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Occurrence of an Alaskan-type complex in theMiddle Tianshan Massif, Central Asian Orogenic Belt:inferences from petrological and mineralogical studiesBen-Xun Su a b , Ke-Zhang Qin a , Patrick Asamoah Sakyi c , Sanjeewa P.K. Malaviarachchi d ,Ping-Ping Liu a b , Dong-Mei Tang a , Qing-Hua Xiao a , He Sun a , Yu-Guang Ma e & Qian Mao ea Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, ChineseAcademy of Sciences, P.O. Box 9825, Beijing, 100029, Chinab Graduate University of Chinese Academy of Sciences, Beijing, 100049, Chinac Department of Earth Science, University of Ghana, P.O. Box LG 58, Legon-Accra, Ghanad Research School of Earth Sciences, The Australian National University, Canberra, ACT,0200, Australiae State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics,Chinese Academy of Sciences, P.O. Box 9825, Beijing, 100029, China

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To cite this article: Ben-Xun Su, Ke-Zhang Qin, Patrick Asamoah Sakyi, Sanjeewa P.K. Malaviarachchi, Ping-Ping Liu, Dong-MeiTang, Qing-Hua Xiao, He Sun, Yu-Guang Ma & Qian Mao (2012): Occurrence of an Alaskan-type complex in the Middle TianshanMassif, Central Asian Orogenic Belt: inferences from petrological and mineralogical studies, International Geology Review,54:3, 249-269

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International Geology ReviewVol. 54, No. 3, February 2012, 249–269

Occurrence of an Alaskan-type complex in the Middle Tianshan Massif, Central Asian OrogenicBelt: inferences from petrological and mineralogical studies

Ben-Xun Sua,b*, Ke-Zhang Qina∗, Patrick Asamoah Sakyic, Sanjeewa P.K. Malaviarachchid, Ping-Ping Liua,b,Dong-Mei Tanga, Qing-Hua Xiaoa, He Suna, Yu-Guang Mae and Qian Maoe

aKey Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing100029, China; bGraduate University of Chinese Academy of Sciences, Beijing 100049, China; cDepartment of Earth Science,

University of Ghana, P.O. Box LG 58, Legon-Accra, Ghana; dResearch School of Earth Sciences, The Australian National University,Canberra, ACT 0200, Australia; eState Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese

Academy of Sciences, P.O. Box 9825, Beijing 100029, China

(Accepted 11 October 2010)

The Xiadong mafic–ultramafic complex lies in the central part of the Middle Tianshan Massif (MTM), along the southernmargin of the Central Asian Orogenic Belt (CAOB). This complex is composed of dunite, hornblende (Hbl) clinopyroxenite,hornblendite, and Hbl gabbro. These rocks are characterized by adcumulated textures and variable alteration. Orthopyroxeneis an extremely rare mineral in all rock units and plagioclase is absent in dunite and Hbl clinopyroxenite. Hbl, Fe-chromite,and Cr-magnetite are common phases. Olivines have forsterite (Fo) contents ranging from 92.3 to 96.6. Clinopyroxenes areCa-rich, Ti-poor diopsides, and mostly altered to tremolites or actinolites. Chromites display low TiO2 and Al2O3 contentsand high Cr# and Fe2+/(Fe2+ + Mg) values. Primary and secondary Hbls show wide compositional variations. These petro-logical and mineralogical features as well as mineral chemistry are comparable to typical Alaskan-type complexes worldwide,which are widely considered to have formed above subduction zones. The chemistry of clinopyroxene and chromite sup-ports an arc plate-tectonic origin for the Xiadong complex. Its confirmation as an Alaskan-type complex implies that theMTM, with Precambrian basement, was probably a continental arc during oceanic plate underflow and further supports thehypothesis of southward subduction of the Palaeozoic Junggar Ocean.

Keywords: Alaskan-type mafic–ultramafic complex; Central Asian Orogenic Belt; mafic–ultramafic complex; MiddleTianshan Massif; continental arc

Introduction

The Central Asian Orogenic Belt (CAOB), reflecting juve-nile crustal growth, is the largest Phanerozoic orogen in theworld, extending 7000 km E–W, from the Siberian Cratonin the north to the Tarim Craton in the south (Figure 1A;Sengör et al. 1993, 2004; Hu et al. 2000; Jahn et al. 2000a,2000b, 2004; Windley et al. 2007; Sun et al. 2008; Xiaoet al. 2009). Its tectonic evolution has been attributed tosubduction, accretion, and collision of an ocean-arc–micro-continent system in the Palaeo-Asian Ocean (Wu et al.1996; Gao et al. 1998, 2006, 2009; Chen et al. 1999; Xiaet al. 2004; Xiao et al. 2004, 2009; Lin et al. 2009).

The Chinese Tianshan Mountains occupy the southernpart of the CAOB and are characterized by widely dis-tributed mafic–ultramafic complexes, most of which havebeen identified as ophiolites or post-orogenic intrusions(Xiao et al. 1992; Qin et al. 2002; Zhou et al. 2004; Maoet al. 2008; Pirajno et al. 2008; Zhang et al. 2008; Sun2009). A great number of Early Permian mafic–ultramaficcomplexes are exposed in the Eastern Tianshan and

∗Corresponding authors. Email: subenxun@mail.igcas.ac.cn; kzq@mail.igcas.ac.cn

Beishan belts and in most host magmatic Ni–Cu sulphidedeposits (Figure 1B; Qin et al. 2002, 2003, 2007; Hanet al. 2004; Zhou et al. 2004; Chai et al. 2006, 2008; Hanet al. 2006; Jiang et al. 2006; Mao et al. 2006; Sun et al.2006, 2007; Mao et al. 2008; Pirajno et al. 2008; Su et al.2009, 2010a, 2010b; Tang et al. 2009; Wang et al. 2009;Liu et al. 2010; Xiao et al. 2010).

Although multiple subduction events in the CAOBhave produced abundant arc-related volcanic rocks andcoeval intrusions, so far no study has reported evidence forAlaskan-type complexes, which are thought to have formedin subduction zone environments (e.g. Williams 1991;Saleeby 1992; Foley et al. 1997; Ayarza et al. 2000; Valliet al. 2004). We discovered a mafic–ultramafic complex inthe Middle Tianshan Massif (MTM) and recognized it to beof Alaskan-type. To our knowledge, this is the first findingof such a complex in the southern margin of the CAOB.

Here we present a detailed description of thepetrological and mineralogical features of the Xiadongmafic–ultramafic complex and compare it with typical

ISSN 0020-6814 print/ISSN 1938-2839 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/00206814.2010.543009http://www.tandfonline.com

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250 B.-X. Su et al.

Figure 1. (A) Location map of the study area in the Central Asian Orogenic Belt and partly in the Tarim Craton (modified after Jahn et al.2000b). (B) Regional geological map of the Eastern Tianshan and Beishan Rift showing the distribution of Palaeozoic mafic–ultramaficcomplexes (modified after Su et al. 2010a).

Alaskan-type complexes. These data are then used to shedlight on the origin and emplacement of the complex.

Geological setting

The MTM, in eastern Xinjiang Uygur AutonomousRegion, is situated between the Jueluotage tectonic belt inthe north and the Beishan Rift in the south, and boundedby the Aqikuduke–Shaquanzi fault in the north and theHongliuhe fault in the south (Figure 1B). Abundant gran-ites and granitic gneisses crop out as a Precambrian crys-talline basement of the MTM (BGMRXUAR 1993; Qinet al. 2002; Xu et al. 2009). Several Early Permian mafic–ultramafic complexes, including Tianyu (280 Ma; Su et al.2010a) and Baishiquan (284.8 Ma; Su et al. 2010a), aredistributed along the northern margin of the MTM. TheXiadong complex is located in the central part of the MTM(Figure 1B).

The Xiadong mafic–ultramafic complex is strip shapedand generally strikes E–W. It is 7 km long and up to 500 mwide with an exposed area of extent ∼2.5 km2 (Figure 2).The country rocks of the complex are dominated by lateProterozoic schist, gneiss, and marbles. Undated graniteand diorite are widely present in the surrounding region andappear to be younger than the mafic–ultramafic complex,

as some granitic and dioritic veins intrude the mafic–ultramafic complex in the horizontal profile (Figure 2).

The rock types that compose the Xiadong complex aredunite, hornblende (Hbl) clinopyroxenite, Hbl gabbro, andminor hornblendite, hereafter called dunite, Hbl clinopy-roxenite, Hbl gabbro, and hornblendite. The dunite bodydominates the northern and western parts of the complex,whereas the Hbl clinopyroxenite and Hbl gabbro are mainlyfound in the southern and eastern parts. The hornblenditeis only observed in the horizontal profile (Figure 2). Themafic rock units (Hbl clinopyroxenite, hornblendite, andHbl gabbro) are mostly gradational over a short distance(approximately several metres), whereas the contactswithin the dunite unit are well defined and display chilledmargins (Figure 3A–3C). The profile demonstrates thatmany veins, including Hbl clinopyroxenite, Hbl gabbro,hornblendite, granite, and diorite, cut through the dunitebodies, suggesting late-stage intrusions within the dunitebodies.

Petrography

Dunite

The dunite occurs as bands of masses aligned in an E–W direction parallel to the elongated direction of the

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International Geology Review 251

Figure 2. Geological map of the Xiadong mafic–ultramafic complex, accompanied by a horizontal profile along A to B showing its rockunits and sampling positions.

Figure 3. Field and outcrop photographs of the Xiadong mafic–ultramafic complex: (A) field survey showing the position of the hori-zontal profile and the relationship of dunite and Hbl gabbro; (B) light yellow dunite intruded by dark green dunite, with chilled marginpresent in the latter; (C) detailed contact between dark green dunite and light yellow dunite; (D) compact and the strongly serpentinizeddark green dunite; (E) occurrence of chromite layer in dark green dunite; (F) coarse-grained, light yellow olivine aggregate within darkgreen dunite; (G) chromite layer well defined in light yellow dunite; (H) the contact between Hbl clinopyroxenite and hornblendite; (I)fresh Hbl gabbro sample.

Xiadong complex. It can be subdivided into two types:dark green dunite and light yellow dunite. The dark greendunite appears to be compact (Figure 3B and 3C) butessentially has been strongly serpentinized (Figure 3D).Most olivines in the dark green dunites are altered to

serpentines, whereas clinopyroxenes (modal abundance

252 B.-X. Su et al.

with light yellow dunite bodies, the dark green dunitebodies frequently exhibit chilled margins ranging from5 to 15 cm in width (Figure 3B and 3C). Some light yellowolivine aggregates can occasionally be observed in the darkgreen dunites (Figure 3F), probably demonstrating that theemplacement of the dark green dunite is later than that ofthe light yellow dunite.

The light yellow dunite consists of olivine and chromitewith accessory Hbl and altered clinopyroxene. Olivinesoccur as cumulate crystals of variable sizes (Figure 4A).Many olivine grains are considerably large (2–4 mm) andirregularly shaped. Other olivine grains are small and round(

International Geology Review 253

Figure 5. Back-scattered images of rocks from the Xiadong mafic–ultramafic complex. (A) Dunite 09XDTC1-28 showing fine-grainedolivine, long prismatic clinopyroxene, spinel rimmed by chromite; (B) Hbl clinopyroxenite 09XDTC1-39 displaying altered clinopyrox-ene, primary hornblende (Hbl), granular magnetite, and dolomite vein; (C) Hbl gabbro 09XDTC1-12 showing the relationship betweenmagnetite and ilmenite; (D) Hbl gabbro 09XDTC1-12 displaying detailed intergrowth of magnetite and ilmenite.

altered to actinolite or tremolite and contain ilmenite lamel-lae. The modal abundance of magnetite and ilmenite in theHbl gabbros can reach up to 15%, which is relatively higherthan that in other rock types (Figure 4C). The magnetites,in most cases, display parallel intergrowth with ilmenitesand occasionally occur as interstitial grains (Figure 5Cand 5D).

Analytical method

Quantitative mineral compositions were determinedby wavelength-dispersive spectrometry using a JEOLJXA8100 electron probe (JEOL, Tokyo, Japan), operatingat an accelerating voltage of 15 kV with 12 nA beamcurrent, 5 µm beam spot, and 10–30 s counting time. Theprecisions of all analysed elements are better than 2.0%.Natural minerals and synthetic oxides were used as stan-dards, and a program based on the ZAF procedure was usedfor data correction. The analyses were done at the State KeyLaboratory of Lithospheric Evolution, Institute of Geologyand Geophysics, Chinese Academy of Sciences. Fe2+–Fe3+ redistribution from electron microprobe analyses wascarried out using the general equation of Droop (1987) forestimating Fe3+. Representative analyses of each of theanalysed phases are given in Tables 1–6.

Mineral chemistry

Olivine

Olivine grains are only observed in the dunites of theXiadong complex. They have Forsterite (Fo) ranging from92.3 to 96.6, MnO from 0.03 to 0.23 wt.% with a rangeof 0.08–0.18 wt.%, NiO from 0.05 to 0.76 wt.%, andextremely low CaO of

254 B.-X. Su et al.

Table 1. Olivine compositions of the Xiadong mafic–ultramafic complex.

Sample Rock type No. SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O NiO Total Fo

09XD-1 Dunite 6 41.8 0.03 0.00 0.00 4.85 0.16 53.3 0.00 0.00 0.01 0.27 100.5 95.209XDTC1-5 Dunite 4 41.2 0.00 0.00 0.05 5.67 0.11 51.8 0.03 0.01 0.00 0.51 99.4 94.309XDTC1-11 Dunite 5 41.7 0.00 0.00 0.00 7.28 0.09 50.9 0.02 0.00 0.01 0.47 100.4 92.609XDTC1-14 Dunite 5 41.9 0.03 0.00 0.01 6.36 0.10 51.3 0.01 0.00 0.00 0.76 100.4 93.609XDTC1-15 Dunite 5 39.9 0.02 0.00 0.06 5.60 0.07 53.0 0.00 0.00 0.02 0.35 99.0 94.509XDTC1-16 Dunite 5 41.0 0.00 0.00 0.02 4.98 0.14 53.3 0.00 0.00 0.00 0.34 99.7 95.109XDTC1-19 Dunite 5 41.5 0.00 0.01 0.00 6.92 0.16 51.2 0.00 0.00 0.00 0.50 100.3 93.009XDTC1-23 Dunite 5 42.0 0.00 0.01 0.00 6.45 0.14 51.6 0.01 0.00 0.01 0.41 100.7 93.509XDTC1-24 Dunite 5 42.1 0.00 0.00 0.03 3.64 0.08 53.6 0.01 0.04 0.02 0.29 99.8 96.409XDTC1-25 Dunite 4 42.4 0.05 0.00 0.00 3.59 0.14 54.1 0.02 0.02 0.00 0.32 100.7 96.409XDTC1-28 Dunite 5 42.4 0.01 0.00 0.00 4.36 0.12 53.5 0.03 0.00 0.02 0.48 100.9 95.709XDTC1-29 Dunite 5 42.2 0.00 0.00 0.04 4.04 0.14 53.2 0.03 0.02 0.02 0.44 100.1 96.009XDTC1-30 Dunite 5 42.4 0.00 0.00 0.01 3.62 0.11 54.3 0.00 0.00 0.00 0.28 100.7 96.409XDTC1-31 Dunite 5 42.1 0.02 0.00 0.02 5.75 0.17 52.2 0.01 0.00 0.00 0.11 100.4 94.209XDTC1-32 Dunite 5 42.0 0.00 0.01 0.00 3.55 0.11 53.9 0.02 0.01 0.01 0.23 99.9 96.509XDTC1-35 Dunite 4 42.1 0.00 0.00 0.09 5.00 0.19 53.1 0.04 0.01 0.03 0.36 100.8 95.009XDTC1-36 Dunite 5 42.3 0.06 0.00 0.00 4.15 0.19 53.9 0.00 0.01 0.03 0.32 100.9 95.909XDTC1-47 Dunite 5 41.8 0.03 0.00 0.01 6.99 0.12 51.6 0.01 0.00 0.01 0.42 100.9 93.0

Note: Fo = 100 × Mg/(Mg + Fe).

have identical compositions in Al2O3 (∼55.0 wt.%),Cr2O3 (∼10.5 wt.%), FeO (∼11.5 wt.%), and MgO(∼12 wt.%). Most chromites have very low Mg#[100 × Mg/(Mg + Fe)] values of

International Geology Review 255

Tabl

e2.

Chr

omit

eco

mpo

siti

ons

ofth

eX

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ngm

afic–

ultr

amafi

cco

mpl

ex.

Sam

ple

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kty

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iner

alN

o.S

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TiO

2A

l 2O

3C

r 2O

3Fe

OM

nOM

gOC

aON

a 2O

K2O

NiO

Tota

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r#M

g#

09X

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ite

Chr

40.

010.

220.

0414

.180

.70.

572.

250.

000.

010.

000.

8198

.799

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7309

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TC

1-3

Dun

ite

Chr

core

20.

110.

087.

7443

.942

.90.

762.

780.

010.

040.

010.

1498

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.4C

hrri

m1

0.02

0.50

0.29

36.7

58.2

0.84

1.41

0.02

0.02

0.00

0.23

98.2

98.8

4.14

09X

DT

C1-

5D

unit

eC

hr3

0.02

0.24

0.09

22.9

70.9

0.89

2.75

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0.00

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0.72

98.5

99.4

6.46

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1.56

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0.00

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1.02

99.0

98.4

3.24

09X

DT

C1-

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unit

eC

hr4

0.00

0.12

0.04

7.54

88.1

0.26

1.31

0.00

0.02

0.00

0.86

98.3

99.1

2.58

09X

DT

C1-

30D

unit

eC

hr3

0.00

0.16

1.62

28.2

64.9

1.01

2.78

0.02

0.03

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0.57

99.3

92.1

7.09

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unit

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1.08

20.1

74.0

0.65

3.48

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0.00

0.00

0.64

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192

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1-35

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3896

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050.

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010.

3798

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180.

0314

.680

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522.

010.

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7398

.799

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2509

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TC

1-37

Hbl

tC

hr2

0.01

0.40

0.08

13.9

81.5

0.46

0.82

0.00

0.01

0.00

0.57

97.8

99.2

1.76

09X

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40H

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pxt

Chr

10.

000.

070.

2610

.885

.40.

310.

20.

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.415

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DE

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blG

brC

hr2

2.43

0.00

11.2

51.9

23.9

2.54

4.43

0.35

0.05

0.35

0.00

97.2

75.7

24.8

Not

es:C

hr,c

hrom

ite;

Cpx

t,cl

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br,g

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;Sp,

spin

el.C

r#=

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256 B.-X. Su et al.

Tabl

e3.

Pyr

oxen

eco

mpo

siti

ons

ofth

eX

iado

ngm

afic–

ultr

amafi

cco

mpl

ex.

Sam

ple

Roc

kty

peM

iner

alS

iO2

TiO

2A

l 2O

3C

r 2O

3Fe

OM

nOM

gOC

aON

a 2O

K2O

NiO

Tota

lM

g#F

sW

oE

n

09X

D-1

2H

blC

pxt

Cpx

50.6

0.28

2.97

0.05

3.45

0.10

15.4

24.7

0.05

0.00

0.03

97.6

88.9

5.49

50.4

44.1

09X

DT

C1-

27H

blG

brC

px55

.70.

050.

490.

000.

680.

2317

.825

.20.

160.

000.

0510

0.3

97.9

1.04

49.8

49.1

Cpx

55.2

0.13

1.23

0.01

0.68

0.16

17.8

25.4

0.06

0.00

0.04

100.

797

.91.

0450

.048

.9C

px55

.80.

050.

670.

080.

510.

1518

.025

.50.

060.

020.

0610

0.9

98.4

0.78

50.0

49.3

Cpx

54.8

0.16

1.13

0.00

0.70

0.10

17.6

25.7

0.07

0.00

0.09

100.

497

.91.

0650

.448

.509

XD

TC

1-40

Hbl

Cpx

tC

px50

.51.

226.

420.

474.

710.

1614

.320

.11.

220.

010.

0099

.184

.58.

3846

.045

.7X

DE

-2H

blG

brC

px52

.90.

001.

160.

291.

190.

0817

.425

.00.

070.

020.

0098

.196

.31.

8449

.748

.4C

px51

.70.

002.

390.

711.

290.

0416

.625

.00.

120.

000.

0097

.895

.82.

0450

.847

.1C

px52

.40.

060.

890.

421.

080.

0717

.525

.00.

200.

000.

0097

.696

.71.

6749

.648

.7C

px52

.30.

000.

550.

511.

100.

0817

.225

.10.

330.

000.

0197

.296

.61.

7150

.248

.1X

DE

-3H

blG

brC

px51

.90.

001.

610.

480.

990.

0517

.525

.40.

060.

010.

0098

.096

.91.

5250

.248

.3C

px52

.40.

030.

900.

070.

890.

0317

.625

.30.

060.

010.

0197

.397

.31.

3750

.048

.6C

px53

.90.

070.

770.

080.

920.

0218

.025

.60.

030.

020.

0399

.497

.31.

3849

.649

.0C

px52

.90.

000.

800.

060.

930.

0217

.825

.50.

030.

010.

0798

.297

.21.

4149

.848

.8X

DE

-4H

blG

brC

px51

.30.

002.

980.

951.

140.

0616

.625

.30.

110.

000.

0298

.596

.31.

8051

.147

.1C

px52

.40.

002.

750.

841.

100.

0616

.824

.60.

170.

010.

1398

.996

.51.

7450

.348

.0C

px53

.00.

030.

760.

000.

930.

0317

.825

.30.

050.

000.

0497

.897

.21.

4249

.748

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px53

.20.

000.

680.

000.

970.

0217

.425

.20.

100.

000.

0797

.797

.01.

5050

.048

.5X

DE

-5H

blG

brC

px53

.50.

031.

380.

061.

030.

0717

.325

.20.

030.

010.

0098

.696

.81.

6050

.248

.2C

px53

.10.

011.

210.

111.

000.

1117

.325

.40.

060.

040.

0598

.496

.91.

5550

.348

.1C

px52

.10.

052.

270.

211.

110.

0516

.825

.20.

040.

000.

0097

.996

.51.

7450

.847

.409

XD

TC

1-14

Dun

ite

Opx

58.6

0.01

0.30

0.10

4.05

0.07

36.8

0.05

0.02

0.00

0.22

100.

294

.25.

760.

194

.1

Not

es:C

px,c

lino

pyro

xene

;Cpx

t,cl

inop

yrox

enit

e;G

br,g

abbr

o;H

bl,h

ornb

lend

e;O

px,o

rtho

pyro

xene

.Mg#

=10

0×M

g/(M

g+

Fe);

Fs=

100

×Fe

/(M

g+

Fe+

Ca)

;Wo

=10

0×

Ca/

(Mg

+Fe

+C

a);

En

=10

0×

Mg/

(Mg

+Fe

+C

a).

Dow

nloa

ded

by [

Inst

itute

of

Geo

logy

and

Geo

phys

ics

of C

AS]

at 1

6:54

21

Nov

embe

r 20

11

International Geology Review 257

Tabl

e4.

Hbl

com

posi

tion

sof

the

Xia

dong

mafi

c–ul

tram

afic

com

plex

.

Sam

ple

09X

D-1

09X

D-7

09X

D-1

009

XD

-12

09X

D-1

309

XD

TC

2-1

XD

ZK

1601

-20

09X

DT

C1-

409

XD

TC

1-6

09X

DT

C1-

709

XD

TC

1-8

09X

DT

C1-

9

Roc

kty

peD

unit

eH

blt

Hbl

tH

blG

brH

blt

Hbl

tH

blG

brH

blG

brH

blC

pxt

Gbr

Dio

Hbl

Gbr

Hbl

tM

iner

alA

mph

Tr

Am

phA

mph

Act

Am

phA

mph

Am

phA

mph

Act

Tr

Act

Act

Am

phA

mph

Am

phN

o.2

12

22

52

21

21

54

25

4

SiO

247

.457

.442

.144

.552

.348

.347

.242

.448

.952

.458

.951

.454

.647

.647

.541

.6T

iO2

0.11

0.00

0.82

0.53

0.09

0.01

0.23

1.30

0.07

0.11

0.04

0.00

0.19

0.30

0.59

0.85

Al 2

O3

9.62

0.60

14.5

11.6

3.59

9.64

11.0

10.7

8.49

5.85

0.03

7.68

2.59

8.97

8.51

13.1

Cr 2

O3

0.64

0.09

0.23

0.04

0.11

0.02

0.22

0.00

0.86

0.29

0.00

0.07

0.02

0.03

0.03

0.00

FeO

3.56

0.76

10.9

16.1

6.31

3.69

4.84

18.0

3.94

3.38

0.96

3.50

9.95

12.2

13.4

15.8

MnO

0.09

0.09

0.26

0.25

0.14

0.07

0.09

0.44

0.08

0.07

0.14

0.05

0.24

0.16

0.28

0.25

MgO

19.4

24.7

13.3

11.5

19.0

18.7

17.7

10.3

20.0

21.0

23.9

19.8

17.2

14.1

13.4

10.3

CaO

12.2

12.3

12.2

10.5

12.5

12.3

13.0

11.5

12.0

12.0

13.2

13.1

11.9

11.8

11.6

11.7

Na 2

O2.

370.

042.

382.

000.

361.

511.

481.

322.

041.

340.

011.

310.

601.

731.

141.

84K

2O

0.08

0.00

0.93

0.24

0.04

0.13

0.15

1.23

0.10

0.05

0.01

0.06

0.08

0.24

0.25

1.19

NiO

0.11

0.10

0.00

0.01

0.07

0.07

0.02

0.02

0.13

0.15

0.00

0.04

0.01

0.00

0.00

0.00

Tota

l95

.696

.097

.697

.394

.594

.496

.097

.196

.696

.797

.397

.097

.497

.296

.696

.6O

xyge

n23

2323

2323

2323

2323

2323

2323

2323

23S

i6.

748

7.79

66.

142

6.47

17.

522

6.91

86.

745

6.36

16.

848

7.25

98.

016

7.18

27.

719

6.88

86.

906

6.25

8T

i0.

011

0.00

00.

090

0.05

80.

010

0.00

10.

024

0.14

70.

008

0.01

10.

004

0.00

00.

020

0.03

20.

064

0.09

6A

l1.

613

0.09

62.

490

1.98

90.

609

1.62

81.

854

1.88

41.

402

0.95

50.

005

1.26

60.

431

1.53

01.

458

2.32

1C

r0.

072

0.01

00.

027

0.00

40.

013

0.00

30.

025

0.00

00.

096

0.03

10.

000

0.00

80.

002

0.00

30.

003

0.00

0Fe

3+0.

385

0.68

60.

355

1.07

30.

352

0.29

20.

170

0.80

00.

592

0.49

60.

099

0.05

60.

286

0.43

00.

628

0.43

2Fe

2+0.

039

0.59

90.

976

0.88

30.

406

0.15

00.

408

1.45

10.

130

0.10

40.

010

0.35

30.

891

1.04

41.

000

1.55

2M

n0.

011

0.01

10.

032

0.03

10.

017

0.00

80.

010

0.05

60.

009

0.00

80.

016

0.00

60.

028

0.01

90.

034

0.03

1M

g4.

120

5.00

22.

888

2.49

04.

071

4.00

03.

763

2.30

14.

176

4.34

34.

850

4.12

93.

622

3.05

22.

907

2.31

0C

a1.

859

1.79

21.

909

1.63

31.

920

1.89

11.

986

1.83

91.

799

1.78

61.

926

1.96

71.

810

1.83

31.

802

1.88

8N

a0.

653

0.01

10.

674

0.56

20.

101

0.41

80.

408

0.38

30.

553

0.36

00.

003

0.35

40.

165

0.48

60.

322

0.53

5K

0.01

50.

000

0.17

30.

044

0.00

70.

024

0.02

70.

236

0.01

80.

010

0.00

20.

011

0.01

50.

044

0.04

60.

228

Ni

0.01

30.

011

0.00

00.

001

0.00

80.

007

0.00

20.

002

0.01

50.

017

0.00

00.

004

0.00

10.

000

0.00

00.

000

Tota

l15

.54

14.8

115

.76

15.2

415

.04

15.3

415

.42

15.4

615

.38

15.1

714

.93

15.3

414

.99

15.3

615

.17

15.6

5M

g#99

.189

.374

.773

.890

.996

.490

.261

.397

.097

.799

.892

.180

.374

.574

.459

.8

(con

tinu

ed)

Dow

nloa

ded

by [

Inst

itute

of

Geo

logy

and

Geo

phys

ics

of C

AS]

at 1

6:54

21

Nov

embe

r 20

11

258 B.-X. Su et al.

Tabl

e4.

(con

tinu

ed).

Sam

ple

09X

DT

C1-

1009

XD

TC

1-12

09X

DT

C1-

1309

XD

TC

1-19

09X

DT

C1-

2109

XD

TC

1-22

09X

DT

C1-

2409

XD

TC

1-25

09X

DT

C1-

2809

XD

TC

1-31

09X

DT

C1-

32

Roc

kty

peH

blC

pxt

Hbl

Gbr

Hbl

tD

unit

eH

blt

Hbl

Gbr

Dun

ite

Dun

ite

Dun

ite

Dun

ite

Dun

ite

Min

eral

Act

Tr

Am

phA

mph

Am

phA

mph

Am

phA

ctT

rT

rA

mph

Act

Act

No.

42

44

35

44

64

35

1

SiO

253

.558

.646

.045

.045

.541

.347

.055

.357

.756

.648

.254

.754

.2T

iO2

0.19

0.00

1.20

0.82

0.32

1.19

0.98

0.14

0.00

0.04

0.17

0.02

0.12

Al 2

O3

4.68

0.34

9.04

11.3

11.5

14.1

8.51

1.42

2.18

2.78

10.1

4.16

4.47

Cr 2

O3

0.00

0.00

0.04

0.03

0.42

0.04

0.05

0.00

0.11

0.06

0.61

0.00

0.09

FeO

3.43

1.63

14.8

11.9

4.80

13.3

13.2

9.20

1.64

1.91

4.01

2.55

2.48

MnO

0.11

0.17

0.44

0.33

0.08

0.32

0.31

0.29

0.09

0.06

0.03

0.08

0.05

MgO

21.5

23.5

12.0

13.9

18.9

12.0

13.4

17.7

23.1

22.9

19.5

22.2

22.0

CaO

12.0

12.7

11.3

11.1

11.7

11.9

11.4

12.5

12.8

12.6

12.5

12.4

12.4

Na 2

O1.

210.

101.

832.

262.

672.

421.

510.

210.

350.

381.

740.

870.

76K

2O

0.05

0.00

0.35

0.20

0.14

0.66

0.24

0.03

0.05

0.00

0.14

0.03

0.07

NiO

0.02

0.03

0.00

0.04

0.12

0.06

0.00

0.00

0.07

0.08

0.16

0.05

0.02

Tota

l96

.897

.096

.996

.896

.197

.396

.796

.898

.097

.497

.297

.196

.7O

xyge

n23

2323

2323

2323

2323

2323

2323

Si

7.38

37.

981

6.78

36.

509

6.43

66.

095

6.85

67.

865

7.79

97.

685

6.72

77.

489

7.44

6T

i0.

020

0.00

00.

133

0.08

90.

034

0.13

20.

108

0.01

50.

000

0.00

40.

018

0.00

20.

013

Al

0.76

10.

055

1.57

01.

922

1.91

42.

446

1.46

30.

238

0.34

80.

446

1.66

60.

671

0.72

5C

r0.

000

0.00

00.

004

0.00

30.

047

0.00

40.

005

0.00

00.

012

0.00

60.

067

0.00

00.

010

Fe3+

0.54

00.

258

0.44

80.

773

0.76

60.

514

0.55

80.

130

0.21

50.

377

0.50

80.

476

0.47

1Fe

2+0.

145

0.07

21.

375

0.67

20.

197

1.12

71.

056

0.96

50.

030

0.15

90.

040

0.18

40.

186

Mn

0.01

30.

020

0.05

50.

040

0.01

00.

040

0.03

80.

035

0.01

10.

007

0.00

30.

009

0.00

6M

g4.

427

4.75

92.

631

2.99

33.

991

2.64

12.

916

3.75

34.

646

4.63

44.

051

4.53

64.

516

Ca

1.77

81.

846

1.77

81.

715

1.77

41.

876

1.78

81.

905

1.85

81.

837

1.86

81.

811

1.83

0N

a0.

324

0.02

70.

523

0.63

40.

733

0.69

10.

426

0.05

80.

091

0.10

10.

471

0.23

20.

203

K0.

008

0.00

00.

065

0.03

60.

024

0.12

50.

044

0.00

50.

008

0.00

00.

025

0.00

60.

011

Ni

0.00

30.

003

0.00

00.

004

0.01

40.

007

0.00

00.

000

0.00

70.

009

0.01

80.

005

0.00

2To

tal

15.1

114

.88

15.3

715

.39

15.5

515

.70

15.2

614

.97

14.9

614

.95

15.3

815

.05

15.0

5M

g#96

.898

.565

.781

.795

.370

.173

.479

.599

.496

.799

.096

.196

.1

Dow

nloa

ded

by [

Inst

itute

of

Geo

logy

and

Geo

phys

ics

of C

AS]

at 1

6:54

21

Nov

embe

r 20

11

International Geology Review 259

Tabl

e4.

(con

tinu

ed).

Sam

ple

09X

DT

C1-

3709

XD

TC

1-39

09X

DT

C1-

4009

XD

TC

1-42

XD

E-1

XD

E-2

XD

E-3

XD

E-4

XD

E-5

XD

E-5

Roc

kty

peH

blt

Hbl

Cpx

tH

blC

pxt

Hbl

tH

blG

brH

blG

brH

blG

brH

blG

brH

blG

brH

blG

brM

iner

alA

ctA

mph

Tr

Tr

Am

phA

mph

Am

phA

ctA

mph

Am

phA

mph

Act

No.

56

22

13

13

55

31

SiO

253

.348

.456

.857

.546

.547

.944

.453

.946

.750

.747

.752

.5T

iO2

0.05

0.23

0.06

0.03

0.45

0.03

0.00

0.03

0.01

0.07

0.02

0.04

Al 2

O3

2.46

8.36

0.03

0.11

9.33

8.09

13.9

2.42

11.5

7.18

10.5

3.91

Cr 2

O3

0.01

0.13

0.02

0.00

0.00

0.37

1.95

0.70

0.56

1.04

1.08

0.03

FeO

9.21

3.91

0.82

1.22

12.8

3.78

2.87

1.46

2.09

1.89

2.19

1.67

MnO

0.18

0.04

0.08

0.08

0.24

0.09

0.07

0.05

0.05

0.08

0.07

0.08

MgO

17.6

19.8

24.1

23.4

13.7

19.5

17.3

22.3

19.1

20.6

19.5

22.1

CaO

12.4

13.3

13.2

13.1

12.3

12.6

13.1

13.5

13.0

13.2

12.8

13.1

Na 2

O0.

620.

620.

050.

071.

011.

242.

080.

191.

691.

091.

560.

46K

2O

0.03

0.06

0.00

0.00

0.30

0.10

0.21

0.03

0.32

0.12

0.23

0.13

NiO

0.00

0.05

0.07

0.00

0.00

0.05

0.14

0.07

0.00

0.08

0.06

0.06

Tota

l95

.994

.995

.295

.596

.893

.796

.094

.695

.196

.095

.894

.0O

xyge

n23

2323

2323

2323

2323

2323

23S

i7.

667

6.88

17.

899

7.98

76.

776

6.91

56.

378

7.63

26.

680

7.13

56.

741

7.45

6T

i0.

006

0.02

40.

006

0.00

40.

049

0.00

30.

000

0.00

30.

002

0.00

70.

002

0.00

4A

l0.

416

1.40

30.

005

0.01

81.

602

1.37

62.

351

0.40

41.

929

1.19

11.

751

0.65

4C

r0.

001

0.01

50.

003

0.00

00.

000

0.04

20.

221

0.07

80.

063

0.11

60.

121

0.00

4Fe

3+0.

234

0.53

10.

220

0.09

00.

559

0.48

40.

000

0.08

30.

125

0.10

10.

270

0.27

0Fe

2+0.

873

0.06

60.

124

0.05

11.

006

0.02

70.

344

0.09

00.

125

0.12

20.

011

0.07

2M

n0.

022

0.00

50.

010

0.01

00.

030

0.01

10.

008

0.00

60.

007

0.00

90.

008

0.00

9M

g3.

780

4.20

84.

982

4.84

12.

980

4.19

63.

698

4.70

44.

070

4.31

94.

118

4.67

5C

a1.

911

2.02

41.

967

1.94

61.

925

1.94

32.

011

2.04

61.

996

1.98

51.

945

1.99

6N

a0.

174

0.17

20.

013

0.01

80.

284

0.34

70.

579

0.05

10.

467

0.29

80.

426

0.12

5K

0.00

50.

011

0.00

10.

001

0.05

60.

018

0.03

90.

005

0.05

90.

022

0.04

10.

023

Ni

0.00

00.

005

0.00

70.

000

0.00

00.

006

0.01

60.

008

0.00

00.

009

0.00

70.

006

Tota

l15

.09

15.2

114

.99

14.9

615

.27

15.3

115

.65

15.1

115

.52

15.3

115

.42

15.1

5M

g#81

.298

.597

.699

.074

.899

.491

.598

.197

.097

.399

.798

.5

Not

e:A

ct,a

ctin

olit

e;A

mph

,am

phib

ole;

Cpx

t,cl

inop

yrox

enit

e;D

io,d

iori

te;G

br,g

abbr

o;H

bl,h

ornb

lend

e;H

blt,

horn

blen

dite

;Gbr

,gab

bro;

Tr,

trem

olit

e.

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260 B.-X. Su et al.

Tabl

e5.

Pla

gioc

lase

com

posi

tion

sof

the

Xia

dong

mafi

c–ul

tram

afic

com

plex

.

Sam

ple

Roc

kty

peM

iner

alN

o.S

iO2

TiO

2A

l 2O

3C

r 2O

3Fe

OM

nOM

gOC

aON

a 2O

K2O

NiO

Tota

lA

nA

b

09X

D-7

Hbl

tP

l2

64.2

0.06

21.2

0.00

0.01

0.02

0.03

2.07

10.7

0.21

0.00

98.5

9.72

90.3

Zo

243

.00.

0223

.60.

010.

100.

000.

0026

.20.

120.

000.

0393

.199

.20.

8109

XD

-10

Hbl

tP

l2

58.5

0.00

24.8

0.04

0.03

0.00

0.00

6.57

7.97

0.02

0.00

98.0

31.3

68.7

09X

D-1

3H

blG

brZ

o4

43.5

0.03

33.7

0.00

0.01

0.01

0.00

17.7

1.07

0.02

0.00

96.2

90.1

9.86

XD

ZK

1601

-20

Hbl

Gbr

Pl

159

.30.

0024

.50.

000.

020.

000.

006.

427.

940.

070.

0198

.230

.969

.109

XD

TC

1-7

Gbr

Dio

Pl

366

.60.

0420

.50.

120.

040.

030.

031.

6910

.90.

050.

0010

0.1

7.86

92.1

09X

DT

C1-

8H

blG

brP

l2

60.5

0.07

24.4

0.00

0.08

0.03

0.01

6.30

8.24

0.03

0.02

99.7

29.7

70.3

09X

DT

C1-

12H

blG

brP

l3

62.4

0.00

23.4

0.00

0.04

0.00

0.01

4.86

8.99

0.06

0.00

99.8

23.0

77.0

09X

DT

C1-

21H

blt

Zo

438

.60.

1325

.70.

058.

920.

150.

0523

.20.

000.

000.

0096

.799

.90.

0209

XD

TC

1-22

Hbl

Gbr

Pl

362

.80.

0023

.60.

000.

060.

000.

015.

008.

990.

090.

0410

0.6

23.5

76.5

09X

DT

C1-

27H

blG

brZ

o5

39.0

0.12

27.0

0.00

7.11

0.09

0.03

23.5

0.01

0.01

0.00

96.9

99.9

0.08

XD

E-1

Hbl

Gbr

Zo

244

.40.

0032

.80.

020.

020.

040.

0016

.61.

870.

020.

0095

.783

.116

.9X

DE

-2H

blG

brZ

o6

38.7

0.02

32.5

0.17

0.37

0.00

0.04

24.0

0.01

0.00

0.00

95.8

99.9

0.07

XD

E-3

Hbl

Gbr

Zo

338

.40.

0033

.00.

030.

660.

010.

0324

.40.

010.

010.

0196

.599

.90.

04X

DE

-4H

blG

brZ

o2

39.0

0.00

32.1

0.02

0.75

0.04

0.03

24.0

0.00

0.01

0.02

95.9

100

0.00

XD

E-5

Hbl

Gbr

Zo

238

.40.

0233

.10.

010.

460.

030.

0323

.90.

010.

000.

0196

.099

.90.

10

Not

es:D

io,d

iori

te;H

bl,h

ornb

lend

e;H

blt,

horn

blen

dite

;Gbr

,gab

bro;

Pl,

plag

iocl

ase;

Zo,

zois

ite.

An

=10

0×

Ca/

(Ca

+N

a);A

b=

100

×N

a/(C

a+

Na)

.

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Table 6. Ilmenite and titanite compositions of the Xiadong mafic–ultramafic complex.

Sample Rock type Mineral SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O NiO Total

09XDTC1-7 Gbr Dio Tita 30.8 38.2 0.85 0.03 0.76 0.01 0.04 27.8 0.00 0.02 0.00 98.509XDTC1-8 Hbl Gbr Ilme 0.00 52.0 0.00 0.02 42.9 3.86 0.10 0.06 0.00 0.01 0.00 98.9

Tita 31.0 39.2 0.57 0.04 0.39 0.04 0.02 28.1 0.01 0.00 0.00 99.409XDTC1-12 Hbl Gbr Ilme 0.03 51.3 0.00 0.02 43.1 4.36 0.10 0.05 0.03 0.00 0.00 99.0

Ilme 0.02 49.0 0.02 0.05 45.9 2.67 0.41 0.04 0.02 0.00 0.02 98.109XDTC1-13 Hblt Ilme 0.02 50.8 0.00 0.04 41.9 6.20 0.10 0.05 0.00 0.03 0.00 99.1

Tita 30.8 39.2 0.64 0.01 0.35 0.06 0.00 28.3 0.00 0.00 0.04 99.309XDTC1-21 Hblt Ilme 0.03 49.7 0.01 0.02 41.3 7.47 0.15 0.03 0.02 0.01 0.03 98.8

Tita 30.8 37.3 1.59 0.00 0.83 0.06 0.00 28.4 0.02 0.00 0.00 99.009XDTC1-22 Hbl Gbr Ilme 0.03 49.4 0.01 0.00 45.9 2.88 0.10 0.04 0.03 0.01 0.00 98.4

Ilme 0.00 49.7 0.00 0.05 47.3 1.68 0.08 0.01 0.00 0.00 0.03 98.8

Note: Dio, diorite; Gbr, gabbro; Hbl, hornblende; Hblt, hornblendite; Ilme, ilmenite; Tita, titanite.

Figure 6. Plots of (A) Fo versus MnO and (B) Fo versus NiO contents of olivines in the Xiadong mafic–ultramafic complex.

Figure 7. Plots of chromite compositions in the Xiadong mafic–ultramafic complex. (A) Fe3+–Cr–Al diagram demonstrating Fe-enrichment trend; (B) Al2O3 versus TiO2 diagram showing the close relationship between the Xiadong chromites and the island-arc field.Alaskan-type field after Alaska complex (Himmelberg et al. 1986; Himmelberg and Loney 1995); Ocean island basalt (OIB), mid-oceanridge basalt (MORB), and island-arc fields after Kamenetsky et al. (2001).

Plagioclase

Plagioclases are completely absent in the dunites and Hblclinopyroxenites, and those present in some hornblenditesand Hbl gabbros are strongly affected by alteration,changing to mostly zoisites. These zoisites have apparentCa enrichment and Si–Al–Na depletion. Some relics of

primary plagioclases have anorthite (An) numbers between9.72 and 30.9 (Table 5).

Ilmenite and titanite

Ilmenites are commonly present in hornblendites and Hblgabbros and have TiO2 in the range of 49.0–52.0 wt.%,

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Figure 8. Plots of clinopyroxene compositions in the Xiadong mafic–ultramafic complex. (A) Al2O3 versus SiO2 and (B) TiO2 versusAlz in clinopyroxene. The fields of the Alaskan-type complexes are from Quetico, Pettigrew and Hattori (2006); Tulameen, Rublee (1994);and Gabbro Akarem, Helmy and El Mahallawi (2003). Non-alkaline and alkaline boundary is after Le Bas (1962) and Alz refers to thepercentage of Al in the tetrahedral sites (100 × AlIV)/2. The arc cumulate, ophiolite, and Mid-Atlantic Ridge trends are after Loucks(1990).

1.0

0.8

0.6

0.4

Mg/

(Mg

+ Fe

2+)

0.2

0.0

1.0

0.8

0.6

0.4

Na

+ K

0.2

0.08.0 8.07.5 7.57.0

Si Si

7.06.5 6.55.56.0 6.0

Actinolite

Ferro-actinolite

Magnesio-hornblende

Ferro-hornblende

Ferro-pargasite

DuniteHbl clinopyroxeniteHornblenditeHbl gabbro

Gabbroic diorite

Tulameen

Quetico

Gabbro AkaremArc cumulates

Pargasite

Tremolite (A) (B)

Figure 9. Plots of hornblende (Hbl) compositions in the Xiadong mafic–ultramafic complex. (A) Hbl classification after Leake et al.(1997); (B) Si versus Na + K contents in Hbl. The fields of the Alaskan-type complexes are from Quetico, Pettigrew and Hattori (2006);Tulameen, Rublee (1994); and Gabbro Akarem, Helmy and El Mahallawi (2003). Arc cumulates field is defined by Beard and Barker(1989).

FeO of 41.3–47.3 wt.%, and MnO of 1.68–7.47 wt.%.All the analysed titanites show homogeneous compositions(Table 6).

Discussion

Comparisons to regional mafic–ultramafic complexes

Abundant mafic–ultramafic complexes are distributed inthe Jueluotage Belt, MTM, and Beishan Rift (Figure 1B).These complexes from the three belts have apparently

different features. The Jueluotage and MTM complexesare generally composed of clinopyroxene/Hbl peridotite,olivine clinopyroxenite, clinopyroxenite, gabbro, norite,and diorite. These rocks often exhibit poikilitic and gab-broic textures. Orthopyroxene, plagioclase, and mica arecommon minerals, but magnetite is minor or absent inthese complexes. Fo contents of olivines range from 78 to86. Clinopyroxenes are classified as diopside and augite.Spinels are mainly Al-rich types and no chromite isobserved. Most of the complexes host magmatic Ni–Cusulphide deposits (Qin et al. 2003, 2007; Zhou et al. 2004;

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Chai et al. 2006, 2008; Mao et al. 2008; Pirajno et al. 2008;Liu et al. 2010; Xiao et al. 2010).

The rock types of the Beishan complexes are mainlydunite, clinopyroxene peridotite, troctolite, gabbro, anddiorite. Plagioclase is present in all rock units, but orthopy-roxene and hydrous minerals such as Hbl and micaare completely absent. Very rare magnetite is observed.Poikilitic (orthocumulated) and gabbroic textures are alsowell developed in the ultramafic and mafic rocks, respec-tively. Olivines have Fo contents in the range of 76–90.Clinopyroxenes are diopsidic and augitic and spinels rangefrom Al-spinel to Cr-spinel. Significant amounts of dissem-inated Ni–Cu sulphides have also been observed in thesecomplexes (Jiang et al. 2006; Su et al. 2009, 2010a, 2010b;Ao et al. 2010).

These regional mafic–ultramafic complexes havewidely been interpreted as evolving from high-Mg tholei-itic magmas from the lithospheric mantle in the post-orogenic extension tectonic setting and/or mantle plume(Zhou et al. 2004; Han et al. 2006; Jiang et al. 2006; Wanget al. 2006; Chai et al. 2008; Mao et al. 2008; Pirajno et al.2008; Zhang et al. 2008; Su et al. 2009, 2010a, 2010b; Sun2009). The Xiadong complex, on the contrary, is distinctlydifferent in petrology, mineralogy, and mineral chemistryfrom other regional complexes, suggesting that the petro-genesis and tectonic environment for the evolution of theXiadong complex is different from the other two.

Comparisons to classic Alaskan-type complexes

Typical features of Alaskan-type complexes have been welldocumented in previous studies (e.g. Taylor 1967; Irvine1974; Rublee 1994; Himmelberg and Loney 1995; Johan2002; Helmy and El Mahallawi 2003; Pettigrew and Hattori2006; Thakurta et al. 2008; Ripley 2009). Morphologically,Alaskan-type complexes have crude concentric zoning inlithologies and, in most cases, are roughly circular orelliptical in shape, pipe-like in cross section, with sizesranging from 12 to 14 km2 (Johan 2002). Petrologically,the Alaskan-type complexes are generally composed ofdunite, wehrlite, olivine clinopyroxenite, Hbl clinopyrox-enite, hornblendite, and Hbl gabbro, but the completesequence of lithologies is rarely observed (Irvine 1974;Himmelberg and Loney 1995). The ultramafic cumulatestend to show adcumulated textures and lack interstitial min-erals crystallized from ‘trapped liquid’ (Thakurta et al.2008; Ripley 2009). Mineralogically, abundant clinopy-roxenes and primary Hbls occur in Hbl clinopyroxenitesand hornblendites. Chromite is commonly concentrated indunite and often forms stratiform segregations and irreg-ular veins (Irvine 1974; Himmelberg and Loney 1995;Johan 2002; Ripley 2009). Orthopyroxene and plagioclaseare rare in the ultramafic rocks, and plagioclase occurs

only in marginal gabbroic rocks (Helmy and El Mahallawi2003; Pettigrew and Hattori 2006). Magnetite is a commonmineral in clinopyroxenite and hornblendite and its modalabundance can range between ∼15 and 20% (Taylor 1967;Himmelberg and Loney 1995).

The mineral chemistry of Alaskan-type complexesis characterized by Mg-rich olivine, Ca-rich diopsidicclinopyroxene, high Fe–Cr, and low Al chromite, andcalcic Hbls with a wide range in composition (Irvine1974; Rublee 1994; Helmy and El Mahallawi 2003).Geochemically, all rock types show low abundances ofincompatible elements such as Y and rare earth elements,low high-field strength elements, and relatively high large-ion lithophile elements (Helmy and El Mahallawi 2003;Pettigrew and Hattori 2006; Ripley 2009).

The Xiadong mafic–ultramafic complex has rock unitsof dunite, Hbl clinopyroxenite, hornblendite, and Hblgabbro, together with a mineral assemblage of high-Mgolivine, diopsidic clinopyroxene, chromite, calcic Hbl,magnetite, and other accessory minerals, which are iden-tical to typical Alaskan-type complexes. On the contrary,the regional mafic–ultramafic complexes in the EasternTianshan and Beishan Rift can be excluded from theAlaskan-type complexes.

Chromite compositions are important indicators todistinguish an Alaskan-type complex, stratiform com-plex, Alpine-type complex, and ophiolite. Relative toAlaskan-type complexes, chromites from both stratiformand Alpine-type complexes have higher Mg#, lowerFe3+/(Fe3+ + Cr + Al) ratios, and slightly lower Cr#(Figure 10A and 10B; Irvine 1967); whereas the chromitesfrom ophiolites show apparently lower Fe2+/(Mg + Fe2+)ratios (Figure 10C; Barnes and Roeder 2001). Allchromites from the Xiadong complex overlap with thefield defined by the typical Alaskan-type complexes anddisplay similar compositional trends to Alaskan-type com-plexes (Figure 10A–10C). Furthermore, it is apparent thatchromite compositions of the Xiadong complex follow adifferentiation (Fe enrichment) trend from an intermedi-ate Cr–Al-rich spinel to Cr-magnetite (Figure 7A). Suchtrend of increasing Fe3+ has been reported for spinels fromAlaskan-type complexes (Snoke et al. 1981; Nixon et al.1990) and is not identified with other igneous complexessuch as ophiolites or layered intrusions (Barnes and Roeder2001). Clinopyroxenes and Hbls from the Xiadong com-plex also have compositional variations similar to someAlaskan-type complexes such as Quetico, Tulameen, andGabbro Akarem (Figures 8A, 8B, and 9B). Olivines fromAlaskan-type complexes worldwide show a Fo range from66 to 95 (Figure 11). The Xiadong olivines show anoma-lously high Fo contents in the range of 92–97, partlyoverlapping the range of those from typical Alaskan-typecomplexes (Figure 11). All the comparisons are summa-rized in Table 7.

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Figure 10. Chromite compositional comparisons betweenXiadong and typical Alaskan-type complexes. (A) Plot of Mg#versus Fe3+/(Fe3+ + Al + Cr) of chromites. The fields in thediagram are from Alaskan-type complexes worldwide, Barnesand Roeder (2001); SE Alaskan-type complexes, stratiform com-plexes, and Alpine-type complexes, Irvine (1967). (B) Plot ofMg# versus Cr# of chromites. All field sources are the same asin (A). (C) Plot of Fe2+/(Mg + Fe2+) versus Cr# of chromites.The fields of Alaskan-type complexes and ophiolite and alterationtrend are after Barnes and Roeder (2001).

Figure 11. Fo content of olivines from the Xiadong mafic–ultramafic complex and typical Alaskan-type complexes (modi-fied after Pettigrew and Hattori 2006). Sources: Turnagain, Clark(1980); Gabbro Akarem, Helmy and Moggesie (2001); BlashkeIsland, Himmelberg et al. (1986); Union Bay, Polaris, and DukeIsland, Irvine (1974, 1976), Tulameen, Rublee (1994); Queticoand Samuel Lake, Pettigrew and Hattori (2006).

These similarities suggest that the Xiadong complexis equivalent to an Alaskan-type complex in terms ofpetrology and mineral chemistry, indicating that they areprobably cogenetic, but without any similarity to strati-form, Alpine-type, and ophiolitic complexes.

Petrogenesis and tectonic significance

A number of hypotheses have been proposed to account forthe Alaskan-type complexes. Taylor (1967) suggested thatfractional melting in the mantle accounted for Alaskan-type complexes. Sha (1995) proposed that the parentalmagmas of Alaskan-type complexes fractionally crystal-lized from the mixture between a mantle-derived maficmagma and a crustal felsic magma. Efimov (1998), onthe contrary, attributed Alaskan-type complexes to tectonicemplacement of fragments of a pre-existing body. Farahatand Helmy (2006) suggested the formation of Alaskan-typecomplexes by fractional crystallization from a commonhydrous parental magma without significant crustal con-tamination. Parental magmas of the Xiadong complex mostlikely contain high Mg contents, evidenced by anomalouslyhigh-Fo olivine (Table 1; Figure 6), high-Mg# clinopyrox-ene (Table 3), and high-Mg Hbl (Table 4; Figure 9A). Onthe contrary, olivines only occur in the dunite unit, andtheir compositions do not show fractional correlations buthomogeneous MnO contents against varying Fo and nega-tive correlation between NiO and Fo (Table 6). The discon-tinuity of mineral modal abundances and intrusive contactswith chilled margins (Figures 2, 3B, and 3C) suggests that

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Table 7. Comparisons between typical Alaskan-type and Xiadong complexes.

Alaskan-type complexes Xiadong complexes

Age Mostly Phanerozoic Late Carboniferous∗Geological setting Close to the end of subduction, prior to

accretion–collisionClose to the end of subduction, prior to

accretion–collision∗Size Most are small in size ranging from 12 to 40 km2 In size of ∼2.5 km2Morphology and zoning Crude concentric zoning of lithologies grading

from olivine-rich ultramafic cores to mafic rimsStrip shape

Sequence of intrusion Gabbroic and dioritic rocks intrude late Gabbroic and dioritic rocks intrude lateLithology Dunite, hornblendite, clinopyroxenite, gabbro;

minor dioritic and syenitic rocksDunite, hornblendite, Hbl clinopyroxenite, Hbl

gabbro; minor diorite and no syeniteTextures Accumulated texture with minor/no trapped

liquidAccumulated texture with minor/no trapped

liquidMineralogy Abundant clinopyroxene, primary hornblende,

magnetite; lack of orthopyroxene andplagioclase in ultramafic rocks

Abundant clinopyroxene, primary hornblende,magnetite; lack of orthopyroxene andplagioclase in ultramafic rocks

Chromite Common occurrence of chromite in dunite Common occurrence of chromite in duniteMineral chemistry High-Mg olivine; diopsidic clinopyroxene;

phlogopitic mica; hornblende is calcic with awide range in composition

High-Mg olivine, diopsidic clinopyroxene;hornblende is calcic with a wide range incomposition

Bulk rock geochemistry Low incompatible elements; relatively high LILEand low HFSE; no Eu anomalies

Relatively high LILE, and low HFSE and REE;no Eu anomalies∗

Mineralization PGE mineralization in olivine-rich cores (dunite)associated with chromite; rare Cu–Nimineralization

Showing potential PGE mineralization in dunite;no Cu–Ni sulphide mineralization∗

Notes: Hbl, hornblende; HFSE, high field strength element; LILE, large-ion lithophile element; PGE, platinum group element; REE, rare earth element.The features of Alaskan-type complexes are after Taylor (1967), Irvine (1974), Rublee (1994), Johan (2002), Helmy and El Mahallawi (2003), Pettigrewand Hattori (2006), Thakurta et al. (2008), and Ripley (2009). Those marked ‘∗’ will be shown elsewhere.

the Xiadong complex was formed by multi-stage emplace-ment of magma, in the sequence of light yellow dunite, darkgreen dunite, Hbl clinopyroxenite, hornblendite, and finallyHbl gabbro. In the genesis stage of dunites, Mn, Ni, andFe are probably preferentially partitioned into chromitesas evidenced by the negative correlation of MnO and thepositive correlation of NiO in chromites with the Fo con-tents of olivines (Figure 12). Fe3+-enrichment trend inchromites (Figure 7A) and associated ilmenite (Figure 5Cand 5D) possibly imply that they were crystallized in a rela-tively high oxidizing environment. A considerable numberof dolomite veins observed in the Xiadong rocks (Figure5B) indicates that the complex most likely has reacted withits country rocks such as marble during its emplacement.

Alaskan-type complexes are always related to the sub-duction environment and arc accretion (Taylor 1967; Irvine1974). For example, the complexes in Alaska intruded thewestern margin of the North American continent during theclosure of the intra-arc basin (Saleeby 1992; Foley et al.1997; Thakurta et al. 2008; Ripley 2009); the Ural mafic–ultramafic complexes intruded during the accretion of arcterrane to the continent (Ayarza et al. 2000); the Queticocomplex formed through the accretion of micro-continentsand arcs to the north, through the subduction of interven-ing oceanic crust (Williams 1991; Valli et al. 2004; Farahatand Helmy 2006). In this study, however, the MTM andJueluotage Belt subduction events took place in Palaeozoic(Han et al. 2006; Wang et al. 2006; Zhang et al. 2008)

and the Dananhu–Tousuquan, Xiaorequanzi–Wutongwozi,and Yamansu are recognized to be island-arc basin, intra-arc basin, and back-arc basin, respectively (Figure 1B; Qinet al. 2002). Thus, the identification of the Xiadong com-plex as an Alaskan-type intrusion implies that the MTMwith Precambrian basement was probably a continental arcduring the subduction process. This tectonic frameworkmay indicate that the Palaeozoic Junggar Ocean located tothe north of the MTM (Ma et al. 1993; Qin et al. 2002; Li2004; Zhang et al. 2004, 2008; Han et al. 2006; Li et al.2006a, 2006b; Wang et al. 2006).

Conclusion

We have conducted a comprehensive study of the petrology,mineralogy, and mineral chemistry of the Xiadong mafic–ultramafic complex. The complex has identical mineralchemistry, as well as similar petrological and mineralogicalcharacteristics as typical Alaskan-type complexes world-wide. The relationships between the rock units, modalcompositions of minerals, and chemical compositionalvariations indicate that the Xiadong complex was formedby multi-stage emplacement mechanisms, accompaniedby reaction with the surrounding country rocks. The dis-covery and the confirmation of the Xiadong complexas an Alaskan-type complex imply that the MTM withPrecambrian basement was most likely a continental arc

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Figure 12. Plots of (A) Fo versus MnO and (B) Fo versus NiO of chromites and olivines from the Xiadong mafic–ultramafic complex.

during the subduction of oceanic lithosphere, which fur-ther supports the hypothesis of southward underflow of thePalaeozoic Junggar Ocean.

AcknowledgementsThis study was financially supported the Nature ScienceFoundation of China (Grant 41030424) by the KnowledgeInnovation Programme of the Chinese Academy of Sciences(Grant KZCX2-YW-107) and the Chinese State 305 Programme(Grant 2006BAB07B03-01).

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