Pyroxene (re-)equilibration in the Precambrian terrain of SW Norway between 1030--990 Ma and reinterpretation of events during regional cooling (M3 stage) FRANS J. M. RIElMEIJER

Rietmeijer F. J. M.: Pyroxene (re-)equilibration in the Precambrian terrain of SW Norway between 1030--990 Ma and reinterpretation of events during regional cooling (M3 stagc). Norsk Geologisk Tidsskrift, Vol. 64, pp. 7 - 20. Oslo 1984. JSSN 0029-196 X.

Pyroxenes in the Precambrian terrain of SW Norway include metamorphic ortho- and Ca-rich clinopyrox­enes and reequilibrated igenous pyroxenes. The compositions indicate a thermal regime of c. 750" to c. 590 •c. They betong to a period of regional cooling (M3) in the metamorphic history of the area around 1030--990 Ma. Events during the M3 stage are reinterpreted and indicate that Rb-Sr whole rock and zircon U-Pb isotope systems may have become reset. This may be coeval with the formation of halogen­bearing amphiboles. Intrusion of the (Quartz) Monzonitic Phase of the Bjerkreim-Sokndal Layered Intrusion took place between c. 1050-1035 Ma and c. 1030--990 Ma.

F. J. M. Rietmeijer, Afdeling Petrologie, lnstituut voor Aardwetenschappen, Budapestlaan 4, 3508 TA Utrecht, the Netherlands. Present address: Mai/ Code SN4, NASA Johnson Space Center, Houston, Texas 77058, U.S.A.

The Precambrian migmatite terrain in Rogaland (SW Norway) has been intruded by a large ig­neous complex consisting of several anorthosite massifs and the Bjerkreim-Sokndal Layered In­trusion (Michot & Michot 1969)_

In gametiferous migmatites (mainly metape­lites) textural arrangements and mineral compo­sitions indicate four successive metamorphic stages (Kars et al. 1980).

The earliest stage (Ml) probably affected the entire Precambrian basement of Rogaland. Min­eral assemblages are indicative for upper-amphi­bolite facies conditions_ Isotopic ages of about 1200 Ma are assigned to this stage (Wielens et al. 1981) .

The so-called granulite-facies stage (M2 ) oc­curs in a c. 15-45 km wide zone surrounding the igneous complex. From the contact with the ig­neous complex, the metamorphic grade in the migmatite complex gradually decreases towards the east from granulite-facies to amphibolite-fa­cies conditions. This change is documented by the pigeonite-in isograde in (Jeuco-) charnockite (Rietmeijer 1980, in Jansen & Maijer 1980), the osumilite-in isograde in metapelites (Meijer et al. 1981) and the hypersthene-in isograde in (leuco) granite (Hermans et al. 1975) (Fig. 1). Also the gradual change of optical and chemical proper­ties of amphiboles (Dekker 1978) and phase rela­tions and compositions in siliceous dolomites

(Sauter 1981) are consistent with this regional pattern.

Metamorphic temperatures at the pigeonite-in and osumilite-in isogrades are 1000 ± 50 °C and >800 °C , respectively. The isogrades closely fol­low the boundary of the igneous complex (Fig. 1). Kars et al. (1980) and Maijer et al. (1981) suggested that the M2 stage may essentially be a thermo-metamorphic event induced by intrusion of the Leuconoritic Phase of the Bjerkreim­Sokndal Layered lntrusion. This phase crystal­lised at c. 1200 oc - c. 920 oc (cf. Table l) at about 9 kbar (Rietmeijer & Champness 1982). The intrusion reset U-Pb isotopic ages of zircons from the migmatite complex to 1045-1030 Ma, which may be the age of intrusion (Wielens et al. 1981).

The M3 stage of medium-grade metamorphic conditions represents a period of regional cooling (Kars et al. 1980) between c. 990-850 Ma, indi­cated by isotopic ages for osumilite (Maijer et al. 1981), hornblendes (Dekker 1978) and brown biotite (Verschure et al. 1980). Isotopic closure temperatures of these minerals indicate regional temperatures between c. 550 oc and 400 oc_

Mosaic microstructures in igneous Ca-rich clinopyroxenes from the Bjerkreim-Sokndal Layered Intrusion (Rietmeijer 1979) and types and compositions of fluid inclusions in quartz from the migmatite complex (Swanenberg 1980)

8 F. J. M. Rietmeijer

59°

58°45'

6500

6490 58°30'

6460

LEGE ND

L2] Ouaternary

INTRUSIVE COMPLEX

� Anorthosite

� Leuconoritic Phase

� (Ouartz-) Monzonitic Phase

MET AMORPHIC COMPLEX

D Caledonides

58°15' D Precambrian

5°30'

NORSK GEOLOGISK TIDSSKRIFr l (1984)

Tonstad

10 km

so

Fig. l. Simplified geological map of southwestern Norway (after Hermans et al. 1975, Rietmeijer 1979). South of dented line the igneous nature of the (Quartz-) Monzonitic Phase obliterates. Charnockitic migmatites occur west of the hypersthene-in isograde.

NORSK GEOLOGISK TIDSSKRIFT l {1984)

indicate a virtually isobaric cooling path at a total pressure slightly in excess of 5 kbar between c. 900-500 oc.

Maijer et al. (1981) argued that only locally in the migmatite terrain adjacent to the igneous complex, formation of M3 mineral assemblages and resetting of some Rb-Sr whole rock and U­Pb zircon systems may have taken place.

Wielens et al. (1981) postulated a metamor­phic event of low-granulite to upper-amphibolite facies conditions at about 950 Ma, during which the (Quartz-) Monzonitic Phase of the Bjerk­reim-Sokndal Layered Intrusion was emplaced. Crystallisation conditions for this phase are c. 950 oc - c. 850 oc at 5-7 kbar (Rietmeijer 1979).

Maijer et al. (1981) pointed out that the model of Wielens et al. (1981) results in an enigmatic relationship between the intrusion age and the regionally prevailing K-Ar isotopic ages of hornblendes. The latter age of 953 ± 10 Ma is present in the igneous and metamorphic com­plexes (Dekker 1978). The recalculated Rb-Sr whole-rock isochron age of the (Quartz-) Mon­zonitic Phase, 928 ±50 Ma, has been interpreted as its intrusion age (Wielens et al. 1981). The new value is higher than the whole-rock isochron age of 857 ± 21 Ma obtained by Pasteels et aL (1979). U-Pb zircon ages for two rocks of the (Quartz-) Monzonitic Phase, viz. 946 ± 14 Ma and 932 ± 5 Ma (Pasteels et al. 1979), are similar to the recalculated whole-rock isochron value.

Finally, a stage of retrograde prehnite-pumpel­lyite facies metamorphism (M4) is locally ob­served throughout the area. Circa 400 Ma old green biotite belonging to this stage is linked to the Caledonian orogenesis (Verschure et al. 1980). A similar age is indicated by the lower intercepts of zircon discordia (Wielens et al. 1981).

I will try to evaluate the temperature regime of the M3-stage, its bearing on the intrusion age of the (Quartz-) Monzonitic Phase and to reinter­pret isotopic age data.

Bjerkreim-Sokndal Layered Intrusion: geologicaVpetrological data

The intrusion is divided in a Leuconoritic Phase and a (Quartz-) Monzonitic Phase (Fig. 1). The synclinally folded Leuconoritic Phase is discord­antly overlain by the sub-horizontal (Quartz-)

Pyroxenes in Precambrian terrain 9

Monzonitic Phase. The former, mainly leuconor­ite and anorthosite, is the most voluminous phase. Its estimated thickness in the axial zone is 5000 metres (Michot & Michot 1969). The latter forms a 350 metre thick sill-like body of two­clinopyroxene monzonite to -syenite (both ± iron-rich olivine) and clinopyroxene-fayalite­quartz syenite to -granite (Rietmeijer 1979).

The contact of the intrusion with the anortho­site massifs and charnockite migmatite complex is tectonic (Rietmeijer 1973, 1979, Michot & Mi­chot 1969). Foliation in the (Quartz-) Monzonitic Phase gradually increases towards the contact. Simultaneously the seriate/interlobate inequi­granular medium-grained textures become com­pletely granulated (Dekker 1978, Rietmeijer 1973). The medium-grained texture occurs in macroscopically undeformed rocks and is be­lieved to represent the original igneous texture.

Recrystallisation is accompanied by formation of extreme poikiloblasts of iron-rich orthopyrox­ene (inverted pigeonites) and iron-rich clino­amphibole. Their c-axes are parallel to the foli­ation (Rietmeijer & Dekker 1978, Dekker 1978). The Ti-content of iron-rich clino-amphiboles in­dicates that they formed 'at high temperatures of granulite facies conditions' (Dekker 1978).

Pyroxene data

In the present study, published and unpublished electron microprobe data for coexisting ortho­and Ca-rich clinopyroxenes have been used. The latter include two unpublished M. Se. theses, viz. Perlin (1980) and van Gaans (1982), and data from the DATA BASE of the Department of Petrology, RU-Utrecht (Tables l and 2 ). Ferric iron is calculated after the method of Hamm & Vieten (1971).

The data set covers every major petrological unit in the area. A detailed petrological study of the area is given by Hermans et al. (1975).

Igneous rocks

Bjerkreim-Sokndal Layered lntrusion

In the Leuconoritic Phase primary orthopyrox­ene crystallises until its Fe-ratio [ = Fe2+ l (Fe2+ +Mg)] reaches c. 0.4, when it is replaced by pigeonite (Duchesne 1973). Ca-rich clinopy-

Tab

le I

.

SAMP

LE NO

.

S1

02

Al

2o

3

T1

02

Fe

O

Fe

2o

3

MnO

M\1()

Ca

O

Na

2o

64

44

Oil X

cpx

53

.0

7

52

.4

2

1.

43

2

.3

9

0.

18

0

.4

3

20

.6

6

8.

71

0.

37

0

.1

7

23

.4

4

13

.7

3

1.

07

2

1.

57

n1

n

a

64

60

opx

CPX

51

.2

3

51

.7

7

1.

24

2

.1

4

0.

14

0

.4

2

26

.6

0

11

.7

7

0.

55

0

.3

0

18

.9

3

12

.5

7

0.

75

2

0.

56

0.

13

0

.6

6

E2

63

opx

cpx

53

.8

8

52

.3

3

1.

52

2

.4

8

0.

21

0

.5

0

15

.8

4

6.

40

0.

27

0

.1

4

26

.5

8

14

.9

1

o.

70

2

2.

00

0.

00

0.

44

R2

5

oøx

cox

49

.4

7

50

.8

7

0.

62

1

.3

7

0.

11

0

.2

0

37

.1

9

16

.3

5

l. 79

o.

75

0

.3

7

11

.2

8

8.

93

0.

96

20

.4

0

0.

00

0.

48

R5

3

DPX

CPX

50

.3

6

51

.2

4

0.

51

1

.3

5

0.

02

0

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0

32

.8

3

14

.7

4

0.

65

0

.3

1

14

.6

0

10

.6

0

0.

74

2

0.

26

0.

00

0.

40

R1

86

OPx

CPx

49

.8

3

50

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7

0.

87

1

.8

7

0.

12

0

.1

0

33

.0

4

15

.9

0

o. 70

0

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7

13

.9

3

10

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0

1.

05

20

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3

na

n

a

R4

86

OPx

CPx

50

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4

51

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5

0.

40

1

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0

0.

00

n

o

36.

33

1

6.

78

0.

58

0

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7

12

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9

9.

34

0.

83

2

1.

06

na

n

a

GA24

8

OPX

CPX

52

.8

9

51

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3

2.

54

3

.8

0

0.

18

0

.6

9

16

.1

5

4.

26

1.

09

3

.1

7

0.

29

0

.1

1

25

.9

1

14

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0

0.

72

2

2.

55

0.

00

0

.6

8

R4

0

OPX

CPX

47

.8

1

49

.0

0

0.

68

1

.0

4

0.

06

0

.2

0

43

.5

3

23

.2

5

1.

06

0.

54

5.

88

5

.2

1

o.

72

20

.5

2

0.

00

0

.2

8

R2

47

R

34

0

opx

cpx

opx

CPX

47

.3

7

50

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7

47

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0

49

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0

1.

91

1

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6

0.

55

1

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0

0.

08

0

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1

0.

20

0

.2

0

42

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7

22

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8

42

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0

22

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5

1.

18

0

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8

0.

88

0

.4

5

5.

08

5

.0

6

7.

10

5

.9

0

1.

35

1

9.

26

0.

22

0

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0

0.

95

1

9.

65

0.

00

o

. 7

0

To

ta

l 1

00

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2

99

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2

99

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4

10

0.

19

9

9.

00

99

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0

10

0.

38

1

00

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6

99

.7

1

99

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0

99

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4

10

0.

74

1

00

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7

10

0.

30

9

9.

77

1

00

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9

99.

74

1

00

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4

99

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6

99

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2

99

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8

10

0.

85

Str

uct

ura

l fo

rmu

lae

calc

ula

ted

to

6 o

xyge

ns

Sl

Al

T1

Fe

2+

Fe

3•

Mn

Mg

Ca

No

to

ta

l

EN

FS

\10

1.

95

9

1.

95

7

1.

96

9

1.

94

7

1.

96

4

1.

94

4

1.

98

2

1.

95

5

1.

98

5

1.

98

0

0.

06

2

0.

10

5

0.

05

5

0.

09

5

0.

06

5

0.

10

9

0.

02

9

0.

062

0

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24

0

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62

0.

005

0

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12

0

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4

0.

01

2

0.

006

0

.0

14

0

.0

03

0

.0

06

0

.0

01

0

.0

06

0.

63

8

0.

27

2

0.

83

6

0.

37

0

0.

48

3

0.

19

9

1.

24

6

0.

52

6

1.

08

2

0.

47

6

0.

05

2

0.

01

2

0.

005

0

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18

0

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09

0

.0

08

0.

00

4

0.

02

5

0.

01

2

0.

02

2

0.

01

0

1.

28

9

0.

76

4

1.

08

5

0.

70

5

0.

04

2

0.

86

3

0.

03

1

0.

82

9

0.

01

0

0.

04

7

4.

007

3

.9

78

4

.00

8

4.

01

4

65

.5

4

0.

4

55

.6

3

7 .

o

32

.4

1

4.

4

42

.8

1

9.

4

2.

1

45

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1

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4

3.

6

1.

44

4

0.

82

6

0.

02

7

0.

87

6

0.

000

0

.0

32

3.

99

7

4.

004

73

.9

4

3.

5

24

.7

1

0.

5

1.

4

46

.0

2

0.

67

3

0.

51

2

0.

85

8

0.

59

6

0.

04

1

0.

84

0

0.

03

1 0

.8

39

0.

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0

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36

0

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0

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3.

99

9

4.

001

4

.0

03

3

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99

34

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2

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3

43

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3

1.

2

63

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2

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0

54

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2

4.

9

2.

1

44

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1

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4

3.

9

8

1.

98

4

1.

95

2

1.

99

1

1.

97

8

1.

92

4

1.

87

9

1.

99

6

1.

96

0

1.

97

5

1.

98

8

1.

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7

1.

96

5

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9

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9

0.

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4

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3

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0

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5

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3

0.

02

7

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0

0.

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4

0.

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3

0.

000

0

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5

0.

01

9

0.

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0

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5

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00

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8

0.

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0

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06

1.

09

4

0.

51

2

1.

20

7

0.

54

0

0.

49

1

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13

1

1.

52

0

0.

77

8

1.

47

5

0.

74

3

1.

48

0

0.

74

9

0.

03

0

0.

08

7

0.

02

3

0.

01

9

0.

02

0

0.

01

2

0.

00

9

0.

00

3

0.

03

7

0.

01

8

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04

2

0.

01

5

0.

03

1

0.

01

5

0.

82

2

0.

59

7

0.

04

5

0.

83

5

4.

01

3

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3

41

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3

0.

7

55

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2

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3

2.

3

43

.0

o.

72

7

0.

53

5

0.

03

5

0.

86

8

3.

99

9

3.

99

2

36

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2

7.

5

61

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2

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8

1.

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44

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1.

40

5

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78

2

0.

02

8

0.

88

6

0.

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0

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4.

00

1

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99

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73

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3.

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25

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49

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36

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31

0

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87

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0.

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0

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98

6

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0

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1

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8

79

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6

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44

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8

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0

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6

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0

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81

5

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9

0.

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3.

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1

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......

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�

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..:::: ·

.....

....

z

o

;.:l

Vl

;>::

Cl

rn

o 5 Cl

c;;

;>::

-l 8 Vl

Vl

;>::

;.:l � j

Tab

le l

con

td.

SAIIP

LE

IK!.

R394

oø

x

cp

x

SID z

47.00

51

.50

A1 2o 3

1. 75

1.

00

Ho 2

o.1o

o.

1o

Fell

44.6

0 22

.70

F�2o 3

MnO

1.

00

0.40

14g0

5.10

4.

30

CaD

1.40

20

.00

Na2o

0.40

o.

oo

073

OØ

lll.

CP

lll.

46.7

4 47

.16

0.34

1.

35

0.08

0.

14

44.84

21

.14

4.16

0.76

0.

72

5.43

5.

23

0.82

19

.52

na

0.

00

F107

0

oo

x

co

x

50.8

1 52

.21

0.65

1.

20

0.28

0.

28

30.7

0 12

.92

1.09

0.

41

0546

OD

X C

PX

50.5

5 52

.42

o. 74

1.

53

0.14

0.

21

37.3

0 18

.25

0.45

0.

26

15.4

5 ll

.39

11.5

4 9.

60

0.76

21

.74

0.95

20

.32

o.oo

0.

60

na

n

a

0521

oo

x

co

x

52.3

5 52

.41

0.67

1.

27

0.13

0.

20

32.4

9 14

.17

0.61

0.

25

15.7

4 ll

.25

0.69

21

.19

na

n

a

To

tal

101.

35 1

00.00

99

.01

99.4

2 99

.74

100.

47

101.

67 1

02.5

9 10

2.68

10

0.74

Stru

ctur

al f

orm

ula

e ca

lcu

late

d t

o 6

oxyg

ens

Sl

Al

Tl Fe

2•

F�3+

Mn

l4g

Ca

No to

ta

l

EN

FS

MO

1.94

6 2.

032

0.08

5 0.

047

0.00

3 0.

003

1.54

5 o.

749

1.98

5 1.

904

0.01

7 0.

064

0.00

3 0.

004

1.59

2 0.

714

0.12

8

1.98

4 1.

971

0.03

0 0.

053

0.00

8 0.

008

1.00

3 0.

408

0.03

5 0.

013

0.02

7 0.

025

0.03

6 0.

013

0.31

5 0.

253

0.34

4 0.

315

0.89

9 0.

641

0.06

2 0.

845

0.03

7 0.

844

0.03

2 0.

879

0.03

2 0.

000

0.00

0 0.

000

0.02

2

4.02

3 3.

942

4.00

3 3.

998

3.99

2 3.

995

16.4

13

.7

80.4

40

.6

3.2

45.7

17.4

16

.8

80.7

38

.1

1.9

45.1

46.5

33

.2

51.9

21

.2

1.6

45.6

1.990

1.

973

0.03

4 0.

068

0.00

4 0.

006

1.22

8 0.

574

0.01

5 0.

008

0.67

7 0.

538

0.04

0 0.

819

3.98

8 3.

986

34.9

28

.1

63.1

29

.7

2.0

42.2

1.98

8 1.

979

0.03

0 0.

057

0.004

0.

006

1.03

2 0.

447

0.02

0 0.

008

0.89

1 0.

633

0.02

8 0.

857

3.99

3 3.

987

45.6

32

.5

52.8

23

.2

1.6

44.3

0555

OP

X

CO

X

46.04

49

.31

0.65

1.

37

0.13

0.

19

39.0

2 20

.53

2.41

0.47

0.

22

8.03

6.

67

1.00

20

.13

na

n

a

97.7

5 98

.42

l. 95

2 1.

970

0.03

2 0.

065

0.00

4 0.

006

1.35

4 0.

686

0.07

7

0.01

7 0.

007

0.50

7 0.

397

0.04

5 0.

862

3.98

8 3.

993

26.6

20

.5

71.0

35

.4

2.4

44.1

F475

oø

x

co

x

50.2

9 50

.83

0.99

1.

76

0.11

0.

21

30.9

8 13

.43

0.45

0.

22

15.8

5 ll

.52

0.63

21

.34

na

n

a

99.3

0 99

.31

1.97

1 1.

948

0.04

5 0.

080

0.00

3 0.

006

1.01

5 0.

431

0.01

5 0.

007

0.92

6 0.

658

0.02

6 0.

876

4.00

1 4.

006

47 .

o 33

.5

51.5

21

.8

1.5

44.7

F106

F1

66

oo

x

co

x

47.7

0 48

.37

1.06

1.

77

O.ll

0.

24

29.6

5 14

.02

2.51

oo

x

co

x

49.7

6 49

.27

1.20

2.

29

0.16

0.

38

35.2

3 16

.07

0.22

0.

19

0.64

0.

23

14.6

7 10

.28

13.8

0 9.

83

0.68

20

.23

0.84

20

.08

na

n

a

na

n

a

96.6

0 95

.10

101.

63

98.1

5

1.94

6 1.

946

0.05

1 0.

084

0.00

3 0.

007

0.98

0 0.

472

0.07

7

0.00

8 0.

006

0.89

2 0.

616

0.03

0 0.

872

3.98

7 4.

003

46.9

31

.6

51.5

24

.0

1.6

44.4

1.94

7 1.

933

0.05

5 0.

106

0.00

5 O.

Oll

1.15

3 0.

527

0.02

1 0.

008

0.80

5 0.

575

0.03

5 0.

844

4.02

1 4.

004

41.7

29

.6

56.2

27

.2

2.1

43.2

L40

0

oo

x

co

x

48.3

3 48

.58

0.96

1.

66

0.11

0.

21

33.8

1 14

.65

0.48

0.

21

Lll9

oox

c

øx

49.2

5 49

.13

0.85

1.

41

0.10

0.

19

36.0

5 15

.88

0.62

0.

16

5031

oox

c

px

52.1

8 50

.48

1.16

1.

92

O.ll

0.

25

22.6

4 8.

40

13.8

3 10

.20

12.1

3 9.

25

o. 9

0 20

.34

1.00

20

.09

0.46

0.

17

21.6

3 13

.74

0.51

21

.69

••

n

a

••

•

•

na

n

a

98.4

2 95

.85

100.

00

96.1

1 98

.69

96.6

5

1.94

9 1.

945

0.04

6 0.

078

0.00

3 0.

006

1.14

0 0.

491

0.01

6 0.

007

0.83

1 0.

609

0.03

9 0.

873

4.02

4 4.

009

41.3

31

.0

56.7

24

.9

2.0

44.1

1.97

0

0.04

0

0.00

3

1.206

0.02

1

0.72

3

0.04

3

4.00

6

36.5

61.4

2.1

1.96

8 1.

973

1.94

6

0.06

7 0.

052

0.08

7

0.006

0.

003

0.00

7

0.53

2 o

. 71

6 0.

271

0.00

5 0.

015

0.006

o. 55

2 1.

219

o. 79

0

0.86

2 0.

021

0.89

6

3.99

2 3.

999

4.00

3

28.4

62

.2

40.4

27.3

36

.7

13.8

44.3

1.

1 45

.8

z

o

"'

"'

;o::

Cl

rn

o

5

Cl

v;

;o::

::l

o

"'

Vl

;o::

"' �

� J � � ;:

:

�

s·

� � <::)- �- �

.,

�

s

·

......

......

Tab

le l

con

td.

SAHI'LE

NO

.

s1

o2

A

12

o3

H

o2

F.O

Fe

2o

3

HnO

HgO

CaO

No 20

F3

/7

0

OPX

CPX

49

.60

4

9.

76

0.

70

1

.7

1

0.

18

0

.3

7

32

.5

7

16

.3

4

0.

63

0

.1

1

14

.7

6

10

.4

9

0.

87

1

8.

79

na

n

a

ll

6A

OPX

c

pic

48

.4

7

49

.6

2

0.

63

1

.2

7

O.

ll

0

.2

0

33

.2

5

14

.2

8

0.

80

0

.2

4

13.

71

1

0.

29

0.

81

2

0.

88

na

n

a

05

13

op

x

co

x

49

.3

2

48

.8

7

1.

07

1

.9

9

0.

10

0

.2

7

34

.6

1

15

.4

8

0.

56

O

.l

l

13

.0

7

9.

65

0,

78

1

9.

62

na

n

a

V1

87

OPX

COX

49

.8

0

50

.9

2

o.

71

1

.5

7

0.

15

0

.2

0

35

.7

2

15

.0

7

o. 7

2

0.

29

13

.0

7

10

.�

0.

93

2

1.

96

0.

07

0

.5

1

R4

0

OPX

COX

48

.8

0

49.

49

1.

80

1

.1

0

0.

31

0

.2

2

43

.6

0

22.

73

no

1}.

58

5.

36

4

.8

9

1.

05

2

0.

67

no

0.

00

R9

5

OOX

co

x

o.�

4

9.

�

1.

90

1

.2

2

0.

10

0

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9

«.

64

�

.�

1.

23

0

.5

3

2.

65

3

.0

3

0.

69

1

9.

46

1.

04

0

.6

1

Til

OOX

co

x

47

.5

8

48

.8

6

0.

47

1

.0

9

0.

19

0

.2

2

44

.1

8

24

.2

6

1.

12

0

.5

3

5.

48

5

.0

8

0.

77

1

9.

77

na

n

a

R9

8

OOX

co

x

47

.0

1

49

.2

3

0.

55

0

.7

0

0.

13

0

.2

2

45

.1

6

26

.0

9

1.

02

0

.6

2

2.

85

2

.9

8

2.

64

1

9.

38

no

0

.6

5

R3

40

OP

X

48

.00

0.

90

0.

00

43

.8

0

1.

20

5.

40

0.

80

0.

00

CPX

50

.1

0

1.

10

0.

00

21

.5

0

0.

60

5.

30

21

.5

0

0.

00

R3

94

OOX

co

x

47

.2

0

51

.5

0

0.

50

1

.00

0.

00

0

.1

0

45

.1

0

22

.7

0

1.

10

0

.4

0

5.

20

4

.3

0

0.

85

2

0.

00

0.

00

0.

00

R4

0

OOX

co

x

47

.5

5

49

.9

1

0.

65

0

.9

4

0.

10

0

.1

2

42

.6

0

21

.4

7

1.

05

0

.5

3

5.

64

5

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7

0.

97

2

0.

73

0.

00

0.

08

R1

53

oo

x c

px

47

.7

4

49

.6

4

0.

40

0

.9

2

0.

20

0

.1

8

42

.0

1

20

.4

0

0.

98

0

.4

7

7.

49

6

.3

2

0.

67

2

0.

15

0.

02

0

.4

5

Tot

al

99

.3

1

97

.5

7

97

.7

8

96

.7

8

99

.5

1

94

.9

9

10

1.

17

1

00

.5

8

10

0.

92

9

9.

68

9

9.

50

9

9.

97

9

9.

79

9

9.

82

9

9.

36

9

9.

86

1

00

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0

10

0.

10

9

9.

95

1

00

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9

8.

56

9

9.

15

9

9.

51

9

8.

53

Str

uct

ura

l fo

rmu

lae

calc

ula

ted

to

6 o

xyge

ns

Si Al

Ti

Fo

2+

Fe

3+

""

Hg

Co

No

toto

l

EN

FS

NO

N

1.

96

6

1.

95

6

1.

96

5

1.

96

3

0.

03

2

0.

07

9

0.

03

0

0.

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9

0.

005

0

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11

0

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3

0.

008

1.

07

9

0.

53

7

1.

12

7

0.

47

3

0.

02

1

0.

004

0.

87

2

0.

61

5

0.

03

7

o. 7

92

4.

01

2

3.

99

4

43

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3

1.

6

54

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2

7.

6

2.

0

40

.8

0.

02

7

0.

008

0.

82

8

0.

60

7

0.

03

5

0.

88

5

4.

01

5

4.

003

43

.0

3

1.

0

54

.9

2

3.

9

2.

1

45

.1

1.

96

7

1.

95

3

1.

96

4

1.

94

9

0.

05

0

0.

09

4

0.

03

3

0.

07

1

o.

oo3

0

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8

o.00

4

o.�

1.

15

5

0.

51

7

1.

17

8

0.

48

2

1.

99

5

1.

97

8

0.

08

8

0.

05

2

0.

01

0

0.

008

1.

490

o.

76

0

0.

01

9

0.

00

4

0.

77

7

0.

57

5

0.

03

3

0.

84

0

4.

004

3.

99

1

39

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2

9.

8

58

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2

6.

8

1.

7

43

.4

0.

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4

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009

0

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20

o. 7

68

0

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74

0

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28

0

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0

0.

03

9

0.

90

1

0.

04

5

0.

88

5

0.

00

5

0.

03

8

0.

000

4.

01

5

4.

03

0

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95

6

3.

99

3

38

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2

9.

3

17

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1

5.

0

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6

80

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9.3

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0

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4

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7

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5

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000

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1.

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8

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0

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3

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0

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99

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99

0

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5

9.

8

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o.�

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0

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5

80

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4

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00

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3

0.

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o.�

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.9

.....

N

�

�

�

:::tl �· "' ..::: ·

<�>"

....

z o

::0

"'

::-: Cl

m

o

r

o

Cl

c;;

::-:

::l

o

"'

"'

::-:

::0 � .....

i

NORSK GEOLOGISK TIDSSKRIFT l (1984)

� <

>< ... u

K ... o

K ... o

g o

�

:ll o

"' (!; ' o

� "'

o

"' "' "' 8 � � o o o

Pyroxenes in Precambrian terrain 13

� "'

,.;

O M \D M .., "' ... M M In ,.... s � � � �

o " e o"": o :;; � 8 �

- o o o O .... CC O

o o o o ..

" .. o

� � � � � � � � �OOsj Occ c:ic:i l.C \() M 0 C7l CP\ N � g å o �

M � ..

c:i o N

� 2 � 2 o W'l o o

o o o o N M Y"' lt'l

... .., O ._ CIO O

o o o o N

.., "' " ... ���a

o Cl) o o !å8� N M M M

8 � (!; 8 o o o o

8 o -o o- ...

"' M Ln o ..n I.I'J ��i å 8 � ... N

" e

o ..

o o N

�. � 8 � o o c ei o o o ...

"' K ... o

"'� � o

o o

; o

....

roxene forms small anhedral crystals or partial rims on Ca-poor pyroxene (Rietmeijer 1973, van Gaans 1982). Subhedral primary orthopyroxene has exsolution lamellae of Ca-rich clino-pyrox­ene /1100. Elongated crystals (//c-axis) of (opti­cally) homogeneous primary orthopyroxene and Ca-rich clinopyroxene are locally present. The Fe-ratio of elongated pyroxenes is 0.25-0.66 (van Gaans 1982).

In the charnockite migmatite complex of Ro­galand elongated to acicular primary orthopyrox­ene of similar compositions is typical in dehy­drated rims of intercalated amphibolite lenses (Rietmeijer 1979).

The grain size of Ca-rich clinopyroxenes in the felsic rocks shows a bimodal distribution (Riet­meijer 1979):

l) large(> l mm) subhedral crystals with up to four generations of exsolution lamellae //'001', '100' and 100 (Rietmeijer & Champness 1980a, 1982) ('hk!' means approximately //hk!). The ini­tial wollastonite con tent [ =Ca/(Fe2+ + Mg+Ca) xlOO] is c. 35-40 mole%.

2) small ( < l mm) anhedral crystals with exso­lution lamellae //'001' only. The initial wollaston­ite content is 40-45 mole%.

��ss o o o o

Primary orthopyroxene also has two distinct ha bits:

l) subhedral crystals (0.5-1.0 mm) with exso­lution lamellae of Ca-rich clinopyroxene 11100. In addition Fe-Ti oxide plates and/or needles //(100) may be present.

2) homogeneous crystals slightly elongated along the c-axis. They are smaller than the for­mer.

Textural evidence indicates that coexisting py­roxene pairs are formed by ortho- and Ca-rich clinopyroxenes from groups l) and 2) (Riet­meijer 1979).

Multiple exsolution patterns are typical for ig­neous clinopyroxenes, whereas metamorphic clinopyroxenes tend to exhibit simple exsolution patterns (Robinson et al. 1972, Jaffe et al. 1975). Thus group l) pyroxenes are interpreted as the original igneous phases and group 2) as meta­morphic phases formed by recrystallisation.

The initial wollastonite contents agree with this interpretation.

Group l) Ca-rich clinopyroxenes may show partial rims of primary orthopyroxene. Riet­meijer (1979) interpreted the texture as an ig-

14 F. J. M. Rietmeijer

20

NORSK GEOLOGISK TIDSSKRIFT l (1984)

10

100

Fet"rosilitr

Fig. 2. Compositions of cocxisting pyroxenes from the Precambrian terrain in Rogaland (SW Norway) plotted on the pyroxene quadrilateral Enstatite-Ferrosilite-Diopside-Hedenbergite (cf. Table 1). The compositions of Ca-rich clinopyroxenes delineate a zone at about 45 mole% CaSi03. For explanation of symbols cf. Table 2.

neous phenomenon. However, Lindsley & An­dersen (1983) showed granule exsolution to be important in slowly cooled Ca-rich clinopyrox­enes. The resulting microstructures are similar to those observed by Rietmeijer (1979). The ortho­pyroxene rims are therefore reinterpreted as sub­solidus reequilibration of igneous Ca-rich clino­pyroxenes.

Botnavatnet and Gloppurdi Igneous Complexes

They are c. 1200 Ma old iron-rich intrusions concordantly intercalated in the migmatites com­plex. Stratiform mafic layers [ amphibolite and (leuco )norite] form an intrinsic part of the intru­sions (Rietmeijer 1979). In some layers iron-rich mafic silicates and Fe-Ti oxides reacted with ad­jacent plagioclase to garnet (Perlin 1980) (Note: similar garnet is observed in a few samples from the Bjerkreim-Sokndal Layered Intrusion).

In sarnple 073 (garnet-bearing norite) garnet forms rims on primary orthopyroxene, pigeonite (Fe-ratio= 0.81) and Ca-rich clinopyroxene (Fe­ratio= 0.73). The pyroxene assemblage indicates crystallisation conditions at about 935 oc (Table 2).

Pyroxene relations are similar to those ob­served in the (Quartz-) Monzonitic Phase of the Bjerkreim-Sokndal Layered Intrusion (Riet­meijer 1979, Rietmeijer & Champness 1980 b).

The low Al content of orthopyroxene (Al203 = 0.25.-0.50 wt%) and the high iron and calcium content of garnet prohibit their use as geobarometer (cf. Harley & Green 1981). The low Al content is typical for orthopyroxene from the igneous rocks in the area (Rietmeijer 1979).

Folded basic intrusions

Hermans et al. (1975) observed igneous masses and zones of pyroxene dioritic composition con­cordantly folded with the surrounding migma­tites. Modal analysis shows the presence of en­derbite, norite and gabbronorite. Textures and colour of constituent amphibole indicate that they reequilibrated under high-grade metamor­phic conditions (Dekker 1978). In sample F107-D (apatite-amphibole monzonorite) the coexist­ing pyroxenes are optically almost homogeneous primary orthopyroxene and small subhedral Ca­rich clinopyroxene with simple exsolution tex­tures. They may be of metamorphic origin (cf. section Bjerkreim-Sokndal Layered Intrusion above).

Metabasites

In the migmatite complex 'mainly banded mig­matites' and 'massive parts' can be distinguished

T

(°Cl

SOU

RCE

SA

MP

LE

S RE

MA

RKS

SY

MB

OLS

l.

IG

NE

OUS

R

OC

KS

1.

1

BJE

RKRE

IM

-S

OK

ND

AL

LAY

ERE

D

92

2,

80R

D

ucn

es

ne

1

973

64

44

, 64

60

Thi

rd

r

hyt

hmi

c u

n�t

; P

ig

eo

ni

te

cr

ys

ta

ll

is

at

io

n

6

IN

TR

US

ION

t

em

pe

ra

tu

re

1

13

5

C

(TH

E

LEU

CON

ORI

TIC

P

HA

SE)

92

0 D

ATA

B

ASE

E2

63

Tni

rd

rhy

thm

ic

un

it;

el

on

ga

te

d

pr

im

ar

y

OPX

•

B74

-

795

van

Ga

an

s 1

982

R

25

, R

53

, R

186

, F

ou

rt

n &

Fi

ft

n r

nyt

nm

ic

un

it

s

su

bne

dr

al

OP

X +

•

(av

. 83

2)

R4

B6,

GA24

8 in

te

rg

ro

wn

Fe

-Ti

oxi

des

; a

cicu

la

r0

0P

X;

Pi

ge

on

it

e

cry

st

al

li

sa

ti

on

t

em

per

at

ur

e

11

98

C

874

-

755

Riet

me

ij

er

19

79

; R

40

, R2

47,

R

34

0,

Fi

ft

n r

nyt

nmi

c u

ni

t;

OPX

ri

ms

on

C

a-

CP

X;

Pi

ge

on

it

e

o

(av

. 81

3)

Riet

me

ij

er

a

nd

R3

94

cr

ys

ta

ll

is

at

io

n

tem

per

at

ur

e

920

°c

(cf

.

Ri

et

mei

je

r

Cha

mp

ne

ss

19

82

1

983

)

1.

2

GL

OP

PU

RD

I I

GN

EO

US

C

OMP

LEX

802

P

er1

1n

1

980

07

3 P

i g

eo

n1 t

e cr

.vs

ta

11

1sa

t1

on

t

em

per

at

ur

e 9

35

°c

•

11

.

FOL

DED

B

ASI

C I

NTR

US

ION

S

810

D

ATA

BA

SE

F

l0

7-

D

cf.

D

ekk

er

19

78

•

11

1.

ME

TAB

AS I

TE

S

866

-

806

Ja

cqu

es

de

D

ixm

ud

e

054

6,

052

1,

05

55

, C

or

re

cte

d f

or

fe

rr

ic

1r

on

•

(a

v.

836)

19

78

F4

75

, F

l0

6,

F1

66

, L4

00

, L

11

9,

S0

31

, F

3/

70

, L

16

A,

05

13

740

DAT

A

BA

SE

V1

87

Su

bne

dr

al

Ca

-C

PX

an

d s

li

gnt

ly

e

lo

ng

at

ed

p

ri

ma

ry

OP

X e

(c

f.

De

kk

er

19

78)

SU

BS

OL

ID

US

PH

AS

ES

(f

or

I

GN

EOU

S

ROC

KS

o

nl

y)

Exs

ol

ve

d

Ca

-CP

X 87

6

-76

9

Du

che

sn

e

19

73

; R4

0,

R9

5,

Til

, o

Ri

etm

ei

je

r 1

97

9;

R9

8,

R34

0,

R39

4

Ri

etm

ei

j e

r

and

C

nam

pn

es

s 1

9R2

Inv

er

te

d P

ig

e�

ni

te

88

7

-7

87

Ri

etm

ei

je

r 1

979

R

40

, R1

53,

R24

7,

o

R31

2,

R788

, A7

, A

l O l

Tabl

e 2. E

quili

brat

ion

tem

per

atur

es a

nd so

urce

s for

met

amor

phi

c an

d re

equi

libra

ted

igne

ous o

rtho

-and

Ca-

rich

clin

opyr

oxen

es u

sed

in F

ig. 2

. Tem

per

atur

es a

re c

alcu

late

d ac

cord

ing

to th

e

pyr

oxen

e ge

othe

rmom

eter

of

Woo

d &

Ban

no (

1973

).

Pige

onit

e cr

ysta

llisa

tion

tem

per

atur

es a

re c

alcu

late

d af

ter

Ishi

i (1

975)

: T

("C

) ==

1270-4

80 X

Fe +

[(10

-5X

Fe).

P);

XFe

= F

e2+/

(Fe2

+ +M

g) p

;g;

P= 9

kba

r (c

f. R

ietm

eije

r an

d C

ham

pne

ss 1

982)

.

Th

e ri

ght-

hand

sid

e co

lum

n p

rovi

des

key

to s

ymbo

ls u

sed

in F

ig.

2.

l

z

o " en

;.:

o

!Tl

o 5 o

c;;

;.:

-1 a en

en

;.: " � j �

.... � � � s·

� 2 3 <::1-

5·

;::: � � s·

....

Vl

16 F. J. M. Rietmeijer

(Hermans et al. 1975). The former is composed of alternating light- and dark coloured layers suggestive of a supracrustal origin. Dark layers in the charnockitic part of the migmatite complex include (leuco)norite, amphibolite, monzonorite and enderbite.

Jacques de Dixmude (1978) suggested that the greater part of the dark layers represent metavol­canics, albeit that they generally show granoblas­tic textures. Several samples (e.g. V187, amphi­bole norite) contain large multiple exsolved Ca­rich clinopyroxenes, small anhedral crystals of homogeneous orthopyroxenes and simple ex­solved Ca-rich clinopyroxenes. The interpreta­tion of pyroxene phase relations compares with groups l) and 2) in the section Bjerkreim-Sokn­dal Layered Intrusion above.

To summarise, igneous pyroxenes in the Pre­cambrian basement show evidence of subsolidus reequilibration. Recrystallisation in the migma­tite complex (Huijsmans et al. 1981) and in the Bjerkreim-Sokndal Layered Intrusion (cf. sec­tion on petrologicallgeological data) took place in the orthopyroxene - Ca-rich clinopyroxene stability field. The wollastonite contents of py­roxenes (Fig. 2) indicate that they (re-) equili­brated on a regional scale (Rietmeijer 1983).

Pyroxene equilibration temperatures

Pyroxene geothermometers by Wood & Banno (1973) or Wells (1977) may be used to calculate equilibration temperatures of coexisting ortho­and Ca-rich clinopyroxenes. The accuracy of the geothermometers is claimed to be within ± 70 °C. All pyroxenes used in the present study are low in Ah03, Ti02, MnO, Na20 and K20 ( < ca 3 wt%) and may be treated as 'pure phases'. Thus compositional errors in calculated temperatures will be negligible. Turnock & Lindsley (1981) showed that application of erroneous thermody­namic data necessitate recalibration of geother­mometer by Wells (1977). Therefore I will use the Wood and Banno geothermometer.

However, the experimentally studied distribu­tion of Mg, Fe

2+ and Ca between coexisting

ortho- and Ca-rich clinopyroxenes at 750 oc and 800 oc (Fonarev & Graphchikov 1982) indicate that, at these temperatures, results obtained with the Wood and Banno geothermometer will be 115-185 oc too high.

For exsolved Ca-rich clinopyroxenes Riet­meijer & Champness (1980b, 1982) compared

NORSK GEOLOGISK TIDSSKRIFT l (1984)

temperatures calculated using the Wood and Banno geothermometer with temperatures ob­tained by cell-parameter modelling. They also concluded that the former method produces tem­peratures which are c. 150 °C too high.

Thus pyroxene equilibration temperatures in Table 2, ranging from 920 oc to 740 oc, must be 'corrected' to c. 770 oc to 590 °C. In a low to intermediate pressure regime, the pressure effect on pyroxene equilibration temperatures will be negligible (Lindsley & Andersen 1983).

Isothermal projections of the orthopyroxene­pigeonite and orthopyroxene-Ca-rich clinopyrox­ene solvi onto the pyroxene quadrilateral plane (Ross & Huebner 1975, Lindsley et al. 1974, Turnock & Lindsley 1981) may also be used to obtain pyroxene equilibration temperatures.

In Rogaland the range of pyroxene equilibra­tion temperatures then becomes c. 700 oc to well below 600 oc (Fig. 3). This is in good agreement with the 'corrected' temperatures.

For comparison, Fig. 3 shows the trends for reequilibrated igneous and metamorphic pyrox­enes from the Adirondack Province (U.S.A.) (Boblen & Essene 1978) and the mangerite-char­nockite intrusives from the Lofoten-Vesterålen Precambrian terrain (Norway) (Malm & Ormaa­sen 1978). Metamorphic temperatures in the Adirondack Province are 790 °C - 760 oc using experimentally determined phase relations on the ferrosilite-hedenbergite join (Jaffe et al. 1978). Malm & Ormaasen (1978) calculated py­roxene equilibration temperatures ranging from 900 oc to 650 oc using the geothermometer of Wood & Banno (1973) (Note: these tempera­tures are to o high).

Rietmeijer (1983) proposed that compositions of orthopyroxenes coexisting with Ca-rich clino­pyroxenes may be used to evaluate the extent of regional pyroxene reequilibration, viz. the wol­lastonite content vs. Fe-ratio for completely re­equilibrated orthopyroxenes will define a straight line in a diagram showing these parameters.

The linear correlation (corr. coeff. 0.83) ob­served for coexisting pyroxenes from Rogaland reflects regional reequilibration at about 600 oc.

The observation that some data points are on the high-temperature side of the reequilibration line was taken as evidence that isolated orthopyrox­enes were not able to ad just to changes of phys­ical conditions (Rietmeijer 1983). It is notewor­thy that these orthopyroxenes are from an area north of the Bjerkreim-Sokndal Layered lntru­sion, where Sauter (1981) reported local occur-

NORSK GEOLOGISK TIDSSKRIFT l (1984)

Diopside

�C!_ _______ _

10 20 50 Enstatite

60

Pyroxenes in Precambrian terrain 17

70

Hedenbefgite 50

BO

10

100 Ferrosilite

Fig. 3. Isothermal projections of the OPX-CaPX solvus at 900 °C (Turnock and Lindsley 1981), at 810 oc (Lindsley et al. 1974) and at 800 °C, 700 oc and 600 oc (Ross & Huebner 1975) plotted on the pyroxene quadrilateral Enstatite-Ferrosilite-Diopside­Hedenbergite. Trends of coexisting pyroxenes are given for recrystallised mangerite-charnockite intrusives (Malm and Ormaasen 1978) (1), metamorphic and reequilibrated igneous pyroxenes from the Precambrian terrain in SW Norway (Il) and metamorphic and reequilibrated igneous pyroxenes from the Adirondacks (Boblen & Essene 1978) (Ill).

Ca-rich clinopyroxenes delineate narrow zones at c. 45 mole % CaSi03. Orthopyroxenes plot on lines at the base of the quadrilateral. Tie-lines indicate the range of FeH/(Fe2++Mg) ratios, viz. squares (I), dots (Il) and asterisks (Ill).

rences of sanidine in rocks of the Faurefjell Me­tasediment Formation.

Discussion In the igneous and migmatite complexes of Ro­galand, pyroxene crystallisation took place be­tween c. 750 oc and c. 590 oc (at !east on the scale of electron microprobe analysis). Reequili­bration of igneous pyroxenes is of the same order of magnitude. The range of temperatures may be partically explained by incomplete readjustment to·subsequent ambient temperature regimes.

Crystallisation temperatures in igneous rocks are considerably higher than the regional crys­tallisationlreequilibration temperatures (Table 2). A comparable situation is reported from me­tamorphosed anorthosites in the Adirondacks (Boblen & Essene 1978).

A Time-Temperature diagram (Fig. 4) sum­marises data from the introduction. During the thermo-metamorphic stage (M2) at about 1050-1035 Ma, temperatures reached c. 1000-800 oc in migmatites adjacent to the igneous complex. Iso­topic ages and closure temperatures for osumilite (>550 °C), hornblendes (ca 550 oq and brown

2 - Geologisk Tidsskr. 1184

biotite ( 450-400 oq indicate that subsequent to the M2 stage the initial cooling rate of c. 6 oc/Ma gradually decreased to c. 1.6 °C/Ma. The thermal regime of regional pyroxene (re-) equilibration existed between c. 1030 - c. 990 Ma (Fig. 4).

Reequilibrated igneous and metamorphic py­roxenes occur in the (Quartz-) Monwnitic Phase, indicating that its solidification was com­pleted before the regional event took place. The structural unconformity between the Leuconori­tic and (Quartz-) Monzonitic Phases indicates a discontinuous emplacement history for the Bjerkreim-Sokndal Layered Intrusion. The ex­tent of the hiatus is presently unknown, but it is tentatively bracketed between (1050-1035) Ma and c. (1030-990) Ma. Maijer et al. (1981) ar­gued that the thermal influence caused by intru­sion of the (Quartz-) Monzonitic Phase was neg­ligible. The period of regional cooling (M3 stage) is tentatively estimated between c. 1030 to c. 850 Ma.

Recrystallisation (granulation) in the (Quartz) Monzonitic Phase is accompanied by formation of extreme poikiloblasts of iron-rich clino-amphi­boles. Amphiboles in the Precambrian basement of Rogaland contain fluorine [F/ (F+OH) = 0.1 -0.19) (Dekker 1978) and chlorine (preliminary

18 F. J. M. Rietmeijer

1200

1100

1000

900

u 800 g_

600

500

NORSK GEOLOGISK TIDSSKRIFT I (1984)

horn blendes

400

1050 1000 950 Time (Ma)

900

Rb-Sr :t!::: 1 brown biotite ..__x-iK-Ar

850

Fig. 4. Time (Ma) vs Temperature ("C) diagram showing the cooling history of the Precambrian charnockitic migmatite terrain in SW Norway between c. 1050 and 850 Ma. The Time- Temperature space for the Leuconoritic Phase of the Bjerkreim-Sokndal Layered Intrusion is indicated in the upper teft-hand corner. Isotopic ages are given for osumilite (Maijer et al. 1981), hornblendes (Dekker 1978) and brown biotite (Verschure et al. 1980). The range of crystallisation temperatures for the (Quart�-) Monzonitic Phase of the Bjerkreim-Sokndal Layered Intrusion, c. 950 "C - c. 850 •c, is shaded. Metamorphic pyroxene crystallisation and reequilibration took place in the range bracketed by the dashed horizontal lines.

data by the author). Both elements will stabilise amphiboles to higher temperatures (amongst others, Kearns et al. 1980). The presence of halogens may explain that the amphiboles could have formed in the temperature regime of pyrox­ene (re-) equilibration.

The sample set used for determination of hornblende K-Ar isotopic ages (Dekker 1978) includes a sample of an extreme poikiloblast. The hornblende isotopic age (953 ± 10 Ma) may thus represent the autometamorphic stage of the Bjerkreim-Sokndal Layered Intrusion suggested by Rietmeijer & Dekker (1978). Alternatively, it may indicate a phase of regional amphibole for­mation implied by Dekker's (1978) observation

that almost all primary amphiboles in the area seem to be of metamorphic origin.

The recalculated Rb-Sr whole-rock isochron age for the (Quartz-) Monzonitic Phase (928 ±50 Ma) conveniently includes the K-Ar hornblende isotopic age as well as the U-Pb zircon ages (Wielens et al. 1981, Pasteels et al. 1979). Petro­logical evidence and isotopic ages do not agree with the interpretation of Wielens et al. (1981) that the intrusion age of the (Quartz-) Monzonit­ic Phase is 928 ± 50 Ma. Instead it seems that Rb-Sr whole-rock and U-Pb zircon isotopic sys­tems have been reset during the M3 stage and may be contemporaneous with the formation of halogen-bearing amphiboles.

NORSK GEOLOGISK TIDSSKRIFT l (1984)

Conclusions l. Metamorphic pyroxene crystallisation and

(re-)equilibration of igneous pyroxenes took place on a regional scale (M3 stage) between c. 750 oc and c. 590 oc around 103�990 Ma.

2. Intrusion of the (Quartz-) Monzonitic Phase of the Bjerkreim-Sokndal Layered Intrusion took place between c. 105� 1035 Ma and c. 103�990 Ma.

3. Halogen-bearing amphiboles formed during a period of regional cooling (M3 stage) and/or during a stage of autometamorphism in the Bjerkreim-Sokndal Layered Intrusion.

4. Rb-Sr and K-Ar isotopic systems may have been reset during the M3 stage and be con­temporaneous with the formation of halogen­bearing amphiboles.

Acknowledgements. - l wish to express my gratitude to the 'Afdeling Petrologie, lnstituut voor Aardwetenschappen' (Utrecht, the Netherlands) for use of its unpublished data. Professor A. C. Tobi, Drs. G. A. E. M. Hermans, P. C. C. Sauter, J. B. W. Wielens and A. G. C. Dekker critically reviewed the manuscript. Special thanks are due to Dr. Cees Maijer, with whom l have spent many hours discussing the possible Time-Temperature schemes for Rogaland.

References

Manuscript received August 1982,

revised April 1983

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Gaans, van, C. 1982: Petrogenesis of orthopyroxene-magnetite symplectites from the Bjerkreim-Sokndal lopolith, SW Nor­way. M.Sc. thesis (unpubl.) Rijksuniversiteit Utrecht (the Netherlands), 123 p.

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Pyroxenes in Precambrian terrain 19

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lshii, T. 1975: The relations between temperature and compo­sition of pigeonite in some lavas and their application to geothermometry. Mineral. J. 8, 48-57.

Jacques de Dixmude, S. 1978: Geothermometrie comparee de roches du facies granulite du Rogaland (Norvege meridion­ale). Bull. Mineral. /01, 57-65.

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Jaffe, H. W., Robinson, P. & Tracy, R. J. 1978: Orthoferrosi­lite and other iron-rich pyroxenes in microperthite gneiss of the Mount Marcy area, Adirondack Mounts. Am. Mineral. 63, 1116-1136.

Jansen, J. B. H. & Maijer, C. 1980: Mineral relations in metapelites of SW Norway. lnt. Colt. Rijksuniversiteit Utrecht, May 8-9, l. V. A. U., 2.

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