Fig. 2. Classification of amphibolites rocks from Cathaysia Block Winchester and Floyd, 1976, 1977. a ZrTiO
2
versus NbY; b TiO
2
versus ZrP
2
O
5
; c NbY versus ZrP
2
O
5
.
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 SiO
2
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 NbY 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 NbY ratios 0.61 – 0.71 and
Group 2 has low NbY ratios 0.11 – 0.29, both forming a trend with nearly constant ZrTiO
2
Fig. 2a. Group 1 samples are transitional to alkali basaltic compositions, whereas Group 2
samples plot in the tholeiite or andesitebasalt fields. In the ZrP
2
O
5
versus TiO
2
and ZrP
2
O
5
versus NbY 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 NbY 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, Mg c 37 – 57 and CaOAl
2
O
3
ratios B 0.7, which may have resulted from different degrees of fractiona-
tion of the basaltic magma. Most are character- ized by high TiO
2
content, 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 Al
2
O
3
16.1 – 17.9 and P
2
O
5
0.22 – 0.33 than those of Group 2 Al
2
O
3
= 13.6 – 16.1, P
2
O
5
= 0.13 – 0.24, but the negative correlation between
during this study were 0.511842 9 8 2s on 10 runs and 0.512627 9 7 on 12 runs, respectively.
The measured
143
Nd
144
Nd ratios are normalized to
146
Nd
144
Nd = 0.7219.
Al
2
O
3
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 CaOAl
2
O
3
and CaO, the negative correlation between CaO TiO
2
and TiO
2
, and the relatively small variations in MgOCaO 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 La
N
= 64 – 85 and La
Yb
N
of 4.8 – 5.5. Group 2 rocks, however, display nearly flat REE patterns with La
N
= 18 – 49 and
LaYb
N
= 1.0 – 2.5. It is noted that samples
LB264 and LB265 have slightly LREE-depleted, convex patterns with LaYb
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 elements 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 TiV of 88 9 5, with the exception of sample LB261 with
slightly lower TiV 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.
Fig. 5. a Ti – V discrimination diagram of Shervais 1982. b Zr versus ZrY 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 TiV ranging from 33 to 47, falling into the field of MORB
Fig. 5a. In the ZrY 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 oNdT 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 oNdT = 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 9 19 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
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
