Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue3-4.2000:

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Precambrian Research 102 (2000) 279 – 301

The roots of the Dabieshan ultrahigh-pressure metamorphic

terrane: constraints from geochemistry and Nd – Sr isotope

systematics

Changqian Ma

_{* , Carl Ehlers}

_{, Changhai Xu}

_{, Zhichang Li}

_,

Kunguang Yang

a_{Faculty of Earth Sciences}_,_{China Uni}₆_{ersity of Geosciences}_,_Wuhan_430074,_{PR China} b_{Department of Geology and Mineralogy}_,_A_,_{bo Akademi Uni}₆_ersity_,_A_,_bo_20500,_Finland

c_{Yichang Institute of Geology and Mineral Resources}_,_Yichang_443003,_{PR China} Received 13 January 1999; accepted 7 March 2000

Abstract

The Dabieshan area between the Sino – Korean and Yangtze cratons in east-central China has become the focus of much recent attention because of the discovery of abundant coesite and rare micro-diamond inclusions in both eclogites and their enclosing country rocks. The Dabieshan metamorphic complex, previously regarded as Archean continental basement of the Yangtze craton, is mainly made up of Precambrian felsic orthogneiss, amphibolite, and migmatitic gneiss with minor eclogite, granulite, ultramafic rock and marble. Our geochemical analyses and Nd – Sr isotope data show that most Dabieshan orthogneisses are distinctly different from the nearby Kongling gneisses of the

Yangtze basement, which are Archean high-Al TTG rocks with an average Nd model age of 3.390.2 Ga with

volcanic arc granitic affinity. The protoliths of the Dabieshan orthogneisses are diverse, and three types of rocks are distinguished: (1) the majority of the felsic gneisses in the eclogite units display geochemical signatures of post-Archean granites and may have resulted from Neoproterozoic magmatism in a rift environment; (2) some of the felsic gneisses in the eclogite units have an affinity with Kongling gneiss, and were presumably derived from the Yangtze basement by tectonic extrusion during Mesozoic exhumation of the ultrahigh-pressure (UHP) metamorphic rocks; and (3) the felsic gneisses of the dome region show geochemical signatures of Archean granitoids and Nd model ages between 3.1 and 1.0 Ga, attributed to mixing between Neoproterozoic mantle-derived material and the Archean Kongling gneisses. Numerical modeling shows that mixing between mantle-derived melts and Kongling gneiss can account for the Nd – Sr isotopic variation of Mesozoic mafic monzodiorites in the UHP eclogite unit, implying that the Kongling complex was extended beneath the Dabieshan terrane possibly during early Mesozoic continental collision. We suggest that the dome region was originally part of the Yangtze craton, and was separated from it by Neoproterozoic rifting. The orogen was later significantly modified, especially by Jurassic – Cretaceous migmatization and magmatism. © 2000 Elsevier Science B.V. All rights reserved.

www.elsevier.com/locate/precamres

* Corresponding author. Tel.: +86-27-87692999; fax: +86-27-87801763.

E-mail address:wbh@wri.com.cn (C. Ma)

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C.Ma et al./Precambrian Research102 (2000) 279 – 301 280

Keywords:Neodymium and strontium isotopes; Archean crust; Yangtze craton; Dabieshan; China

1. Introduction

The Qinling-Dabieshan orogen of east-central China (Fig. 1) is the suture zone formed by the Triassic collision of the Yangtze and Sino – Ko-rean cratons (e.g. Li et al., 1993; Ames et al., 1996). It is truncated at its eastern end by the sinistral Tan – Lu fault, one of the world’s largest continental strike-slip faults (Xu et al., 1987), which offsets the Dabieshan terrane by a mini-mum of 530 km northward to the Sulu area (Okay et al., 1989). Of particular interest is the extensive distribution of coesite and its pseudo-morphs as inclusions in garnet, omphacite, kyan-ite, epidote, dolomkyan-ite, zoiskyan-ite, and zircon in eclogites, ultramafic rocks, felsic gneisses, carbon-ate rocks and jadeite quartzite (e.g. Okay et al., 1989; Wang et al., 1989; Yang and Smith, 1989;

Hirajima et al., 1993; Wang and Liou, 1993; Zhang et al., 1996; Carswell et al., 1997; Liou et al., 1997). This ultrahigh-pressure (UHP) meta-morphism has affected the three easternmost ter-ranes of the collision belt, the Xinxian terrane (also called Hong’an), Dabieshan terrane and Sulu terrane (Fig. 1), and diamond inclusions have been discovered in eclogite of the Dabieshan terrane (Xu et al., 1992). The occurrence of co-esite- and diamond-bearing UHP metamorphic rocks indicates that the crustal rocks have been

buried to great depths of \100 km, significantly

exceeding the 75 km of the present mountain root in the Himalayas. Thus these UHP rocks and their enclosing country rocks may provide very important clues for unravelling the deep thermal and compositional structure and the mechanics of mountain belts.

Fig. 1. Simplified tectonic map of the Qinling – Dabieshan – Sulu collisional belt showing the major structural units and distribution of UHP and HP metamorphic zones. Inset shows the location of the collisional belt in relation to the major structure in China.

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C.Ma et al./Precambrian Research102 (2000) 279 – 301 281 In contrast to the extensively studied UHP

eclogites, the Precambrian felsic orthogneisses in this orogen have received remarkably little atten-tion. A key issue, still unresolved, is the age and nature of the protoliths of the Dabieshan or-thogneiss and other country rocks enclosing the UHP eclogites. Many workers (Wang et al., 1990; Okay and Sengo¨r, 1992) have suggested that the

Dabieshan metamorphic complex represents

deeply subducted Archean continental basement of the Yangtze craton. However, recent Sm – Nd isotopic data have yielded Nd model ages ranging from Archean to Neoproterozoic for Precambrian felsic orthogneisses (e.g. Ames et al., 1996; Chav-agnac and Jahn, 1996; Liou et al., 1997; Chen and Jahn, 1998) and U – Pb and Pb – Pb zircon data on the same orthogneisses have given a wide spec-trum of ages, ranging from late Archean (Chen et al., 1996), Paleoproterozoic (Jian et al., 1999), Neoproterozoic (Ames et al., 1996; Rowley et al., 1997; Xue et al., 1997) to Cretaceous (Xue et al., 1997; Hacker et al., 1998). This indicates that the orogenic history of the Dabieshan is complex.

In this study we present new geochemical and

Nd – Sr isotope data for Precambrian felsic

gneisses, some amphibolites and eclogites, and Mesozoic intrusive rocks from the Dabieshan ter-rane. Our objective is (1) to delineate crustal age provinces within the Dabieshan terrane and North Yangtze craton, based on the new geo-chemical and Nd – Sr isotopic data and those al-ready published in the literature; (2) to identify

ancient crustal components in contaminated

mantle-derived rocks by recognizing mixing rela-tionships with the aid of Nd – Sr isotope systemat-ics; and (3) to provide evidence for involvement of the Archean Kongling gneisses of the Yangtze craton in continental subduction and the exhuma-tion of the UHP and high-pressure (HP) rocks.

2. Geologic setting

2.1. The Dabieshan terrane

The Dabieshan terrane, a fault-bounded Pre-cambrian metamorphic complex, is bounded to the south by a foreland fold-thrust belt along the

middle and lower reaches of the Yangtze River, and to the north in Beihuaiyang by a greenschist facies fold-thrust belt called the Fuziling Group (Fig. 1). It can be subdivided, from north to south, going upwards structurally, into units of different lithology, metamorphic facies and tec-tonic style (Wang et al., 1998), the Dabieshan

orthogneiss domes, the UHP/HP eclogite-bearing

units, and a blueschist-bearing fold-thrust belt (Fig. 2).

(1) The Dabieshan orthogneiss domes (hereafter referred as ‘the dome region’), including the Luo-tian and Yuexi domes, are the footwall unit of the Dabieshan terrane (Wang et al., 1998). Both domes form the western and northeastern parts of the Dabieshan complex and have similar tectonic and lithological features. The term ‘Dabieshan complex’ (DBC) has been used to refer to various metamorphic rocks in both the dome region and

the UHP/HP eclogite-bearing units. The DBC in

the dome region is composed of amphibolite- and granulite – facies felsic gneiss (75% of the total area), of a supracrustal sequence (24%), and of

metabasic – ultramafic rocks (B1%) (Sang et al.,

1997). A supracrustal sequence is mainly made up of metavolcanic amphibole – biotite gneiss, amphi-bolite and a small proportion of marble and magnetite quartzite (You et al., 1996). The gneisses underwent intense migmatization, which

mainly formed stromatic and ptygmatic

migmatites. The radial dips of foliation and the predominant NW, and, SE plunges of mineral lineations outline the structural character of both domes (Fig. 2) (Wang et al., 1998). The metamor-phic grade of the dome region decreases out-wards, from granulie – facies in the cores of the domes to upper amphibolite – facies conditions on the flanks of the domes. The core of the Luotian dome in western Dabieshan consists of tonalitic diatexites with granulite blocks and amphibolite enclaves. The intensity of migmatization decreases gradually towards the flanks of the dome where metatexites predominate (Wang et al., 1998). Very few eclogites have been found in this dome region. A few attempts have been made to date the granulites and migmatitic orthogneisses. Chen et al. (1996) obtained a U – Pb zircon upper-intercept

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C.Ma et al./Precambrian Research102 (2000) 279 – 301 282

Fig. 2. Generalized geologic map of the Dabieshan (modified from Ma et al., 1998) showing sample localities, and distribution of Nd model ages for felsic gneisses and intrusive rocks,. and ofo_Ndvalues for amphibolites and eclogites. Fault zones: (1) Tan – Lu fault; (2) Xincheng – Xishui strike-slip fault; (3) Shangcheng – Macheng fault; and (4) Mozitan strike-slip fault.

sample from Huangtuling Primary School in the core of the Luotian dome (point M9 in Fig. 2), and Jian et al. (1999) reported a Pb – Pb zircon age

of 245697 Ma for the same granulite. Recently,

the protoliths of some orthogneisses in the dome region have been taken to be Neoproterozoic and Cretaceous in age from U – Pb zircon ages of

756.690.8 Ma on an orthogneiss from northern

Dabieshan (Xue et al., 1997), and of 133.792.3

Ma and 134.092.8 Ma on orthogneisses from

northern and western Dabieshan (Xue et al., 1997; Hacker et al., 1998). However, Ma (1999) prefers to interpret the Cretaceous ages as representing the time of intense migmatization and doming.

(2) The UHP/HP eclogite-bearing units

(‘eclog-ite units’ hereafter) comprise amphibol(‘eclog-ite facies felsic gneisses with minor amphibolite, eclogite, garnet-bearing peridotite, jadeite quartzite, and

marble (Liou et al., 1997). The Dabieshan eclog-ites have a predominant assemblage of

jadeite-bearing clinopyroxene (omphacite or chloro

melanite) and garnet, and may contain glau-cophane, kyanite, orthopyroxene, coesite and dia-mond (You et al., 1996). The eclogite units can be further differentiated into two subzones with dif-ferent P – T regimes, a coesite- and diamond-free HP unit in the south, and an UHP unit containing coesite- and diamond-bearing eclogites in the north (Fig. 2) (Okay, 1993; Carswell et al., 1997; Wang et al., 1998). It has been suggested that the

coesite-bearing UHP eclogites reached peak

metamorphic conditions of 680 – 850°C and 2.6 – 3.9 GPa (Hacker et al., 1995; Carswell et al., 1997), while the HP eclogites attained metamor-phic conditions of 600 – 700°C at about 2.2 GPa (Carswell et al., 1997). The eclogite facies

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meta-C.Ma et al./Precambrian Research102 (2000) 279 – 301 283 morphism in the Dabieshan terrane has been dated

at 245 – 220 Ma by various geochronological meth-ods (e.g. Li et al., 1993; Ames et al., 1996; Chav-agnac and Jahn, 1996; Rowley et al., 1997), and these ages have been interpreted as representing the timing of north-directed underthrusting of conti-nental crust of the Yangtze craton (e.g. Li et al., 1993). The protolith ages of the Dabieshan eclog-ites are not well constrained though the possibility of their being Neoproterozoic has been discussed (Ames et al., 1996; Rowley et al., 1997; Jahn, 1998). Rowley et al. (1997) have obtained an upper

intercept age of 772.599.5 Ma on zircons from a

felsic gneiss that is a host rock of eclogites. Their result agrees well with an upper intercept age for a gneiss dated by Ames et al. (1996). Both papers interpreted the age as representing a protolith age for felsic gneisses in a rift environment along the northern margin of the Yangtze craton.

(3) The blueschist-bearing fold-thrust belt is composed of Meso – Neoproterozoic metasedimen-tary and metavolcanic rocks (‘Susong group’), lower Sinian metavolcanic rocks (‘Yaolinghe group’) and upper Sinian sedimentary rocks (You et al., 1996). The metamorphism of the Susong group attained conditions of the medium to high pressure greenschist facies. Epidote blueschists are extensively exposed in the belt, and the blueschist facies metamorphism has been dated at 230 – 195 Ma by Sm – Nd crossite-whole rock isochrons

(Yang et al., 1994) and40_Ar

/39_{Ar phengite plateau}

ages (Eide et al., 1994) on blueschists from near Hong’an.

The DBC is intruded by abundant middle Juras-sic- early Cretaceous granitic plutons, and minor late Triassic mafic monzodiorite and early Creta-ceous gabbroic rocks with shoshonitic and high-K calc-alkaline affinities (Ma et al., 1998; Jahn et al., 1999). Three groups of Mesozoic intrusive rocks have been identified (Ma et al., 1998). Group I

consists of late Triassic (210 Ma) mafic

monzo-diorites (the Liujiawa stock, Fig. 2), which could have been generated by partial melting of enriched subcontinental lithospheric mantle or by crustal assimilation of mantle-derived magma. Group II comprises middle Jurassic – early Cretaceous (160 – 120 Ma) quartz monzonites, monzogranites and syenogranites, and could have been produced by

crustal assimilation and fractional crystallization of mantle-derived magmas. Group III is represented by Cretaceous (125 – 95 Ma) granitic stocks and granitic porphyries, which could have been derived by anatexis of Dabieshan felsic gneisses and subse-quent fractional crystallization (Ma et al., 1998; Ma, 1999).

2.2. The Kongling complex

The oldest known basement of the Yangtze craton is the Kongling complex (KLC). It outcrops

in an area of about 150 km2 _{in the Three Gorges}

region of the Yangtze River, Western Hubei (Fig. 1). It is composed of gneisses, including grey gneisses with amphibolites, and supracrustal rocks (Ma et al., 1997). The grey gneisses, composed of banded orthogneisses with medium to fine-grained high-Al trondhjemite, tonalite, and granodiorite (TTG) compositions, give U – Pb zircon upper

intercept ages of 2800 – 3000 Ma (e.g. 2936998

Ma, Ames et al., 1996; 2850915 Ma, Ma et al.,

1997). Ma et al. (1997) have interpreted these ages as the time either of TTG intrusion, or metamor-phism of the TTG to grey gneisses. Amphibolites occur as foliated enclaves in the grey gneisses, and they yield a Sm – Nd whole-rock errorchron age of

32909170 Ma (Ma et al., 1997). Supracrustal

rocks form a younger unit, which has been subdi-vided into a lower khondalite series and upper amphibole schists. Zircons from an amphibolite layer in the khondalite have yielded a U – Pb upper

intercept age of 203194 Ma, which is considered

an approximate estimate of the age of the basaltic protolith (Ma et al., 1997).

3. Analytical methods

Analyses of major, trace and rare earth element compositions were made at the Analytical Institute of the Hubei Bureau of Geology and Mineral

Resources. SiO2 and H2O+ were determined by

gravimetry; TiO2and P2O5by spectrophotometry;

Al2O3, Fe2O3, FeO and CO2 by volumetry; and

MnO, MgO, CaO, Na2O and K2O by atomic

absorption spectrometry. Analyses of trace ele-ments and rare earth eleele-ments (REE) were made by ICP-AES. Analytical precision (relative

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stan-C.Ma et al./Precambrian Research102 (2000) 279 – 301 284

dard deviation) is usuallyB1% for major

ele-ments except H2O

+_, _B_{4% for REE and Y, and}

5 – 10% for trace elements.

Sr and Nd isotopic analyses were performed at the Isotope Laboratory of the Yichang Institute of Geology and Mineral Resources, by procedures described by Ma et al. (1998). Repeated

measure-ments (n=6) of the NBS987 and La Jolla Nd2O3

standards were taken throughout the analytical period and yielded the following average ratios:

87_Sr

/86_Sr

=0.7102695 (2 SD), and 143_Nd

144

Nd=0.511845910 (2 SD). 87

Rb/86

Sr and

147_Sm_/144_{Nd ratios were determined to a precision}

of90.7% and90.2%, respectively. Total

proce-dural blanks are insignificant: B1 ng for Sr, and

86 pg for Nd.

Parameters used for calculations are: (143_Nd

144

Nd)CHUR, today=0.512638, (

147

Sm/144

Nd)CHUR,

today=0.1967. DM values for Nd are (143Nd/

144_Nd)

today=0.51315, (

147_Sm

/144_Nd)

today=0.2137.

Decay constantl_{: for} 87_Rb₌_1.42_×₁₀−5_Ma−1_,

and for 147_Sm₌_6.54_×₁₀−6 _Ma−1_{. The}

nota-tions of o_Nd (0) and o_Nd (t) are defined as:

o_Nd ₍₀₎₌_[(143_Nd_/144_Nd)

meas./(143Nd/144Nd)CHUR,

today−1]×104, oNd (t)=[(143Nd/144Nd)initial./

(143_Nd

/144_Nd)

CHUR, t−1]×10

4_{. Where, meas}

measurement, (143_Nd_/144_Nd)

initialis the initial ratio

of the sample suite at the time of its formation (t)

and is calculated from the expression (143

Nd/

144_Nd)

initial=(143Nd/144Nd)meas−(147Sm/144Nd)meas

(elt₋_{1). (}143_Nd_/144_Nd)

CHUR, t is the isotope ratio

of CHUR at timetand is given by the expression:

(143_Nd_/144_Nd)

CHUR,t=0.512638−0.2137×(e

lt₋

1). Sm – Nd model ages (TDM) were calculated

using a linear isotopic ratio growth equation

(Peu-cat et al., 1989): TDM=l−1ln {1+

((143_Nd

/144_Nd)

meas.−0.51315)/((

147_Sm

/144_Nd) meas.

−0.2137)}.

4. Results and discussion

VSB4.1.Geochemical characteristics of felsic and

grey gneisses

Representative analyses of major, trace and rare earth elements for felsic and grey gneisses are

presented in Tables 1 and 2. Among them, the Kongling grey gneisses were mainly analyzed by Li Fuxi and Ma Daquan (1991, unpublished data) and Ma et al. (1997), while a few data for the Dabieshan felsic gneisses are from You et al. (1996), Sang et al. (1997) and Wu et al. (1998).

The Kongling grey gneisses are high-Al TTG rocks, according to the criteria of Barker (1979)

and Drummond and Defant (1990), with Al2O3\

15 wt.% at the 70 wt.% SiO2level, high Sr (\300

ppm), and low Rb/Sr (B0.15), K/Rb (typically

B550), Y (B15 ppm), and Nb (B10 ppm,

ex-cept in sample H44). The average concentrations

of Rb (36914ppm), Sr (594963 ppm) and Ni

(1192 ppm) are also similar to the values

pro-posed by Condie (1981) for ‘high Al2O3’ Archean

gneisses. Leat et al. (1986) have argued that the

TiO2 versus Zr plot of Pearce (1980) can

effec-tively separate peralkaline from sub-alkaline felsic volcanics, and that the low-Zr group is

subalka-line (B350 – 700 ppm Zr) (Fig. 3). The Kongling

gneisses fall into the intermediate and silicic ‘vol-canic arc’ field below line A-B on Fig. 3, and are

all depleted in TiO2 and Zr relative to ‘within

plate’ rocks. Being low in Zr content (B350

ppm), rocks from the KLC are subalkaline (Leat et al., 1986), and fall into the calc-alkaline field of Fig. 3. Their REE patterns are highly fractionated

[(La/Yb)n=48.85−94.60], and may be divided

into two subgroups with negative and positive Eu anomalies (Fig. 4). In the chrondrite normalized

La/Yb versus Yb diagram (Fig. 5A), the Kongling

grey gneisses display typical Archean features (Jahn et al., 1981; Martin, 1986). Immobile trace elements are also used to test the tectonomag-matic affinity of the grey gneisses on the discrimi-nation diagram of Pearce et al. (1984). The data confirm the orogenic affinity of their protoliths, and all the grey gneisses from the KLC fall in the field of volcanic arc granitoids (Fig. 5B).

The Dabieshan felsic gneisses display a large variation in chemical composition. Compared with high-Al trondhjemitic gneisses fron the KLC,

they are richer in Zr and TiO2 (Fig. 3), and are

not high-Al granitoids. However, their low Zr

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C . Ma et al . / Precambrian Research 102 (2000) 279 – 301 285 Table 1

Chemical compositions of the Dabieshan felsic gneisses and granulitea

Dome region Eclogite units

Z083 94095 94084 Sh2-3 TP5-1 TS23-1

SG-2 LJW-1

Sample _XXW-1 _XXW-2 _Htl-M4 109-1§

M9 M16 M16 M23 M24-1 M25 M27 M28 M29

Nos. in Fig. 2 M3 M4 M9

(2) (3) (3) (1) (4) (4)

(1) (1)

Refs. (1) (1) (1) (1)

SiO2(wt.%) 67.04 71.93 66.21 60.77 66.31 68.99 72.41 65.44 68.57 68.75 72.98 75.48

0.49 0.30 0.70 0.35 0.40 0.43

0.69 0.26

0.37 0.69

TiO2 0.82 0.49

14.75

14.66 13.48 16.14 13.37 14.97 12.88 15.65 15.89 14.96 13.47 11.71

Al2O3

2.07

1.58 0.71 0.70 0.77 1.17 1.32 1.69 0.81 1.18 1.17 0.87

Fe2O3

1.29 1.87 3.09 2.02 2.31 1.29

2.77 1.45

FeO 3.05 2.05 2.52 7.84

MnO 0.10 0.05 0.06 0.08 0.15 0.03 0.06 0.10 0.05 0.05 0.05 0.08

0.92 0.43 1.01 1.12 1.18 0.89

1.93 0.76

1.74

MgO 1.27 2.18 7.63

3.28

3.21 2.68 4.26 1.58 2.32 1.33 2.17 2.50 3.47 2.33 1.37

CaO

4.69

4.44 4.42 5.17 2.84 4.37 3.46 4.15 5.40 4.59 4.10 3.56

Na2O

4.02 4.55 4.38 2.11 2.06 1.42

2.15 3.62

K2O 1.93 1.79 1.22 2.17

0.21 0.07 0.17 0.24 0.12 0.10

P2O5 0.27 0.17 0.13 0.09 0.09 0.06

0.40 0.29 0.55 0.64 0.70 0.93

0.75 0.43

0.65 1.50

H2O+ 0.76 0.49

0.04

0.07 0.11 0.20 0.25 0.04 0.20 0.19 0.03 0.20 0.14 0.04

CO2

99.65

99.67 99.34 99.81 99.58 99.21 99.25 99.29 99.74 99.97 99.30 99.74

Total

69.9 132.2 110.4 45.4 40.6

83.0 49.7 34.9 91.7

36.1 50.9

75.4 Rb (ppm)

Sr 363 481 507 178 252 941 111 227 469 742 238 184

2780 682 1547 765 1563 534

1066 767

536 874

Ba 603 978

0.90

0.28 0.24 0.07 0.79 1.11 4.10 3.00 0.50 0.45 0.64 0.94

8.9

8.2 7.0 6.0 10.2 7.7 22.3 35.9 9.5 4.8 4.7 13.1

7.0 nd. nd. 7.9 2.7 2.1

4.7 8.2

Hf 4.1 7.4 5.2 4.9

192 243 538.2 304 137 152

Zr 126 243 156 149 151 221

7.93 41.00 63.80 14.21 7.61 15.82

33.77 25.50

12.91 13.88

Y 10.54 7.21

7.2

5.2 4.3 3.1 11.3 7.9 nd. nd. 19.6 9.1 5.5 9.1

0.20

0.57 0.27 0.78 0.21 0.29 2.40 2.60 4.50 0.59 0.71 1.65

21.8 nd. nd. 8.9 22.9 B2

55.9 115.5

Cr 15.6 26.5 115.5 1014.9

11.0 8.9 12.4 4.9 14.9 3.9 4.1

Ni 6.6 6.9 68.5 376.3 12.9

8.4 6.0 6.1 6.7 12.1 6.1

14.0 5.6

9.7 7.7

Co 12.6 44.1

16.15 9.41 11.28 26.51 58.50 54.90 66.70 97.11 55.58 36.07 31.38

La (ppm) 19.66

35.71 21.37 24.87 43.33 100.80 104.00 122.70 186.2 90.43 66.55 71.90

Ce 39.27

10.73 11.90 15.20 20.92 8.71 7.58

4.90 8.46

Pr 4.40 2.95 3.49 4.42

32.57 40.60 59.10 73.95 25.56 27.22

Nd 17.71 11.57 13.16 15.92 19.18 31.44

4.78 8.80 14.20 9.89 3.60 4.86

4.11 5.67

2.81 2.93

Sm 3.15 2.04

1.15

1.03 0.75 0.88 0.94 1.34 0.91 1.82 2.40 0.92 0.97 0.88

5.04

3.09 2.01 2.38 2.51 3.03 7.21 12.80 5.26 2.43 3.66 5.26

0.29 1.14 1.89 0.64 0.25 0.47

0.89 0.82

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Precambrian

Research

102

(2000)

279

–

301

286

Table 1 (Continued)

Dome region Eclogite units

Z083

Sample XXW-1 XXW-2 Htl-M4 109-1§ _SG-2 94095 94084 Sh2-3 TP5-1 TS23-1 LJW-1

M16 M23 M24-1 M25 M27 M28

M16 M29

Nos. in Fig. 2 M3 M4 M9 M9

(1) (1) (2) (3) (3) (1) (4) (4) (1)

(1)

Refs. (1) (1)

1.64 7.02 11.30 2.59 1.60

2.49 2.86

Dy 2.13 1.50 2.32 5.47 4.34

0.44 0.30 0.45 1.25 0.31 1.57 2.46 0.59 0.32 0.57 0.89

Ho 0.51

0.71 4.50 6.95

Er 1.08 0.76 1.32 1.48 3.67 1.57 0.74 1.74 2.44

0.11 0.78 1.03 0.29 0.11 0.29

0.59 0.38

Tm 0.14 0.11 0.23 0.27

0.60 4.72 6.29 1.89 0.59 1.94

Yb 0.71 0.68 1.24 1.61 3.32 2.17

0.10 0.77 0.98 0.27 0.09 0.32

0.54 0.34

Lu 0.11 0.12 0.20 0.25

215.51

SREE 86.28 53.84 65.05 92.69 119.95 248.32 323.42 403.54 190.93 155.10 166.32

65.89 7.86 7.17 34.72 63.66

8.25 12.56

6.15 10.53 9.77

(La/Yb)n 15.37 9.35

Eu/Eu* 1.00 1.11 1.01 1.03 0.77 1.01 0.34 0.41 0.92 0.90 0.68 0.46

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C.Ma et al./Precambrian Research102 (2000) 279 – 301 287 affinity, and may be classified further into high-K

or calc-alkaline types (Fig. 3). Their REE patterns

are relatively flat (Fig. 4). Most felsic gneisses from the dome region do not show negative Eu Table 2

Chemical compositions of the Kongling grey gneissesa

H45 H46

H34

Sample H36 H37 H44

(5) (5) (5) (5) (5)

Refs. (5)

71.44 67.84

70.82

SiO2(wt.%) 71.62 68.69 69.40

0.18 0.19 0.35 0.26 0.30

TiO2 0.58

15.42 15.19

16.95 16.41

Al2O3 15.85 14.98

0.31 0.90 0.67

Fe2O3 0.36 0.63 0.59

1.85

1.21 1.55 1.85 2.11 3.06

FeO

0.05 0.02

MnO 0.02 0.03 0.04 0.03

0.85 0.51

MgO 0.45 0.51 0.85 0.73

2.36

1.89 2.20

2.83

CaO 2.67 2.99

4.67 5.06

5.56 5.06

Na2O 5.44 5.26

1.22

K2O 1.52 1.52 2.30 2.73 1.37

0.81

0.07 0.08 0.11 0.08 0.21

P2O5

0.81 0.86

H2O+ 0.58 0.64 0.70 0.93

100.28

99.67 99.33 99.71 99.70 99.47

Total

28 53

Rb (ppm) 20 33 38

620 760 490 505 540

Sr 380

700 390

600

Ba 740 440 780

0.8

Ta 0.4 0.8 1.2 1.6 0.8

6.1

1.8 4.3 5.9 13.9 8.1

133

128 118 141 164 466

3.39

4.41 3.39

2.14

Y 1.52 2.84

20.7 4.7

Th 2.7 6.7 3.7 8.9

1.1 1.0 1.0

U 1.0 0.8 0.7

134

95 143 66 107 113

10 56

Ni 10 nd 14 9

3 3 5 2 7

33.00 140.00

47.80

La (ppm) 12.00 24.50 19.10

21.00 35.20 27.30 78.90 49.90

Ce 339.00

19.50 5.23

1.98

Pr 3.65 2.77 8.23

101.00

Nd 6.98 13.10 9.94 30.70 21.00

3.48

1.10 2.17 1.57 5.21 13.50

0.69

0.56 0.66 0.54 0.81 0.83

4.92

2.33 1.62

0.86

Gd 0.67 1.15

0.25

0.13 0.19 0.16 0.37 0.78

0.47

Dy 0.80 0.63 1.76 3.60 1.09

0.20

0.09 0.16 0.12 0.26 0.51

1.08 0.40

Er 0.20 0.33 0.23 0.51

0.17 0.07

Tm 0.03 0.05 0.04 0.09

1.00

0.40 0.34

0.23

Yb 0.17 0.31

0.05

0.03 0.05 0.04 0.08 0.17

63.53 626.05 117.32

SREE 45.40 82.32 177.44

48.85 53.06 56.12 81.57 64.82

(La/Yb)n 94.60

1.84 1.15 1.29 0.61 0.78

Eu/Eu* 0.26

a_{Data sources for Table 1 and Table 2: (1) this study; (2) Sang et al. (1997); (3) Wu et al. (1998);(4) You et al. (1996); (5) Li Fuxi} and Ma Daquan (1991, unpublished report). § Sample 109-1 is an intermediate granulite. nd, not determined. Chondrite normalizing values for REE are from Taylor and McLennan (1985).

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C.Ma et al./Precambrian Research102 (2000) 279 – 301 288

Fig. 3. Plot of Zr vs TiO2for the Dabieshan felsic gneisses and the Kongling grey gneisses. Fields of ‘volcanic arc’ and ‘within plate’ rocks from Pearce (1980). Rocks below line A-B, draw from co-ordinate TiO2=0.2%, Zr=10 ppm to TiO2=1.0%, Zr=100 ppm (Pearce, 1980), are interpreted as having inter-mediate or silicic compositions. It is suggested that Zr content ranging from 350 to 700 ppm can effectively divide intermedi-ate and silicic rocks into peralkaline and subalkaline types, and subalkaline rocks can be further divided into high-K and calc-alkaline (Leat et al., 1986).

In a plot of Nb versus Y, most of the Dabieshan felsic gneisses display volcanic arc affinity except for two gneisses of the UHP eclogite unit, which fall in the field of within-plate granites (Fig. 5B). Fig. 6 shows that chondrite-normalized REE patterns and primitive mantle-normalized incom-patible element patterns for two felsic gneiss sam-ples from the eclogite units match those of the Kongling grey gneisses well, implying that some of the felsic gneisses in the eclogite units may have been derived from the KLC perhaps during Meso-zoic exhumation of the UHP metamorphic rocks.

4.2. Rb–Sr and Sm–Nd isotopic characteristics

Twenty-four felsic gneiss, amphibolite, eclogite and other metamorphic rock samples, and twelve samples of Mesozoic intrusive rocks were col-lected from the Dabieshan terrane. Published Nd and Sr isotope data for the DBC, and for the KLC have been compiled from available literature sources. The isotopic data are given in Table 3 and Table 4, and sample locations for the Da-bieshan rocks are shown in Fig. 2.

anomalies, and they display Archean features (Fig. 5A). In contrast, the majority of felsic gneisses from the eclogite units show significant negative Eu anomalies (Fig. 4), and they straddle both Archean and post-Archean fields in Fig. 5A.

Fig. 4. Chondrite-normalized REE patterns for selected samples of Dabieshan and Kongling gneisses. Chondrite normalizing values used are from Taylor and McLennan (1985).

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C . Ma et al . / Precambrian Research 102 (2000) 279 – 301 289 Table 3

Rb–Sr and Sm–Nd isotopic data for metamorphic rocks and selected granitoid rocks from the Dabieshan

87_Rb_/86_Sr

No. of samples 87_Sr_/86_Sr₉₂s Sm (ppm) Nd (ppm) 147_Sm_/144_Nd _T

143_Nd_/144_Nd

Rock types Rb (ppm) _{Sr (ppm)} oNd(0) oNd (at 760 Sourcesb

No. in Fig. 2

Ma) (Ga) 92s

0.7092393 5.98 39.16 0.0923 0.51152598 −21.7

131.20 472.10 2.0 −11.6 (1)

Felsic gneiss 96-58

M1 0.8015

746.30 0.3408 0.7087693 7.01 52.81 0.0803 0.511821919 −15.9 1.5 −4.6 (1)

M2 96-53 Felsic gneiss 88.20

0.7084692 3.43 19.09 0.1088 0.512276912 −7.1 1.3

0.4100 1.5

XXW-1 (1)

M3 Felsic gneiss 75.59 363.66

X-1 Amphibolite 3.87 12.72 0.1838 0.512738913 2.0 2.1 3.2 (1)

M3(1)

0.7085197 4.35 27.40 0.0961

M4 X-2 Felsic gneiss 47.47 378.62 0.3610 0.51219897 −8.6 1.2 1.2 (1)

0.7077496 4.74 14.93 0.1920 0.51200091 −12.4 7.9

0.1270 −12.0

M4(1) XXW110 Amphibolite 15.38 347.87 (1)

0.7101791 9.03 34.55 0.1582 0.51238396 −5.0 2.1

M5 96-12 Amphibolite 8.64 287.50 0.0867 −1.2 (1)

0.7070591 4.70 22.48 0.1266 0.51189598 −14.5 2.2

0.2176 −7.7

M6 96-48-1 Felsic gneiss 36.00 276.90 (1)

0.8647

96-63-2 Felsic gneiss 139.40 464.60 0.71215924 7.29 40.48 0.1090 0.51161695 −19.9 2.2 −11.4 (1)

0.71298910 5.08 29.45 0.1043 0.51136497 −24.9 2.5

0.3913 −15.9

96-110 641.50 (1)

M8 Felsic gneiss 87.02

213.80

Intermediate 1.1800 0.7464293 5.19 27.95 0.1123 0.511044913 −31.1 3.1 −22.9 (1)

109-1 87.13

granulite

0.7418998 7.20 28.01 0.1555 0.51172198 −17.9 3.7

0.5815 −13.9

Felsic gneiss (1)

M9(1)a _96-M4 _57.58 _286.50

283.10

Amphibolite 0.3338 0.71243921 6.57 30.42 0.1305 0.51156999 −20.9 2.9 −14.4 (1)

96-M2 32.77

M9(2)a

xenolith

3.8 15.45 0.1489 0.51234798 −5.7

M10 d144 Felsic gneiss 1.9 −1.0 (2)

0.706594 5.96 24.21 0.1490 0.512511911 −2.5 1.5

0.1819 2.2

96-238 (1)

M11 Felsic gneiss 54.89 870.00

0.4368

96-111 Felsic gneiss 57.73 381.00 0.7072791 4.88 22.90 0.1290 0.512122912 −10.1 1.8 −3.5 (1)

M12

0.7059492 4.63 17.29 0.1620 0.51233497 −5.9

M13 96-212 Amphibolite 30.23 467.80 0.1863 2.4 −2.6 (1)

0.7063894 7.58 48.44 0.0947 0.51181099 −16.2 1.7

0.2738 −6.2

91.36 962.00 (1)

M14 LG2 Felsic gneiss

0.0590

SG-1 Felsic gneiss 17.42 849.53 0.7066195 10.04 50.38 0.1205 0.512032910 −11.8 1.8 −4.4 (1)

M15

0.7087494 4.05 19.51 0.1689 0.51233999 −5.8 2.7

M15(1) SG-2 Amphibolite 70.8 286.56 0.7120 −3.1 (1)

7.47 40.36 0.1118 0.51201197 −12.2 1.7 −4.0

d118 (2)

M16 Felsic gneiss

0.001

DB-91-34 Eclogite 0.01 47.4 0.7049492 4.74 20.08 0.143 0.512173910 −9.1 2.1 −3.9 (3)

M17

0.7088691 3.88 18.72 0.1255

M18 Bh-12 Felsic gneiss 76.57 529.60 0.4169 0.511895911 −14.5 2.2 −7.6 (1)

0.7064791 2.52 8.04 0.1894 0.512098910 −10.5 6.5

0.1473 −9.8

M18(1) Bh-9b Eclogite 6.01 117.50 (1)

0.70611911 5.94 26.13 0.1376 0.51202899 −11.9 2.2

M18(2)a _Bh-7 _Eclogite _11.76 _570.70 _0.0594 ₋_6.2 ₍₁₎

0.7069892 10.94 79.47 0.0833 0.51141099 −24.0 2.0

0.1329 −13.0

M19 96-218 Felsic gneiss 85.71 1859.00 (1)

d143 Felsic gneiss 12.6 89.09 0.0855 0.51133597 −25.4 2.1 −14.6 (2)

M20

6.58 40.37 0.0984 0.51158999 −20.5 2.1 −10.9

d105 (2)

M21 Felsic gneiss

d123 Felsic gneiss 9.04 59.25 0.1088 0.51137996 −24.6 2.6 −16.0 (2)

M22

0.418

BJ93-01 Felsic gneiss 28.65 198.35 0.71084798 8.0 43.53 0.1111 0.51240795 −4.5 1.1 −3.8 (4)

M23

0.70375595 1.96 7.51 0.1578 0.51252094 −2.3 1.7

0.001 1.5

0.14 456.33 (4)

M23(1) BJ93-03 Eclogite

0.072

BJ93-04 Eclogite 5.81 232.31 0.70402698 1.63 5.55 0.1778 0.51260896 −0.6 2.3 1.3 (4)

M23(2)a

0.71076896 4.0 21.5 0.1125 0.51212998 −9.9 1.5

M24 BJ93-26 Felsic gneiss 49.61 361.23 0.398 −1.7 (5)

0.72403199 3.28 14.82 0.1338 0.511420918 −23.8 3.3

1.041 −17.7

BJ93-32 (6)

M25 Felsic gneiss 88.85 247.21

0.005

BJ93-31 Eclogite 0.19 112.12 0.71078196 4.57 16.89 0.1636 0.51169597 −18.4 4.4 −15.2 (6)

M25(1)

M25(2) BJ93-36 Eclogite 2.7 487.28 0.016 0.70488997 3.72 14.89 0.151 0.51222997 −8.0 2.2 −3.5 (6)

7.33 34.89 0.1272 0.51217699 −9.0 1.7 −2.3

T16-1 (7)

M26 Felsic gneiss

1.9370

LJW-1 Felsic gneiss 91.40 182.52 0.7194997 5.13 22.76 0.1363 0.511941938 −13.6 2.4 −7.7 (1)

M27

0.7132592 1.62 7.36 0.1330 0.512153915 −9.5 1.9

M28 D12 Felsic gneiss 1.71 39.30 1.1120 −3.3 (3)

7.14 37.70 0.1144 0.51200599 −12.3 1.8 −4.4

D132 (2)

M29 Felsic gneiss

D142 Susong schist 5.47 24.07 0.1374 0.512362912 −5.4 1.6 0.4 (2)

M30

0.4842 0.7096392 8.39 46.41 0.1093 0.51212797 −10.0 1.5

G1 XXW-N11 Quartz 80.84 481.43 (8)

monzonite

0.7234091 5.57 31.86 0.1058 0.51161297 −20.0

124.03 2.2

Granite 243.57 5.6703 (8)

G2 B11-2

B1-2 Granite 218.60 3.0430 0.7167594 9.80 58.58 0.1012 0.51152097 −21.8 2.2 (8)

G2 207.30

0.70598935 2.73 12.96 0.1275 0.511928910 −13.8 2.2

G3 109-2 Tonalite 73.13 606.90 0.3474 (1)

0.70841914 3.43 10.25 0.2022 0.51277397 2.6 4.9

0.398

Mafic enclave (1)

G3(1)a _96-M11 _30.63 _221.9

1.6030

96-63-1 Granite 147.80 266.10 0.7186192 7.39 50.73 0.0811 0.51140298 −24.1 2.0 (1)

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Table 3 (Continued)

87_Sr_/86_Sr₉₂s

No. in Fig. 2 No. of samples _{Rock types} _{Rb (ppm)} Sr (ppm) 87Rb/86Sr Sm (ppm) Nd (ppm) 147_Sm_/144_Nd 143_Nd_/144_Nd oNd(0) TDM oNd (at 760 Sourcesb

Ma) (Ga) 92s

0.7080494 9.27 59.14 0.0949

G5 96-34 Granite 154.30 637.30 0.6981 0.51166598 −19.0 1.9 (1)

0.7085596 8.41 47.55 0.1070 0.511617910 −19.9 2.2

0.4378

96-116 (1)

G6 Granite 1110 731.20

0.5268

96-117 Granite 110.90 607.10 0.7087591 2.78 15.08 0.0914 0.511688919 −18.5 1.8 (1)

1096.60 0.2140 0.7083892 14.54 94.08 0.0935 0.51160498 −20.2 2.0 (1)

Granite

G8 94-1 81.41

0.7144698 6.68 40.23 0.1004 0.511841940 −15.5 1.8

3.3160

Granite 215.42 187.43 (1)

G9 94-6

1123.50

Quartz 0.1951 0.7077191 5.18 32.52 0.0964 0.51144397 −23.3 2.2 (8)

089 76.04

G10

monzonite

D102 Granite 6.37 44.23 0.0870 0.51137595 −24.6 2.1 (2)

G11

0.7083295 12.54 76.52 0.0991 0.51172998 −17.7 1.8

0.3828

G12 YH61 Monzonite 94.73 713.50 (8)

0.7094693 12.38 82.55 0.0907 0.51169997 −18.3 1.8

G12(1)a _A12 _{Quartz syenite} _153.75 _948.47 _0.4674 ₍₈₎

10.08 65.53 0.0959 0.51169594 −18.4 1.9

Granite (1)

G13 D1211

302.80

Quartz 0.4577 0.7095294 3.90 17.81 0.1326 0.512341913 −5.8 1.5 (1)

Shiguan1 48.06

G14

monzonite

7.05 55.97 0.0800 0.51153093 −21.6 1.8

G15 D1351 Granite (1)

0.1238 0.70806912 9.86 44.13 0.1352 0.51219698 −8.6

523.90 1.9

96-217 (8)

G16 Quartz 22.47

monzonite

A1 Monzodiorite 24.91 0.0818 0.7080292 9.75 48.73 0.1211 0.51162999 −19.7 2.5 (8)

G17 878.36

G17(1)a _A5 _Monzodiorite _42.43 _736.37 _0.1660 _0.7078094 _6.26 _33.17 _0.1141 _0.51157196 ₋_20.8 _2.4 ₍₈₎

0.332 0.70889911 5.16 33.77 0.0925 0.51135497 −25.0

700.07 2.3

Quartz 80.61 (1)

G17(2)a _103-2

monzonite

94-2 Granite 81.70 0.2721 0.7067792 2.64 16.62 0.0961 0.51164197 −19.4 2.0 (8)

G18 865.71

G19 MC-1 Monzonite 81.24 879.45 0.2663 0.7062294 4.68 29.31 0.0966 0.51177896 −16.8 1.8 (8)

10.12 67.34 0.0909 0.511338911 −25.4 2.2 (2)

G20 D1412 Granite

a_{The sample has not been located in Fig. 2, but it has a sampling location similar to that before it.}

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Table 4

Rb–Sr and Sm–Nd isotopic data for the Kongling complex from the Yangtze basementa

Rock typesb 87_Rb_/86_Sr 87_Sr_/86_Sr₉₂_s _{Sm (ppm)} _{Nd (ppm)} 147_Sm_/144_Nd 143_Nd_/144_Nd₉₂_s _o_Nd(0) _T DM(Ga) Rb (ppm)

No. of Sr (ppm)

samples

9.08 70.70

H45 gg 60.56 460.87 0.3879 0.7191199 0.0776 0.510315917 −45.3 3.2

3.96 26.40 0.0906 0.51057916 −40.3

0.7155292 3.2

H44 64.37 560.79 0.33166

0.7125295

gg 45.42 506.20 0.21608 1.40 8.63 0.0983 0.510511912 −41.5 3.5

H37

0.10096 0.7074494 7.07 0.09 0.0907 0.510548917 −40.8 3.2

H34 gg 24.29 693.47

6.07 0.15 0.1528 0.511789917 −16.6

0.7068893 3.4

363.96 0.08737 HvI-7 amph 11.03

amph 2.63 8.43 0.1888 0.512621912 −0.3 3.2

B17

amph 2.79 8.96 0.1888 0.512578926 −1.2 3.5

B19

4.27 15.28 0.1691 0.512142925 −9.7 3.4 B20 amph

amph 3.30 10.72 0.1860 0.5125397 −2.1 3.4

B21

5.35 24.94 0.1299 0.511289927 −26.3 3.4 amph

B22

amph 3.68 11.76 0.1894 0.5126599 0.2 3.1

B56

6.15 30.21 0.1231 0.51121198 −27.8

B57 amph 3.2

2.15 6.83 0.1896 0.512653913 0.3 3.1 amph

B61

amph 2.81 9.45 0.1800 0.51242797 −4.1 3.3

B62

4.66 17.54 0.1608 0.51207199 −11.1 3.1 B65 amph

4.80 15.47 0.1879 0.512531914 −2.0 3.6 B66 amph

amph 3.01 11.72 0.1553 0.511838915 −15.6 3.4

B70

3.15 24.19 0.0786 0.51031916 −45.4 3.2 B73 gg

a_{Sources of isotopic data: H45 to Hv17 from Li Fuxi and Ma Daquan (1991, unpublished report) and B17 to B73 from Ma et al. (1997).} b_{Rock types: gg, grey gneiss, and amph, amphibolite.}

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C.Ma et al./Precambrian Research102 (2000) 279 – 301 292

Fig. 5. Plots of Ybnvs. (La/Yb)n(A) and Nb vs. Y (B) for the Dabieshan felsic gneisses and Kongling grey gneisses. For Ybn vs. (La/Yb)nplot: Chondrite normalizing values used are from Taylor and McLennan (1985), and fields for Archean and Post-Archean granitoids are indicated after Jahn et al. (1981) and Martin (1986). Data points from the KLC plot in the Archean field, whereas felsic gneisses from the Dabieshan straddle both Archean and post-Archean fields. The Nb vs. Y plot shows tectonomagmatic grid of Pearce et al. (1984). VAG=volcanic arc granitoids, WPG=within-plate grani-toids; ORG=ocean-ridge granitoids. Symbols as in Fig. 3.

amphibolite sample (HVI-7) from the KLC shows

similar 87_Sr_/86_{Sr and} 143_Nd_/144_{Nd ratios to the}

Dabieshan amphibolites.

Fig. 6. Chondrite-normalized REE patterns and primitive mantle-normalized incompatible element patterns, showing that some felsic gneisses in the eclogite units have geochemical features which match those of the Kongling grey gneisses. Percentages in brackets represent SiO2contents of the samples. Chondrite normalizing values used are from Taylor and McLennan (1985), and primitive mantle normalising factors are from Hofmann (1988).

Fig. 7. 143_Nd_/144_{Nd vs.} 87_Sr_/86_{Sr isotopic variation for felsic} gneisses, amphibolites and eclogites from the Dabieshan and Yangtze basement.

In a plot of 87_Sr_/86_{Sr versus} 143_Nd_/144_{Nd (Fig.}

7), the Dabieshan felsic gneisses have a higher

average 87_Sr

/86_{Sr ratio of 0.712 than the}

Da-bieshan eclogites and amphibolites (:0.707). The

felsic gneisses from the eclogite units show a wide

range, especially in 87_Sr_/86_{Sr ratio, probably}

reflecting fractionation between Rb and Sr.

How-ever, the87

Sr/86

Sr ratios for the majority of meta-morphic rocks from the DBC and KLC are lower than an average ratio of continental crust (0.72; Hofmann, 1997). Felsic gneisses from the

Da-bieshan terrane have much higher 143_Nd_/144_Nd

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C.Ma et al./Precambrian Research102 (2000) 279 – 301 293

Fig. 8. Plot of 87_Rb_/86_{Sr vs.} 147_Sm_/144_{Nd for felsic gneisses,} amphibolites and eclogites from the Dabieshan and Yangtze basement. Also shown for comparison are the parent/daughter isotopic ratios for average continental crust (CC) and depleted mantle (DM). For Sm – Nd, CC value is taken from Taylor and McLennan (1985), DM value from Peucat et al. (1989). Rb – Sr ratios for CC and DM are taken from Mo¨ller et al. (1998). Symbols as in Fig. 7.

ing the formation of new continental crust from depleted mantle, that processes in the crust in-cluding partial melting and high-grade

metamor-phism did not affect the Sm/Nd systematics on

a whole-rock scale, and that all the material in the sample was derived from the upper mantle during a single event (Arndt and Goldstein, 1987; Milisenda et al., 1994). If this is the case,

measurement of 143_Nd_/144_{Nd and} 147_Sm_/144_{Nd in}

a crustal rock would yield a Nd model age

(TDM) which represents the latest time that Nd

was in isotopic equilibrium with depleted mantle (DM). A complication in interpreting the Nd model ages of orthogneisses is the possible effect of mixing between juvenile material and old crust. If that has occurred, the model ages would be intermediate between the actual times of derivation of crustal material from depleted mantle and the model residence age of the old crust, and would not correspond to crust-forma-tion events (Arndt and Goldstein, 1987).

Assuming that the Sm – Nd components of a crustal sample did not fractionate since addition of the sample to the continental crust from the mantle, calculated Nd model ages of felsic gneisses provide a powerful tool for insights into the premetamorphic crustal history of a high-grade terrane. However, the significance of Nd model ages for mafic rocks is ambiguous, as

many of them have mantle-like Sm/Nd ratios

(Fig. 8). It is clear that most amphibolites and

eclogites in Tables 3 and 4 have high 147_Sm

144_{Nd ratios of} _\_{0.15, which would give}

unre-alistically high Nd model ages with no

geological significance. We have thus limited ourselves to Nd model ages of the Dabieshan felsic gneisses in this paper (Fig. 9).

Grey gneisses from the KLC have a mean Nd

model age of 3.390.1 Ga (at 1 S.D.), which is

distinctly older than Nd model ages of the major-ity of the Dabieshan felsic gneisses (Fig. 9), most of which range from 1.0 to 2.9 Ga (Fig. 9). A Susong schist from the blueschist-bearing fold-thrust belt has a model age of 1.6 Ga. However, an intermediate granulite (sample 109-1) from the Luotian dome and a felsic gneiss (BJ93-32) from Thirty-six felsic gneiss samples from the

Da-bieshan yield a mean 147

Sm/144

Nd ratio of

0.1290.02, which is within the typical range

(0.09 – 0.13) for crustal rocks (Taylor and

McLennan, 1985). 147_Sm

/144_{Nd ratios of the}

Kongling amphibolites (0.12 – 0.19, see Table 4)

and most Dabieshan eclogites (\0.12, Fig. 8)

are higher than the typical range of crustal rocks, while grey gneisses from the KLC have

lower 147

Sm/144

Nd ratios (0.08 – 0.10) than the

mean 147_Sm_/144_{Nd ratio (0.12) of crustal rocks}

(Fig. 8) (Milisenda et al., 1994; Mo¨ller et al., 1998). The Dabieshan felsic gneisses display 87

Rb/86

Sr ratios ranging from 0.059 to 1.937

with a mean value of 0.54690.102, which is

higher than average continental crust (0.4098, Faure, 1986), whereas the Dabieshan eclogites

show a relatively wide range in 87

Rb/86

Sr values (Fig. 8).

A major application of Sm – Nd isotope sys-tematics to old orogens is to delineate provinces of different ages of ‘crust-formation’, ‘crustal ex-traction’, or ‘crustal residence’ indicated by Nd model ages (e.g. DePaolo, 1981; Arndt and Goldstein, 1987; Dickin et al., 1990; Milisenda et al., 1994; Mo¨ller et al., 1998). Basic assump-tions in using such model ages are that major fractionation between Sm and Nd occurred

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dur-C.Ma et al./Precambrian Research102 (2000) 279 – 301 294

Fig. 9.o_Nd(0) vs.TDM(Ga) variation for felsic gneisses from the Dabieshan and Yangtze basement. Symbols as in Fig. 3.

bieshan yield Proterozoic model ages (Table 3, Fig. 2), possibly suggesting that they came from, or were contaminated by, crustal sources and rocks of the Dabieshan and Yangtze basement (discussed blow).

Rowley et al. (1997) and Ames et al. (1996) have argued that an episode of rift-related mag-matism took place between 600 and 800 Ma. We

calculated initial o_Nd _{values of the Dabieshan}

fel-sic gneisses, amphibolites and eclogites at 760 Ma

(Table 3), and also show the o_Nd _{(at 760 Ma)}

values of a few samples of the Dabieshan

amphi-bolites and eclogites in Fig. 2. The o_Nd _{values of}

the amphibolites and eclogites, ranging from

+3.2 to −15.2, are similar to those of the

Da-bieshan felsic gneisses (+3.8 to −17.7).

4.3. Geochronological significance

On a conventional Rb – Sr isochron diagram, most of the felsic gneisses, amphibolites and eclogites from the Dabieshan plot close to a 493 Ma reference line (Fig. 10). In contrast to the Qinling and Tongbai metamorphic rocks, which experienced high-temperature metamorphism be-tween 480 and 430 Ma (zircon Pb data by Kro¨ner et al., 1993; Ar – Ar hornblende data by Zhai et al., 1998), the geological significance of the Or-dovician chronological results from the Da-bieshan terrane has been a matter of some debate. Ames et al. (1996) suggested that the Dabieshan metamorphic rocks did not experience an early Paleozoic metamorphic event at all. However, You et al. (1996) obtained a U – Pb zircon lower

intercept age of 47192 Ma (MSWD=0.3) and a

Sm – Nd garnet-whole rock isochron age of 4819

25 Ma for the Dabieshan eclogites, and they considered these ages as another eclogite-facies metamorphic event in addition to the well-known Triassic HP and UHP metamorphism (Yang et al., 1994; You et al., 1996). Given the relative mobility of Rb and Sr, the Rb – Sr isotopic system might readily be disturbed either by influx of fluids or by a later thermal event. Thus it appears that the Rb – Sr errorchron of 493 Ma in Fig. 10 may indicate an early Paleozoic metamorphism in the Dabieshan terrane.

the eclogite units yield Archean model ages simi-lar to those of the KLC.

The regional distribution of the Nd model ages for the Dabieshan felsic gneisses and Mesozoic intrusive rocks is shown in Fig. 2. Most younger model ages of 1.0 – 1.6 Ga occur near the north-ern and southnorth-ern margins of the Luotian dome, and the northern margin of the eclogite units. Granulite facies rocks with a depleted-mantle model age of 3.1 Ga are exposed in the core of the Luotian dome, and the oldest model age of 3.3 Ga was found in a felsic gneiss that encloses UHP

diamond-bearing eclogite blocks from the

Shuanghe region (Xu et al., 1992; Liou et al., 1997). All Mesozoic intrusive rocks in the

Da-Fig. 10. Rb – Sr isochron diagram for felsic gneisses, amphibo-lites and eclogites from the Dabieshan. Reference age is calcu-lated with the Isoplot program (Version 2.95) of Ludwig (1997). Symbols as in Fig. 7.

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C.Ma et al./Precambrian Research102 (2000) 279 – 301 295

Fig. 11. Sm – Nd isochron diagram for felsic gneisses, amphi-bolites and eclogites from the Dabieshan and Kongling com-plex. Ages and error are calculated with the Isoplot program (Version 2.95) of Ludwig (1997). Discussion see text. Symbols as in Fig. 7.

with o_Nd (T)= +2.8. The high MSWD value of

30.2 indicates that the variation along the regres-sion line is much larger than that caused by analytical error. This implies that the Sm – Nd isotopic system of these samples was not in equilibrium.

The scatter in the measured Sm – Nd ratios of the Dabieshan felsic gneisses indicates that either the Sm – Nd system was differentially mobilized or the rocks did not have the same initial ratios. The isotopic variation of gneiss samples on the mixing line 1 may be associated with mixing between (Neoproterozoic?) mantle-derived magmas and Kongling gneisses, because crustal assimilation by a primary mafic magma during ascent and em-placement combined with fractional crystalliza-tion (AFC), presumably generated the protolith of felsic gneisses (Ma, 1999). Three samples of Da-bieshan eclogites and amphibolite on mixing line 2 possibly represent a mixing relationship between a younger mafic magma and Kongling gneiss.

Among the Dabieshan metamorphic rocks, the Huangtuling intermediate granulite (sample 109-1, Table 3) has the oldest Nd model age of 3.1 Ga, and falls on line L2 in Fig. 11. In order to constrain the age of its protolith and the timing of granulite – facies metamorphism, Sm – Nd isotopic analyses were performed on a set of samples including intermediate granulite, host tonalite (fel-sic) gneiss and two mafic enclaves from the core of the Luotian dome (point M9 in Fig. 2). A few mineral separates from the granulite were also analyzed. The data are given in Table 5 and Table 3. Interestingly, the majority of data points in-cluding granulite, mafic enclaves, tonalite (felsic) gneiss, plagioclase and orthopyroxene define an The majority of Sm – Nd data for the DBC

displays rather a broad array on a Sm – Nd isochron diagram (Fig. 11). Five samples of

Da-bieshan eclogites and amphibolites yield a 21479

456 Ma reference line (L1) with an initial o_Nd of

+4.9 (age calculated using model 1 of Ludwig

(1997) in which scatter from a straight line is attributed to analytical errors alone), and indicat-ing derivation from a LREE-depleted mantle reservoir. Although the reference line does not meet the critical isochron prerequisites of White-house et al. (1996), we consider that the age provides an approximate estimate of the protolith age for some Dabieshan eclogites and amphibo-lites. In contrast to the Dabieshan eclogites, thir-teen data points for the Kongling amphibolites define a reference line (L2) whose slope

corre-sponds to an age of 32449145 Ma (95% conf)

Table 5

Sm–Nd isotopic data for the constituent minerals of the Huangtuling intermediate granulite and an amphibolite enclave Mineral or rock

Samples Sm (ppm) _{Nd (ppm)} 147_Sm_/144_Nd 143_Nd_/144_Nd

92s 2.70

Garnet from 109-1a

Grt 4.59 0.3564 0.512475912

Biotite from 109-1

Bt 0.28 1.29 0.1302 0.511419922

Orthopyroxene from 109-1 1.67

Opx 8.70 0.1165 0.51112599

Plagioclase from 109-1 1.52 9.85 0.0934 0.510785921

Amphibolite enclave 3.43 10.25 0.2022 0.51277397

96-M11

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C.Ma et al./Precambrian Research102 (2000) 279 – 301 296

Fig. 12. Sm – Nd isochron diagram for the Huangtuling inter-mediate granulite (sample 109-1), its constituent minerals, a host felsic gneiss of the granulite (sample 96-M4) and amphi-bolite xenolith and enclave (96-M2 and 96-M11) from the core of the Luotian dome. Data and abbreviations are given in Table 3 and Table 5.

4.4. Contribution of the Archean Kongling gneiss

to the Mesozoic Dabieshan mafic monzodiorite

The Mesozoic mafic monzodiorites of Group I (e.g. A1 and A5, Table 3) are characterised by

high Th/Yb ratios, relative enrichment in

incom-patible elements, negative Nb anomalies, low

o_Nd_, _high _initial 87_Sr_/86_Sr _ratios _and

Pale-oproterozoic Nd model ages, suggesting either interaction between crustal material and mafic magmas during their ascent through the crust, or enrichment of the mantle source by subduc-tion and recycling of crustal material (Ma et al., 1998). In order to examine the nature and ex-tent of possible crustal components which were involved in the petrogenesis of the Liujiawa mafic monzodiorites (Group-I), we have per-formed isotopic mixing calculations.

The element concentrations and isotope ratios of mixtures of two components are described by the following equations (Boger and Faure, 1976; Gray, 1984):

CI=f·CC+(1−f) ·CM (1)

and

CI· RI=f·CC·RC+(1−f) ·CMRM (2)

where CI, CC and CM are the concentrations of

element, ppm Nd, Sm, Sr, or Rb in isotopically mixed intrusion rocks (I), a crustal component

and RM are their respective isotopic ratios

87_Sr

86_{Sr or} 143_Nd_/144_{Nd. The factor} _f _{is the}

weight-ing factor of the crustal component in any given

mixture. For example, for the ratio 143_Nd_/144_Nd

in a mixture, Eq. (2) becomes:

C(Nd)I(143Nd/144Nd)I

=f·C(Nd)C(143Nd/144Nd)C

+(1−f) ·C(Nd)M(

143

Nd/144Nd)M (3)

To examine the effects of mixing on the Nd

model ages (TDM), a parent/daughter isotopic

ratio (147_Sm

/144_{Nd) in the mixture must be}

cal-culated. An equation for the calculation can be given by finding the molar concentrations of

147_{Sm and} 144_{Nd in the mixture, following the}

procedure outlined by White et al. (1967) and using the atomic fractions (atom%) reported by Rosman and Taylor (1998):

errorchron whose slope corresponds to an age

of 27899650 Ma (95% conf.) (Fig. 12), but a

data point from garnet in the granulite does not lie on the line. Since biotite is most likely to be a non-primary phase, it was excluded from re-gression calculations. The very high MSWD value of 511 implies that the samples cannot be regarded as cogenetic or as equilibrium assem-blages. Thus line L1 may reflect only a mixing relationship. The inset in Fig. 12 shows a whole-rock (WR)- orthopyroxene-plagioclase isochron

age of 22389300 Ma with an initial o_Nd₌

−6.6 for sample 109-1 (MSWD=1). The

non-linearity of data points from the granulite

sam-ple, orthopyroxene and plagioclase in a

143_Nd_/144_{Nd – 1}_/_{Nd plot (not shown) suggests}

that the straight line L2 is a true isochron and not a mixing line. Consistent within the bounds of uncertainty with a Pb-Pb zircon age of

245697 Ma for the same granulite exposure

re-ported by Jian et al. (1999), the isochron age of

22389300 Ma probably represents the timing

of granulite – facies metamorphism. Therefore, the age of the protolith to the granulite must be

older than 22389300 Ma. Petrographical study

shows that garnet occurs as rim around a ca. 2 mm wide felsic vein cutting the granulite, imply-ing that the garnet resulted from anatexis after granulite – facies metamorphism.

(1)

4 .

Contribution of the Archean Kongling gneiss

to the Mesozoic Dabieshan mafic monzodiorite

The Mesozoic mafic monzodiorites of Group

I (e.g. A1 and A5, Table 3) are characterised by

high Th

/

Yb ratios, relative enrichment in

incom-patible elements, negative Nb anomalies, low

o

_Nd

_,

_high

_initial

_Sr

_/

_Sr

_ratios

_and

Pale-oproterozoic Nd model ages, suggesting either

interaction between crustal material and mafic

magmas during their ascent through the crust,

or enrichment of the mantle source by

subduc-tion and recycling of crustal material (Ma et al.,

1998). In order to examine the nature and

ex-tent of possible crustal components which were

involved in the petrogenesis of the Liujiawa

mafic monzodiorites (Group-I), we have

per-formed isotopic mixing calculations.

The element concentrations and isotope ratios

of mixtures of two components are described by

the following equations (Boger and Faure, 1976;

Gray, 1984):

C

=

f

· C

+

(1

−

f

) ·

C

(1)

and

C

· R

=

f

· C

· R

+

(1

−

f

) ·

C

R

(2)

where

C

,

C

and

C

are the concentrations of

element, ppm Nd, Sm, Sr, or Rb in isotopically

mixed intrusion rocks (I), a crustal component

(C) and a mantle component (M), and

R

,

Rc,

and

R

are their respective isotopic ratios

_Sr

/

_{Sr or}

143

_Nd

_/

144

_{Nd. The factor}

_f

_{is the}

weight-ing factor of the crustal component in any given

mixture. For example, for the ratio

143

_Nd

_/

144

_Nd

in a mixture, Eq. (2) becomes:

C

(Nd)I

(

143

Nd

/

144

Nd)

=

f

· C

(Nd)C

(

143

Nd

/

144

Nd)

+

(1

−

f

) ·

C

(Nd)M

(

143

Nd

/

144Nd)

(3)

To examine the effects of mixing on the Nd

model ages (T

), a parent

/

daughter isotopic

ratio (

147

_Sm

/

144

_{Nd) in the mixture must be}

cal-culated. An equation for the calculation can be

given by finding the molar concentrations of

147

_{Sm and}

144

_{Nd in the mixture, following the}

procedure outlined by White et al. (1967) and

using the atomic fractions (atom%) reported by

Rosman and Taylor (1998):

errorchron whose slope corresponds to an age

of 27899

650 Ma (95% conf.) (Fig. 12), but a

data point from garnet in the granulite does not

lie on the line. Since biotite is most likely to be

a non-primary phase, it was excluded from

re-gression calculations. The very high MSWD

value of 511 implies that the samples cannot be

regarded as cogenetic or as equilibrium

assem-blages. Thus line L1 may reflect only a mixing

relationship. The inset in Fig. 12 shows a

whole-rock (WR)- orthopyroxene-plagioclase isochron

age of 22389300 Ma with an initial

o

_Nd

₌

−

6.6 for sample 109-1 (MSWD

=

1). The

non-linearity of data points from the granulite

sam-ple,

orthopyroxene

and

plagioclase

in

a

143

_Nd

_/

144

_{Nd – 1}

_/

_{Nd plot (not shown) suggests}

that the straight line L2 is a true isochron and

not a mixing line. Consistent within the bounds

of uncertainty with a Pb-Pb zircon age of

24569

7 Ma for the same granulite exposure

re-ported by Jian et al. (1999), the isochron age of

22389

300 Ma probably represents the timing

of granulite – facies metamorphism. Therefore,

the age of the protolith to the granulite must be

older than 22389

300 Ma. Petrographical study

shows that garnet occurs as rim around a ca. 2

mm wide felsic vein cutting the granulite,

imply-ing that the garnet resulted from anatexis after

granulite – facies metamorphism.

(2)

(

147

_Sm

/

144

_Nd)

=

[C

(Sm)I

(532.78

+

142.91(

143

_Nd

/

144

_Nd)

]

/

(1003.09C

(Nd)I

)

(4)

where

C

(Sm)I

,

C

(Nd)I

, and (

143

Nd

/

144

Nd)

in the

mixture can be calculated using the Eq. (1) and

Eq. (3).

In order to numerically model isotopic mixing,

an average isotopic composition for four Permian

gabbro samples from the Xinxian block, ca. 70

km west of the Dabieshan terrane (Zhang et al.,

1995) and three Cenozoic basalt samples from the

Tan – Lu fault belt (Zhi and Chen, 1992) is used as

the mantle component. The other crustal

compo-nents that may contribute to the final composition

of the Mesozoic Dabieshan mafic intrusive rocks

include those of the Yangtze basement

repre-sented by the Kongling gneisses and Dabieshan

intermediate and felsic gneisses

/

granulite. Thus,

two felsic gneisses and one intermediate granulite

with distinct Nd model ages are examined in turn

as possible crustal end members (H44 from the

Kongling complex; 96-110 and 109-1 from the

Dabieshan dome region). Mixing calculations

show that the addition of 27% Archean Kongling

gneiss will yield a magma composition which

closely matches the compositions of the Liujiawa

mafic monzodiorites in Nd and Sr isotopes (Fig.

13), and that calculated T

of the resulting

magma (ca. 2.5 Ga) agrees well with the Nd

model ages of the samples A1 and A5 (Table 3),

whereas mixing with components from the

Da-bieshan terrane (samples 96-110 and 109-1) would

not result in isotopic compositions of the Group I

mafic monzodiorite stock. Fig. 13 shows that half

the samples of the Group II lie on the trend line

between the mantle-derived component and the

Kongling

grey

gneiss.

It

appears

that

the

Kongling grey gneisses have been involved in the

Group II magmatism, and that the Dabieshan

complex may have played a significant role in the

petrogenesis of the Group III intrusive rocks (Fig.

13).

Distinguishing between the effects of crustal

contamination and subduction enrichment (i.e.

source contamination) is critical for assessing the

implications of our results for crustal structure.

The most compelling evidence for crustal

contam-ination would be a correlation between isotopic

ratios of Sr and Nd and chemical compositions

with a fractionation indicator (e.g. SiO

, MgO, or

Mg-number), because such a correlation would

imply that changes in isotopic composition were

produced during differentiation, while the magma

was ascending and emplaced in the crust.

Accord-ing to the Nd model ages, rocks of the KonglAccord-ing

complex are not a major rock type exposed in the

Dabieshan terrane and therefore composition

ef-fects of the Kongling grey gneiss on the

mantle-derived mafic magmas may be indicative of

recycling of the Yangtze basement into the mantle

via continental subduction. However, an

alterna-tive possibility is that the KLC underlies the

Dabieshan terrane, so that interaction between

Fig. 13. o_Nd(0) vs. 87_Sr/86_{Sr diagram showing the isotopic}

mixing relationship between mantle-derived material and crustal components. Also shown are composition fields of the Kongling complex in the Yangtze basement, Permian gabbros from the Xinxian block (Zhang et al., 1995) and Paleogene basalts from the Tan-Lu fault belt (Zhi and Chen, 1992). Trace element and isotopic composition of mantle-derived material used in the mixing calculation is the average composition of four Permian gabbros (Zhang et al., 1995) and three Cenozoic basalts (Zhi and Chen, 1992): Rb: 8.048 ppm, Sr: 424.101 ppm, Sm: 2.311 ppm, Nd: 9.394 ppm, 87_Rb/86_Sr=0.01641, 87_Sr/86_Sr=_0.70405; 147_Sm/144_Nd_=0.17644, _and 143_Nd/ 144_Nd_{=0.51267. Three crustal end members include the} Yangtze basement gneiss (H44), the Dabieshan felsic gneiss (96-110) and the Huangtuling granulite (109-1) (see Table 4 and Table 3 for their compositions). Figures on the mixing lines refer to mass fractions (wt.%) of the crustal components in the mixed intrusive magmas. Other symbols as in Fig. 3.

(3)

Kongling grey gneiss and the Group-I magma

could occur at a lower crustal depth (Ma et al.,

1998). By comparing the two Liujiawa mafic

mon-zodiorite samples (Table 3), it becomes clear that

a sample (A5) with more ‘primary’ chemical

com-position, e.g. a Mg-number of 0.56 and a SiO

/

Al

O

ratio of 3.5 (Ma et al., 1998), close to those

of primary mantle melts (Kempton et al., 1997),

does have less contaminated isotopic ratios of Nd

and Sr than the other sample (A1) with a lower

Mg-number of 0.45. The possibility of subduction

enrichment therefore seems to be more plausible.

Regardless of which mixing mechanism is

pre-ferred, the Archean KLC may have been present

below the Dabieshan terrane.

5. Implications for the crustal structure of the

Dabieshan terrane

Analysis of geochemical and Nd and Sr isotope

data of the DBC and KLC indicates that the

protoliths of the Dabieshan felsic gneisses and

Kongling grey gneisses have diverse origins. The

Kongling grey gneiss is an Archean high-Al TTG

rock with an average Nd model age of ca. 3.3 Ga,

whereas the felsic gneisses of the DBC, which

display a large variation in chemical composition

and Nd and Sr isotopic ratios, constitute a

meta-morphic thrust-stack sheet of welded slices that

have different tectonic and metamorphic histories,

as suggested by Okay and Sengo¨r (1992).

Re-cently, Ames et al. (1996) attributed the protolith

formation of the Dabieshan orthogneisses and

eclogites mainly to rift magmatism between ca.

600 and 800 Ma. However, we argue that

Archean components are present in the

Da-bieshan terrane using four lines of evidence: (1)

Some felsic gneisses in Dabieshan display the

geochemical signature of Archean granitoids on a

plot of chondrite-normalized Yb versus La

/

Yb

(Fig. 5). Among them, two gneiss samples from

the eclogite units show REE and incompatible

element patterns that are identical to those of an

Archean grey gneiss in the KLC (Fig. 6). (2) The

Huangtuling intermediate granulite with a Nd

model age of 3.1 Ga yields a WR-mineral

isochron of 2238

9300 Ma, which probably

rep-resents the age of granulite – facies metamorphism.

This implies that the protolith age of the granulite

must be older than 2238

9300 Ma. (3) Besides

these Archean Nd model ages, Archean U – Pb

zircon ages have been noted, not only in the dome

region (e.g. Chen et al., 1996) but also in the

eclogite units (Chavagnac and Jahn, 1996). Cao

and Zhu (1995) separated zircon from a single

Bixiling eclogite sample (M23 in Fig. 2) and

ob-tained a four-point discordia with an

upper-inter-cept age of 2774

9 24 Ma, and a lower-intercept

age of 452

9 78 Ma. (4) Isotopic mixing

calcula-tions also indicate that the Archean Kongling

grey gneisses have been involved the petrogenesis

of the Liujiawa mafic monzodiorites intruded into

in the eclogite units, and of some of the Group II

rocks, implying that the KLC probably extended

beneath the Dabieshan terrane during early

Meso-zoic continental collision. It follows that the felsic

gneisses with KLC affinity within the eclogite

units may have resulted from tectonic extrusion of

Yangtze basement beneath the Dabieshan during

Mesozoic exhumation of the UHP metamorphic

rocks.

The results of this study indicate that the

ma-jority of the felsic gneisses in the eclogite units

have post-Archean features that are distinct from

those in the dome region. The Dabieshan eclogites

show volcanic arc to within-plate affinities in

chemistry (Ma, 1999). Two felsic gneiss samples

(94080 and 94095) from the eclogite units with a

U – Pb zircon age of ca. 629 Ma (Wu et al., 1998)

also display within-plate affinity in the Nb versus

Y plot of Pearce et al. (1984), indicating an

episode of rift-related magmatism between ca. 600

and 800 Ma, as suggested by Ames et al. (1996).

Although the felsic gneisses in the dome region

display chemical and isotopic features of Archean

granitoids, there is only a limited effect on the Nd

model ages of felsic gneisses. A possibility is that

the Nd – Sr isotopic systematics and the

geochem-istry of the felsic gneisses in the dome region

resulted from mixing between Neoproterozoic

mantle-derived magmas and Archean Kongling

gneisses with volcanic arc affinity. In such a case,

the resulting granitoids would display

post-Archean Nd model ages that would not represent

‘crust-formation ages’ (Arndt and Goldstein,

(4)

1987) and would have geochemical features

inter-mediate between typically Archean and

post-Archean granitoids.

Therefore, we consider that the Dabieshan dome

region was originally part of the continental

base-ment of the Yangtze craton that formed as a late

Archean to Early Proterozoic continental

mag-matic arc, and later was separated from the

Yangtze craton by Neoproterozoic rifting. After

north-directed underthrusting of continental crust

of the Yangtze craton and the Dabieshan

micro-continent beneath the Sino – Korean craton,

indi-vidual

terranes

with

distinct

tectonic

and

metamorphic histories amalgamated during early

Mesozoic time. The orogen was then modified by

intensive migmatization and magmatism, especially

during Jurassic and Cretaceous time (Wang et al.,

1998; Ma et al., 1998). Such a model can account

for the wide range of geochemical features and

isotope ages of the DBC, reflecting a variety of

thermal and tectonic events and distinct crustal

components.

Acknowledgements

This work was financially supported by the

National Natural Science Foundation of China

(Numbers 49572100 and 49972022), with

addi-tional support from a grant (94-96) from the Fok

Yingtung Education Foundation. MCQ is grateful

to the China University of Geosciences at Wuhan

and A

,

bo Akademi University for having supported

his research effort at A

,

bo. We are grateful to Ma

Daquan for providing us with unpublished data, to

Chris Gray for helpful suggestions on the isotopic

mixing calculations, to Roger Mason for checking

the manuscript, and to Tao Jidong and Ai Xiaoling

for their help during the study. We appreciate the

careful and constructive reviews by Michael Raith

and Andreas Mo¨ller, which led to substantial

improvement of the manuscript.

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