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

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Geochemical and Sm – Nd isotopic study of amphibolites in

the Cathaysia Block, southeastern China: evidence for an

extremely depleted mantle in the Paleoproterozoic

X.-H. Li

a,b,

_{* , M. Sun}

_{, G.-J. Wei}

_{, Y. Liu}

_{, C.-Y. Lee}

_{, J. Malpas}

b a_{Guangzhou Institute of Geochemistry}_,_{Chinese Academy of Sciences}_,_{PO Box}₁₁₃₁_,_Guangzhou₅₁₀₆₄₀_,_Guangdong_,_{PR China}

b_{Department of Earth Sciences}_,_{The Uni}

6ersity of Hong Kong,Pokfulam Road,Hong Kong c_{Department of Geology}_,_{National Taiwan Uni}

6ersity,245Choushan Road,Taipei106-17,Taiwan Received 3 June 1999; accepted 29 February 2000

Abstract

Geochemical and Sm – Nd isotopic results are reported for late Paleoproterozoic mafic amphibolites from SW Zhejiang and NW Fujian, parts of the Cathaysia Block of SE China. Two suites of contemporaneous amphibolites are distinct in their geochemical characteristics. Group 1 samples, from NW Fujian, have chemical compositions of transitional and alkali basalts, show LREE-enriched patterns and plot mainly in the field of within-plate basalt on a number of trace element discrimination diagrams. Group 2 rocks, from SW Zhejiang, have tholeiitic compositions and are characterized by flat to LREE-depleted patterns and fall into the MORB field. All the amphibolite samples have highoNd(T) values of +5.6 to +8.5 (T=1766919 Ma). A positive correlation betweenoNd(T) and Nb/Th suggests possible mixing of a mantle-derived magma and a crustal component, with the least-contaminated samples having very highoNd(T) values (+8+8.5) and Nb/Th ratios of 13. The geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto-oceanic basin (e.g. the Gulf of Tadjoura). The exceptionally high o_{Nd(T) values of up to} +8.5 for some of the amphibolites suggest the presence of a time-integrated extremely depleted mantle source beneath Cathaysia during the Paleoproterozic. © 2000 Elsevier Science B.V. All rights reserved.

Keywords:Amphibolite; Paleoproterozic; Cathaysia Block; Southeastern China; Sm – Nd isotopes; Depleted mantle

www.elsevier.com/locate/precamres

1. Introduction

South China consists of two major tectonic blocks separated by the Jiangshan – Shaoxing Fault (Fig. 1). Most isotope age studies have shown that the Cathaysia Block is of Pale-oproterozoic-Mesoproterozoic age (e.g. Shui, * Corresponding author. Fax: +86-20-85514130.

E-mail address: gzisolab@public.guangzhou.gd.cn (X.-H. Li)

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X.-H.Li et al./Precambrian Research102 (2000) 251 – 262 252

1987; Jahn et al., 1990; Hu et al., 1991; Li et al., 1992; Gan et al., 1993, 1995; Chen and Jahn, 1998), although a few workers have suggested that it is as old as Archean, based on Sm – Nd ‘isochron’ and inherited zircon U – Pb ages (Fu et al., 1991; Zhou, 1997). More recently, Li (1997a) reported a SHRIMP U – Pb zircon age of 17669

19 Ma for the oldest amphibolites exposed in NW Fujian, confirming a Late Paleoproterozoic age for these rocks. The Precambrian tectonic evolu-tion of Cathaysia is, however, still poorly under-stood, largely due to the paucity of detailed geochemical, petrological and structural data for the metamorphic rocks.

This paper presents the results of a detailed geochemical and Sm – Nd isotopic study of amphi-bolites from NW Fujian and SW Zhejiang, within the Cathaysia Block, and provides evidence for the existence of an extremely depleted mantle

source and for the development of magmatism in a rift/proto-ocean basin system in the Paleoproterozoic.

2. Geological background

Precambrian basement of the Cathaysia Block is mainly exposed in an area between the Jiang-shan – Shaoxing and Lishui – Haifeng Faults in NW Fujian and SW Zhejiang Provinces (Fig. 1). On the basis of lithological, structural and meta-morphic features, the basement rocks are divided into two metamorphic sequences. The lower se-quence, termed the Mayuan Group in NW Fujian and the Badu Group in SW Zhejiang, consists mainly of leptynites, schists, granitic gneisses, and mafic and felsic meta-volcanics (Hu et al., 1991; Jin et al., 1992). Granitic gneisses crop out mostly

Fig. 1. Simplified geological map of Cathaysia Block, SE China, illustrating the distribution of Precambrian basement (modified after Hu et al. (1992) and Gan et al. (1993)).

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in the Tangyuan Formation (the lowest part of the Badu Group), and display a transitional rela-tionship with the surrounding leptynites, with a few granitic gneisses showing intrusive contacts with the country rocks (Hu et al., 1991). These rocks have undergone amphibolite facies meta-morphism. The upper sequence, i.e. the Mamian-shan Group in Fujian and the Longquan Group in Zhejiang, mainly comprise schists, meta-vol-canics, Fe-bearing quartzites and marbles. These rocks have been metamorphosed to lower green-schist facies (Hu et al., 1991; Jin et al., 1992).

Samples of amphibolite analyzed in this study were collected from two localities along road-cuts in Tianjingping, Jianning County, NW Fujian and Zhulu, Longquan County, SW Zhejiang (Fig. 1). Samples from NW Fujian include LG24, LG28, LG29 and LG35 with chemical compositions of transitional and alkali basalts (Group 1), while those from SW Zhejiang include LB258, LB259, LB2622, LB263, LB264 and LB265 with tholeiitic compositions (Group 2), and one sample, LB261, with transitional basalt composition similar to Group 1 samples. A 50 kg amphibolite sample (LG28), collected from NW Fujian yields a SHRIMP U – Pb zircon age of 1766919 Ma which is interpreted as the crystallization age for the protolith of the amphibolites (Li, 1997a). This zircon date provides the best estimate of the for-mation age of the Mayuan Group in NW Fujian. Amphibolites in SW Zhejiang have not been di-rectly dated, but U – Pb zircon dates of 1832980 Ma, 1870936 Ma, 1889995 Ma and 1975980 Ma obtained by conventional U – Pb techniques were reported for the granitic gneisses (Hu et al., 1992; Gan et al., 1993, 1995). Because the granitic gneisses are intrusive into the surrounding lep-tynites and amphibolites, the lowest part of the Badu Group, their ages (1.8 – 1.98 Ga) are sup-posed as minimum age for the Badu Group. It is noted, however, that all the zircons from the granitic gneisses are highly discordant, and the significance of the upper intercept ages of 1.8 – 1.98 Ga is equivocal. A precise207_Pb

/206_{Pb age of} 174398 Ma by evaporation techniques has been reported for a pegmatite which intrudes the Tangyaun Formation (Gan et al., 1995). This age provides a minimum age for the amphibolites in

SW Zhejiang. Geological and chronological data therefore suggest that the amphibolites from NW Fujian and SW Zhejiang are likely contemporane-ous, although the precise age of the SW Zhejiang amphibolites is not yet known.

3. Analytical methods

Major element oxides were determined using a Rigaku RIX 2000 X-ray fluorescence spectrome-ter (XRF) at the Department of Geology, Na-tional Taiwan University. The analytical un-certainties are generally better than 5% for most elements. The detailed analytical procedures for major element analysis by XRF are described by Lee et al. (1997).

Trace elements were analyzed using a Perkin-Elmer Sciex ELAN 6000 inductively-coupled plasma mass spectrometer (ICP-MS) at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The detailed procedures for trace element analysis by ICP-MS are described by Li (1997b). About 50 mg sample powders were dissolved in Teflon bombs using a HF+HNO3 mixture. An internal standard solution containing the single element Rh was used to monitor drift in mass response during counting. The international standard BCR-1 was chosen to calibrate element concentrations of measured samples. In-run ana-lytical precision for most elements is less than 3%, whilst reproducibility is generally less than 5% (see LB264 in Table 1). Trace elements are also presented for basalt standard BHVO-1, and are generally in good agreement with compiled values (Govindaraju, 1994).

Nd isotopic compositions (unspiked aliquots) were determined using a multi-collector Finnigan MAT-262 mass spectrometer operated in static multi-collector mode at the Research Center of Geoscience, Chinese Academy of Sciences in Bei-jing. Sm and Nd concentrations (spiked with mixed 146_{Nd –}149_{Sm tracers) were measured on a} VG-354 mass spectrometer operated in dynamic multi-collector mode at the Guangzhou Institute of Geochemistry. The 143_Nd_/144_{Nd ratio of the La} Jolla standard and USGS basalt standard BCR-1 measured on this MAT-262 mass spectrometer

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X .-H . Li et al . / Precambrian Research 102 (2000) 251 – 262 254 Table 1

Major and trace element analyses of the amphibolites from the Cathaysia Block, SE China

Standard

Group 2B Group 2A

Group 1

LB258 LB259 LB262 LB263 LB264 LB264b _LB265 _BHVO-1 Sample LG24 LG28 LG29 LG35 LB261

46.18 50.21 47.74 48.46 47.08

49.03 47.23

SiO2 48.47 47.50 47.28 47.87

2.56 2.21 2.28 1.82 2.42 1.76 2.20 2.13 1.50 1.43

TiO2 2.22

13.64 15.92 15.92 15.19 15.84 16.08

Al2O3 16.78 17.90 16.31 16.11 17.02

14.93 12.18 13.88 15.67 13.20

12.70 13.73

14.45 13.37 SFe2O3 13.86 13.05

0.17

0.20 0.19 0.21 0.20 0.26 0.22 0.20 0.18 0.22 0.23

MnO

8.08 5.22 5.28 3.87 6.02

MgO 4.48 5.08 5.71 5.28 5.25 5.93

8.96 10.01 9.59 8.86 9.96

9.47 9.58

7.33

CaO 7.91 7.81 7.94

3.31

4.08 3.90 3.39 2.88 2.67 2.97 3.01 3.40 2.98 2.83

Na2O

1.04

1.06 1.00 1.30 1.36 0.92 0.78 1.04 1.27 1.08 1.53

K2O

0.24 0.21 0.22 0.17 0.13

0.22 0.13

0.33

P2O5 0.31 0.28 0.31

99.08

99.15 99.05 99.02 98.70 98.30 99.48 99.08 99.20 98.01 98.70

Total

57 51 48 37 52

Mgca ₄₄ ₄₈ ₄₉ ₄₈ ₅₀ ₅₁

309 236 295 357 233 223

205 259

184 320

V 143 165 146

689

78.7 36.5 58.1 28.0 379 543 362 165 546 522 539 292

245

38.0 64.0 74.0 70.8 195 224 168 62.2 231 220 236 125

42.9 20.4 43.0 59.0 46.6 45.3

40.4 82.7

59.8 46.5 9.8

Rb 32.5 28.1

315

448 739 535 442 221 257 282 244 235 236 265 410

28.6

26.7 22.3 26.9 29.1 45.6 33.3 38.7 46.7 29.9 30.3 32.4 30.8

158 131 140 147 90.5 89.6

159 90.1

Zr 191 140 179 216 181

18.4 17.4 7.93 9.72 8.30 7.67 3.76 3.85 3.63 20.4

Nb 17.1 15.9 16.4

1.50 1.27 2.50 2.25 1.78 1.80

2.85 2.87

5.82 4.80 0.11

Cs 3.66 1.75

155

469 623 401 380 90.9 100 110 89.6 87.2 83.4 144 144

8.96 11.5 9.94 8.20 4.20 4.14

La 17.6 15.2 17.7 17.1 20.1 4.74 16.0

22.8 26.9 24.0 20.8 12.2 12.1

42.0 13.3

42.7 39.5

Ce 35.8 42.0 40.7

5.28

5.81 4.29 5.56 5.40 3.70 3.49 3.35 3.14 1.95 1.99 2.22 5.65

20.9

25.0 19.8 23.2 23.2 18.9 15.2 16.4 15.1 9.93 10.1 11.0 25.6

5.48 4.34 4.90 4.96 3.41 3.45

4.97 3.62

5.62 5.56 6.48

Sm 6.21 4.90

1.91

2.40 1.89 2.09 1.95 1.81 1.59 1.67 1.83 1.35 1.33 1.54 2.15

5.11

6.73 4.87 5.99 5.51 6.52 5.43 6.31 6.43 4.85 4.68 4.95 6.44

1.20 0.92 1.09 1.26 0.82 0.79

0.89 0.90

Tb 1.02 0.75 0.94 0.91 0.99

7.40

Dy 5.95 4.35 5.34 5.13 4.77 5.79 6.74 7.41 5.43 5.53 5.25 5.42

1.61 1.23 1.46 1.77 1.17 1.13

1.05 1.23

1.16 1.01

Ho 0.84 1.06 1.03

2.94

3.13 2.29 2.81 2.72 4.44 3.47 3.97 4.98 3.31 3.20 3.47 2.70

0.42

0.43 0.31 0.38 0.39 0.68 0.49 0.60 0.75 0.49 0.52 0.51 0.32

4.23 3.22 3.65 4.67 3.12 3.08

2.64 3.29

Yb 2.65 1.98 2.41 2.37 2.06

0.62 0.48 0.53 0.72 0.47 0.48 0.50 0.30

Lu 0.39 0.28 0.35 0.33 0.41

3.86 3.06 3.38 3.81 2.15 2.10

3.88 2.37

4.25 5.40 4.56

Hf 4.54 3.12

1.05

0.99 0.89 0.93 1.15 0.50 0.61 0.47 0.44 0.21 0.22 0.21 1.24

1.16

Th 2.00 1.26 1.70 1.54 2.61 1.36 0.63 0.78 0.29 0.30 0.39 1.16

0.99 0.34 0.10 0.25 0.20 0.19 0.08 0.44

0.62

U 0.67 0.32 0.56 1.88

a_Mg_c₌_{100 Mg}_/_(Mg₊_Fe2+_{), assuming Fe}

2O3/(FeO+Fe2O3)=0.20. b_{Duplicate analysis.}

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Fig. 2. Classification of amphibolites rocks from Cathaysia Block (Winchester and Floyd, 1976, 1977). (a) Zr/TiO2versus Nb/Y; (b) TiO2versus Zr/P2O5; (c) Nb/Y versus Zr/P2O5.

4. Results

4.1. Petrochemical classification

Eleven mafic amphibolite samples from the lower sequence in Zhulu and Tianjingping were analyzed for major and trace elements, and the results are presented in Table 1. According to SiO2 contents, all the mafic amphibolites are basaltic. K, Na and the low-field-strength ele-ments (LFSE: Cs, Rb, Sr, Ba) were likely remobi-lized during amphibolite facies metamorphism (e.g. Humphris and Thompson, 1978), Thus, only immobile elements such as the high-field-strength elements (HFSE: Ti, Zr, Y, Nb, Ta, Hf), Th and the rare earth elements (REE) are used in the following discussion to identify the magmatic affinity of the basaltic protoliths.

On the basis of Nb/Y (an index of alkalinity of volcanic rocks), the two groups of mafic amphibo-lites in the study area are clearly distinguished. Group 1 has high Nb/Y ratios (0.61 – 0.71) and Group 2 has low Nb/Y ratios (0.11 – 0.29), both forming a trend with nearly constant Zr/TiO2 (Fig. 2a). Group 1 samples are transitional to alkali basaltic compositions, whereas Group 2 samples plot in the tholeiite or andesite/basalt fields. In the Zr/P2O5 versus TiO2 and Zr/P2O5 versus Nb/Y (Fig. 2b – c) diagrams of Winchester and Floyd (1976, 1977), Group 1 samples mostly plot in the tholeiite field, but close to alkali basalts. Sample LG28 with the highest Nb/Y ratio of 0.71 plots within the alkali basalt field. Group 2 rocks are clearly tholeiitic in composition.

4.2. Major and trace element geochemistry

Overall, the mafic amphibolites have relatively low and variable MgO (3.87 – 8.08%), Mgc (37 – 57) and CaO/Al2O3 ratios (B0.7), which may have resulted from different degrees of fractiona-tion of the basaltic magma. Most are character-ized by high TiO2content, with Group 1 ranging from 1.82 to 2.56% and Group 2 from 1.43 to 2.42%. Group 1 samples possess somewhat higher Al2O3 (16.1 – 17.9%) and P2O5 (0.22 – 0.33%) than those of Group 2 (Al2O3=13.6 – 16.1%, P2O5= 0.13 – 0.24%), but the negative correlation between during this study were 0.51184298 (2s) on 10

runs and 0.51262797 on 12 runs, respectively. The measured 143_Nd_/144_{Nd ratios are normalized} to 146_Nd_/144_Nd₌_0.7219.

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X.-H.Li et al./Precambrian Research102 (2000) 251 – 262 256

Al2O3 and MgO and absence of Eu anomalies (Fig. 3) indicate that fractional crystallization of plagioclase was not significant. Fractional crystal-lization of clinopyroxene does appear to have been an important process however, as evidenced

by the positive correlation between CaO/Al2O3 and CaO, the negative correlation between CaO/

TiO2and TiO2, and the relatively small variations in MgO/CaO (0.44 – 0.90).

Group 1 and 2 amphibolites have distinct REE abundances and patterns (Table 1 and Fig. 3). Group 1 samples show LREE-enriched patterns with chondrite-normalized LaN=64 – 85 and (La/ Yb)Nof 4.8 – 5.5. Group 2 rocks, however, display nearly flat REE patterns with LaN=18 – 49 and (La/Yb)N=1.0 – 2.5. It is noted that samples LB264 and LB265 have slightly LREE-depleted, convex patterns with (La/Yb)N:1. Group 2 sam-ples can be subdivided into two sub-groups in terms of their REE patterns, i.e. Group 2A for samples LB264 and LB265 with LREE-depletion and Group 2B for the other four samples with nearly flat to slightly LREE-enriched patterns.

Fig. 4a and b show MORB-normalized trace element patterns (Pearce, 1982) for the two groups of amphibolites. Group 1 samples have ‘humped’ patterns characterized by variable en-richment in all the trace elements except Y and Yb. Such patterns, without clear Nb – Ta deple-tion, suggest formation in a within-plate setting with little crustal contamination, and resemble many alkali basalts formed in continental rifts or oceanic islands. Group 2A samples show enrich-ment in LIL eleenrich-ments (from Sr to Th) and ap-proximately the same abundance of HFS and REE elements (from Ta to Yb) as average MORB. Because of the unreliability of the mobile LIL elements in tectonic discrimination, the close similarity between Group 2A samples and average MORB in HFS and REE abundances is taken as evidence that these rocks were most likely derived from a MORB- or MORB-like mantle source. Group 2B samples have similar patterns but slightly higher contents of HFSE and REE rela-tive to those of Group 2A, possibly as a result of smaller degrees of partial melting.

In other discrimination diagrams, Group 1 am-phibolites plot mainly in the field of within-plate basalt, whilst Group 2 samples lie in the MORB field. For example, on the Ti versus V diagram of Shervais (1982), Group 1 rocks have high Ti/V of 8895, with the exception of sample LB261 with slightly lower Ti/V of 53, consistent with a within-Fig. 3. Chondrite-normalized REE diagrams for meta-volcanic

rocks. Normalization values after Sun and McDonough (1989).

Fig. 4. MORB-normalized incompatible element spidergrams of Pearce (1982) for meta-volcanic rocks.

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Fig. 5. (a) Ti – V discrimination diagram of Shervais (1982). (b) Zr versus Zr/Y discrimination diagram of Pearce and Norry (1979). (c) Zr – Nb – Y discrimination diagram of Meschede (1986). (d) Ti – Zr – Y discrimination diagram of Pearce and Cann (1973). Group 1 and 2 amphibolites fall into the field of within-plate basalt and MORB fields, respectively in all the above discrimination diagrams. IAT, island-arc tholeiite; BAT, back-arc basin tholeiite; MORB, mid-ocean ridge basalt; WPB, within-plate basalt; VAB, volcanic-arc basalt; E-MORB, E-type MORB; N-MORB, N-type MORB; CAB, calc-alkaline basalt.

plate setting. Group 2 samples have Ti/V ranging from 33 to 47, falling into the field of MORB (Fig. 5a). In the Zr/Y versus Zr diagram of Pearce and Norry (1979), most Group 2 samples lie in the MORB field, whilst Group 1 samples plot as within-plate basalts (Fig. 5b). In the Ti – Zr – Y diagram of Pearce and Cann (1973), Group 1 and Group 2 samples clearly fall into the within-plate basalt and MORB fields, respectively (Fig. 5d). It is notable that Group 2 samples have relatively low Nb contents (3.6 – 9.7 ppm) and plot in the N-MORB field in the Nb – Zr – Y diagram of Meschede (1986) (Fig. 5c).

4.3. Sm–Nd isotopes

Sm – Nd isotopic data for Group 1 and Group 2 amphibolites are presented in Table 2. Group 1 and Group 2 samples have distinct 147_Sm

/144_Nd ranges of 0.1362 – 0.1425 and 0.1649 – 0.1970,

re-spectively, but all have relatively high oNd(T) values of +5.6 to +8.5. Regression of Sm – Nd isotopic data for all the samples yields an impre-cise age of 17009242 Ma with oNd(T)=6.99

2.3. The scattering of the data (MSWD=27.5), which is far in excess of experimental uncertainty, is mainly attributed to variations in initial 143_Nd_/ 144

Nd. Nevertheless, this Sm – Nd age is in agree-ment within error with the SHRIMP U – Pb zircon age of 1766919 Ma, and indicates that crustal contamination was insignificant in the genesis of the basaltic protoliths. This is consistent with the absence of clear negative Nb – Ta anomalies in all the samples (Fig. 4). On the other hand, the presence of inherited zircons of 2.0 – 2.8 Ga (Li, 1997a) suggests that a crustal component in the protoliths of the amphibolites cannot be com-pletely ruled out. The exceptionally high o_Nd(T) values, up to +8.5, demonstrate that some basaltic protoliths (particularly those of Group 2)

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X.-H.Li et al./Precambrian Research102 (2000) 251 – 262 258

Table 2

Sm–Nd isotopic data for amphibolites from the Cathaysia Block, SE China

Nd (ppm) 147_Sm_/144_Nd

Sample _{Sm (ppm)} 143_Nd_/144_Nda _o

Nd(T)b

Group1

24.60

LG24 5.78 0.1425 0.51239597 7.7

LG28 4.84 21.08 0.1388 0.512395910 8.5

22.99 0.1407 0.51239098

LG29 5.35 8.0

22.96 0.1392

5.29 0.51235497

LG35 7.6

LB261 4.77 21.17 0.1361 0.51223098 5.9

Group2

19.45

LB258 6.08 0.1892 0.512860914 6.1

15.50 0.1649

4.18 0.512570915

LB259 6.0

4.65

LB262 16.50 0.1702 0.51276199 8.5

4.64

LB263 14.69 0.1908 0.512950915 7.5

10.32 0.1970

3.36 0.51292999

LB264 5.6

3.38

LB264c _10.31 _0.1983 _0.512923₉₁₂ _5.4

11.25

LB265 3.56 0.1910 0.51291999 6.8

USGS standard

28.9 0.1364 0.51264597

BCR-1 6.52

a 143_Nd_/144_{Nd ratios have been adjusted relative to the La Jolla standard}₌_0.511860.

b_T₌_{1766 Ma obtained for sample LG24 by SHRIMP U–Pb zircon analysis (Li, 1997a), representing the crystallization age of} the amphibolites.

c_{Duplicate analysis.}

were derived from an extremely depleted mantle source in the late Paleoproterozoic.

5. Discussion

5.1. Magma genesis and tectonic setting

Given the transitional to alkaline nature of some amphibolites, their close association with granitic gneisses and the presence of inherited zircons, it is of interest to assess the degree of crustal contamination, if any, of the basalt pro-toliths. Nd isotopes and some trace element ratios such as Nb/Th and Nb/La are very sensitive to even small degrees of crustal contamination. MORB and oceanic island tholeiites commonly have Nb/Th of 14 – 20 (Sun and McDonough, 1989), and the amphibolites from SE China have somewhat lower Nb/Th ratios of 7 – 13. Further-more, E-type MORB and ocean island tholeiites usually have Nb/La ratios of considerably greater than 1.0 (commonly 1.3) (Sun and Mc-Donough, 1989), whereas comparable Group 1 amphibolites have slightly lower Nb/La ratios of

0.87 – 1.07. Such features could suggest a small crustal component in the original basaltic mag-mas. In theoNd(T) versus Nb/Th plot (Fig. 6), all samples but one (LB264) display a positive corre-lation, suggesting a possible mixing of depleted mantle-derived magma and a crustal component. Sample LB264 deviates significantly from the lin-ear trend, but duplicate analysis (Tables 1 and 2)

Fig. 6. Positive correlation between Nb/Th ratios and initial oNd values of the amphibolites. Symbols as in Fig. 2.

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confirm the analytical data, which yield the rela-tively low oNd(T) value of +5.4+5.6 and high Nb/Th of 13. The reason for such a devia-tion is not known. A few samples with high

oNd(T) values (+8+8.5) have high Nb/Th of

13, close to that of a depleted mantle source, whilst samples with low oNd(T) values of +6 have low Nb/Th ratios of 7. Assuming that the mantle source of the basaltic magmas had normal Nb/Th ratios of 14 – 20, o_{Nd(T) values up to} +9+10.5 can be estimated in terms of the linear trend. If the crustal contaminant possessed Nb/Th ratio similar to that of average crust (Nb/

Th:3, Taylor and McLennan, 1985), its o_Nd(T)

value should be approximately+4.5, correspond-ing to a TDMage of 1.9 Ga consistent with that of crustal rocks in the Cathaysia Block (Li et al., 1992; Chen and Jahn, 1998). It is noted that these estimated oNd(T) values of +9+10.5 for the depleted mantle source would be the highest ever recorded for depleted mantle sources in the Late Paleoproterozoic. Even the oNd(T) values of

+8.5 for the least-contaminated samples with Nb/Th:13 are exceptional but not too dissimilar to those meta-tholeiites (o_{Nd(T) values up to} +8.2, T=1767 Ma) from the Harts Range, cen-tral Auscen-tralia (Sivell and McCulloch, 1991).

Geochemical and Nd isotopic signatures clearly indicate that Group 1 and 2 amphibolites are similar to present-day within-plate basalts and MORB, respectively. The close association of the transitional to alkali and tholeiitic amphibolites and the absence of arc-type volcanism indicates that these volcanic rocks were most likely formed in an environment similar to a modern ensialic rift developing into a proto-oceanic basin such as the Tadjoura Gulf (Barrat et al., 1990, 1993). Basalts erupted in Tadjoura prior to opening of the gulf are dominantly of transitional to alkali type, whilst tholeiitic basalts (E-type, T-type and N-type MORBs) form the new oceanic crust.

5.2. Implications for the extremely depleted mantle source in Paleoproterozoic

The exceptionally high oNd(T) values (up to

+8.5) of Group 2 amphibolites in this study indicate that an extremely depleted mantle source

could have existed in Cathaysia during the Pale-oproterozoic. It should be noted however, that the Sm – Nd isotopic system could behave as an open-system during metamorphism. For example, com-bined Hf and Nd isotopic studies indicate that the highly positive oNd(T) values previously sug-gested for 4.0 – 3.6 Ga old rocks have been pro-duced, at least in part, by whole-rock geochemical disturbance (Vervoort et al., 1996). Thus, it is necessary to verify if the Sm – Nd system of the amphibolites has been modified by metamor-phism. As mentioned above, an imprecise Sm – Nd isochron age of 17009242 Ma is in agreement within error with the SHRIMP U-Pb zircon age of 1766919 Ma, suggesting that neither crustal contamination in the genesis of the basaltic pro-toliths nor modification of the Sm – Nd system by subsequent metamorphism were significant, al-though positive correlation ofoNd(T) and Nb/Th reveals a small crustal component in some of the amphibolites (Fig. 6). Note that oNd(T) and Nb/

Th are two independent parameters related to the involvement of crustal components and that their co-variation suggests that Nb, Th and REEs were immobile during subsequent metamorphism. Thus, we believe that the very higho_{Nd(T) values}

are a property of basaltic precursors to the am-phibolites, rather than the result by subsequent geochemical disturbances. Sample LB264 is prob-ably the only exception in terms of its significant deviation from the linear trend between oNd(T) and Nb/Th (Fig. 6).

It is widely believed that that extraction of juvenile continental crust from the mantle over the past four Ga has led to a progressive deple-tion of incompatible elements in the upper mantle. The evolution of the depleted mantle dur-ing geological history however, is still not well known. Linear growth models for depleted mantle are generally assumed, with Nd isotopes evolving from a chondritic source (o_Nd(T)₌_{0) since the}

Early Archean, to the present-day MORB source (oNd(T):+10+12) (e.g. Patchett and Arndt, 1986; DePaolo et al., 1991). This global depleted mantle source has been widely used for calcula-tion of Nd model ages for crustal rocks. Fig. 7 shows Nd isotopic results for the amphibolites in this study and other mafic rocks of 1.5 – 2.0 Ga. It

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X.-H.Li et al./Precambrian Research102 (2000) 251 – 262 260

Fig. 7. oNd versus age plot showing Nd isotopic results of amphibolites in Cathaysia Block and comparison with mafic rocks of 1.5 – 2.0 Ga in the world. SN, southern Norway (Menuge, 1985); RM, Rocky Mountains (Nelson and De-Paolo, 1984); OM, Østfold-Marstrand belt (Ahall and Daly, 1989); AI, Arunta Inlier (Zhao, 1994); HRMC, Harts Range meta-igneous complex (Sivell and McCulloch, 1991); FRC, Front Range, Colorado (DePaolo, 1981); SGr, southern Greenland (Patchett and Arndt, 1986); NC, northern Canada (Patchett and Arndt, 1986; Chauvel et al., 1987; Lucas et al., 1992); SW, Sweden (Skiold and Cliff, 1984; Claesson, 1987; Valbracht, 1991); SES, southeastern Saskatchewan (Patchett and Arndt, 1986); FIN, Finland (Patchett and Kouvo, 1986; Huhma et al., 1990); FG, French Guiana (Gruau et al., 1985). Depleted mantle evolutionary trajectory after Patchett and Arndt (1986).

HRMC mantle source is thus considered as a part of a ‘fossil’ mantle wedge which had been highly depleted during a major episode of continental growth in the late Archean (2.7 Ga) (Sivell and McCulloch, 1991). However, the Cathaysia am-phibolites in this study were most likely formed as basaltic volcanics in a rifting to proto-oceanic environment, suggesting that an extremely de-pleted mantle source in Cathaysia, which would also have formed in the Late Archean, must have been isolated from the bulk of the convecting asthenosphere and preserved until 1.8 Ga.

6. Conclusions

Two groups of Late Paleoproterozoic amphibo-lites are recognized from the Cathaysia Block, SE China, based on geochemical signatures. Group 1 rocks with transitional to alkali basaltic composi-tion are similar to within-plate basalts, whilst Group 2 rocks, with tholeiitic composition, resem-ble MORB. They were most likely formed in an environment similar to the modern ensialic rift developing into a proto-oceanic basin (e.g. the Gulf of Tadjoura). The exceptionally higho_Nd(T)

values up to +8.5 for some of these meta-basalts indicate an extremely depleted time-integrated mantle source in Cathaysia during the Paleoproterozioc.

Acknowledgements

We thank Shen-su Sun for his constructive suggestions during the preparation of this paper. XHL thanks the Croucher Foundation of Hong Kong for a 6 month visiting fellowship to the Department of Earth Sciences, The University of Hong Kong. The paper has benefited from the helpful and constructive comments and some syn-tax polish of B.M. Jahn and an anonymous re-viewer. This work was supported by the National Natural Science Foundation of China (Grant No. 49725309) and the Chinese Academy of Sciences (Grant No. 981105) to XHL. This is a contribu-tion to IGCP 440.

can be seen that most data fall around the de-pleted mantle evolutionary trend, with two excep-tionally highoNd(T) values from the Harts Range meta-igneous complex (HRMC) of central Aus-tralia (Sivell and McCulloch, 1991) and the Cathaysia Block of SE China. It appears that the mantle must be heterogeneous in both time and space, and therefore no single curve will be uni-versally applicable. It is noted that the amphibo-lites in Cathaysia are strikingly similar in age and

o_{Nd(T) values to the HRMC meta-tholeiites in}

central Australia, which are of 1767 Ma and have

o_{Nd(T) values up to} ₊_{8.2 (Sivell and McCulloch,}

1991), although the origin and tectonic settings of these two mafic associations are clearly different. The HRMC meta-tholeiites, with geochemical features similar to back-arc basin basalts, were generated in a subduction-related setting. The

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plate setting. Group 2 samples have Ti/V ranging

from 33 to 47, falling into the field of MORB

(Fig. 5a). In the Zr/Y versus Zr diagram of Pearce

and Norry (1979), most Group 2 samples lie in

the MORB field, whilst Group 1 samples plot as

within-plate basalts (Fig. 5b). In the Ti – Zr – Y

diagram of Pearce and Cann (1973), Group 1 and

Group 2 samples clearly fall into the within-plate

basalt and MORB fields, respectively (Fig. 5d). It

is notable that Group 2 samples have relatively

low Nb contents (3.6 – 9.7 ppm) and plot in the

N-MORB field in the Nb – Zr – Y diagram of

Meschede (1986) (Fig. 5c).

4 .

3 .

Sm–

Nd isotopes

Sm – Nd isotopic data for Group 1 and Group 2

amphibolites are presented in Table 2. Group 1

and Group 2 samples have distinct

147

_Sm/

144

_Nd

ranges of 0.1362 – 0.1425 and 0.1649 – 0.1970,

re-spectively, but all have relatively high

o

Nd(T)

values of

+

5.6 to

+8.5. Regression of Sm – Nd

isotopic data for all the samples yields an

impre-cise age of 1700

9 242 Ma with

o

Nd(T)

=6.9

9 2.3. The scattering of the data (MSWD

=

27.5),

which is far in excess of experimental uncertainty,

is mainly attributed to variations in initial

143

_Nd/

144

Nd. Nevertheless, this Sm – Nd age is in

agree-ment within error with the SHRIMP U – Pb zircon

age of 1766

919 Ma, and indicates that crustal

contamination was insignificant in the genesis of

the basaltic protoliths. This is consistent with the

absence of clear negative Nb – Ta anomalies in all

the samples (Fig. 4). On the other hand, the

presence of inherited zircons of 2.0 – 2.8 Ga (Li,

1997a) suggests that a crustal component in the

protoliths of the amphibolites cannot be

com-pletely ruled out. The exceptionally high

o

_Nd

(T)

values, up to

+8.5, demonstrate that some

basaltic protoliths (particularly those of Group 2)

(2)

Table 2

Sm–Nd isotopic data for amphibolites from the Cathaysia Block, SE China

Nd (ppm) 147_Sm/144_Nd

Sample _{Sm (ppm)} 143_Nd/144_Nda _o

Nd(T)b

Group1

24.60

LG24 5.78 0.1425 0.51239597 7.7

LG28 4.84 21.08 0.1388 0.512395910 8.5

22.99 0.1407 0.51239098

LG29 5.35 8.0

22.96 0.1392

5.29 0.51235497

LG35 7.6

LB261 4.77 21.17 0.1361 0.51223098 5.9

Group2

19.45

LB258 6.08 0.1892 0.512860914 6.1

15.50 0.1649

4.18 0.512570915

LB259 6.0

4.65

LB262 16.50 0.1702 0.51276199 8.5

4.64

LB263 14.69 0.1908 0.512950915 7.5

10.32 0.1970

3.36 0.51292999

LB264 5.6

3.38

LB264c _10.31 _0.1983 _0.512923₉₁₂ _5.4

11.25

LB265 3.56 0.1910 0.51291999 6.8

USGS standard

28.9 0.1364 0.51264597

BCR-1 6.52

a 143_Nd/144_{Nd ratios have been adjusted relative to the La Jolla standard=0.511860.}

b_{T=1766 Ma obtained for sample LG24 by SHRIMP U–Pb zircon analysis (Li, 1997a), representing the crystallization age of}

the amphibolites.

c_{Duplicate analysis.}

were derived from an extremely depleted mantle

source in the late Paleoproterozoic.

5. Discussion

5 .

1 .

Magma genesis and tectonic setting

Given the transitional to alkaline nature of

some amphibolites, their close association with

granitic gneisses and the presence of inherited

zircons, it is of interest to assess the degree of

crustal contamination, if any, of the basalt

pro-toliths. Nd isotopes and some trace element ratios

such as Nb/Th and Nb/La are very sensitive to

even small degrees of crustal contamination.

MORB and oceanic island tholeiites commonly

have Nb/Th of 14 – 20 (Sun and McDonough,

1989), and the amphibolites from SE China have

somewhat lower Nb/Th ratios of 7 – 13.

Further-more, E-type MORB and ocean island tholeiites

usually have Nb/La ratios of considerably greater

than 1.0 (commonly

1.3) (Sun and

Mc-Donough, 1989), whereas comparable Group 1

amphibolites have slightly lower Nb/La ratios of

0.87 – 1.07. Such features could suggest a small

crustal component in the original basaltic

mag-mas. In the

o

Nd(T) versus Nb/Th plot (Fig. 6), all

samples but one (LB264) display a positive

corre-lation, suggesting a possible mixing of depleted

mantle-derived magma and a crustal component.

Sample LB264 deviates significantly from the

lin-ear trend, but duplicate analysis (Tables 1 and 2)

Fig. 6. Positive correlation between Nb/Th ratios and initial oNd values of the amphibolites. Symbols as in Fig. 2.

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confirm the analytical data, which yield the

rela-tively low

o

Nd(T) value of

+

5.4 +5.6 and

high Nb/Th of 13. The reason for such a

devia-tion is not known. A few samples with high

o

Nd(T) values (

+

8 +8.5) have high Nb/Th of

13, close to that of a depleted mantle source,

whilst samples with low

o

Nd(T) values of

+6

have low Nb/Th ratios of

7. Assuming that the

mantle source of the basaltic magmas had normal

Nb/Th ratios of 14 – 20,

o

_{Nd(T) values up to}

+9

+

10.5 can be estimated in terms of the

linear trend. If the crustal contaminant possessed

Nb/Th ratio similar to that of average crust (Nb/

Th

:

3, Taylor and McLennan, 1985), its

o

_Nd(T)

value should be approximately

+

4.5,

correspond-ing to a T

age of

1.9 Ga consistent with that

of crustal rocks in the Cathaysia Block (Li et al.,

1992; Chen and Jahn, 1998). It is noted that these

estimated

o

Nd(T) values of

+9

+10.5 for the

depleted mantle source would be the highest ever

recorded for depleted mantle sources in the Late

Paleoproterozoic. Even the

o

Nd(T) values of

+8.5 for the least-contaminated samples with

Nb/Th

:

13 are exceptional but not too dissimilar

to those meta-tholeiites (

o

_{Nd(T) values up to}

+8.2, T=

1767 Ma) from the Harts Range,

cen-tral Auscen-tralia (Sivell and McCulloch, 1991).

Geochemical and Nd isotopic signatures clearly

indicate that Group 1 and 2 amphibolites are

similar to present-day within-plate basalts and

MORB, respectively. The close association of the

transitional to alkali and tholeiitic amphibolites

and the absence of arc-type volcanism indicates

that these volcanic rocks were most likely formed

in an environment similar to a modern ensialic rift

developing into a proto-oceanic basin such as the

Tadjoura Gulf (Barrat et al., 1990, 1993). Basalts

erupted in Tadjoura prior to opening of the gulf

are dominantly of transitional to alkali type,

whilst tholeiitic basalts (E-type, T-type and

N-type MORBs) form the new oceanic crust.

5 .

2 .

Implications for the extremely depleted

mantle source in Paleoproterozoic

The exceptionally high

o

Nd(T) values (up to

+8.5) of Group 2 amphibolites in this study

indicate that an extremely depleted mantle source

could have existed in Cathaysia during the

Pale-oproterozoic. It should be noted however, that the

Sm – Nd isotopic system could behave as an

open-system during metamorphism. For example,

com-bined Hf and Nd isotopic studies indicate that the

highly positive

o

Nd(T) values previously

sug-gested for 4.0 – 3.6 Ga old rocks have been

pro-duced, at least in part, by whole-rock geochemical

disturbance (Vervoort et al., 1996). Thus, it is

necessary to verify if the Sm – Nd system of the

amphibolites has been modified by

metamor-phism. As mentioned above, an imprecise Sm – Nd

isochron age of 1700

9 242 Ma is in agreement

within error with the SHRIMP U-Pb zircon age

of 1766

919 Ma, suggesting that neither crustal

contamination in the genesis of the basaltic

pro-toliths nor modification of the Sm – Nd system by

subsequent metamorphism were significant,

al-though positive correlation of

o

Nd(T) and Nb/Th

reveals a small crustal component in some of the

amphibolites (Fig. 6). Note that

o

Nd(T) and Nb/

Th are two independent parameters related to the

involvement of crustal components and that their

co-variation suggests that Nb, Th and REEs were

immobile

during

subsequent

metamorphism.

Thus, we believe that the very high

o

_{Nd(T) values}

are a property of basaltic precursors to the

am-phibolites, rather than the result by subsequent

geochemical disturbances. Sample LB264 is

prob-ably the only exception in terms of its significant

deviation from the linear trend between

o

Nd(T)

and Nb/Th (Fig. 6).

It is widely believed that that extraction of

juvenile continental crust from the mantle over

the past four Ga has led to a progressive

deple-tion of incompatible elements in the upper

mantle. The evolution of the depleted mantle

dur-ing geological history however, is still not well

known. Linear growth models for depleted mantle

are generally assumed, with Nd isotopes evolving

from a chondritic source (

o

_Nd(T)

₌

_{0) since the}

Early Archean, to the present-day MORB source

(

o

Nd(T)

:

+10

+12) (e.g. Patchett and Arndt,

1986; DePaolo et al., 1991). This global depleted

mantle source has been widely used for

calcula-tion of Nd model ages for crustal rocks. Fig. 7

shows Nd isotopic results for the amphibolites in

this study and other mafic rocks of 1.5 – 2.0 Ga. It

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HRMC mantle source is thus considered as a part

of a ‘fossil’ mantle wedge which had been highly

depleted during a major episode of continental

growth in the late Archean (

2.7 Ga) (Sivell and

McCulloch, 1991). However, the Cathaysia

am-phibolites in this study were most likely formed as

basaltic volcanics in a rifting to proto-oceanic

environment, suggesting that an extremely

de-pleted mantle source in Cathaysia, which would

also have formed in the Late Archean, must have

been isolated from the bulk of the convecting

asthenosphere and preserved until

1.8 Ga.

6. Conclusions

Two groups of Late Paleoproterozoic

amphibo-lites are recognized from the Cathaysia Block, SE

China, based on geochemical signatures. Group 1

rocks with transitional to alkali basaltic

composi-tion are similar to within-plate basalts, whilst

Group 2 rocks, with tholeiitic composition,

resem-ble MORB. They were most likely formed in an

environment similar to the modern ensialic rift

developing into a proto-oceanic basin (e.g. the

Gulf of Tadjoura). The exceptionally high

o

_Nd(T)

values up to

+8.5 for some of these meta-basalts

indicate an extremely depleted time-integrated

mantle

source

in

Cathaysia

during

the

Paleoproterozioc.

Acknowledgements

We thank Shen-su Sun for his constructive

suggestions during the preparation of this paper.

XHL thanks the Croucher Foundation of Hong

Kong for a 6 month visiting fellowship to the

Department of Earth Sciences, The University of

Hong Kong. The paper has benefited from the

helpful and constructive comments and some

syn-tax polish of B.M. Jahn and an anonymous

re-viewer. This work was supported by the National

Natural Science Foundation of China (Grant No.

49725309) and the Chinese Academy of Sciences

(Grant No. 981105) to XHL. This is a

contribu-tion to IGCP 440.

can be seen that most data fall around the

de-pleted mantle evolutionary trend, with two

excep-tionally high

o

Nd(T) values from the Harts Range

meta-igneous complex (HRMC) of central

Aus-tralia (Sivell and McCulloch, 1991) and the

Cathaysia Block of SE China. It appears that the

mantle must be heterogeneous in both time and

space, and therefore no single curve will be

uni-versally applicable. It is noted that the

amphibo-lites in Cathaysia are strikingly similar in age and

o

_{Nd(T) values to the HRMC meta-tholeiites in}

central Australia, which are of 1767 Ma and have

o

_{Nd(T) values up to}

₊

_{8.2 (Sivell and McCulloch,}

1991), although the origin and tectonic settings of

these two mafic associations are clearly different.

The HRMC meta-tholeiites, with geochemical

features similar to back-arc basin basalts, were

generated in a subduction-related setting. The

(5)

References