Geochemistry of a Rapakivi Granite Suite in aProterozoic Rift Trough in Beijing and Its Vicinity

Yu Jianhua, Fu Huiqin, Zhang Fenglan and Guan MeishengBeijing Centre of Geological Research and Laboratory

Abstract

Controlled by E-W- trending faults, a Proterozoic (1.4-1.8 Ga old) rapakivi granite suite was intruded in

Beijing and the area to its east (within Hebei Province), forming three parallel belts of igneous rocks. The

isotopic, trace element and rare earth element geochemical data of a bimodal rock association made up of

anorthosite, gabbro and alkali basalt and olivine-bearing quartz-syenite, rapakivi granite and trachyte as well

as potassic A- type granites and anorogenic granites-- all suggest that there exists an incipient rift in the

study area. Fractional crystallization of a mixed magma formed by the magma derived from the upper mantle

and the magma derived by small degrees of fusion of the lower crust produced anorthosite cumulates. The

water-deficient granitic magma was differentiated into a subalkaline series. When the fractional crystallization

was incomplete, rhythmic eruptions took place.

Features of the Beijing Rapakivi Granite Suite

Anorogenic magmatism in the interior of the Proterozoic craton or on its margins gaverise to a gigantic rapakivi granite-anorthosite belt extending from the southwestern UnitedStates northeastwards through Labrador of Canada and across southern Greenland intothe Baltic shield in the northern hemisphere (Anderson, 1983; Haapala, 1985; Emslie, 1978).An anorogenic rapakivi granite suite of similar nature is also developed in a MiddleProterozoic rift zone (Chen, 1981) or aulacogen (Wang et al., 1984) in Beijing and its vicini-ty in the northern part of the North China platform (Fig. 1). The northern boundary of therift trough is a deep fault stretching along the southern edge of the Inner Mongolian axis,while its southern boundary is covered by the Quaternary in Hebei Province. From the be-ginning of the Middle Proterozoic on, the craton mainly experienced two stages: (1) the rift-ing stage from the deposition of the Changzhou Formation to the deposition of theDahongyu Formation, during which a suite of sedimentary and volcanic rocks withtaphrogenic features was formed; (2) the wholly downwarping stage beginning with thedeposition of the Gaoyuzhuang Formation. At the end of the deposition of the QingbaikouSystem, about 800 Ma ago, the whole area was uplifted and became a vast land and thenentered the stage of stable development of a Palaeozoic platform; so the Sinian is absent.Substantial geophysical work (Liu, 1978) has revealed that the E-W-trending deep fault is alithospheric one dissecting the upper mantle and that there are distinct steplike anomalies of

Vol. 4 No. 2 ACTA GEOLOGICA SINICA June 1991

the gravity field in the rift trough. The appearance of the platform line due to the mantleuplift on the Moho shows the sign of possible mantle diapirism. Controlled by theE- W- trending deep fault there occurred a suite of intrusive and volcanic rocks withanorogenic features concurrent with the development of rifting within the platform.Althongh the suite has been cut and covered in a NE direction by the Yanshanian(Mesozoic) structures and a sequence of caic- alkaline magmatic rocks when it extendswestwards to the Badaling Hills and the Western Hills, three parallel magmatic rock beltsare still clearly recognizable in the E-W-trending fault belt.

V

4Changping

+V Y?

V (

I- Huairou

- V Vw V /-' Innerf y ¡' ,/ Mongolian AxisChangshaoying

Gubeikou

Yanshan Rift Trough

Beijing

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Beijing CityflpoperIIII 2

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Fig. 1. Sketch map showing the distribution of the tectonics and igneousrocks in the northeastern part of Beijing.

1. Archaean gabbro diorite; 2. Proterozoic anorthosite; 3. Proterozoic granitoids; 4. Proterozoic gabbro and syenite

porphyry; 5. breccia of volcanic neck facies; 6. volcanic rock series of the Dahongyu Formation of the Changcheng Sys-

tem; 7. Mesozoic diorite-granitoids; 8. Jurassic-Cretaceous volcanic rocks.

The northern intrusive rock belt Along the Chicheng- Changshaoying- Gubeikoufault, there occurs a 100 km long rock belt made up of quartz-syenite and K-rich granite in-trusions, and on its northern side there occur early-intruded anorthosite and gabbro thatare parallel to the belt. Previously these rocks were assigned to Archaean migmatite orYanshanian intrusions, but the phenomenon that they are intruded into Archaeanmetamorphic rocks and covered by the Jurassic can still be recognized The isotopic age ofthe quartz- syenite is consistent with the age of the Damiao complex (consisting ofanorthosite / norite and monzonite) (Table 1), both being the product of magmatism in theearly stage of rifting (1400-1800 Ma). Granitic rocks exhibit coarse- grained equigranular

170 Vol.4 NO. 2 ACTA GEOLOGICA SINICA June 1991

4.

- -yV y

texture and some of them exhibit murmekitic texture, no rapakivi texture being observed.Their common feature is that they are rich in K-feldspar (more than 50%). The Lanjing

Table 1. Isotopic Age Data of Proterozoic Igneous Rocks

1)Zr-U/ Pb; zircon U-Th-Pb age; K/ Ar; hornblende-biotite K-Ar age (Ad=0.581x l0°a, Aß =4.962x 10°a); Rb/ Sr; whole-rock Rr-Sr isochron age (A= 1,42± 10_lOaH); Sr0; initial Sr isotopic ratio;

Nd0; initial Nd isotopic ratio.

2) Unpublished data.

quartz- syenite in the western part of the belt contains fayalite, hypersthene, Fe- rich angiteand hornblende, so should be termed olivine- bearing quartz- syenite. It closely resemblesmedium- grained fayalite- bearing tirilite in the Viborg intrusion of Finland or olivine- bear-ing mangerite of North America (Anderson, 1983; Haapala, 1985; Emslie, 1978). They are

Locality / lithologyIsotopic dating

methodIsotopic age Ma0 Initial isotopic ratio Data source

Damiaocomplexin Sm/Nd 1735±239 Nd0 = 0.5101 Jiaetal., 1978Hebei Province;

40Ar/ 39Ar 1656± 15 Hu et al., 19782)anorthosite-norite andmonzonite

Rb / Sr 1686 ± 194 Sr0 = 0.7040 Jia et al., 1978

Northern intrusive rock Zr-U/Pbbelt of Beijing; Hb.K / Ar 1790 ± 24olivine-bearing quartz-synite in northern partof Huairou County

Dilution method1455 ± 18

1735± 23Sm/NMiddle intrusive rock Zr-U / Pb 1715 ± 31

Jia et al., 1978belt of Beijing; hornb- °

Ar / 39Ar-Hb1716 ± 21

Hu et al., 19782)lende-biotite-plagioclase Rb/ Sr 1686± 193

Hu et al., 19782)rapakivi granite in Sha-

Bi-K / Ar1588 ± 50

Zhao, 1964chang intrusion, Miyun

Bi-K / Ar1562 ± 50

Sr0 = 0.7070 Zhao, 1964County (first intrusion) Rb/ Sr 1311 ±

± 0.0049R=0.9978

Porphyritic granite in1078 ± 107 Sr0 = 0.7286

Shachang intrusion(second intrusion)

Rb / SrR = 0.9719 ± 0.0161

Medium-grainedbiotite-granite in

Bi-K / Ar 1280±50 Zhao, 1964Shachang intrusion(third intrusion)

Southern volcanic rockbelt in southern part of Upper horizon

Sr0 = 0.7082Beijing; 1371±30

±0.0012Dahongyu Fm. of Rb / Sr R = 0.9995Changcheng Sys. and Rb / Sr Lower horizon

Sr0 = 0.7045alkali basalt and 1543 ± 69

± 0.0084trachyte in Beishan, R = 0.9980Pinggu County

Geochemistry of Rapakivi Granite Yu et al. 171

all the oldest granitoid intrusions in the batholith, but the rocks in the study area are coarsein grain size and have a higher content of K-feldspar. In the Gudonggou and Gubeikou in-trusions in the middle and eastern parts of the belt, the quartz content increases, thedark-coloured minerals are dominated by hornblende and biotite and their K-feldspar con-tent (55%+) and K20 content (5.9%) are similar to those in the middle belt, but the rocksshow no rapakivi texture. Therefore, in order to highlight their K-enriched character, theyare termed K- rich granite. Anorthosite intruded by quartz- syenite is composed ofcoarse- grained lathiike plagioclase cumulates with minor augite, hornblende, apatite andtitanomagnetite. There is veined vanadiferous titanomagnetite in the intrusions. As sodiumzoisite is very well developed, most of the plagioclase is andesine (An = 3 5-45).

The middle intrusive rock belt Along the Miyun- Qiangzilu fault there occur theShachang plagioclase rapakivi granite intrusion and gabbro dykes. The Shachang intrusionhas a length to width ratio of 6:1 and is composed of granites resulting from three pulsatoryintrusions. The early- intruded rapakivi granites, whose age is similar to that of the intru-sions of the northern belt, are the product of the late stage (> 1400 Ma) of the suite.Late-intruded granites, whose ages are all over 1000 Ma, are the product of the late stagewith a time interval possibly continuing until the Late Proterozoic Jixianian Period. Theplagioclase rapakivi texture is common in rapakivi granites. In order to give prominence toits oval phenocrysts, the texture is called rapakivi texture for short. The features of the tex-ture has been described by Zhao Chonghe et al.© Through observations the present au-thors want to emphasize the following four points: (1) Ovoids composed of several clearlydefined K-feldspar grains in a petal-shaped poikilitic texture protrude locally into the rimsof plagioclase, showing the features of regular intergrowths formed by magmaticcrystallization. Their formation is obviously related to the nature of magma. (2) Among thethree rock belts, only the hypabyssal rocks with a porphyritic texture in the middle belt ex-hibit the rapakivi texture, implying that one of the essential conditions for its formation isrelated to the tectonic position during the high- level near- surface localization of magmas.(3) The rapakivi ovoids are replaced by plagioclase rims to some extent, dark-coloured ma-trix minerals fill along the fissures and the K-feldspar ovoids in various dyke rocks (e. g.diabase dykes) are resorbed and injected by the matrix all these phenomena indicatethat the sequence of crystallization is different from that of normal granites, and it is evi-dent that K- feldspar crystallizes out preferentially. (4) The later- intruded porphyriticbiotite- granite also contains a small amount of rapakivi ovoids, but they are small in sizeand plagioclase rims are seldom present; the porphyritic texture predominates within the in-trusions. The rapakivi texture no longer appears in medium and fine-grained biotite- or twomica- granite resulting from the third intrusion. They are distinguished from rapakivi gran-ite by their increasing quartz content, decreasing K-feldspar content, dominance of albite inplagioclase and coexistence of bio tite and muscovite. It is apparent that the increase of acid-ity and albitization and muscovitization have modified the nature of the magma that pro-duces the rapakivi texture.

The southern volcanic rock belt The Changcheng System is mainly represented bythe first member of the Dahongyu Formation. In Beishan of Pinggu County there occurs a

(fZhao Chonghe, 1964. Geological and petrological research on plagioclase rapakivi granite in Miyun, Beijing, fl:

Proceedings of the First Conference of Mineralogy, Petrology 4nd Geochemistry sponsored by the Geological Society

of China(Rock Part), pp. 124-143.

172 Vol. 4 NO.2 ACTA GEOLOGICA SINICA June 1991

Geochemistry of Rapakivi Granite Yu et al. 173

K- rich volcanic rock belt about 150 km long from east to west and 5-20 km wide fromnorth to south, which is marked by a rhythmic eruptive rock series composed of alkalibasalt and trachyte in the thickness ratio of 7:3. Five weak to strong eruption cycles may belargely distinguished. The whole volcanic rock series and the lower parts of various cyclesconsist mainly of alkali basalt, while upwards trachyte increases. According to the classifi-cation of volcanic rocks, basic rocks consist mainly of alkali basalt with some basanite andless pyroxene basalt. The phenocrysts of the rocks include labradorite (An = 53-63),Ti-bearing augite (Ti =2%), olivine and K-feldspar. Part of trachyte might be phonolite; therocks are made up of K-feldspar arranged in the comb form. They have a far higher alkali(particularly potassium) content than the calc-alkaline rocks. There are also a few dozens ofbreccia pipes of volcanic neck facies in the volcanic rock belt, and the rocks includeultramafic and K-rich basic volcanic breccia. Along the secondary faults there occur clus-ters of hypabyssal rock bodies such as gabbro, syenite gabbro and syenite porphyry bossesand apophyses. The belted distribution of volcanic rocks and arrangement of hypabyssalintrusions imply that the southern belt is controlled by E-W-trending hidden faults.

Fig. 2. QAP modal diagram of Proterozoic granitoids (after Strekeisen,1979; modal areas after Bowden, 1984).

1. A- type granite field; 2. I-type granite field; 3. s- type granite field; 4. transitional zone between the I- and

S-type granite fields; 5. quartz-syenite; 6. K-rich granite; 7. plagioclase rapakivi granite; 8. porphyritic grainte;

9. medium- to fine -grained grainte.

Petrological and Mineralogical Features of the

Rapakivi Granite Suite

The petrological and mineralogical features of the anorogenic rapakivi granite suiteobviously reflect that the magma itself is rich in potassium, iron and fluorine. So K-feldspar

crystallized earlier than plagioclase. This suite is distinguished from other granite suites byits iron-rich dark-coloured mineral association and fluorite as one of the accessory mineralassociation.

Table 2 Compositions of Dark-coloured Minerals in Proterozoic Granitoids

Notes; 1,2,3 and 5 denote quartz-syenite; 6 and 10, K-rich granite; 4,7,8 and 11, plagioclase rapakivi gran-

ite; 9 and 12, porphyritic granite. 01 = olivine. Py = clinopyroxene, Nb = hornblende, Bi = biotite.* Result of electron probe analysis, and others are results of chemical analysis.

On the QAP modal diagram for the petrological classification of Proterozoicgranitoids (Fig. 2), the data points are distributed continuously from the quartz-syenite fieldto the K- feldspar granite field, forming an evolutionary series with increasing silicic acids.The early-stage rapakivi granites and other rocks mostly lie in the transition zone betweenthe two fields, while the late- stage granites tend to fall on the edge of the alkali feldspargranite field. Modal areas of various types of granite are distinguished on Bowden' s (1984)diagram. The granitoids in the study area plot in th A-type granite field without exception.Their distribution and evolutionary trend are similar to those of A- type alkali granites ofNigeria or Australia. The Yanshanian (Mesozoic) calc-alkaline granitoids in the area plot in

% 1-0l 2-Py 3-Hb 4-Hb 5_Hb* 6_Hb*

Si02 29.01 47.64 39.30 39.10 40.39 30.97Ti02 0.00 0.37 4.85 1.71 1.87 2.00

Al203 0.00 0.50 9.12 11.26 8.47 8.25Fe203 8.12 8.59FeO 67.20 31.03 22.86 20.70 31.86 29.58MnO 1.92 1.06 0.49 0.51 0.59 0.57MgO 0.87 1.64 1.47 3.95 1.05 2.72CaO 0.11 16.32 6.91 7.50 9.79 9.64

Na7O 0.45 0.62 1.51 1.41 2.13 2.13K20 0.04 0.00 1.27 1.20 1.39 1.47P705 0.00 0.22 0.16 0.00 0.00 0.00

F - - 0.40 0.75 - -tot% 100.01 99.51 96.52 96.84 97.63 97.37

7_Hb* 8-Bi 9-Bi 10_Bi* 11_Bi* 12_Bi*

Si07 39.88 30.95 32.70 34.48 36.33 36.26Ti02 1.14 3.35 2.50 3.31 3.24 3.12

Al201 9.25 14.40 17.10 11.75 12.75 14.93Fe203 18.15 16.33FeO 29.04 13.85 14.50 34.05 28.83 30.16MnO 0.56 0.26 0.38 0.41 0.46 0.30MgO 3.37 5.50 1.50 1.41 4.65 2.12CaO 10.04 1.10 0.20 0.21 0.09 0.00Na20 2.44 0.07 0.10 0.43 0.30 0.41K20 1.60 4.32 7.80 8.63 9.02 8.95P205 0.00 0.06 0.04 0.00 0.00 0.00

F - 0.28 0.46 - - -tot% 97.40 92.09 93.61 95.15 95.97 96.46

1 74Vo1. 4 NO.2 ACTA GEOLOGICA SINICA June 1991

Geochemistry of Rapakivi Granite Yu et al. 175

the I-type granite field. It is evidentthat they and Proterozoic granites belong to differentpetrogenic series.

Plagioclase in the early- stage granitoids is mostly ordered andesine and oligoclase(An = 18-40; = 0.8-1.0); for rapakivi granites, andesine (An = 34-36) is dominant. Thelate- stage biotite- or two mica- granite shows relatively strong albitization, and theplagioclase contained therein is mostly albite (An= 5-10). X-ray diffraction analysis ofK-feldspar indicates its three-peak method ordering degree = 0.60-0.70, so it is intermediatemicrocline. The degree of ordering (= 0.80-0.95) of K-feldspar in the late-stage granites ishigher and approximates that of maximum microcline. Electron microprobe analysis(China University of Geosciences, 1988) has shown that K-feldspar in granitoids and vol-canic rocks contains 11.1-15.6% K20, 0.4-3.6% Na20 and 0.0-0.1% CaO the so-dium and calcium contents are very low.

The dark- coloured mineral composition (Table 2) clearly shows the iron- rich mineralassociation. The Fe2[SiO4] molecules in olivine (1OL) of olivine-bearing quartz-syenite at-tain 96.11%, so it is the rare iron- rich fayalite approaching the endmember of fayalite.There is also some tephroite (accounting for 2.16%) which occurs as isomorphs.Clinopyroxene, with Fe / Fe+Mg =91%, plots in the iron-rich augite field on the Fs-En-Wodiagram. Hornblende in the granitoids, with Fe / Fe+Mg = 0.90-0.97, belongs toferrotschermakite of the istisuite group in the chmical classification; biotite, withFe/ Fe+Mg = 0.89-0.97, belongs to the lepidomelane group. Therefore all the dark-col-oured minerals are rich in iron and poor in magnesium and belong to iron-rich species. Thisfeature is markedly different from that of the Archaean or Mesozoic calc- alkaline granites:the dark- coloured minerals of the latter are rich in magnesium and poor in iron. TheFe / Mg ratio in granitoids is also very high, with Fe / Mg = 9.37-26.47, the highest onebeing over four times higher than the average value of the world granite, which clearly re-flects the iron- enriched character that is different from that of the caic- alkaline graniteseries.

The accessory mineral association of the Proterozoic granitoids is ofmagnetite-apatite-fluorite type. Magnetite has a high Ti content (TiO4= 4.20%) and lowCr, Co and V contents. Another feature is the appearance of quite a few fluorine-bearingminerals, particularly in the late-stage biotite granites. Fluorite makes up 16.8% of the totalamount of accessory minerals. Measurements of the fluorine content in rocks and dark-col-oured minerals indicate that the fluorine content in rocks and hornblende or biotite increaseprogessiveIy with differentiation of granitic magmas, which conforms to the progressiveircrease of the amount of fluorite in the accessory minerals. This suggests that fluorine ingranitic magmas is the main volatile component which exerts an important effect on the na-ture of the magmas and tends to be concentrated in residual melts with crystallizationdifferentiation. Other accessory minerals often include hypersthene, augite, ilmenite, orthiteand monazite. The accessory mineral zircon has a characteristic light rose-purple tinge, andits HO2 content increases from the early-stage quartz-syenite (1.30%) and rapakivi granite(1.53%) to late-stage biotite-granite (1.86%). The total U and Th contents also increasefrom 0.019% and 0.029% to 0.151%. Therefore the Zr/ Hf ratio decreases from 42.8 and36.2 to 29.2. This regularity of variation proves that vQlatiles and some large-ion lithophileelements such as Hf, U and Th become enriched continuously in the residual magmas withthe progress of magmatic differentiation.

176Vol.4 NO.2 ACTA GEOLOGICA SINICA June 1991

The chemical compositions of various rock types in the rapakivi granite suite areshown in Table 3. With the ratio A / CNK ranging from 0.80 to 1.10 the rocks arepetrochemically of normal type or alumina-oversaturation type; in particular volcanic rocksand late- stage granites are mostly of alumina- oversaturation type. This coincides with thecase of Proterozoic anorogenic granitoids in other parts of the world (Anderson, 1983;

40

Major Element Geochemistry

A

NN

T

lo joS1020%

Fig. 3. The total alkali-silica variation diagram of Proterozoic igneous rocks (after Kuno, 1965).A = alkali basalt series; AL = high-Al (cale-alkali) basalt series; T = thole jite series. 1. Granitoids; 2. trachytes; 3.

basalts; 4. anorthosites.

Emslie, 1978); i.e., their petrochemical type is intermediate between the normal type and thealumina oversaturation type and may be called the near alumina- oversaturation type. Butmeanwhile it is also similar to A-type granites (Collins et al., 1982) and has by far higher al-kalis than the caic- alkaline series. The character of enrichment of alkalis, particularlypotassium, in the rapakivi granite suite of the study area is very typical in the rapakivi gran-ites of the world. The average alkali content (8.79%) and K20 content (5.45%) of thegranitoids are both noticeably higher than those of quartz-syenite or granites of China, andin all the rocks the potassium content is higher than the sodium content. With the evolutionof the magmas, the K/ Na ratio rose from 1.20 to 1.80-1.96 and then fell to 1.30. Thosewith the highest K / Na ratio are K-rich granites and rapakivi granites. The presence of thehigh-K magma is probably one of the essential conditions for the formation of the rapakivitexture. The volcanic rocks in the area have a much higher alkali content and potassiumcontent than their analogues around the world. For example, the average K20 content oftrachyte is 11.79%, over two times as high as that of its analogue around the world; the av-erage K20 content of alkali basalt is 5.61%, being also more than double that of itsanalogue. Even basic anorthosite is also rich in alkalis and poor in calcium. The anorthosite

o N 01' 15-z 'N

N .2+o s.

N

O

NN N

cj3

5o 60

10

5

Geochemistry of Rapakivi Granite Yu et al. 177

in the northern belt of the area is similar to the Damiao anorthosite of Hebei in chemicalcomposition. Its potassium and sodium contents are both far higher than the average con-tents of the world anorthosite, while its calcium content is lower. This is probably becausethe basicity of China' s anorthosite is low and labradorite seldom occurs. Various kinds ofrocks in the suite are commonly rich in alkalis (potassium), suggesting that the magma itselfis rich in potassium. and that this K- rich character is not due to local K- feldspathization.Substantial K- feldspar is mainly the product of cooling and crystallizaiton of the magma.Some late-stage granites approximate K-feldspar granites in relation to their features, but ingeneral they may be assigned to the K-rich subalkaline rock series.

Table 3. Average Chemical Compositions of Proterozóic Igneous Rocks

1. Olivine-bearing quartz-syenite (3); 2. K-rich granite (1); 3. K-rich granite (2); 4. K-rich granite (2); 5.

Plagioclase rapakivi granite (5); 6. Porphyry biotitegranite (7); 7. Medium- grained two- mica granite (3); 8.

Fine-grained leucogranite (1); 9. Anorthosite (3); 10. Gabbro (2); 11Alkali diabae (1); 12. Pyroxene-basalt (7);

13, 14. Alkali basalt (25); 15, 16. Trachyte (10).

The figures in parentheses denote the number of samples.

Owing to enrichment in alkalis (particularly potassium), the intrusive and volcanicrocks both plot in the field of the alkali basalt series on the total alkali-silica variation dia-gram of igneous rocks (Fig. 3), forming an essentially continuous zone. Trachyte, whichgrades to quartz- syenite and rapakivi granite, undoubtedly falls in the field of alkali basaltseries. The late- stage biotite- granite and two- mica granite, which show increasing silicicacids but decreasing total alkalis, plot at the edge of the field of high- Al (or caic- alkali)

% Si02 Ti02 Al203 Fe203 FeO MnO MgO CaO Na20 K20 P205 F H20 Total

1 63.63 0.47 15.35 1.90 2.71 0.09 0.43 2.32 4.62 5.55 0.12 0.06 3.00 100.25

2 71.96 0.33 12.80 0.98 2.15 0.04 0.43 0.82 2.80 5.50 0.04 0.05 2.60 100.50

3 66.99 0.53 14.59 2.98 1.92 0.08 0.31 2.14 3.67 5.90 0.04 0.04 0.84 100.03

4 69.15 0.36 13.63 1.71 1.68 0.09 0.16 2.10 3.23 5.90 0.02 0.13 1.93 100.03

5 68.69 0.40 14.55 0.40 1.37 0.05 0.56 1.80 3.20 5.51 0.16 0.26 2.73 99.68

6 73.88 0.15 13.21 1.01 1.40 0.03 0.20 1.05 3.10 5.26 0.03 0.38 0.81 100.51

7 75.20 0.12 12.92 0.63 1.26 0.03 0.18 0.94 3.08 5.10 0.02 0.64 0.40 100.52

8 79.15 0.00 11.41 0.23 0.03 0.00 0.08 0.87 3.43 4.46 0.00 0.00 0.20 99.86

9 51.49 0.58 23.87 1.81 1.79 0.05 1.13 9.42 4.59 1.35 0.18 - 3.21 99.47

lO 55.11 0.66 13.94 2.40 6.42 0.13 6.96 7.62 3.33 0.76 0.25 - 1.99 99.57

11 50.26 2.20 13.46 6.54 7.69 0.19 4.05 4.91 3.00 3.33 0.72 - 3.14 99.49

12 47.33 167 17.14 5.09 5.40 0.16 5.52 6.32 2.87 3.06 0.38 - 5.65 100.49

13 47.90 2.31 16.04 8.65 3.93 0.07 4.21 2.32 1.58 7.58 0.78 - 5.05 100.42

14 47.70 2.00 16.28 7.29 4.30 0.12 5.02 3.91 1.96 5.61 0.56 - 5.74 100.49

15 54.48 1.12 15.71 6.41 1.76 0.08 2.52 2.27 0.64 11.70 0.36 - 3.50 100.44

16 59.13 0.79 16.61 0.18 0.54 0.08 1.05 1.29 0.41 13.36 0.15 - 2.70 100.29

basalt series. Anorthosite and alkali basalt, which show much higher alkalis than clac-alka-line rocks though their silicic acid content is relatively low, both lie in the alkali basalt field.The total alkali- silica variation relationship reflects that the intrusive rocks and volcanicrocks are comagmatic products under different geological conditions though there existsdifference in petrology between them. Meanwhile, in essence the rapakivi granite suiteshould belong to the alkaline series and is obviously different from the calc- alkaline ortholeiite series in nature of the magma. As anorogenic granitoids, some late- stagehigh- silicic acid granite may bear some resemblence to the differentiaton product of theS- type or I- type granitic magmas, but in general, the rapakivi granites should belong toanorogenic potassic A-type granites. It is generally considered that A-type graintes originateby partial melting of felsic granulite and are formed by residual magmas generatinganorogenic granitic magmas. The authors hold that the independent potassic subalkalinegranite series originating by partial melting of the lower crust in an anorogenic rift envi-ronment should be regarded as an important component part of A-type granites.

Trace Element and Rare Earth Element (REE) Geochemistry

As compared with the acid rocks around the world, Proterozoic granitoids in the areahave rather high concentrations of large-ion lithophile elements such as Rb, Ba, Ga, Y, Zrand Nb (especially Ba, Zr and Y); meanwhile the low Sr and V concentrations are veryprominent too. Besides, the trace element distributions in granitoids and anorthosite, par-ticularly th distributions of the element pairs Rb- Sr and Zr- V, have revealed thephonomenon of element fractionation produced by fractional crystallization. The Rb con-tent in granitoids is close to or in excess of its abundance in the world acid rocks and the Zrcontent is 1.5 times higher than the abundance, whereas the Sr content is only 1 / 2 or morethe abundance and B only about 1 / 7. On the contrary, the Sr and V contents ofanorthosite are both close to or greater than their abundances in basic rocks, while the Rband Zr contents are only about half their abundances. The Rb / Sr ratio (0.67) of granitoidsis greater than that of the world acid rocks, while the Rb / Sr ratio of anorthosite is only1 / 5 that of the world basic rocks. The mutual element compensation between the twoend- member rock types has proved the existence of bimodal magmatic fractionalcrystallization, which results in the concentration of large-ion lithophile çlements in graniticmagmas and removal of the elements Sr and V in the process of accumulation ofplagioclase.

Trace elements show a wide regular variation in granitoids, which is related to thecrystallization differentiation of granitic magmas. The early-stage rocks with a low contentof silicic acids have relatively high abundances of such elements associated with the basicityof the rocks as Ba, Sr, Ti, Zr, P and Zn, but very low abundanced of Rb, Be and Cu. Thelate-stage rocks with a high silicic acid content show much higher contents of such elementsassociated with the acidity and alkalinity of rocks as Rb, Be and Cu (in particular Rb in-creases by nearly three times); while those elements with relatively high abundances men-tioned above are reduced noticeably l-4 times. Accordingly the Rb / Sr ratio rises from 0.34to 0.86-0.94 in the early-stage granites, and may go up rapidly to 1.63-2.47 in the late-stagegranites. Of course, autometamorphism that is well developed in the late-stage granites hasto some extent changed the a.bundances of trace elements, producing significantly high Rb

178Vo!. 4 NO.2 ACTA GEOLOGICA SINICA June 1991

Trace elements: 1. Olivine-bearing quartz-syenite (3); 2. K-rich granite (7); 3. K-rich granite (13); 4.

Rapakivi granite (23); 5. Porphyritic biotite- granite (20); 6. Medium- grained two- mica granite (9); 7.

Arerage of granitoids; 8. Anorthosite (9). The figures in parentheses denote the number of samples.

REE, 1 Olivine- bearing quartz- syenite; 2. Rapakivi granite; 3. K- rich granite; 4. K- rich granite; 5.

Porphyritic biotite-granite; 6. Anorthosite; 7. Alkali basalt; 8. Trachyte.

The RFE contents and features of the rocks of this suite are the same as those ofanorogenic granites abroad: the total REE content ranges from 400 to 800 ppm, theLREE / HREE ratio is intermediate, and there is distinct negative Eu anomaly. Granitoidsin the area also show the features of differentiation and evolution and progressive increasein silicic acid and total REE content (mainly the LREE content from the early to latestages, and meanwhile the negative Eu anomaly tends to be more distinct. The REE distri-bution patterns (Fig. 4) of various types of granitoids are very similar, lying at relativelyhigh positions of the diagram. The REE distribution patterns are relatively LREE-enrichedwith the LREE part sloping to the right and the HREE part being nearly flat, and there aredistinct negative anomaly depressions. This kind of REE distribution pattern is similar tothose of A- type alkali granites in Nigeria, Australia and the southeastern coast of China(Pitcher, 1983). On the other hand, the total REE content of anorthosite is only less than1 / 10 that of granitoids and its LREE / HREE ratio is relatively low, but there appearshigh positive Eu anomaly derived by accumulation of plagioclase. Its REE distributioncurve lies at a low position and slopes gently to the right, with the slope of the LREE part

and low Sr anomalies and Eu anomaly in some rocks, but the regular variation in elementabundance brought about by differentiation is pronounded.

Table 4. Trace Element and REE Average Contents of Proterozoic Igneous Rocks

Ti Ba Sr Zr Cr Ni V P Ga Rb N Be Cu Pb Zn Y Rb / Srb

1 2272 1653 269 669 8 11 8 411 15 93 31 2 11 24 140 33.7 0.34

2 2535 712 114 861 9 lO 3 209 21 92 34 1 14 21 130 45.9 0.86

3 2316 1172 140 603 6 11 3 248 21 131 34 2 12 26 124 49.4 0.94

4 2675 2113 314 394 31 7 10 487 24 173 19 4 17 24 66 48.2 0.55

5 1190 596 157 282 29 11 5 103 22 257 25 3 39 21 48 62.9 1.63

6 776 345 107 224 27 12 3 91 22 263 22 6 44 32 41 - 2.47

7 2149 1256 194 496 18 10 6 294 22 163 27 3 16 24 91 48.0 0.84

8 3057 1165 780 48 21 31 181 51 18 17 44 1 15 28 85 5.8 0.02

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu R E L/H Eu/Eu*E

1 86.7 199 23.6 95.1 17.2 3.0 12.0 2.1 8.6 1.5 4.3 0.2 3.9 0.4 458 12.8 0.65

2 118 245 31.4 118 20.2 3.3 13.6 1.6 10.2 2.0 5.6 0.8 5.2 0.8 578 13.5 0.61

3 105 243 28.9 115 20.5 3.2 14.1 2.6 10.9 2.0 5.7 0.4 5.1 0.7 556 12.4 0.57

4 123 269 29.8 108 18.4 1.8 12.6 2.2 10.5 2.1 5.9 0.6 5.7 0.8 591 13.6 0.36

5 171 358 37.3 123 20.2 1.3 13.1 2.1 11.5 2.3 7.0 0.9 7.4 1.1 757 15.6 0.25

6 8.2 18 2.0 9.5 1.9 1.0 1.6 0.3 1.4 0.3 0.6 0.1 0.6 0.1 44 8.0 1.85

7 34.1 82.8 9.0 37.7 7.4 2.3 6.5 1.1 5.5 1.0 2.6 0.4 2.2 0.4 193 8.7 1.10

8 39.5 102 11.5 51.5 11.7 2.6 10.2 1.7 8.6 1.5 3.9 0.5 2.8 0.4 248 7.3 0.78

Geochemistry of Rapakivi Granite Yu et al. 179

180 Vol. 4 NO. 2

being low as compared with granitoids, and the positive Eu anomaly has a sharp peak. Thedifferent distribution patterns and opposite Eu anomalies of the two end- member rocksmay probably reflect that both were derived from different source regions, i.e., the uppermantle and the lower crust, and the complementary distribution patterns also imply thatthe initial magma underwent REE fractionation in the process of accummulation ofplagioclase. The total REE content of volcanic rocks is intermediate between the two typesof intrusive rocks. Their LREE / HREE ratio is relatively low, and their distribution curvesslope gently to the right and lie between the curves of the two types of intrusive rocks. Alka-li basalt exhibits slight positive Eu anomaly and trachte exhibits slight negative Eu anoma-ly, but these anomalies are not as distinct as those of the intrusive rocks. The REE distribu-tion patterns of volcanic rocks indicate that they and intrusive rocks are comagmatic andwere formed in the process of similar fractional crystallization, but rapid eruptions causedthe REE fractionation of bimodal rocks to fail to proceed thoroughly, thus there occurredrhythmic repeated eruptions of the end-member rocks.

ACTA GEOLOGICA SINICA June 1991

Lade 1r Ñd tn,SmEuGd ±bôyHolrTrn Yb

Fig. 4. REE distribution pattern of Proterozoic intrusive and volcanic rocks.L Quartz-syenite; 2. plagioclase rapakivi granite; 4. K-rich granite; 5. porphyritic granite; 6. anorthosite; 7. al-

kali basalt; 8. trachyte.

Granites may be subdivided according to the tectonic setting in which they are in-truded into four main groups ocean ridge granites (ORG), within plate granites (WPG),volcanic arc granites (VAG) and collision granites (COLG). Various tectionic types of gran-ites provide systematic criteria for the distinction of geochemical patterns not only in re-spect to the mineral association and chemical composition but also in respect to trace ele-ments. On the basis of the studies of Pearce et al. (1984) and Harris et al. (1986), the traceelement contents in Proterozoic anorogenic granites in the area have been normalized to the

300

200

100

4?

50gç)

10

5-

hypothetical ocean ridge granite (ORG) values furnished by them and the element distribu-tion curves have been constructed in order to correlate with various tectonic types of gran-ites in the world (Fig. 5). As shown in Fig. 5-A, the trace element distribution curves of va-rious types of Proterozoic granites are very similar: they all show a relatively low Sr ratio,very high K, Rb and Ba ratio peaks, high concentrations of Ce and Sm relative to their ad-jacent elements and ratios of Nb to Yb that are close to the normalizing values (i.e. close to

o

Geochemistry of Rapakivi Granite Yu et al. 181

'F

)F cQ ÇRh

p pm

I 01)

Sr K20 Rh Ra Nb Ce Zr Sm Y Yb

A- -

unity). This distribution pattern closely resembles the distribution pattern of various ele-ments of within plate granites (WPG) illustrated in Fig. 5. Despite their intra-group varia-bility, the within plate granite patterns have the following features: (1) high values of K, Rband Th; and (2) values of Hf (Zr) to Yb that are close to the normalizing value. Many with-

Fig. 5. Trace element distribution patterns of Proterozoic granites (A) and Rb- (Y+Nb) (B) and Nb-Y (C)discriminant diagrams (after Pearce and Harris, 1984, 1986).

ORG = ocean ridge granites, WPG = within plate granites. 1. Quartz. syenite; 2. K-rich granite; 3. plagioclase

rapakivi granite; 4. porphyritic granite; 5. trace element distribution region of WPG (Pearce, 1984); 6. region of

Archaean and Mesozoic calc.alkaline rocks.

lOO 5(0Y + Nb ppm

lo

182 Vol.4 NO.2 ACTA GEOLOGICA SINICA June 1991

in plate granites notably exhibit higher Y, HREE and Nb contents. On the other hand thevolcanic arc granites or collision granites are characterized by enrichment in K, Rb, Ba andTh relative to Ta, Nb, Hf, Zr, Y and Tb, and a further significant feature of theirs is the lowvalues of Hf (Zr) to Yb relative to the normalizing composition. However. it shouldbepointed out that the Proterozoic granites in the area do not show a large negative Baanomaly characteristic of some within plate granites; on the contrary they have a very highBa ratio, which is a regional feature of the area. Trace element distribution patterns furtherdemonstrate that the trace element distribution patterns of the Proterozoic anorogenicrapakivi granite suite in the area are similar to those of A-type alkali granites intruded intointracontinental rifts or grabens (i.e. the granites from Nigeria, Sudan and the Oslo graben)or introduced into strongly attenuated continental crust (i.e. granites form Greenland andScotland) of the within plate granites. This idea has been further demonstrated by the Rb-(Y+Nb) and Nb- Y discriminant diagrams (Fig. 5- B and C). In the two diagrams theProterozoic granites in the area all plot in the within plate granite field, exhibitig the featureof high abundances of Rb, Y and Nb. As far as their distribution is concerned, they are inessence different from Archaean or Mesozoic caic- alkaline intrusive rocks that plot in thevolcanic arc granite field.

Discussion of Tectonic Setting and Petrogenesis

The anorogenic rapakivi granite suites restricted to the Mid- Proterozoic distinguishthemselves by their salient geological, petrological and geochemical characteristics. It notonly forms a gigantic rock belt extending from North America to North Europe, but re-ports of its traces also keep pouring from China and Africa. So it represents a major eventof transformation of oceanic crust to continental crust in the global Precambrian geologicalhistory. It seems that the very similar petrological geochemical features of the rapakivigranite suites could indicate that they have similar tectonic settings, magma sources andevolutionary processes. It is evident that their tectonic environments are different fromthose of calc-alkaline plutonic rocks that are intruded into them and were formed in earlierorogenic movements, so they cannot have any connection with earlier orogenic tectonicsand are also somewhat different from the presently developing tectonics, e.g. the EastAfrica rift system. Anderson and Cullers (1978) and Emslie (1978) have noted the similarityof these anorogenic granites to the kinds of rocks found in extensional tectonic settings.Anderson (1983) pointed out that the most appropriate tectonic regime must be one that al-lows some forms of mantle diapirism and is probably an incipient rift that was aborted atan early prerift stage due to a failure to integrate into a world- wide plate system. Themidcontinent gravity high must certainly represent an incipient rift that failed.

The rapakivi granite suite in the Beijing area provides an example of magmatism (bothplutonism and volcanism) accompanied by various shallow- level and graben- type (Wangand Qiao, 1984) sedimentation in this period in an intracratonic rifting environment of thegigantic rock belt of the northern hemisphere. Its essential difference from the precedingand subsequent tectono-magmatic activity lies in the following: it is located in the interiorof a craton that had been solidified after the long-continued Precambrian orogeny but notlonger; it is the product of the non- orogenic graben or incipient rift- type extensional envi-ronment that first appeared in the area after the interval of the Early Proterozoic; mantlediapirism and development of deep faults are bound to induce a series of igneous activity

Geochemistry of Rapakivi Granite Yu et al. 183

originating in the mantle and passing upwards into the curst. Analogous tectono-magmaticenvironments have occurred in varions Proterozoic cratons (e.g. the North American plat-form and the Baltic shield) all over the world, and what are different are merely their scaleand extent and the distinctness of their manifestations. It is an anorogenic potassic A-typerapakivi granite suite bearing both similarities to and differentes from its subsequentintraplate continental rift A-type granite suite.

A central question is whether the rapakivi granite suite represents derivative meltsfrom a mafic, mantle- derived magma or a parent melt derived directly from fusion ofcrustal material. From the petrological geochemical features of the rapakivi granite suite itmay be confirmed that the bimodal rock association of this anorogenic complex iscogenetic. Besides isotopic data also indicate that the basic end-member anorthosite in thearea has an isotopic composition of mantle- derived magmas (Nd0 = 0.5 10, Sr0 = 0.704),which is consistent with that of anorogenic anorthosite (Sr0 = 0.702-0.705) from varionsparts of the world (Anderson, 1983), so it should be formed by plagioclase accumulation ofmagmas derived from the mafic mantle. The isotopic ratios of granitoids are all higher thanthose of anorthosite. The initial Sr isotopic ratios of the early-stage rapakivi granite (Sr0 =0.707) and the lower part of the Dahongyu Formation (Sr0 = 0.7045) imply that theyshould be the product of partial melting of undercraton lower crust material. On the otherhand the late- stage biotite and two- mica granites and the upper part of the DahongyuFormaiton exhibit very high initial Sr isotopic rotios (Sr0 = 0.7286 and Sr0 = 0.7082 and0.7 126 respectively). Such wide variation of initial Sr isotopic ratios in granitoids can alsobe found in A-type alkali granites (e.g. Sr0 = 0.702-0.752 for alkali granite in Nigeria) andrapakivi granites in North Europe and North America; in particular, younger two- micagranite may have very high initial Sr values. This phenomenon is interpreted as the result ofdifferent degrees of crustal contamination during the ascent of magmas in the extensionaltectonic setting or suggests that there is possibility that deep faults might trigger fusion ofcrust or infracrust material (Pitcher, 1983). The whole-rock oxygen isotopic composition(518O = +9.13%) of the plagioclass rapakivi granites in the area also explains this idea.Therefore, the genetic relationship of the bimodal rock association does not seem to be oneof a comagmatic lineage but rather successive melting events originating in the mantle andpassing upwards into the crust. The magmatism involves both the basic magma derivedfrom the upper mantle and the granitic magma generated by partial melting of the lowercrust. As for the latter, the involvement of a significant crustal component in its source ma-terial seems mandatory.

The lower crust, as source material of rapakivi granites, comprises large amounts ofcalc- alkaline syntectonic tonalitic to granodioritic rocks in Proterozoic undercratonicsource regions. In view of the feature of the source material, as a consequence of the lowwater content of the metaigneous source, the amount of fusion was small (20+10%). Thisamount was favourable for the formation of a marginally peraluminous, potassic, iron-rich,rather dry magma. This idea has been confirmed in the study of anorogenic granites ofNorth America. As far as most anorogenic plutonism is concerned, a very small amount ofpartial melting of the tonalitic to granodioritic lower crust is very characteristic. In somebatholiths, a metasedimentary contribution, coupled with a greater amount of H20 andpartial melting, may aid in accounting for the composition of the more stronglyperaluminous two-mica granites which have lower K / Na and Fe / Mg ratios, higher initialSr values and lower total REE. Therefore, it may be envisaged that the activity of deep

faults during extensional rifting brought about mantle diapirism, thus leading to the ascentof magmas derived from the upper mantle. The huge heat flow gave rise to small degrees ofpartial melting of lower crust material, thus producing a less acid, relatively dry, highlyviscous, potassic and iron- rich granitic magma. In the anorogenic environment, owing tothe immiscibility and fractional crystallizaton of the mixed mafic and granitic magma in themagma chamber, anorthosite and basic intrusive rocks were formed first by plagioclase ac-cumulation from gabbroic nragma. Then owing to magmatic differentiation and pulsatoryemplacement coupled by continuous involvement of crust material, residual potassic,iron- rich rapakivi granitic magmas evolved into leucocratic two- mica granites with in-creasing acidity and hydrous minerals, forming a series of potassic A-type granites. As a re-sult of fractional differentiation the bimodal endmember rocks display similarities with re-spect to the rock and mineral association, chemical composition and trace elements andREE and exhibit the phenomenon of mutual compensation of fractionation. When thefractional crystallization of magmas was incomplete and magmas induced by faults roserapidly and erupted at the surface, the dry magmas were able to intrude into the upper lev-els of the crust with shallow pluton yielding regional volcanic activity. Thus there occurredrhythmic eruptions of alkali basalt and trachyte of the Dahongyu Formation.

Ah

Fig. 6. Q-Ab- Or normative diagram of Proterozoic granites, in comparison with the experimentally de-termined ternary minima at 0.5 to 10 kb from Bower and Tuttle, 1958, and Luth et al., 1964.

1. Isobaric minimum; 2. maximum concentration area of the world granitic rocks (Winkler, 1981); 3.

quartz-syenite; 4. K-rich granite; 5. plagioclase rapakivi granite; 6. porphyritic granite;

7. medium and fine-grained granite.

Plagioclase rapakivi texture is a texture peculiar to Proterozoic anorogenic granites,and its formation is related to the nature of granitic magmas in such tectonic settings. Acomparison of the bulk composition of the rapakivi granites with the experimental resultsof the "granite system" (Fig. 6) shows that the Proterozoic granites in the area mainly plot

184Vol.4 NO. 2 ACTA GEOLOGICA SINICA June 1991

2 3 A 4

Geochemistry of Rapakivi Granite Yu et al. 185

closer to the Or (K- feldspar) apex of the alkali feldspar field, far from the minima andeutectic points, implying difference from the maximum concentration area of the worldgranites. So it is considered to be formed by low-acidity, K-rich dry magmas. On the dia-gram the early- stage rocks fall into the K- feldspar field. Therefore K- feldspar firstcrystallized and coalesced to form oval phenocrysts in relatively water- deficient,high-viscosity, dry magmas, and then plagioclase was removed and mantled the ovoids toform rims. Finally, when the melt composition reached the eutectic ratio, dark- colouredminerals and felsic minerals crystallized to form the matrix. The data points of late- stagçgranites are closer to the minimum "M". Apparently, with an increase in acidity and vola-tiles of melts, their K-rich character is not pronounced and their composition is closer tothat of normal granites, showing the ternary minimum composition; therefore the rapakivitexture does not appear. An experimental study of the effects of fluorine in granitic magmas(Manning, 1982) indicates that at a given pressure the introduction of small amount offluorine can obviously reduce the crystallization temperature and cause the ternary minimato shift from the quartz field and approach the alkali feldspar field. Therefore a highfluorine content may produce a potassium content greater than the eutectic ratio and a rela-tively high water pressure in the melts during the crystallization of rapakivi granites. Theincrease of the fluorine content in late- stage rocks and minerals suggests that the residualmelts were saturated with volatiles at that time and therefore closer to the melts at theternary minima and crystallized at relatively low temperatures and pressures into normalgranites. But the appearance of the rapakivi texture requires a tectonic environment of rela-tively rapid cooling apart from the condition for the nature of the melt. The K-rich granitesof the northern belt of the area is similar to the rapakivi granites of the middle belt in com-position, but no rapakivi texture appears there. Only in the case of high-level emplacement,could early- crystallized K- feldspar coalesce into spheroidal phenocrysts with the minimumsurface tension and then be mantled by plagioclase as the melts were not apt to attain astate of equilibrium crystallization and was liable to release volatiles and produce pressuredifference. Hence the rapakivi texture in the area only occurs in the early- stage Shachangintrusion that was emplaced into the higher levels. This is consistent with the fact that therapakivi texture in Proterozoic anorogenic granites in varions parts of the world mainly ap-pears in high-level batholiths.

Acknowledgements

The authors wish to express his thanks to Zhang Xiaoben and Yin Jun of the ResearchGroup of Igneous Rocks for their cooperation in completing this research, to Shi Jishangand Sun Shanping and Prof. Hong Dawei for their help and valuable opinions and to Prof.Weng Shiji and Prof. K. C. Condie of the United States for their opinions of revising thepaper.

Chinese manuscript received Oct. 1989accepted Mar. 1990

Translated by Fei Zhenbi

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Yu Jianhua Born in September 1937; graduated from theFaculty of Geochemistry, Department of Geology, Peking Universityin 1961; having been engaged in research on granites and relevant min-erals for a long time; now working as a senior Engineer with BeijingCentre of Geological Research and Laboratory. Correspondence: 24Huangsi Street, Beijing 100011, China.

186Vol.4 NO.2 ACTA GEOLOGICA SINICA June 1991