The Aln6 complex: tectonics of dvke emplacement
P. KRESTEN
L1THOS
Kresten, P. ,1980: The Aln6 complex: tectonic~ of dyke emplacement. Lithox 13, 153.-158. Osio.
ISSN 0024-4937.
The model of dyke emplacement proposed by v. Eckermann could not be confirmed by the present
study. A new model is suggested, involving up-doming of the wall-rock due to the ~atrusion of
magma (accompanied by the formation of radi~! dykes and two sets of cone ~heets), followed by
subsidence (formation of s6vite ring dykes and two other sets of cone sheets).
P. Kresten, Geological Survey of Sweden, Box 670, S-75128 Uppsala, Sweden.
A well4 ~own model for emplacement of the AIn6
dykes wa~ advanced by v. Eckermann (1942, 1948,
1958, 1966). He found that 'marie dykes" (aln6ites,
c,uachitites, tinguaites) define a radial dyke pattern
and carbonatite dykes a cone-sheet pattern. S6vite
forms the core of the intrusion (at the bottom of
Klingerfj~irden bay, see Fig. 1), and also ring dykes
within the main complex at Aln6 island. A projection of strike directions for radial dykes indicated a
centre situated about 1.5 km north of the northern
:~hore of Aln6 island (v. Eckermann 1948, 1958).
~, vertical pro.~ection of cone sheet dykes through
Ibis centre indicated the existence of a series of
explosion loci. The final version of the model is
(v. Eckermanr~ 1966):
(1) S6vi~e focus at I km depth; (2) Alvikite focus
at 2 km depth; (3) Focus of Mg-rich alvikites at
3.5 km depth; (4) Beforsite focus at 7-8 km depth;
(5) Two sets of radial dykes related to the 2 and
7-8 l~n foci.
Figures for the depth of the foci refer to the
pre~nt erosion surface. The amount of eroded
material since the time of formation was estimated
to be about 2 km (v. Eekermann 1948, 1966).
The object of the present study is to test v.
Eckenv, ann's model in the ligh~ of more recent
studies, toveriag more than 1000 dyke observations
in a much l~Lrger area (some 1000 kmZ). A brief
account has been given earlier (Kresten 1979).
General outlines of the geology
Magnetometric surveys of the area (Kresten 1976a,
1979) have indicated that the Alnfi inU~sion does
not continue across Klmgerfj/ird~m bay onto the
mainland north of AI~6 island, as had been
suggested by v. Eckennaan (1948). Consequently,
the existence of a 'central s6vite cone' (v. Eckemmnn
1948, 1966) could not be confirmed. At pre~ent, the
following centres of igneous activity are known
(Fig. 1):
(1) The main complex at Aln6 island (piutonic
alkaline rocks, sOvites).
(2) The Bfir~ng vent, at the western margin of the
main complex (s0vites).
(3) The Salskfir breccia, attached to the main
intrusion at its NNW boundary. So far, only
boulders of carbonatite breccia with fragments of
melilitite and abundant autoliths have been found;
boundaries in F:,g. 1 according to mal~aetometric
measurements (Kresten !979).
(4) The S6ff&er intrusion, on ,he mainland north
of Aln6 island (melil~te-rich plmonic rocks, s6vites).
In addition, a centre within the Avike Bay ~rea
has been sugg~tted (S6derstr6m 1966; K re.ten
1976b). More recent evidence has limber substantiated the existence of an Avike Bay centre (Kre-=ten
1979). Within the main complex a~ Aln6 is|and~ the
northernmost parts of the intrusion differ in several
respects from the remainder of the complex, as will
be discussed below. Therefore, the main intrusion
at Aln6 island could represent two sepa-rate intm.~ive
events.
The sOvites
The pattern of s6vite in~xusions is different in the
northern and southern parts of the main intrusion
at Aln6 island (Fig. 1, inset). In the north, s~vites
seem to form a ring dyke pattern, together with
melteigite, pyroxenit¢ and some ijo!ite. The :~ovite
ring dyke is between |00 and 200 metres wide.-,and
contains abundant fragments of alkaline rocks and
fenites. The southern boundary of the r~ng dTke is
characterized by a zone of rhcomorphic breccias,
containing fragments of various fenites and alkaline
154 P. Kresten
LITHOS 13 (1980)
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,~WKE BAY
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ALNO
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5 km
rocks in a highly variable matrix. Such breccias
have not been found in other parts of the complex.
The s6vite ring dyke is intruded both by younger
s6vites and by ijolite. Elsewhere in the complex,
s6vites always post-date the intrusion of ijolhes.
Radial off-shoots of the s6vite ring dyke are com-mon along the northern shore of Aln/5 island.
North of the s6vite ring dyke indicated in Fig. 1,
exposures are too scarce, and the magnetometric
map is too inconclusive, to pemait ~, more detailed
1 km
"...
.....
Fig. 1. Some prominent dykes
of the AIn6 area, in relation to
the known centra of plutonic/
volcanic activity. Inset: pattern
of s6vite dykes (exaggerated
thickness) within the main
complex (margins:dotted line).
discussion of the geology. However, large masses
of s6vite wi~h intercalated pyroxenite seem to be the
rnost abundant rock types there.
The remainder of the complex is occupied by
fenites with intrusions of pyroxenite (possibly a
fragmented ring dyke pattern), melteigite to ijolite,
and nepheline syenites. All the rocks are cross-cut
by s6vites, arranged in a complex pattern of
anostomosing veins and dykes (Fig. 1). The general
impression is that of a rather complicated ring dyke
like pattern, with several off-shoots. The s6vites of
the Bear/ing vent show a similar pattern, there
intruded into fenites only.
The raditaldyke pattern
Fig. 2. Poles to all (1097) dykes of the AIn6 area. Lower
hemisphere projectit,n. contour intervals ~--I-2-4%.
Radial dykes are rather common in the area, as
indicated in Fig. 2 (steeply dipping dykes with
highly variable strike dirc~:tions). A plot of extended
strike directions for radial dykes wider 'than 10 cm
frc,m the whole area (Fig. 6 in Kresten 1979) shows
sev,~ral maxima of constructed dyke intersections,
mainly in the vicinity of the main intrusion at Aln6
island. The emanation of all these dykes from one
distinct centre, as proposed by v. Eckermann (1948,
1958) could not be confirmed. Furthermore, steeply
dipping dykes in the whote area show two preferred
directions ofstrike, rougl" ly N-S, and E-W (Fig. 2).
These strike directions, which are also common to
diabase dykes substantially older than the Aln6
intrusion, most likely represent a chanell/ng of the
AinU5dykes into the pre-existing joint pattern of the
Aln~ complex 155
LITHOS t3 (1980)
i~
15
,10
.
.
.
5
0_
.
0
/.,I.IZO-EARBONATITES/
/////~
/ /
zo
is
ALVIKITES
\ I
'~-15 ~
s
o.
15
20
zo
\115
10
5
0
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OITES
1
5
is
lO
BEFORSITES
s
o
.o
15
Fig. 3. Vertical projections of cone sheet dykes of the AIn6 area. (Kms.)
wall-rock. Intrusions of Aln6 dykes into joint zone
accompanying diabase dykes, as well as within the
diabase dykes, are fairly common.
Taking into account the fact that several centres
of igneous activity occur in the area (Fig. 1), it seems
unlikely that radial dykes would have emanated
from one spot only. According to the model
presented below, radial dykes form during the
intrusive stage, in connection with the up-doming
of the wall rock. Necessarily, large intn~sions of
magma, like the main intrusion at Aln6 island,
will exel'clse much higher stress onto the wall-rock
than, for example the small intrusion at $OrAker.
Therefore, the radial dyke pattern around the Aln~
intrusion (as a whole) will be expected to predominate. It is also well substantiated that radial dyke
patterns are particularly sensitive to pre-existing
joint pat~.erns in the wall-rock, and to major
structural inhomogeneities (Johnson 1961, 1968).
The radial dykes are not 'mainly aln6itic' (v.
Eckermann 1948); all varieties of dyke rocks have
been found to occur as radial dykes.
The cone sheet pattern
The plot of the poles to all dykes registered ia the
area (Fig. 2) shows that steeply dipping dykes
(mainly radial dykes) prevail. Dykes dipping 10-75 °
are common, mainly in the southern pan: of the
plot. This could be interpreted as being in fe,vour
of v. Eckermann's cone sheet model: the areas
north, north-west, north-east and east of his centre
are covered by the sea, and thus no observations
can be made. A maximum for dykes with more or
less horizontal dips is also evident from the plot
(Fig. 2).
~n order to test the model of v. Eckermann, a
vel~ical plot of the cone sheet dykes had to be
constructed. The centre was chosen with the
156 F Kresten
20
LITHOS 13 (1980)
15
10
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1/
//
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,,
/
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,/
/
i[
DI I
/
/
/
/
//
//
//
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5
//
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0
0
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1Okra
10
I/
Fig. 4. T h e cone sheet pattern.
I
I
I
l
'
kill
co-ordinates 693000/! 58500 (Rikets Ndt); it lies ju st
off the northern coast of Aln6 island. I would like
to emphasize that the centre chosen is only a centime
for geometric projections, ,~d not a centre in the
meaning of v. Eckermann. All dips ~have been
corrected for deviations from calct~lated strike
directions (i.e, 90 ° to the vector between centre and
dyke outcrop). From the papers by v Eckermana
0942, 1948, 1958), it is not evident whether he used
uncorrected or corrected dip values. The use of
corrected dip values seems to be the most correct
approach.
The projections (Fig. 3) have been plotted rocktype wise. "[ne resulting patterns are strikingly different from those shown by w Ecke~mann: no
,distinct fi~ci are evident, only slight diffi;rences are
found in the projections for different ~ock types.
Perhaps the most striking difference is that the
projections of v. Eckermann do not show any outward-dipping dykes. The outward-dipping dykes
could be interpreted as belonging to ano'ther centre
of igneous activity far away from the one chosen
for the geometric projections. This is evidently no~
the case: outward-dipping dykes show a rather
~imilar d;=tribution in the area to inward-dipping
dykes, and the two often occur together at the same
outcrop.
The cone sheet system, ~s presented in this study,
is shown simplified in Fig. 4. Four types are
distinguished:
A. Inward-dipping dykes with ,,,hallow to raoderate
dips (less than 45'~),
B. lnwardi-dipping dykes wit~ steep dips (about
6O-80°).
C. Outward-dipping dykes with shallow dips (less
than 30°).
D. Outward-dipping dykes with steeper dips (more
than 4:;°).
Left: this study, simplified f r o m
Fig. 3. R~ght: t h e m o d e l
p r o p o s e d by v. E c k e r m a n n .
The pairs A-D, and B-C, respectively, define
two orthogonaf sets of joints. The relative frequency
of cone sheet types is (Fig. 3): A (most frequent)
- C - B - D (least frequent). Looking at different
rock types, type D dykes are not found among pure
carbonatites (alvikites and beforsites), which perhaps is explained merely by the small number of
dykes in these groups. Type C dykes seem rare for
beforsites.
It might be added that the c~ne sheet projections
of Fig. 3 have tK:en carried out on dykes from the
whole area showing field dips of less than 75 °, a
:dmplification made in order to save time during
data processing. Therefore, some B and D dykes
may have been lost.
The tectonic model
The proposed model (Fig. 5) has emerged as a
synthesis from various observations on volcanic ring
structures and models for their formation (Anderson 1936; Billings 1945; Reynolds 1956), applicable
data on carbonatitecomplexes (Garson 1965, 1966;
Le Bas 1977) and diapiric intrusions, particularly
salt ~omes (Currie 1956; Gussow 1968). Necessarily,
tile model is idealized, ,~suming the wallurock to be
a h~mogeneous t-ody, as well as picturing the
ascending magma as being one single (even if
internally differentiated) bc~dy, contained in a
magma chamber.
Intrusion of a majo~" mass of igneous material
will cause up-doming of the overlying coumry rock,
this will be especially pronotmced when the intrusion has reached high ~evels of the cr~t. Radial
dykes form, which are legarded as an ~i,filling of a
radial crack pattern in the wall-rock around the
central ~atrusion, ~bllo~ng Crajector'es of radial
A/n~ compiex
LITHOS i3(1980)
stress (i.e. directions of maximum compression).
The cracks are kept open by the wedging action of
the intruding dyke rock (magma and/or mush). As
pointed out above, pre-existing planes of weakness
in the wall-rock have great influence on the resulting
pattern. Dramatic explosive evems are not a necessary, nor even a likely, pre-requisite for the emplacement of radial dy;¢es. Radial fracture patterns are
very common in connection with salt diapirs
(Gussow 1968), which are obviously rather quiet
and undramatic diapific intrusions.
The stress field created by the upward movement
of the magma gi~es rise to the formation of two
sets of joints (Fig. 5): tension joints, commonly
with an inward dip of ,45° or less: the classic cone
sheets of Anderson (1936). Shear joints form as
well (Fig. 5); dykes fillir~gthese shear joints seem to
be absent in most volcanic ring dyke intrusions
(Anderson 1936; Billings 1945). However, the
formation of thes~ steeply outward-dipping joints
is evident from tectonic considerations, and well
docamented from salt diapirs (Currie 1956; Gussow
1968).
As a consequence of the intrusion of magma into
radial, tension and (to a limited extent) shear joints,
the pressure in the magma chamber decreases,
prol~bly not so rauch because of mass reduction
but rather due to degassing. When the pressure in
the magma chaml:er becomes lower than the pressure exerted by the overlying rocks, subsidence of
the wall-rock occurs (Fig. 5, following the model of
Anderson 1936). A set of shear joims above the
magma chaml:er will give rise to a series of ring
dykes, with steep outward to vertical dips at higher
levels above the magma chamber (Anderson 1936;
Billings 1946), and inward dips close to the magma
chamber (Reynolds 1956). Cauldron subsidence
will occur along these ring fractures. Another set
of shear joints will show steep inward dips (Fig. 5).
Tension joints in contortion with the subsidence
show horizontal to flat outward dips (Fig. 5, see
Anderson 1936).
The model proposed seems applicable to the
observations made on the Aln6 rocks in every
particular. S6vite is thought to occupyring dykes,
which obviously are not as ideal as shown in Fig. 5.
Cauldron subsidence-strictly speaking-may have
occurred mainly for the northern ring dyke complex
~tt Aln6, as indicated by the rheomorphic breccias,
which most likely contain much material from the
,,)verlymg wall-rock. The cauldron subsidence pro~:~osed by v. Eckermann (1966) could not be confirmed, mainly due to eytensive overburden. For
the cone sheet system, the relative frequency of dyke
I 1 - - Lithos 2/~0
157
I NTRUSIOI¢
~;hear join|si
D
SUBSIDENCE
\
Tension JOintS
Icone sheets)
A
Sheor joints
Ififlgdykes]
"--
Tensionjoints
C
Sheor joints
43
Fig. 5. Proposed model for the emplacement of Ain6 dykes.
Letters refer to dyke types distinguished in Fig. 4 (left).
types ( A - C - B - D ) is readily explained by the model:
A and C type dykes occupy tension joints, type A
being in direct connection with the magma chamber
(Fig. 5). B and D type dykes occupy shear joints,
of which type D joints possibly are kept more closely
together than type B joints. The latter may also be
in connection with the magma at greater depths.
Again, it will not be necessary to assume dramatic
explosive events for the formation of the cone sheet
pattern as has often b ~ n advocated (v. Eckermann
1948, 1966, Garson 1965, 1966; Le Bas 1977).
Preliminary observations on the s6vite dykes a~-o
suggest a rathe:r 'quiet' mode o!"intrusion, in a state
of fairly high viscosity. Similar ideas have been
proposed for the s6zites of the Fen complex
(Saether 1957). In principle, the process of uplift
followed by subsideno,', could have been operative
several times, with var:able intensities.
The model proposed here differs from the clas;ic
cone sheet model of v. Eckermann in several
respects. However, fl~e present model seems to
explain all the observations in a satisfactory w~:y.
Von Eckermann's model cannot accoun~ for tae
formation of C type dykes, which, after all, are
fairly common. A further weak point in the model
of v. Eckermann is concerned witah the depth c |
origin of the dykes. According to his model, cone
sheets emerged from foci as deep as 9-10 km
below the surface during the time of formation.
Extremely violen,t processes must have bee.l active
in order to create, and open up, a joint pattern at
such depths, as well as transporting magma (or
crystal mush) in these commonly narrow cracks
for distances of 20 km and more, pushing upwards
huge piles of rock.
158 F. Kresten
For ~he present model, a depth of less than 2 km
(roof of magma chamber to surface during time of
intrusion) is postulated. This remains a more or
less informed guess for the time being, based on
possible depths of fo~'~nation for tension joints
(Hobbs et al. 1976). Also, the vent at S~il,~k~irmost
likely reached the surface, even if the age of this
vent in relation to the Aln6 complex remains
uncertain (Brueckner & Rex, this volume).
A cknt~wledgements. - Financial support from the Geological
Survey of Sweden and the Swedish Natural Science Research
Count;il (NFR), as well as a stipend received from the
Norwegian Council for Technical and Scientific Research
(NTNF) is gratefully acknowledged.
References
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cone-sheets, ring-dykes, and cauldron-subsidences. Proc.
Roy. Soc. Edinb. 56, 2, 128-163.
Billings, M. P. 1945: Mechanics of igneous intrusion in New
Hampshire. Am. J. Sci. 248-A, 40-68.
Currie, J. B. t956' Role of concurrent deposition and
deformation of sediments in development of salt-dome
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forskningsr6n inorn AIn6 alkalina omr~tde. Geol. Fi~ren.
Stockh. Fi#h. 64, 399-455.
Eckermann, H. v. 1948: The alkaline district of Ain6 Island.
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c: the AIn6 formation on the mainland north-west of
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Nr 2, 61 pp
LITHOS 13 (1980)
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Accepted for publication NovEmber 1979
Printed April 1980