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
b, G.-J. Wei
a, Y. Liu
a, C.-Y. Lee
c, J. Malpas
baGuangzhou Institute of Geochemistry,Chinese Academy of Sciences,PO Box1131,Guangzhou510640,Guangdong,PR China bDepartment of Earth Sciences,The Uni
6ersity of Hong Kong,Pokfulam Road,Hong Kong cDepartment 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 oNd(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
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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)
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
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 precise207Pb
/206Pb 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).
X
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
Cr
245
38.0 64.0 74.0 70.8 195 224 168 62.2 231 220 236 125
Ni
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
Sr
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
Y
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
Ba
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
Pr
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
Nd
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
Eu
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
Gd
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
Er
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
Tm
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
Ta
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
aMgc=100 Mg/(Mg+Fe2+), assuming Fe
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
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. 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 147Sm
/144Nd 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 143Nd/ 144
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) 147Sm/144Nd
Sample Sm (ppm) 143Nd/144Nda 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 143Nd/144Nd ratios have been adjusted relative to the La Jolla standard=0.511860.
bT=1766 Ma obtained for sample LG24 by SHRIMP U–Pb zircon analysis (Li, 1997a), representing the crystallization age of the amphibolites.
cDuplicate 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)
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, oNd(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 oNd(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 (oNd(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 highoNd(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 (oNd(T)=0) since the
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 highoNd(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
oNd(T) values to the HRMC meta-tholeiites in
central Australia, which are of 1767 Ma and have
oNd(T) values up to +8.2 (Sivell and McCulloch,
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