BASIC PLUTONIC INTRUSIONS OF THE RISÖR­SÖNDELED AREA, SOUTH NORWAY: THE ORIGINAL LITHOLOGIES AND THEIR METAMORPHISM

IAN C. STARMER

Department of Geology, University of Nottingham, England

Present address: Dept. of Geo/ogy, Queen Mary College, London, E. l

STARMER, I. C.: Basic plutonic intrusions of the Risör-Sändeled area

South Norway: The original Jithologies and their metamorphism

Norsk Geologisk Tidsskri/1, Vol. 49, pp. 403-431. Oslo 1969.

The emplacement of Pre-Cambrian basic intrusions is shown to have

been a polyphase process and to have occurred Jargely between two

major periods of metamorphism. The original igneous lithologies are

discussed and a regional differentiation series is demonstrated, with

changes in rock-type accompanied by some cryptic variation in the

component minerals. Metamorphism after consolidation converted the

intrusives to coronites and eaused partial amphibolitisation. Corona

growths are described and attributed to essentially isochemical recrystal­

lisations in the solid-state, promoted by the diffusion of ions in inter­

granular fluids. The amphibolitisation of the bodies is briefly outlined,

and it is concluded that the formation of amphibolite involved the intro­

duction of considerable amounts of extraneous water. The criterion

for demoostrating the onset of amphibolitisation is thought to be the

intemal replacement of pyroxene by homblende, contrasting with the

replacement of plagioclase by various amphiboles during corona growth.

Both amphibolites and coronites have been scapolitised, and this may

have been an extended process which certainly continued after amphi­

bolitisation. Regional implications are considered and it is suggested

that a !arge mass of differentiating magma may have underlain the

whole region in Pre-Cambrian times.

INTRODUCTION

The 'hyperites' of the Kragerö region, to the north, were intensively studied

by Brögger (1934) and were re-investigated from a genetical viewpoint by Reynolds & Predrickson (1962). More recently, Frodesen has carried out a detailed geochemical study of a coronite body immediately to the north of

the preseht area (Frodesen 1968 a, b).

In the Risör-Söndeled district the genesis of the hyperites can be divided

into a number of phases and processes. After differentiation and intrusion

some of the rocks underwent late-magmatic or deuteric alteration. Clouding

and marginal alteration of plagioclase then occurred in conjunction with

corona growths around ferromagnesian minerals and intense metamorphism

converted the rocks to amphibolite. Chlorine metasornatism locally eaused

the scapolitisation of both coronite and amphibolite.

The period of intrusion can be placed in a geologkal time-sequence and

the original igneous lithologies are seen to follow a differentiation trend in-

404 IAN C. STARMER

LOCATION OF

HYPERITIC

Fig. J. The generallocation of the hyperite bodies.

volving a troctolite-troctolitic norite series, a troctolite-olivine gabbro series,

and a number of late-stage, olivine-free gabbros. The differentiation is on

a regional scale and only small parts of the series are shown by individual

masses at the present level of exposure.

In the north of the area, around Söndeled, amphibolite bands are often

concordantly interlayered with metasediments and the whole complex has

been affected by a series of interneting fold phases. Some bands show affini­

ties with the large, discordant coronite bodies, but others represent intrusives,

lavas or tuffs in the sedimentary. sequence (Starmer 1967). The present work

is concemed only with the larger, discordant masses which exhibit good

corona growths and igneous differentiation trends.

FIELD RELA TIONS

The general distribution of the hyperites is shown on Fig. l and a more

detailed indication of their field relations and rock-type is given on Figs. 2-4.

The exposed surfaces of some bodies consist largely of amphibolite or

metagabbro but this is often a thin veneer with relatively unaltered coronite

projecting through sporadically. For this reason the maps (Figs. 2-4) show

only the general distribution of coronite and amphibolitised derivatives since

small patches of the former occur in amphibolite and vice versa. This type

of alteration is the result of differential permeation of water during meta­

morphism. Considered in three-dimensions, most bodies have a dominantly

coronite core which passes outwards into an amphibolitised margin.

The masses are often stock-like or lensoid in shape and form distinct

topographic features. The larger bodies are markedly discordant to the

earlier-formed lithological banding and regional foliation, but smaller masses

mayshow some concordance at the present level of exposure.

05km.

ICS 1967

Legend _..r'Vert. } -<" 75_85• Fo!iation -<' 65_75• �-nke and -< 55-85" lp ........ <55"

BASIC PLUTONIC INTRUSIONS 405

IN THE AREA

EAST OF S0NDELED

�·"' Faults _- Minor Folds EJ Amphibolite � Metagabbro 0 Pegmatite

Re �l �2 lliiiiil3 �4

Fig. 2. The area East of Söndeled. The coronites are: -C) clinopyroxene-rich gabbro,­

l) noritic troctolite,- 2) troctolitic gabbro,- 3) olivine gabbro,- 4) gabbro.

In the north of the area, around Söndeled, (Fig. 2), the basic rocks intruded

a complex of quartzites, amphibolites, and biotite-rich schists and gneisses,

which had previously undergone Upper Amphibolite facies metamorphism

and accompanying folding. Along the Risör peninsula (Fig. 3) they invaded

a zone of migmatites consisting of the above lithologies intimately mixed

with 'granitic' gneisses (sensu lato). In both of the above areas, the solidified

intrusions were subjected to a later metamorphism of Upper Amphibolite

grade which eaused extensive amphibolitisation. Deformations bent the sur­

rounding rocks around these resistant masses, which often controlied the

wavelengths of major structures (e.g. at Avreid, Fig. 4). Marginal shearing

also occurred towards the end of this period of metamorphism and deforma­

tion.

In the South of the area, around Laget (Fig. 4), basic rocks intruded

Granulite facies lithologies which had retrogressed (under Upper Amphibolite

conditions) and had undergone regional granitisation (Starmer 1967). The

largest body consists of troctolitic and olivine gabbro coronite with a small

mass of somewhat sheared olivine-free gabbro on its western flank. Am­

phibolitised margins are extremely thin and are in no way comparable with

those of bodies to the north, but some amphibolite has formed adjacent to

pegmatites and in a series of north-south striking shears and fissures. Around

5

406 IAN C. STARMER

�·' Faults � Minor Folds _..,.. Vertical ..-<" 75·85" ....r 65·75" _.<' 55·65" .--r <55"

lkm . t RIS0R PENINSULA

AND

�3

Fig. 3. The Risör peninsula and Barmen. The coronites are:- l) troctolite, clinopyroxene

and bronzite troctolite,- 2) noritic troctolite, troctolitic norite,- 3) troctolitic gabbro,-

4) olivine gabbro, - 5) gabbro.

this main body, 'granitic' augen gneisses have been transformed to an adinole of plagioclase-quartz gneiss containing abundant rutile, sphene and occasional diopside.

Exposures of noritic troctolite coronite to the southeast (Fig. 4) are

heavily amphibolitised and seem to represent discordant, dike-like bodies within the augen gneiss.

The coronites retain their original igneous texture; the amphibolites may be massive, gneissic or even schistose (when rich in biotite), but the term

'metagabbro' is reserved for rocks with a remnant coarse igneous-textured

fabric of granoblastic andesine aggregates pseudomorphing original labradorite

laths (reaching 5 cm length) and horobiende aggregates replacing the origi­

nal ferromagnesian minerals. Retrogression of both amphibolite and metagab­

bro has occasionally produced biotite-rich schists.

Many coronites have a megascopic purple coloration imparted by the

plagioclase laths but others may be black or brownish grey in colour.

Several lithologies are related to the 'hyperites'. Anthophyllitejgedrite­

rich rocks which have formed by magnesia metasomatism, show a selective

concentration around the intrusives, and when they occur at some distance

from the exposed margins ma y be related to sub-surface extensions.

ICS 1967

BASIC PLUTONIC INTRUSIONS 407

-

t � ..,.

"' . ..,.

Fig. 4. The Avreid-Laget area. The coronites are: - l) troctolite, clinopyroxene and bronzite

troctolite, - 2) noritic troctolite, - 3) troctolitic gabbro, - 4) olivine gabbro, - 5) gabbro.

Apatite dikes have developed (particularly at Hasdalen) and rutile-hearing

veins occur at Laget and Stamsöy.

Plagioclase-rich segregations (consisting of oligoclase or andesine, usually

with associated quartz) have formed layers, lenses, ptygmatic-folds, and

veins in amphibolite and metagabbro. The adjacent rock is frequently

enriched in homblende andfor biotite and both of these minerals may occur

within the felsic segregations. Larger borlies of plagioclase-rich pegmatite

(consisting of albite, oligoclase or sodic andesine with varying amounts of

quartz, homblende or biotite) have intrusive relationships and appear to

have been derived from extemal sources.

Large 'granitic' pegmatites are often associated with the hyperite masses

which are thought to have provided zones of low pressure to which the pegma­

titic fluids could migrate.

Where pegmatites cut coronite they cause local amphibolitisation and

where they have intruded amphibolite, the latter is often enriched in biotite.

08 IAN C. STARMER

SPINEL -

""d �� x' m � z .. m .:C 3

Fig. 5. The recalculated modes of the original intrusions, showing some cryptic variation of

component minerals.

THE ORIGINAL IGNEOUS LITHOLOGIES

Significant trends in the modal analyses tend to be masked by the volume

for volume replacements of magmatic minerals by corona growths. Detailed

investigation of the replacements has enabled recalculation of the modes

of the original intrusives, and the rocks are considered as coronites derived

from these primary igneous lithologies. The expression 'olivine-gabbro

coronite' therefore describes a rock formed from olivine gabbro, in which

corona growths have partially or completely replaced some of the primary

minerals (i.e. in such a rock all the original olivine may now be replaced

by secondary orthopyroxene).

The nomenciature used for the igneous lithologies is summarised in

Fig. 5 and is based on the olivinej(olivine + pyroxene) percentages of the

recalculated modes. The rocks are referred to a troctolite-norite or troctolite­

gabbro series according to the dominance of ortho- or clino-pyroxene in the

original lithologies.

Representative modes of a few of the least-altered coronites are presented

in Table l and have been recalculated in terms of primary magmatic

minerals in Fig. 5 to show the assemblages of the original intrusives. The

compositional variations of the component minerals have also been included

25

c

20

al k 6

4

0.7

mg

0.5

®

0®

00 0

0 0 0

90

+

x 0

+ .

.... . .

+

. t � ..

+

..,x+

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+ + ..

...

t

100

si

$

+ 6)

+ +

.

$ e+ .

+ +

e +e +

+

110

0

®

0

0

BASIC PLUTONIC INTRUSJONS 409

e 25 e

+ + +

+ al 0 + X•:

_..+ . .

0 ..

20

0 ... +

50 x f m

0 + +

+ +

e e

90 100 110 40

si Fig. 6. Plot of Niggli al, fm, c, alk, mg against si for the coronites of Table 2. Symbols repre­

sent:

® troctolite

�t gabbro (without olivine) + olivine gabbro

x clinopyroxene troctolite trocolictic gabbro

+ noritic troctolite

bronzite troctolite + troctolitic norite 0 gabbroic troctolite

on this diagram, which is therefore restricted to rocks in which these could

be adequately deterrnined. (The lower values for the plagioclase indicate the

maximum extent of the marginal zoning within a given rock.)

Three trends are apparent from Fig. 5: a troctolite-troctolitic norite series,

a troctolite-olivine gabbro series, and a series of olivine-free gabbros. The

rocks of the latter have been arranged in order of decreasing pyroxene

content and they appear separate from the olivine gabbros, which contain ca.

10 % or more original modal olivine.

In these least-altered coronites, differentiation trends are shown by changes

�

o

Ta

ble

l.

Mo

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me

of

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oro

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Sp

ec.

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. l

2

3

4

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6

7

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1

2

13

1

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1

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ry M

iner

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r

Pla

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38

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7

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9.2

9

54

.47

53

.84

4

3.3

7

48

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6.7

1

39

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5

4.4

9

41

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8

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vin

e +

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lt.

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-

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;;::

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1

9.2

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23

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2

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1

8.2

4

23

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4

2.0

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2

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2

2.3

7 "'

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Ort

ho

px

. (±

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-

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Fe

Ore

1

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2

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1.6

7 2

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3

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3

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1

.62

2

.07

2

.06

1

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1

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5

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3

.58

3

.10

4

.72

Sp

ineJ

0

.11

0

.09

0

.08

T

r -

0.1

5

Sec

on

da

ry M

iner

als

Am

ph

ibo

le (

± S

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.)

39

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2

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6

21

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1

9.6

1

13

.79

1

5.3

6

10

.92

4

.05

2

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9

24

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2

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0

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1

5.5

4

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2

7.3

0

28

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ho

px

. 3

0.0

9

13

.18

1

1.1

7

12

.76

1

1.5

7

12

.60

1

3.4

9

9.1

1

10

.95

9

.93

9

.03

Ga

rnet

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--

5.0

3

--

8.6

4

5.4

2

-8

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Bio

tite

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0

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1

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-

--

1.4

0

Acc

esso

ry P

rim

ary

Min

era

ls

Ap

ati

te

T r

T r

T r

T r

T r

T r

T r

T r

T r

0.8

6

T r

T r

1.1

9

T r

T r

l. T

roct

oli

te c

oro

nit

e, S

E o

f A

vre

id.-

2.

Tro

cto

lite

co

ron

ite

wit

h c

lin

op

yro

xen

e, F

ran

säse

n.-

3.

Cli

no

py

rox

ene

tro

cto

lite

co

ron

ite,

Avreid.-

4.

Ga

bb

roic

tro

cto

lite

co

ron

ite,

Ha

sda

len

.-5

-10

. T

roct

oli

tic

ga

bb

ro c

oro

nit

es:

5,

6,

9 L

ag

et;

7,

8

Ha

sda

len

; 1

0

SE

o

f A

vre

id.

-1

1.

Oli

vin

e g

ab

bro

co

ron

ite,

L

ag

et.

12

-16

. G

ab

bro

s: 1

2,

13

; 1

5 H

asd

ale

n;

14

La

get

; 1

6 P

lass

en.

Table J (cont.).

Spec. No.

Primary Minerals

Plagioclase

Olivine + Deuteric alt.

Clinopx (± Hb)

Orthopx (± Hb)

Fe Ore

SpineJ

Secondary Minerals

Amphibole (± Spinel.)

Orthop x

Gamet

Biotite

Accessory Primary Minerals

BASIC PLUTONIC INTRUSIONS 411

2A JA 4A 5A 6A

52.89 46.97 J8.62 4J.71 64.44

0.54 9.00

0.49

1.66 J.J2 6.22 J.52 6.45

J.92 1.37 2.JJ 2.J2 1.71

0.12 T r 0.04

25.57 JJ.JO J4.11 JJ.88 11.62

15.68 14.55 18.18 9.14 6.75

7.07

0.16 O.J7

7A 8A

51.80 60.98

12.28 6.80

1.04

12.55 12.J8

2.18 J.4J

0.06 0.09

11.46 11.80

7.49 4.51

l. OJ

0.11 T r

Apatite Tr T r T r T r T r T r T r

2A. Troctolite coronite with bronzite, Fransäsen; JA-6A. Bronzite troctolite coronites:

JA, 4A, 5A. Avreid , 6A. Barmen; 7A. Noritic troctolite coronite, N of P1assen; 8A. Trocto­

litic norite coronite, Barmen.

in rock type involving modal variations of the primary minerals coupled

with the appearance and disappearance of certain phases (olivine, spinel,

clino- and ortho-pyroxene). The minerals show some cryptic variation in

the troctolite-troctolitic norite and the troctolite-olivine gabbro series, with

decreasing CajNa ratios in plagioclase and decreasing Mg/Fe ratios in olivine

and pyroxene.

Table 2 shows the chemical variations in a selection of coronites which

show no marked metasomatic alteration (particularly by scapolitisation and granitisation). Corona growths will be considered later but are thought to have occurred by essentially isochemical reconstitution of the original

intrusives.

These analyses are of individual rock types and are not representative of

the full chernical variation in the particular bodies from which they were

collected. In fact, although there are lithological changes across some masses,

these are usually small compared to the complete regional variation.

Niggli numbers calculated from the analyses have been plotted on Fig. 6

and show that the rocks all belong to one major differentiation trend. The

presurnably early-differentiate troctolite rocks form a somewhat distinct

group but, at the other extreme, olivine-free lithologies are not clearly sep­

arated from the rest of the series. The position of noritic rocks relatively

'early' in the trend reflects their troctolitic character in the present area.

Noritic lithologies rnay have been emplaced prior to gabbroic rocks,

412 IAN C. STARMER

Table 2. Chemical analyses and Niggli numbers of some coronites and chemical analyses of a few amphibolites

Oxide percentage

l 2 3 4 5 6 7 8 9 10 11 12

Si02 43.84 44.87 45.48 46.62 46.51 45.87 46.54 45.93 45.84 46.10 46.27 45.86

Al20.a 17.95 18.83 17.97 17.29 17.56 17.64 17.61 16.82 16.96 17.59 17.30 17.79

F�03 2.74 1.75 2.05 3.76 3.18 3.32 2.60 4.08 2.50 2.94 2.09 2.74

FeO 7.68 7.54 7.83 7.08 6.92 8.15 7.36 8.38 7.94 8.51 8.96 7.71

M gO 12.95 12.93 12.15 10.94 9.83 9.43 8.88 9.54 8.36 8.42 9.23 7.20

Ca O 9.93 9.96 10.15 9.52 10.55 9.97 10.09 8.83 10.18 9.40 9.56 10.90

Na20 2.07 2.15 2.11 2.09 2.18 2.66 2.74 2.13 2.88 2.71 2.52 2.78

K20 0.20 0.23 0.20 0.27 0.21 0.30 0.56 0.27 0.57 0.52 0.38 0.74

Ti02 0.25 0.27 0 .94 0.86 0.73 0.65 1.30 0.79 1.42 1.83 1.43 1.66

M nO 0.19 0.21 0.25 0.24 0.16 0.21 0.18 0.20 0.10 0.14 0.17 0.15

P20s 0.16 0.18 0.10 0.11 0.19 0.14 0.22 0.15 0.20 0.18 0.23 0.20

H20+ 1.14 1.02 0.86 0.93 1.56 1.31 1.55 1.40 1.23 1.20 1.14 0.84

H! O- 0.11 0.10 0.12 0.09 0.09 0.14 0.11 0.15 0.11 0.13 0.11 0.10

Sum 99.21 100.04 100.21 99.80 99.67 99.79 99.74 98.67 98.29 99.67 99.39 98.67

Niggli numbers

al 20.6 21.8 21.2 21.3 22.1 22.0 22.7 21.5 22.2 22.9 22.1 23.5

f m 54.5 53.1 52.8 52.8 49.0 49.7 47.0 53.1 46.6 48.4 49.9 43.3

c 20.7 20.8 21.7 21.3 24.1 22.5 23.7 20.6 24.3 22.2 22.1 26.1

alk 4.2 4.4 4.3 4.6 4.8 5.8 6.6 4.8 6.9 6.5 5.9 7.1

mg 0.69 0.71 0.68 0.65 0.64 0.60 0.62 0.58 0.59 0.57 0.60 0.55

si 85.5 87.9 90.8 97.6 99.3 96.8 102.0 99.8 101.9 101.8 100.3 102.8

l. Troctolite coronite, SE of Avreid. - 2. Troctolite coronite (with clinopyroxene), Fransäsen.- 3. Troctolite coronite (with bronzite), Fransäsen.- 4. Bronzite troctolite coronite, Avreid.- 5. Clinopyroxene troctolite coronite, Avreid. - 6. Noritic troctolite coronite, Barmen.- 7. Gabbroic troctolite coronite, Hasdalen.- 8. Troctolitic norite coronite, Barmen.- 9-11. Troctolitic gabbro coronites: 9 SE of Avreid; 10 Hasdalen; 11 Stamsöy.- 12-14. Olivine gabbro coronites, Hasdalen.- 15-16. Gabbro (without olivine): 15 Hasdalen. 16 Laget.- (17-25 are samples from west to east across the main Laget body.) 17-23. Troctolitic gabbro coronite.- 24. Olivine gabbro coronite.- 25. Troctolitic gabbro coronite (patch in olivine gabbro).

Analysis methods - Titrimetric, Colorimetric. Analyst - I. Starmer (University of Nottingham).

hut the evidence is inconclusive. To the east of Laget, noritic troctolite

coronites are heavily amphibolitised whereas an adjacent mass of troctolitic

gabbro shows much less alteration. Noritic rocks immediately to the south­

west are also extensively amphibolitised (Rodwell 1968). Batey (1965) and

Ryan (1966), working on similar coronites between Brevik and Kragerö,

considered that noritic rocks probably represented earlier differentiates and

intrusions than gabbroic lithologies.

There is definite evidence that the olivine-free gabbros representoo late­

stage intrusions. They are often emplaced on the flanks of more basic masses

(e.g. at Hasdalen, Laget, and east of Avreid) and at Hasdalen a sharp chilied

contact is still preserved against troctolitic gabbro coronite.

13

47.53

17.78

2.57

8.43

7.85

9.71

2.80

0.41

1.75

0.18

0.23

0.96

0.13

14 15 16 17 18

48.09 47.81 48.69 46.06 47.11

17.62 18.62 18.43 16.92 16.00

2.38 1.98 2.15 2.91 3.05

7.83 7.62 7.38 8.80 9.17

7.35 7.23 7.14 8.69 8.48

10.14 10.69 10.01 8.97 8.81

2.65 2.75 2.92 2.52 2.73

0.60 0.85 0.81 1.02 0.49

1.81 1.58 1.90 2.15 1.50

0.15 0.12 0.16 0.15 0.18

0.22 0.21 0.17 0.18 0.15

0.89 1.06 0.86 0.72 1.07

0.17 0.09 0.11 0.10 0.09

BASIC PLUTONIC INTRUSIONS 413

19 20 21

46.67 48.26 47.14

16.36 17.05 16.65

2.91 3.30 3.16

8.96 8.85 8.35

8.54 7.51 8.87

9.26 8.62 9.58

2.93 2.74 2.81

0.87 0.63 0.59

1.35 0.86 0.50

0.16 0.17 0.15

0.17 0.19 0.13

0.93 0.82 0.67

0.12 0.11 0.18

22 23 24

46.89 47.21 47.50

16.53 16.80 17.19

3.34 2.67 3.39

8.58 8.64 7.96

8.91 9.12 7.42

9.07 9.15 9.33

2.79 2.61 2.80

0.55 0.60 0.61

0.71 0.64 0.93

0.16 0.18 0.18

0.15 0.15 0.14

0.87 0.82 1.01

0.18 0.13 0.10

25

46.41

16.62

3.94

8.38

8.17

9.26

2.79

0.64

1.36

0.17

0.18

0.97

0.12

100.33 99.90 100.61 100.73 99.19 98.83 99.23 99.11 98.78 98.73 98.72 98.56 99.01

23.5 23.8 24.7 25.0 22.0 21.2 21.2 23.0 21.4 21.4 21.7 23.2 21.7

46.6 44.5 42.1 42.7 49.9 51.1 49.6 48.7 49.5 50.6 50.4 46.8 49.4

23.3 24.9 25.9 24.6 21.2 21.2 21.8 21.1 22.4 21.2 21.5 22.9 22.0

6.6 6.8 7.2 7.7 6.8 6.6 7.4 7.1 6.7 6.7 6.4 7.1 6.9

0.56 0.56 0.58 0.57 0.57 0.55 0.57 0.53 0.58 0.58 0.59 0.54 0.54

106.5 110.3 107.9 112.1 101.9 105.7 102.7 110.8 102.7 103.0 103.7 109.0 102.8

Table 2 (cont.). Amphibolites- chemical analyses

Si02

A1203

Fe.03

FeO

M gO

Ca O

Na20

K20

Ti02

MnO P206 H20+

H20-

Sum

AM l

49.81

14.23

3.18

7.46

6.99

8.71

3.69

1 .05

1.94

0.16

0.22

2.35

0.11

99.90

AM2

47.15

15.55

2.68

8.80

6.22

9.62

2.90

0.90

2.48

0.12

0.17

2.86

0.13

99.58

AM3

45.67

15.30

3.52

9.76

6.61

10.o2

3.12

0.54

1.72

0.17

0.24

2.13

0.09

98.89

AM4

48.16

15.98

2.75

7.14

6.93

9.53

2.75

0.75

2.59

0.15

0.29

2.00

0.14

99.16

AM5

48.05

15.93

3.42

7.35

6.70

8.80

2.69

0.81

2.34

0.12

0.26

1.96

0.21

98.64

AM6

49.11

14.20

2.34

9.63

6.21

9.52

2.83

0.68

2.65

0.18

0.19

2.15

0.10

99.79

414 IAN C. STARMER

THE PETROGRAPHY OF THE CORONITES

The original igneous assemblagesand textures

Although coronas have formed about the primary magmatic minerals, the

original textures have often not been obliterated.

The igneous fabrics were frequently ophitic or subophitic, but some

troctolitic rocks show an orthocumulate type of texture with cumulus

plagioclase and olivine and intercumulus pyroxene. The alignment of platy

feldspar laths produces an igneous lamination in many lithologies but this

tends to be sporadic in occurrence. Where amphibolitisation is patchy, the

lamination has been pseudomorphed by a lepidoblastic fabric of homblende

and andesine and is therefore continuous through the patches of amphibolite.

Plagioclase was the first mineral to crystallise but olivine often formed

subophitic intergrowths. The feldspar developed subidiomorphic laths (reach­

ing 5 mm length) and the olivine xenomorphic crystals (reaching 4 mm size).

Iron ores then started to form, only rarely attaining subidiomorphic out­

lines and reaching a maximum size of 2-3 mm. Interstitial relationships are

common (particularly in the troctolitic rocks) and, in many cases, iron ores

may partially or completely enclose the smaller plagioclase and olivine

crystals. The ores consist of titanomognetite, with ilmenite (usually subordi­

nate) forming part of the same mass or developing separate crystals. Pyritic

patches occur in these minerals and in some rocks discrete pyrite has formed.

Iron ores continued to grow as pyroxenes started to crystallise and were

only rarely included in the latter. In olivine-rich rocks, the pyroxenes are

normally interstitial to olivine and plagioclase, but in lithologies which were

olivine-poor, clinopyroxenes frequently formed subophitic growths with

plagioclase. The pyroxenes are usually less than 5 mm size but in the latter rocks

highly irregular, interdigitating masses may form continuous growths over

areas of 2-3 cm2 (Plate l, Figs. l & 2). Green pleonaste spinel is occasionally developed as discrete crystals, but

is more commonly associated with the margins of iron ores and may be partially endosed by the latter. lt formed ubiquitously in rocks originally

containing 15 %, or more, modal olivine and sporadically in noritic lithologies

containing 10-15 %. The crystals are always small(� 0.2 mm diam.) and are

only present in minor concentrations ( � 0.5 % of mode).

Apatite is often present as a minor accessory and occurs in small crystals

(rarely reaching 0.4 mm size) included in both plagioclase and olivine.

The olivine-free gabbros were sometimes very coarse-grained with plagio­

clase reaching 3 cm lg, and some of the metagabbros may contain pseudo­

morphed plagioclase laths up to 5 cm in length.

In many rocks, olivine crystals were deuterically altered prior to the

formation of their coronas (Plate l, Figs. 3 & 4). Various mixtures of bowl­

ingite, iddingsite, antigorite, magnetite, and haematite replace the olivine

totally, partially, or merely along the ubiquitous cracks.

2

3

PLATE l

Figs. J & 2. lnterstitial relations of clinopyroxene to plagioclase (Fig. l) and to plagioclase

and olivine (Fig. 2). The pyroxenes show a continuous growth over some areas and have

schiller structures and non-uniform clouding. Oriented rutile needles can also be seen. The

clouded plagioclase has formed clear margins (poorly developed in Fig. l) against hornblende

coronas. (Figs. l & 2 x 40.)

Figs. 3 & 4. Deuteric alteration of olivine inside the bronzite corona (p). Fig. 3 shows altera­

tion to bowlingite (b) and iron-ores, the latter as speeks in bowlingite and along eraeks in

olivine. Fig. 4 illustrates replacement by antigorite (a) and iron-ore, which both become

disseminated in the ingrowing bronzite corona. - (F i g. 3 x l()(); Fig. 4 x 40.)

416 IAN C. STARMER

The alteration products are normally endosed by the ingrowing bronzite corona, and erack-systems retained in the orthopyroxene are partially oblit­erated with the deuteric products disseminated around them.

The plagioclase

Plagioclase laths are usually clouded with dust and inclusions but the outer parts of most crystals consist of clear, more sodic feldspar which may, or may not, be zoned (Plate 2, Figs. l, 3, 4, 5, 7).

Albite polysynthetic twinning is often developed in both cores and rims and in the former may be combined with pericline or Carlsbad twins.

The variation in composition of the original plagioclase (An66_ 42) and

the maximum extent of marginal zoning within a given rock have been summadsed in Fig. 5. The rims in one thin-section may show both 'normal' or 'reverse' zoning (i.e. to successively more sodic or more calcic forms, respectively) or may be entirely unzoned. In all cases, the feldspar of the rim is more sodic than that of the core, although the latter may show a

slight 'normal' zonation.

Recrystallisations in some rocks have eaused the development of stubby andesine-labradorite laths, and then granoblastic andesines (An35_ 48).

PLATE 2

Fig. J. Clouded plagioclase with clear rims. This type (also shown in Fig. 7) has a non­

uniform distribution of larger inclusions and should be compared with those of Figs. 3, 4 and 5 which have a more uniform clouding of fine dust, producing a megascopic purple colora­

tion. (x 40.) Fig. 2. Titanomagnetite crystal with partial corona of biotite crystals (b) and with outer

symplectite of green-brown hornblende and spinel. Secondary iron-ore occurs around the

upper biotite (x 40). Fig. 3. Olivine crystals (o) with bronzite corona (p) and outer actinolite-spinel symplectite (a).

The latter is being replaced by green-brown hornblende (h) which contains a few, remnant

spineJ vermicules. The lower hornblende replacement appears to emanate from the corona

around a small iron-ore. (x 35.) Fig. 4. Olivine (o) partially altered to bowlingite (b) and magnetite inside a corona of bronzite.

The outer actinolite-spinel symplectite (a) is still preserved in places but is largely replaced

by hornblende (h) developing from coronas around titanomagnetite and spineJ (s). (x 35.) Fig. 5. Coarse actinolite-spinel symplectite around secondary bronzite (p). The clear rim on

the clouded plagioclase normally follows the symplectite margin but the latter has grown

through it on the right of the photograph. (x 30.) Fig. 6. Secondary bronzite aggregate (p) which has completely replaced original olivine.

The outer corona is green hornblende with a few spinel vermicules. In the bronzite aggregate,

radiating·rods surround larger granoblastic crystals which replaced the final olivine. (x 35.) Fig. 7. Plagioclase and interstitial diallage forming subophitic intergrowths. The pyroxene

has a thin corona of brown hornblende and has a densely clouded centre with clearer margins.

A late-stage veinlet of antigorite and bastite cuts the primary minerals and coronas (x-x).

The marginal zoning on plagioclase (which has non-uniform clouding) is either absent or

poorly developed adjacent to the thin hornblende corona on pyroxene. (x 40.)

PLATE 2

BASIC PLUTONIC INTRUSIONS 417

2 3

4 5

6

418 IAN C. STARMER

PLATE 3

3

5 6

Fig. l. Garnet coronas (g) around titanomagnetite have grown inwards replacing hornblende

growths and concentrating at the margin of the iron-ore. The crystal "g" is beginning to

develop outwards. Garnet (y) also replaces hornblende around secondary bronzite. (x 40.)

BASIC PLUTONIC INTRUSIONS 419

The clouding gradually disappears during these changes, which mark the

onset of amphibolitisation. A number of different feJdspars may therefore

coexist in one thin section, with these recrystallised forms adjacent to

labradorile laths with wide, clear borders and lobate, sutured margins.

The fine dust clouding in the cores of the plagioclase consists of magnetite,

haematite, and spinel and varies in density from one lath to another withn1

some thin sections. Plagioclases with a megascopic purple coloration often

tend to have a uniform clouding of fine dust whereas others have dust

and larger inclusions of magnetite and haematite which are more randomly

distributed.

Frequently green and brown hornbiendes have developed along grain

boundaries between adjacent feJdspars and as inclusions within them. These

inclusions are most abundant against amphibole coronas and appear to be

genetically related to these growths. Around both intergranular and included

hornblende, the plagiocJase has developed clear rims in which zoning can

rarely be detected. Spinel inclusions also occur in plagioclase adjacent to

coronas of amphibole (± spinel).

The development of clear, more-sodie plagioclase along the margins of

laths and around horobiende inclusions appears to have started after slight

clouding of the original feldspar, but to have been largely synchronous with

the formation of amphibole coronas around ferromagnesian minerals. Dust

and inclusions from the rims were probably partly resorbed into amphibole

growths and partly redistributed through the plagioclase. Small masses of

included magnetite may have resulted from this redistribution.

Well-developed coronas of amphibole (± spinel) rarely transgress the

clear rims and grow into the clouded centres, occasionally pseudomorphing

twin-lamellae of the original plagioclase. The dust within the cores may be

partically resorbed, but some has recrystallised to small magnetite crystals

adjacent to the corona.

The author considers the rims to be related to corona growths and virtually to represent ingrown feldspar coronas.

The scapolitisation of plagioclase in some coronites wilJ be considered

later.

Fig. 2. Clinopyroxene (from olivine-free gabbro) with poor horobiende corona and patchy

intemal replacement. (x 40.) Fig. 3. Oinopyroxene with thin green-brown horobiende corona and brown horobiende

developing along the cleavages. (x40.) Fig. 4. The nebulous reaction zone between sericitised plagioclase (se) and the horobiende

corona around secondary bronzite (p). Granoblastic andesine and horobiende are forming

in this zone. (x 35.) Figs. 5 & 6. Coronite containing scapolitised plagioclase under plane-polarised light and

erossed nicols, respectively. Note the retention of the clouded centre and clear rim of the

replaced plagioclase. 'H' is a horobiende corona and 'h' a similar corona around clino­

pyroxene. (x 35.)

420 IAN C. ST ARMER

The olivine and its coronas

The compositions of the primary allvines have already been summarised

(Fig. 5) and vary with original rock-type from chrysolite to hyalosiderite

(Fa26_ 36) showing no apparent zoning.

Olivine may develop an ingrown bronzite corona, a seeond earona of

amphibole ( + spinel), and an ou ter growth of gamet.

The bronzite earona replaces olivine inwards, leaving a sharp outer margin.

(The small size of individual crystals makes composition determination

difficult, but most seem to lie in the range En75-En84.) The bronzite is

usually in the form of granoblastic crystals ( � 0.2 mm size) or radiating

rods (� 0.3 mm lg) normal to the original olivine margins. Rarely the­

orthopyroxene forms a continuous growth around the olivine and occasionally

larger bronzite crystals ( � 1.5 mm size), which may have slightly clouded

centres, grow and replace the above coronas. All these varieties are endosed

within the seeond amphibole growths.

The total replacement of olivine by orthopyroxene produces either grano­

blastic aggregates or radial rods surmunding larger granoblastic crystals (� 3 mm size). Often these masses contain disseminated, interstitial speeks

of magnetite which may be concentrated along the original olivine bound­

ary, or may rarely form dendritic intergrowths with the bronzite. Within

continuous growth rims of secondary orthopyroxene, the final olivine has

sometimes been replaced by a single, large crystal.

Exceptionally the bronzite earonas are in contact with plagioclase but

normally an outer growth of amphibole or amphibole-spinel symplectitc

develops outwards from the original boundary, replacing the plagioclase. Commonly it consist of granoblastic crystals or radiating rods of actinolite­spinel symplectite, green or green-brown homblende, or actinolitic-homblende.

Spinel vermicules normally comprise about 10 % of the symplectite corona and can be evenly distributed or concentrated in either the inner or outer portions. Frodesen (1968 a & b) found that spinel was always formed in the outer parts of the amphibole growth, which he considered to represent two

separate coronas. The spinel was found to be Concentrated in the inner

portions by Reynolds & Predrickson (1962) and in the outer by Brögger (1934).

Continuous rims or granoblastic crystals of green-brown or brown horn­

blende may replace the symplectite and often contain rernnant spinel vermi­cules. In many rocks, however, earonas of horobiende appear to have

been formed without the replacement of any pre-existing symplectite. Where

it is observed, the replacement has occasionally resulted from horobiende

growths around adjacent iron ores (Plate 2, Figs. 3 & 4).

The outer earona of almandine gamet, which is not always developed, forms as gmnoblastic crystals or as a continuous growth rim. Brögger (1934) considered that it only formed in the later stages when most of the olivine

BASIC PLUTONIC INTRUSIONS 421

had been replaced and Reynolds & Predrickson (1962) that it only formed after complete replacement of olivine by bronzite. Within the present area, gamet develops around cores of both secondary orthopyroxene and rernnant olivine. The seeond corona in all cases consists of horobiende and not of actinolite-spinel symplectite.

The outermost coronas of gamet about olivine, pyroxene, and iron ore are often continuous and seem to have formed contemporaneously. They fre­quently grow inwards replacing horobiende coronas and develop outwards as poikiloblastic masses w hen amphibolitisation starts (P late 3, Fig. l).

The pyroxenes and their coronas

Both primary clinopyroxenes and orthopyroxenes commonly exhibit a

schiller structure and are frequently clouded with dust and inclusions. The degree of dust-clouding is variable within one thin-section and often

within a single crystal. It may be sufficiently dense to render the crystals opaque and in some cases variations in its density have a crude zonation. The dust is largely composed of magnetite and haematite and these minerals often form somewhat larger inclusions in the pyroxenes together with oriented rutile needles (Plate l, Figs. l & 2).

The clinopyroxenes are of augite composition and frequently contain fine exsolution lamellae of iron-magnesium pyroxene.

The primary orthopyroxenes are mainly of hypersthene and bronzite composition (En68 .89) and may contain exsolution lamellae of calcium-rich pyroxen e.

Both primary clino- and ortho-pyroxenes develop thin (O:: 0.2 mm wide) coronas of green, green-brown, or brown horobiende adjacent to plagioclase. Rarely these growths may consist of actinolite-spinel symplectites around clinopyroxene or green homblende-spinel symplectites around orthopyroxene. The coronas are occasionally incomplete but normally form a continuous crystal growth which may, or may not, be orientated paraHel to the pyroxene crystal directions. Sometimes the amphiboles form granoblastic crystals and rarely larger horobiende aggregates develop outwards.

Small plagioclase laths endosed in clinopyroxene have thin rims of green homblende.

Outside the hornblende coronas garnet is sporadically developed as grano­blastic or xenoblastic crystals, which grow inwards and develop poikio­blastically outwards; in both cases enclosing and resorbing hornblende from the adjacent coronas.

The homblende coronas around pyroxenes are always thin and appear to be less advanced than growths around olivines and iron ores. It seems unlikely that this is due to instigation at a late stage, since when a seeond garnet corona has formed it is often continuous with, and apparently con­temporaneous with, those around olivine and iron ore. The thin homblende

6

422 IAN C. STARMER

growth was, therefore, broadly synchronous with amphibole coronas around the latter minerals and its poor development is taken to indicate that pyroxene-plagioclase was almost a stable assemblage.

The iron ores (+ spinel) and their earonas

Iron ores, consisting of magnetite, titai:wmagnetite, ilmenite, and subordi­nate pyrite, often have associated primary spineJ and develop an inner corona of brown or red-brown biotite, an outer growth of horobiende or homblende-spinel symplectite, and sometimes an outermost corona of gamet. These secondary growths formed at the expense of adjacent plagioclase, although in the rare case where the inner biotite corona is well-developed, a little erosion of the primary iron ore has sometimes occurred. When spineJ formed discrete crystals, with no associated iron ores, the biotite corona was not developed and horobiende ( + spin el) was the inner growth.

The biotite corona around iron ores may be entirely absent and is normally incomplete, bein�. represented by one or two individual laths (Plate 2, Fig. 2),

which rarely contain zircon inclusions with pleochroic-haloes. The horobiende corona forms against plagioclase or against amphibole

growths around olivine and pyroxene, where any intervening plagioclase has been replaced. The corona normally consists of radiating rods or granoblastic crystals of brown horobiende with, or without, pleonaste as a symplectic intergrowth. Occasionally the horobiende is green-brown or green, and rarely a little brown biotite occurs towards the outer margin.

These horobiende growths are normally well-developed (often > l mm

thick) and are usually the largest coronas in the rock. Where they are adjacent to actinolite-spiiiel symplectites around olivine, they may replace them.

Sometimes a partial or complete third corona of gamet is developed, normally as granoblastic crystals but rarely forming a continuous crystal growth (.e 0.8 mm wide).

The gamet tends to grow inwards replacing the horobiende and may grow through the Iatter, becoming Concentrated along the margin of the iron ore but still containing partially resorbed bomblendes in its outer portions. Poikiloblastic gamets (.e 1.5 mm size) sometimes grow out from the corona and enclose adjacent minerals (Plate 3, Fig. 1).

When primary iron ore is included in clinopyroxene, it develops a thin, green horobiende corona.

The earonas (general remarks)

With the exception of the gamet growths, which are very sparadie in occurrence and tend to be restricted to certain bodies, the coronas described above developed in all lithologies except the olivine-free gabbros. Within the latter rocks ]arge granoblastic horobiende crystals formed a type of partial

BASIC PLUTONIC INTRUSIONS 423

corona around primary clinopyroxenes and iron ores: The hornblende is green, green-brown, or blue-green in colour and has also formed intemal replacement patches in the pyroxene (i.e. amphibolitisation started before coronas developed properly).

Other metamorphic minerals

The amphibolitisation will be considered later, but within relatively unaltered coronites a number of metamorphic, non-corona minerals have formed.

The plagioclase has recrystallised to stubby labradorite-andesine laths in many coronites and eventually forms granoblastic andesine during amphibolit­isation. The clouding of the feJdspar disappears during the recrystallisations. In some rocks granoblastic hornblende and andesine are produced from a nebulous reaction zone at the contact of amphibole coronas and plagioclase (Plate 3, Fig. 4).

Hornblende develops in patches and along eraeks and cleavage in both primary and secondary pyroxenes (Plate 3, Figs. 2 & 3). Most of the olivine has disappeared by this stage. Further alteration leaves patches of pyroxene in hornblende crystals and complete pseudomorphous replacement may retain the dust-clouding and schiller structures of the primary minerals. A few augites pass through an intermediate stage of actiDolite formation.

Granoblastic hornblende aggregates (with crystals � l mm size) grow out from coronas and may poikiloblastically enclose other minerals. lnterstitial speeks of iron ore frequently occur within these growths and larger masses ma y

poikiloblastically include homblende. Many ores grew contemporaneously with the horobiende but some represent primary iron ores to which additional material has nucleated.

The green and green-brown hornbiendes in the corona growths are sporadically replaced by chlorite and fibrous, blue homblende.

It has already been noted that where gamet coronas developed, they often grew inwards replacing horobiende and also grew poikiloblastically outwards.

All the above reactions essentially mark the onset of amphibolitisation, although the rocks are still typically coronites.

In addition to the above metamorphic effects, a little sericite formed in plagioclase, biotite developed from homblende, and both biotite and horn­blende were replaced by chlorite. A few quartz blebs appeared in horn­blendes and hornblende aggregates and orthopyroxenes were occasionally altered to bastite before amphibolitisation. Late-stage veinlets of antigorite and bastite cut both corona growths and plagioclase crystals and may have developed before, during or after amphibolitisation (Plate 2, Fig. 7).

THE AMPHIBOLITES

The initial effects of amphibolitisation in coronites have been described above, but this process ultimately led to the formation of stable amphibolites, which

424 IAN C. STARMER

are fine to medium grained rocks (usually less than 5 mm grain size) con· taining essential hornblende and andesine (An35_48) in subequal proportions.

Biotite and almandine garnet are often present in varying concentrations and

quartz occurs in some places.

Associated with the amphibolites are coarser metagabbros with a relict

igneous textured fabric. Retrograde biotite-rich schists form on the outer

margins of bodies and within intemal shear zones.

Adjacent to 'granitic' gneisses and pegmatites, amphibolites become bio·

tite-rich and with marked enrichment in K20, Si02 and H20 may develop

microcline porphyroblasts.

The plagioclases and hornbiendes of the amphibolites and metagabbros

are often granoblastic or xenoblastic equant crystals, but hornbiendes may

become lepidioblastic or poikiloblastic, enclosing iron ores, sericitised plagio·

clase, and quartz blebs. The hornbiendes often form aggregates and are

normally green, green-brown, or rarely blue-green in colour. Occasionally

actinolite may occur. A little remnant pyroxene rarely exists in a state of

partial alteration within amphibole crystals or aggregates.

Red-brown biotite develops either subidioblastic laths or xenoblastic crys­

tals, occasionally forming patches in hornblende and rarely poilciloblastically

enclosing the Iatter. It frequently appears to be secondary in origin, forming

from the hornblende and rarely from garnet.

Quartz, when present, occurs as interstitial, xenoblastic and irregular crys­

tals and as blebs within hornblende or garnet.

The smaller garnet crystals are xenoblastic and relatively free of inclusions

bu t larger crystals (reaching l O cm diam.) are poikiloblastic with inclusions of

quartz and iron ores.

Accessories include titanomagnetite and apatite with occasional ilmenite, pyrite, haematite, rutile, sphene, and tourmaline. Alterations have sometimes produced chlorite, scapolite, prehnite, and sericite.

The amphibolites sporadically contain patches which are rich in sphene, and this mineral may then form large idioblastic crystals (reaching 8 cm length) in felsic segregations.

Some scapolitisation of amphibolites has occurred and severe local retro­gressive metamorphism has sporadically produced Greenschist facies assem­

blages. The latter commonly consist of plagioclase and actinolite (or actinolitic hornblende) but also contain tremolite, chlorite, epidote (pistacite or clino­

zoisite), and quartz with accessory biotite and magnetite.

Comparison of analyses of coronites and amphibolites (Table 2) indicates

that the latter are generally richer in Ti02 and poorer in MgO.

Analyses 17-25 Successively represent a traverse from west to east across

the main Laget coronite body and show a marginal enrichment in TiO�.

probably more significantly demonstrated by the calculated Niggli number!i.

Titanium is thought to have been very mobile during the metamorphism of

this area and these changes are not considered entirely to reflect original, mag­

matic variations.

BASIC PLUTONIC INTRUSIONS 425

The outward migration of Ti02 is weil shown by the sparadie sphene-rich

patches in the bordering amphibolites and the development of marginal

rutile ore bodies (particularly at Laget and on Stamsöy).

The reduction in MgO content is considered to result from horobiende

replacing both primary and secondary pyroxene, instead of replacing plagio­

clase as in earona growth. lt is most marked in original troctolitic rocks

(rich in secondary orthopyroxene) and noritic lithologies (rich in primal)'

orthopyroxene) From thin section observations, some of the iron released

during recrystallisations was absorbed in the growth of secondary iron ores.

The exit of some magnesium from the hyperite bodies probably contributed

to magnesia metasarnation which produced anthopyliite/gedrite-bearing rocks

around them.

THE SCAPOLITISA TION OF CORONITES AND AMPHIBOLITES

Scapolitisation eaused by chlorine pneumatolysis is restricted to certain bodies,

particularly those aröund Hasdalen and Plassen, to the east of Söndeled.

Although the effects are most marked around the margins of individual mas­

ses, sparadie alterations also occur in the cores.

Both coronite and adjacent amphibolite have been affected, the metasorna­

tism sometimes being concentrated around small fissures. Veins of scapolite,

which often carry actinolite as a major component, cut coronite and amphi­

bolite and have eaused intensive alteration of the surrounding rock. These veins

frequently contain minor amounts of rutile and sphene.

In the coronites, single plagioclase laths were replaced inwards by a

number of small scapolite crystals (of mizzonite and dipyre composition).

In most cases, relict features of clouded centres and clear rims have been

retained, although the dust and Iarger inclusions have been samewhat

redistributed (Plate 3, Figs. 5 & 6). This replacement occurred after the

growth of the amphibole coronas and has rarely attacked the latter.

The alteration is partial in many rocks, with some plagioclase completely

replaced, same marginally affected and some completely unaltered. The

scapolitisation tends to promote horobiende formation and in the ultimate

stages produces scapolite-hornblende stone or ödegårdite.

The fact that both coronites and adjacent amphibolites are affected suggests

that the main metasomation either post-dated or was contemporaneous with

amphibolitisation.

Frodesen (1968 b) considered that the scapolitisation may have occurred

during the later stages of earona growth and concluded that 'the magmatic

phase bad not come to an end before the regional metamorphic phase

started'.

In the present area, the process definitely commenced after the clouding

and formation of clear rims in plagioclase and the growth of amphibole

coronas. No garnet coronas have been observed in scapolitised specimens

426 IAN C. STARMER

although they have sometimes formed in adjacent unaltered rock. The scapolitisation could, therefore, have started before gamet coronas formed,

but it certainly continued after the amphibolitisation.

Often coronite adjacent to thoroughly scapolitised amphibolite shows severe alteration only within 5 to 10 cm of the contact; thereatter the scapolitisation is partial. This could indicate that the fluids eausing amphibolit­isation also carried chlorine etc. and scapolitised the coronite immediately prior to amphibolitising it. Much of the amphibolite was then scapolitised as it formed.

The evidence of particularly severe alteration around open fissures in both coronite and amphibolite and the occurrence of scapolite-rich veins suggests intense activity after amphibolitisation and indicates that scapolit­isation may have been an extended process.

PETROGENESIS

The field-relations of the hyperites clearly indicate that they were intruded largely duting an interval between intense metamorphic phases, although the country rocks were still depressed in the crust.

A regional differentiation trend has been demonstrated within the present area and variations within individual bodies are generally insignificant.

The original igneous lithologies formed a differentiation series from troctolite to troctolitic norite and from troctolite to olivine gabbro, with the development of separate olivine-free gabbros. The latter, from field evidence,

represent late-stage intrusions normally emplaced at the sites of earlier, more basic bodies. The olivine-free gabbros sometimes form chilied margins against earlier troctolitic gabbros (e.g. at Hasdalen) and although amphibolit­ised, possess only poor corona growths.

Late-stage magmatic or deuteric effects eaused the alteration of olivine to bowlingite, iddingsite, iron ores, and antigorite before the development of coronas.

There seems little doubt that corona growth and amphibolitisation were both metamorphic processes which affected the intrusives after their emplace­ment and consolidation.

Most authors consider the coronas to be metamorphic and not magmatic phenomena (e.g. Brögger 1934, Gjelsvik 1952, Murthy 1958, Reynolds &

Predrickson 1962). Murthy (1958) has summansed the features of coronites and has made a

number of distinctions between coronas and magmatic reaction rims. Electron-probe analyses carried out by Mason (1967) on a troctolite

coronite from Sulitjelma suggested origins by solid state diffusion in an aqueous medium, rather than by magmatic crystallisation. It was thought

that such processes could occur during metamorphism or during relatively slow cooling after consolidation.

Frodesen (1968a) considered that around completely replaced olivines the

BASIC PLUTONIC INTRUSIONS 427

marginal orthopyroxene rims and the inner amphibole growths were 'deuteric'

or 'autometamorphic' and that secondary bronzite-iron ores and outer

amphibole-spinel symplectites were the result of regional metamorphism.

Coronas obviously represent the partial readjustments of igneous assem­

blages which were unstable during subsequent elevated PT conditions. Thin

section observations indicate that they formed by solid-state transformations

and volume for volume replacements of the primary magmatic minerals,

hut the mechanisms involved are not immediately obvious.

The olivinejbronzite transformation was thought by Reynolds & Pred­

rickson (1962) to be due to metasornatism from siliceous solutions with

10 to 12 per cent by weight of silica added to the rocks for the complete alteration of olivine. Murthy (1958) considered the prime agent in corona

formation was intergranular water which acted as a transporting medium for

the ions in an essentially closed system, hut Mason (1967) thought that a

little water might have been introduced from the country rocks.

Analyses of coronites from the present area (Table 2) suggest that they

resulted from essentially isochemical recrystallisations of normal troctolitic­

noritic-gabbroic rocks. Silica and water contents are not partieularly high

and fall within the range of values expected for these lithologies, indicating

that large scale silica metasornatism did not occur and little, if any, water

was introduced during corona growth.

The low water content of the rocks must have inhibited the growth of

amphiboles, which could only have developed by concentration of inter­

granular fluids at their sites of formation, or by the iritroduction of extremely

small amounts of extraneous water.

The growths around olivine involved the mobility of Fe, Mg and to a lesser

extent Al, Ca, and Si. The olivinefbronzite transformation eaused the exit

of Fe and Mg and an increase in the Si content (possibly accompanied by

the introduction of small amounts of Al and Ca). The replacement of

plagioclase by amphibole (-+- spinel) required the accession of Mg, Fe, and H20 and the relative reduction of Ca, Al, and Si contents.

Values calculated from Mason's work (1967) on similar coronas in the Sulitjelma troctolite indicate a small increase in MgO/FeO ratio from olivine (2.60) across orthopyroxene (2.78) to amphibole-spinel symplectite (2.83).

Reynolds & Predrickson (1962) and Murthy (1958) found that the con­

version of olivine to bronzite always involved a marked increase in the Mg/Fe ratio, although Murthy showed that a variety of orthopyroxene com­

positions could result from similar olivines.

Mason's values of MgO/FeO ratios for amphibole-spinel symplectites

have been quoted above. Horobiende coronas are obviously somewhat

variable in composition hut values quoted by Murthy (1967) from Buddington (1952) suggested MgOf(Fe, Mn)O ratios of between 3/1 and 6/1.

lt therefore appears that during the formation of secondary orthopyroxene

and amphibole, some iron was liberated in excess of that combined with

magnesium in these coronas. This iron could have eaused clouding of

428 IAN C. STARMER

plagioclase before and during the development of amphibole. Some of the free iron also formed interstitial speeks of ore in the bronzite coronas.

The type of plagioclase clouding found in these rocks was thought by Poldervaart & Gilkey (1954) to be the result of ionic diffusions into the crystal along discontinuity surfaces developed during unmixing. It occurred as the result of prolonged high ternperil-tures in the presence of water vapour and a supply of iron from the original rock; conditions which are fulfilled in

the coronites of the present area. The replacement of plagioclase by the outer gamet coronas would absorb

all the liberated iron and magnesium and thin-section observations indicate that clouding did not increase at this stage.

Since olivine is the only primary fermmagnesian mineral ·which shows

significant intemal replacement, it is suggested that iron and magnesium released from its conversion to brQnzite, diffused along grain boundaries to assist in the formation of biotite. and horobiende coronas at iron orefplagio­clase interfaces. This process explains why the primary iron ore has generally not been eroded and suggests that this interface provided a site of high chemical activity which eventually produced the best-developed coronas. The low potash contents of these rocks (Table 2) account for the poor development of the inner .biotite. growths.

The diffusion of ions in an intergranular film also explains the sporadic development of horobiende along grain boundaries of adjacent plagioclase laths and as inclusions within the feldspar where the network of intemal dislocations was penetrated. Both the intergranular and included horn­blende may form clear rims in the plagioclase similar to the clear margins against amphibole coronas. This phenomenon is discussed below. Small spinel inclusions in the feldspar are considered to have developed by similar processes.

The replacement of labradorite by actinolite or actiriolitic homblende would have released some excess alumina, which probably eaused the for­mation of spineJ as a symplectic intergrowth. When this corona was replaced by homblende, more alumina could be accommodated and the spineJ vermicules tended to be resorbed.

Plagioclase developed clear rims (often zoned) against coronas and against adjacent feldspar. The more sodic nature of the rims relative to the unaltered cores results from the instability of the primary igneous minerals, which in

metamorphic terms, formed assemblages representative of higher facies con­ditions than those prevailing during corona growth.

The variability of the marginal zoning, or the complete lack of it, probably

reflects local and often very marked differences in metamorphic conditions on a microscopic scale, resulting from variable concentrations of intergranular

fluids. The calcium liberated is thought to have been absorbed into these fluids

and used in corona growths (particularly of amphibole). Dust and inclusions

in these peripheral regions may have been partly resorbed into growing

IlASIC PLUTONJC JNTRUSJONS 429

coronas or redistributed in the plagioclase cores. The recrystallised rims did

not presurnably contain the network of dislocations developed in the cores and

were not therefore clouded. The removal of calcium from the plagioclase was largely a function of the

stability of the feldspar in its own very localised metamorphic environment, but may also have depended on the extent of corona growth in a given rock.

The variation of metamorphic environments on a microscopic scale and the state of overall disequilibrium in these rocks is very evident from plagioclase alterations in the coronite stage and the early phases of amphibolit­isation. All the ferromagnesian minerals have changed to homblende in

some thin sections, but whereas some plagioclase has recrystallised to granoblastic andesine, some has changed to stubby laths of andesine­labradorile composition and some rnay remain as clouded labradorile laths with wide, altered rims and lobate, sutured margins against adjacent feldsp:u.

Corona growths involved the replacement of plagioclase by amphibole, but the onset of amphibolitisation is marked by the intemal replacement of both primary and secondary pyroxene by homblende. This process tended to reduce the MgO contents of some rocks, and the expulsion of magnesium may have been partly responsible for metasornatism around the hyperites.

Amphibolites formed by the introduction of considerable amounts of extraneous water, whereas the coronites formed in an essentiaiiy closed sy­stem. It has already been suggested that the poorly-developed homblende coronas around primary pyroxene probably indicated that pyroxene-plagio­clase was almost a stable assemblage (i.e. that the coronites formed under localised Homblende-Granulite rather than Upper Amphibolite facies con­ditions). The amphibolite margins of the hyperite bodies represent equilibrium assemblages produced under the regional Upper Amphibolite facies meta­morphism, and the coronite cores would have completely retrogressed to

stable amphibolite if sufficient water had permeated them. Corona growth may not have resulted from the intense metamorphism

which formed amphibolite, but could have been eaused by prolonged, elevated

PT conditions deep in the crust, between major metamorphic episodes. In this respect it is interesting that late-stage, olivine-free gabbros exhibit very poor coronas but frequently show heavy amphibolitisation. lt must be remembered, however, that in these rocks no iron and magnesium for corona growth could be derived from the conversion of olivine to bronzite.

At this point it is informative to consicler the intrusions in a regional con text.

In the Risör-Sändeled district the coronites show an igneous differentiation series, over the area as a whole and an extended period of emplacement.

Olivine-free gabbros were late-stage and there is a little evidence to suggest that noritic rocks may have preeecled gabbroic lithologies. These features could have resulted from intrusion of differentiated fractions over a period of time from a large, underlying mass of magma (probably of olivine gabbro

composition).

430 IAN H. STARMER

Bugge's survey of the Koilgsberg-Bamble Series (Bugge 1943) indicated the existence of similar 'hyperites' from Brevik to Arendal and supports the idea of intrusion on a regional scale. More intensive studies have shown coronites similar to those of the present area in many of these borlies (e.g. Brögger 1934, Reynolds & Fredtickson 1962, Batey 1965, Ryan 1966, Prode­

sen 1968a & b, and Rodwell 1968). Within this region of South Norway, Smithson (1965) demonstrated a large,

positive gravity anomaly which increased markedly towards the coast. This was attributed to a thickening wedge of supracrustals which were denser than the migmatites presurned to underlie them. lt seems possible that this anomaly could be partly due to large masses of sub-surface basic rock related to the exposed hyperites.

Although scapolitisation is restricted to certain masses, it is often very intensive and must have involved large quautities of chlorine-rich, pneumotoly­

tic fluids. In the author's opinion these are unlikely to have emanated from the individual hyperite borlies which bad consolidated prior to corona growth. The fluids could have been derived from a large underlying mass of magma, and this would explain an extended period of scapolitisation.

ACKNOWLEDGEMENTS

The author would like to thank Professor The Lord Energlyn and the staff of the Department of Geology, University of Nottingham, for the use of facilities and help in the preparation of material.

Special thanks are extended to the people of the Risör-Söndeled area

for their help and warm friendship during the author's fieldwork.

8th Apri/1969

REFERENCES

Dept. of Geo/ogy, University of Nottingham,

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Present address:

Dept. of Geology, Queen Mary College,

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BASIC PLUTONIC JNTRUSIONS 431

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