Journal of Southeast Asian Earth Sciences, Vol. 3, Nos 1-4, pp. 283-291, 1989 0743-9547/89 $3.00 + 0.00 Printed in Great Britain Maxwell Pergamon Macmillan plc

Characteristics and evolution of the Lower Precambrian basement of the North China platform

CUI WENYUAN,* WANG SHIQI,* WEI CHUNJING* a n d TAO QUAN t

*Department of Geology, Peking University, Beijing; l"Beijing Research Institute of Uranium Geology, Beijing, People's Republic of China

Abstract--The Lower Precambrian basement of the North China platform is composed of Lower, Upper Archaean and Lower Proterozoic metamorphic complexes. The Lower Archaean metamorphic complex, distributed mainly in the North Border sub-domain, consists of granulite, gneiss, amphibolite and pyroxenite. The metamorphic grade is up to granulite facies. The Upper Archaean metamorphic complex, distributed extensively in the whole region, is composed of gneiss, leptynite and amphibolite with minor granulite and magnetite quartzite. The metamorphic grade is predominated by amphibolite facies with minor granulite and greensehist facies locally. The Lower Proterozoic metamorphic complexes occur mainly in some peripheral and internal downwarped belts of the Archaean craton. The rock formations may clearly be divided into eugeosynclinal and miogeosynclinal facies. The metamorphic grade is chiefly of greenschist facies.

According to the studies of some geological characteristics, the evolutional history of the Precambrian basement can be summarized as follows. At 3400-3500 Ma, a simatic protocrust might be present in the region. The first epoch of metamorphism accompanied by some charnockite intrusions occurred at about 2800-3000 Ma. This was followed by the second epoch of metamorphism accompanied by abundant intrusions of tonalite, trondhjemite and granodiorite at about 2500 Ma. The early Proterozoic tectonic settings were remarkably different from the Archaean as different geosynclines developed at the peripheries and interior of the Archaean craton. At about 1800 Ma, the basement of the North China platform was consolidated by the Luliang movement.

INTRODUCTION

Tim NORTH China platform is bounded at 30-45°N and 105--128°E with extensive exposures of Precambrian metamorphic rocks, especially Archaean metamorphics. Since the 1970s, a number of geologists have done much work on their primary rock formations, metamorphism, magmatism, geochemistry, geochronology, tectonic framework and crustal evolution and made a great progress in their understanding (Dong et al. 1986, Cheng et al. 1986, Zhao 1980, Qian et al. 1985, Chen et al. 1981).

On the basis of earlier work, this paper gives the characteristics and evolution of the Lower Precambrian basement of the platform as a result of the studies made by the authors in recent years.

CHARACTERISTICS OF THE LOWER PRECAMBRIAN BASEMENT OF NORTH

CHINA PLATFORM

The Precambrian of the North China platform can be divided into the Archaean and the Proterozoic according to primary rock formation, metamorphism, isotopic ages and tectonic movements. The Archaean can be subdivided into the Lower and Upper Archaean, and the Proterozoic into the Lower and Mid-Upper Proterozoic. Moreover, four tectonic movements and stages have been identified (see Table 1). The Mid-Upper Pro- terozoic unmetamorphosed cover will not be dealt with in this paper.

The basement of the whole North China platform can be divided into four sub-domains on the basis of

its geological characteristics, regional distributions and other criteria (Dong et al. 1986, Cheng et al. 1986). They are as follows: the North Border, Shanxi- Hebei-Shandong-Henan, Jiao-Liao (east Shandong and Liaoning) and Huaiyang sub-domains (Fig. 1).

Lower Archaean geological characteristics

The Lower Archaean metamorphic complex chiefly occurs in the North Border sub-domain. These rocks, making up a granulite belt 1500 km long and 200 km wide, are distributed from Linhe, Inner Mongolia in the west, eastward to Jingyu, Jilin Province. The metamor- phic strata mainly include the Qianxi Group, Guzuizi Group, Miyun Group, Lower Jining Group, lower part of Jianping Group, lower part of Anshan Group and Longgang Group. The Qianxi Group, exposed in east Hebei Province, is composed of pyroxenite (amphib- olite)--granulite formation in the lower part and biotite-plagioclase gneiss-granulite formation in the mid-upper part. The former is mainly basic granulite and basic granulite interlayered with acidic granulite, and the latter mainly biotite--plagioclase gneiss, hornblende-plagioclase gneiss and diopside amphibolite with minor intermediate-basic granulite. Both of them are intercalated with magnetite quartzite and amphib- olite. The protoliths may be intermediate-basic volcanic rock-greywacke formations.

The metamorphic complex of granulite facies exposed to the east of Zhangiiakou includes the Guzuizi Group, the Miyun Group, the lower part of the Jianping Group and the lower part of the Anshan Group in Qingyuan and Huadian areas is similar to those of the Qianxi Group.

283

284

o o 03

.o e L

<

CuI WENYUAN et al.

Table I. Division and main tectonic events of the Precambrian in the North China platform

Tectonic movement, cycle and stage

p ~

a~ Middle-late Proterozoic epoch ~ ~

, , ,~ , , ,~ ,~ ,~ ,~ L iilia ng movement

03

o Lfiliang epoch o

.o ~ Wutai movement

U~ Wutai epoch

~ Fuping movement ~

< o

03

,r,

<

Fuping epoch

Qianxi movement

Qianxi epoch

g, o

t ~

0 3 0

03

O

8

Time (Ma)

1800 + I00

2300 + I00

2500 + 100

0 ..~., e~

~_ 2800-3000

._~ e ,

2

3400-3500

Bndons

t ~ Lower Archaean

Upper An:haea n

Lower pfll~cneOlC

• Bounda~, hc;ween ~ub dc~maifl ~ 200 400kin

I I

Fig. 1. Schematic map of the distribution of early Precambrian complexes in the North China platform.

Lower Precambrian basement of North China platform 285

The Lower Jining Group, exposed in Datong, Yang- gao and Mashikou areas to the west of Zhangiiakou, is composed of intermediate granulite and minor basic and acidic granulite intercalated with lenses of biotite- plagioclase gneiss, plagioclase pyroxenite (amphibolite) and thin layered magnetite quartzite. The protoliths may be chiefly tonalite (or dacite) and minor tholeiite in the lower part, and intermediate-acidic volcanic rocks, clas- tic rocks and siliceous ironstones in the upper part.

In addition, some intrusive charnockites outcrop in Xianjinchang of Qingyuan, Jianping of west Liaoning Province, Qian'an, Taipingzhai, Shangchuan, Xiao- heiting and Shirengou of east Hebei Province.

The isotopic age of about 3400-3500 Ma obtained from the metamorphic rocks of the Qianxi Group may represent the age of the protolith formation (Table 2, nos 1-3, 5-7); the age of 2800-3000 Ma may be that of the first epoch of granulite facies metamorphism (Table 2, nos 4, 8-13); and the age of 2700Ma may be that of the extensive intrusions of charnockite (Table 2, nos 14-17).

Four formulae of two-pyroxene geothermometer (Wood and Banno 1973, Wells 1977, Lindsley et al.

1981, Lindsley and Dixon 1976) and three formulae of geobarometer (Newton and Perkins 1982, Wells 1979, Wood 1974) have been used to calculate the temperature and pressure of metamorphism for each area. The results, listed in Table 3, are shown in Fig. 2. It is obvious that the metamorphoic temperature of the Gehuyao area is the highest; that of Xuanhua area lowest; and that of the other areas intermediate. The metamorphic pressures of Gehuyao, Miyun and Jian- ping areas are the highest while that of Baotou and Xuanhua areas is the lowest and that of the other areas intermediate. This indicates that the heat flow and burial depth during metamorphism were different in different areas.

The data of rare earth elements (REE) determined by neutron activation analysis show that the REE patterns from the amphibolite of the Qianxi Group may be divided into two types• Type I (Fig. 3A), which is flattened, is characteristic of oceanic tholeiite. Type II resembles the REE pattern of continental tholeiite which is slightly enriched in light REE and shows mostly negative Eu-anomalies (Fig. 3B). The granulites of the Qianxi Group are enriched in light REE, depleted in heavy REE and show negative Eu-anomalies (Fig. 3C). The felsic gneisses are enriched in light REE and with positive Eu-anomalies (Fig. 3D); and the charnockites in Qianxi area are intermediately enriched in light REE and mostly with positive Eu-anomalies (Fig. 3E), which is similar to most Archaean tonalites.

The basic granulites from the Lower Jining Group have two types of REE pattern. Type I is similar to that of oceanic theoliite which is slightly enriched in light REE and with negative Eu-anomalies (Fig. 4A). Type II is similar to that of continental tholeiite which is en- riched in light REE and with negative Eu-anomalies (Fig. 4B). The intermediate granulites from the Lower Jining Group are strongly enriched in light REE, slightly

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Table 3. Temperature and pressure of the Archaean metamorphism in the North China platform

Area

Temperature (°C) Pressure (GPa)

Newton and Perkins Quantity 1982 Wells Wood Quantity

of mineral 1979 1974 of mineral pairs Tl T 2 T 3 T 4 800°C 850°C 800°C 800°C assemblages

Baotou 18 847 894 852 826 8.22 8.32 9.8 9.76 10 Tuguiwula 22 878 869 867 766 C_rehuyao 18 894 961 912 942 11.75 11.14 13.46 1 Huai'an 20 832 874 871 793 8.83 9.08 10.47 9.95 6 Xuanhua 17 800 817 819 703 8.40 8.64 10.17 2 Miyun 15 852 866 796 761 11.69 12.11 13.09 11.86 6 Shirengou- 11 840 851 815 738 9.85 10.15 11.52 10.88 4 Shangzhuang Taipingzhai 15 849 895 850 824 10.72 11.06 12.52 10.73 2 Qian'an 8 825 870 891 794 Jianping 26 908 951 783 899 10.96 11.35 12.65 14.14 8

* T I, Wood and Banno (1973); T 2, Wells (1974); T 3, Lindsley et al. (1981); /'4, Lindsley and Dixon (1976).

depleted in heavy REE and with negative Eu-anomalies, which resembles that of tonalite (Fig. 4C).

Upper Archaean geological characteristics

The Upper Archaean metamorphic complex is distrib- uted in all the four sub-domains.

1. North Border sub-domain. The upper part of the Anshan Group, outcropping in Anshan, Benxi and Qingyuan areas may be the representative of the east part of this sub-domain. In Qingyuan area, there are amphibolite and minor pyroxene amphibolite, biotite-plagioclase gneiss and leptynite in the lower part; amphibolite interlayered with biotite leptynite and lep- rite in the middle part; and biotite-leptynite, two-mica- quartz schist, magnetite--quartzite, quartzite and marble in the upper part. Moreover, there are large quantities of intrusive tonalite and monzonitic granite. The proto- liths of the metamorphic rocks might be intermediate to basic volcanic rocks in the lower part; intermediate-basic volcanic rocks intercalated with intermediate-acidic vol- canic rocks and pyroclastic rocks in the middle part and

sedimentary rocks in the upper part, which make up a complete volcano-sedimentary cycle.

Luanxian Group in east Hebei Province and the Dantazi Group between Zhangjiakou and Longhua areas may represent the central part of this sub-domain. The metamorphic rocks are amphibolite with biotite- leptynite in the lower part and biotite-leptynite and two-mica schist interlayered with magnetite-quartzite in the upper part, the protoliths of which may be a series of volcanic rocks and greywackes.

The Upper Jining Group and Wulashan Group may represent the west part of this sub-domain. The Upper Jining Group is predominated by a khondalite series which includes rocks such as sillimanite-feldspar gneiss, leptite and biotite-plagioclase gneiss interlayered with cordierite bearing sillimanite-plagioclase gneiss, granu- lite and marble, the protoliths of which may be clay- stone, siltstone with intermediate-acidic volcanic rocks and pyroclastic rocks. Wulashan Group consists mainly of granulite, leptynite and hornblende (biotite)-- plagioclase gneiss intercalated with amphibolite, magne- tite--quartzite and thin layered marble, the protoliths of

1000

50O B

0.5 1

p ( G P a )

1 I . I 1.2 1.3

1000

500

Fig. 2. P-Tdiagram of the granulite facies metamorphism from the North Border sub-domain. A, B, the dry and wet solvus of granite, respectively. 1. Baotou area; 2. Gehuyao area; 3. Huai'an area; 4. Xuanhua area; 5. Miyun area; 6. Shirengou

area; 7. Taipingzhai area; 8. Jianping area.

Lower Precambrian basement of North China platform 00t 00

100:

50- ~ ~ .

10 - ~ ' - - ~ ~ A

[:a de l~r I~d Sm E'u ~ T'b [~ I~o E'r I;~, ~ l:u

Fig. 3. Chondrite-normalized REE distributions in the rocks of Qianxi Group. A. Amphibolite (type I); B. Amphibolite (type II); C. Granulite; D. Felsic gneiss; E. Charnockite.

287

which range from intermediate-basic volcanic rocks in the lower part to clastic rocks and carbonates in the upper part.

The metamorphic rocks from the west part of this sub-domain belong mainly to the granulite facies, locally

to amphibolite facies. The P - T conditions of metamor- phism are listed in Table 3. Most of them belong to the medium-pressure type. It is remarkable that cordierite is present in the aluminium-rich gneiss from Baotou and Liangcheng areas of Inner Mongolia, and garnet has not

I00

50:

~ 50'

10

' r i i i ,

t~a Ce ! ~, Nd Sm E'u Gd ]Jb I~ lqo Er TmYb Lu

Fig. 4. Chondrite-normalized REE distributions in the rocks of Lower Jining Group. A Basic granu]ite (type I); B. Basic granulite (type II); C. Intermediate granulite.

288 CuI WENYUAN et al.

50

100

10

i °

100- ~ ;10

10 A ~ 10

l~', (~c I~r N~t Sm Eu (.~ TTb r~y I~, l~rTmYbl'.u Fig. 5. Chondrite-normalized REE distributions in the rocks of Luanxian, Upper Jining and Wulashan Groups. A. Amphibolite of Luanxian Group; B. Hornblende pyroxcnite of Upper Jining Group; C. Sillimanite--garnct-plagioclase gneiss of Upper Jining Group; D. Pyroxene arnphibolite of Wulashan Group; E. Granulite of Wulashan Group; F.

Biotitc-plagioclasc gneiss of Wulashan Group.

been found in the meta-basic rocks from Tuguiwula, which indicates that some low-pressure type of meta- morphism may be present.

The metamorphic rocks from central and east parts are predominated by amphibolite facies with minor greenschist facies locally.

A number of isotopic ages of 2500 Ma (Dong et al. 1986; Table 4, nos 1 and 2) obtained may represent the age of late Archaean metamorphism.

Accompanying the late Archaean metamorphism there occurred extensive intrusions of tonalite, trond- hjemite, granodiorite and some potassium-rich granites whose isotopic ages are listed in Table 4 (nos 6-10).

It has been found that the REE patterns from the amphibolites of Luanxian Group, the hornblende pyrox- enites of the Upper Jining Group and the pyroxene amphibolites and granulites of the Wulashan Group (Fig. 5A, B, D, E) are similar to that of tholeiite which is slightly enriched in light REE. The former two have negative Eu-anomalies which may be related to the fractionation of plagioclase. The biotite-plagioclase gneiss of the Wulashan Group is strongly enriched in light REE and depleted in heavy REE (Fig. 5F). The sillimanitc-garnet-biotitc-plagioclase gneisses of the Upper 'Jining Group are highly enriched in light REE

and with strong negative Eu-anomalies (Fig. 5C), which is similar to that of Archaean sedimentary rocks from west Australia.

2. Jiao-Liao sub-domain. The Upper Archaean meta- morphic strata in this sub-domain include the upper part of the Anshan Group outcropping in the Xinjin area of the south Liaodong Peninsula and Jiaodong Group in Shandong Peninsula. They are mainly composed of biotite leptynite, biotite-amphibolite and amphibolite with minor biotite-quartz schist, magnetite-quartzite and marble. In addition, some eclogite lenticles are present in Jiaodong Group. Most of these rocks belong to amphibolite facies and only a few of them at the top belong to greenschist facies.

An isotopic age of 2680 + :5 Ma (Table 4, no. 3) has ~a been obtained from Jiaodong Group which belongs to the late Archaean metamorphic epoch.

3. Shanxi-Hebei-Shandong-Henan sub-domain. The Upper Archaean Taishan Group outcropping in Taishan area of west Shandong Province is made up of biotitc-plagioclase gneiss, hornblendc-biotitc- plagioclase gneiss, amphibolite, biotitc-leptynite, leptite and magnetic-quartzite. Their protoliths are pre- dominated by intermediate-basic volcanic rocks and greywackes with minor sedimentary rocks on the top.

Lower Precambrian basement of North China platform 289

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The metamorphic grade is mainly of high-amphibolite facies• It should be pointed out that some low pressure facies series type with andalusite are present in Yanling- guan area. Most isotopic age data published are around 2500 Ma (Dong et al. 1986, and Table 4, no. 4) which belongs to the late Archaean metamorphic epoch•

The Upper Archaean metamorphic strata outcropping in Henan Province include the Dengfeng Group and the Taihua Group• The Dengfeng Group is composed of amphibolite, leptynite and leptite, with minor mag- netite-quartzite, and protoliths of which may be volcano-sedimentary rock series• In addition, there are extensive outcrops of granitic rocks and migmatitic gneisses. The metamorphism of the main epoch belongs to amphibolite facies of medium pressure type. An isochronic age of 2562 Ma (Zhang et al. 1985) has been obtained by the Rb-Sr method from the metamorphic rocks of Dengfeng Group which belongs to the late Archaean metamorphic epoch. The Taihua Group con- sists of supracrustal rock series and felsic gneisses. The former includes rocks such as gneiss, amphibolite, minor granulite and marble, the protoliths of which may be a series of volcano-sedimentary rocks. The major part of Taihua Group is made up of felsic gneisses which corre- sponds to tonalite in chemical composition. The meta- morphism of the Taihua Group is up to the granulite and amphibolite facies, the temperature is 680-760°C, and the pressure is 0.8-1.0 GPa. A metamorphic age of about 2620 Ma (Zhang et al. 1985) has been obtained by the Rb-Sr method. Obviously, it belongs to the late Archaean metamorphic epoch.

The Upper Archaean metamorphic rocks outcropping in Taihang-L/Jliang area may be divided into lower and upper rock suits. The former includes Fuping, Zan- huang, Huoxian and Taiyueshan Groups which are mainly composed of several rhythmic sequences of biotite--plagioclase gneiss, leptite and amphibolite, locally, intercalated with sillimanite gneiss. Some two- pyroxene-plagioclase granulite and garnet-hyperthene-- magnetite-quartzite are found in the lower part of the Fuping Group. The latter includes the Longhuahe Group, the lower part of Wutai Group and Yun- zhongshan Group, etc. The rocks are mainly biotite leptynite with amphibolite, leptite, gneiss and marble, which make up a few rhythmic sequences. The protoliths might be intermediate-basic volcanic greywackes, clastic rocks and carbonates, which make up a volcano- sedimentary rock series of flysch type. An isotopic age of 2800-I-~0 ° Ma has been obtained from perfectly rounded zircon in the basal part of Fuping Group (Liu 1984, Dong et al. 1986), which indicates that the sedi- mentation occurred later than 2800 Ma and belonged to the late Archaean. The metamorphic facies in this area are mostly amphibolite facies with minor greenschist and granulite facies. Progressive metamorphic belts may be present in some areas.

4. Huaiyang sub-domain. The Dabieshan Group oc- curring in this sub-domain is distributed in the border area of Hubei, Henan and Anhui Provinces, is com- posed mainly of biotite--leptynite, biotite-plagioclase

290 CUI WENYUAN et al.

gneiss, amphibolite and biotite--hornblende-plagiodase gneiss intercalated with leptite and magnetite-quartzite. Their protoliths may be intermediate-basic volcanic rocks and greywackes with clear rhythm which are interlayered with pelitic rocks and are associated with eclogite and ultramafic intrusive bodies.

Metamorphisms of the Dabieshan Group belong to the regional medium-high temperature type. Most of them are of amphibolite facies.

Lower Proterozoic geological characteristics

In the North China platform, Lower Proterozoic metamorphic complexes are distributed in some periph- eral and internal downwarped belts of the Archaean craton. They are represented by the Sanheming (Dong- wufenzi), Erdaowa, Hongqiyingzi (Upper Chongli group), Zhangjiafen, Zhuzhangzi, Beiliaohe and Laoling Groups, etc., from west to east in the North Border sub-domain; Liiliang, Wutai, Hutuo, Gantaohe, Zhong- tiao, Songshan, Jining (in Shandong Province) Groups, etc., in Shanxi-Hebei-Henan-Shandong sub-domain; Nanliaohe, Fenzishan, Wulian and Jiaonan Groups in Jiao-Liao sub-domain and Luzhenguan and Fuziling Groups in the Huaiyang sub-domain.

Since the beginning of the Proterozoic, the rock formations of the North China craton have differentiated into eugeosynclinal and miogeosynclinal facies. The former is represented by the Wutai, San- heming and Zhuzhangzi Groups which are mainly composed of chlorite schist, amphibole schist, plagioclase-amphibole schist, biotite-amphibole schist, actinolite-quartz schist intercalated with magnetite- quartzite in the lower part, and sericite-quartz schist, two-mica leptynite, quartzite and meta-conglomerate in the upper part. The protolith sequences may be a series of intermediate-basic and intermediate-acidic volcano- sedimentary formations in the lower part and clastic rocks and daystones deposited in shallow water in the upper part. The latter is represented by the Hutuo and Songshan Groups which are made up of meta- conglomerate, quartzite, sericite-quartz schist, phyllite and dolomitic marble, the protoliths of which may be terrigenous dastic rocks, claystones, semi-claystones and carbonates.

The early Proterozoic metamorphism in this region is mainly of greenschist facies with minor low amphibolite facies in some areas. According to the data from Wutai, Liiliang and Zhuzhangzi Groups, the metamorphic tem- perature is from 350 to 550°C and the pressure between 0.25 and 0.7 GPa.

Most of the isotopic age data obtained are between 2300 and 1800 Ma (Table 4, nos 11-16), belonging to the early Proterozoic.

EVOLUTION OF THE LOWER PRECAMBRIAN BASEMENT OF NORTH CHINA PLATFORM

The basement of the North China platform is com- posed of Archaean and Lower Proterozoic metamorphic

compiexes. The former makes up the major part of the basement, which is the oldest one in China. The latter is chiefly present in the peripheral and internal down- warped belts and makes up different geosyndinal sys- tems.

The Lower Archaean metamorphic formations are as follows: the lower part is pyroxenite (amphib- olite)-granulite formation; and the upper part is biotite-plagioclase gneiss-granulite formation. The chemical compositions of basic granulite and amphib- olite are similar to those of tholeiite and calc-alkaline basalt. Most of their REE distribution patterns are similar to those of continental tholeiite, and a few of them to those of oceanic tholeiite. These rocks may represent the simatic protocrust which was already in existence at around 3400-3500 Ma based on some avail- able isotopic age data. At about 2800-3000 Ma, the first epoch of regional medium-high temperature metamor- phism took place, and an embryonic form of the ancient crystalline basement developed. This period is named the formation stage of the continental nucleus. Some tonal- ites and charnockites were then intruded successively.

The Qianxi movement accompanying the first epoch of metamorphism made the ancient continental nucleus strongly deformed. It has been deduced that the struc- tural trend at that time was in a nearly east-west direction.

The P-Tconditions of granulite facies metamorphism are 0.8-1.3 GPa and 750-900°C, respectively, which correspond to a burial depth of 28.5-45km during metamorphism. Obviously, the Lower Archaean meta- morphic rocks exposed on the earth's surface at present may be near or at the base of protocrust at that time. It can be deduced that the protocrust may be much larger than the granulite belt now exposed.

The late Archaean tectonic settings can be divided into east and west parts bordering at Zhangjiakou. The east part, controlled by the ancient continental nucleus of east-west extension, includes the Upper Archaean Lu- anxian Group, the Dantazi Group, and the upper part of the Anshan Group, which are composed of intermediate-basic or intermediate-acidic volcano- sedimentary formation and flyschoid formation. In the west part, especially in the Jining-Fengzhen area, a series of aluminium-rich sedimentary rocks and carbon- ates are developed; in the other areas with exposed late Archaean metamorphic rocks, some intermediate- basic volcanic rocks and greywackes are developed, which indicates that a differentiation of original rock formations occurred since the beginning of the late Archaean. At about 2500Ma the second epoch of metamorphism took place on a large scale. The meta- morphism made the Upper Jining Group and Wulashan Group metamorphose to the granulite facies, and made the metamorphic rocks of the granulite facies to the east of Datong remetamorphose to the granulite facies for some areas, or retrogressively metamorphose to amphib- olite facies elsewhere. Accompanying this epoch of met- amorphism, a large quantity of tonalite, trondhjemite, granodiorite and some potassium-rich granites were intruded in Miyun, Qianxi of Hebei Province; Jianping,

Lower Precambrian basement of North China platform 291

etc., of Liaoning Province; Dengfeng area of Henan Province and Hengshan area of Shanxi Province. The strong activity of intermediate-acidic magma indicated that the crust was unstable at that time.

The Fuping movement, corresponding to the second epoch of metamorphism, again deformed the Archaean rocks. The main structural trend was varied with different areas. In the North Border sub-domain, some highly folded structures with a north-south trend were developed.

By the end of the late Archaean, the crystalline basement had consolidated. This period is named the accretion stage of the continental nucleus.

During early Proterozoic time, the Archaean crust continued to be mobile, which resulted in the splitting and migrating of the Archaean craton. Some aulacogens were developed at the periphery and interior of the craton. Because of the difference of tectonic settings, the original rock formations were highly complicated compared to the Archaean. Eugeosynclinal, miogeo- synclinal and some intermediate facies have been identified. The granites emplaced at that time were mainly potassium-rich ones which were very limited in quantities. Tonalite, trondhjemite and granodiorite are very rare. The structural styles of the Lower Proterozoic are mainly linear composite folds. In south and north peripheries, the structural trends are of north-south direction, but in the interior, they are N.E. to N.N.E. Metamorphic grade is up to greenschist and amphibolite facies. At 1800 Ma, the Liiliang movement formed the stable crystalline basement of North China platform, and thus the evolutional history of the early Pre- cambrian came to a close.

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