Metagabbros in the Modum Complex, southern Norway: an important heat source for Sveconorwegian metamorphism

Precambrian Research, 52 ( 1991 ) 97-113

97

Elsevier Science Publishers B.V., Amsterdam

Metagabbros in the Modum Complex, southern Norway: an important heat source for Sveconorwegian metamorphism Ingrid Anne Munz and Randi Morvik

*

Mineralogisk-Geologisk Museum, Sarsgt. I, 0562 Oslo 5, Norway (Received May 9, 1990; revised and accepted January 24, 1991 )

ABSTRACT Munz, I.A. and Morvik, R., 1991. Metagabbros in the Modum Complex, southern Norway: an important heat source for Sveconorwegian metamorphism. Precambrian Res., 52: 97-l 13. The Precambrian Modum Complex in southern Norway is characterized by supracrustal rocks penetrated by numerous gabbro intrusions, occurring as elongated bodies subparallel with the regional foliation. These gabbroic bodies show a partial alteration to amphibolites. The best preserved parts of the metagabbros show a subophitic texture with olivine and plagioclase as the major cumulus phases and clinopyroxene, _+orthopyroxene and ilmenite as the major intercumulus phases. Subsequent alteration of these igneous phases has led to formation of coronas around olivine in contact with plagioclase and recrystallization of clinopyroxene. The igneous phases indicate intrusion temperatures in the range of I 100-1000 oC and pressures below 5-8 kbar. Formation of coronas and recrystallization of clinopyroxene occurred in the pressure-temperature range of 7-10 kbar and 800-600 °C. A 1224 +_15 Ma Sm-Nd mineral isochron obtained from igneous phases indicates emplacement at the start of the Sveconorwegian metamorphism. A complex evolution is suggested for the Sveconorwegian metamorphic event, involving both magmatism and high-grade metamorphism.

Introduction

The Precambrian basement in southern Norway mainly consists of late-Proterozoic gneisses and supracrustals ( ~ 1600-900 Ma). Oftedahl (1980) subdivided the region into different sectors. The two high-grade sectors, Kongsberg and Bamble (Fig. 1, inset), are characterized by numerous mafic intrusions. These mafic intrusions range in composition from ultramafic rocks to gabbros and norites (Brickwood and Craig, 1987). In the Bamble Sector the dominating mafic intrusions are locally known as hyperites, and are mostly olivine gabbros (Bragger, 1934). The corresponding mafic intrusions in the Kongsberg Sector are the Vinor diabases and Vinor me*Present address: Oljedirektoratet, Postboks 600, 4001 Stavanger, Norway.

0301-9268/91/$03.50

tagabbros (Bugge, 1917; Bugge, 1937; Starmer, 1977, 1985 ). The mafic intrusions occur in high-grade gneisses and supracrustals. The Kongsberg and Bamble Sectors are now separated by the Permian Oslo graben. Due to the similar lithologies in the two sectors, they are assumed to represent a former continuous area (Bugge, 1936; Bugge, 1943; Josang, 1966; Starmer, 1976). Peak metamorphic conditions in the Bamble Sector are estimated by thermobarometry to 750-800°C and 7 kbar (Jansen et al., 1985; Lamb et al., 1986). These pressure and temperature estimates are in agreement with the results obtained from whiteschists and orthoamphibole-cordierite rocks in the Modum Complex, situated within the Kongsberg Sector (Munz, 1990). On the basis of Rb-Sr chronology, however, discussion has arisen as to whether such high-grade conditions were ob-

© 1991 - - Elsevier Science Publishers B.V.

98

Fig. 1. Distribution of metagabbros in the Modum Complex (filled rectanglewithin the KongsbergSector). Relics of igneousminerals occur in the numbered bodies. tained during one or two separate metamorphic events. According to Jacobsen and Heier (1978), the western part of the Kongsberg Sector was affected by two periods of highgrade metamorphism. A granulite facies metamorphism 1500-1600 Ma ago was followed by an amphibolite facies metamorphism in Sveconorwegian time, 1100-1200 Ma. A similar polymetamorphic evolution was also described from the Bamble Sector (O'Nions and Baadsgaard, 1971 ). The high-grade nature of the Sveconorwegian event was, however, questioned by Field and Rhheim (1979, 1980, 1981 ), Smalley et al. ( 1983, 1988) and Field et al. (1985). These authors argued that the Sveconorwegian event represented only a minor greenschist facies alteration of older highgrade rocks. Their conclusions have later been questioned by Weis and Demaiffe (1983), Baadsgaard et al. (1984) and Starmer ( 1990 ), based on structural and radiometric data. The metamorphic conditions of the Sveconorwegian event are thus still a matter of debate.

I.A. MUNZ AND R. MORVIK

In this discussion, the mafic intrusions have been used as structural markers, separating the two metamorphic events (Starmer, 1969, 1985 ). A poorly constrained Rb-Sr whole-rock isochron giving an age of 1200 Ma for the Vinor metagabbros (Jacobsen and Heier, 1978 ) suggests that the metagabbros intruded at the onset of the Sveconorwegian event. This paper describes the igneous mineralogy and the subsequent alteration of metagabbros in the M o d u m Complex, situated in the eastern part of the Kongsberg Sector (Fig. 1 ). The metamorphic evolution of southern Norway is discussed on the basis of the data from these metagabbros. Igneous minerals from one of the metagabbros have been dated by the Rb-Sr and S m - N d methods, to establish the age of intrusion. Pressure and temperature conditions for the intrusion and for the subsequent alteration are estimated.

Field relations Metagabbros are c o m m o n in the M o d u m Complex, occurring as large, elongated bodies, constituting approximately 30-40% of the total area. These metagabbros are partly altered to amphibolites, but relics of igneous textures and minerals are still present in most of the bodies. The distribution of the metagabbros is shown in Fig. 1. Metagabbros with a preserved igneous mineralogy are marked with a number. Samples from all of these numbered bodies have been used in this study. The metagabbros in the M o d u m Complex are relatively homogeneous, consisting of coarse-grained rocks with coronite textures. There is no field evidence of compositional variation except for one locality with igneous layering in the Hoghs metagabbro. The metagabbros are locally altered to amphibolites. This alteration occurred in several stages. The first stage is the development of coronas, present in even the least altered samples. With further alteration, the igneous texture is commonly preserved, while the minerals, especially

99

METAGABBROS IN THE MODUM COMPLEX, SOUTHERN NORWAY

preserve primary features. Since the alteration of metagabbro to amphibolite was accompanied by formation of a foliation subparallel to the regional trend, the metagabbro-amphibolite transition must represent a regional metamorphic event. Extensive hydrothermal alteration occurs in the contact zones between the metagabbros and their country rocks. This hydrothermal activity has resulted in the formation of concordant lenses or bands of orthoamphibole-cordierite rocks (Munz, 1990) and cobalt sulfide/arsenide mineralizations and cross-cutting veins and dykes ofscapolite, albite and calcite (Munz, in prep. ).

pyroxenes and olivine, are replaced. Major minerals in the amphibolites are hornblende, plagioclase, +_garnet, +_biotite, +_orthopyroxene. Complete alteration leads to recrystallization and formation of foliated amphibolite. According to Josang (1966) the metagabbroamphibolite transition is smooth, from a metagabbro in the core to amphibolite at the borders of the bodies. Our field observations suggest, however, that the transition is somewhat more complicated, with alternations of metagabbro and amphibolite on a scale of a few metres. The gabbros outcrop as oval bodies elongated in N-S direction. The country rocks consist mainly of metasediments: quartzites, micaschists and sillimanite-bearinggneisses. The contacts between the metagabbros and their country rocks are subparallel to the foliation in the supracrustals. This foliation strikes N-S, bending around the metagabbros. The occasional foliation developed in the amphibolitized parts of the metagabbros also follows this trend. The deformation of the Modum Complex was clearly heterogeneous, with the gabbroic bodies acting as competent lenses able to

Petrography Primary igneousassemblage All the metagabbro bodies in Fig. 1 contain (pseudomorphic) igneous textures. However, only the numbered bodies still contain igneous minerals. The igneous mineral assemblages of samples from these different metagabbros are listed in Table 1.

TABLE l Mineralogy Sample

No.

Primary igneous minerals

Metamorphic minerals Coronas

Ol Plag Cpx Opx Ilm Sp RM-216 P-183 IA-402 IA-403 IA-400C IA-400B IA-6

1 1 2 2 3 3 4

IA-29B

4

IA-368F G-7 IA-378A 1A-378B

5 6 6 6

× X

X X X X

X

X X X X X

X X X X X

X X

X

X

X

X X X X X X

X X X X X X

X X X X X X

X

X X

X X X X X

Ap

Bi Opx Cpx A + S p

X X X X X

X X X

Recrystallized assemblage

X X X X

X × X X X

X X

Gnt Scap Cpx A

X X X X X

X X X X

X

X

X

X X X X X X

X X X X X X

X X

Opx Gnt Scap

X X X

x x X

X X

X

X

x(v)

x(v)

x x

X(R)

x

x(v)

X

(V) Vein; ( R ) recrystallized and vein. Abbreviations: Ol = olivine; Plag = plagioclase; Cpx = clinopyroxene; Opx = orthopyroxene; Ilm = ilmenite; Sp = spinel; Ap = apatite; Bi = biotite; A, Amph = amphibole; Gnt = garnet; Scap = scapolite; WR = whole rock.

J

iI

101

METAGABBROSIN THE MODUMCOMPLEX,SOUTHERNNORWAY

Olivine, plagioclase, clinopyroxene, _orthopyroxene, ilmenite, +- spinel, _ biotite and _+apatite constitute the igneous mineral assemblage in the metagabbros. All samples show a subophitic, coarse-grained texture with olivine and plagioclase as the cumulus phases and pyroxenes and ilmenite as the major intercumulus phases. Olivine occurs as euhedral grains with multiple coronas of orthopyroxene, _+clinopyroxene, amphibole+spinel, +_garnet at the contact with plagioclase. These coronas may replace olivine completely. Olivine now remains only in half of the samples of Table 1. However, coronas, with or without olivine in the core, are ubiquitous, demonstrating that all the metagabbro bodies originally carried olivine. Plagioclase. The plagioclase laths are often clouded and zoned (Fig. 2 ). Clinopyroxene in optical continuity fills most of the interstices, enclosing both olivine and plagioclase grains. Very small needles of ilmenite give the clinopyroxene a dark colour. Occasionally, ilmenite also occurs as exsolutions in clinopyroxene. In some sections, the clinopyroxene is partly altered to amphibole, particularly in the outer part of the grains. Orthopyroxene has previously only been reported in the Morud metagabbro (J~sang, 1966 ), but the present study shows that it also occurs in the Knatten (No. 3) and the Overbykollen (No. 6) bodies. Orthopyroxene constitutes, however, less than 5 modal % of the rocks and occurs as an interstitial phase, but

unlike clinopyroxene the grains are not in optical continuity. Ilmenite is present in all samples, both as irregular, interstitial grains and as inclusions in clinopyroxene. The interstitial ilmenite grains are often rimmed by coronas of amphibole or amphibole+garnet. Spinel. A few grains of green spinel form an intergrowth with ilmenite in two samples. Biotite. When present, biotite constitutes less than 2-3% of the mode, and is mostly secondary. However, some of the biotite grains may represent a late igneous phase. Apatite occurs as rounded, 0.1-0.3 m m grains, constituting less than 1% of the mode.

Secondary alteration Olivine coronas. When olivine occurs in contact with plagioclase, multiple coronas are invariably present. These coronas range in size from a width of 0.1 m m in the least altered sample (IA-368F) to a complete replacement of olivine. From inner to outer zone the coronas consist of: orthopyroxene; clinopyroxene; amphibole + spinel; garnet. The sequence is, however, not always complete. The zones of orthopyroxene and amphibole+ spinel are always present. The spinel occurs as clouds of inclusions within the amphibole. Garnet is common, usually occurring as more or less continuous zones outside the amphibole + spinel zone, or in some cases occurring

Fig. 2. Microtextures in metagabbros. (A) Subophitic texture of zoned plagioclase laths and interstitial, dark, igneous clinopyroxene, growing in optical continuity (RM-216). (B) Interstitial, igneous orthopyroxene (G-7). (C) The simplest olivine coronas with inner orthopyroxene and outer amphibole + spinel (IA-368F). (D) The most complex coronas consisting of four, more or less continuous zones: orthopyroxene; clinopyroxene; amphibole + spinel; garnet. Garnet also occurs within the amphibole + spinel zone (P-183 ). (E) The primary igneous clinopyroxene is rimmed by a secondary amphibole. The clinopyroxene grain in the centre of the photo also shows recrystallization to a secondary clinopyroxene at one end. The alteration is accompanied by loss of the dark colour in the relics of the igneous clinopyroxene (IA-403). (F) A mosaic texture consisting of a small, secondary clinopyroxene grain completely filling the interstice. Possibly formed by alteration of a primary clinopyroxene grain (IA-6). (G) Euhedral grains of garnet growing in plagioclase (IA-403). (H) Ilmenite corona of amphibole and garnet (IA-403).

102

within the amphibole + spinel zone, in contact with the orthopyroxene. Clinopyroxene is only present in the coronas in the Morud gabbro, where it occurs between the orthopyroxene and amphibole + spinel zones. Scapolite is found as the outer rim in the coronas in sample G-7, where the inner zones are orthopyroxene and amphibole + spinel. Alteration of pyroxenes. The primary igneous clinopyroxenes often show alteration to amphibole a n d / o r to a secondary clinopyroxene. The first stage of the alteration is a loss of colour, with amphibole occurring in the outer part of the grains or as patches within them. In some samples, small grains of a recrystallized clinopyroxene form a mosaic texture in the outer part of the primary clinopyroxene. Similar small, recrystallized clinopyroxene_+ orthopyroxene grains also fill some interstices completely, possibly replacing single primary clinopyroxene grains (Fig. 2 ). The primary igneous orthopyroxene usually shows less alteration than the clinopyroxene. In the Morud metagabbro, orthopyroxene shows a thin rim of secondary clinopyroxene and amphibole. Alteration ofplagioclase. Plagioclase is usually the least altered primary phase. At the contact with olivine, the coronas grow into the outer parts of the plagioclase laths. In sample IA-403, small grains of euhedral garnet have also grown in the plagioclase (Fig. 2). Most parts of the plagioclase laths show little or no alteration. Even in amphibolites, where the olivine and pyroxenes are completely altered to amphiboles and garnets, the primary igneous texture of the plagioclase laths may be preserved. Ilmenite coronas. Ilmenite often shows a rim of amphibole or less frequently of biotite. In some samples, garnet occurs with the amphibole. Scapolite veins. In some samples, scapolite occurs in ca. 0.1-1 mm wide veins. Sample IA400 shows a wider vein, 4-5 mm, of scapolite and amphibole.

I.A. MUNZ AND R. MORVIK

Mineral chemistry The mineral analyses were performed using a CAMEBAX microbeam with WDS at Mineralogisk-Geologisk Museum, Oslo. Peaks and backgrounds were counted for 10 s at an accelerating voltage of 15 kV and a beam current of 10-20 n. A summary of the XMg ratios for the igneous and metamorphic mineral assemblages in each sample is given in Tables 2 and 3. In general, the Morud (No. 1 ) and Knatten (No. 3) metagabbros have the most Mg-rich mineral compositions. Representative mineral analyses are given in Table 4. Primary igneous and corona pyroxenes from three samples are plotted in Fig. 3, after the method of Lindsley (1983). The orthopyroxenes show a clear distinction between metamorphic and igneous grains, with lower Ca content and higher Mg content in the metamorphic pyroxenes. A similar difference is not equally obvious for the clinopyroxenes. Igneous clinopyroxenes show a spread particularly in Ca content, producing an almost continuous transition to the metamorphic clinopyroxenes. A limited subsolidus equilibration of the clinopyroxene chemistry is thus indicated. This reequilibration is related to the loss of colour of the clinopyroxene during alteration. Igneous clinopyroxenes which have lost colour, always show high Ca contents.

Primary igneous assemblage Olivine. No zoning is present in the olivine grains. The variation range in each sample is due to differences between grains. Forsterite compositions range approximately from Foso to Fo7o. Clinopyroxene shows more compositional variation within each grain than the olivine, but no systematic zoning was detected. The XMg range is 0.58-0.80. Ca content is in the range of 18 to 22 wt% CaO. The loss of colour of the igneous clinopyroxene during alteration to amphibole and/or secondary clinopyroxene

103

METAGABBROS IN THE MODUM COMPLEX, SOUTHERN NORWAY TABLE 2 P r i m a r y igneous m i n e r a l a s s e m b l a g e Sample

RM-216 P-183 IA-402 IA-403 IA-400C IA-400B IA-6 IA-29B IA-368F G-7 IA-378A IA-378B

No.

1 1 2 2 3 3 4 4 5 6 6 6

An Plag

XMs = M g / M g + Fe 01

Cpx

Opx

0.67-0.71 0.57-0.64 0.65-0.67 0.49-0.55 0.53-0.58 0.59-0.61 0.61-0.64

0.72-0,80 0.77-0,79 0.67-0,74 0.58-0,61 0.76-0.81 0.72-0.76 0.69-0.72 0.68-0.71 0.75-0.80 0.70-0.74 0.67-0.72 0.68-0.74

0.70-0.72 0.64-0.71

48-61 33-62 41-62 27-55 54-62 59-73 23-63 32-62 47-64 43-63 31-57 34-58

0.66-0.70

0.61-0.64 0.57-0.61

For a b b r e v i a t i o n s see Table 1.

TABLE 3 Metamorphic mineral assemblages Sample

No.

C o r o n a minerals; XMg = M g / M g + Fe Opx

Cpx

Amph

Gnt

0.83-0.84 0.80-0.83 -

0.67-0.73 0.54-0.56 0.50-0.54 0.60-0.71 0.69-0.73 0.62-0.64 0.62-0.68 0.63-0.70 0.49-0.59 0.47-0.58 0.56-0.64

0.41-0.44 0.26-0.31 0.22-0.26 0.41-0.45 0.46-0.51 0.31-0.34 0.33-0.41

RM-216 P-183 IA-402 IA-403 IA-400C IA-400B IA-6 IA-29B IA-368F G-7 IA-378A IA-378B

1 I 2 2 3 3 4 4 5 6 6 6

0.73-0.75 0.71-0.73 0.61-0.63 0.51-0.57 0.71-0.74 0.73-0.75 0.64-0.68 0.65-0.67 0.67-0.69 0.65-0.69 0.66-0.69 0.67-0.71

Sample

No.

Recrystallized m o s a i c assemblage; XMs= M g / M g + Fe

RM-216 P-183 IA-403 IA-6 IA-29B

1 1 2 4 4

Opx

Cpx

Gnt

0.40-0.43 -

0.63-0.70 0.76-0.77 0.52-0.56 0.55-0.59 0.30-0.35

0,16-0.18

*Intergrowths with spinel: too fine-grained for reliable analysis. For a b b r e v i a t i o n s see Table 1.

0.20-0.34

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I.A. M U N Z A N D R. M O R V I K

TABLE 4 Representative mineral analyses Primary igneous minerals: Ol O1 O1 RM-216 P-183 IA-6 SiO2 TiO2 A]203 FeO MnO

MgO

O1 IA-368F

Cpx RM-216 52.64 0.48 2.70 7.35 0.16 15.81 19.74 0.76 99.64

37.66

36.77

35.64

36.53

27.63 0.37 34.49

32.31 0.24 31.11

38.26 0.54 25.74

33.83 0.69 29.21

100.43

100.18

100.26

CaO Na20 K20 Total

Structural formula based on: 4 oxygen Si Ti AI Fe Mn Mg Ca Na K Cations

SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 K20 Total

Cpx IA-403

Cpx IA-400B

Cpx IA-368F

Cpx IA-378B

52.38 0.48 2.90 7.32 0.22 15.41 20.26 0.70

51.10 0.73 1.75 13.16 0.20 11.60 20.37 0.67

51.11 0.54 5.62 7.76 0:23 12.87 21.09 1.06

52.85 0.98 2.49 8.52 0.14 15.76 18.59 0.51

50.97 0.86 2.04 10.93 0.29 13.43 20.65 0.58

99.67

99.58

100.28

99.84

99.75

1.94 0.01 0.12 0.23

1.93 0.01 0.13 0.23 0.85 0.80 0.05

1.89 0.02 0.24 0.24 0.01 0.71 0.83 0.08

1.95 0.03 0.11 0.26

0.87 0.78 0.05

1.95 0.02 0.08 0.42 0.01 0.66 0.83 0.05

0.87 0.73 0.04

1.92 0.02 0.09 0.34 0.01 0.75 0.83 0.04

4.00

4.00

4.02

4.02

3.99

4.00

Plag IA-400B core

Plag IA-400B rim

6 oxygen

1.00

1.00

1.00

1.00

0.62 0.01 1.37

0.73 0.01 1.26

0.90 0.01 1.08

0.78 0.02 0.20

3.00

3.00

2.99

2.00

Primary igneous minerals: Opx Opx Opx RM-216 P-183 IA-400C

Opx G-7

Opx IA-378B

53.71 0.16 1.57 17.13 0.36 25.02 2.02 0.10

52.58 0.31 0.61 22.30 0.30 22.03 1.71 0.04

53.37 0.22 1.00 18.58 0.46 24.50 1.30 0.04

52.71 0.22 0.47 22.21 0.59 21.87 1.41 0.04

52.29 0.23 0.49 25.34 0.65 20.25 1.30 0.02

100.07

99.88

99.47

99.52

100.57

Structural formula based on: 6 oxygen Si Ti AI Fe Mn Mg Ca Na K Cations

1.96

Cpx P-183

Plag P- 183 core

Plag P- 183 rim

53.52

59.99

50.20

51.35

30.00 0.21

25.17 0.14

31.93 0.15

30.84 0.08

12.02 4.76 0.11 100.62

6.95 7.65 0.19 100.09

14.88 3.11 0.01 100.28

13.64 3.85 0.08 99.84

2.41

2.67

2.28

2.34

1.59

1.32

1.71

1.66

0.58 0.42 0.01 5.01

0.33 0.66 0.01 4.99

0.72 0.27

0.67 0.34

4.98

5.01

8 oxygen

0.07 0.52 0.01 1.36 0.08

1.97 0.01 0.03 0.70 0.01 1.23 0.07

1.97 0.01 0.04 0.57 0.01 1.35 0.05

1.98 0.02 0.70 0.02 1.22 0.06

1.97 0.01 0.02 0.80 0.02 1.14 0.05

4.00

4.02

4.00

4.00

4.01

METAGABBROSIN THE MODUM COMPLEX, SOUTHERN NORWAY

105

TABLE 4 (continued) Corona minerals: Opx Opx RM-216 P-183 SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 KaO Total

54.34 0.01 2.00 16.13 0.42 27.10 0.28 0.02

53.19 0.03 1.84 18.49 0.35 25.78 0.35 0.01

100.30

100.04

Opx IA-400C

Opx IA-29B

Opx IA-378B

Cpx RM-216

53.55

52.40 0.01 1.49 21.66 0.35 23.30 0.38 0.04

52.97 0.02 1.11 20.54 0.57 24.06 0.21 0.02

52.44 0.03 3.40 5.32 0.09 14.84 22.59 0.86

52.63 0.01 2.56 6.10 0.07 14.69 22.48 0.83

99.63

99.50

99.57

99.37

2.17 17.93 0.34 25.23 0.19

99.41

Cpx P-183

Structural formula based on: 6 oxygen Si Ti AI Fe Mn Mg Ca Na K Cations

1.94

1.96

1.95

1.97

1.93

1.95

0.09 0.49 0.01 1.45 0.01

0.08 0.56 0.01 1.40 0.01

0.09 0.55 0.01 1.38 0.01

0.07 0.68 0.01 1.29 0.02

0.05 0.64 0.02 1.33 0.01

0.15 0.16

0.11 0.19

0.82 0.89 0.06

0.81 0.89 0.06

4.00

4.00

4.00

4.02

4.02

4.01

4.01

SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 K20 Total

39.60 0.04 22.41 22.06 0.76 8.98 6.96

100.S1

37.97

Gnt IA-400C

Gnt IA-6

Grit IA-29B

Gnt IA-378A

40.46 0.36 16.13 11.58 0.05 12.57 11.54 3.08 0.69 96.46

6.10 0.09 2.56 1.62

6.03 0.04 2.83 1.44

2.83 1.78 0.89 0.14 16.01

2.79 1.84 0.89 0.13 15.99

Recrystallized minerals: Cpx Cpx Opx IA-403 IA-6 IA-403

Gnt IA-403

21.42 29.58 1.01 4.71 5.83

39.47 0.02 22.40 22.68 1.07 9.35 5.48

38.56 0.02 21.69 25.31 1.05 6.37 6.92

39.25 0.02 22.42 24.10 0.95 7.66 6.68

38.25 0.05 21.81 28.70 2.21 5.87 3.94

51.30 0.11 1.03 15.83 0.22 10.11 21.13 0.70

51.52 0.07 1.21 13.85 0.16 10.30 21.31 0.83

49.90 0.05 0.51 33.96 0.38 13.93 0.57

37.91 0.03 21.07 28.84 1.35 3.49 7.42 0.02

100.52

100.47

99.92

101.08

100.83

100.43

99.25

99.32

100.11

Structural formula based on: 12 oxygen Si Ti Al Fe Mn Mg Ca Na K Cations

40.78 0.76 14.49 12.98 0.07 12.69 I1.11 3.06 0.71 96.65

Amph IA-29B

23 oxygen

1.95

Corona minerals: Gnt Gnt P- 183 IA-403

Amph IA-6

6 oxygen

12 oxygen

2.99

2.99

2.99

3.00

2.99

2.98

1.96

1.98

1.98

3.00

2.00 1.39 0.05 1.01 0.56

1.98 1.94 0.07 0.55 0.49

2.00 1.44 0.07 1.06 0.45

1.99 1.64 0.07 0.74 0.58

2.01 1.53 0.06 0.87 0.54

2.00 1.87 0.15 0.68 0.33

0.05 0.51 0.01 0.58 0.87 0.05

0.06 0.44 0.01 0.59 0.88 0.06

0.02 1.13 0.01 0.83 0.02

1.97 1.91 0.09 0.41 0.63

8.00

8.02

8.01

8.02

8.00

8.01

4.03

4.02

3.99

8.01

For abbreviations see Table 1.

106

I,A. MUNZ AND R. MORVIK

Plagioclase laths always show zoning. An decreases from the core towards the rim. Typical An contents of the cores are in the range of 6070. An rim values vary between 20 and 50 (Table 2 ).

t"

IA

Secondary alteration v

Ilml ,,d

v

Olivine coronas. The most pronounced chemical difference between the igneous orthopyroxene and the orthopyroxene in the olivine coronas lies in the Ca content. The corona orthopyroxene only contains 0.1-0.4 wt% CaO. The XMg values in corona orthopyroxene are higher than in the igneous orthopyroxenes. The orthopyroxene zones also often show a zoning in AI content, with AI increasing outwards. Clinopyroxene in the olivine coronas from the Morud metagabbro shows an increase in XMg compared to the igneous clinopyroxene. The CaO content of the corona clinopyroxene is also higher than in the igneous clinopyroxene (22-23 wt%). According to the nomenclature of Leake ( 1978 ), the corona amphiboles classify as pargasites or ferroan pargasites. The garnets in the olivine coronas are almandinepyrope-grossular solid solutions with a compositional range of Alma2_66PY 18-44Gro 11-23"

!

(][3o

RM-216 ? 3 / ,, ~ 2O

Recrystallized mosaic assemblages. The

En 10 20 30

50 Fs

Fig. 3. Wo-En-Fs triangle. Igneous and corona pyroxenes from sample RM-216, P-183 (Morud metagabbro) and 1A-378B (Overbykollen metagabbro). @=igneous clinopyroxene; • = igneous orthopyroxene; © = corona clinopyroxene; [] = corona orthopyroxene.

is usually accompanied by high Ca content (ca. 22 wt% CaO). Orthopyroxene. XMg for orthopyroxene varies between 0.57 and 0.71. The igneous orthopyroxene is characterized by a high Ca content, above 1 wt% CaO.

small, rounded clinopyroxene grains formed by recrystallization of igneous clinopyroxene have higher Fe content than both the igneous clinopyroxene and the corona clinopyroxene. The Ca content is somewhat lower than in the corona clinopyroxene, in the upper range of the igneous compositions (around 21-22 wt% CaO ). In sample IA-403, small orthopyroxene grains grow together with the recrystallized clinopyroxene. These orthopyroxene grains are very Fe-rich (XMg=0.40--0.43). The euhedral garnet grains that have grown in the plagioclase in sample IA-403 (Alm65_68PY13_laGro19_ 21 ), are also more Fe-rich than the corona garnet.

METAGABBROS IN THE MODUM COMPLEX, SOUTHERN NORWAY

107

Thermobarometry

bodies overlap. The third body containing igneous orthopyroxene, the Knatten metagabbro (sample IA-400C), only contains reequilibrated clinopyroxenes with high Ca content. Only minerals from the same textural situation were combined to estimate the metamorphic temperatures. In sample IA-403, where orthopyroxene and garnet occur both in coronas and in mosaic microstructures, a marked difference in chemistry between the two microstructures is demonstrated (Table 3 ). Disequilibrium between minerals from the different microstructures was therefore assumed, and no attempt was made to combine a mineral from the corona with a mineral from the mosaic microstructure. However, the results from the coronas and from the mosaic assemblages are identical (Table 5). Clinopyroxene-orthopyroxene and clinopyroxene-garnet

Both the igneous and the metamorphic stage contain mineral assemblages suitable for thermometry. Coexisting orthopyroxene + clinopyroxene occur in both stages, and clinopyroxene + garnet and orthopyroxene -t- garnet occur in the metamorphic stage. The thermometers of Wells (1977), Ellis and Green ( 1979 ) and Harley (1984) were used. The resuits for each sample are presented in Table 5. Temperatures for the igneous stage were obtained for two of the metagabbro bodies (Morud, No. 1; Overbykollen, No. 6). The spread in temperatures is more than 200 ° C: from 840 to 1070 ° C. A restricted reequilibration of the clinopyroxene chemistry (Fig. 3) probably gave rise to the lower range of the temperature interval. The temperature ranges for the two TABLE 5 Thermobarometry Sample

No.

T e m p e r a t u r e ( ° C) Cpx-Opx a

Cpx-Gnt b

Opx-Gnt c

Pressure (kbar) (at 650 °C) Opx-Gnt a

Primary igneous mineralassemblage RM-216 P-183 G-7 IA-378B

1 1 6 6

850-1070 970-1070 840-965 850-1015

Corona minerals RM-216

1

730-810

-

-

-

P-183 IA-402 IA-403 IA-400C IA-400B IA-6 IA-29B IA-368F G-7

1 2 2 3 3 4 4 5 6

790-835 . .

650-720 520-650 580-660 625-725 650-765 560-660 560-660

6.5-7.5 7.5-9.5 8-10 4-6 8.5-9.5 7.5-9 8-10

. .

735-810 . .

IA-378A IA-378B

6 6

.

400-625

9-11

.

640-685

11-12

. .

.

.

RecrystaHizedmosaica~emblages IA-403

2

a Wells ( 1 9 7 7 ) . b Ellis a n d G r e e n ( 1 9 7 9 ) . c Harley (1984). a Harley and Green (1982).

775-800

715-795

108

I.A. MUNZAND R. MORVIK

TABLE 6 Nd and Sr data for the Morud metagabbro Sample

Nd (ppm)

P-183 WR 4.42 P-183 CPX 13.81 P-183 PLAG 1.41

Sm (pore)

143Nd/144Nd-t-lo " 147Sm/144Nda Sr ( X 10 -6 ) (ppm)

Rb (ppm)

87Sr/86Sr_+ lo(×10--6)

87Rb/86Sr b

1.62 6.79 0.37

0.512912_+3 0.513532_+9 0.512418_+9

6.45 3.31 9.16

0.704021 + 3 0.704888_+4 0.703682 _+4

0.0724 0.1477 0.0494

0.2235 0.2988 0.1603

256.4 64.9 537.1

a 1a=0.25%. b 1a=0.50%.

thermometry show temperatures in the range of 700-800°C. Orthopyroxene-garnet thermometry shows consistently lower temperatures in all samples, mostly between 600 and 700°C. The pressure during m e t a m o r p h i s m was estimated by the garnet-orthopyroxene barometer of Harley and Green (1982). A temperature of 650°C was used in the calculations. This value is approximately the average temperature obtained from the Fe-Mg exchange between these two minerals. The results (Table 5 ) show a large scatter ranging from 4 to 12 kbar. Most of the estimates fall, however, in the range between 7 and 10 kbar. The scatter is mostly due to variable A1 content in the orthopyroxene. Some of the coronas show an AI zoning in orthopyroxene, with A1 content increasing outwards. Mongoltip and Ashworth (1983) show that the diffusion of A1 in coronas is slow. The AI zoning in some of the coronas may indicate that total equilibrium was not obtained.

MORUD METAGABBRO

J,"

P-183 CPX

AGE=1224+/-15m.y. I.R.=0.51112+/-0.00005 MSWD=2.55

z~

f/"

//" /

0.5130

J / -~1~ P183 WR

Z

/ j-

I

// /" /~P

.183 PLAG

I

A 024

026

028

0.3

0 32

147Sm/ 144 Nd ,/. i

AGE=865+/-16m.y. I.R.=0.70309+/-0.00002 MSWD=65.0

P 183CP~

' i

/

i 1I

/- ./

¢j~ o 7o4S

f3~

//

c~

./J o 7040

P 183 WR ./ / J

..-~ ....

B

07035

Age determination

Igneous clinopyroxene and plagioclase were separated from sample P-183 from the Morud metagabbro. Only grains without any alteration were selected. Isotopic compositions of the mineral separates and the whole-rock sample are given in Table 6. The analytical procedure has been described by Mearns ( 1986 ). Isochrons were calculated after the method of York (1969). Decay constants used were

003

005

0o7

009

0~1

0~3

015

87Rb/86 Sr Fig. 4. Mineral isochrons for sample P-183 ( M o r u d metagabbro).

2147Sm=6.54× 10 -12 yr -~ and ~.87Rb= 1.42× 10 -I1 yr -l. A S m - N d isochron of clinopyroxene, plagioclase and whole rock for sample P-183 gives an age of 1224 _+ 15 Ma (Fig. 4). The 1224 Ma age is based on analyses of fresh igneous min-

109

METAGABBROS IN THE MODUM COMPLEX, SOUTHERN NORWAY

with olivine and plagioclase as the major cumulus phases and clinopyroxene, + orthopyroxene and ilmenite as the major intercumulus phases. The mineral chemistry indicates only a small variation in Fe-Mg ratios. The Morud and Knatten metagabbros are slightly more Mg-rich than the other metagabbros. Thermobarometry from the different bodies show the same P - T path. All bodies are elongated with contacts subparallel to the regional N-S trending foliation. This foliation is also present within the amphibolitized parts of the bodies. Albitization is present around all of the metagabbros. These mineralogical and structural data strongly suggest that the metagabbros represent one generation of intrusions. An emplacement of the gabbros during two different periods, as found for two metagabbros in the Bamble Sector (de Haas et al., 1990), is highly unlikely. In contrast to the Vinor basic intrusions in the western part of the Kongsberg Sector where three different intrusion phases have been demonstrated (Starmer, 1985), the metagabbros in the Modum Complex are more homogeneous. The texture of these metagabbros is similar to the main phase with coronite stocks of the Vinor basic intrusion as described by Starmer (1985). The concordant contacts of the Modum metagabbros are also in contrast to the frequently cross-cutting relationships of the Vinor basic intrusions (Bugge, 1917; Bugge, 1937; Starmer, 1985). This could, however, be due to the lack of equivalents of the late phase Vinor intrusions in the Modum Complex. The 1224 Ma S m - N d mineral age of the Morud metagabbro shows that the intrusions were emplaced at the start of the Sveconorwegian metamorphism. The age is in good agreement with the 1200 Ma Rb-Sr whole-rock age of the Vinor metagabbros (Jacobsen and Heier, 1978 ) and the 1150 Ma SmNd whole-rock age of the Vestre Dale metagabbro in the Bamble Sector (de Haas et al., 1990).

Intrusive conditions Temperatures estimated from igneous clinopyroxene-orthopyroxene pairs for two of the metagabbros range from 840 to 1070°C. The lower range of this temperature interval was probably caused by a restricted subsolidus reequilibration of the clinopyroxene chemistry. Textural relationships indicate that olivine and plagioclase were the first phases to crystallize, followed by crystallization of clinopyroxene, ___orthopyroxene and ilmenite. Phase diagrams in the system CaO-MgO-A1203-SiO2 show that contemporaneous crystallization of olivine and plagioclase is only possible at pressures below 5 kbar (Presnall et al., 1978 ). Natural systems, however, contain additional components, most important Na20 and FeO. Melting experiments on olivine tholeiitic composition show that olivine is a liquidus phase at pressures up to 8 kbar (Thompson, 1975). The crystallization of plagioclase and olivine therefore suggests intrusion at moderate depth ( ~<5-8 kbar region).

Metamorphic conditions The first stage of alteration of the igneous assemblage caused formation of coronas around olivine in contact with plagioclase and alteration and recrystallization of the igneous clinopyroxene. Thermometry shows that these two microstructures were formed at the same temperatures, 600-800 ° C. Two-pyroxene and clinopyroxene-garnet thermometry gives somewhat higher temperatures, 700-800 °C, than orthopyroxene-garnet thermometry, 600700 °C. Whether this discrepancy is due to disequilibrium between the minerals, to different closure temperatures in different systems or to inconsistency between the different thermometers is not obvious. Coronas are reaction structures and are in principle indicators of a disequilibrium situation. The recrystallized, mosaic assemblage, however, shows textural

I 10

indications of equilibrium, such as 120 ° triple points at grain boundaries. As noted in the thermobarometry section, the garnet-orthopyroxene pressure estimations show some scatter due to the variable A1 content in orthopyroxene. The clustering of pressure estimates in the range of 7 to 10 kbar, however, indicates that the coronas were formed within this pressure range. For similar coronas from one sample of metagabbro from Risor (Bamble Sector) Joesten (1986) suggested a formation by direct crystallization from the igneous melt. This hypothesis is in conflict with much previous work on coronites (Griffin and Heier, 1973; Mongoltip and Ashworth, 1983) and was questioned by Ashworth (1986), who argued for a "solidstate replacement". Mineral chemistry of orthopyroxenes and the thermometry in the present study clearly demonstrate the metamorphic character of the coronas. Igneous orthopyroxenes, easily recognized as large, interstitial grains, show Ca contents almost an order of magnitude higher than those of the corona orthopyroxenes. Two-pyroxene thermometry shows that igneous crystallization temperatures are well preserved for interstitial pyroxenes. The lower temperatures derived for the coronas must therefore represent the temperature conditions of their formation. Joesten (1986) does not describe interstitial, high-Ca orthopyroxenes. The textural description given by Joesten ( 1986 ), with orthopyroxene occurring as rims around olivine, and the mineral composition he reports for these orthopyroxenes suggest that they are metamorphic and not primary igneous.

The nature of the Sveconorwegian metamorph&m The country rocks to the metagabbros are mainly metasediments: quartzites, micaschists, sillimanite-bearing granitic gneisses and a marble unit. The presence of migmatites in the

I.A. MUNZ AND R. MORVIK

,

600

'

7bo

I

j2

9bo

800

L

~ooo

c

13

Fig. 5. Pressure-temperaturepath for the Modum Complex. The reactionsand P, Tbox 1 showthe metamorphic evolution ofwhiteschistsand orthoamphibole-cordierite rocks (Munz, 1990). Boxes2, 3 and 4 representP, Testimates from the present study: 2=igneous stage; 3=twopyroxene and clinopyroxene-garnet temperatures; 4 = orthopyroxene-garnettemperaturesof corona and recrystallizedminerals. The metagabbrosshow a P-T path coincidingwith the metamorphicevolutionof their country rocks. granitic compositions indicates temperatures >~650°C for these rock types. Extensive hydrothermal activity occurs in the contact zones between the metagabbros and the metasediments (Munz, 1990; Munz, in prep.). From orthoamphibole-cordierite rocks, which belong to the hydrothermal parageneses, Munz (1990) has demonstrated a cooling path in the same pressure-temperature range as that obtained for the coronas and the recrystallization in the present study. Garnet-biotite thermometry in these rocks gives temperatures of 700800°C when garnet cores are combined with matrix biotite, while garnet rims combined with biotites in contact with garnets give reequilibrated temperatures in the range of 600650°C. The common thermal evolution of the metagabbros and their country rocks (Fig. 5 ) suggests that these P - T conditions (600800°C and 6-8 kbar) represent a major metamorphic event, that affected the entire area. These results are in good agreement with the metamorphic conditions estimated for the Bamble Sector (Jansen et al., 1985; Lamb et al., 1986). Field and R~theim (1979, 1980, 1981),

111

METAGABBROS IN THE MODUM COMPLEX, SOUTHERN NORWAY

Smalley et al. (1983, 1988) and Field et al. ( 1985 ) argue that the major high-grade metamorphism in the Bamble Sector occurred 1540 Ma ago, and that the Sveconorwegian metamorphism only represented a greenschist-facies alteration. However, the intrusion age and the P-Tpath of the metagabbros, shown in the present study, clearly demonstrate the highgrade nature of the Sveconorwegianevent. SmNd systematics of well equilibrated granulitefacies mineral assemblages from the Bamble Sector also suggest a major deformational and high-grade metamorphic event in Sveconorwegian time (Kullerud and Dahlgren, 1990). Furthermore, the high-grade intrusions of Gjerstad and Morkheia only give Rb-Sr wholerock ages of ~ 1250 Ma (Smalley et al., 1983, 1988). The gneissic foliation in these intrusions is interpreted as mylonitic deformation along the Porsgrunn-Kristiansand fault zone, and not related to any major metamorphic event. Falkum and Pedersen (1980) show however that the foliation in the Telemark Sector is rotated through a transition zone around the Porsgrunn-Kristiansand fault into the NEtrending foliation in the Bamble Sector. The gneissic foliation of the Gjerstad intrusion may thus represent the regional foliation of the Bamble Sector. In the Kongsberg Sector the regional foliation trends N-S (Bugge, 1936 ). The concordant contacts of the metagabbros and the presence of this foliation in the metagabbros suggest that the regional N-S foliation is of Sveconorwegian age. The presence of older ages within the Sveconorwegian Province suggests that the Rb-Sr whole-rock system and the Sm-Nd mineral system do not easily reset, even under high-grade metamorphic conditions. The preservation of a Sm-Nd age through a high-grade metamorphism is due to the incomplete recrystallization of the rock, which has also been demonstrated for highpressure metamorphism (Mark and Mearns, 1986). Simple tectonic models with thickening of

the crust and conduction as the only heat transfer process are not applicable for the Sveconorwegian metamorphism in southern Norway. Calculation using such models shows a clockwise P - T - t path (England and Thompson, 1984). The P - T - t path of southern Norway is however an isobaric or near-isobaric cooling path (Touret, 1985; Munz, 1990; this study). The large quantities of mafic magma in the Modum Complex represent considerable heat sources. The major heat transfer process in the area was probably advection, first by magma and later by hydrothermal activity. An isobaric cooling path is best explained as the result of equilibration after a period with extension and intrusions. Recrystallization and formation of foliation occurred along this PT - t path, demonstrating a major deformational event after the intrusions. A complex model involving both magmatism and highgrade metamorphism in the middle to lower crust is necessary to explain the Sveconorwegian event.

Acknowledgements We thank T. Andersen, H. Austrheim and B. Jensen for comments on the manuscript. We also appreciate the reviews on the manuscript by D. Bridgwater, W.L. Griffin and I.C. Starmer. Microprobe analyses were supported by Grosserer Cand. Jur. Halvdan Bjorums Legat. This is ILP contribution no. 124.

References Ashworth, J.R., 1986. The role of magmatic reaction, diffusion and annealing in the evolution of coronitic microstructure in troctolitic gabbro from Risor, Norway: a discussion. Mineral. Mag., 50: 469-473. Baadsgaard, H., Chaplin, C. and Griffin, W.L., 1984. Geochronology of the Gloserheia pegmatite, Froland, southern Norway. Nor. Geol. Tidsskr., 64:11-119. Brickwood, J.D. and Craig, J.W., 1987. Primary and reequilibrated mineral assemblages from the Sveconorwegian mafic intrusions of the Kongsberg and Bamble areas, Norway. Nor. Geol. Unders. Bull., 410: 1-23.

1 12

I.A. MUNZ AND R. MORVIK

Brogger, W.C., 1934. On several Archean rocks from the south coast of Norway, II. The south Norwegian hyperites and their metamorphism. Det Nor. Vidensk. Akad. Oslo, Skr. Mat.-Nat., 1, 421 pp. Bugge, A., 1936. Kongsberg-Bamble formasjonen. Nor. Geol. Unders., 146, 117 pp. Bugge, A., 1937. Flesberg og Eiker, beskrivelse til de geologiske karter F350 og F35V. Nor. Geol. Unders., 143, 118 pp. Bugge, C., 1917. Kongsbergfeltets geologi. Nor. Geol. Unders., 82,272 pp. Bugge, J.A.W., 1943, Geological and petrological investigations in the Kongsberg-Bamble formation. Nor. Geol. Unders., 160, 150 pp. de Haas, G.J.L.M., Huijsmans, J.P.P. and Verschure, R.H., 1990. The magmatic evolution of the Vestre Dale gabbro, Bamble, south Norway, and the Sm-Nd systematics of two hyperites (abstr.). Geonytt, 17 ( 1 ): 51. Ellis, D.J. and Green D.H., 1979. An experimental study of the effect of Ca upon garnet-clinopyroxene Fe-Mg exchange equilibria. Contrib. Mineral. Petrol., 71: 1322.

England, P.C. and Thompson, A.B., 1984. Pressure-temperature-time paths of regional metamorphism, I. Heat transfer during the evolution of regions of thickened continental crust. J. Petrol., 25: 894-928. Falkum, T. and Petersen, J.S., 1980. The Sveconorwegian orogenic Belt, "a case of Late-Proterozoic plate-collision. Geol. Rundsch., 69: 622-647. Field, D. and Rhheim, A., 1979. Rb-Sr total rock isotope studies on Precambrian charnockitic gneisses from south Norway: evidence for isochron resetting during a low-grade metamorphic deformational event. Earth Planet. Sci. Lett., 45: 32-44. Field, D. and R~heim, A., 1980. Secondary geologically meaningless Rb-Sr isochrons, low 87Sr/S6Sr initial ratios and crustal residence times of high-grade gneisses. Lithos, 13: 295-304. Field, D. and Rhheim, A., 1981. Age relationships in the Proterozoic high-grade gneiss region of southern Norway. Precambrian Res., 14: 261-275. Field, D., Smalley, P.C., Lamb, R.C. and Rhheim, A., 1985. Geochemical evolution of the 1.6-1.5 Ga old amphibolite-granulite facies terrain, Bamble Sector, Norway: dispelling the myth of Grenvillian high-grade reworking. In: J.L.R. Touter and A.C. Tobi (Editors), The Deep Proterozoic Crust in the North Atlantic Provinces. NATO ASI Series, D. Reidel, Dordrecht, pp. 567-578. Griffin, W.L. and Heier, K.S., 1973. Petrological implications ofsome corona structures. Lithos, 6:315-335. Harley, S.L., 1984. An experimental study of the paritioning of Fe and Mg between garnet and orthopyroxene. Contrib. Mineral. Petrol., 86: 359-373. Harley, S.L. and Green, D.H., 1982. Garnet-orthopyroxene barometry from granulites and peridotites. Nature, 300: 697-701.

Jacobsen, S.B. and Heier, K.S., 1978. Rb-Sr isotope systematics in metamorphic rocks, Kongsberg Sector, south Norway. Lithos, 11: 257-276. Jansen, J.B.H., Blok, J.P., Bos, A. and Scheelings, M., 1985. Geothermometry and geobarometry in Rogaland and preliminary results from the Bamble area, S Norway. In: J.L.R. Touret and A.C. Tobi (Editors), The Deep Proterozoic Crust in the North Atlantic Provinces. NATO ASI Series, D. Reidel, Dordrecht, pp. 499-516. Joesten, R., 1986. The role of magmatic reaction, diffusion and annealing in the evolution of coronitic microstructure in troctolitic gabbro from Risor, Norway. Mineral. Mag., 50: 441-467. Josang, O., 1966. Geologiske og petrografiske undersokelser i Modumfeltet. Nor. Geol. Unders., 235, 148 pp. Kullerud, L. and Dahlgren, S., 1990. Timing,of the highgrade metamorphism in the Bamble Sector, South Norway (abstr.). Geonytt, 17( 1 ): 68. Lamb, R.C., Smalley, P.C. and Field, D., 1986. P-T conditions for the Arendal granulites, southern Norway: implications for the roles of P,T and CO2 in deep crustal LILE-depletion. J. Metamorph. Geol., 4:143-160. Leake, B.E., 1978. Nomenclature of amphiboles. Can. Mineral., 16: 501-520. Lindsley, D.H., 1983. Pyroxene thermometry. Am. Mineral., 68: 477-493. Mearns, E.W., 1986. Sm-Nd ages for Norwegian garnet peridotite. Lithos, 19: 269-278. Mongoltip, P. and Ashworth, J.R., 1983. Quantitative estimation of an open-system symplectite-forming reaction: restricted diffusion of AI and Si in coronas around olivine. J. Petrol., 24: 635-661. Munz, I.A., 1990. Whiteschists and orthoamphibole-cordierite rocks and the P - T - t path of the Modum Complex, south Norway. Lithos, 24:181-200. Mork, M.B.E. and Mearns, E.W., 1986. Sm-Nd isotopic dating of minerals and whole-rocks from gabbros showing different degrees of transition to eclogites. Lithos, 19: 255-267. Oftedahl, C., 1980. Geology of Norway. Nor. Geol. Unders., 356, 167 pp. O'Nions, R.K. and Baadsgaard, H., 1971. A radiometric study of polymetamorphism in the Bamble region, Norway. Contrib. Mineral. Petrol., 34: 1-24. Pedersen, S., 1981. Rb/Sr age determinations on late Proterozoic granitoids from the Evje area, south Norway. Bull. Geol. Soc. Den., 29: 129-143. Presnall, D.C., Dixon, S.A., Dixon, J.R., O'Donnell, T.H., Brenner, N.L., Schrock, R.L. and Dycus, D.W., 1978. Liquidus phase relations on the join diopside-forsterite-anorthite from 1 arm to 20 kbar: their bearing on the generation and crystallization of basaltic magmas. Contrib. Mineral. Petrol., 48:179-203.

METAGABBROSIN THE MODUMCOMPLEX,SOUTHERNNORWAY Priem, N.A.H., Boelrijk, N.A.I.M., Hebeda, E.H., Verdurmen E.A.Th. and Verschure R.H., 1973. Rb-Sr investigations on Precambrian granites, granitic gneisses and acidic metavolcanics in Central Telemark: metamorphic resetting of Rb-Sr whole rock systems. Nor. Geol. Unders., 289: 37-53. Skjernaa, L. and Pedersen, S., 1982. The effects of penetrative Sveconorwegian deformations on Rb-Sr isotope systems in the Romskog-Aurskog-Holand area, SE Norway. Precambrian Res., 17:215-243. Smalley, P.C., Field, D. and Rhheim, A., 1983. Resetting of Rb-Sr whole-rock isochrons during Sveconorwegian low-grade events in the Gjerstad augen gneiss, Telemark, southern Norway. Isot. Geosci., 1: 269-282. Smalley, P.C., Field, D. and Rhheim, A., 1988. Rb-Sr systematics of a Gardar-age layered alkaline monzonite suite in southern Norway. J. Geol., 96:17-19. Starmer, I.C., 1969. Basic plutonic intrusions of the RisorSondeled area, south Norway: the original lithologies and their metamorphism. Nor. Geol. Tidsskr., 49: 403431. Starmer, I.C., 1976. The early major structure and petrology of rocks in the Bamble series, Sondeled-Sandnesfjord, Aust-Agder. Nor. Geol. Unders., 327: 77-97. Starmer, I.C., 1977. The geology and evolution of the

113 southwestern part of the Kongsberg series. Nor. Geol.Tidsskr., 57: 1-22. Starrner, I.C., 1985. The geology of the Kongsberg district and the evolution of the entire Kongsberg sector, south Norway. Nor. Geol. Unders. Bull., 401: 35-58. Starmer, I.C., 1990. Rb-Sr systematics of a Gardar-age layered alkaline monzonite suite in southern Norway: a discussion. J. Geol., 98:119-123. Thompson, R.N., 1975. Primary basalt and magma genesis, II. Snake River Plain, Idaho, USA. Contrib. Mineral. Petrol., 52: 213-232. Touret, J.L.R., 1985. Fluid regime in southern Norway: the record of fluid inclusions. In: J.L.R. Touret and A.C. Tobi (Editors), The Deep Proterozoic Crust in the North Atlantic Provinces. NATO ASI Series, D. Reidel, Dordrecht, pp. 517-550. Weis, D. and Demaiffe, D., 1983. Age relationships in the Proterozoic high-grade gneiss regions of southern Norway: discussion and comment. Precambrian Res., 22: 149-155. Wells, P.R.A., 1977. Pyroxene thermometry in simple and complex sygtems. Contrib. Mineral. Petrol., 62: 129139. York, D., 1969. Least squares fitting of a straight line with correlated errors. Earth Planet. Sci. Lett., 5: 320-324.