GEOCHEMISTRY AND METAMORPHISM OF DOLERITE DIKES …

GEOCHEMISTRY AND METAMORPHISM

OF DOLERITE DIKES FROM AUSTVÅGØY

IN LOFOTEN

SURYA N. MISRA & WILLIAM L. GRIFFIN

Misra, S. N. & Griffin, W. L. 1972: Geochemistry and metamorphism of dolerite dikes from AustvågØy in Lofoten. Norsk Geologisk Tidsskrift, Vol. 52, pp. 409-425. Oslo 1972.

A dolerite swarm was intruded into the deep-seated metamorphic and igneous rocks of the Lofoten-Vesterålen area late in the Precambrian tectonic cycle. The dolerites are localized along E-W shear zones and were intruded before movement on the shears had ceased. The interplay of cooling, shearing and hydration has produced rocks ranging from coronite dolerites through granulites to epidote and garnet amphibolites. The dolerites are alkali basaltic in composition, but have abnormally high K/Rb ratios. The metamorphism has been essentially isochemical, except for an apparent leaching of K which led to lower, more normal K/Rb ratios in the amphibolites. There is some indication that Rb and Sr have been separated during the retrograde metamorphism. Comparison of major element compositions shows that the Lofoten dolerites most closely resemble alkali basalts of the continental margins, and that they are similar in this respect to several other Scandinavian dolerite occurrences.

S. N. Misra, Mineralogisk-geologisk Museum, Sars gate l, Oslo 5, Norway.

Present address: C-12, Vanivihar Campus, Bhubaneswar-4, India.

W. L. Griflin, Mineralogisk-geologisk Museum, Sars gate l, Oslo 5, Norway.

Present address: Institutt for geologi, Universitetet i Oslo, Blindern, Oslo 3, Norway.

The Lofoten-Vesterålen island group in north Norway is underlain by a

complex of deep-seated metamorphic and igneous rocks. Part of this terrain was described by Heier (1960) and coordinated mapping of the entire area is now in progress. The oldest rocks are polymetamorphic gneisses, at least some of which are obviously metasediments (marhles, quartzites, iron for

mations and graphite schists). The entire gneiss complex has been tightly folded and subjected to granulite-facies conditions, but retrogression to amphibolite-facies assemblages is widespread, especially in the northern part of Lofoten. Some aspects of the metamorphism were discussed by Griffin & Heier (1969). The gneisses are intruded by two groups of mangeritic to

charnockitic igneous rocks, which are separated in time by a period of iso

clinal folding.

One of the last stages of tectonic activity in Lofoten was the development

of numerous E-W shear zones. They are characterized by intense foliation,

relatively narrow width (1-30 meters) and complete retrogression of gra

nulites and mangerites to mica ( ± garnet) gneisses. Despite this obvious deformation, however, the maximum offset so far proven along any of the

shears is about 20 meters.

410 S. N. MISRA & W. L. GRIFFIN

These shears have served as loci of intrusion for a swarm of dolerite dikes

ranging from a few centimeters to about 30 meters in width. The margins,

where unaltered, are typically chilled to fine-grained dolerite, whereas the

centers of the thickest dikes have gabbroic textures. The margins are gener

ally planar, parallel to the shearing direction, and apophyses of dolerite are

rare. The dolerites were clearly intruded while the shears were still active.

Cases are observed in which foliated rocks are intruded by completely mas

sive dolerite, but more commonly dolerites themselves are sheared parallel to

their strike, so that hydrous zones occur at the margins andfor irregularly

within the dikes. In same cases movement along the shears has reduced the

dolerites to layers of amphibolite, and the thinnest of these layers may be

deformed and pulled apart. Thus a gradation is present in the field from

apparently fresh dolerites, through recrystallized garnetiferous rocks to

strongly foliated amphibolites.

Several of the dikes have been net-veined by material melted out of the

country rocks by the heat from the dolerite intrusion. RbfSr dating of this

remobilized material from several dikes has given an isochron age of

1795 ± 20 m.y. for the intrusion of the dikes (Heier, pers. comm.). By

comparison with other published (Heier & Compston 1969) and unpublished

(Heier, pers. comm.) data, this isochron age suggests that the dolerite intru

sion followed very closely after the intrusion of the second series of man

geritic rocks. This age is also similar to that inferred for late-tectonic dale

rite intrusion in the Kristiansund area of western Norway (Pidgeon, pers.

comm.).

This paper is a study of a suite of dolerites and metadolerites collected

from the western half of AustvågØy in Lofoten. After a brief petrographic

description, their chemistry is discussed in detail and compared quantita

tively to that of other basaltic rocks.

Petrography

The textural variation observed in the field corresponds to a mineralogical

change from dolerites with a variety of corona structures, through pyroxene

and garnet-amphibolites, to epidote amphibolites. Many of the rocks are

dominated by disequilibrium textures reflecting progressive hydration with

declining temperature. For convenience we will describe the rocks in groups,

but all gradations between groups are present, aften within a single dike.

Dolerites: The dolerites, where not chilled, have subophitic textures, with

subhedral to euhedral plagioclase, olivine and apatite, surrounded by inter

stitial clinopyroxene, orthopyroxene and amphibole. Ilmenite-magnetite is

abundant, and the opaque grains are typically surrounded by rosettes of

dark red biotite. This biotite rim is certainly a late-magmatic feature, as it is

aften intergrown with, and occasionally surrounded by, the interstitial clino

pyroxene. Interstitial spaces are locally filled by a pyroxene-plagioclase-

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biotite symplectite. The primary hornblende is dark green-brown and often

heavily pigmented; the primary clinopyroxene is often nearly opaque with

black pigment. The plagioclase is labradorite, and commonly is full of dusty inclusions which give it a brown color.

The chilled margins generally have longer, thinner feldspar laths, more

abundant ore and biotite and less pyroxene and olivine than the cores of

the dikes, though the bulk composition does not differ significantly (Table 1).

The pyroxenes are stubby and not usually subophitic, and olivine may be completely oxidized to ore. Apatite may occur as abundant thin needles rather than as blocky prisms.

Even the freshest dolerites show development of thin coronas around the

olivine grains. The coronas consist of an inner orthopyroxene zone and an

outer zone of fibrous clinopyroxene intergrown with spinel. The corona

development is to be expected during cooling of the dolerites as olivine

reacts with plagioclase (Griffin & Heier 1969, Griffin 1971). In troctolites

further west in Lofoten a second reaction produces a garnet corona at the

expense of the clinopyroxene corona (Griffin & Heier 1969) but garnet was not observed to form in this way in the dolerites studied here. Garnet does,

however, form at an early stage in the retrogression of the dolerites, by

breakdown of the biotite to give a new, finer-grained biotite and garnet. This

process proceeds concurrently with recrystallization and amphibolitization

of the clinopyroxene, so that biotite is surrounded by garnet and pyroxenes

by hornblende intergrown with quartz. At this stage olivine in some rocks

is altered to a mass of granular ore and an unidentified high-birefringence

material.

Hornblende granulites: The rocks so designated contain two pyroxenes as well as hydrous mafic minerals. In some cases the pyroxenes and the hydrous minerals are obviously in disequilibrium and the rock retains a relict doleritic texture in which laths of plagioclase separate rounded sievelike masses of pyroxenes and dark green hornblende. Garnet often forms a 'fishnet' texture outlining the plagioclase laths. In some rocks, however, we find equigranular to well-foliated mosaics of two pyroxenes, hornblende,

biotite and plagioclase, which, except for their cross-cutting field relations,

are indistinguishable from mafic layers in the granulite-facies country rocks.

Amphibolites: Rocks containing the association clinopyroxene-hornblende

biotite-plagioclase appear to have formed by hydration of both coronites and

hornblende granulites. Blue-green hornblende is clearly in disequilibrium

with pyroxene in all cases, producing ragged masses of amphibole and bio

tite heavily sieved with plagioclase and quartz as well as with opaque

minerals. The elimination of pyroxene leads to amphibolites with or without garnet as an essential mineral. Much of the garnet appears to have formed

at the expense of biotite and ore, or from alteration of pyroxene in the

dolerites; recrystallization produces euhedra with abundant inclusions in

GEOCHEMISTRY AND METAMORPHISM OF DOLERITE DIKES 413

their cores. Some amphibolites have recrystallized to equigranular mosaics

with moderately well-developed biotite foliation, but many have the same

sort of ragged textures seen in the pyroxene amphibolites. Opaque grains are irregular, poikiloblastic and often rimmed with sphene.

Epidote appears only in a few samples and appears to be a late development. It tends to form long curved prisms which grow across and enclose all of the other minerals. In one specimen the epidote grows as irregular rims

on masses of garnet. The petrographic study suggests that the retrograde metamorphism of the

dolerites followed several different paths. The coronites represent dolerites

which have cooled without tectonic disturbance and without an appreciable

introduction of water. Judging from the chilled margins, the thinness of the coronas and the lack of garnet in many of them, the cooling may have been relatively rapid. In cases where the reactions were hastened and recrystallization induced by shearing of the dry rocks, hornblende granulites with varying textures have been produced. In most cases, however, the introduction of water has apparently occurred at some stage during the cooling-shearing process. The interplay between cooling, shearing and hydration can easily

explain the range of mineralogy and textures found in the amphibolitic meta

dolerites.

Chemistry

The precision and accuracy of the analytical methods used were checked

by running all analyses in duplicate and calculating the pooled mean variance

Table 2. Analytical methods and precision of the data.

Dolerite W1 W1 AGV- 1 AGV-1

Oxide Method So2 This Recom. This Recom. average

work va1ue work va1ue

Si� XRF 47.19 0.213 52.27 52.64 58.02 58.97

A1203 XRF 14.53 0.015 14.62 14.85 16.66 17.01

Ti02 XRF 2.85 o 1.10 1.07 1.09 1.08

FC203 XRF 5.04 0.006 10.80 11.09 6.89 6.80

FeO Vol.* 10.02

Mn O XRF 0.22 o 0.16 0.17 0.09 0.09

MgO XRF 5.67 0.028 6.55 6.62 2.05 1.49

Ca O XRF 7.49 0.001 10.95 10.96 4.92 4.98

Na20 FP 3.75 2.15 4.33

K20 XRF 2.09 o 0.54 0.64 2.82 2.89

P205 XRF 1.82 o 0.22 0.14 0.44 0.48

Rb XRF 22 0.451 20 22 69 67

Sr XRF 670 6.465 180 180 658 657

Zr XRF 210 6.538 91 100 227

XRF = X-ray fluorescence. Vol. = volumetric. FP = Flame photometer. * Mean of the two burette readings was taken for calculation. S02 = Pooled mean variance for all analyses done in duplicate. The standard deviation of replicate analyses for Sr = 0.894 and for Rb = 3.714.

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3

.60

2.5

4

1.2

9

1.9

1

1.2

0

99

.12

1

00

.51

21

.07

7

.73

20

.57

3

1.2

9

12

.75

2

1.9

4

6.5

5

0.9

0

--

10

.74

1

2.3

2

13

.91

1

5.7

8

6.0

6

4.3

5

4.3

4

3.1

4

4.0

0

2.5

5

20

1

9

64

2

31

9

19

9

12

3

81

0

56

4

.03

1

.060

.72

.7

5

22

1

9

�

....

� Sil ;z: � - Vl

� P.a � i' o � - '"r1 '"r1 z

�


10r---------------------------------------�

•

•

•

; 5 � o z

o 4�0��4�5��5�0��5�5��60

- sio2 Fig. l. Si� vs. Na20 + K20. Boundary lines separating the fields for tholeiitic basalts, high alumina basalts and alkali basalts are from Kuno (1968).

of the results, and by re-analyzing two international rock standards (Table 2). Ferrous iron was determined by standard potassium dichromate titration. Na20 was determined with a Beckman flame photometer. All other elements were determined with a Philips manual X-ray fluorescence spectrograph using a dilution technique for major element analysis and the mass absorption technique of Norrish & Chappel (1966) for the trace elements. The

trace elements were determined on pressed powders whereas Na-tetraborate 9:1 fused pellets were used for the major elements.

The major element data and the normative compositions are given in

Table 3. The major element compositions of the samples are very similar

regardless of degree of metamorphism. The chemistry of the freshest rocks

compares well with the modal compositions, olivine being present in rocks

with a high MgO content. The high P205 in these rocks reflects the abund

ance of apatite in the modes. TiG.?, K20 and MgO show large dispersions,

relative to the mean deviations of other elements. The dolerites, together

with a gabbro (M275b) from the same area, are plotted on Na20 + K20 VS.


20 19 18 17 16

o 15 ;f 14

t 13 12 11

20 19 18 17 16

o 15 _ .. oc( 14

t 13 12 11

20 19 18 17 16

.. 15 o

4. .. 14

t 13 12 11

•

•

•

2 3 4

• •

2 3

2 3 4

•

• • • • • •

5

•

5

•

•

5 - Na20+K20

6

6

•

6

• •

5 i 02 • 45.00-4 7.50

7 8 9

•

7 8 9

•

Si02 • 5Q01-5250

7 8 9

Fig. 2. Alkali-alumina diagram; boundary lines from Kuno (1968) separate fields for

tholeiite, high alumina and alkali basalts.

Si� (Fig. l) and Na20 + K20 VS. Al20a (Fig. 2) diagrams as suggested by

Kuno (1960). All the rocks fall in the alkali basalt field in the two diagrams.

The dolerites are mostly 01 and Hy normative with a few instances of Ne


together with 01 in the norm. In the AFM diagram (Fig. 3) the rocks plot in

the field above the mugearite-trachyandesite boundary (Uchimuzu 1966) and

belong to the basalt-mugearite-trachyte series. In this respect they are similar

to the rocks of Scotland, Hawaii and Sidara (Kuno 1968). Fig. 4 shows variations in the mcide content in relation to the solidification

Index (SI= MgO X 100/MgO + FeO + F�03 + K20 + Na20). The vari

ation trends in general agree with the trends observed in other alkaline rock suites. The trend of TiO<.J shows the maximum at SI = 20 noted in alkaline

rocks by Kuno (1968). There is a general increase in alumina content above SI = 22; CaO increases systematically with increasing SI and �O decreases.

The data for the trace elements are given in Table 3 together with some

representative geochemical ratios. The arithmetic mean of 648 ppm Sr in the Lofoten dolerites is slightly lower than the average of 77 4 ppm for alkali basalts (Prinz 1968). The 27 ppm average rubidium content compares well with the estimated arithmetic mean of 18-33 ppm for basaltic rocks (Prinz

1968). The RbjSr ratios mostly fall in or near the normal basaltic range (Fig. 5) and the mean RbfSr ratio of 0.05 is identical to the average for

FeO

•

• •

14;

�· •

Fig. 3. AFM diagram with the boundary for the trachyandesite-mugearite fields (Uchimuzu 1%6).


� o

� o

. <>

00

o

<SID

o

.

o·

o

o

o

c

o

O I

o

0-

o

o

o

o o o

O C11D

o

o

o

o

o o

o o

a::>O

CD<IID

o

o

o

o

o

Ca O

o p o o

( 00 o

= Oa:J

CIE> o tiD

o

o

o

o

Mg O w . "' <> ...

.

o .

<Il ..

o CC . <D

O O <IX> O -

o .

o .

.

o .

Ti� o ... .., w o

o o

o .

o o o o . o 00 . o .

l l

..,., l ... _

! o o .

o o .

�

o l. !

o o :,. '

l

Fig. 4. Variation diagram showing the relations of various oxides to the Solidification lndex (SI= MgO X 100/MgO + FeO + F1:203 + Na20 + K20).

alkali basalts estimated by Prinz (1968). It is noteworthy, however, that most

of the scatter in Fig. 5 is due to the metadolerites, suggesting differentiation

of Rb and Sr during the retrograde metamorphism.


All but one of the fresh dolerites have K/Rb > 800, which is abnorrnally

high for basaltic rocks of these high K contents (Fig. 6). The Rb analyses do not appear to be biased, certainly not by the arnount necessary to cause this anomaly (Table 2). The normal RbjSr ratios also suggest that the Rb

values are reliable. Fig. 6 demonstrates that the KjRb ratio decreases dramatically as hydration and retrograde metamorphism proceed; only two of the metadolerites have retained high KjRb. This decrease is not due to any significant overall rise in Rb content, but to an apparent depletion in K.

Heier & Thoresen (1971) and Heier & Brunfelt (1970) used a large

sarnple of gneisses and intrusive rocks from Lofoten-Vesterålen to demon

strate that elements such as Rb, U, Th, and Cs are selectively removed during prograde metamorphism from amphibolite to granulite facies. Heier

& Thoresen (1971) further demonstrated that regional retrograde meta

morphism of the granulites did not modify the high K/Rb, KJV and KjTh ratios produced in the prograde metarnorphism. The data in Table 3 and Fig. 6 agree with those of Heier & Thoresen in that there has been no de-

0·2

Rbtsr

0·1

Antarctic and Tasmanicin Tholeiit es

o

o

/Sub

marine

/

Tholeiites

20

-----------�Continental, Island -are Island Tholeiites; Alkali Basalts

40

Rb(ppm) 60 80

Fig. 5. Rb/Sr vs. Rb diagram; boundary lines from Condie & Barsky (1969). Open circles are metadolerites; filled circles are 'fresh' dolerites.


1000

800

KJRb 600

400

250

0·01

Basaltic Achondrite

Continental Tholeiites....___

Antarctic and Tasmanian

0·1 Tholeiites\

o

1·0 %K

•

.,

2

• •

3

Fig. 6. K/Rb vs. K diagram; boundary lines from Condie & Barsky (1%9). Open circles are metadolerites; filled circles are 'fresh' dolerites.

monstrable introduction of Rb during the retrograde metamorphism of the dolerites. The apparent leaching of K is probably related to special meta

somatic processes operating within the shear zones where the dikes were

intruded. The average value of 208 ppm zirconium is relatively high (130 ppm

average for alkali basalts, Prinz 1968). The zirconium values show a strong negative correlation with the iron enrichment index.

In order to study compositional similarities with basalts from other areas, the method proposed by Ragland et al. (1969) has been adopted. The analyses are recalculated as

a= x-y

and a2 = (x-y)2

y y

where x = wt % oxide in the comparison basalts and y = wt % oxide in the

Lofoten dolerite. The cumulative total for all oxides gives two values for

each comparison basalt and a rank is assigned for each value. The combined

rank orderings provide a comparison with the Lofoten dolerite composition.

A low value in the rank ordering indicates close similarity of the two rock

types.

It has been noted by Leeman & Rogers (1970) that in such a comparison

differences in FeO, Ti(h, Na20, and K20 exercise a strong influence on the

rank orderings. This influence is supposed to be advantageous in that FeO


Table 4. Average and range of composition for dolerites used in the comparison with representative basalts.

Oxide 2 3 4

Si<>2 47.11 47.19 45.56-51.76 44.Q0-51.76 AI203 14.57 14.53 13.44-15.22 13.44-18.44 Ti<>2 2.82 2.85 2.46- 3.03 0.89- 3.28

FejJ03 4.75 5.04 3.62- 6.17 1.62- 8.30 FeO 10.63 10.02 7.01-12.43 6.83-12.43 Mn O 0.22 0.22 0.20-- 0.25 0.17- 0.27

MgO 5.83 5.67 3.96- 6.31 3.96- 6.48

Ca O 7.90 7.49 6.01- 7.95 6.01-10.00

Na20 3.21 3.75 2.96- 4.56 1.96- 5.01

K20 1.82 2.09 1.64- 3.56 0.68- 3.56

P203 1.67 1.82 1.25- 2.42 0.32- 2.75

l. Average composition of the two chilled margin rocks used for comparison after recalculation.

2. Average of the fresh dolerites. 3. Range of chemical composition included in the average of column 2. 4. Range of the chemical composition of rocks from AustvågØy which have been

analysed for the present study.

Table 5. Chemical composition of comparison basalts.

No. a Si<>2 AI203 Ti<>2 *FeO MgO Ca O Na20 K20 b

l 2 49.94 17.25 1.51 8.71 7.28 11.86 2.76 0.34 10

2 3 49.36 13.94 2.50 11.89 8.44 10.30 2.13 0.38 181

3 6 46.61 15.47 2.94 11.12 7.32 11.32 2.89 1.06 8

4 7 52.3 14.5 1.1 13.62 5.1 10.0 2.1 0.4 11

5 9 49.15 13.23 2.87 13.90 5.50 9.79 2.87 0.49 lO

6 11 46.0 18.5 1.4 10.99 5.4 11.9 2.6 1.6 l

7 13 47.26 14.91 2.48 11.87 8.05 9.98 3.13 1.20 5 8 14 48.18 16.10 2.31 14.13 3.90 9.89 3.49 1.56 19

9 18 48.11 15.55 1.72 9.88 9.31 10.43 2.85 1.13 7

lO 19 46.87 13.98 2.72 11.95 9.82 10.47 2.84 0.68 7

11 20 47.04 14.82 3.05 12.20 8.29 1D.40 2.96 0.85 28 12 21 48.16 18.31 2.91 9.71 4.87 8.79 4.05 1.69 lO

13 22 46.54 15.36 2.23 11.35 9.12 9.54 3.46 1.27 7 14 23 43.1 13.1 4.1 13.45 9.0 12.4 2.7 1.6 3 15 24 46.7 17.3 3.6 10.52 4.7 9.7 4.1 3.0 10 16 25 47.7 15.2 3.2 10.77 9.7 8.9 2.7 1.6 2

17 30 50.35 13.55 2.51 14.05 5.27 10.42 2.57 0.64 15 18 38 52.33 14.91 1.35 10.44 7.39 10.12 2.08 0.83 l

19 2 47.2 15.8 2.5 12.2 7.7 10.5 2.7 0.7 9

20 4 49.3 13.9 2.9 14.5 4.0 7.7 3.6 2.0 4

21 6 47.6 15.8 2.3 13.6 6.8 9.3 3.3 0.5 6

22 7 49.1 16.6 2.9 11.2 5.7 8.3 3.5 1.6 8

23 8 47.6 16.3 3.6 11.7 6.3 8.7 3.8 1.1 4

24 9 48.4 13.3 4.3 12.0 8.2 7.7 3.5 1.6 5

25 13 49.5 15.4 1.8 9.8 7.6 9.7 3.6 1.5 36

Data from compilations. Analyses 1-18 are from Ragland et al. 1969 (Tab le 5, p. 72) and 19-25 are from Leeman & Rogers 1970 (Table 5, p. 15). * Total iron as FeO. a: Refers to the original Table as cited above. b: Number of analyses involved in calculating the average.


Table 6. List of comparison basalts with rank ordering.

No. b* a Rank a2 Rank Sum

20 Basalts from Craters of the Moon, Idaho 0.70 2 0.79 3

2 22 Carboniferous-Permian alkali-olivine basalt of Scotland 0.66 1.30 2 3

3 8 Basalt from the inner district of South Island, New Zealand 1.15 3 1.47 3 6

4 23 Alkali basalt from St. Helena 1.33 5 1.60 4 9

5 21 Basalt of Yellowstone area 1.43 7 1.65 5 12

6 5 Thingmuli volcano, Iceland 1.35 6 1.82 6 12

7 7 Basalt from peripheral dt. of South Island, New Zealand 1.35 6 2.28 8 14

8 12 Pacific alkali basalt 1.23 4 3.24 12 16

9 24 Alkali basalt from Tutuila 1.46 8 2.48 9 17

lO 17 lndian Deccan basalt 1.58 11 2. 08 7 18

11 3 Azores plg. & ol-basalt 1.57 lO 3.27 13 23

12 11 Hawaiian alkali-o!. basalt 1.61 13 2.83 11 24

13 19 Snake river basalt, Idaho 1.80 15 2.81 10 25

14 16 Gough Island basalt 1.50 9 3.98 17 26

15 25 Basin range alk-ol basalt 1.60 12 3.29 14 26

16 13 Pribilof Island alk. basalt 1.64 14 3.36 15 29

17 4 Izu peninsula, Japan, basalt 2.34 22 3.80 16 38

18 15 Tristan da Cunha trachybasalt 2.13 18 4.66 20 38

19 10 Hawaiian alkali-o!. basalt 2.04 17 4.88 21 38

20 2 Hawaiian ol tholeiite 2.33 21 4.50 18 39

21 6 Bogoslof Island basalt 1.% 16 5.47 23 39 22 18 Chillzone of Palisades sill 2.37 23 4.59 19 42 23 9 Japanese al-ol. basalt 2.22 20 5.46 22 42 24 14 Tristan da Cunha alk-basalt 2.13 19 6.48 27 46

25 l Pacific and Atlantic tholeiite 2.89 27 7.67 29 56

*b refers to the analysis number in Table 7.

Table 7. Analyses of rocks from Norway and Sweden used for comparison.

Si� Al20:l Ti� FeO MgO Ca O Na20 K20.

l 48.45 14.48 3.36 13.53 6.22 9.07 2.85 1.12 2 51.66 16.59 2.31 10.62 4.41 7.35 4.09 2.20 3 49.98 14.48 1.32 11.13 7.34 11.62 2.75 0.20 4 50.36 17.09 1.85 13.87 5.04 7.31 3.50 0.61 5 46.71 17.44 1.93 14.47 6.63 8.16 2.75 1.22 6 47.22 13.46 3.26 16.57 5.30 9.38 2.75 1.63 7 50.18 16.72 1.63 11.26 6.52 9.38 2.75 1.32 8 51.80 13.69 3.09 14.39 4.42 7.93 2.78 1.13 9 48.24 15.30 2.55 14.75 6.42 8.87 2.55 1.22

lO 47.20 17.50 2.10 12.78 7.60 8.50 2.60 0.80 11 47.16 17.17 2.22 13.51 6.46 9.39 2.92 0.80 12 46.85 15.28 2.90 14.63 7.04 9.02 2.72 1.09 13 47.98 15.44 1.77 11.81 7.98 9.37 2.48 0.92

The analyses from 1-9 are from Table 9, p. 109 and lQ-11 are from Table 8, p. 108 of Gjelsvik (1950). Analysis 12 is from T. H. Green (unpub.) and 13 is from Griffin & Råheim (1972). The total iron is taken as FeO and the analyses are recalculated to 100 % on water free basis.


Table 8. Rank orderings for similar rocks from Norway and Sweden.

No. from

Name a2 Rank Sum Tab le a Rank

7

9 Asby diabase, Loos Hamra region 0.92 2 0.56 l 3

6 Hellefors dolerite 0.87 l 0.72 2 3

l Diabase dykes of southern coast of Norway 1.02 4 0.75 3 7

12 Dolerite from Vestvågøy 0.99 3 0.79 4 7

8 Konga diabase 1.06 5 1.18 5 10

5 Asby diabase, Nordingrå 1.19 6 1.21 6 12

11 Hyperite of Kongsberg-Bamble formation 1.43 7 1.62 7 14

4 tlje diabase 1.56 9 1.98 8 17

2 Oslo diabase 1.48 8 2.69 11 19

7 Breven dolerite 1.58 lO 2.41 10 20

13 Kristiansund dolerite 2.52 12 1.53 7 19

10 SunnmØre dolerite 1.72 11 2.30 9 20

3 Støren basalt 2.59 12 5.42 12 24

and the other components, though low in abundance in basaltic rocks, are

sensitive to processes of differentiation.

For purposes of comparison, the average of the two chilled margins (G57e

and M80d) was assumed to approximate the original composition of the

Lofoten dolerite magma. This is also equivalent to using the average com

position of the fresh dolerites (Table 4). All analyses have been calculated

to 100% (water free) and the total iron calculated as FeO, as the F�03jFeO

ratio may be affected by post-magmatic events and may thus introduce

spurious differences in the comparison. Table 5 gives the analyses of the

comparison basalts and the rank orderings are given in Table 6.

In the present study, comparison has been made with about 50 different

analyses of various kinds of basaltic rocks from different provinces. The

comparison was done on two levels: a world-wide comparison designed to test similarity to basalts from various tectonic environments (Table 5) and a comparison with other Scandinavian basalts to test for the presence of a

chemical province (Table 7). Only selected analyses of rocks are included in Tables V and VI.

On a world-wide scale, the Lofoten dolerites are most closely similar to the basalts from Craters of the Moon, Idaho (Leeman & Rogers 1970) and

the Carboniferous-Permian alkali-olivine basalts of Scotland (Tomkeieff

1937). It is apparent from Table 8 that the Lofoten dolerites are similar in

general to alkali basalts from continental margins and show little affinity

to the alkali basalts from oceanic islands. In particular, the Lofoten rocks

have lower contents of MgO and CaO, and higher contents of FeO than the

J apanese and Hawaiian alkali basalts. In the case of continental tholeiites,

e.g. Indian Deccan basalts (Sukheswala & Poldervaart 1958), the differences

in alkalis, alumina and lime are overshadowed by the similarity in Ti�,

FeO and MgO. Other continental tholeiites, such as the Palisades sill, con-


tain high TiG.! and MgO and low FeO with respect to the Lofoten dolerites.

A comparison with other Norwegian and Swedish dolerites and basalts shows a significantly dose similarity. The chemical composition of these

rocks is given in Table 7 and the rank orderings are given in Table 8. In a

grand rank ordering of these rocks together with other comparison basalts,

seven of the first eleven positions are occupied by the Norwegian and Swe

dish dolerites and basalts. This remarkably dose compositional similarity of

rocks from a wide geographic area within Norway and Sweden suggests the

existence of a unique chemical province of Norwegian and Swedish basaltic

rocks. Same of these comparison basalts (numbers 10, 12, 13) are known or

inferred to be similar in age to the Austvågøy dolerites (1800 m.y.), but others (2, 11) are certainly younger. The chemical similarities may indicate

uniformity in the mantle composition andfor similarity in the tectonic envi

ronment and rate of heat flow during the origin and emplacement of these

rocks.

The dose similarity of the Lofoten dolerite magmatic composition to

alkali basalts from continental margins also suggests the possibility of a similarity in the tectonic environments during emplacement of these rocks.

The distribution of alkali basalts in the Western United States, Japanese

islands and Eastem Australian regions correlates with high heat flow and

thin crust (Kuno 1959, Morgan 1968, Howard & Sass 1964, Leeman &

Rogers 1970). A similar tectonic environment may have existed in the

Lofoten islands during the emplacement of the Lofoten dolerites.

Acknowledgements. - The authors. wish to thank Professor K. S. Heier for critically reading the manuscript and for invaluable advice during the work. The writers benefited greatly from discussions with Dr. P. Weigand and Mrs. B. B. Jensen. Special thanks are due to the late Professor T. F. W. Barth for helpful discussions and his critical comments. The work was supported by financial grants from Norsk Utviklingshjelp, and from Norges Geologiske Undersøkelse.

January 1972

REFERENCES

Condie, K. C. & Barsky, C. K. l%9: Geochemistry of Precambrian diabase dikes from Wyoming. Geochim. et Cosmochim. Acta 33, 137-138.

Gjelsvik, T. 1950: Metamorphosed dolerites in the gneiss area of Sunnmøre on the west coast of Southern Norway. Norsk geo[. tidsskr. 30, 33-134.

Griffin, W. L. 1971: Genesis of coronas in anorthosites of the Upper Jotun Nappe, Indre Sogn, Norway. Jour. Petrology 12, 219-243.

Griffin, W. L. & Heier, K. S. 1969: Paragenesis of garnet in granulite facies rocks, Lofoten-Vesteraalen, Norway. Contr. Mineral. and Petro[. 23, 89-116.

Heier, K. S. & Compston, W. l%9: Interpretation of Rb-Sr age patterns in highgrade metamorphic rocks, North Norway. Norsk geo[. tidsskr. 49, 257-283.

Heier, K. S. 1960: Petrology and geochemistry of highgrade metamorphic and igneous rocks on LangØy, Northern Norway. Norges geo[. undersØkelse 207.

Heier, K. S. & Brunfelt, A. O. 1970: Concentration of Cs in high grade metamorphic rocks. Earth Planet. Sei. Let. 9, 416--420.

Heier, K. S. & Thoresen, K. 1971: Geochemistry of high grade metamorphic rocks, Lofoten-Vesteraalen, North Norway. Geochim. et Cosmochim. Acta 35, 89-99.

Howard, L. E. & Sass, J. H. 1964: Terrestrial heat flow in Australia. Jour. Geophys. Res. 69, 1617-1926.


Kuno, H. 1959: Origin of Cenozoic petrographic provinces of Japan and surrounding areas. Bull. volcanologique 20, 37-76.

Kuno, H. 1960: High alumina basalts. Jour. Petrology l, 121-145.

Kuno, H. 1968: Differentiation of basaltic magmas. In Hess, H. H. & Poldervaart, A. (Eds.): Basalts. John Wiley and Sons Inc.

Leeman, W. P. & Rogers, J. J. W. 1970: Late Cenozoic alkali-olivine basalts. Contr. Mineral. Petro[. 25, 1-24.

Morgan, W. R. 1968: The geology and petrology of Cenozoic basaltic rocks in the Cocktown Area, North Queensland. Jour. Geol. Soc. Australia 15, 65-78.

Norrish, K. & Chappel, B. W. 1966: X-ray fluorescence spectrography. In J. Zussman (Ed.): Physical Methods in Determinative Mineralogy. Academic Press, London.

Prinz, M. 1968: Geochemistry of basaltic rocks. In Hess, H. H. & Poldervaart, A. (Eds.): Basalts. John Wiley and Sons Inc.

Ragland, P. C., Rogers, J. J. W., Justus, P. S. 1969: Origin and differentiation of Triassic dolerite magmas North Carolina, U.S.A. Contr. Mineral. and Petro/. 20, 57-80.

Sukheswala, R. N. & Poldervaart, A. 1958: Deccan basalts of the Bombay area, India. Bull. Geol. Soc. America 69, 1475-1494.

Tomkeieff, S. l. 1937: Petrochemistry of the Scottish Carboniferous-Permian igneous rocks. Bull. volcanologique 2, 59-87.

Uchimuzu, M. 1966: Geology and petrology of alkali rocks of Dogo, Oki islands. Univ. Tokyo Jour. Fac. Sei. Ser. Il, 16, 85-159.

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