in the hidaka metamorphic belt, hokkaido

Geochemical Journal, Vol. 23, pp. 321 to 337, 1989

Anatexis and chemical evolution of pelitic rocks

during metamorphism and migmatization

in the Hidaka metamorphic belt, Hokkaido

MICHIO TAGIRII, MASATOSHI SHIBA2 and HITOSHI ONUKI2

Department of Earth Sciences, Ibaraki University, Mito 310',and Department of Earth Sciences, Hirosaki University, Hirosaki 0362, Japan

(Received November 29, 1989; Accepted February 21 , 1990)

The southernmost Hidaka metamorphic belt consists mainly of cordierite tonalite intrusions and

pelitic metamorphic rocks ranging from the greenschist to the granulite facies. Anatectic migmatites are common in the higher amphibolite and granulite facies zones. Compositional changes in major, rare earth elements and some other trace metals are so small that they are undetectable among the pelitic metamorphic rocks of zones A + B + C and D, but they are large enough to be detected in the higher am

phibolite (zone D) to the granulite facies rocks (zone E). The enrichment of Fe, Mg, Na, Eu, and Sc, and the depletion of K, P, La, Ce, Nd, Cs and Rb are statistically significant in pelitic granulites, while heavy REEs are very variable. The chemical variation of pelitic granulite was derived from the accumulation of plagioclase +garnet. This suggests that more than 50-60% of the total volume of pelitic granulite was melted to produce a large amount of tonalitic magma, leaving pelitic granulite as a restite. Migmatites of the higher amphibolite facies are anatexites, and their K, P, Cs, Rb and light REE content is the same as that of lower grade metamorphic rocks. Migmatites of the higher amphibolite facies melted incipiently to segregate only a small amount of melt, and could not produce a large magmatic mass such as the cordierite tonalites. Cordierite tonalites are S-type granites, and their major elements, Cs, Rb and light REE concentra

tions are similar to those of lower grade metamorphic rocks. The chemical variation of cordierite tonalites is explained by the extraction of plagioclase +garnet from a tonalitic magma and the variation of original sedimentary rocks. The small chemical difference between the cordierite tonalites and the lower grade metamorphic rocks suggests that the former was derived from a massive melting of meta

pelites or that much of the restite is retained. The material migration among higher amphibolite facies rocks, pelitic granulites, migmatites and cordierite tonalites took place through mineral /melt interaction in the lower crust.

INTRODUCTION

The chemical changes of metamorphic rocks during progressive metamorphism have been a

persistent subject of geochemistry. For example, Shaw (1954) studied this problem in the Littleton Formation in New Hampshire (USA), where the

grade of the regional metamorphism of the same formation varied from unmetamorphosed to sillimanite-grade. He showed that the concentrations of most major, minor and trace elements remained constant with the exception of

potassium. Uno (1961) showed the concentrations of major elements of pelitic metamorphic rocks in the Tsukuba district, Japan, remained constant with the exception of H20 during pro

gressive metamorphism of the amphibolite facies. Eade and Fahrig (1971) studied the

geochemical evolution of the continental crust in New Quebec, and by applying statistic analysis, concluded that A1203, total Fe, P205, CaO and Ti02 were enriched in the granulite facies rocks compared with the amphibolite facies rocks, whereas Si02, K20 and H20 were depleted. They

321

322 M. Tagiri et al.

concluded that the continental crust has a chemical zonation with decreasing Si, K and H contents with depth. Holland and Lambert

(1972) also statistically studied chemical evolution in the Canadian Shield and the Precambrian of Scotland, and concluded that K and probably Ti contents decreased with depth. More recently, Shaw et al. (1986) have estimated the composition of the upper and lower crusts as follows: The lower crust has less Si, K, Nb, Rb and Zr, but more Ti, Fe, Mg, Co, Cr, Cu, Ni, Sc, V and Y than the upper crust. These results are summarized as follows: The chemical change was very small up to the amphibolite facies metamor

phism with the exception of decreasing K and water contents. In the granulite facies, pelitic metamorphic rocks are depleted in Si, K and H,

whereas enriched in total Fe than the am

phibolite facies equivalent. However, the results on other elements are inconsistent with each other.

On the other hand, Barr (1985) reported a chemical variance of migmatites and nonmigmatites in the Moines of northern Scotland. After applying statistic methods, he concluded that Si02, CaO and Na20 were enriched in migmatites, whereas A1203, Fe203 and K20 were depleted, and that these chemical differences between migmatites and non-migmatites corres

pond to changes of mineral composition during migmatization such that the migmatites tend to be plagioclase-rich and the non-migmatites to be micaceous. The large variance of Zr, Y and rare earths were considered to be a spurious

phenomena resulting from the chemical inhomogeneity of the samples. The chemical differentiation of migmatites forming leucosomes and melanosomes is strongly influenced by the chemistry of the original protolith.

In this study, the chemical variance of pelitic

metamorphic rocks, pelitic migmatites and

tonalites will be discussed after statistical treat

ment. Compositions of metamorphic rocks

change systematically over metamorphic grades.

Migmatization and the genesis of granitic

magma will be explained in relation to the

material transport.

OUTLINE OF GEOLOGY OF THE SOUTHERNMOST

HIDAKA METAMORPHIC BELT

The Hidaka metamorphic belt consists of a Western Zone of metamorphozed ophiolite and

a Main Zone of metamorphozed terrigenous sediment (Komatsu et al., 1983). According to these authors, the Main Zone is representative of a rock sequence found in the continental-type crust which was obducted on an accretion com

plex along the collision zone of two island arcs. The crustal section consists of granulite facies

gneisses at the base, amphibolite facies gneisses in the middle to upper and the greenschist facies rocks and unmetamorphosed sediments at the

top. The area reported in this study is located in the Main zone, which is chiefly composed of

pelitic metamorphic rocks. The low to medium metamorphic grade portion is derived from the Mesozoic Nakanogawa Group (Kontani, 1978). The area is divided into five metamorphic zones

based on the mineral assemblage of pelitic rocks

(Fig. 1). Zone A is characterized by the chlorite + muscovite + biotite + plagioclase assemblage, and belongs to the greenschist facies. Zones B and C are characterized by the mineral assemblage cordierite + muscovite + biotite +

plagioclase, and cordierite + K-feldspar + muscovite +biotite + plagioclase, respectively. Zone D is characterized by sillimanite + cordierite + K-feldspar + biotite + plagioclase. Zone E is distinguished by the appearance of orthopyroxene in pelitic metamorphic rocks from zone D, and belongs to the granulite facies

(Tagiri et al., 1990). Sillimanite appears in both D and E zones, and andalusite appears in both C and D zones. Zones B, C, and D belong to the amphibolite facies. The mineral assemblage of orthopyroxene + garnet + cordierite is common in zone E. Zone C' is a contact aureole around the igneous complex (Shiba, 1988). Metamor

phic conditions were estimated by Osanai et al. (1986) and Shiba (1988) to be 6-6.5 kb, 650750°C in the higher grade part of zone D, and 6.5-7 kb, 750-800°C for zone E. These metamor

Anatexis and chemical evolution of pelitic rocks 323

ST'

}

y

+ 4

+ +++

+ +

+

+

+ +

+++++

+++++ + + + + +

+ +

4 + '~

+ + + +

+ ++ +

+ ++ +

++

ST S + +5 + +

HoromanH

++

T

+

+ +

+ + . Toy t Q T

+t+

+++ +

I

+I

M oni +

+T+ +

+ +

m +++++ + +

I I I I I I

I I I I I I I I I I I/ I,,=

+ +

+

+ +

++++

III I I I III

I I I I

4km

Erimo

I

Meguro

Hjdaka

Shoya

S14Perrro

4a

'C H

LEGEND

~] Zone A I I Zone B

® Zone C ® Zone C' ® Zone D ssss Zone E

Migmatites + ,F CT & BT

® Gabbros Peridotites

ST-ST Saruru-Toyoni

shear zone

H-H Horoizumi shear zone

Fig. 1. Metamorphic zonal map of the southernmost Hidaka metamorphic belt . CT: cordierite tonalites. biotite tonalites.

BT.'

phic conditions are sufficient for the pelitic gneisses to melt (e.g. see Vielzeuf and Holloway, 1988).

Muscovite decomposes in zone D, and migmatites occur commonly in zones D and E. Zone D migmatites are mainly distributed in and around a tonalitic granitic body. In this paper, migmatites are classified as a rock having a characteristic texture that indicates in situ melting, which is described in detail by Tagiri et al. (1990), and as a rock consisting of both of leucosome and melanosome. The following textures are taken to indicate in situ partial melting: Presence of idiomorphic plagioclase which has a normal type zonal structure, and quartz-pools of submillimeter size which include idiomorphic plagioclase and biotite. In the samples studied, K-feldspar was rarely found in the grain boundaries. These characteristic textures may have resulted from the crystallization of quartz and plagioclase in granitic melt. In some migmatites the quartz-pools (the melt portion) are sporadically distributed, and at times are linked together to produce a veinlet.

Many types of intrusive rocks, such as gab

bros and granites, occur in the area. Among

them, the gabbros and granites are closely associated with migmatites, and two types of granitic rocks occur. One is cordierite tonalite, and the other is biotite tonalite, the latter being designated "hornblende tonalites" by Owada and Osanai (1989). The sequence of intrusion has been summarized by Tagiri et al. (1988). Ac

cording to these authors, gabbros were intruded at the earliest stage, cordierite tonalites during the middle stage and biotite tonalites at later stages. The cordierite tonalites and biotite tonalites produced thermal effects on the metamorphic rocks of zones A, B, C and D, and

produced migmatites in and around the granitic bodies (Fig. 1). Radiometric ages of the metamorphic rocks range from 40 to 23 Ma, and those of the intrusive rocks from 54 to 17 Ma

(Osanai et al., 1990).

BULK ROCK CHEMISTRY OF PELITIC

METAMORPHIC ROCKS AND GRANITOIDS

About 1000 cm' rocks of uniform facies were

collected, and crushed to get about 100 ml

powder. In the area studied, meta-pelites are fine-grained, and have thin compositional ban

324

Table 1.

M. Tagiri et al.

Chemical variances of metamorphic rocks and granitoids

Group Zone A Zone ABC Zone D Zone E

wt% S'02 Ti02 A1203 Fe203 MgO CaO Na20 K20 P205 ppm

La Ce Nd Sm Eu Gd Dy Er Yb Lu Y Co Cr Cs Nb Ni Rb Sc Sr Zn Zr Mg/Mg+Fe K/Rb x 100

Av.(No.) 64.32(8) 0.84(8)

16.90(8) 6.57(8) 2.55(8) 3.00(8) 3.53(8) 2.64(8) 0.22(8)

21.9(5) 49.6(5) 22.0(5) 4.87(5)1.00(5) 3.6(5) 3.04(5)1.61(5) 1.08(5)0.11(5)

13.7(5) 14.9(5) 167(5) 6.04(5) 5.8(5)

81(5) 81.5(5) 12.5(5) 385(5) 81(5)

135(5) 0.250(5) 2.88(5)

Dev.

1.39

0.17

0.63

0.52

0.34

0.81

0.45

0.56

0.07

3.30

7.37

3.68

0.85

0.19

0.56

0.33

0.15

0.17

0.04

1.18

2.4

126

1.84

1.5

51

18.3

1.9

128

14

30

.012

0.15

Av.(No.) 64.73(12) 0.81(12)

16.72(12) 6.64(12) 2.52(12) 2.71(12) 3.40(12) 2.78(12) 0.22(12)

21.3(7) 48.7(7) 21.9(7) 4.87(7) 0.99(7) 3.6(7) 3.03(7)1.57(7) 1.07(7)0.10(7)

13.8(7) 16.2(7) 169(7) 6.40(7) 6.6(7)

96(7) 89.5(7) 13.3(7) 342(7) 83(7) 135(7) 0.247(12) 2.84(7)

Dev.

1.81

0.15

0.67

0.65

0.30

0.86

0.63

0.55

0.07

2.95

6.37

3.09

0.72

0.16

0.49

0.31

0.15

0.14

0.04

1.33

4.0

104

1.69

1.9

51

20.8

3.3

128

17

25

.012

0.18

t95

1.15

0.10

0.43

0.41

0.19

0.55

0.40

0.35

0.04

2.72

5.89

2.86

0.66

0.15

0.45

0.29

0.14

0.13

0.04

1.23

3.7

96

1.56

1.8

47

19.2

3.1

118

16

23

.008

0.17

Av.(No.) 64.73(19) 0.86(19)

17.05(19) 6.97(19) 2.69(19) 2.59(19) 3.13(19) 2.63(19) 0.15(19)

21.5(12) 49.1(12) 21.8(12) 5.11(12)1.09(12) 3.7(12) 2.71(12)1.76(12) 0.86(12) 0.10(12)

10.9(12) 15.8(12) 168(12) 4.98(12) 8.2(12)

88.3(12) 82.6(12) 15.4(12) 284(12) 105(12) 143(12) 0.249(19) 2.77(12)

Dev.

1.69

0.08

0.79

0.65

0.36

1.03

0.63

0.35

0.07

2.27

5.25

2.12

0.56

0.10

0.46

0.49

0.85

0.35

0.06

2.68

1.8

52

1.55

1.0

21.4

11.6

4.5

25

14

14

.024

0.24

Av.(No.) 63.20(17) 0.90(17)

16.80(17) 8.07(17) 3.26(17) 3.14(17) 3.84(17)1.17(17) 0.10(17)

17.4(11) 39.9(11) 17.4(11) 4.13(11)1.29(11) 3.1(11) 2.66(11) 2.04(11)1.49(11) 0.20(11)

12.4(11) 20.8(12) 245(12)

1.57(11) 7.5(12)

132(12) 23.9(12) 19.2(12) 265(12) 98(12) 133(12) 0.258(17)5.04(12)

Dev.

1.89

0.10

1.13

0.89

0.47

1.24

0.72

0.72

0.04

2.67

7.05

3.85

1.01

0.23

0.99

1.52

1.94

1.26

0.20

9.64

8.4

59

1.23

1.9

25

17.7

7.1

90

31

35

.027

3.47

Av.: Average value, (No.): Number of samples, Dev.: Sample standard deviation, t95: t-distribution of 95% reliability,

dings of several centimeters thickness, so that some kinds of uniform bands of meta-pelites were analyzed. As pelitic migmatites are fine

grained heterogeneous rocks, different facies of pelitic migmatites including leucosome and melanosome were analyzed. The compositions of bulk rocks for major, trace and rare earth elements (REE) were determined by means of Xray fluorescence analysis, photon-activation analysis (Yoshida et al., 1986), and inductively coupled argon plasma emission spectrophotometry (Tagiri, et al., 1989), respectively. Analyzed rocks were put into three metamorphic groups according to the metamor

phic zones A + B + C, D and E, three granitoid

groups of zone D migmatites, cordierite tonalites and biotite tonalites. The average chemical com

positions, standard deviations and numbers of samples for each group are presented in Table 1.

Raw data are available from the first author

(M.T.). For comparison of the chemical com

positions of the two types of tonalites with that of metamorphic rocks, tonalites with Si02 con

tent of 60 to 70 wt% were selected. In this study, only zone D migmatites are separately discussed.

Zone E migmatites are included in zone E

metamorphic rocks.

All metamorphic rocks of the study are of

pelitic composition according to the ACF diagrams of White and Chappell (1977) (Fig. 2).


Table 1. (continued).

Group Migmatite Cord-Tonalite Bi-Tonalite

wt%

Si02

Ti02

A1203

Fe203

MgO

CaO

Na20

K20

P205

ppm La Ce Nd Sm Eu Gd Dy Er Yb Lu Y Co Cr Cs Nb Ni Rb SC Sr Zn Zr Mg/ Mg +Fe2 K/Rbx 100

Av.(No.) 63.15(5)0.93(5)

17.72(5)8.15(5) 3.11(5) 2.79(5) 3.32(5) 2.71(5) 0.17(5)

21.4(9) 49.4(9) 22.2(9)5.13(9) 1.08(9) 3.7(9) 2.89(9) 1.48(9) 1.04(9) 0.13(9)

11.9(9) 19.1(2) 184(2) 4.95(2) 9.0(2)

95.0(2) 80.9(2) 18.5(2) 311(2) 91(2) 152(2) 0.248(5) 2.58(2)

Dev.

1.74

0.05

1.43

0.60

0.11

0.20

0.18

0.23

0.05

2.07

5.03

2.56

0.62

0.11

0.48

0.53

0.36

0.30

0.07

2.71

1.4

21

0.04

0.84

23.8

4.8

4.8

29

11

4

0.008

0.11

Av.(No.) 66.17(15)0.84(15)

16.62(15)6.55(15) 2.51(15) 2.27(15) 3.08(15) 2.85(15) 0.17(15)

21.4(9) 46.5(15) 21.4(9)5.02(8) 1.06(9) 3.6(9) 2.25(9) 1.39(9)0.64(9) 0.07(9)8.6(9)

15.3(8) 149(8) 5.71(8) 8.0(8)

88.6(8) 89.7(8) 14.6(8) 297(8) 92(8) 152(8) 0.246(15) 2.61(8)

Dev.

1.55

0.11

0.66

0.81

0.48

0.56

0.62

0.34

0.07

2.70

6.50

2.60

0.32

0.20

0.40

0.17

0.28

0.11

0.04

0.80

2.6

23

1.53

0.8

12.0

10.0

2.2

32

17

21

0.022

0.21

Av.(No.) 65.53(7)0.75(7)

16.14(7)5.84(7) 2.13(7) 3.48(7) 3.87(7) 2.46(7) 0.21(7)

20.7(6) 45.0(6) 20.1(6(4.86(6) 0.96(6) 3.6(6) 2.90(6) 1.47(6) 1.11(6) 0.13(6)

12.5(6) 11.8(7) 129(7) 5.14(7) 7.1(7)

71.8(7) 79.7(7) 12.8(7) 314(7) 63(7) 179(7) 0.245(7) 2.56(6)

Dev.

1.19

0.16

0.97

1.26

0.31

0.37

0.14

0.34

0.10

12.7

26.3

11.4

2.27

0.08

1.50

1.26

0.67

0.64

0.09

6.00

2.0

20

1.81

1.7

11.8

11.0

3.1

54

11

53

0.061

0.13

Cordierite tonalites are the S-type granite, and the composition of biotite tonalites is between that of the S-type and the I-type granites. Chemical variances of major element contents are shown in Fig. 3 which also shows the sample

standard deviations. The variances of major elements of all the metamorphic groups are very similar to each other. Zone D migmatites have a relatively small variance as compared with the metamorphic rocks. Cordierite tonalites exhibit a variance similar to those of the metamorphic rocks. The Mg/Mg+Fe variance of biotite tonalites is larger than that of cordierite tonalites, the large variance resulting from the fractionation of mafic minerals in the granitic

magma. Figure 4 shows the variance of the REE.

The variances of light REE show similar orders

of magnitude among metamorphic rocks, zone

D migmatites and cordierite tonalites in contrast

to the large variances of heavy REE in zone E

metamorphic rocks.

The variances of the trace metals are also shown in Fig. 5. However, those of zone D migmatites are for only two samples. The variances of Co, Cs, Nb, Sc, Zn and Zr and the K/Rb ratio of lower grade metamorphic rocks have similar orders of magnitude in contrast to the large variances of zone E metamorphic rocks. Cordierite tonalites and biotite tonalites have a similar variance to each other with the ex


A A

I-type I S-type I-type S-type

T

T x+,

Mig E

CGranitoids

F MetamorphicsF

Fig. 2. ACF diagrams of metamorphic rocks and granitoids. The thick lines represent the boundary of the land S-type granitic rock fields (after White and Chappell, 1977). Mig: zone D migmatites. CT: cordierite tonalites. BT: biotite tonalites. ABC: zones A + B + C metamorphic rocks. D: zone D metamorphic rocks. E: zone E metamorphic rocks.

ception of the large variances of Nb and Zr.

COMPOSITIONAL CHANGES OF PELITIC

METAMORPHIC ROCKS

The chemical variances among the three

metamorphic groups were statistically analyzed.

The results are presented in Tables 1 and 2. The

t-distribution of 95% reliability of the chemical

variance of zones A + B + C metamorphic rocks

is listed in Table 1, and shown in Fig. 3. The

monodimensional analyses of variance on the three metamorphic groups are indicated in Table

2, in which 0, and 02 are the degrees of freedom

of the variance analysis. F(0.0125) is a 1.25% value of F-distribution, and F a ratio of the pre

sent variances among the three metamorphic

groups. "Yes" means that the chemical compositions of the three metamorphic groups are

significantly different at the 1.25% significance level. The concentrations of 12 elements were

different among the three metamorphic groups.

Statistically, pelitic granulites (zone E) are richer

in total Fe, MgO, Na20, Eu, and Sc, and are

poorer in K20, P205, La, Ce, Nd, Cs and Rb than the lower metamorphic grade rocks (zones A, B, C and D).

Generally, pelitic sediments are not chemical

ly homogeneous. However, as shown in Figs. 3,

4 and 5, K20, P205, Cs, and Rb decrease, but

total Fe, MgO, Eu and Sc increase with increas

ing metamorphic grade from zones A + B + C to

zone E. The variations of REE are clearly

recognized in the chondrite-normalized patterns

(Fig. 6). The negative Eu anomaly decreases with increasing metamorphic grade, eventually the

anomaly becoming positive in zone E. Pelitic

sediments showing a positive Eu anomaly have not been reported in literature (e.g. Fleet, 1984).

It is concluded that the granulite facies metamorphism has given rise to the increase of Eu

content and its positive anomaly of pelitic meta

morphic rocks. The Eu enrichment of zone E cor

responds to that pelitic granulites tend to be rich

in plagioclase and poor in biotite. The

plagioclase of pelitic granulites is calcic


wt %

SiO2 A ABC D E68

64

60

Mig C-T B-T Al 20 3 A

TiO 2

ABC D E Mig C-T B-T

4~

327

18

16

14' IFe2O3

9

T_

1.0

0.8

0.6

1~7

MgO

5 r

CaO

5 F.

4

3

2

3 k4lNa2O

5

4

3

P2O

0.3

0.2

0.1

0

q,

1l_

41

2

0l Mg/Mg+Fe

0.3

0.2

Fig. 3. Variance of major element contents with the sample standard deviation . Two wedges A + B + C represent a t-distribution of 95% reliability. Abbreviations are the same as in Fig . 2.

of zones

andesine. The large variance of heavy REE, Co,

Sc, Zn and Zr, and the relative enrichment of heavy REE and Sc occur contemporaneously

with the Eu enrichment in zone E. The similar

magnitude of variances of major and light REE contents and the systematic chemical changes of

12 elements with increasing metamorphic grade

suggest that a mass balance exists among the

three metamorphic groups. This is discussed below based on the assumption of chemical

homogeneity of protolith.

CHEMICAL VARIATION OF PELITIC MIGMATITES

AND TONALITES

Figures 3, 4 and 5 show the chemical

variances of zone D migmatites, cordierite

tonalites and biotite tonalites, and Fig. 7 shows

the chondrite-normalized REE patterns of

migmatites and tonalites. Figure 8 shows varia

328

PPM A

30 La

20

10

ABC D E Mig CT BT

M. Tagiri et al.

A ABC D E Mig CT BT

60

40

20

Ce

30

20

10

Nd6

4

2

1.4

1.0

0.6

Eu

Eq~l5

4

3

2

Gd

4

3

2

Dy

2

0.4

0.3

0.2

0.1

0

Fig. 4.

id

1

0

20

10

0

Variance of REE with the sample standard deviation

ppm

20

10

A ABC


D E Mig CT BT A ABC D E

CO

Mig CT BT

300

200

100

CS

Cr

8

6

4

2

8

6

4

Nb

100

140

100

60

Ni60

20

20

10

Sc

120

100

80

60

Fig. 5.

500

400

300

200

220

180

140

100

ISr

Variance of trace metals with the sample standard deviation.

329


Table 2. Monodimensional analysis of variance for the three metamorphic groups.

Element P01, 02) F(0.0125) F Significance

Si02 Ti02 A1203 Fe203 MgO CaO Na20 K20 P205 Mg/Mg+Fe La Ce Nd Sm Eu Gd Dy Yb Lu Y Co Cr Cs Nb Ni Rb SC Sr Zn Zr

2,45

2,45

2,45

2,45

2,45

2,45

2,45

2,45

2,45

2,45

2,27

2,27

2,27

2,27

2,27 2,27

2,27

2,27 2,27

2,27

2,28

2,28

2,27

2,28

2,28

2,28

2,28

2,28

2,28

2,28

5.13

5.13

5.13

5.13

5.13

5.13

5.13

5.13

5.13

5.13

5.49

5.49

5.49

5.49

5.49

5.49

5.49

5.49

5.49

5.49

5.45

5.45

5.49

5.45

5.45

5.45

5.45

5.45

5.45

5.45

3.28

1.55

1.66

7.59

12.63

4.06

5.88

53.41

14.33

2.60

8.28

7.37

7.11

4.66

7.72

2.06

0.34

1.79

1.93

0.49

3.27

3.68

20.45

4.57

3.06

33.66

5.59

2.67

1.58

1.17

No

No

No

Yes

Yes

No

Yes

Yes

Yes

No

Yes

Yes

Yes

No

Yes

No

No

No

No

No

No

No

Yes

No

No

Yes

Yes

No

No

No

tion diagrams for tonalites. Zone D migmatites

are similar to zone D metamorphic rocks in the

concentrations of K20, P205 and light REE, and

in the magnitude of the Eu negative anomaly,

but resemble zone E metamorphic rocks in abun

dance of Si02, Ti02, total Fe, and MgO. That is,

zone D migmatites have a intermediate chemistry

between that of zones D and E metamorphic

rocks.

Cordierite tonalites are characterized by the relatively large variance of Eu, the very small variances of heavy REE and the low concentration of heavy REE. Cordierite tonalites are very similar to zones A + B + C and D metamorphic rocks in the concentrations of major elements, light REE, Cs, Nb, Rb, Sc and Sr, and in the magnitude of the negative Eu anomaly. The Eu content and the magnitude of the positive Eu

anomaly of cordierite tonalites decrease with increasing K20 content and decreasing CaO content. These Eu variation trends may have resulted from plagioclase fractionation in the cordierite tonalite magma. The average of zones A + B + C metamorphic rocks also plots in the variation field of cordierite tonalites in Fig. 8. Basic cordierite tonalites which are mainly com

posed of orthopyroxene + cordierite + plagioclase with a small amount of biotite + quartz show a positive Eu anomaly, and their patterns and concentrations are similar to that of the zone E metamorphic rocks which show a

positive Eu anomaly (Fig. 6). These facts suggest that the basic cordierite tonalites and some of the zone E metamorphic rocks were produced through a common mechanism to enrich

plagioclase. Biotite tonalites have a very different

chemical composition compared with the zone D migmatites and cordierite tonalites in the ap

pearance of high CaO (Fig. 2), low total Fe and MgO and wide variances of light REE and Zr. The biotite tonalites have larger negative Eu anomalies than the former two. A strong

positive Eu anomaly was observed in an analyzed biotite tonalite, which is highly depleted in all the REE (Fig. 7). In biotite tonalites the La content increases with enrichment of Sr, and with depletion of Rb (Fig. 8). Phlogopite/melt distribution coefficient of Rb is over 1 and that of La below I (Henderson, 1984). Biotite fractionation might be one of the mechanisms to have produced the La-Rb variation trend of biotite tonalite. The wide Mg / Mg + Fe variance of biotite tonalites might be also derived from biotite fractionation. La content of biotite tonalites increases with progressive magmatic differentiation. The negative Eu anomaly increases with enrichment of La. This variation trend may be correlated with the plagioclase fractionation during differentiation of the biotite tonalite magma, because plagioclase / dacitic melt distribution coefficient of Eu is over 1 and that of La below 1 (Henderson, 1984).

100

50


10

50

ABC zones

D zone 5

10

4v c 0 r U

d CL E CO C

50

10

5

1

0.5

Fig. 6.

5

5

1

a -10.5 I I 1 I i 1 1 1 I I

La Ce Nd Sm Eu Gd Dy Er Yb Lu

Chondrite-normalized REE patterns of pelitic metamorphic rocks.


100

50

10

D zone

50

Migmatite

m

V C 0

a) o.

10

,

b.

Cord-Tonalite

A

100

50

10

5

1

0.5

5

,

Bi-Tonalite,

,

R

K~b

c..q

b

i

-d

r

i

,,

,

b

5

5

1

0.5

Fig. 7.

varieties.

La Ce Nd Sm Eu Gd Dy Er Yb Lu

Chondrite-normalized REE patterns of granitoids. Broken lines of cordierite tonalites are for basic The broken line of biotite tonalites is for a depleted rock.


Eu

2

1

0

ppmCord-Tonalite

•• .. *

.

La

30

10

La

ppm

°

0

Bi-Tonalite

°

00

Sr ppm

5 CaO%

ppm°

300

Eu ppm

1

1

.

2 3

000

4

30

10

00

400

0

° * Rb ppm

1 O

80 90

AEu

+10

1

1

2

2

3 4 K20%

4

.

5 CaO%

.

La ppm

-101

Fig. 8.

30

% .*

10-5 -10

Fig. 8. Variation trends of tonalites. The star indicates the average value of zones A + B + C metamorphic rocks. A Eu is the magnitude of the europium anomaly.

SYSTEMATICS OF MIGMATITES

AND TONALITES

Petrographically, zone D migmatites are anatexites which are composed of leucosome and melanosome, and cordierite tonalites and biotite tonalites are magmatic rocks. Discussions will concentrate on the hypothesis that cordierite tonalite magma, an S-type granite, was produced

by the melting of meta-pelite of the Hidaka belt

(Owada and Osanai, 1989). As described previously, zone D migmatites to a great extent chemically resemble zones A + B + C and D metamorphic rocks with the exception of the Fe and Mg concentration. Cordierite tonalites are also chemically similar to zones A + B + C and D metamorphic rocks with the exception of the depletion of Ca, Na and heavy REE. These are clearly seen in Fig. 9, where the average values are used in the calculations. According to

Vielzeuf and Holloway's (1988) melting ex

periments on pelitic rock, the melt composition was rich in K20 for a small degree of melting, whereas K20 concentration decreases with the increased melting. This chemical change is due to the melting of plagioclase and garnet in the system. In the sample preparation for zone D migmatites, the melt portion (leucosome) could not be separated from the host, since the size of the melt portion is less than 1 mm. Zone D migmatites have a K20 concentration similar to that of zone D metamorphic. rocks which are the

precursor of zone D migmatites (Fig. 9). From the small difference of K20 and light REE content among zone D migmatites and zones A + B + C and D metamorphic rocks, it can be concluded that zone D migmatites may have been melted incipiently, but only a small amount of melt segregated; most of melt remained in situ. If the small of melt segregated and formed


K Na Ca Al Fe Mg

1.4

1.0

0.6 0

N

U m 4

Y v 0 D: 1.8

Q

1.4

1.0

.o

0

0. R -o k k X o

x xx xo '~ X~ G

0

--.0Mig

-o D + ++o

E

0.6

SF~~a

Xx ~b oX

X0+

o BT

E

BT

.j° Mig

Arc CT ++++ oX

La Ce Nd Sm Eu Gd Dy Y Er

Fig. 9. Material balance normalized by zones A +B+C metamorphic rocks.

the calculations. Abbreviations are the same as in Fig. 2.

Yb Lu

The average values are used for

the cordierite tonalite mass shown in Fig. 1, a

large amount of zone D metamorphic rocks and

zone D migmatites must exist in this district in

comparison with the volume of cordierite

tonalites. The volumetric ratio of zone D

metamorphic rocks to cordierite tonalites is

about 1 to 1 in this district. Accordingly, the

melting of zone D migmatites could not produce

a large magmatic body such as the cordierite

tonalites.

Zone E metamorphic rocks are distinct from those of zones A + B + C and D by the low concentration of K, P, Cs, Rb, La, Ce and Nd, the enrichment of Fe, Mg, Eu and Sc, and the large variances of Dy, Yb, Lu and Y. Average zone E metamorphic rocks are rich in Yb and Lu, though the statistical analyses do not show a significant value. Using the distribution coefficients between minerals and melt (Henderson, 1984), the depletion and enrichment of these elements are consistently explained as the

change of mineral assemblage from zone D to zone E. Garnet has large values of mineral/ melt distribution coefficient of Fe, Mg, Yb and Lu, orthopyroxene those of Fe, Mg, Sc, Yb and Lu, and plagioclase (andesine) that of Eu. In contrast to these elements, these three minerals have small distribution coefficients of K, P, Cs, Rb, Ce and Nd. The depletion and enrichment of these elements suggests that some zone E metamorphic rocks showing the positive Eu anomaly and the large variance of heavy REE are restites. That is, plagioclase concentrated in zone E metamorphic rocks since plagioclase was the mineral phase to crystallize first in the melt as has been discussed by Tagiri et al. (1990). Here, it is noted again that zone E metamorphic rocks contain migmatites. The chemical characteristics of cordierite tonalites are contrasted with those of zone E metamorphic rocks as compared with zone D metamorphics (Fig. 9). The large amount of plagioclase fractionation in cordierite tonalite


La/Gd

8

6

4

e

4

e

Pp

N

A

It _OP%

PP o,,

Dy/Yb

4

3

2

20

i

d a \. :.;

' X

% %* *

i

30

La ppm

C0...

x

40

P1

A

POOr

2 4 6 Dy ppm

oABC • D *E

* Mig ACT A BTFig. 10. REE systematics. The arrows represent vectors for the 10% extraction of the minerals. Alla: allanite. Amp: amphibole. Ap: apatite. Gar: garnet. Opx: orthopyroxene. Or: orthoclase. PI. plagioclase. Zir: zircon. Other abbreviations are the same as in Fig. 2.

magma, indicated in Fig. 8, seems to be compen

sated by the plagioclase-concentration of zone E

metamorphic rocks.

REE systematics are presented in Fig. 10. Some fractionation trends of representative minerals are also displayed in the diagrams. The vectors representing 10% of the mineral fractionation were calculated using the mineral/ melt distribution coefficients of Henderson (1984). An arrowhead corresponds to melt composition after subtracting 10% mineral from the starting melt. The fractionation trend of 1 to 10% can be approximated with a straight line. In the diagram of La/Gd-La, zones A, B, C and D metamorphic rocks, zone D migmatites and cordierite tonalites are plotted in the same narrow region and do not show any apparent fractionation trend. The indistinct chemical variation of

cordierite tonalites can be explained as a result of orthopyroxene+plagioclase fractionation. Zone E metamorphic rocks have a garnet fractionation trend, which is consistent with the

previous discussion on the enrichment of Fe, Mg, and heavy REE. Vielzeuf and Holloway (1988) distinguished three main stages of melting. The first is the stage of incipient melting of muscovite, the second being the stage of extensive melting of biotite, and the third is the stage of melting of garnet and spinel. In the second stage, melt coexists with

garnet, plagioclase disappears in the melt, and the proportion of melt reaches 50-60%. The first stage corresponds to the event of zone D migmatites. Taking zone E migmatites into consideration, the garnet fractionation trend of zone E metamorphic rocks was probably derived from the second stage melting. That is, the

precursor of zone E metamorphic rocks (zones A, B, C and D metamorphic rocks) melted more than 50-60% to produce migmatites in which

garnet coexisted with melt. Garnet fractionation primarily occurred in melt was separated to produce the chemical trend of zone E metamorphic rocks. Plagioclase crystallization started in a cooling stage of melt. The very small differences in La/Gd-La ratio among cordierite tonalites and zones A, B, C and D metamorphic rocks in


dicate an extensive melting of meta-pelites, as described in the discussion of the K and light REE contents. The fractionation trend of biotite tonalites differs very clearly from that of cordierite tonalites, but its mechanism is unknown because of the absence of the REE distribution coefficients for biotite. Any other effective mineral fractionation indicated in Fig. 10 is not

plausible. For example, both of apatite and allanite fractionations of biotite tonalites in the diagram of La/ Gd-La are denied by their variation trends in the diagram of Dy/Yb-Dy. Zircon fractionation trend of zone E metamorphic rocks is not consistent with their Zr concentration of Fig. 5.

In the diagram of Dy/Yb-Dy, zones A, B, C and D metamorphic rocks, zone D migmatites and cordierite tonalites are also plotted in the

same region. The apparent variations of zones A, B, C and D metamorphic rocks may be original trends of pelitic sediments. Zone E metamorphic rocks occupy a wide field which can be explained by the concentration of

garnet + plagioclase. Low values of Dy/ Yb of zone E metamorphic rocks are probably produced by the concentration of garnet, because

garnet has a very large value of mineral/melt distribution coefficient of Yb comparing with that of Dy. The trend of cordierite tonalites seems to be a superposition of the fractionation of garnet + plagioclase and the chemical variation of original protolith. Consequently, cordierite tonalites were produced by extensive melting of meta-pelites in which some zone E metamorphic rocks remained as restites after the cordierite tonalite magma had been extracted

(Owada and Osanai, 1989).

SUMMARY AND CONCLUSIONS

Compositional changes of pelitic metamor

phic rocks from the lower amphibolite facies (zones A + B + C) to the higher amphibolite facies (zone D) are so small that they are undetectable. However, from the higher amphibolite facies to the granulite facies (zone E) they are large, especially in the concentration of total Fe,

MgO, K20, P205, La, Ce, Nd, Eu, Cs, Rb and

Sc. Zone D migmatites, which are petro

graphically anatexites, retain as a whole the

original chemical characteristics of the metamor

phic rocks during migmatization. It is concluded that zone D migmatites melted incipiently, pro

ducing only a small amount of melt that remain

ed in situ. The zone D migmatites could not pro

duce a large magmatic mass.

When compared with the metamorphic rocks

of A, B, C and D zones, zone E metamorphic

rocks, which are composed of granulites and

anatectic migmatites, are depleted in K, P, Cs, Rb, La, Ce and Nd, but enriched in total Fe,

Mg, Na, Eu, Yb, Lu and Sc by the concentration

of plagioclase + garnet + orthopyroxene. The

plagioclase + garnet variation trend of zone E metamorphic rocks suggests that zone E metamorphic rocks melted more than 50-60% of

its total volume to produce a large amount of

tonalitic magma. Zone E metamorphic rocks are

chemically contrasted with cordierite tonalites, and have a chemical characteristics of restite

after cordierite tonalite magma has been ex

tracted.

Cordierite tonalites are petrographically magmatic, but chemically they resemble those of the metamorphic rocks of A, B, C and D zones. The chemical variation of cordierite tonalites is a

superposition of plagioclase +garnet fractionation and the chemical variation of original pelitic sedimentary rocks. The small difference in K, P, Cs, Rb, La, Ce and Nd between cordierite tonalites and zones A, B, C and D metamorphic rocks suggests that the cordierite tonalite magma was produced by a massive melting of meta

pelites or that they consist of melt + restite. Biotite tonalites probably originated from a

parental magma different from that of cordierite tonalite. On the basis of the petrographic and

geochemical studies, the compositional changes among zone D and zone E metamorphic rocks, zone D migmatites and cordierite tonalites ap

pear to have taken place through mineral /melt interaction.

Acknowledgments-The authors express their thanks


to Prof. S. Banno of Kyoto University and Prof. M. Hashimoto of Ibaraki University for critically reading the manuscript, and Dr. A. Fujinawa of Ibaraki University for helpful discussion. They are also grateful to Prof. K. Aoki, Dr. H. Fujimaki and Dr. T. Yoshida of Tohoku University for their help with the experiments. This study was funded by a Grant in Aid for Scientific Research from the Ministry of Education, Japan (Grant No. 60121005, 62103006 of Prof. T. Iiyama of Chiba University: No. 62540626 of M. S. K. Ochiai of Science Education Institute of Osaka Prefecture; No. 63540643 of A. Fujinawa of Ibaraki University).

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Henderson, P. (1984) General geochemical properties

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