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 tec- tonic cycle. e dolerites are localized along E-W shear zones and were intruded before movement on the shears had ceased. e 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 amphi- bolites. 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 chaockitic 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) eisses. Despite this obvious de- formation, however, the maximum offset so far proven along any of the shears is about 20 meters.
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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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412 S. N. MISRA & W. L. GRIFFIN
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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8.8
7
10
.00
9
.31
7
.65
3.7
0
3.7
3
3.1
0
3.5
4
0.6
8
0.9
5
1.1
3
2.2
6
0.3
9
0.3
7
0.4
0
2.7
5
100
.17
1
00
.52
10
0.5
0
100
.40
4.0
2
5.4
2
6.7
4
13
.65
33
.28
2
5.7
4
23
.44
3
2.5
0
31
.50
2
8.3
5
27
.10
1
6.8
3
-3
.97
2
.81
-
6.4
7
--
5.3
2
8.0
3
13
.81
1
3.5
4
2.8
8
11
.82
1
6.2
9
18
.30
9
.41
3.7
9
4.1
9
4.7
6
8.8
7
0.2
8
1.4
8
2.4
6
4.6
7
0.8
2
0.7
5
0.8
4
5.8
8
11
2
9
36
2
8
63
5
93
3
64
7
55
4
23
5
33
9
21
4
19
0
51
3
27
2
26
1
67
0
.01
7
.03
1
.05
6
.05
1
.69
.6
2
.71
.7
4
24
2
9
23
2
0
29
3
33
1
45
.56
4
6.7
4
3.1
1
2.2
2
14
.27
1
5.2
4
5.7
9
4.1
0
10
.10
1
1.7
1
0.2
2
0.2
7
5.8
8
5.1
4
7.7
8
9.0
0
1.%
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
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5
20
1
9
64
2
31
9
19
9
12
3
81
0
56
4
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9
�
....
� Sil ;z: � - Vl
� P.a � i' o � - '"r1 '"r1 z
�
GEOCHEMISTRY AND METAMORPHISM OF DOLERITE DIKES 415
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.
416 S. N. MISRA & W. L. GRIFFIN
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
GEOCHEMISTRY AND METAMORPHISM OF DOLERITE DIKES 417
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).
418 S. N. MISRA & W. L. GRIFFIN
� 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.
GEOCHEMISTRY AND METAMORPHISM OF DOLERITE DIKES 419
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.
420 S. N. MISRA & W. L. GRIFFIN
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
GEOCHEMISTRY AND METAMORPHISM OF DOLERITE DIKES 421
Table 4. Average and range of composition for dolerites used in the comparison with representative basalts.
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.
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.
422 S. N. MISRA & W. L. GRIFFIN
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
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.
GEOCHEMISTRY AND METAMORPHISM OF DOLERITE DIKES 423
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-
424 S. N. MISRA & W. L. GRIFFIN
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
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GEOCHEMISTRY AND METAMORPHISM OF DOLERITE DIKES 425
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