crustal evolution and phanerozoic crustal growth in northern xinjiang: nd isotopic evidence. part i....

Crustal evolution and Phanerozoic crustal growth innorthern Xinjiang: Nd isotopic evidence. Part I. Isotopic

characterization of basement rocks

Aiqin Hua, Bor-ming Jahnb,*, Guoxin Zhanga, Yibing Chena, Qianfeng Zhanga

aGuangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, People's Republic of ChinabGeÂosciences Rennes (UPR CNRS 4661), Campus de Beaulieu, UniversiteÂ de Rennes 1, 35042 Rennes Cedex, France

Abstract

The Central Asian Orogenic Belt (CAOB) is known as the most important site of juvenile crustal growth during the

Phanerozoic. In order to examine the processes of such crustal generation and the role of Precambrian crust in the magma

genesis, we conducted geochemical and Nd isotopic studies on both the basement rocks and Phanerozoic granites from the

major tectonic terranes in northern Xinjiang: Altai, Junggar, Tianshan and North Tarim. In this paper only the results on the

basement rocks are reported.

The North Tarim Terrane is composed of Archean bimodal suite (TTG gneisses and amphibolites) and Proterozoic granitic

gneisses. This terrane is a fragment of ancient continental crust and is tectonically dissociated from the CAOB. The other

terranes in northern Xinjiang (Altai, Junggar, and Tianshan) belong to the CAOB. The Altai and Tianshan are composite

terranes probably formed by accretion of Phanerozoic subduction complexes with entrained Proterozoic basement rocks as

microcontinental blocks. Geochemical characteristics of amphibolites from Altai and Tianshan suggest their formation in island

arc settings. Sm±Nd model ages of the Tianshan basement rocks vary from 1.2 to 2.2 Ga, but mainly concentrated in 1.7±

2.1 Ga. Similarly, amphibolites and gneisses of the Altai terrane have TDM in two apparently discrete groups at 0.9±1.5 Ga and

2.4±2.6 Ga. The initial Nd isotope ratios or 1Nd(T ) values indicate that a large proportion of basement gneisses from Tianshan

were derived from remelting of Paleo- to Mesoproterozoic protoliths (1.7±2.1 Ga). However, this is not the case for the Altai

basement gneisses.

Granitic gneisses and metasedimentary rocks (schists and phyllites) from the East and West Junggar terranes have much

younger TDM ages of 0.7±1.4 Ga. Presence of a minor Precambrian crustal component is possible as inferred from the model

ages, but no data have shown Precambrian ages for the Junggar basement rocks. The model age and eNd(T ) data support the idea

that the Junggar terrane is composed of young island arc assemblages, and the Junggar Basin itself could be a trapped Paleozoic

oceanic crust.

Together with some reliable radiometric ages for basement rocks, the present isotopic data and TDM ages indicate that ancient

microcontinental blocks constitute a signi®cant proportion of the continental crust in northern Xinjiang, and probably also in the

entire CAOB. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: crustal evolution; microcontinental block; granitic gneisses; Central Asian Orogenic Belt (CAOB); Sm±Nd isotopes; Altaid Collage

1. Introduction

Continental Asia has long been recognized as a

huge tectonic mosaõÈc composed of stable Precambrian

Tectonophysics 328 (2000) 15±51

0040-1951/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.

PII: S0040-1951(00)00176-1

www.elsevier.com/locate/tecto

* Corresponding author. Tel.: 133-299-286-083; fax: 133-299-

281-499.

E-mail address: [email protected] (B. Jahn).

cratons (Siberian, Sino-Korean, Tarim, Yangtze,

Indian, etc.) welded by inter-cratonic mobile orogenic

belts, which often include numerous dispersed minor

terranes. Among the mobile belts, the Central Asian

Orogenic Belt (CAOB, Coleman, 1989; Zonenshain et

al., 1990), otherwise known as the Altaid Tectonic

Collage (SengoÈr et al., 1993; SengoÈr and Natal'in,

1996), represents the largest Phanerozoic orogen of

the world. This orogen is accretionary in nature, as

opposed to the collisional orogens such as the

Hercynian and Alpine chains (Windley, 1995).

According to SengoÈr et al. (1993), nearly 50% of

materials accreted were formed in subduction zones

and derived from the upper mantle. If this can be

veri®ed, then the CAOB would represent a very

important addition of juvenile crust in the Phanero-

zoic. Combining the results already obtained from

Sierra Nevada (DePaolo, 1980), Canadian Cordillera

(Samson et al., 1989; 1990), Arabia (Stern, 1994;

Stein and Hofmann, 1994), and possibly the Lachlan

orogenic belt of Australia (Coney, 1992; Collins,

1996), as well Central Asia (Kovalenko et al.,

1996), the production of new crust in the Phanerozoic

appears to be very signi®cant. Consequently, the

existing models of continental development, in

which little to no growth is commonly assumed,

would have to be re-evaluated.

Because of the foreseeable important implications

for continental evolution, a new IGCP project (No.

420) was launched in 1997 to study the eastern part

of the CAOB, which encompasses Siberia, Kazakh-

stan, Mongolia, Xinjiang, Inner Mongolia, and NE

China. The northern half of Xinjiang in NW China

(Fig. 1) is part of the CAOB and it has been subjected

to intense resource-oriented studies in the last ten

years by Chinese geologists. Parallel geochronology

and Nd±Sr isotopic tracer studies have also been

carried out in order to better understand the crustal

evolution of northern Xinjiang as a whole (e.g. Hu

1982; Hu and Rogers, 1992; Hu et al., 1982, 1986,

1995, 1997 and 1998a,b; Zhao et al., 1993; Han et al.,

1997). In the course of study, it was realized that

abundant granitic rocks were emplaced in late

Paleozoic times and many of them have isotopic

compositions indicating their juvenile characters,

whereas others showing variable contributions of old

crustal rocks in their petrogeneses (Zhao et al., 1993;

Han et al., 1997; Hu et al., 1998b).

The purpose of this paper and the companion paper

(Part II) is to report new Sm±Nd (and some Rb±Sr)

isotopic data as well as chemical analyses for both

Precambrian basement and Phanerozoic intrusive

granitic rocks. This work represents by far the most

comprehensive isotopic tracer study on any segments

of the CAOB. Owing to the very large data set (221

analyses), Part I will deal with geochemical and

isotopic characterization of basement rocks. Pertinent

age information will also be provided. Part II will

address the genesis of Phanerozoic granitic rocks

and discuss the problems of juvenile crustal growth

and the role of the basement rocks in the generation of

these granitoids. The term ªbasement rocksº used here

is loosely de®ned and it means Precambrian meta-

morphic rocks intruded by the granitic plutons.

Some strata of probable early Paleozoic to late

Precambrian age from the Altai Terrane, as well as

some ophiolitic complexes of the Junggar Terrane, are

also included in ªbasement rocksº. Following the

customary Chinese usage, northern Xinjiang can be

divided into the following tectonic units or terranes

(from north to south): Altai, West Junggar, East

Junggar, Junggar Basin, Turpan Basin, West

Tianshan, East Tianshan, and Northern Tarim. These

terms will be employed throughout the papers. The

samples used in this study came from all the above

terranes except Junggar and Turpan basins, which are

overlain by as much as 15 km of Cenozoic sediments

(BGMRX, 1993).

2. General geologic setting of northern Xinjiang

The boundaries of major tectonic terranes in

northern Xinjiang appear to represent the original

boundaries of accretion of Precambrian crust and

younger oceanic and island arc terranes, but they are

complicated by transpressional faulting owing to

intracontinental extrusion tectonics resulting from

the collision of India with Eurasia in the early Tertiary

(Tapponnier et al., 1982; Ma et al., 1987; Coleman,

1989). A synthesis of terrane accretion in northern

Xinjiang from Precambrian to the present has been

attempted by several authors (e.g. Ma et al., 1987;

Coleman, 1989; Xiao et al., 1992; Hu, 1998a,

1998b). Xiao et al. (1992) considered northern

Xinjinag as a convergent zone of three principal

A. Hu et al. / Tectonophysics 328 (2000) 15±5116

A.

Hu

eta

l./

Tecto

nophysics

328

(2000)

15

±51

17

. . .

BoleJinghe

Lake Ebinur

LakeSayram

Xingxingxia

QingheQinghe

ErtaiErtai

Fuyun

JunggarBasin

Northern Xinjiang

Tarim Basin

UlungurLake

Changji Qitai

Barkol

Hami

ShanshanTurpan

Qingir

Balguntay

Korla

Bayanbulak

Kuqa

Wenquan

KuytunShihezi

Karamay

Urumqi

China

Xinjiang

Cenozoic cover

Mesozoic strata

Paleozoic strata

Mesoprotero. basement

Neoprot. - Paleo. basem.

Archean andPaleoprotero. basement

Mesozoic granites

Late Paleozoic granites

Early Paleozoic granites

Proterozoic granites

Basic rocks

Ophiolites

88 92

100

48

44

100

40

84

76

44

40

80

84

88 92

96

96

80

48

0 200 400 km

Intermediate rocks

Hantengri Feng

Baicheng

TS60

Toksun

NT42-47

Yining

Aksu

Keyishan

NT28-36

NT26

Weiya

TS61-65TS67-70

Xinyuan

Tekes

TS49-59

ALT6-11

ALT12-17

ALT1

ALT18-23

ALT2-5

JUG1-8

TS48

TS44-47

NT27

NT1-25

NT48-50

TS39-43

TS20

TS21

JUG9-10

TS17-19

TS1-16

Paleo-Mesoprot. basem.

Sample

ALT24

ALT25

TS22-38

NT37-41

Xiamaya

Lake Bosten

Yuli

Baihaba

Laba

Xingdi

Yili BasinTurpan BasinII

I

III

Altay

E

EEE

E

EE

N

E E E

E

N

N

N

N

EE

N

Fig. 1. Simpli®ed geological map of northern Xinjiang showing the major stratigraphic units and intrusive rocks (modi®ed after the unpublished geologic map of 1:5,000,000

produced by Xinjiang BGMR, 1994). Sample localities are indicated by small letters with number. Three terrane boundary faults are given by Roman letters. I:Eerqisi Fault, II:

Tianshan Main Fault±Shaquanzi Fault, III: Qinqir Fault.

plates Ð Siberia, Kazakhstan and Tarim, following

the total subduction of the Paleo-Asian Ocean. Each

individual plate is further composed of major ªcrustal

segmentsº and minor ªcrustal slabsº as the secondary

and tertiary tectonic or structural units, respectively.

In this tectonic classi®cation, the Altai Mountains in

northernmost Xinjiang were considered as a southern

part of the Siberian plate, and Tianshan Mountains as

part of the Kazakhstan plate (Xiao et al., 1992). This is

contrary to our current thinking that all terranes to the

north of the Tarim craton, including their ªKazakhstan

plateº and the Altai Mountains, should belong to the

CAOB, and the southern margin of the Siberian plate

lies much further to the north in the Baikal region

(SengoÈr et al., 1993).

In order to avoid the controversy about the tectonic

af®nity, we choose to describe only individual

terranes. As de®ned by Coney et al. (1980), terranes

are distinct geologic tracts that have recorded different

history from nearby or adjacent terranes. Based on

this concept, Coleman (1989) divided Central Asia

into 13 terranes; but they cautioned that the number

and con®guration of terranes would be modi®ed as

more data become available. For example, terrane

boundaries in the complex Tianshan Mountains have

been variously described by different authors

(Coleman, 1989; Windley et al., 1990; Xiao et al.,

1992; Gao et al., 1998). Nevertheless, their grouping

of terranes into genetically oriented broad categories

is instructive and is adopted here. These categories

are: (1) continental fragments, including Precambrian

cratons and microcontinents; (2) oceanic crust assem-

blages, including ophiolites and overlying sediments;

(3) island arc assemblages; and (4) composite terranes

consisting of two or more of the above categories in

stratigraphic sequence. In the following we provide

brief description of the six terranes (from north to

south) from which our samples were collected. We

call attention that our terrane boundaries are loosely

constrained and we hope that our isotopic Nd model

age data would help re-de®ne the boundaries more

precisely and signi®cantly in the future.

2.1. Altai Terrane

This northernmost terrane is a ªcomposite terraneº

consisting of Precambrian gneiss complexes, Paleo-

zoic strata and intrusive granites. The discovery of

Meso-Proterozoic (Jixian System, <1400±1000 Ma)

plant microfossils (ªcoronary plantº) by M.Y. Zhao in

marbles (RGSP1, 1986) from the Fuyun area (Fig. 1)

has ®rmly established the existence of Precambrian

rocks in the Altai Terrane. The Precambrian gneiss

complexes have recently been subdivided into two

groups: Kemuqi and Fuyun Groups (Li and Bespaev,

1994). The Kemuqi Group is composed of granitic

gneiss, migmatite, metagabbro, amphibolite, biotite±

quartz±schist, marble lenses, and metavolcanic tuff.

Although a number of ages have been published

(RGSP1, 1987; Zhang, et al., 1994; Hu et al., 1995;

1997; Wang et al., 1989; Zhang et al., 1996a,b), few

were good enough to con®rm the existence of Precam-

brian rocks in the Altai Terrane. A garnet-bearing

gneiss of the Kemuqi Group from west of Fuyun

County gave an imprecise U±Pb upper intercept age

of 2349 ^ 226 Ma, and a depleted-mantle based Sm±

Nd model age (TDM) of 2.6 Ga (Hu et al., 1995; Table

1; all errors quoted in this paper are 2s). At present

this is the only zircon U±Pb age for the metamorphic

rocks to show the existence of Precambrian rocks in the

Altai Terrane (Hu et al., 1995; Hu et al., 2000). Further-

more, a Sm±Nd isochron age of 1357 ^ 52 Ma, with

1Nd(T )�16.7 (MSWD� 0.25), was obtained for a

suite of rocks from a gneiss complex of the Kemuqi

Group near Altay city (Hu et al., 2000; Table 1).

The Fuyun Group is lithologically similar to the

Kemuqi Group, only with amphibolites being the

dominant component. A Sm±Nd isochron age of

706 ^ 65 Ma with 1Nd(T )�16.5 (MSWD� 0.38)

was obtained for a gneiss complex of the Fuyun

Group (Hu et al., 2000). Interpretation of the two

Sm±Nd ages are dif®cult because the isochrons are

constructed using data of different lithologies (granitic

gneisses and amphibolites). However, it seems to

support Proterozoic ages for these rocks. In addition,

Sm±Nd model ages (TDM) for six amphibolites with

ªlowº 147Sm/144Nd ratios (ca. 0.14) gave a tight range

of ages from 915 to 948 Ma (the samples from

Wuqiagou to the east of Fuyun, Fig. 1), which may

represent the time of their protolith formation. The

Precambrian rocks have been thermally affected by

massive granitic intrusions in late Paleozoic, which

resulted in young hornblende and biotite 40Ar/39Ar

plateau ages of 278 and 271 Ma, respectively (Hu et

al., 1997).

It should be underlined that before the recent


A. Hu et al. / Tectonophysics 328 (2000) 15±51 19

Table 1

Age summary of basement rocks in northern Xinjiang, China

Age Method References

Altai TerraneW. Fuyungranitic gneiss 2349 ^ 226 Ma zircon U±Pb (u.i.) Hu et al., 1995

2.6 Ga Sm±Nd TDM Hu et al., 1995granitic gneisses 706 ^ 65Ma Sm±Nd isochron Hu et al., in review

E. Fuyun6 amphibolites 915±948 Ma TDM (fSm/Nd < 0.3) Hu et al., 1995hb (gneiss) 278 ^ 2 Ma Ar/Ar Hu et al., 1997bio (gneiss) 271 ^ 2 Ma Ar/Ar Hu et al., 1997

Altaygneiss/amph 1375 ^ 52 Ma Sm±Nd isochron Hu et al., 2000

East Junggar TerraneE. Barkolgneisses 1908 Ma zircon Pb evap Zhang, 1989

S. Xiamayagneisses 671 ^ 140 Ma Sm±Nd isochron Zhang et al., 1996a,b

1.1±1.4 Ga TDM Hu et al., 1997

West Junggar Terranemetavol and phyllites 0.7 Ga and 1.4 Ga TDM Hu et al., 1995ophiolites 523 ^ 7 Ma Pb±Pb isochron Kwon et al., 1989

489±395 Ma Sm±Nd isochron Zhang and Huang, 1992

East Tianshan TerraneXingxingxiagneiss/amph 1829 ^ 143 Ma Sm±Nd isochron Hu et al., 1997gneisses 1400 ^ 42 Ma zircon U±Pb (u.i.) Hu et al., 1986gneiss 1404 ^ 18 Ma zircon U±Pb (u.i.) unpublishedhornblendes 402 ^ 16 Ma Hu, 1982 Hu, 1982muscovites 284 ^ 15 Ma Hu, 1982 Hu, 1982biotites 235 ^ 27 Ma K±Ar Hu, 1982microcline 198 ^ 20 Ma K±Ar Hu, 1982

Balguntaybiotite (schist) 344 ^ 7 Ma Ar/Ar Hu et al., 1997

West Tianshan TerraneWunquanamphibo. (10) 1600±1800 Ma TDM Hu et al., 1995granitic gneisses ,1800 Ma TDM Hu et al., 1997granitic gneiss 821 ^ 11 Ma zircon U±Pb (u.i.) Chen, 1999granitic gneiss 798 Ma zircon U±Pb (con.) Chen et al., 1999

Du±Ku Highway (Laerdun±Daban)tonalitic gneiss 882 ^ 33 Ma zircon U±Pb (u.i.) Chen et al., 2000a

South of Muzha'ertegranodioritic gneiss 707 Ma zircon U±Pb (con.) Chen et al., 2000b

North Tarim TerraneQingiramphibo. (10) 3263 ^ 129 Ma Sm±Nd isochron Hu and Rogers, 1992TTG gn. (8) 2.9±3.0 Ga TDM Hu and Rogers, 1992granodioritic gneiss ca. 2.6 Ga zircon U±Pb (SIMS) unpublishedgranitic gneiss 2071 ^ 37 Ma U±Pb (u.i.) Gao, 1990biotite (gneiss) 499 ^ 10 Ma Ar/Ar Hu et al., 1997

Xishankou (NE Korla)granitic gneiss 2488 ^ 5 Ma zircon Pb evap. Gao, 1990

Yulitonalitic gneisses 2100±2300 Ma zircons U±Pb (7/6) Gao, 1990biotite (schist) 636 ^ 6 Ma Ar/Ar Hu et al., 1997

Kuokesu(southern part of N Tarim)hb (granodio.) 787 ^ 5 Ma Ar/Ar Hu et al., 1997biotite (gr. gn.) 766 ^ 7 Ma Ar/Ar Hu et al., 1997

con®rmation of Precambrian ages, the Kemuqi and

Fuyun Groups were erroneously regarded as lower

Devonian (Kangbutiebao Group), or upper Carboni-

ferous (Kalae'erqisi Group) gneisses, or even late

Paleozoic granitic rocks. It is possible that the

Precambrian rocks represent part of a microcontinent

entrained in the vast CAOB.

The Paleozoic strata are represented by meta-

morphosed ¯ysch deposits, composed of schists,

crystalline limestones, metagreywackes, meta-

volcanic rocks and phyllites (BGMRX, 1993).

Abundant granitic intrusions have been emplaced in

this terrane during Paleozoic to early Mesozoic. Most

of them have late Paleozoic ages of 350±250 Ma; a

few were emplaced in early Paleozoic (390±408 Ma)

and early Mesozoic (210±180 Ma) times (Wang et al.,

1989; Zhang et al., 1996a,b; Zhang et al., 1994; Hu et

al., 1997). Granitic rocks occurring along the Ulungur

River were intruded at about 300 Ma and are charac-

terized by their peralkaline compositions and juvenile

crustal nature (Han et al., 1997).

2.2. Junggar Basin, west and east Junggar Terranes

The Junggar Basin is covered by Cenozoic desert

and thick continental basin sediments ($10 km) as

old as Permian (BGMRX, 1993). A number of drilling

records indicate little deformation within the basin,

suggesting stable con®guration of the basement at

least since the Permian (Xie et al., 1984; Coleman,

1989). The nature of the Junggar basement has been

much debated; some considered that the basin repre-

sents a microcontinent with Precambrian basement

(Ren et al., 1980; Wang, 1986; Watson et al., 1987;

Zhang et al., 1984; Wu, 1987), whereas others

regarded it as trapped Paleozoic oceanic crust of

various origins (Li et al., 1982; Jiang, 1984; HsuÈ,

1989; Feng et al., 1989).

Surrounding the Junggar Basin, numerous ophio-

lites are exposed in East and West Junggar Terranes

as well as in its southern margin (Fig. 1). Coleman

(1989) considered these terranes as oceanic arc

assemblages and compared them with those in the

present western Paci®c. In these terranes no rocks of

Precambrian ages have been positively documented.

The sediments in these accreted oceanic terranes are

cherts, shales, limestone, volcaniclastic sediments and

tuff. (BGMRX, 1993). Minor amounts of granitic

gneisses occur in East and West Junggar Terranes,

but none of them have been precisely dated. Zhang

et al. (1989) reported a zircon Pb evaporation age of

< 1.9 Ga for a granitic gneiss from the east of Barkol

region in the East Junggar, but the age has not been

con®rmed by subsequent work. Sm±Nd isotope

analyses on ®ve granitic gneisses from the south of

Xiamaya in East Junggar (Fig. 1) yielded an imprecise

Sm±Nd isochron age of 671 ^ 140 Ma with

1Nd(T )�13.6 ^ 0.2 (MSWD� 0.6) (Zhang et al.,

1996a,b).

Ophiolites from the Junggar Terranes have been

subjected to more geochronological studies, particu-

larly using the Sm±Nd isochron technique. Some

published results (Zhang and Huang, 1992) are

summarized below: (1) Tongbale ophiolites

(West Junggar): 489 ^ 53 Ma, 1Nd(T )�15.8 ^ 0.6

(MSWD� 1.6) for gabbros; 447 ^ 56 Ma,

1Nd(T )�17.3 ^ 0.5 (MSWD� 0.4) for basalts;

(2) Dalabute ophiolites: 395 ^ 12 Ma, 1Nd(T )�18.9 ^ 0.1 (MSWD� 0.1) for gabbros; (3) Honggu-

leleng ophiolites: 444 ^ 27 Ma, 1Nd(T )�17.2 ^ 0.2

(MSWD� 0.9) for basalts. Moreover, a Pb±Pb

isochron age of 523 ^ 7 Ma was obtained for a leuco-

gabbro of the Tangbale ophiolites and it was inter-

preted as the formation age of the oceanic crust

(Kwon et al., 1989). Duplicate analyses using ®ve

sphene grains and a plagioclase fraction from the

same leucogabbro were performed in the same labora-

tory (Univ. California, Santa Barbara) and yielded a

new Pb±Pb isochron age of 508 ^ 20 Ma (Xiao et al.,

1992, p. 26). In either case, the Pb±Pb and Sm±Nd

ages for the Tangbale ophiolites are in agreement

within error limits. (4) Armantai ophiolites in north-

east Junggar: Sm±Nd isochron age of 561 ^ 41 Ma

with 1Nd(T )�16.1 ^ 0.9 (MSWD� 0.7) for WR

gabbros, diabases, andesitic porphyrites and plagio-

clase (Huang et al., 1997). In view of the overall

age information (Table 1), the presence of Precam-

brian basement rocks in the Junggar Terranes is not

strongly supported.

A variety of granitic rocks were emplaced in the

East and West Junggar Terranes (Fig. 1). They range

in composition from M-type plagiogranites, A-type

peralkaline granites, to I-type granodiorites and

granites (Xiao et al., 1992). The principal periods of

intrusion are in late Paleozoic (320±250 Ma, Lu et al.,

1989; Zhou, 1989; Shen et al., 1993); a minor amount


of granitoids were emplaced in early Paleozoic

(<400 Ma, Hu et al., 1997).

2.3. East and west Tianshan Terranes

The Tianshan Range is a complex orogen with

composite terranes and doubly sutured belts (Windley

et al., 1990; Xiao et al., 1992). It extends east±west

for at least 2500 km in central Asia. Towards the west

of Balguntay, the Tianshan Range is split into north-

ern and southern branches separated by the Yili Basin

(Fig. 1). From the structural point of view, it might be

more logic to separate Tianshan into North, Central

and South Tianshan Terranes. However, our sampling

localities dictate the usage of East and West Tianshan

Terranes for convenience.

Paleo- to Mesoproterozoic basement rocks occur

widely in the uplifted zones from east to west

Tianshan. The eastern end of Tianshan is situated

near Xingxingxia (Fig. 1). The East Tianshan Terrane

is represented by the Xingxingxia Group, which

comprises banded and augen gneisses, migmatites,

schists, amphibolites, hornblende quartz schists and

marbles, and is exposed to the south of the Shaquanzi

Fault (see Geological map of K-46-XXIII of

200,000:1; Hu et al., 1982b) in East Tianshan Ð

from Xingxingxia in the easternmost Tianshan to

Balguntay in the middle section (SW of Urumqi,

Fig. 1). In the West Tianshan Terrane, several litho-

logic groups have been proposed by the Chinese

geologists (BGRMX, 1993): (1) the Nalati Group,

composed of hornblende±plagioclase gneiss, migma-

tite, and minor greenschist, quartzite and dolomitic

marble, is distributed in the northern slope of Haerke

Mountains and along the Nalati Mountains (north-

weast of Bayanbulak); (2) the Muzha'erte Group,

composed of granitic gneisses, leptynite, marbles,

and ®ne-grained metasediments, occurs in the area

of the Muzha'erte River in the southern slope of

Haerke mountains (northwest of Baicheng); and (3)

the Wenquan Group, made up of amphibolites,

amphibole±quartz schists, banded and augen

gneisses, migmatite, mica schists and marbles, is

exposed in the Wenquan and east of Sayram lake

areas in the northern branch (Fig. 1). The northern

branch of West Tianshan (near Wenquan) coincides

with the Yili Terrane of Coleman (1989).

Thus, the lithologies of Precambrian basement

rocks of the Tianshan Terranes include amphibolites,

banded and augen gneisses, migmatites, schists, quart-

zites and marbles. Some of them have undergone

granulite facies metamorphism, which is best repre-

sented by the granulites of Weiya in East Tianshan

(Chen et al., 1998). So far, these basement rocks have

not been extensively dated, but the following age

information suf®ces to indicate the Precambrian

history of the Tianshan Terranes.

In East Tianshan, a whole-rock Sm±Nd isochron

age of 1829 ^ 143 Ma with 1Nd(T )�14.5 ^ 0.4

(MSWD� 2.6) was determined for four granitic

gneisses plus one amphibolite of the Xingxingxia

Group (Hu et al., 1997). All rocks have a tight

range of TDM about 2.0 Ga (1.9±2.1 Ga) (Table 4).

Moreover, a U±Pb upper intercept age of

1400 ^ 42 Ma for 15 zircon fractions from ®ve

gneisses of Xingxingxia Group was obtained (Hu et

al., 1986). Duplicate analyses of four zircon fractions

from a granodioritic gneiss yielded an identical upper

intercept age of 1404 ^ 18 Ma (Hu, unpublished).

The discrepancy of the Sm±Nd and zircon ages

(Dt < 400 Ma) is dif®cult to interpret. Two explana-

tions are equally possible: (1) the zircons have been

completely reset at 1400 Ma and the age of

<2000 Ma represents the crustal residence time of

the gneiss and amphibolite protoliths; and (2) the

protolithic magmas for gneisses were produced at

1400 Ma by melting of mixed sources comprising

older granitoid and ma®c rocks of Archean or Paleo-

proterozoic ages. The amphibolite could be a basaltic

rock formed at 1400 Ma and was contaminated by an

older crustal component. Such a large discrepancy

between Sm±Nd and zircon ages has been observed

in Precambrian basement of Alberta (Theriault

and Ross, 1991) and the Arunta Inlier, central

Australia (Zhao and McCulloch, 1995; Chauvel et

al., 1985).

K±Ar analyses of hornblendes, muscovites, biotites

and microcline separated from different samples of

gneisses, hornblende schists, and mica schists from

Xingxingxia to Tianhu in the central uplift zone in

the East Tianshan gave cooling ages from 400 to

200 Ma (Hu, 1982; Hu et al., 1982; Table 1). This

suggests that an important thermal effect was asso-

ciated with late Paleozoic massive granitic intrusions

in Tianshan.

In West Tianshan, Precambrian metamorphic


rocks of Wenquan, Nalati and Muzha'erte Groups

have been recently dated, and the results are

summarized here: (1) a tonalitic gneiss of the Natati

Group sampled at 748 km (north of Bayanbulak)

along the Du±Ku highway yielded an U±Pb upper

intercept age of 882 ^ 33 Ma (Chen et al., 2000a);

(2) a U±Pb upper intercept ages of 821 ^ 11 Ma

and a concordant age of 798 ^ 8 Ma were obtained

for granitic gneisses of the Wunquan Group from

the south of Wenquan County and from east of

Lake Sayram, respectively (Chen, 1999; Chen et

al., 1999); (3) a U±Pb concordant age of

707 ^ 7 Ma was obtained for a granodioritic gneiss

of the Muzha'erte Group from the south slope of

Haerke Mountains (northwest of Baicheng; Chen

et al., 2000b). These results indicate that protoliths

of the orthogneisses were emplaced in late Protero-

zoic (700±882 Ma, Chen, 1999; Table 1). Moreover,

10 amphibolites of the Wenquan Group gave tightly

grouped Sm±Nd model ages of about 1.7 Ga (TS5-

14, Table 4), and all gneisses have a small range of

TDM about 1.9 Ga (1.8±2.0 Ga) in West Tianshan

(Hu et al., 1995; Table 1). Two amphibolites with

coarse texture (TS15 and TS16, Table 1) from the

Wenquan area have TDM of about 1.4 Ga (Hu et al.,

1997).

In East Tianshan early Paleozoic strata are rela-

tively minor in comparison with late Paleozoic

sequences which include island arc volcanic rocks,

volcaniclastic and some marine sediments. The

moderate distribution of ophiolites has been used to

delineate paleosutures (Windley et al., 1990). Simi-

larly, in West Tianshan, early Paleozoic arc-trench

sequences were developed in the Wenquan±Jinghe

area (Xiao et al., 1992).

Granitic rocks were ubiquitously intruded in the

Tianshan Mountains. Most of them have late

Paleozoic ages (ca. 300 Ma) and a few were

emplaced in early Paleozoic times (400±450 Ma)

or in Neoproterozoic (1200±960 Ma, Hu et al.,

1986).

2.4. Northern margin of Tarim block ± North Tarim

Terrane

This terrane refers to the region south of the Qingir

Fault and is locally named as the Kuruktag (Fig. 1). It

is a genuine continental terrane characterized by

widespread Precambrian crystalline basement rocks

and Paleozoic stable platform sediments (Huang et

al., 1987; Xiao et al., 1992). An Archean complex

(local term, the Tuogelakebulake Group) occurs near

Qingir village, and is composed of grey gneiss,

amphibolite, and actinolite±mica±schist, mica±

quartz±schist, migmatite and marble. The geochem-

ical characteristics of the predominant grey gneisses

are of the typical Archean TTG suite (tonalite±

trondhjemite±granodiorite; Hu et al., 1997). Amphi-

bolites commonly occur as enclaves within grey

gneisses and their protoliths are of continental

tholeiite composition. A whole-rock Sm±Nd isochron

age of 3263 ^ 129 Ma with 1Nd(T )�13.2 ^ 0.7

(MSDW� 3.57) was obtained for 10 amphibolite

samples (Hu and Rogers, 1992). This age was inter-

preted as the formation time of their basaltic proto-

liths, and the initial 1Nd(T ) value of 3.2 falls near the

Nd isotope evolution curve for the depleted mantle

(DM). Sm±Nd model ages for eight gneisses range

from 2.94 to 3.03 Ga (Hu and Rogers, 1992; Table

1), which may be interpreted as the period of their

protolith formation. However, a recent ion microp-

robe analysis using CAMECA IMS-1270 (Nordic

Consortium, Swidish Museum of Natural History,

Stockholm) on zircon grains from a granodioritic

gneiss (NT17, Table 2) gave a U±Pb upper intercept

age of about 2.6 Ga (unpublished). This con®rms

the existence of late Archean crystalline basement

rocks.

Moreover, late Archean to early Proterozoic

gneisses, amphibolites and schists also occur in the

areas of Xingdi and Xishankou in the western

Kuruktag. A zircon Pb evaporation age of

2488 ^ 5 Ma was obtained for a granitic gneiss

from Xishankou (between Korla and Qingir) (Gao,

1990; Gao et al., 1993) and Sm±Nd model ages of

2.4±2.6 Ga were obtained for the gneisses from the

southeastern area of Yuli (Hu et al., 1997). In addition

to the Archean and early Proterozoic metamorphic

complexes, some Proterozoic granitic intrusions are

also recognized. These rocks show gneissic

texture. Five whole-rock samples of these granitic

rocks plus two constituent minerals (hornblende

and biotite) were analyzed for Sm±Nd isotopes and

yielded an isochron age of 2239 ^ 104 Ma with

1Nd(T )�14.3 ^ 0.4 (MSWD� 3.8) (Hu et al.,

1997).


2.5. General tectonic scenario

According to Xiao et al. (1992), the paleo-oceans of

Junggar and South Tianshan were closed after Devo-

nian time and all the above terranes were essentially

accreted by ca. 360 Ma. Epeiric seas and shallow

basins were formed for deposition of terrigenous sedi-

ments. Silicic magmatism started to take place. By the

end of the Carboniferous, marine environments in

northern Xinjiang disappeared entirely following the

collision of terranes. Intense intra-continental

orogeny, manifested by post-collisional uplifts and

simultaneous subsidence of major basins (Tarim,

Junggar and Turpan), took place since the Permian.

Regarding the Tianshan Range, Cheng et al. (1986)

and Windley et al. (1990) presented evidence for two

Paleozoic sutures that mark the accretion of an island

arc (Junggar) from the north in the late Carboniferous

and a passive continental margin (Tarim) from the

south in the late Devonian to a central continental

block. The sedimentation and structural patterns in

Tianshan and elsewhere in northern Xinjiang have

been further complicated by Cenozoic deformation

induced by the India±Eurasia collision.

2.6. Paleozoic magmatism

In northern Xinjiang, the presence of Paleozoic

magmatism is manifested by the deposition of thick

marine volcaniclastic and volcanic ¯ows, as well as

the intrusion of diorite, granodiorite and granite

plutons. The huge amount of calc-alkaline intrusion

may suggest a substantial crustal growth in Central

Asia by arc magmatism (Coleman, 1989). Coleman

(1989) further noted that the most spectacular event

that accompanied the late Paleozoic convergence and

consolidation of Central Asia was the widespread and

pervasive intrusion of high-level granites, some of

them belong to A-type granites produced in post-

orogenic extensional periods (Han et al., 1997).

They are not con®ned to any particular terranes but

crosscut major boundary faults that separate the

terranes. Available Nd±Sr isotopic data indicate that

a large proportion of the granites from Kazakhstan,

Xinjiang, Mongolia, Transbaikalia, Inner Mongolia to

NE China were formed as juvenile addition to the

continental crust (Kwon et al., 1989; Zhao et al.,

1993; Kovalenko et al., 1996; Han et al., 1997; Hu

et al., 1998b; Wu et al., 2000; Litvinovsky et al., 2000;

Heinhost et al., 2000; Jahn et al., 2000; this paper).

Their petrogenesis might not be directly related to

subduction zone magmatism. This aspect will be

discussed in another paper (Part II).

3. Sampling and analytical methods

In order to delineate the patterns of Phanerozoic

crustal growth and to estimate the proportion of

recycled ancient crust in the Phanerozoic magmagen-

esis, our sampling strategy is to collect not only Paleo-

zoic granitoids but also Precambrian basement rocks

covering all the tectonic terranes with maximum

geographic distribution. We emphasize that many

sampling localities are hardly accessible. The wide

geographic distribution is to ensure the representativ-

ity of the basement characteristics. The sampling

localities are shown in Fig. 1.

Major element compositions were analyzed using

the conventional wet chemical method at Institute of

Geochemistry (CAS, Chinese Academy of Sciences)

in Guiyang. Trace element contents were analyzed

using ICP-MS at Guangzhou Institute of Geochemis-

try, CAS and ICP-AES in Wuhan Analytical Centre of

Rocks and Minerals. The analytical procedures of ICP-

MS at Guangzhou can be found in Liu et al. (1996).

The uncertainty of major element analyses is ca. 1%

and that of trace elements varies from 5 to 10% judging

from replicate analyses of standard rock samples.

Nd, Sr isotopic compositions were analyzed during a

time span of 10-year in several laboratories: Guangzhou

Institute of Geochemistry (CAS, Chinese Academy of

Sciences), Institute of Geology, Beijing (CAS), Institute

of Geology, Beijing (CAGS, Chinese Academy of

Geological Sciences), Yichang Institute of Geology

and Mineral Resources (CAGS), Centre of Geos-

ciences, Beijing, and Isotope Laboratory of SURRC,

East Kilbride, U.K. The analytical procedures of indi-

vidual laboratories can be found in Mao et al. (1989);

Zhang and Ye (1987); Qiao (1987); Huang and Wu

(1990); Li et al. (1988); Paterson et al. (1992).

4. Results and discussion

The results of chemical analyses are given in Tables

2 and 3, and isotopic data are presented in Table 4.



Table 2

Major and trace element compositions for metamorphic basement rocks from Tarim, Altai and Junggar Terranes


Table 2 (continued)


Table 3

Major and trace element compositions for metamorphic basement rocks of the West (Samples 1±23) and East (Samples 24±48) Tianshan

Terrane


Table 3 (continued)

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32Table 4

Sm±Nd Isotopic compositions for basement rocks from Northern Xinjiang (1, Hu et al., 1992; 2, Hu et al., 1997; 3, Zhang et al., 1996; a, SURRC, East Kilbride, UK; b, Guangzhou

Institute of Geochemistry, CAS; c, Chinese Academy of Geological Sciences (CAGS); d, Institute of Geology, CAS; e, Yichang Geology and Mineral Institute, CAGS. f, Centre of

Geosciences, CAS)

Sample

no.

Field

no.

Rock type Location T

(Ga)

Sm

(ppm)

Nd

(ppm)

147Sm/144Nd

143Nd/144Nd

2sm fSm/Nd 1Nd(0) 1Nd(T) TDM21 TDM22a Data

source

Labor-

atory

Tarim Terrane

1 NT1 Amphibolite Qingir 3.3 4.01 13.24 0.1832 0.512521 41 20.07 22.3 3.4 3.13 1 a










11 NT11 Gneiss Qingir 2.6 16.36 105.5 0.0937 0.510818 28 20.52 235.5 21.3 2.94 3.07 1 a





16 NT20 Granitic gn Qingir 2.1 8.97 48.70 0.1114 0.511124 25 20.43 229.5 27.0 3.00 3.10 1 a



19 NT25 Schist Qingir 2.4 2.84 15.81 0.1087 0.511250 36 20.45 227.1 20.1 2.74 2.80 2 a

20 NT26 Granitic gn Kuokesu 0.9 4.25 28.15 0.0912 0.511361 30 20.54 224.9 213.2 2.22 2.62 2 a

21 NT27 Granitic gn Dongdashan 0.8 7.47 38.94 0.1160 0.511459 25 20.41 223.0 214.8 2.62 2.69 2 a

22 NT28 Gneiss Kuokesu 2.2 6.93 35.03 0.1197 0.511721 16 20.39 217.9 4.1 2.31 2.32 2 b





27 NT33 Dioritic gn Kuokesu 0.9 7.52 44.93 0.1012 0.511417 10 20.49 223.8 213.2 2.34 2.62 2 b



30 NT36 Amphibolite Kuokesu 2.5 12.53 54.32 0.1395 0.511889 16 20.29 214.6 3.7 2.58 2 b

31 NT37 Gneiss Kuokesu 1.3 6.13 31.00 0.1196 0.511583 12 20.39 220.6 27.8 2.52 2.53 b





36 NT42 Gneiss SE Yuli 2.1 6.09 32.57 0.1131 0.511298 29 20.43 226.1 23.7 2.79 2.85 2 a

37 NT43 Amphibolite SE Yuli 2.5 4.54 20.63 0.1330 0.511825 32 20.32 215.9 4.5 2.49 2 a

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Table 4 (continued)

Sample

no.

Field

no.


(Ga)

Sm

(ppm)

Nd

(ppm)

147Sm/144Nd

143Nd/144Nd


source

Labor-

atory



40 NT46 Migmatite SE Yuli 2.3 2.23 15.06 0.0895 0.511043 25 20.55 231.1 0.4 2.57 2.68 2 a

41 NT47 Gneiss SE Yuli 2.3 1.24 7.81 0.0958 0.511124 46 20.51 229.5 0.1 2.61 2.71 2 a

42 NT48 Amphibolite Korla 3.3 8.56 38.14 0.1357 0.511491 25 20.31 222.4 3.3 3.22 1 b

43 NT49 Amphibolite Korla 3.3 7.46 28.52 0.1582 0.511967 22 20.20 213.1 3.1 3.22 b

44 NT50 Amphibolite Korla 0.8 6.34 31.00 0.1235 0.511683 30 20.37 218.6 211.2 2.47 b

Tianshan Terrane

45 TS1 Augen gn Wenquan 0.8 19.56 95.10 0.1243 0.512104 36 20.37 210.4 23.4 1.78 1.72 2 a

46 TS4b Granitic gn Wenquan 1.0 4.52 23.29 0.1174 0.512181 12 20.40 28.9 1.2 1.53 1.54 bc

47 TS5 Amphibolite Wenquan 1.7 12.10 48.04 0.1523 0.512459 42 20.23 23.5 6.1 1.71 2 a

48 TS6 Amphibolite Wenquan 1.7 10.64 41.05 0.1567 0.512548 7 20.20 21.8 7.1 1.61 c









57 TS15 Amphibolite Wenquan 1.4 4.05 19.92 0.1229 0.512322 38 20.38 26.2 7.0 1.39 a

58 TS16 Amphibolite Wenquan 1.4 3.61 15.32 0.1426 0.512506 28 20.28 22.6 7.1 1.38 a

59 TS17b Gneiss E Sayram 0.8 6.85 29.38 0.1409 0.512202 20 20.28 28.5 22.7 1.98 1.71 bf

60 TS18 Gneiss E Sayram 0.9 8.36 39.72 0.1272 0.512048 7 20.35 211.5 23.5 1.94 1.85 d

61 TS19 Mig-Gneiss E Sayram 0.9 6.70 32.01 0.1265 0.512040 7 20.36 211.7 23.7 1.93 1.86 d

62 TS20 Granitic gn Tekes 0.9 3.61 16.11 0.1354 0.512288 23 20.31 26.8 20.1 1.67 1.54 2 a

63 TS21 Gneiss S Muzha'erte 0.7 6.35 36.61 0.1048 0.511953 11 20.47 213.4 25.2 1.67 1.82 bf

64 TS22 Amphibolite L-d Daban 1.0 11.52 53.19 0.1309 0.512465 11 20.33 23.4 5.0 1.26 c

65 TS23 Gneiss L-d Daban 1.0 8.20 39.41 0.1257 0.512292 18 20.36 26.7 2.3 1.48 1.45 c




69 TS27 Schist L-d Daban 1.0 6.67 36.87 0.1093 0.511857 15 20.44 215.2 24.1 1.88 1.98 c




73 TS32 Meta-gabbro L-d Daban 0.7 2.50 10.43 0.1450 0.512190 12 20.26 28.7 24.2 2.12 1.73 c

74 TS33 Meta-Gabbro L-d Daban 1.0 5.31 22.33 0.1438 0.512472 9 20.27 23.2 3.5 1.48 1.35 c

75 TS34 Amphibolite L-d Daban 1.0 7.24 37.72 0.1160 0.511707 22 20.41 218.2 27.9 2.24 b

76 TS35 Schist L-d Daban 1.0 1.44 4.67 0.1871 0.512387 34 20.05 24.9 23.7 4.32 1.94 b

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34Table 4 (continued)

Sample

no.

Field

no.


(Ga)

Sm

(ppm)

Nd

(ppm)

147Sm/144Nd

143Nd/144Nd


source

Labor-

atory




80 TS39 Granitic gn Balguntay 0.7 6.46 38.29 0.1019 0.511541 23 20.48 221.4 212.9 2.19 2.46 2 a

81 TS40b Dioritic gn Balguntay 0.7 4.76 26.15 0.1101 0.511795 8 20.44 216.4 28.7 1.99 2.11 bc

82 TS44 Granitic gn Kumishi 0.8 6.64 54.38 0.0739 0.511991 12 20.62 212.6 20.1 1.26 1.48 b

83 TS45 Amphibolite Kumishi 0.8 4.75 18.54 0.1547 0.512424 23 20.21 24.2 0.1 1.87 a

84 TS46 Gneiss Kumishi 0.8 11.70 73.34 0.0964 0.512277 12 20.51 27.0 3.2 1.13 1.22 b

85 TS47 Schist Kumishi 0.8 15.91 100.4 0.0958 0.512246 35 20.51 27.6 2.7 1.17 1.26 a

86 TS48 Gneiss Kangkuertage 0.8 2.60 11.32 0.1389 0.512316 21 20.29 26.3 20.4 1.70 1.51 d

87 TS49 Granulite Weiya 0.8 5.34 25.86 0.1248 0.511918 10 20.37 214.0 26.7 2.10 2.03 c


89 TS54 Gar-Bi-Q-

Schi

Weiya 0.8 10.05 55.67 0.1091 0.511871 10 20.45 215.0 26.0 1.86 1.97 c

90 TS56 Schist Weiya 0.8 5.28 23.14 0.1380 0.511886 19 20.30 214.7 28.7 2.53 c




94 TS60 Granitic gn Xingxingxia 1.3 6.53 31.86 0.1238 0.511980 26 20.37 212.8 21.0 1.98 1.95 2 a

95 TS61 Gneiss Xingxingxia 1.3 12.72 70.15 0.1096 0.511850 30 20.44 215.4 20.8 1.90 1.96 2 a

96 TS62 Gneiss Xingxingxia 1.4 3.66 23.80 0.0930 0.511631 6 20.53 219.6 21.1 1.91 2.06 d



99 TS66 Amphibolite Xingxingxia 1.8 4.25 15.29 0.1681 0.512521 30 20.15 22.3 4.3 2.09 2 a

100 TS67 Gneiss Xingxingxia 1.2 5.58 32.21 0.1047 0.511812 8 20.47 216.1 22.5 1.87 1.98 b

101 TS68 Gneiss Xingxingxia 0.8 2.95 15.31 0.1166 0.512254 10 20.41 27.5 0.7 1.40 1.42 b

102 TS69 Amphibolite Xingxingxia 0.8 4.36 17.37 0.1516 0.512517 44 20.23 22.4 2.2 1.55 b

103 TS70 Amphibolite Xingxingxia 0.8 3.30 14.00 0.1425 0.512518 8 20.28 22.3 3.2 1.35 b

Altai Terrane

104 ALT1 Ga-Bi gneiss Fuyun 2.3 7.22 36.23 0.1205 0.511549 6 20.39 221.2 1.6 2.60 2.62 c

105 ALT2 Gneiss Fuyun 0.9 4.64 25.18 0.1114 0.512170 9 20.43 29.1 0.7 1.46 1.51 c

106 ALT4 Granitic gn Fuyun 0.9 5.37 21.50 0.1509 0.512404 6 20.23 24.6 0.7 1.81 1.50 c

107 ALT5 Gneiss Fuyun 0.9 5.03 25.93 0.1173 0.512207 33 20.40 28.4 0.7 1.49 1.50 a

108 ALT6 Mig-Gneiss Altay 1.4 4.97 20.12 0.1493 0.512558 9 20.24 21.6 6.9 1.40 1.41 e

109 ALT7 Granitic gn Altay 1.4 7.58 37.43 0.1224 0.512313 8 20.38 26.3 6.9 1.40 1.40 e

110 ALT8 Granitic gn Altay 1.4 29.38 192.3 0.0924 0.512040 13 20.53 211.7 7.0 1.39 1.40 e

111 ALT9 Mig-Gneiss Altay 1.4 7.82 30.00 0.1576 0.512472 9 20.20 23.2 3.7 1.84 1.66 e

112 ALT10 Leptynite Altay 1.4 5.27 27.26 0.1169 0.512275 6 20.41 27.1 7.2 1.38 1.38 e

113 ALT11 Amphibolite Altay 1.4 3.37 10.82 0.1883 0.512899 9 20.04 5.1 6.6 1.50 e

114 ALT12 Granitic gn Fuyun 0.7 9.13 43.24 0.1276 0.512658 7 20.35 0.4 6.6 0.87 0.86 c

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Table 4 (continued)

Sample

no.

Field

no.


(Ga)

Sm

(ppm)

Nd

(ppm)

147Sm/144Nd

143Nd/144Nd


source

Labor-

atory




118 ALT16 Amphibolite Fuyun 0.7 2.10 6.58 0.1933 0.512951 18 20.02 6.1 6.4 1.49 c


120 ALT18 Amphibolite Wuqiagou 0.7 11.20 47.04 0.1439 0.512720 7 20.27 1.6 6.3 0.94 e






126 ALT24 Phyllite Baihaba 0.5 6.88 34.11 0.1220 0.512590 34 20.38 20.9 3.8 0.93 0.92 b

127 ALT25 Phyllite Baihaba 0.5 3.74 18.75 0.1206 0.512377 16 20.39 25.1 20.2 1.26 1.25 b

Junggar Terrane

128 JUG1 Gneiss S. Xiamaya 0.7 4.02 19.87 0.1222 0.512496 12 20.38 22.8 3.9 1.09 1.08 3 d

129 JUG2 Schist S. Xiamaya 0.3 9.75 40.85 0.1443 0.512830 11 20.27 3.7 5.4 0.70 0.58 d




133 JUG6 Leptynite S. Xiamaya 0.7 3.12 14.51 0.1298 0.512512 9 20.34 22.5 3.5 1.16 1.11 3 d

134 JUG7 Leptynite S. Xiamaya 0.7 15.67 77.18 0.1227 0.512458 12 20.38 23.5 3.1 1.16 1.14 3 d


136 JUG9 Phyllite Laba 0.5 7.73 39.95 0.1170 0.512238 5 20.41 27.8 23.2 1.44 1.45 2 b

137 JUG10 Metavol rock Laba 0.5 6.05 28.06 0.1303 0.512785 36 20.34 2.9 6.7 0.67 0.64 2 b

a Two-stage Nd model age preferred when f(Sm/Nd) values are greater than 20.2 or smaller than 20.6.b Sm and Nd values from ICP-MS with error of 2±6%.

Sm±Nd model age (TDM) is calculated assuming a

linear evolution of DM from 1Nd� 0 at 4.55 Ga to

110 at the present time. In most cases, a single-

stage evolution is assumed. However, for a few highly

differentiated granitoids, a two-stage model was

employed in order to minimize the effect of Sm±Nd

fractionation on model age calculation. Like in

DePaolo et al. (1991), the two-stage model assumes

that the Nd isotopic composition of the rock in ques-

tion has evolved with Sm/Nd ratio of the average

continental crust in the ®rst stage, followed by an

evolution with Sm/Nd ratio of the rock in the second

stage. The equations used for single and two-stage

TDM calculations are:

Single stage

TDM � 1=l ln {1 1 �0:51315 2 �143Nd=144Nd�S�=

�0:2137 2 �147Sm=144Nd�S�}

Two-stage

TDM � �eDM 2 eS 1 Q £ Tc £ � fS 2 fcr��=�Q £ � fDM 2 fcr��

where l � 0:00654 Ga21; S� sample, Q� 25.1

Ga21, eDM � e value for the present depleted

mantle�110, eS�measured 1Nd(0) value for

sample, Tc� age of crystallization of sample,

f � fSm=Nd � ��147Sm=144Nd�S=0:1967�2 1, fS� f

value for sample, fcr� f value for the average conti-

nental crust�20.4, fDM� f value for the present

depleted mantle� 0.086.

Other de®nitions

eNd�0� � ��143Nd=144Nd�S=0:512638 2 1�p10000

eNd�T� � eNd�0�2 Q £ f £ T

where T� time of crystallization or any assumed

time (in Ga).

The chondrite values (in ppm) used for REE

normalization are from Masuda et al. (1973) divided

by 1.2: La (0.315), Ce (0.813), Pr (0.115), Nd (0.595),

Sm (0.193), Eu (0.0722), Gd (0.259), Tb (0.05), Dy

(0.325), Ho (0.073), Er (0.213), Tm (0.0305), Yb

(0.208), Lu (0.0323).

The primitive mantle (PM) values (in ppm) used

for spidergram construction are from Sun and

McDonough (1989): Rb (0.635), Ba (6.99), Th

(0.056), U (0.021), K (249), Nb (0.713), La (0.687),

Ce (1.775), Sr (21.1), P (96), Nd (1.354), Zr (11.2),

Sm (0.444), Eu (0.168), Ti (1300), Gd (0.596), Dy

(0.737), Y (4.55), Er (0.48), Yb (0.493), Lu (0.077).

4.1. Geochemical characteristics of the basement

rocks

4.1.1. North Tarim

In the North Tarim Terrane the principal lithology is

represented by amphibolites and TTG gneisses occur-

ring in the vicinity of Qingir (Table 2). The amphibo-

lites follow the tholeiitic trend in the AFM diagram

(Fig. 2a), and fall in the continental basalt ®eld in the

TiO2±K2O±P2O5 plot (Fig. 2b). However, they may be

separated in two groups based on their TiO2, MgO, Ni±

Cr contents and REE patterns. The ®rst group (NT1, 2,

6, 7, 10, Table 2) has TiO2 ranging from 1.14 to 1.78%,

MgO from 5.1 to 6.8%, Ni about 40 ppm, Cr about

60 ppm and slightly LREE-depleted to moderately

LREE-enriched REE patterns (Fig. 3a); whereas the

second group (NT8 and NT9) has lower TiO2 (0.59,

0.63), higher MgO (9.20, 9.90), higher Ni

(<145 ppm) and Cr (<540 ppm) and LREE-enriched

patterns with distinct negative Eu anomalies (Fig. 3a).

Such differences may indicate that their basaltic proto-

liths were formed by melting of two separate sources.

Moreover, the very systematic variation of MgO with

the abundances of HREE is remarkable for both groups,

suggesting their cogenetic relationship between the

group members except NT2. An amphibolite from

southeast of Yuli has a LREE-enriched pattern

(NT44, Fig. 3a) similar to that of the second group of

Qingir amphibolites, only with the exception of posi-

tive Eu anomaly. Three spidergrams of amphibolites

(NT2, 7, 9; Fig. 4a) show negative anomalies for Nb

and Ti, and variably enriched LIL elements (K, Rb,

Ba). These characteristics appear to indicate their

protolith formation in subduction zones.

The grey gneisses from North Tarim are mainly of

TTG compositions. They have SiO2� 58±73%,

Al2O3� 13.7±17.6%, and Na2O/K2O� 3.8±1.0

(Table 2). They have the ASI (aluminium saturation

index) or A/CNK (� molecular proportion of Al2O3/

(CaO 1 Na2O 1 K2O)) ranging from 0.8 to 1.3, about


half of them less than unity (Fig. 5d). Three

trondhjemitic gneisses (NT15, 18, 19) and one

tonalitic gneiss (NT47) have low REE abundances

(LaN� ca. 25 and 40) with LREE enrichment and

positive Eu anomalies (Fig. 3b). Besides, a tonalitic

gneiss (NT42), three trondhjemitic (NT12, 13, 14) and

one granodioritic gneisses (NT17) have very high

REE (LaN� ca. 100 and 250±350), with (La/Yb)N

ratios of about 79±13 (Fig. 3b). Two granitic

gneisses (NT20 and NT21) possess SiO2 < 76%,

Al2O3 < 11%, and Na2O/K2O , 1, one of them

(NT21) has low REE abundances (LaN < 30) with

only a moderate LREE-enrichment and a weak nega-

tive Eu anomaly (Table 2; Fig. 3b). The spidergrams

of two trondhjemitic gneisses (NT12, 13) show nega-

tive anomalies of Nb, Sr, P and Ti, indicating that they

were not only formed in continental margin settings

but also subjected to signi®cant fractionation of feld-

spars, apatite and sphene or other opaques (Fig. 4a).

Two other trondhjemitic gneisses (NT18, 19) show

roughly similar spidergrams as those of gneisses

NT12 and NT13, only with much lower abundances,

and one (NT18) has no Sr and Ti anomalies. The

Paleoproterozoic gneisses (NT29, 30, 32, 33,

Table 2) from the southern part of North Tarim have

SiO2 (60±68%), high Al2O3 (17.10±18.63%),

Na2O . K2O, and low ASI (0.73±1.0) except a

trondhjemitic gneiss NT26 (ASI , 1.30; Table 2,

and Fig. 5a and d).

The overall lithological characteristics (bimodal)

and elemental abundances of the North Tarim base-

ment rocks are typical of all Archean Terranes

(Condie and Hunter, 1976), and are quite comparable

with that of the Sino±Korean Craton (Jahn and Zhang,

1984; Jahn et al., 1987). North Tarim is undoubtedly

the only Archean Terrane in northern Xinjiang.

4.1.2. Junggar

In our opinion, the Junggar Terranes are basically

ªisland arc assemblagesº, although the nature of the

Junggar basement, Precambrian microcontinent or

trapped Paleozoic oceanic crust, has been much

debated. In addition to early Paleozoic ophiolite

complexes, minor amounts of granitic gneisses of

ambiguous ages occur in East and West Junggar

Terranes. Seven gneisses and leptynites collected

from an area south of Xiamaya in East Junggar

Terrane have SiO2 from 61 to 68%, high Al2O3 (16±

20%) and high ASI (1.0±1.2, and 2±3.5; Fig. 5d).

Four analyses (JUG2, 3, 5, 8) have tonalitic to trondh-

jemitic compositions, whereas three granitic gneisses

(JUG1, 6, 7) have granitic compositions (Table 2, and

Fig.5b). The REE patterns of two tonalitic gneisses

(JUG3, 5) show moderate LREE enrichment, absence

of HREE depletion, and low (La/Yb)N ratios (ca. 3±5;

Fig. 3c). This is akin to granitic plutons formed in

young oceanic arc settings (Chappell and White,


Fig. 2. (a) AFM diagram (Irvine and Baragar, 1971) showing the

tholeiitic trend of the amphibolites from North Tarin. (b) The TiO2±

K2O±P2O5 discrimination diagram for basalts (after Pearce et al.,

1975) shows all of the amphibolites in the northern Xinjiang plot-

ting in the continental ®eld. Filled diamonds Ð North Tarim, ®lled

triangles Ð East Tianshan, open triangle Ð West Tianshan, open

square Ð Altai.

A.

Hu

eta

l./

Tecto

nophysics

328

(2000)

15

±51

38

Fig. 3. Chondrite-normalized REE patterns for (a) amphibolites from North Tarim, (b) TTG gneisses from North Tarim, (c) tonalitic gneisses from East Junggar, (d) amphibolite and

gneisses from Altai, (e) amphibolites and gneisses from West Tianshan, (f) amphibolites and gneisses from Laerdundaban area (L-d Daban), West Tianshan, (g) gneisses from

Balguntay and Xingxingxia, East Tianshan, and (h) granulites and schists from Weiya, East Tianshan.

A.

Hu

eta

l./

Tecto

nophysics

328

(2000)

15

±51

39

Fig. 3. (continued)

A.

Hu

eta

l./

Tecto

nophysics

328

(2000)

15

±51

40

Fig. 4. Primitive mantle (PM)-normalized spidergrams for (a) amphibolites and TTG gneisses from Qingir, North Tarim, (b) amphibolites and metagabbro from Wenquan and

Laerdundaban, West Tianshan, and tonalitic gneiss from East Junggar (c) granitic gneisses from Wenquan, east of Sayram, Tekes and Muzha'erte, West Tianshan, (d) granitic

gneisses from Balguntay and Xingxingxia, East Tianshan.

1974). Like all granitic rocks, the common negative

anomalies in Nb, (Sr), P, Zr, and Ti are also observed

in the spidergram of tonalitic gneiss (JUG5, Fig. 4c).

Additional chemical analyses of some Junggar base-

ment rocks are shown in Table 2. Two greywackes

(JUG9, 10) have SiO2 about 72%, Na2O/K2O� 1.6±

7.2 and SiO2/Al2O3� 5.5 and 4.5. This may indicate

their formation in active continental margin based on

SiO2 vs K2O/Na2O discriminant diagram (after Roser

and Korsch, 1986).

4.1.3. Altai

The Altai composite terrane consists of

Precambrian gneiss complex, Paleozoic strata and

intrusive granites. The Precambrian gneiss

complex is composed of granitic gneiss, metagabbro,


Fig. 5. Classi®cation of felsic rocks based on the normative An±Ab±Or composition (after O'connor, 1965). (a) for North Tarim, open

diamonds-Archean grey gneisses, open circles - Proterozoic gneisses and granites, and ®lled circles Ð granitic gneisses. (b) for Altai and

Junggar, open squares represent gneisses from Altai, and ®lled squares from Junggar. (c) for Tianshan, open triangles Ð West Tianshan, ®lled

triangles Ð Xingxingxia Group, ®lled circles Ð Balguntay, in East Tianshan. (d) A/NK±A/CNK diagram for granitic gneisses of northern

Xinjiang. A/NK� Al2O3/(Na2O 1 K2O), A/CNK�Al2O3/(CaO 1 Na2O 1 K2O) in molecule numbers. A/CNK is equivalent to ASI

(aluminum saturation index). Open diamonds denote North Tarim, other symbols are the same as for (b) and (c).

amphibolite, biotite±quartz±schist, marble lens and

interlayered metamorphic tuff. The Paleozoic strata

are represented by metamorphosed ¯ysch deposits,

including mica schists, marbles, metagreywackes

and phyllites (Dong et al., 1986; BGMRX, 1993).

The results of chemical analyses for various rock

types are presented in Table 2. Nine gneisses from

the Altai Terrane show peraluminous with ASI . 1

(1.1±2; Fig. 5d) of which four are granodioritic

gneisses (ALT4, 7, 10, 14) with SiO2� 65±75%

and Na2O . K2O; ®ve are quartz monzonitic gneisses

(ALT1, 2, 3, 5, 12) with SiO2� 61±70%, and

Na2O , K2O (Fig. 5b). One amphibolite (ALT23)

and two quartz monzonitic gneisses (ALT1, 2) were

analysed for trace elements. The amphibolite shows a

rather high TiO2 (ca. 2.4%) and high REE abundances

with LaN� 112 and YbN� 24 (Fig. 3d). By contrast,

the two quartz monzonitic gneisses have lower but

more fractionated REE patterns. Similar REE distri-

butions have been found in biotite-bearing granitic

gneisses and their enclaves from the Altai Terrane

(Zhao et al., 1993).

4.1.4. Tianshan

The Precambrian basement rocks of the Tianshan

Terranes are characterized by amphibolites, banded

and augen gneisses with some migmatites, schists,

quartzites and marbles. Some of these rocks have

undergone granulite facies metamorphism, most

notably for those occurring near Weiya in East

Tianshan (Chen et al., 1998).

Basement rocks (48 samples) were analysed and the

data are given in Table 3. Seven amphibolites (TS5,

12, 15 from Wenquan; TS22, 29, 30, 31 from Laer-

dundaban in the south of Du±Ku Highway, north of

Bayanbulak) and two metagabbros (TS32, 33 from

Laerdundaban) in the West Tianshan Terrane have

moderately fractionated REE patterns with LaN from

40 to 150 and YbN 6±30 (Figs. 3e and 3f). All of them

show different degrees of negative Eu anomalies in

REE patterns and negative Nb and Ti anomalies in

spidergrams (Fig. 4b). These are typical features of

island arc basaltic volcanic rocks (Gill, 1981). In a

TiO2±K2O±P2O5 diagram these amphibolites plot in

the continental basalt ®eld (Fig. 2b). A cumulate

origin of a metagabbro (TS32) may be inferred from

its relatively low REE abundances and high Ni

(155 ppm) and Cr (397 ppm) contents.

The gneisses of the West Tianshan Terrane show a

large compositional range from TTG to granites (Fig.

5c). Their SiO2 contents vary from 61 to 77%, Al2O3

from 12 to 19%, CaO 1.4±4.7%, and MgO 0.3±2.3%.

All of them are peraluminous (Fig. 5d). Two tonalitic

gneisses (TS4, TS20) and a granodioritic gneiss

(TS21) from Wenquan, Tekes and Muzha'erte,

respectivly, have REE patterns almost identical to

those of amphibolites (TS5 and TS15; Fig. 3e). The

remarkable similarity of REE abundances between

tonalitic gneiss (TS24), trondhjemitic gneiss (TS26),

amphibolites (TS22, 29, 30, 31), and metagabbro

(TS33) is also observed in Laerdundaban (L±d

Daban, Fig. 3f). However, such coincidence cannot

be used to suggest a direct genetic relationship.

Augen gneiss (TS1) from Wenquan has the highest

REE pattern with distinct negative Eu anomaly (Fig.

3e). A high degree of fractional crystallization is

likely to have occurred in the generation of the

augen gneiss (granitic gneiss in Fig. 5c). Granitic

gneisses (TS17, 18, 19) from east of Lake Sayram

have similar REE as the augen gneiss of Wenquan,

only with lower abundances (Fig. 3e). All gneisses

(tonalitic, granodioritic and granitic gneisses) from

West Tianshan show similarity in their spidergrams

(Fig. 4c).

In the middle section of Tianshan near Balguntay

(Fig. 1), a dioritic and a granitic gneiss were analysed

(Fig. 5c and d). The dioritic gneiss (TS40) has

K2O . Na2O and ASI� 1.1, showing common REE

pattern with LREE enrichment and negative Eu

anomaly. However, a high-K granite (TS39,

SiO2� 71%, K2O� 5.7%, Na2O� 2.9%) shows a

highly fractionated REE with unusual HREE

depletion (LaN� 150, LuN� 2; Fig. 3g). More extra-

ordinarily, it shows a high Cr (141 ppm), Ni (76 ppm),

Th (25 ppm) and very high W (94 ppm) (Table 3). We

have not reached an appropriate interpretation for the

unusual chemical characteristics. The two rocks show

the common spidergrams for granitic rocks, with

LILE enrichment and distinct negative anomalies in

Nb, (Sr), P, Zr, and Ti (Fig. 4d).

In East Tianshan Terrane, the basement rocks of the

Xingxingxia Group occurring in the easternmost part

of Tianshan are composed of granodioritic (TS62 and

TS63) and granitic gneisses (TS60, 61, 64, 65, Table3

and Fig. 5c). The granodioritic gneisses have

ASI . 1, Al2O3� 15±16%, and Na2O/K2O < 1,


whereas granitic gneisses have Na2O/K2O� 0.3±0.5

and moderate Al2O3� 14% and ASI of 0.9±1.2 (Fig.

5d). All of them display the usual REE patterns for

granitoids. However, the granitic gneisses (TS60, 61,

64, 65) show nearly parallel REE patterns; one of

which has very high abundances with LaN ca. 600

(Fig. 3g).

The basement rocks in the Weiya area, East Tian-

shan, have been metamorphosed to the granulite

facies (Chen et al., 1998). Two of them (TS49 and

TS53) have low SiO2 (ca. 43%), moderate Al2O3

(ca. 14%) and very high CaO (ca. 29%; Table 2).

Their REE patterns are identical and show LREE

enrichment, negative Eu anomalies, and ¯at HREE

(Fig. 3h). The protolith of the rocks is most likely of

sedimentary calc-silicate origin. Through granulite

facies metamorphism, carbonate and silicate minerals

reacted to form diopside and CO2 was largely lost.

The present mineral assemblage is composed of

diopside and garnet with minor plagioclase (Chen et

al., 1998). A similar kind of metamorphic rocks from

Madagascar has recently been studied in detail by

Boulvais et al. (1998).

Two more granulites from Weiya have gabbroic

(TS58) and high-Mg andesitic (TS57) compositions

(Table 3). The gabbroic granulite has a slightly

enriched LREE and a positive Eu anomaly, while

the high-Mg andesitic granulite shows more enriched

LREE (LaN� ca. 140), ¯at HREE (ca. 10 £chondrites) and negative Eu anomaly (Fig. 3h). The

protoliths of the two granulites are most likely basic

rocks formed in an arc tectonic setting.

Two schist samples (TS55 and TS56) have variable

SiO2 (44 and 57%), MgO (8.7 and 4.1%), but charac-

terized by very high Al2O3 (29 and 22%, Table 3).

They have REE patterns typical of common shales

(Taylor and McLennan, 1985) but more distinct

negative Eu anomalies (Fig. 3h). The two schists are

most likely derived from terrigenous sediments in a

subduction zone.

In summary, the geochemical analyses allow us to

reassess the nature of basement rocks in northern

Xinjiang. The North Tarim Terrane has a bimodal

lithology (amphibolites and TTG gneisses) and

geochemical characteristics that are typical of

Archean terranes. This terrane is undoubtedly an

ancient continental fragment of the Tarim craton.

The geochemical characteristics of tonalitic,

trondhjemitic and granitic gneisses from East Junggar

Terrane (and probably West Junggar as well) and

graywackes from west Junggar appear to support the

idea of oceanic island arc assemblage, hence denying

the concept of an old microcontinent. With no excep-

tion, all amphibolites from composite terranes (Altai

and Tianshan) are characterized by overall frac-

tionated REE with LREE-enrichment. This indicates

that either their basaltic protoliths were derived from

enriched mantle sources or they have been subjected

to signi®cant crustal contamination during their petro-

genesis, hence suggesting the presence of ancient

continental crust in the composite terranes.

4.2. Isotopic compositions of the basement rocks

New analyses (80) and published Sm±Nd isotopic

data (80) are used to characterize the basement rocks

of the individual terranes, to constrain the accretion

history of northern Xinjiang and to assess the role of

basement rocks in the genesis of Paleozoic intrusive

granites.

4.2.1. Sm±Nd model ages

Fig. 6 is a diagram of 1Nd�0� vs fSm/Nd showing all

the analyses for the basement rocks of northern

Xinjiang. This is a modi®ed version of the conven-

tional isochron diagram. The data are shown with

three reference isochrons of 1±3 Ga using a DM

composition of 1Nd(0)�110 and fSm/Nd�10.086.

Both 1Nd(0) and fSm/Nd values vary widely with

1Nd(0) from 110 to 240 and fSm/Nd from 10.5 to

20.6. In general, samples of basic lithologies

(amphibolites, metagabbros, metaultrabasic rocks)

have higher fSm/Nd and 1Nd(0) values than granitic

gneisses. Individual terranes have their own charac-

teristic isotopic ®elds. The basement rocks of the

North Tarim Terrane fall between 2 and 3 Ga refer-

ence isochrons and many of them sit around the 3 Ga

isochron regardless of their lithologies (Fig. 6). Ten

amphibolites from Qingir formed a Sm±Nd isochron

age of 3263 ^ 129 Ma with 1Nd(T )�13.2 ^ 0.7 (Hu

and Rogers, 1992). These data are shown to fall on a

3.2 Ga model isochron (Fig. 6). In comparison with

the rocks from other parts of northern Xinjiang, the

granitic gneisses from North Tarim have the lowest

1Nd(0) (220 to 240) and highest TDM (about 3 Ga).

Undoubtedly, the oldest continental crust in northern


Xinjiang is preserved in the region of Qingir of the

North Tarim Terrane. Elsewhere in the same terrane

(Korla, Kuokesu, southeast of Xingdi and southeast of

Yuli) amphibolites and granitic gneisses possess late

Archean to early Proterozoic TDM ages (Table 4;

Fig. 6).

The Tianshan basement rocks have 1Nd(0) values

ranging from 24 to 221, and fSm/Nd from 20.3 to

20.6. The data are roughly con®ned between the

reference isochrons of 1 and 2 Ga, and most of them

are around 2 Ga, regardless of the complex lithologies

including amphibolites, ortho- and para-gneisses, and

a variety of granulites and schists (Fig. 6). The range

of TDM is quite consistent with the available radio-

metric age data. As reviewed earlier, the basement

rocks from Xingxingxia of the East Tianshan Terrane

have radiometric ages of 1.8, 1.4 and 1.2 Ga, respec-

tively (Hu et al., 1986; 1995, 1997); whereas those

from Wenquan, south Muzha'erte and Laerdondaban

of the West Tianshan Terrane have younger ages of

0.9±0.7 Ga (Chen, 1999; Chen et al., 1999; 2000a,b).

The Altai Terrane is composed of Proterozoic crus-

tal fragments and Paleozoic continental margin rocks.

Their Sm±Nd isotopic compositions can be distin-

guished from those of the Tianshan Terrane. The fSm/

Nd values of Altai rocks range from 0 to 20.5, and

their 1Nd(0) from 16 to 221 (Table 4). Their model

ages (TDM) show a large variation from 2.6 to 0.9 Ga

(Table 4; Fig. 6). The two samples with very low

1Nd(0) are a garnet±biotite gneiss of the Kemuqi

Group from Fuyun (221) and a phyllite from Hanasi

(217; Zhao et al., 1993).

The basement rocks of the Junggar Terrane have

the youngest model ages about 1 Ga and the highest

1Nd(0) from 110 to 24, with only one exception of

JUN9, which has eNd�0� � 27:8 and TDM � 1:4 Ga

(Table 4; Fig. 6). The analysed samples include

granitic gneisses, leptynites, phyllite, metavolcanic

tuff, schists (Table 4) and ophiolites (Zhang et al.,

1992). They were formed in late Precambrian to

early Paleozoic arc or continental margin environ-

ments with little contribution from much older conti-

nental crust. Many basic rocks (ophiolites) have

isotope compositions about the DM reference point

(Fig. 6), hence the calculated model ages are not

meaningful.

We re-examine the range of model ages (TDM) using

another representation as shown in Fig. 7. The 160

basement rock data are plotted in a diagram of147Sm/144Nd vs TDM. Model ages calculated using147Sm/144Nd higher than 0.16 are not meaningful,

hence they are excluded from the following discus-

sion. Most basic and ultrabasic rocks (shown in black

symbols) would belong to this category. On the other


Fig. 6. 1Nd(0) vs fSm/Nd diagram for the basement rocks from northern Xinjiang. DM model isochrons for 1, 2 and 3 Ga are shown for reference.

Black symbols are for amphibolites and basic rocks; open symbols for felsic gneisses.

hand, very low 147Sm/144Nd ratios (,0.08) would

suggest that the rocks have been strongly fractionated,

hence a two-stage model was used to calculate their

model ages (DePaolo et al., 1991). The rest of the data

points with 147Sm/144Nd # 0.16 in Fig. 7 are mainly

composed of granitic gneisses (open symbols). A

scrutiny of the data distribution allows de®nition of

the ranges of TDM as shown by the bar symbols. North

Tarim has a range from 3.3 to 2.2 Ga, but it may also

be separated in three age groups (3.0, 2.5±2.6 and

2.2±2.3 Ga), which is consistent with the results of

radiometric dating (Table 1).

The majority of the Tianshan samples (22 out of 32)

have TDM in the range of 1.7±2.1 Ga (Table 4; Fig. 7).

The rest of ten samples have model ages ,1.7 Ga,

among which four samples of young model ages

(,1.4 Ga) are two biotite±plagioclase gneisses

(TS22, 24) of the Nalati Group collected from the

Laerdundaban area in the fault zone, in West

Tianshan, and two biotite gneisses (TS44, 46) from

Gangou near the Kumishi Fault (south of Toksun) in

East Tianshan. Despite of strong late Paleozoic

magmatism and tectonic activities, the basement

rocks have preserved model ages of 1.7±2.1 Ga.

This period may indicate a signi®cant crust-forming

event as recorded in North America (DePaolo, 1981;

Nelson and DePaolo, 1985; Bennett and DePaolo,

1987; Chauvel et al., 1987; Patchett and Arndt,

1986; Patchett and Ruiz, 1989; Samson and Patchett,

1991a,b); South Greenland (Patchett and Bridgwater,

1984), the Svecofennian of Sweden (Patchett and

Todt, 1987), the Svecokarelia of Finland (Patchett

and Kouvo, 1986), and West Africa (Abouchami

and Boher, 1990; Boher et al., 1992).

The basement rocks and overlying Palaeozoic phyl-

lites from the Altai Terrane show Sm±Nd model ages

ranging from 1.5 to 0.9 Ga, except two samples older

than 1.8 Ga (Fig. 7). A garnet±biotite gneiss (ALT1)

of Fuyun has an imprecise zircon U±Pb upper inter-

cept age of 2349 ^ 226 Ma and a Sm±Nd model age

of 2.6 Ga (Hu et al., 1995; Table 4). This is most

exceptional among the Altai gneisses. Moreover,

Zhao et al. (1993) reported a model age of ca.

2.4 Ga for a lower Palaeozoic phyllite from Hanasi.

The nature of the basement rocks in Junggar is still

open to question. The rock types that have been

hypothesized to represent the basement are: (1) a

suite of granitic gneisses, metavolcanic rocks with

gneissic texture and schists from an area about

40 km south of Xiamaya in East Junggar; and (2)

phyllites and metavolcanic rocks of the Laba Group

in West Junggar. Most gneisses and schists of

Xiamaya have Sm±Nd model ages ranging from 1.0

to 1.2 Ga. An older model age of 1.4 Ga was obtained


Fig. 7. 147Sm/144Nd vs TDM diagram for the basement rocks from northern Xinjiang. Model ages for rocks with 147Sm/144Nd ratios close to the

DM value (0.21 ^ 0.05) are not meaningful and aberrant ages (, 0 or $4 Ga) may result. Only TDM for rocks with 147Sm/144Nd ratios #0.16

are taken into consideration in the present discussion. A younging shift of model age ranges from Tarim (continental segment) to Tianshan/

Altai (composite terranes) to Junggar (island arc assemblages) is shown by thick bars.

for phyllite (JUG10, Table 4) of the Laba Group in

West Junggar. Otherwise, Sm±Nd model ages for

other rock types, including metavolcanic rocks from

the Laba Group, metavolcanic rocks and schists from

Xiamaya are con®ned in the range of 0.7±0.8 Ga.

Note that all the gneisses, leptynites and phyllites

with TDM of 1.0±1.4 Ga occur in the southern margin

of the Junggar Basin. It is not impossible that these

rocks may belong to the Tianshan Terrane.

In conclusion, based on the Sm±Nd model ages for

the basement rocks of northern Xinjiang, the funda-

mental terrane boundaries between North Tarim,

Tianshan, Junggar and Altai are established. We

suggest the Eerqisi Fault to be the boundary between

the Altai and Junggar Terranes; the Tianshan Main Ð

Shaquanzi Fault separates Junggar from Tianshan;

and the Qingir Fault (in the northern margin of

Tarim basin) separates North Tarim from Tianshan

(Fig. 1). This scheme does not coincide with previous

tectonic division in Xinjiang (Xiao et al., 1992; Cheng

et al., 1986; etc.). The four principal terranes have

their own characteristic model age ranges: 3.3±

2.2 Ga for North Tarim; 2.1±1.7 Ga for Tianshan;

1.5±1.3 Ga for Altai and 1.2±1.0 Ga for Junggar.

4.2.2. Nd isotopic compositions

Fig. 8 summarizes the initial Nd isotopic composi-

tions, expressed as 1Nd(T ), of the basement rocks as a

function of their intrusive or primary ages. Amphibo-

lites of the North Tarim Terrane have positive 1Nd(T )

values, which are consistent with the mantle deriva-

tion of their basaltic protoliths. As for the granitic

gneisses, their 1Nd(T ) values vary from positive to

negative and the data seem to follow the trend of

continental or granitoid isotopic evolution (Fig. 8).

It is probable that the protoliths of younger granitic

gneisses were derived from recycling of older

gneisses. Nevertheless, the positive values of the

late Archean and Paleoproterozoic granitic gneisses

indicate their origin by melting of short±lived

mantle-derived basalts and are consistent with their

TTG nature.

The basement rocks of Tianshan have an isotopic

®eld distinguished from those of North Tarim (Fig. 8).


Fig. 8. 1Nd(T) vs primary age plot of the basement rocks from northern Xinjiang. Nd isotope evolution trends for the DM and the continental or

average granitoids are shown by thick half-tone lines. Basic rocks (amphibolites and metagabbros) are denoted by solid symbols, and granitic

gneisses by open symbols. The isotope ®elds for the four terranes are easily distinguished. The granitoid or continental ªslopeº is de®ned with

the parameter fSm/Nd�20.4. Data of the West Junggar ophiolites from Zhang and Huang (1992).

The Mesoproterozoic amphibolites (1.7±1.8 Ga) of

Tianshan have 1Nd(T ) values close to the DM line,

and the granitic gneisses have slightly positive to

highly negative 1Nd(T ) values (1 to 210). The trend

appears to follow the average continental or granitoid

isotopic evolution. Together with model ages (TDM) of

1.7±2.1 Ga, a signi®cant crust-forming event at ca.

1.8 Ga may be implied. The isotopic characteristics

suggest that the granitic gneisses were likely derived

by remelting of Paleo- to Mesoproterozoic protoliths.

As for the Junggar Terrane all the ophiolites and

basement rocks, except a phyllite from Laba, are char-

acterized by positive 1Nd(T ) values. This indicates its

juvenile nature and argues against the idea that the

Junggar Basin is underlain by Precambrian (or Pre-

Sinian) rocks as advocated by some geologists (Ren et

al., 1980; Wang, 1986; Watson et al., 1987; Zhang et

al., 1984; Wu, 1987).

The Altai Terrane is a composite terrane consisting

of Proterozoic basement rocks and Phanerozoic strata,

volcanics and intrusive granites. However, all the

Altai basement rocks have positive 1Nd(T ) values

regardless of their rock types (Fig. 8; Tables 1 and

4). This is unique, and is the principal difference

from the other composite terrane Ð Tianshan. Note

also that the Altai samples are distributed in a wider

®eld overlapping the other three terranes (Fig. 8).

Consequently, the Precambrian basement rocks of

the Altai Terrane are not only ªjuvenileº but also

highly variable in age, with preservation of a few

Paleoproterozoic continental fragments.

5. Conclusions

The geochemical and Nd isotopic characterization

of the basement rocks for the major tectonic terranes

in northern Xinjiang has led the following conclu-

sions:

1. The North Tarim Terrane is composed of Archean

bimodal suite (TTG gneisses and amphibolites)

and Proterozoic granitic gneisses. They have

model ages (TDM) ranging from 3.2 to 2.2 Ga,

based on single- or two-stage model calculation.

The initial Nd isotopic compositions indicate that

the Archean and Paleoproterozoic gneisses were

formed by remelting of short-lived mantle-derived

rocks, whereas the younger (Neoproterozoic)

gneisses could have been produced by recycling

of older gneisses. Their overall isotopic ®eld is

distinguished from the other terranes. This terrane

forms the northern part of the Tarim craton, and is

tectonically separated from the CAOB or the Altaid

Tectonic Collage.

2. The other terranes in northern Xinjiang (Altai,

Junggar, and Tianshan) belong to the CAOB. The

presence of Proterozoic basement rocks in the

composite terranes (Altai and Tianshan) demon-

strates that the CAOB was probably formed by

accretion of ªyoungº subduction complexes with

ªolderº entrained microcontinental blocks.

Tianshan amphibolites possess REE patterns with

uniform enrichment in LREE and negative Nb

anomalies. This is indicative of their formation in

arc tectonic setting. Sm±Nd model ages of

Tianshan basement rocks fall in the range of 1.2±

2.2 Ga, but most from 1.7 to 2.1 Ga. In terms of

initial Nd isotopic compositions, Tianshan base-

ment rocks can be distinguished from those of

North Tarim, and Tianshan gneisses also follow

the continental trend with slightly positive to

highly negative 1Nd(T ) values; the latter were

probably derived by remelting of Paleo- to Meso-

proterozoic protoliths.

3. The amphibolites and gneisses of the Altai Terrane

have TDM in two apparently discrete groups at 0.9±

1.5 Ga and 2.4±2.6 Ga. The model ages are consis-

tent with available Sm±Nd isochron and zircon

U±Pb ages (Hu et al., 1995 and Hu et al.,

1995,unpublished). The Altai gneisses are unique

in having only positive 1Nd(T ) values regardless of

their rock types, which indicate their relative

ªjuvenileº characters. Besides, they are highly

variable in age, with preservation of a few

Paleoproterozoic continental fragments.

4. The granitic gneisses and metasedimentary rocks

(schists and phyllites) from the East and West

Junggar Terranes have very young TDM ages of

0.7±1.4 Ga. Presence of a minor Precambrian crus-

tal component is possible as inferred from the

model ages, but no data have shown Precambrian

ages for the Junggar basement anywhere. The

present data thus con®rm the nature of young

island arc assemblages for the East and West

Junggar Terranes, and the Junggar Basin itself


could well be a trapped Paleozoic oceanic crust as

proposed by previous authors (Li et al., 1982;

Jiang, 1984; HsuÈ, 1989; Feng et al., 1989).

5. Together with some reliable radiometric ages for

the basement rocks, the present isotopic and TDM

age data indicate that ancient continental crust or

microcontinental blocks constitute a signi®cant

proportion at least in northern Xinjiang, and

probably in the entire CAOB.

Acknowledgements

The senior author likes to thank Dr G. Rogers for

his assistance and encouragement during her visit

(1989±1990) in East Kilbride, UK, and Prof Z.G.

Wang for his guidance in ®eld study and sample

collection in West Junggar. She is also grateful to

the laboratory of GeÂosciences Rennes for a warm

reception during a three-month visit in Fall 1997 for

the preparation of this article. The following labora-

tories have contributed to the acquisition of the

isotope data reported herein: Guangzhou Institute of

Geochemistry (CAS), Institute of Geology, Beijing

(CAS), Yichang Institute of Geology and Mineral

Resources (CAGS), and Centre of Geosciences

(CAS). Chemical analyses were performed in

Guiyang Institute of Geochemistry (CAS) and

Guangzhou Institute of Geochemistry (CAS). Prof

Randy Van Schmus and Prof Scott Samson made

critical reviews and provided useful suggestions for

improving the ®nal draft. This research has been

supported by Chinese National Key Project Ð

Xinjiang ª305º during 1987±2000 (Nos. 75-56-01-

27, 85-902-06-02, 96-915-07-05A) and National

Science Foundation of China (No. 49633250; for

1997±2000). We thank Q.X. Li, S.R. Hu, G.L. Wu,

Prof S.K. Fan and Prof B.Q. Zhu for their cooperation

and help in the realisation of this project. B-m.J

acknowledges the support of French research grants

INSU-DTT (1997) and IT (1998). This is INSU

contribution No. 231. This work also represents a

contribution to IGCP-420 (Continental growth in the

Phanerozoic).

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