Table 2 Sm–Nd isotopic data for amphibolites from the Cathaysia Block, SE China
Nd ppm
147
Sm
144
Nd Sample
143
Nd
144
Nd
a
Sm ppm o
Nd
T
b
Group
1
24.60 LG24
0.1425 5.78
0.512395 9 7 7.7
LG28 4.84
21.08 0.1388
0.512395 9 10 8.5
22.99 0.1407
0.512390 9 8 LG29
8.0 5.35
22.96 0.1392
5.29 0.512354 9 7
LG35 7.6
LB261 21.17
4.77 0.1361
0.512230 9 8 5.9
Group
2
19.45 LB258
0.1892 6.08
0.512860 9 14 6.1
15.50 0.1649
4.18 0.512570 9 15
LB259 6.0
4.65 LB262
16.50 0.1702
0.512761 9 9 8.5
4.64 LB263
14.69 0.1908
0.512950 9 15 7.5
10.32 0.1970
3.36 0.512929 9 9
LB264 5.6
3.38 LB264
c
10.31 0.1983
0.512923 9 12 5.4
11.25 LB265
0.1910 3.56
0.512919 9 9 6.8
USGS standard 28.9
0.1364 0.512645 9 7
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 NbTh and NbLa are very sensitive to
even small degrees of crustal contamination. MORB and oceanic island tholeiites commonly
have NbTh of 14 – 20 Sun and McDonough, 1989, and the amphibolites from SE China have
somewhat lower NbTh ratios of 7 – 13. Further- more, E-type MORB and ocean island tholeiites
usually have NbLa ratios of considerably greater than 1.0 commonly
1.3 Sun and Mc- Donough, 1989, whereas comparable Group 1
amphibolites have slightly lower NbLa ratios of 0.87 – 1.07. Such features could suggest a small
crustal component in the original basaltic mag- mas. In the oNdT versus NbTh 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 NbTh ratios and initial o
Nd values of the amphibolites. Symbols as in Fig. 2.
confirm the analytical data, which yield the rela- tively low oNdT value of + 5.4 + 5.6 and
high NbTh of 13. The reason for such a devia- tion is not known. A few samples with high
o
NdT values + 8 + 8.5 have high NbTh of 13, close to that of a depleted mantle source,
whilst samples with low oNdT values of + 6 have low NbTh ratios of 7. Assuming that the
mantle source of the basaltic magmas had normal NbTh ratios of 14 – 20, oNdT values up to
+
9 + 10.5 can be estimated in terms of the linear trend. If the crustal contaminant possessed
NbTh ratio similar to that of average crust Nb Th : 3, Taylor and McLennan, 1985, its oNdT
value should be approximately + 4.5, correspond- ing to a T
DM
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 oNdT 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 oNdT values of +
8.5 for the least-contaminated samples with NbTh : 13 are exceptional but not too dissimilar
to those meta-tholeiites oNdT values up to +
8.2, T = 1767 Ma from the Harts Range, cen- tral Australia 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 oNdT 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 oNdT 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 9 19 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 oNdT and NbTh reveals a small crustal component in some of the
amphibolites Fig. 6. Note that oNdT 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 oNdT 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 oNdT and NbTh 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 oNdT = 0 since the Early Archean, to the present-day MORB source
oNdT : + 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
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
