re( (received may 26, 1980; accepted march 3, 1981)

Geochemical Journal, Vol. 15, pp. 17 to 23, 1981 17

Rb-Sr and 40Ar 39Ar geochronological studies on the

Precambrian rocks in the Minnesota River Valley

HIDEO TSUNAKAWA 1 and MASAHISA Y ANAGISAWA2

Geophysical Institute, University of Tokyo, Bunkyo-ku, Tokyo 113,' and Institute of Space and Aeronautical Science, University of Tokyo,

Meguro-ku, Tokyo 153,2 Japan

Re( (Received May 26, 1980; Accepted March 3, 1981)

Some granitic rocks and basaltic dikes of the Minnesota River Valley were dated by Rb-Sr and40Ar-39Ar methods. The obtained Rb-Sr mineral isochron ages are as follows: Morton gneiss 2.55 ± 0.06 (la)b.y. and its biotite ages 2.38b.y., Sacred Heart granite 2.37 ± 0.05 (la)b.y., Montevideo gneiss 1.82 ± 0.09 (1a)b.y. Each age may be corresponding to the respective geological event. Whole rock samples of the Morton and Montevideo gneisses, however, do not define a Rb-Sr isochron age. From 40Ar-39Ar dating results, a mild geological disturbance which might have occurred about 1.0b.y. ago was inferred. These results imply a complex history of the gneisses and a metamorphism in this region.

INTRODUCTION (the Algoman orogeny) from the amphibolite to the granulite facies took place about 2.6 b.y.

GOLDICH and HEDGE (1974) have reported a ago. About 1.8 b.y. ago, a mild metamorphism 3.80b.y. Rb-Sr whole rock isochron age for the reset only the K-Ar ages and the Rb-Sr biotite

gneiss exposed in the Minnesota River Valley, ages of gneisses in the Granite Falls-Ortonville southwestern Minnesota, U.S.A. If this age is area. This event included the intrusion of established, the gneiss in the Minnesota River granitic stocks and mafic dikes into the gneiss Valley will be the oldest crustal rock ever found. in the vicinity of Granite Falls.

A reconnaissance survey of the Precambrian GOLDICH and HEDGE (1974) reported a rocks in this region was made by LUND (1956). Rb-Sr whole rock isochron age of 3.80b.y. with CATANZARO (1963) reported the first radio an initial 87Sr/86Sr ratio of 0.700 for the Morton metric age over 3b.y. for North America. and Montevideo gneisses. However, FARHAT and GOLDICH and his coworkers (GOLDICH et al., WETHERILL (1975) pointed out that, as these 1961, 1966, 1970; GOLDICH and HEDGE, 1962, gneisses were affected by metamrophism, it was 1974; GOLDICH and GAST 1966; GOLDICH and possible to obtain apparent ages either too old MUDREY, 1969) have reported data on gneisses or too young, if the rocks were only partially in the Minnesota River Valley by K-Ar, U-Pb equilibrated with respect to the Rb-Sr system. and Rb Sr methods. HANSON and HIMMELBERG The latter authors found that their samples of

(1967) performed a geochronological study of the Montevideo gneiss did not lie on the 3.80 dikes in this region. Mainly on the basis of b.y. isochron and reported an age of about 3.2 these results, SIMs and MOREY (1972) reviewed b.y. for the gneiss on the basis of their zircon the geochronological evolution of this region. data. MICHARD-VITRAC et al. (1977) suggested According to these authors, the geological that their U Pb data on single zircons from the history may be summarized as follows: Sub Morton gneiss supported FARHAT and WETHERILL sequent to the primary igneous processes (the (1975)'s interpretation. Mortonian event) which occurred prior to an We made Rb-Sr and 40Ar-39Ar geochronoevent of 3.0b.y. ago, a high-grade metamorphism logical studies of the Minnesota River Valley

18 H. TSUNAKAWA and M. YANAGISAWA

Precambrian rocks, hoping (1) to resolve more

detailed metamorphic history of this region, (2)

to compare the 40Ar-39Ar and Rb-Sr dating

results and (3), if possible, to establish the age of the parent rocks of the gneisses in this region.

SAMPLES AND ANALYTICAL METHODS

All the sample localities are shown in Fig. 1. Based on the classification of rock types in the Minnesota River Valley by GRANT (1972), the samples can be divided into the five groups:

(1) granitic phase of the Morton gneiss (sample Nos. 02-1, 02-2-1, 02-2-2, 02-3), (2) tonalitic

phase of the Morton gneiss (sample Nos. 03-1, 03-2, 03-3), (3) Sacred Heart granite (sample No. 05), (4) Montevideo gneiss (sample Nos.

07-1, 08-1, 08-2, 09) and (5) dikes (sample Nos. 01, 06, 07-2). Detailed description of these samples is given in the appendix.

In all cases, sample sizes of 1-2kg were crushed and the fraction of 80-100 mesh or 100-150 mesh was taken for analysis. Plagioclase, potassium-feldspar and biotite were separated from several samples. Plagioclase fractions contain some quartz grains. Chemical compositions of the whole rock samples were measured by the X-ray fluorescence method. Rb and Sr concentrations were determined by the isotope dilution method. The precision for 87Rb/86Sr ratios is estimated to be less than

± 3 %(1 a). All the 87Sr/86Sr ratios were nor

malized to the 86Sr/88Sr ratio of 0.1194. Re

peated analyses of the 87Sr/86Sr ratio for the Eimer and Amend SrCO3 reagent during the

course of this study gave an average of 0.7079 ± 0.0002 (la). Used values of decay constant is X(87Rb) = 1.39 X 10-" yr-'. The isochron ages and the corresponding initial 87Sr/86Sr ratios

(=I) were calculated by the York method (YORK, 1969). All errors of the calculated values are

quoted at 1 a level. For the 40Ar-39Ar analyses, samples were irradiated with a fast neutron dose of about 1018nvt in a JMTR reactor with a standard sample (Bern 4M muscovite; the age = 18.7 Ma), K2SO4 and CaF2. The irradiated samples were heated in seven steps from 600°C to the melting point in a vacuum. The detailed

experimental procedures for the 40Ar-39Ar dating are described elsewhere (SAITO and OZIMA, 1977). The decay constants of X(40K) = 5.543 X 10-10yr' and Xe(40K) = 0.581 X 10-'Oyr-' are used in the calculation of ages.

Ortonville

g 2 Montevideo 07-1

Sac nite Falls o °05

06

• sample locality 02 0 40 km 02-2-2

~~ 02-3

MINNESOTA

\1 \ 110 0L.Superior

Minneapolis

X

Gra red Heart 03-1 03-2

~,~ Morton

2-, i 1Q,1"

Minnesota

River

Fig. 1. Map of the Minnesota River Valley showing the localities of samples.

RESULTS AND DISCUSSION

Rb-Sr dating results Chemical composition

of the representative samples and all the data

of Rb-Sr analyses are shown in Tables 1 and 2,

respectively.

Plagioclase, potassium-feldspar and the whole

rock sample from the granitic phase of the Morton gneiss (02-3) yielded a good mineral

isochron with an age of 2.55 ± 0.06b.y. and I =

0.7076 ± 0.0002 (Fig. 2). We conclude that the

age of 2.55 b.y. corresponds to the metamor

phism during the Algoman orogeny in the Morton area. The whole rock samples 02-1,

02-2-1 and 02-2-2 also lie on this 2.55 b.y. iso

chron within the analytical error. Hence, these four sample specimens (02-1, 02-2-1, 02-2-2, 02

3) are likely to have been equilibrated with

respect to the Rb-Sr system in the metamor

phism of 2.55b.y. ago. However, the data points for the 02-2-1 and 02-3 biotites lie systematically

below the 2.55b.y. isochron. Both dotted lines

in Fig. 2 connecting the points of the whole

rock and biotite of 02-2-1 and 02-3 samples

give the same age of 2.38b.y. It is very likely

Rb-Sr and 40Ar-39Ar geochronological studies 19

that the Rb-Sr

biotite did not

system

become

of the 02-2-1 and 02-3

completely closed until

2.3 8 b.y. ago.

age is discussed

Geological

later.

significance of this

Table 1. Chemical composition (water free, recalculated to 100%) of the representative samples

Sample 01 02-1 03-3 05 07-1

87Sr

86Sr

Si02

TiO2

A1203

FeO

MnO

MgO

CaO

Na20

K20

P205

49.4

1.07

13.2

13.6

0.18

7.46

10.5

2.38

1.42 0.14

72.4

0.15

14.7

1.62

0.00

0.60

1.53

2.52

4.95

0.09

56.9

0.77

21.0

4.46

0.00

1.59

6.28

5.17

0.94

0.07

71.8

0.31

15.8

2.48

0.00

0.54

1.49

2.90

4.48

0.12

73.8

0.10

16.3

1.05

0.00

0.14

1.54

3.31

2.63

0.08

0.750

Total 99.4 98.5 97.2 99.9 98.9 0.700

Morton gneiss

whole rock o plagioclase e K-feldspar biotite

a 02-3 02-3

02-1..». 02-2-202-2-1

02~ ;p 03-1 .

•03-2' 03-3

A'02-3

6 O~

'Lhh

/ 0221

ti5 ~e;~y.

02-2-1.02-3 Biotite

20

10

0 200 400 6000

01: Dike, 02-1: granitic phase of the Morton gneiss, 03-3: tonalitic phase of the Morton gneiss, 05: Sacred Heart granite, 07-1: Montevideo gneiss. These chemical compositions are measured by the X-ray fluorescence method.

0 0.5 1.0 1.5 2.0 87Rb/86Sr

Fig. 2. Rb-Sr isochron diagram for the Morton gneiss. Solid line: Mineral isochron of sample 02-3. Dotted lines: Mineral age measured on the biotite and

the whole rock of samples 02-2-1 and 02-3.

Table 2. Rb-Sr analytical data

Sample Rb (ppm)

Sr (ppm)

87Rb/86Sr 87Sr/86Sr

Morton gneiss

02-1

02-2-1

02-2-2

02-3

03-1

03-2

03-3

W.R.

W.R.

Bi.

W.R.

W.R.

K-f.

Pl.

Bi.

W.R.

W.R.

W.R.

Sacred Heart granite

05 W.R.

K-f.

Pl.

Bi.

Montevideo gneiss

07-1

08-1

08-2

09

W.R.

W.R.

K-f.

Pl.

W.R.

K-f.

Pl.

W.R.

K-f.

Pl.

Bi.

113

107

686

133

113

232

7.35

637

77.1

63.8

37.7

172

438

23.9

580

46.5

107

382

11.8

124

385

21.7

94.0

308 31.4

655

365

373

3.13

401

350

573

423

19.8 1359

1315

1328

499

549

712

93.7

391

469

435

332

417

444

291

499

674 541

24.0

0.900

0.823

635.2

0.960

0.932

1.172

0.0504

93.15

0.164

0.141

0.0822

0.998

2.309

0.0965

17.93

0.345

0.661

2.545

0.107

0.860

2.504

0.218

0.545 1.325

0.162

79.10

0.7402

0.7371

21.72

0.7406

0.7406

0.7489

0.7094

3.7881

0.7079

0.7051

0.7035

0.7380

0.7804

0.7081

1.2947

0.7179

0.7468

0.7865

0.7263

0.7508

0.7885

0.7297

0.7324

0.7536

0.7235

2.6890

W.R. = whole rock K-f _ potassium feldspar Pl. = plagioclase Bi. = biotite


In Table 1, the tonalitic phase of the Morton

gneiss (03-3) has the chemical composition apparently distinct from that of the granitic

phase (02-1). The data points of samples 03-1, 03-2 and 03-3 are scattered (Fig. 2). Therefore,

the Rb-Sr systems of these sample specimens were not completely equilibrated in the metamorphism of 2.55b.y. ago.

A good mineral isochron age of 2.37 ± 0.05 b.y. with I = 0.7049 ± 0.0003 was obtained for

plagioclase, potassium feldspar and the whole rock sample from the pluton, Sacred Heart

granite (05) (Fig. 3). The 05 biotite also lies on this isochron. We conclude that the mineral

isochron age of 2.37 b.y. for sample 05 corresponds to the time when the Sacred Heart

granite was intruded into the surrounding gneisses. The good agreement between this age (2.37 b.y.) and the biotite ages of the granitic

phase of the Morton gneiss (2.38b.y.) may suggest that. the Algoman orogeny persisted until 2.37 2.38b.y. ago. We may speculate further that prior to 2.55 b.y. ago the original rocks of the Morton gneiss were burried and metamorphosed and that 2.3 7 2.3 8 Ky. ago a disturbance which might have been associated with an upheaval occurred in this region with the intrusion of the Sacred Heart granite. The 05 isochron is younger and its I value is higher than those obtained by GOLDICH et al. (1970), who

gave a Rb-Sr age of 2.70b.y. and I = 0.702 for potassium-feldspar and whole rock sample from

their five sample specimens, although their data

points were considerably scattered. GRANT (1972) indicated the significant inhomogeneity of the Sacred Heart granite. Hence, there is a

possibility that each rock unit of the Sacred Heart granite has different I value, causing the

scattered data points of their Rb-Sr analyses. Plagioclase, potassium-feldspar and whole

rock sample from sample 09 of the Montevideo

gneiss constitute a well defined isochron with an age of 1.82 ± 0.09b.y. and I = 0.7193 ± 0.003

(Fig. 4). The 09 biotite also lies on this isochron. The very high I value of sample 09 must show that the 1.82b.y. isochron age represents

the time when this sample specimen of the Montevideo gneiss was completely re-equilibrated to reset the Rb-Sr mineral isochron age. This result indicates that fairly intensive metamor

phism occurred at the site of sample 09 1.82 b.y. ago. On the other hand, the data for samples 08-1 and 08-2 of the Montevideo gneiss, whose sampling sites are only about 100m distant from

09's site, do not lie on a straight line (Fig. 4). The apparent isochron ages constructed from a

pair of plagioclase and whole rock data of 08-1 and 08-2 are 2.7 b.y. and 2.4 b.y., respectively. Those between potassium-feldspar and whole rock data give a 1.6b.y. age for sample 08-1

875r 86Sr

87Sr 86Sr

0.750

0.700

Sacred Heart granite

whole rock

a plagioclase

e K-feldspar

• biotite

5

ti31/

05

05

° 05

ey

05 Biotite

1.5

0 10

1.0

0.720

0 0.5 1.0 1.5 2.0 87R06Sr

Fig. 3. Rb-Sr mineral isochron of the Sacred Heart

granite (sample 05).

0.750

Montevideo gneiss

whole rock plagioclase n K-feldspar • biotite 08-2

08-1 ;.:

08-2,ti:=ef 08-1 1 y .~ 09 / 09

. 07-1

08-2,

08-1

a09

09 Biotite

0 50 100

3

2

1

0

0.70C 0 0.5 1.0 1.5 2.0 87Rb/8651

Fig. 4. Rb-Sr isochron diagram for the Montevideo

gneiss (samples 07-1, 08-1, 08-2, 09). Solid line: Mineral isochron of sample 09. Dotted lines: Reference lines between the plagioclase

and whole rock and between the potassium-feldspar and whole rock.


Table 3. Minimum age in 40Ar-39Ar age spectrum with the percentage of 39Ar and the temperature at the step which gives the minimum age Total fusion age corresponds to the K Ar age

SampleMinimum age

(b.y.)Percentage of 39Ar (%)

Temperature (°C)

Total fusion age (b.y.)

01

02-1

03-1

06

07-1

07-2

08-1

08-2

09

W.R.

W.R.

W.R.

W.R.

W.R.

W.R.

W.R.

K-f.

Pl.

W.R.

K-f.

Pl.

W.R.

K-f.

P1.

1.47

1.44

2.04

1.95

1.17

1.49

1.04

1.19

1.02

1.08

1.27

1.26

1.16

1.29

1.39

29

11

14

64

16

15

8

32

8

9

16

20

19

42

16

950-1000

950

1000

1050-1100

900-1000

950-1000

800 900

750-1000

950

900

800

900

950

900-1000

1000

1.82

1.96

2.24

2.16

1.47

1.94

1.56

1.42

2.20

1.33

1.63

2.08

1.81

1.67

2.36

Correction factor: (36Ar/3'Ar)ca = 3 X 10-4, (39Ar/37Ar)ca = 1 X 10-3, = 9 X 10-2. X(40K) = 5.543 X 10-10yr-i X (40K) = 0.581 X 10-10yr-1

(40A r/39 Ar)K

and a 1.7 b.y. age for sample 08-2. Those ap

parent ages can be explained by the way that

although the plagioclase of samples 08-1 and

08-2 retains the record of the Algoman orogeny,

the loss of radiogenic 87Sr of the potassium

feldspar might have occurred in a metamor

phism of 1.8b.y. ago, which is inferred from the isochron age of sample 09. The results of

samples 08-1, 08-2 and 09 of the Montevideo

gneiss imply that the intensity of the meta

morphism of 1.8 b.y. ago varied significantly

from . place to place in the Montevideo area.

From the whole rock data of samples 07-1, 08

1, 08-2 and 09, no isochron is obtained.

In Table 3, the minimum age in the age spec

trum for each sample is shown with the cor

responding temperature and percentage of the 39Ar released . In Fig. 5, it can be seen that all

minimum ages are older than 1.0b.y. and that a

peak may be recognized at 1.0 1.2 b.y. This may suggest a geological disturbance in the

Minnesota River Valley which occurred 1.0 b.y.

ago. The fact that the age of the last geological

disturbance inferred from the 40Ar-39Ar system

atics does not appear in Rb-Sr systematics may

be attributed to the general characteristic that

the former systematics is more sensitive to the

'OA r-39Ar dating results Although the 40Ar

39Ar stepwise-heating dating on the samples from

the Minnesota River Valley does not yield a

definite age, we can still obtain some useful

information about the record of the geological disturbances which affected the 40Ar-39Ar system

of the samples. It is generally regarded that the minimum age in the 40Ar-39Ar age spectrum

represents an upper limit of the age of the last

geological disturbance, as discussed by FITCH et al. (1969) and LANPHERE and DALRYMPLE (1971)

and also inferred from the diffusion model of

the 40Ar-39Ar age spectrum by TURNER (1968).

U)

a E 8 0 r 6

0 4

2 Z 0

0.8 1.0 1.2 1.4 1.6

Minimum Age

1.8 2.0 2.2 2.4 b.y.

Fig. 5. Histogram for the minimum ages in the 40Ar39Ar age spectra. This may suggest that a geological disturbance occurred about 1.0b.y. ago.


geological disturbance. This 1.Ob.y. event coin= cides with the age of the Duluth gabbro and the North Shore volcanic group in the west of Lake Superior (FAURE et al., 1969).

CONCLUSION

The Rb-Sr mineral isochron ages and 40Ar39Ar dating results in this study reveal more

detailed metamorphic event in the Minnesota River Valley, as follows.

(1) The activity of the Algoman orogeny in this region reached a climax prior to 2.55 b.y. ago resulting a high-grade metamorphism and

persisted until 2.3 2.4b.y. ago. The upheaval in this region might have occurred with the intrusion of the Sacred Heart granite 2.3 2.4 b.y. ago.

(2) About 1.8 b.y. ago, a relatively intensive metamorphism occurred in the Montevideo area. The intensity of this metamorphism varied significantly from place to place.

(3) A last mild geological disturbance may be inferred from the 40Ar-39Ar systematics to have occurred over the Minnesota River Valley region about 1.Ob.y. ago.

Acknowledgments-We greatfully acknowledge the continuing encouragement and guidance of Prof. M. OZIMA, University of Tokyo. We thank Prof. P. W. WEIBLEN and Dr. R. BAYER, University of Minnesota for their collaboration in sampling. We would like to thank Prof. S. ARAMAKI, University of Tokyo, Dr. K. SAITO, University of Yamagata and colleagues in Geophysical Institute, University of Tokyo for valuable help and

counsel.

REFERENCES

CATANZARO, E. J. (1963) Zircon ages in southwestern Minnesota. J. Geophys. Res. 68, 2045

2048.

FARHAT, 3. S. and WETHERILL, G. W. (1975) Inter

pretation of apparent ages in Minnesota. Nature257, 721-722.FITCH, F. J., MILLER, J. A. and MITCHELL, J. G. (1969) A new approach to radio-isotopic dating in orogenic belts. in Time and place in orogeny (Geological Society of London) 157-195.

FAURE, G., CHAUDHURI, S. and FENTON, M. D. (1969)

Ages of the Duluth gabbro complex and of the

Endion sill, Dulth, Minnesota. J. Geophys. Res. 74,

720-725.

GOLDICH, S. S. and GAST, P. W. (1966) Effects on weathering on the Rb-Sr and K-Ar ages of biotite

from the Morton gneisses, Minnesota. Earth Planet. Sci. Lett. 1, 372-375.GOLDICH, S. S. and HEDGE, C. E. (1962) Dating of

the Precambrian of the Minnesota River Valley, Minnesota. J. Geophys. Res. 67, 3561-3562.GOLDICH, S. S. and HEDGE, C. E. (1974) 3800-Myr granitic gneiss in southwestern Minnesota. Nature 252, 467-468.

GOLDICH, S. S., HEDGE, C. E. and STERN, T. E. (1970) Age of the Morton and Montevideo gneisses and

related rocks, southwestern Minnesota. Geol. Soc. Am. Bull. 81, 3671-3696.GOLDICH, S. S., LIDIAK, E. G., HEDGE, C. E. and

WALTAHLL, F. G. (1966) Geochronology of the midcontinent region, United States, Part 2, northern

area. J. Geophys. Res. 71, 5389-5408.GOLDICH, S. S. and MUDREY, M. G., JR. (1969) Dilatancy model for discordant U-Pb zircon ages. Geol. Soc. America Abs. with Programs 80.GOLDICH, S. S., NIER, A. 0., BAADSGAARD, H., HOFFMAN, J. H. and KRUGER, H. W. (1961) The Precambrian geology and geochronology of Minne

sota. Minn. Geol. Survey Bull. 41, 193.GRANT, J. A. (1972) Minnesota River Valley, south

western Minnesota. in Geology of Minnesota, P. K. SIMS and G. B. MOREY, eds. (Minnesota Geological Survey) 177-196.

HANSON, G. N. and HIMMELBERG, G. E. (1967) Ages of mafic dikes near Granite Falls, Minnesota. Geol.

Soc. Am. Bull. 78, 1429-1432.LANPHERE, M. A. and DALRYMPLE, G. B. (1971) A

test of the 40Ar-39Ar age spectrum technique on some terrestrial materials. Earth Planet. Sci. Lett. 12, 359-372.

LUND, E. N. (1956) Igneous and metamorphic rocks of the Minnesota River Valley. Geol. Soc. Am. Bull. 67,1475-1490.MICHARD-VITRAC, A., LANCELOT, J., ALLEGRE, G. J. and MOORBATH, S. (1977) U-Pb ages on single zircons from the early Precambrian rocks of west

Greenland and the Minnesota River Valley. Earth Planet. Sci. Lett. 35, 449-453.SAITO, K. and OZIMA, M. (1977) 40Ar-39Ar geo

chronological studies of submarine rocks from the western Pacific area. Earth Planet. Sci. Lett. 33,

353-369.SIMS, P. K. and MOREY, G. B. (1972) Resume of

geology of Minnesota. in Geology of Minnesota, P. K. SIMS and G. B. MOREY, eds. (Minnesota Geolog

ical Survey) 3-17.


TURNER, G. (1968) The distribution of pottasiumand argon in chondrites. in Origin and distribution of elements, L. H. AHRENS, ed. (Pergamon) 387-398.

YORK, D. (1969) Least squares fitting of a straight line with correlated errors, Earth Planet. Sci. Lett. 5,

320-324.

APPENDIX

Morion gneiss

(i) Granitic phase (Sample Nos. 02-1, 02-2-1. 02-2-2,02-3) 02-1, 02-2-1. 02-2-2 and 02-3 are medium

grained, equigranular and gray to pink quartzofeldspathic gneisses. These were collected from a block

less than 2m in size. Main minerals are plagioclase,

quartz, biotite, potassium-feldspar, myrmekite and apatite.

(ii) Tonalitic phase (Sample Nos. 03-1, 03-2, 03-3) 03-1, 03-2 and 03-3 are medium-grained, equi

granular and gray quartzofeldspathic gneisses, which were collected from a block less than 2m in size.

Main minerals are plagioclase, quartz, hornblende

and biotite.

Sacred Heart granite (Sample No. 05) 05 is a salmon-pink granite which was intruded

into the surrounding gneisses. Main minerals are potassium-feldspar, quartz, plagioclase, biotite and chlorite.

Montevideo gneiss (Sample Nos. 07-1, 08-1, 08-2, 09) These rocks are medium-grained, equigranular, pink to red quartzofeldspathic gneisses. The sampling site for 07-1 is almost in contact with the dike (07-2). 08-1 and 08-2 were collected within 1 m distance from each other. Sampling sites for 09 is about 100m apart from those of 08-1 and 08-2. Main minerals of these Montevideo gneisses are potassium-feldspar, plagioclase, quartz, myrmekite, biotite and apatite.

Dikes (Sample Nos. 01, 06, 07-2) Sample 01 was collected from a basaltic dike which

was intruded into the Morton gneiss. Main minerals

of 01 are hornblende and plagioclase. Samples 06 and

07-2 are tholeiitic diabase including plagioclase pheno

cryst, which were intruded into the Montevideo gneiss.

re( (received may 26, 1980; accepted march 3, 1981)

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