Vol. 11 No .2 CHINESE JOURNAL OF GEOCHEMISTRY 1992

REE Geochemical Evolution and Its Significance of Early Precambrian Metamorphic

Terrain, Wuyang, Henan

C H E N YANJING ( ~ J : : l ~ )

(Institute of Geochernistry, Academia Sinica, Cru~ang )

F U SHIGU (~:~7,~-) , Hu SHouxa (~]~J~) (Department of Earth Sciences, Nanjing University )

AND ZHANG YUANYOU ( ~ K ~ ) (Geological Team 4, Henan Branch of China Non-ferrous Metal Industry Corapany )

Abstract

The supracrustal rocks of the Wuyang metamorphic terrain are divided into/.he Zhao 'anzhuang, Tieshanmiao and Yangshuwan Formations. These three Formations were dated at 3000-2550 Ma , 2550-2300 Ma and 2300-2200 M a , respectively. EREE and La /Yb)n of the Zhao'anzhuang For- mation volcanic rocks are obviously higher than those of the Tieshanmiao Formation equivalents, sug- gesting a sedimentary gap (2550 Ma boundary) between these two. formations. The Zhao'anzhuang Formation is older than the Tieshanmiao Formation. The sediments of these two Formations show no obvious differences in REE and are generally characterized by low EREE and positive Eu anomalies. On the contrary, the sediments of the Yangshuwan Formation are characterized by high ~REE and negative Eu anomalies. Detailed discussions demonstrate that the Yangshuwan Formation was depos- ited in an oxidizing environment whereas the other two formations were formed in a reducing environ- ment .At the end of the evolution of the Tieshanmiao Formation about 2300 Ma a g o , the sedimentary environment was transformed from reducing to oxidizing. On the basis of the SHAB (sott /hard acid and base) theory, an oxidation-reduction model for sedimentary REE evolu- tion has been established. It is proposed that the mantle tends to become gradually depleted in REE, especially in LREE, and the indices ~REE and L a / Y b ) n of mantle-derived volcanic rocks al- so tend to become lower and lower.

Geological Setting

The Wuyang Early Precambrian metamorphic terrain is located in the Huaxiong block at the southern margin of the North China craton. In the 1950's, the Tieshanmiao-type Fe deposits were found in this terrain by Chu Xinchun et al. In 1960 a larger Fe d e p o s i t - the Zhao'anzhuang deposit was found there by Zhang Yuanyou et al. Since then, much work has been done on the ge- ology of those Fe deposits. The metamorphic strata of the terrain are subdivided from the bottom upwards into the Zhao'anzhuang Formation, the Tieshanmiao Formation and the Yangshuwan Formation. These three formations were dated at Archaean. The correlation between the stratigraphic division and the stratigraphic sequence has further been confirmed. It follows that the ages of these three forma- tions are 3000-2550 M a , 2550-2300 Ma and 2300-2200 Ma respectively, and are petrographically distinguished as the primary greenstone belt, the secondary greenstone belt and the khondalite series (Fig. 1 ) ( Chela et a l . , 1988, 1989, 1991; Hu , 1988).

134 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y Vol. 11

2200Ma . . . . , . . . . , , • " '1

i'. :. "Jd : : :: .i | O I • e O 0 |

I - - " " - • - - " - - I Ymtsctmmm F r . ¢ . . _ . . . . .

- - ~ *~o0~ N01

2550 b l a _ ~ *~0( ]2 N03 • o~N) 3 N012

Tieshanmia. Ft . [ v v v v v W "~031 N011 V V V V ¥

I V V V V :

2300 b l a _ _ ~ ~ o 0 1 4 N06

~ o ) 0 2 4 NO10

Zhaoanzhu.. ~ ' A ~ O ~ 0 2 7 N08

Fig.1. The stratigraphic sequence of the Wuyang metamorphic terrain and sample localities.

1. Banded iron formation (BIF); 2. volcanic rock; 3. fine elastic rock; 4. coarse elastic rock ; 5. graphite-bearing elastic rock ; 6. biocaleilutite ; 7. Ai-rich sediment ; 8. dolomitite and dolomitic limestone.

No. 7, Wo15" quartz 97% metamorphic elastic rock.

Samples and Analysis

Listed in Table 1 are the contents of major and rare-earth elements in the supracrustal rocks of different horizons in the Wuyang metamorphic terrain. The major element data were provided by Zhang Yuanyou. The REE contents were analyzed with the ICP technique at the geological laboratory of Hubei Provincial Geological Bureau. Most of the samples were collected from drill cores, with no sign of being weathered. In the following are the petrography and mineralogy data for the samples :

No. 1, Wo26 : diopside 10% ; carbonate minerals 90% ; chemical sediment.

No. 2 , Wo30: diopside 40%; quartz 40% ; magnetite 20% ; chemical sediment.

No. 3 , Wo03: diopside 57%; quartz 20% ; magnetite 20% ; hematite 3% ; chemical sediment.

N o . 4 , Wo33 : graphite 6% ; carbonate minerals 10% ; quartz 20% ; feldspar 20% ; others (actinolite, epidote, chlorite, biotite) 44% ; biocalcilutite.

N o . 5 , Wo34: quartz 40% ; feldspar 20*/0 ; calcite 5% ; biotite 35% ; calcareous shale.

No. 6 , Wo14: quartz 90% ; feldspar 10% ; clastic rock.

; feldspar 2 % ; muscovite 1%; migmatized

N o . 8 , Wo27: quartz 20%; andesine 20%; garnet 30%; amphibole 30%; pyroxene 10% ; komatiite.

N o . 9 , Wo21 : magnetite 45% ; serpentine 48% ; calcite 6% ; apatite 1% ; al- tered ultrabasic volcanic rock.

No. 10, Wo24; magnetite 5 % ; 10% ; altered ultrabasic volcanic rock.

No. 11, Wo31 : magnetite 13% ultrabasic volcanic rock.

No. 12, Wo03 : magnetite 5% ; pyroxene 95% ; ultrabasic volcanic rock.

serpentine 50% ; pyroxene 35% ; olivine

serpentine 85% ; phlogopite 2% ; altered

Environments of the Sediments in Special Reference to Their REE Geochemisti'y

From Tables 1 - 2 and Figs. 1 - 2 it is shown that all the samples, except No. 7 which was migmatized, share the following two characteristic features: (1) The index E u / E u * for the sediments younger than 2300 M a ( N o s . 4 - 5 ) is

No.2 CHINESE JOURNAL O F GEOCHEMISTRY 135

<0 .80 , older than 2300 M a (No. 1, No. 2 , No. 6 ) > 1.00 and at about 2300 Ma (No .3 ) 0 . 8 0 - 1 . 0 0 . This trend is consistent with the variation of E u / E u * in the 31 samples from the Early Precambrian sediments at the southern margin of the North China craton (Chen et a l . , 1990). (2 ) The ZREE of the sediments younger than 2300 Ma is obviously higher than that of the sediments older than 2300 M a .

A 0

\

_~ 2

m ¢1)

.~ 1

0

( a ) (a)

+ ' ~ + - - 4 - - +---+ No. 4

No. 6

i I i I i

" t ~ + ~ ' 4 " ' ~ ' + " ~ " ~ ' t ' ~ ' t ~ ' t ~ ÷ No . 4

~ ~ ~ ~ j ~ q ~ ~ ~ No .I No.3 No. 2

La Ce Pr d S m E u Gd b Dy Ho E r T m Y b Lu

Fig.2. Chondrite-normalized REE diagram of sediments in the Wuyang metamorphic terrain. (a) : Clastic rock; (b) : chemical sediment.

As proposed by Taylor et a l . ( 1 985 ) , the post-Archaean sediments are de- pleted in E u , and such a Eu-depletion is attributed to the development of K-granites at the end of the Archaean rather than to other factors such as oxidation-reduction. Moreover , in the Wuyang terrain and its vicinity, no K-granite older than 2200 Ma has yet been reported, and the sediment Eu-depletion occurred at about 2300 M a . Hence, such a Eu-depletion cannot be simply attributed to the emplacement of K-granites. We consider that the Eu anomaly of the sediments should be determined by the geochemical behavior of Eu and sedimentary environment. It is well known that Eu can exist either as Eu 2÷ or as Eu 3+ while the other REE can exist as trivalent ions only , i . e . , R 3+ . On the basis of the SHAB (soft / h a r d acid and base) theory (Dai , 1987), Eu 3+ and R 3+ are both hard a c i d s , thus sharing much in common with respect to their properties, Eu anomalies seem to have resulted from the separation of Eu

136 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y Vol. 11

Table 1 • The contents of major elements (wt .%) and REE (in ppm) in the supracrustal rocks from different horizom of the W u y n g metamorphic terrain

No. 1 2 3 4 5 6 7 8 9 10 11 12

SiO2 3.03 40.53 43.63 57.44 62.84 92.30 95.15 48.45 15.58 40.72 29.93 40.53

TiO2 0.02 0.072 0.05 0.64 0.63 0.15 0.04 1.13 1.11 0.16 0.66 0.07

?d203 0.61 0.86 1.07 11.44 13.72 3.57 1.28 13.71 2.29 3.33 7.21 0.86

Fe203 1.12 17.82 27.47 2.48 2.65 0.11 0.11 1.44 35.29 5.29 11.38 17.82

FeO 1.94 16.63 13.73 2.22 3.47 1.31 1.43 12.11 16.41 3.44 6.27 16.63

MnO 0.00 0.391 0.43 0.15 0.11 0.16 0.19 0.23 0.19 0.15 0.09 0.39

MgO 19.76 6.01 3.41 6.23 2.32 0.13 0.00 7.45 14.31 35.70 31.16 6.01

CaO 30.37 13.50 6.91 11.24 3.00 0.37 0.22 9.28 5.50 1.63 1.70 13.50

Na20 0.00 0.653 1.00 0.91 2.02 0.22 0.22 2.59 0.16 0.13 0.10 0.17

K20 0.00 0.173 0.39 1.61 4.08 1.74 0.78 0.85 0.11 0.01 0.20 0.05

P205 0.01 / / 0.11 0.24 0.01 0.01 0.10 1.83 0.03 0.61 0.65

Others 42.24 3.05 1.78 5.30 3.07 0.37 0.20 1.93 6.47 9.47 10.87 2.93

La 2.89 1.64 1.98 40.53 62.49 6.30 15.72 4.33 34.67 36.60 0.90 7.54

Ce 4.16 2.52 3.75 73.46 94.85 9.96 26.05 10.63 59.90 58.55 1.53 11.91

Pr 0.52 0.27 0.47 9.39 10.33 1.31 3.20 1.71 6.70 6.13 0.22 1.20

Nd 1.50 0.86 1.58 32.77 29.92 3.78 10.29 7.88 20.34 15.35 0.59 3.22

Sm 0.25 0.15 0.28 6.91 4.36 0.58 1.73 2.41 3.28 2.18 0.18 0.52

Eu 0.096 0.071 0.078 1.442 0.875 0.191 0.336 0.897 0.491 0.284 0.037 0.161

Gd 0.233 0.165 0.244 6.781 3.275 0.399 1.483 2.888 2.528 1.405 0.151 0.479

Tb 0.042 0.029 0.040 1.109 0.486 0.085 0.253 0.527 0.398 0.201 0.028 0.087

Dy 0.228 0.178 0.215 6 .608 2.113 0.313 1.661 3.404 1.924 0.872 0.186 0.417

Ho 0.057 0.043 0.048 1.365 0.410 0.067 0.353 0.680 0.396 0.199 0.044 0.092

Er 0.173 0.134 0.121 3.857 1.023 0.177 1.059 1.879 0.938 0.515 0.146 0.270

Tm 0.026 0.020 0.022 0.591 0.154 0.035 0.156 0.290 0.154 0.100 0.030 0.044

Yb 0.150 0.121 0.132 3.618 0.772 0.197 0.984 1.666 1.052 0.699 0.197 0.251

Lu 0.025 0.020 0.021 0.546 0.107 0.033 0.153 0.244 0.181 0.120 0.035 0.040

Y 3.10 1.82 1.86 42.63 11.79 1.96 11.40 18.12 11.67 6.08 1.27 4.01

E u / E u 1.21 1.39 0.90 0.64 0.69 1.16 0.63 1.05 0.51 0.47 0.67 0.98

Y~REE 10.35 6.22 8.98 188.98 211.17 23.43 63.43 39.44 132.95 123.21 4 . 2 7 26.23

L a / Y b ) n 11.16 7.85 8.69 6.49 46.90 18.53 9.26 1.51 19.09 30.34 2.65 17.40

Table 2 . FeO/Fe203, Eu/En* and XREE of the sedhnents from different stratigraphic

horizons of the Weyang metamorphic terrain

S~mple and stratigraphy

Yangshuwan Formation

Tieshanmiao Formation

Zhao 'anzhuang Formation

Chemical sediment

No.of

4 0 .88 0 .64 188.98

3 0.50 0.90 8.98

2 0.93 1.39 6.22

1 1.73 1.21 10.35

Oasfic ~ediment 1

No.of I Eu/Eu * samples I FeO/Fe203, [ ~REE 4 0.88 0.64 188.98

5 1.31 0.69 211.17

6 11.91 1.16 23.43

No.2 CHINESE JOURNAL OF GEOCHEMISTRY 137

from the R E E . The ratio of Eu 3 + / E u 2+ depends upon the nature and degree of such separation. Clearly, the value of Eu 3 + / E u 2+ is controlled by fO2 (oxygen fugacity ) in the sedimentary environment. In the case of low fOE, EU 2+ is predominating over Eu3+; the main aqueous anions are H S - , S 2- , S C N - , $202-3, C O - , CH4, etc. These species are ready to combine with E U 2+ , forming stable complexes and then be precipitated from the water. Meanwhile , a great amount of R 3+ and minor Eu 3+ will remain in the water as free ions because they cannot combine with soit bases. So the sediments formed under reducing condi- tions are characterized by low YREE and positive Eu anomaly . In the case of high f O 2, Eu 3 + / E u 2+ is high , with Eu 3+ being dominan t ; the main aqueous anions are SO2-4, CO2-3, O H - , etc. It is easy for Eu 3+ and R 3+ to combine with these species into stable complexes and then to be precipitated, while Eu 2+ remains as a free ion in water. Thereby, the sediments formed under oxidizing conditions are characterized by high YREE and negative Eu anomaly .

In terms of the oxidation-reduction model described above, the sediments older than 2300 Ma from the Wuyang terrain, characterized by positive Eu anomalies and low E R E E , should have been formed under reducing conditions, while those younger than 2300 M a from the Yangshuwan Formation, marked with negative Eu anomalies and high E R E E , must have been deposited in an oxidizing environ- ment. The sediments at the top of the Tieshanmiao Formation, with moderate EREE and E u / E u * , are likely to have been formed in an environment changing from reducing to oxidizing at about 2300 M a . Variations in FeO/Fe20 3 of the sediments (Table 2) lend strong support to the above conclusions. Therefore, it can be further deduced that 2300 Ma witnessed a change in environment from reducing to oxidizing, that the oxygen-rich atmosphere appeared at about 2300 M a , that the sediments younger than 2300 Ma are mostly depleted in Eu ~ and that the sediments depleted in Eu are most likely to have been deposited in an oxidizing environment after 2300 M a .

REE Evolution of Volcanic Rocks and Its Significance

The lanthanum contraction effect is a factor leading to a decrease in ion ra- dius with increasing atomic number of the lanthanum-family elements. The HREE are smaller than the LREE in radius. During partial melting of the mantle, both LREE and HREE would find their way into the melt as incompatible elements. Larger in radius and lighter in weight, the LREE are easier than the HREE to in- corporate into the melt. As a result, the melt would be higher in L a / Y b )~ and EREE than the residual phase. Partial melting could cause the mantle to become gradually depleted in R E E , especially in LREE and the values of EREE and La/Yb)n for the mantle-derived volcanics to decrease steadily. The validity of this model can also be strongly evidenced by the fact that the values of EREE and La /Yb)n for tholeiites from Early Precambrian greenstone belts are much higher than those of the contemporary oceanic tholeiites. Therefore, the volcanic rocks at the lower levels of a greenstone belt should be higher in La/Yb)~ and YREE than the equivalents at the upper levels.

From Table 1 and Figs. 1 - 3 it can be seen that the ZREE of ultrabasic vol- canic rock( No. 9) and ore-bearing ultrabasic volcanic rock ( N o . 10) of the

138 CHINESE JOURNAL OF GEOCHEMISTRY Vol.ll

~z

0

,..i,

~ N o .8 \ ~ - ~ No. 9

~ ~ ~ N o , I O

I I ¢ I i l i i I I | I i I

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig.3. Chondrite-normalized REE diagram of volcanic rocks in the Wuyang metamorphic terrain.

Zhao'anzhuang Formation is almost ten times higher than that ( N o . 11, No. 12) of the Tieshanmiao Format ion . The former also has higher L a / Y b ) ° than the latter. So the Zhao'anzhuang Formation must have been deposited be- fore the Tieshanmiao Format ion . The volcanics of these two formations are unlikely to be formed as a result of continuous partial melting of the mant le , and there should be a sedimentation hiatus between them.

Sample No. 8 is a komatiite sample from the Zhao 'anzhuang Format ion , with L a / Y b ) n = 1.51, and it would be the product of intensive (high percent )partial melting of the mantle. To some extent, it represents the composition of the mantle at that t ime. Its EREE is higher than that of the Tieshanmiao Formation volcan- ic rocks , indicating that the REE abundance of the mantle is higher at the time the Zhao 'anzhuang Formation was formed than that at the time the Tieshanmiao Formation was formed.

Conclusions

1. The values of EREE and La/Yb)n of volcanic rocks of the Zhao 'anzhuang Formation are obviously higher than those of the equivalents of the Tieshanmiao Formation, indicating that the Zhao 'anzhuang Formation is older than the Tieshanmiao Format ion , with a sedimentation hiatus between them.

2. The mantle tends to become gradually depleted in R E E , especially in LREE ; ~REE and La/Yb)n of the mantle-derived volcanic rocks tend to decrease subsequently. Thereafter, the volcanic rock formations can be stratigraphically divided.

3. The sediments formed under oxidizing conditions are characterized by nega- tive Eu anomalies and high Z R E E , while those formed under reducing conditions are marked with positive Eu anomalies and low E R E E . This model can be used to describe the stratigraphy and sedimentary environment of the sediments.

4. In the Wuyang terrain the sediments, older than 2300 M a , of the Zhao ' anzhuang and Tieshanmiao formations are marked with low ZREE and positive Eu anomalies, those, younger than 2300 M a , of the Yangshuwan Format ion are characterized by high ZREE and negative Eu anomalies, and those deposited at the top of the Tieshanmiao Formation were dated at about 2300 M a ,

No.2 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y 139

demonstrating a sedimentary environmental change from reducing to oxidizing at ~ 2300 M a .

5. The oxygen-rich atmosphere came into being at about 2300 M a . As a re- sult, the sediments formed after 2300 Ma must be depleted in Eu but high in EREE. The sediments depleted in Eu are likely to have been deposited after 2300 M a .

Acknowledgements

The authors wish to thank Professors Dai Anbang, Wang Dezi, Ouyang Ziyuan, Qiu Yuzhuo and Xie Hongsen for their advice.

References

Chen Yanjing, 1989, Catastrophes in the geological environment at about 2300 M a , Proceedings of the 3rd conference on the comprehensive study of cosmology, Earth Science and Biology : Beijing, Science and Technology Press, p. 78 - 82 (in Chinese ) .

Chen Yanjing, Fu Shigu and Hu Shouxi, 1988, The major elements in different types greenstone belts at the southern margin of the North China Platform and their significance: J . Nanjing University(Earth Sci- ences), n .1, p .70-83 (in Chinese with English abstract).

Chen Yanjing and Fu Shigu, 1991, Variations in REE patterns of Early Precambrian sediments: Kexue Tongbao (Bulletin of Science ) , v .36, p. 1 I00 - 1104.

Dai Anbang, 1987, Coordination chemistry: Beijing, Science Press, 750 p . (in Chinese). Hu Shouxi, 1988, Geology and metallogeny of the collision belt between the South China and North China

paleoplates : Nanjing, Nanjing University Press, 558 p . (in Chinese). Taylor, S . R . and S. M . Mclennan, 1985, The continental crust: its composition and evolution:

Blackwell, Oxford, 312p.