watersedimentary association (Fig. 2) suggest a rather longperiod of eruption on a continental margin. Elsewhere inCentral Asia, Carboniferous subduction-related and/orsyncollisional igneous rocks with calc-alkaline afnitiesare reported in the South Tianshan belt of Kyrgyzstan(Hamrabaev and Simon 1984 ; cited by Solomovich andTrifonov 2002 ) and around the Tu-Ha basin of the Chineseeastern Tianshan (Li et al. 2001 , 2006a , b; Sun et al. 2006 ).Therefore, Carboniferous magmatic rocks have mostprobably been generated in an active margin setting.

Usually, the Permian high-K calc-alkaline and alkalineigneous rocks were considered to be post-collisional, e.g. inthe South Tianshan of Kyrgyzstan (Solomovich and Tri-fonov 2002 ; Solomovich 2007 ). Particularly, the Permianhigh-K calc-alkaline and transitional granites of the Boro-horo plutons also have subduction components according

(c) B95

2 6 0

3 0

0

3 4 0 3

8 0

1 high common Pb zircon

Age=266 6Ma (95 % conf.)3 of 12 rejected, MSWD=10.1Probability=0.0

0.04

0.06

0.08

0.09

0.05

16 18 20 2822 26

0.07

24

238 206U / Pb

2 6 0

2 8 0

3 0 0

3 2 0

3 4 0

Age=280 5Ma(95 % conf.)1 of 12 points, MSWD=1.2Probability=0.28

0.04

0.06

0.08

0.10

0.12

0.14

18 20 22 24 26

(d) B102

2 0 7

2 0 6

P b /

P b

2 0 7

2 0 6

P b /

P b

238 206U / Pb

(a) B101

3 0 0

4 0 0 5

0 0 6

0 0 7 0

0 8 0 0

Age=294 7 Ma (95 % conf.)3 of 12 rejected, MSWD=3.5Probability=0.001

0.04

0.06

0.08

0.09

0.05

6 10 14 26

0.07

22

0.10

18

(e) KKS5

2 4

0

2

8 0

3 2 0

3 6 0

4 0 0

common Pb

0.04

0.06

0.08

0.10

18 22 26 3014

Age=277 3Ma(95 % conf.)5 of 12 rejectedMSWD=0.92Probability=0.48

2 0 7

2 0 6

P b /

P b

(f) KMX13

2 4 0

2 8 0

3 2 0

3 6 0 4

0 0

0.04

0.05

0.06

0.07

0.08

14 20 22 2418 26 2816

Age=252 4Ma (95 % conf.)4 of 12 rejected, MSWD=0.66Probability=0.71

(b) B94

2 3 0

2 5 0

2 9 0

3 1 0

3 3 0

Age=272.8 6 Ma (95 % conf.)6 points, MSWD=1.3Probability=0.26

Age=293 0.5 Ma (95 % conf.)5 points, MSWD=0.8Probability=0.52

0.04

0.06

0.08

0.10

18 20 22 24 26 28 30

Fig. 7 Inversed Concordiadiagrams of zircon UPb LA-ICPMS dating results on thegranitoids from the WesternChinese Tianshan. Filled graycircles refer to the plots used tocalculate the concordant ages,and open circles representrejected plots


123


14/24

T a

b l e 3

A s y n t h e s i s o f W h o l e r o c k g e o c h e m i s t r y o f P e r m i a n m a g m a t i c r o c k s i n B o r o h o r o , A l a t a w , S

o u t h e r n T i a n s h a n , N

a l a t i a n d B o g d a a r e a s , W e s t C h i n e s e T i a n s h a n

L o c a t i o n s

B o r o h o r o

A l a t a w

N a l a t i ( A i k e n d a b a n F m

. )

S a m p l e n o .

B 9 4

B 9 5

B 1 0 1

B 1 0 2

k w s y 1 2

k w s y 3

k w s y 2 7

k w s y 1 7

a 1 - 3

9

a 1 - 4

5

a 1 - 3

8

a 1 - 4

A g e

2 8 5 M a

2 6 6 M a

2 9 4 M a

2 8 0 M a

2 9 8 M a

2 8 5 M a

2 9 2 M a

2 7 1 M a

P 2

P 1

P 1

P 1

R o c k s


K - g r a n i t e


K - g r a n i t e

G r a n i t e

G r a n i t i c d y k e

G r a n i t e

R h y o l i t e

S h o s h o n i t e ( ? )

T r a c h y t e

a n d e s i t e

T r a c h y t i c

a n d e s i t e

T r a c h y t e

R e f e r e n c e s

T h i s s t u d y

L i u e t a l . (

2 0 0 6 )

C h e n e t a l .

( 2 0 0 4 a

, b )

S i O

2

7 1 . 4

7 1 . 3

5

7 2 . 7

9

6 9 . 3

9

7 3 . 0

5

7 6 . 9

7

7 7 . 1

6

7 4 . 4

5

4 1 . 6

9

5 3 . 4

6

5 4 . 7

9

5 9 . 9

9

T i O

2

0 . 2 3

0 . 2 2

0 . 2 4

0 . 4 7

0 . 2 9

0 . 0 8

0 . 1

0 . 2 5

0 . 9 1

1 . 3 1

1 . 1 9

0 . 9 4

A l 2 O

3

1 4 . 0

1

1 3 . 7

8

1 4 . 0

6

1 3 . 9

3

1 3 . 3

9

1 2 . 4

3

1 2 . 3

6

1 1 . 9

5

1 8 . 2

9

1 5 . 2

1 7 . 2

3

1 5 . 6

3

F e 2

O 3

2 . 6 6

2 . 5 4

1 . 7 7

3 . 9

2 . 4 3

0 . 9 3

1 . 3 8

2 . 2 8

4 . 1 4

3 . 1 3

3 . 3 1

2 . 6

M n O

0 . 0 5

0 . 0 6

0 . 0 5

0 . 0 8

0 . 0 5

0 . 0 1

0 . 0 2

0 . 0 4

0 . 2 3

0 . 1 6

0 . 1 3

0 . 1 5

M g O

0 . 1

0 . 0 9

0 . 5 1

0 . 4 9

0 . 5 2

0 . 0 4

0 . 0 3

0 . 1 4

4 . 7 8

7 . 4 6

3 . 9 2

3 . 3 7

C a O

0 . 8 8

0 . 8

2 . 0 9

2 . 0 3

1 . 6 5

0 . 4 4

0 . 4 4

1 . 3 1

2 0 . 4

3

5 . 0 1

5 . 3 1

0 . 7 6

N a 2 O

3 . 9

3 . 7 8

3 . 7 2

4 . 0 2

3 . 7

3 . 9 1

3 . 8 5

3 . 3 5

0 . 3 3

4 . 7 5

4 . 2 2

4 . 1

K 2

O

5 . 7 5

5 . 6 5

3 . 3

3 . 7 2

4 . 3 7

4 . 5 7

4 . 8

3 . 9 3

0 . 1 1

1 . 7 5

4 . 2 6

5 . 7 2

P 2 O

5

0 . 0 4

0 . 0 5

0 . 0 8

0 . 1 2

0 . 0 7

0 . 0 1

0 . 0 1

0 . 0 6

0 . 2

0 . 3 3

0 . 3 4

0 . 4 7

L O I

0 . 2 9

0 . 3 3

0 . 7 8

0 . 6 6

0 . 6 8

1 . 0 9

0 . 6

1 . 0 7

5 . 5 5

2 . 9 5

2 . 3 3

3 . 1 5

T o t a l

9 9 . 3

9 8 . 6

4

9 9 . 3

7

9 8 . 8

2

1 0 0 . 2 1

1 0 0 . 4 7

1 0 0 . 7 5

9 8 . 8

2

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