Precambrian Research, 27 (1985) 63--84 63 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

REE G E O C H E M I S T R Y O F E A R L Y P R E C A M B R I A N C H A R N O C K I T E S A N D T O N A L I T I C - - - G R A N O D I O R I T I C G N E I S S E S O F T H E Q I A N A N R E G I O N , E A S T E R N H E B E I , N O R T H C H I N A

WANG KAIYI 1 , YAN YUEHUA 1 , YANG RUIYING 2 and CHEN YIFEI ~

~Institute of Geology, Academia Sinica, P.O. Box 634, Beijing (People's Republic of China) 21nstitute of High Energy Nuclear Physics, Academia Sinica (People's Republic of China) 3Research team of Geological Bureau, Hebei (People's Republic of China)

ABSTRACT

Wang, K.-Y., Yan, Y.-H., Yang, R.-Y. and Chen, Y.-F., 1985. REE geochemistry of early Precambrian charnockites and tonalitic--granodioritic gneisses of the Qianan Region, eastern Hebei, North China. Precambrian Res., 27: 63--84.

The charnockites and gneisses from the Qianan area, eastern Hebei Province, China, were investigated for their isotope and trace element abundances.

The majority of the charnockites are acid in composition, only a few samples have a composition corresponding to intermediate to basic rocks. A Rb--Sr whole-rock isochron of the charnockites gives an age of 2.65 ± 0.05 Ga. The highly fractionated REE patterns and the variable (Ce/Yb)N ratios in these rocks are interpreted as being a result of igneous processes. It is believed that the charnockitic rocks are related to each other by fractional crystallization under high temperatures and low PIJ 20 conditions; the major precipitating phases are thought to be orthopyroxene and plagioclase.

By contrast, the Qianan gneisses are mainly of tonalitic and granodioritic composition. However, a few samples have composition corresponding to granite (s.s.), probably pro- duced by metasomatic processes. A Rb----Sr whole-rock isochron of the Qianan gneisses yields an age of 2.13 -" 0.06 Ga and this age probably represents a remobilization event. The Qianan gneisses do not contain hypersthene; they have strongly fractionated REE patterns similar to those found in many Archaean tonalitic rocks of the world. In addi- tion, the total REE abundances and the magnitudes of the Eu anomaly show regular variation with the SiO 2 content. This may suggest that the Qianan gneisses might have evolved under hydrous conditions with hornblende exerting a major control on their trace element chemistry.

INTRODUCTION

T h e s t u d y area is s i t u a t e d in t he Q i a n a n u p l i f t e d b l o c k or the Q i a n a n g r a n i t e - g n e i s s e s d o m e in e a s t e rn H e b e i P r o v i n c e . F i g u r e 1 show s a s i m p l i f i e d geo log ica l m a p o f t he b l o c k . T h e r eg iona l g e o l ogy o f th i s area has b e e n de- sc r ibed b y Bai et al. ( 1 9 8 0 ) , Q i a n ( 1 9 8 1 ) a n d S u n a n d Wu ( 1 9 8 1 ) . O n l y s o m e i m p o r t a n t f ea tu re s are o u t l i n e d here . T h e u p l i f t e d b l o c k is b o u n d e d to t h e

0301-9268/85/$03.30 © 1985 Elsevier Science Publishers B.V.

64

~ . ~ / ///x~A87 CHI ~ I e n g z h u a n g

l aoz u.n

"=kin ! I'I I ~ r l r 0 ') ~ I ~ r i :

.,-.S.h u ichangj

hli a n ~ 72 73 74 74 q)" ~4,1J ~+ QT S 1A. , ~---~N2 A71 L o n g h u s h a n g . ~

,dlY angyas /~(~Q L H 1 (

D3 96 Da4 Man shan Bixinzhuang

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ri'a~l~n lllgm I t~ Qianan 11111 o i ,J ll l l

( ~ : ~ QSF2a QSF3a QSF3b

QBCI

Quaternary ~ Q ianxi Group

Sinian Suberathem [ - ~ Qianan Gneisses

Fig. 1. S impl i f i ed geologic map of the Qianan upl i f ted block area. Small inset map shows locat ion o f Fig. 1 in Hebei Province.

65

north, east and south by faults, but there is a boundary fold zone to the west, in which there exist synkinematic and post-kinematic N--S trending faults. Therefore, the block may be divided into two tectonic units: the granite--gneisses dome and the boundary folds {Qian, 1981). The tonalitic-- granodioritic gneisses consti tute the essential part of the dome. The charn- ockites occur in several areas along the N--S trending faults within the western boundary folds. Both gneisses and charnockites contain a great variety of enclaves of supracrustals which are generally called the Qianxi Group and have undergone amphibolite and granulite facies metamorphism.

The Qianan uplifted block took shape during the time of the Fuping orogeny ~2.5 Ga ago (Ma et al., 1979). Later, during the Wutai orogeny at ~2.1 Ga ago, the block was further reworked by another thermal event (Zhao, personal communication, 1982).

For several years petrological and geochemical studies have concentrated on the granulite facies rocks of eastern Hebei, because these rocks occupy a unique role in our understanding of the early crustal evolution in North China. In this paper geochemical and Rb--Sr isotopic age data of the tonalitic--grano- dioritic gneisses and charnockites of the Qianan region are presented. These data are then used to discuss the origin and petrogenesis of these rocks.

FIELD AND PETROGRAPHIC RELATIONSHIPS

Charnockites

Charnockites occur in several areas along the N--S trending faults within the western boundary fold zone. They extend discontinuously from Shui- chang in the north to Sonting in the south (Fig. 1). The charnockites engulf the older Qianxi supracrustal rocks. Some basic rocks (enclaves) in the Caozhuang area have recently been dated at ~3 .5 Ga by the Sm--Nd method (quoted in Jahn and Zhang, 1984). These basic rocks are the oldest rocks found in China so far. The contact between the charnockites and the supra- crustal rocks is discordant, and there is only a weakly developed foliation within the charnockites. These features suggest that the intrusions are post- kinematic.

According to Streckeisen's {1974) classification, most of the charnockite samples fall into the fields of charnockitic and charnoenderbitic granolite, and only a few samples fall into the field of hypersthene-pyroclase granolite. A simplified terminology will be adopted here: charnockite for the former, and basic charnockite for the latter.

Charnockite The charnockites occur in the Shuichang, Yangyashang and Sonting areas

(Fig. 1}. They are medium to coarse grained and exhibit a granoblastic tex- ture although the minerals are of variable grain size. They are comprised of plagioclase {45--60%), quartz (15--30%), hypersthene (5--15%), alkali

66

feldspar (5--10%), biotite (2--15%) and hornblende (1--3%). Accessory minerals include zircon, apatite and opaques. Plagioclase and hypersthene commonly form large, subidiomorphic grains. Most hypersthene crystals are fresh, only a few have been altered to uralite. Plagioclase shows irregular lobate grain boundaries and is seen to be replaced by microcline along grain boundaries.

Basic charnockites The basic charnockites are found only in the Sonting area and make up

only a small portion of the entire charnockite complex. The rocks (A87 and Chl) exhibit automorphous and hypidiomorphic equigranular textures and are composed chiefly of medium to coarse grained plagioclase (65--80%) with smaller amounts of orthopyroxene (15--20%) and biotite (tr--5%). Accessory minerals include apatite and opaques. In view of the plagioclase- dominated mineral assemblage, the term "leuconoritic charnockite" will be used for these rocks. On the other hand, some specimens, such as sample A9, contain a basic mineral assemblage that is somewhat different from leuconoritic charnockites. The principal mineral constituents are amphibole (40%), plagioclase (35%), diopside (10%), hypersthene (8%), biotite (5%) and small amounts of opaques.

Qianan gneisses

The Qianan gneisses are located in the core of the Qianan uplifted block and occur as several plutons in the Mangshan, Tashan, Longhushan, Bixin- zhuang and Mengzhuang areas (Fig. 1). They were intruded into the amphi- bolites and granulites of the lower Qianxi Group. The gneissic nature of the rocks becomes evident in areas close to the contacts with the supracrustals. Enclaves of arnphibolite are found throughout the rocks and become abun- dant near the contacts. The contacts of the gneisses with supracrustals ap- pear to be concordant. The platy flow structure in the Qianan gneisses is parallel to the schistosity of the supracrustals. These structural features suggest that the Qianan gneisses were generated by folding and uplift during block (or dome) formation (Tan and coworkers, unpubl.).

According to the nomenclature of Streckeisen {1973), most of the Qianan gneisses fall in the tonalitic and granodioritic fields (Fig. 2). The gneisses consist of plagioclase (45--55%), quartz (20--35%), biotite (3--15%) and microcline (tr--lO%), with small amounts of hornblende and clinopyroxene. Accessory minerals include apatite, sphene, zircon and opaques. Sample QSF2a is dioritic in composition and contains up to 30% hornblende, 30% plagioclase, 15% clinopyroxene, 10% biotite and 10% quartz. Some Qianan gneisses have been affected by metasomatism, and K-feldspar is locally im- portant so that it causes some samples to fall in the granite field. The granites contain up to 50% of microcline and perthite, 30% quartz, 10% sodic plagioclase and small amounts of biotite. The granites do not show the

67

A

/ / \ \ Fig. 2. Modal proportion of quartz (Q)-plagioelase (P)--alkali feldspar (A) in rocks from the Qianan gneisses. Nomenclature after Streckeisen ( 1973 ).

gneissic texture as the common Qianan gneisses. They are thought to be formed by post-kinematic potassium metasomatism along the margins of the intrusions.

ANALYTICAL METHODS

Sc and rare earth elements (REE) were determined by NAA. The methods used are similar to those described by Deng et al. (1981). Total analytical errors are estimated at ~10% for La, Sm, Eu, Yb, Lu and Sc, and 15% for Ce, Nd, Gd and Tb. Cr, Ni and Ba were determined by ICP spectrometry, the errors are estimated at ~ 10%. Some Rb and Sr values were obtained by the XRF and atomic absorption methods, and errors are estimated from 5 to 10%, depending on elemental abundances. In the samples used for age deter- mination, Rb and Sr contents were determined by isotope dilution. A brief description of analytical procedures can be found in ASIG (1978). The de- cay constant ~,STRb used in the age calculation is 0.0142 Ga -1. Isochron calculation follows the method of York {1969}.

Rb--Sr WHOLE ROCK DATA

Tables I and II present the results of the Rb--Sr isotopic measurements. The data for the charnockites define an isochron with an age of 2647 -+ 53

(20) Ma and an initial 87Sr/~6Sr ratio of 0.70223 -+ 20 (2a) (Fig. 3). As there is little evidence that the analyzed samples have been altered by later geologic events, the 'normal ' Rb and K/Rb and Rb/Sr ratios of the dated charnockite

68

TABLE I

Rb--Sr data for charnockites from Qianan area

Sample No. 8~Rb/s6Sr* ~TSr/86Sr (2o)

A72 0 .445 0 .71945 _+ 10 A73 0 .436 0 .71928 -* ]4 A74 0 .170 0 .70848 *- 4 A82 0 .822 0 .73352 -+ 18 A83 0 .520 0 .72176 -+- 14 A85 0 .659 0 .72676 -* 14 A86 0.477 0 .72069 ± 14 A87 0 .034 0 .70356 -*- 8 A88 1.231 0 .74964 -~ 14 A9 0 .089 0 .70613 ~ 8

* U n c e r t a i n t i e s a r e 2%. Analys ts : I so tope L a b o r a t o r y , I n s t i t u t e of Geology, Academia Sinica, Beijing (ASIG).

TABLE II

Rb--Sr data for Qianan gneisses

Sample No. *TRb/8'Sr* ~Sr/8~Sr (20)

Q T S l a 0 .536 0 .72055 + 20 QSF1 0 .555 0 .72117 -+ 14 Q S F 2 a 0 .482 0 .71905 ~ 14 Q S F 3 a 0 .327 0 .71397 ~_ 18 Q S F 3 b 0 .336 0 .71403 ± 16 QLH1 0 .179 0 .70956 ± 14 Q L H 4 a 0 .199 0 .71022 ± 10 Q L H 4 b 0 .509 0 .71968 ± 12

* U n c e r t a i n t i e s are 2%. Analys ts : I so tope L a b o r a t o r y of ASIG.

samples suggest that the Sr-isotopic system has not been disturbed or partial- ly reset by later geologic events. The isochron age of 2647 Ma is therefore interpreted to represent the time of intrusion of the charnockite complex. During the last few years many isotopic ages have been reported for the granulites from the Taipingzhai and Guojiagou areas (Chung et al., 1979; Pidgeon, 1980; Liu et al., 1984; Jahn and Zhang, 1984). These results show coherent ages of ~2.5 Ga. The present data, together with the above publish- ed results, indicate that the Qianan-Qianxi area has undergone an important tectono-thermal event during the late Archaean.

T h e d a t a f o r t h e Q i a n a n g n e i s s e s d e f i n e a n i s o c h r o n w i t h a n a g e o f 2 1 2 9 -+

5 8 ( 2 a ) M a a n d i n i t i a l SVSr/86Sr r a t i o o f 0 . 7 0 4 0 4 + 2 8 ( 2 o ) ( F i g . 3 ) . T h i s age ,

69

significantly lower than that for the charnockites, is difficult to interpret, especially in view of the intrusive relation with the supracrustal rocks. As stated above, the Qianan uplifted block took shape during the Fuping orogeny ~2 .5 Ga ago (Ma et al., 1979), and the generation of the Qianan gneisses may be related to folding and uplift during block (or dome) forma- tion, so that the formation age of the gneisses is thought to be older than 2.5 Ga. Field evidence suggests that the Qianan gneisses have experienced post- kinematic metasomatism. It appears, therefore, that the age of 2129 -+ 58 Ma represents a younger thermal event.

II 7 51i

• 26;7 • 5.i : :a . M a

~ T S r , . ~ z .

~ 6 S r i 7:~t

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l ) . ~ ' ( i . 4 0 . 6 I ~ 1 . 0 1 . 2 1.

b T R b / ~ 6 S r

Fig. 3. W h o l e - r o c k R b - - S r i s o c h r o n s for the c h a r n o c k i t e s and t h e Q i a n a n gne i s ses .

GEOCHEMISTRY

C h a r n o c k i t es

Twenty samples of charnockite were analyzed for major and trace ele- ments, and the results are given in Table III. Sample A9 is basic in com- position. Samples A87 and Chl axe leuconoritic in composition. They differ slightly from most norites in containing more Si and less Ca and Mg. In composition they axe intermediate between norite and diorite or between

7O

T A B L E I l l

Major and trace element contents for charnockites

Intermediate--basic charnockites Charnockites

A9 A87 A86 Ch l A74 A y l D3 N2 A88

SiO~ 48.8 52.55 54.71 55.04 56.26 61.87 62.68 65.94 66.8 TiO 2 0 .84 0 .56 1.46 0 .42 1.18 0.77 0.78 0 .49 0 .36 A1203 12.17 14 .43 14.61 21 .23 16 .43 15.98 16.13 14.94 13.97 Fe 2 03 4 .55 8 .65 8.77 1.42 5 .55 0 .97 0 .00 2.05 0 .28 FeO 9 .79 3.27 2.90 3.77 3 .50 5.99 6.67 3 .18 5.73 MnO 0 .27 0 .20 0 .23 0.07 0 .16 0.08 0 .09 0 .06 0.09 MgO 8.15 6 .62 5,28 2 .50 0.49 2.85 3.61 2.41 2.07 CaO 9 .74 7.87 4.32 7.99 9 .00 3.27 3 .72 3.27 3.24 Na20 2.45 3.75 3 .40 5.05 4 .10 3 ,46 3.68 3,63 3.8 K20 0 .80 1.00 2.85 1.09 1.90 3.47 2.16 2.88 3.45 P:O s 0 .39 0.07 0.29 0 .52 0.05 0 .28 0 .06 0 .15 0.14

Sc (ppm) 25 13 10 7 13 10 15 32 Cr 436 362 68 300 37 220 170 Ni 120 90 38 12 62 94 140 73 Rb 12 14 73 15 42 88 90 147 Sr 379 1055 442 2338 719 400 340 344 Ba 144 558 1170 765 513 1125 387 639 La 34 22 21 27 75 53 170 83 Ce 81 37 35 52 145 96 313 132 Nd 38 15 14 17.8 62 37.5 110 40.2 Sm 7 2.4 2 3.3 10 4.4 9.8 6.3 Eu 2.2 1.2 1.3 1.5 1.9 1.2 1.4 1.1 Gd 3.6 Tb 1.1 0.4 0.3 0.78 0.6 0.7 0 .34 Dy 2.6 1.7 Yb 2.1 0.6 0.9 0.8 1.96 1.0 1.0 0 .85 Lu 0.5 0.12 0 .39 0.19 K / R b 553 593 324 603 375 327 199 195 Rb /Sr 0 .03 0 .013 0 .16 0 .006 0 .058 0 .22 0 .26 0 .43 (Ce/Yb) N 7.4 14 8.9 13 5.8 22 71 35

anorthosite and diorite. Samples A86 and A74 are dioritic in composit ion, and the rest o f the samples falls in the granodiorite, adamellite and tonalite fields on an A b - - A n - O r normative diagram (Fig. 4).

Sample A71 was taken from an enclave within the charnockites; it is a biotite--plagioclase gneiss whose major and trace elements contents are also given in Table III.

Sr contents in the charnockites range from 344 to 719 ppm, with a mean value of 589 ppm. Ba concentrations vary from 144 to 1890 ppm, with a mean value o f 794 ppm. These values of the charnockites are comparable with those of the Nfik gneisses of western Greenland {McGregor, 1979) and the Lewisian gneisses of northwest Scotland (Weaver and Tarney, 1981) . The leuconoritic charnockites have the highest Sr contents (average 1700 ppm) and the lowest Rb/Sr ratios (~0 .01 ) . Except for leuconoritic charnock- ites, the Rb contents range from 42 to 147 ppm with a mean value of 91

Fig . 4. T e r n a r y p l o t o f n o r m a t i v e A n - - A b - - O r for the c h a r n o c k i t e s ( so l id c i rc le s ) and Q i a n a n g n e i s s e s ( o p e n c irc les ) . T h e c l a s s i f i c a t i o n b o u n d a r i e s are f rom O ' C o n n o r ( 1 9 6 5 )

71

Enclave

A73 D5 A85 D6 A y 5 Da4 A83 A u l A 8 2 A72 Da7 A71

66 .87 68 .74 69 .13 71 ,54 62 .94 63 .23 63 .27 64 .16 65 .2 65 .63 65 .67 61.9 0 .40 0 .37 0 .57 0 ,15 0 .21 0 .59 0 .30 0 .52 0 .59 0 .60 0 .74 0 .68

14.98 14.86 13.88 14,88 16.44 15.12 14 .25 15.41 15.16 15 .53 15.48 16 .52 2.05 0 .69 2 .72 0 ,28 1.68 0.61 6 .70 0 .28 3 .15 3 .12 0.51 1.73 1.97 3.43 1.73 1,21 4 .85 6.51 1.07 5.73 2.11 1.80 4 .96 5 .34 0 .06 0 .06 0 .06 0 ,02 0 .11 0 .10 0 .16 0 .09 0 .08 0 .05 0 .07 0 .09 1.79 2.02 4 .07 0 ,73 2.41 3 .57 3 .70 2 .83 2 .02 3 .55 2 .40 2.87 3.33 2 .65 1.91 2 ,70 4 .67 3 .65 3 .26 3 .45 2 .78 2.07 3 .24 4.77 3 .40 3.08 3 .40 3 .11 4 .54 3 .48 3 .40 3 .45 3 .35 3 .85 3 .50 3 .05 4 .00 3.89 2 .30 4 .80 1.27 2 .66 2.65 3 .25 5.00 2 .90 2.81 1.50 0.11 0 .05 0 .11 0 ,40 0 .33 0 .22 0 .22 0 .22 0 .12 0 .19 0 .03 0 .22

7.9 7 10 83 250

43 52 12 90 84 21

599 368 340 1350 468 342

25.2 24 56.2 30.5 39 95 10.5 14 33.6

1.9 2 5.5 1.3 1.3 1.36 1.4 4.68 0.23 0.2 0.46 1.4 0 .58 0 .6 0 .78 0 .17 0 .12

369 227 502 0 .15 0 .23 0 .06

11.9 1~ 28

39.9 56.6 16.2

2.8 1.2 3.5 0 .42 2.1 0 .96

13.3

16 13 13 9 13 18 105 180 81 85 161 52 52 57 13 81 103 134 74 99 61

448 340 470 483 366 357 873 1125 1890 687 675 827

52.5 195 86 330 32.6 110

6.4 10 1.4 1.3 5.2 0.54 3.2 0 .75

272 262 0 .18 0 .30

26

35 36 26.7 53 62 56.7 13 17 24.6

2 2.4 3.9 1.3 1.3 1.3

0.8 0.3 0 .39 0 .64 1.8

0.9 0.7 0 .8 1.27 0 .2 0 .14 0 .24

310 325 235 204 0.29 0.15 0.27 0.17

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ppm; and the Rb/Sr ratios show a significant variation (Fig. 5). The K/Rb ratios of the charnockites range from 195 to 592 with an average value of 354 (Fig. 6). They are comparable with the K/Rb ratios of the charnockites from Madras, southern India (Weaver, 1980) but are substantially different from those of the intermediate and acid granulites from the Taipingzhai and Guojiagou areas (Jahn and Zhang, 1984).

The REE patterns of the charnockites are strongly fractionated with marked light REE enrichment and heavy REE depletion (Figs. 7--10). The wide range in (Ce/Yb)N ratios (8.9--83) is similar to that found in many Archaean tonalites (Jahn et al., 1981) and in the Qianxi granulites (Jahn and Zhang, 1984). Many of the samples have positive Eu anomalies, but negative Eu anomalies are also found in samples with higher total REE contents. Sam- ple A82 has the highest total REE content and also the largest (Ce/Yb)N. Although the charnockites have a wide range in light REE abundance (Ce 40--400 X chondrite), the heavy REE and Eu seem to show much more re- stricted variation (Eu 16--20 X chondrite; Yb 3--4.5 X chondrite). This

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Da4 ~.~ ~D6 ~ \ Charnockites

La Ce Nd Sm Eu Gd Tb Dy Yb Lu Fig. 7. REE patterns normalized to chondrite values of Haskin et al. (1968) for the charnockites from Yangyashan.

74

= c

200

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Charnockl tes

J 73 i i I i i I ~ J

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Yb Lu

75

feature is also observed in the Lewisian granulite-facies tonal i tes and t rond- hjemites (Weaver and Tarney , 1980) .

Two in termedia te to basic samples (A9 and A74) have REE pat terns less f rac t iona ted than those o f the o ther charnocki tes . Their (Ce/Yb)N ratios vary f rom 5.8 to 7.4. The enclave sample (A71) , a biot i te--plagioclase gneiss, has a REE pat te rn comparab le with tha t o f A74 (Fig. 10).

±

L )

200

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50

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~ r n ( , c k i t e s

I

l.a Ce Nd Sm Eu Tb Yb Lu

Fig. 10. REE patterns for the charnockites (A74") and the enclave of supracrustal rocks in the charnockites (A71 - biotite--plagioelase gneiss).

F.eOitotal i

Na~O+ MgO K20

Fig. 11. AFM diagram for the Qianan gneisses (solid circles). Some data according to Y.X. Zhang, H.Q. Yan and W.H. Zhao (unpubl.).

76

Qianan gneisses

Fourteen samples of Qianan gneisses were analyzed for major and trace elements and the results are given in Table IV.

In the AFM diagram the Qianan gneisses are shown to follow a calc-alkaline or trondhjemitic trend (Fig. 11). However, they are distinguished from the 'normal' calc-alkaline suite by an increase in Na20 and decrease in K20 from intermediate to silicic rocks (Arth et al., 1978). In Fig. 4, except for sample QSF2a which is dioritic in composition, most Qianan gneisses fall in the tonalite and granodiorite fields, with only a few samples falling in the K-rich granite field (Fig. 4). Moreover, Ti, Ca, Mg and Fe contents decrease regular- ly with increasing SiO2 (Fig. 12).

The Sr values of the Qianan gneisses range from 260 to 738 ppm (mean 550 ppm), the Ba values range from 340 to 2136 ppm (mean 1091 ppm), and the Rb values range from 37 to 114 ppm (mean 71 ppm). These values are comparable with those of the charnockites. The Rb/Sr ratios of the Qianan gneisses range from 0.06 to 0.21 (Fig. 5), and the K/Rb ratios are also not much different from those of the charnockites (Fig. 6).

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77

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• , S i 0 2

• ' - ' - ~ " 52 . 79

A I i i

Ce Nd Sm Eu Gd Tb Ho

q~lsI~x 5 9 . 7

~ (41.t44 6 5 . 6

ql.It I 67.6 • QsF3B 70.6

~ ,@_, .~ ,QSl-3A 69. 7

Y b Lu

Fig. 13. R E E pat terns for the t o n a l i t i c - - g r a n o d i o r i t i c gneisses f rom the Q i a n a n area. Sam- ple Q S F 2 a is d ior i t ic in c o m p o s i t i o n .

The Qianan tonalitic--granodioritic gneisses have strongly fractionated REE patterns with (Ce/Yb) N ratios of 8 .5--32 (Figs. 13 and 14). Both the total REE contents and the magnitude of the Eu anomalies show a regular inverse variation with SiO2 (Fig. 13).

Near the contacts with the supracrustal rocks there are dark bands con- sisting chiefly of hornblende, plagioclase and clinopyroxene. Sample QSF 2a was collected from one of the dark bands and shows a very high total REE content, a slightly fractionated REE pattern with (Ce/Yb)s -- 2.7 and a clear negative Eu anomaly (Fig. 13).

Figure 14 shows the REE patterns for two granodioritic gneisses and a granite. They are less fractionated than those of the tonalitic and some granodioritic gneisses. Their (Ce/Yb)N ratios range from 6.6 to 12.5, and a negative Eu anomaly is only present in one sample.

DISCUSSION AND PETROGENESIS

The geochemical results, particularly the REE data, show that there are significant differences between the charnockites and the Qianan gneisses.

TA

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QSI-1

~BX I

Fig. 14. REE pat terns for the Qianan gneisses. QBX1 and QBC2 are granodiori t ic in com- pos i t ion with Na20/K~O ratios < 1.5. Sample QSF1 is granitic in compos i t ion .

This may be explained by different conditions under which the parental magmas have evolved.

Petrogenesis of charnockites

The close association of anorthosite/leucogabbro and charnockitic rocks is well known (Windley, 1978). There is, however, controversy concerning the consanguinity of these rock types and the nature of the primary magma. This problem is briefly reviewed below.

The leuconoritic rocks (or basic charnockites) in the present study area are associated with charnockites, although they make up only a small propor- tion of the entire charnockite suite. The leuconoritic charnockites (A87, Chl) have REE patterns with lower total REE contents as compared with other charnockites (Fig. 9). The Rb--Sr isochron relationship (Fig. 2) further suggests that the basic charnockites and charnockites are probably cogenetic.

Field et al. (1980) suggested that progressive crystallization under low PH20 (granulite facies) conditions of average Phanerozoic dacitic melt could produce feldspar~lominated cumulates corresponding to low REE contents and with positive Eu anomalies, Furthermore, the experimental results for crystallization of anhydrous quartz diorite magma suggest that, under appropriate temperature conditions, separation of crystals from liquid by a filter-pressing mechanism during deformation may result in compositions

80

from gabbro through gabbroic anorthosite to anorthosite, together with some acid rocks (Green et al., 1969, 1972).

The relative low contents of Ca and Mg (Table III) suggest that the leuco- noritic member in this area is unlikely to represent a crystallized phase from basic magma. However, the high Sr content (1700 ppm) and positive Eu anomalies (Fig. 9) for leuconoritic charnockite exhibit the characteristics of crystallization from a probable dacitic melt. Because the crystal--liquid distribution coefficients of Sr and Eu for plagioclase in equilibrium with a dacitic melt are high, this implies that the nature of the parental magma for the charnockites might be andesitic--dacitic in composition. It is presumed that the parental andesitic--dacitic magma crystallized slowly under high T P conditions first to generate a plagioclase-dominated cumulate phase corre- sponding to the leuconoritic composition. Subsequently the magma rose continuously and differentiated at higher levels to produce a series of charn- ockitic rocks with various REE distributions. Thus the leuconoritic member and the charnockites could be related by progressive fractional crystalliza- tion under granulite facies conditions. These processes have operated in the crust, and anorthositic rocks might be left as the main component of the lower crust, while the low melting acidic fractions have risen and intruded at higher crustal levels (Green, 1969). Therefore, the charnockite complex with various composit ions in this area probably represents variable levels of the lower to middle Archaean continental crust in North China.

An example of a geochemical model calculation follows. The parental magma for the charnockites occurring in the Yangyashan area is assumed to have a REE composit ion similar to that of sample Da4, which shows moder- ate LREE enrichment. The crystallizing phases give a composit ion of plagio- clase4s hypersthenel0 biotite15 quartz2s hornblendes zircon0.1, approximating to the modes of sample A85. The mineral/melt distribution coefficient used in the calculations is taken from Arth (1979) and Nagasawa (1970) for a dacite melt composition. The bulk distribution coefficient D s = Z7 X ~ X h d i, where X i is the weight fraction of a given mineral i in the precipitating phase and h d i is the mineral/melt distribution coefficient for a given trace element for mineral i, yields the following values: Dee = 0.24, Dsm = 0.31, Dzu = 1.08, Dvb = 0.83. It is clear that the bulk distribution coefficients for Eu and Yb are close to 1, so their contents are constant either in the precipitating phase or in the residual.liquid phase.

During fractional crystallization the Rayleigh fractionation law can be used to describe the concentration of a given trace element in the melt C 1 relative to the parent melt Co by the equation C~/Co = F ( D - 1), where F is the fraction of melt left and D is the bulk distribution coefficient for the mineral settling out o f the melt. A~uming F = 0.3 and 0.08, the calculated results (Fig. 15) show that the REE patterns in residual melts after 70--92% crystallization resemble those of samples A88 and A82, respectively.

A few intermediate to basic samples (A9 and A74) have slightly different REE compositions from the other chamockites which might be caused by

81

L9

5U)

I0(}

I0

" ~ . ( ) ~ • Ax2

mag ma \ , \ / / @ " ~

C¢. Sm l':u "~ b

Fig. 15. Chondrite-normalized plots of results of crystallization modelling discussed in the text; F is the fraction of melt remaining.

contamination with the supracrustals with which the charnockites are asso- ciated. As described before, the REE pattern of charnockite sample A74 is roughly similar to the REE pattern of enclave sample A71.

Petrogenesis of Qianan gneisses

The REE patterns of the Qianan gneisses vary systematically with the SiO2 contents. A similar variation has been observed in a differentiated gabbro--diorite--tonalite--trondhjemite suite in SW Finland where horn- blende appears to be the dominant crystallizing phase (Arth et al., 1978). For the Qianan gneisses, having SiO2 contents from 60 to 71% and Na20/ K20 >t 1.5, the REE abundances decrease with increasing SiO2 content, accompanied by a change from positive Eu anomalies to small negative Eu anomalies. Corresponding variations in mineralogy have also been observed. The samples with higher total REE content generally contain hornblende, whereas the samples with lower total REE content generally do not contain hornblende. Therefore, hornblende may be a dominant crystallizing phase similar to that observed in SW Finland.

82

The REE pattern of sample QSF2a that was collected from a dark band near the margin of the pluton shows a high total REE content, a slightly fractionated distribution with (Ce/Yb) N = 2.7 and a pronounced negative Eu anomaly. Its origin is not clear at present.

The Qianan gneisses do not display a differentiated suite, like in SW Fin- land, from basic to acid rock. There are no basic intrusions closely asso- ciated with the gneisses. This suggests that the Qianan gneisses might not have been derived from a basic magma.

The Qianan gneisses with marked light REE enrichment and strong heavy REE depletion are similar to most Archaean tonalite--trondhjemite--grano- diorites or the TTG suite {e.g., Jahn et al., 1981). These rocks could be derived from a source rich in light REE by partial melting with garnet or hornblende in the residue (Arth and Hanson, 1972, 1975; Arth et al., 1978; O'Nions and Pankhurst, 1974; Glikson, 1976, 1982; Barker and Arth, 1976; Condie and Hunter, 1976; Compton, 1978; Hanson, 1978, 1981; Barker et al., 1981; Jahn et al., 1981).

The REE patterns in the more potassic members of the Qianan gneisses with Na20/K20 ratios < 1.5 (including granite) do not display regular varia- tions with increasing SiO2. Their heavy REE appear slightly higher and have no significant Eu anomalies. The more potassic members of the Qianan gneisses are considered to be affected by potassium metasomatism, and this implies that the REE pattern could have been modified by a post-crystalli- zation metasomatic event.

CONCLUSIONS

In the Qianan area the granite--gneiss block (or dome) took shape dur- ing the late Archaean, and the Qianan gneisses and the chamockites were generated during a tectono-thermal event resulting in block (or dome) forma- tion (Ma et al., 1979; Qian, 1981).

In view of the field relations the Qianan gneisses might represent syn- kinematic intrusions, whereas the charnockites might be related to in-situ dry anatexis and represent post-kinematic intrusions. Therefore, the primary age o f the gneisses is thought to be 2.65 Ga or older.

During the early Proterozoic the Qianan block (or dome) was further re- worked by a second thermal event (Zhao, personal communication, 1982}. The age of 2129 -+ 58 Ma for the Qianan gneisses might represent this younger thermal event. On the other hand, it appears that the elevated temperature had been maintained until ~2 .1 Ga ago. By contrast, the charnockites show little direct evidence of significant remobilization by this younger thermal event.

ACKNOWLEDGEMENTS

The isotopic data were obtained by the laboratory of the Institute of Geology, Academia Sinica, Beijing. We are grateful to Zhongpu Zhao for dis-

83

cuss ions and advice t h r o u g h o u t t he s t u d y . We t h a n k B o r m i n g J a h n , S h e n s u S u n , J. T a r n e y and U.G. C o r d a n i for t he i r c o m m e n t s a nd sugges t ions o n the m a n u s c r i p t .

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