loumal of the Geological SocietJ, London, Yo|. 147, 1990,
/.
-t
pp. 439-450,
11 figs, 2 tables.
Printed in Northcrn Ireland
The Caledonian Heilhornet Pluton, north-central Norway: geological setting'
radiometric age and implications for the Scandinavian Caledonides
O. NORDGULEN' & B. SCHOUENBORG2
tNorwegian Geological Suruey, PO Box 3006, N -7002 Trondheim, Norway
2
Institute of Geology, IJniuersity of Lund., Sdluegatan 13, 5-223 62 Lund, Sweden
Abstract: The Heilhornet Pluton is a medium-sized (c.5okm']). sub-circular, discordant intrusion
which penetrates the thrust zone separating the Helgeland Nappe Complex from paragneisses in the
Vestranden region, north-central Norway. The intrusion consists of biotite + hornblende granite and
ranges in composition from 61.5 to 72.5wr.Ea StOz. The initial Sr ratio (c. 0.7070) shows that crustal
sources have becn involved in the petrogenesis of the granite. A U-Pb zircon age ol 444 + 11 Ma for
the pluton indicates that juxtaposition of the nappe complex with metasedimentary rocks in Vestranden took place prior to or during the late ordovician. A consequence of this is that the general
correlation of the melasedimentary rocks in Vestranden with the Sevc Nappes must be revised Final
castward thrusting of the complex, together with a yet unknown part of the Vestranden supr2rctustal
sequencc, probably took place during the middle Silurian to early Devonian, Scandian, orogenjc
complex the igneous complex on the island of Hortavier has
given a Rb-Sr date of 471 + 5 Ma (Gustavson & Prestvik
Several granitoid batholiths and isolated granite intrusions
are known from the Scandinavian Caledonides, most of
Despite their abundance, very little work has been done on
1979). Compilation of isotope studies of the Bindal
Batholith has been provided by Nissen (1986) and
Torudbakken & Mickelson (1986). New Rb Sr isotope data
them. and they hare receired only cursor) altention in
recent syntheses of the development of the Scandinavian
Bindal Batholith commenced duing the Cambrian
them occurring in westernmost Norway and in the Upper
and Uppermost Allochthons (Stephens et al.
1985a).
indicate that the intrusive activity in the southern part of the
Previous work has suggested that the granitoids of the
Bindal Batholith are confined to the Uppermost Allochthon
(Stephens et al. 1985a). The western limit of the Helgeland
Nappe Complex can be followed from close to Grong
northwards to the vicinity of the Heilhornet Pluton (Fig. 1).
The basal thrust generally dips at low to intermediate angles
to the east and northeast and is locally folded by open,
The Heilhornet Pluton is one of about 50 major
intrusions which constitutes the Bindal Batholith, an
intrusive complex occupying a large part of the Helgeland
Nappe Complex in the Uppermost Allochthon of north-
central Norway (Fig. 1).
In this paper we describe the pluton and its geological
setting before discussing the isotope data. Finally. we
consider the significance of our obseryations with respect to
models
for the evolution of the central part of
and
spanned a period of more than 100 Ma (Nissen 1988).
Caledonides (e.9. Stephens et al. 1985b).
northeast plunging folds.
Detailed mapping has shown that the Heilhornet Pluton
cuts into an imbricate zone which marks the boundary
between the nappe complex in the east and subjacent
undifferentiated supracrustal rocks collectively referred to as
the Vestranden sequence in the west. This boundary has
been mapped and described between the Grong area in the
south and Bindal in the north (Kollung 1967; Roberts el dl.
1983; Nissen 1986; Nordgulen & Bering 1987; Husmo &
Nordgulen 1988). In the imbricate zone, lenses and thin
slivers of metasedimentary rocks and various intrusions are
the
Scandinavian Caledonides.
Geological setting and previous work
The Heilhornet Pluton is a roughly circular intrusion about
7 km in diameter. lt is situated some 25 km east of Leka
(Fig. 1), and cuts the contact between the Helgeland Nappe
Complex and supracrustal rocks of the Vestranden region.
The geology of the nappe complex is dominated by the
voluminous granitoids of the Bindal Batholith (Kollung
1967; Myrland 1972; Nordgulen 191t4; Gustavsor 1988) in
Nordland and Nord-Trondelag (Fig. 1). Most of the plutons
are medium- to coarse-grained and sometimes megacrystic
granitoids, although intermediate and basic rocks also occur.
There are few detailed radiometric studies of the individual
plutons. A granodiorite in the southeastern part of the
Helgelaod Nappe Complex has been dated by the Rb Sr
method (Nissen 191t6) and gave an age of 526110Ma.
Rb Sr work from the northern part of the compiex yielded
ages of 433tl1Ma for granitic dykes and 420+8Ma
(mineral age) for the MosjOen gabbro (Torudbakken &
Mickelson 1986). In the southwestern part of the nappe
stacked
in a
complicated manner. This is well-illustrated
immediately south of the Heilhornet Pluto. where a
basement cover couplet consisting of metagabbroic rocks
unconformably overlain by mafic and calcareous conglomerates, marble and metasandstone is present (Fig.2). The
mafic conglomerates, which lie directly on metagabbro south
of SOrfjorden, consist essentially of metagabbroic clasts in a
hornblende-rich matrix. They are interpieted as unconformable deposits on the metagabbro (Husmo & Nordgulen
1988). The basement cover couplet has been telescoped by
low-angle thrusts
into an imbricate structure which
was
subsequently transected by the pluton (Husmo & Nordgulen
1988). To the east, the pluton cuts rocks which are regarded
439
O. NORDGULEN & B. SCHOUENBORG
440
Fig.
The map is modified from
1. Overview of the central part of the scandinavian caledonides and location of thc Heilhornct Pluton
as part of the Helgeland Nappe Complex. These include
metagabbros and various ultramalic rocks which are
intruded by quartz-monzonitic rocks of the Bindal
Batholith. The mafic and ultramafic rocks, together with
associated pillow lavas and greenstones, which outcrop
northeast
Gee c/
dl. (198s).
of the pluton, have been interpreted as the
remnant of a dismembered ophiolite (Husmo
& Nordgulen
1988).
Contact relationships and intemal structure
As described above, the contact of the pluton is discordant.
In the south, east and north, the contact is irregular and
quite steep where exposed. The western contact dips to the
east at a fairly shallow angle (c.30') and is sub-parallel to
the regional S2-foliation in the adjoining metasedimentary
rocks. However at outcrop, the contact is strongly
a
discordant: apophyses can be followed from the pluton into
the surrounding rocks (e.g. Fig. 3). At the southern contact,
the Heilhornet Pluton can be seen to intrude the
metagabbroic and metasedimentary rocks of the imbricate
zone. Following the contact from the western part of the
pluton to Bursvikbotn in the north, it is seen that it transects
lithological boundaries in the metasedimentary rocks of tbe
Vestranden sequence (Fig. 2).
Locally, xenoliths of the wall-rocks occur near the
contact. This is well exemplified along the northern margin
of the Heilhornet Pluton at Bursvikbotn. where they include
calc-silicate rocks. micaceous schists and gneisses (Fig.4)
Away from the contact area, the granite is virtually devoid
of xenoliths. In the eastern part of the pluton, on
Hildringsfjell, there is a major raft of grey. medium-grained
diorite which probably represents an earlier member of the
comparatively late stage of the magmatic development in
the area (Nordgulen 1984; Nordgulen & Mitchell 1988)
Sheets of llne-grained aplite occur at a few localities in
the pluton and may be regarded as late-stage differentiates
of the granite.
Bindal Batholith.
Only a brief description of the internal structure of the
pluton will be given in the Present account AlQng the
western and northwestern contact, an S-type fabric (Pitcher
& Berger 1972) of variable intensity is developed (Fig. 2) It
In the west and northwest the Heilhornet Pluton intrudes
supracrustal rocks belonging to the Vestranden sequence
(Kruhl 19114; Nordgulen & Bering 1987; Schouenborg 1988).
Evidence of amphibolite-facies metamorphism is found in
mica-rich lithologies. Growth of kyanite and porphyroblasts
of garnet, staurolite and later also sillimanite pre-dates the
development of a penetrative foliation which is cut by the
Heilhornet Pluton. Post-emplacement mineral associations
include the paragenesis actinolite, clinozoisite, chlorite and
quartz.
The Heilhornet Pluton is cut by a number of malic
dykes, up to 10m wide, which strike approximately
NW SE. Most of them are strongly sheared and
retrogressed to biotite schist, but igneous textures are still
preserved in some. The dykes are probably part of a suite of
mafic rocks which appears
to have intruded at
HEI LHORNET PLUTON. NORTH-CENTRAL NORWAY
5,--)(.( t )', ""
F
^/
Hf '-*
/,-I
\ \
\
\ ,i\
\
t-r
l1ooo"" ,,40
-1-
ri.
{i*'/ c o o(
o+s+
l1;1i\ i;.-
)
I
l
FOCKS BELONGING TO THE HNC
SUPRACRUSTAL & GFAN TOID ROCKS
OF THE VESTBANDEN SEOUENCE
t.7
m
ffi
N
ANATECTIC. PARTLY PORPHYR TIC GRANITE
CA SCHIST
SEMIPEL T
ffi
l't
CALC SILICATE SCH ST N,4ARBLE &
C SCHIST
oton
rr
& [,4oNZoDroR
TF
ou,qRrz rvroNzoNrrF
roLrntro cnarurrr
r - tv Io\tIt!- vF-A. _DrV-\ ARj
L Il l Roors & cRAN TorDs
[I - l I Jt tcA\/lATtC RO.r< CABBqO
CaLt \S , O\. , OL ap ,/-VO\,/O _RANn
[iI] ror ot.o poRpHyRocLASrc cBANrrE
Ir
rl l
srnprlrrrrurrr
Cnnr\Oo Onrlo GNE SS
t ----l
LJ
\\4FTACABBRO JND,I
TECTONIC POSIT ON
OUATERNARY COVER
rc & cAr
fL -_l I,IETASANDSTONF
'uo,
MA RB LE
,
FOLIATION
^
FABRIC IN THE HEILHORNET
-
geLLloRruer cRANrrF
lil
t-
INTRUSIONS
ffi
ffi
..
Fig.
C
BIOTITE RICH PARTLY ANATECTIC SCH ST
M
-
0
al
L VL
TOURIMALINE GRANITE
FELS
Z L] .l
fl.-.'l
t- I
E-D-\
IAILD
(APr or s.oNCLo\4FqA'r
GRANITE
---^ ^
"
FAI]LT
THRUST
STRETCHING LINEATION
& Bering (1987) and Husmo & Nordgulen (191lil). Hd.
of
Figs 3 and 4 are also marked
The
locations
Hf,
Hildringsfjcll.
Horndalen;
2. ceologicat map of the srudy
area. The map is modified from Nordgulen
0
)
O. NORDGT]LEN & B SCIIOTJI'NRORG
:'a
1a
)r.<
\ta
l? "',
" "" ""'-
llll
Granite dyke
Calcdreous metasedimenlary rocks
I'ig. 3, Photograph showing Sranite apophyses enranating from the Heilhornet Pluton and intruding metascdimentary rocks of the Veslranden
sequeoce (UTM-coord: 36355 722100). A simplified drawing is also shown to clarify the cross curling relarionships.
wrapping around variably recrystallized feldspars. Away
from the westcrn contact thc fabric becomes less obvious.
but it is still broadly concordant with the contact. In the
Measurements in the area betwcen Sorfjorden and
Hiidringsfjell (Fig.2) show considerable variation in both
strike (from N S to E-W) and dip (20 50') of the fabric.
The origin of mineral orientation fabrics in granitoids is
dimcult to assess. ln a recent review, Hutton (1988) pointed
out that early-formed fabrics rnay be imposed on the rock
during crystallization resulting in a texture with few signs of
ductile strain. Commonly, such fabrics tend to become
modined or even obliterated by ductile strain during final
consolidation of the pluton. These ductile etfects may be
due to emplacement-induced and/or regional strains. In
mafic minerals. A weak mineral oricntation occurs localLy,
but no systcmatic pattcrn cmcrged from our measurements.
The castern part of the pluton is wcll cxposed, and in most
addition. post-emplacement deformation may also affect the
pluton. To evaluate these possibilities. detailed knowledge
on thc structural evolution of the wall-rocks with respect to
emplacement is required (Hutton 1988. Paterson et dl.
places
1989).
is roughly parallel to the contact and is defined by rcriented
ma6c minerals and elongate grains of quartz. Locally, there
is a very strong. protomylonitic fabric characterized by
anastomosiflg bands of llnely recrystallized minerals
central wcstcrn part of the pluton. a poorly-dellDed
prcferred mineral orientation is present, frequently too
wedk for svstematic measurement. In the less well cxposed,
north central part of the pluton (Horndalen, Fig. 2), the
granite has a somewhat liner grain-size and contains more
a mineral orientation is developed (Fig.
2).
In the Heilhornet Pluton. the variation in attitude and
intensity of the mineral orientation may be partly related to
processes operating during emplacement. However, in the
area immediately west
of the pluton,
large scale post-
Silurian folding has taken place (Schouenborg 1988). Thus,
the contribution to the fabric from regional deformation
may be considerable. especially along the western contact.
Throughout thc pluton the mineral orientation fabric is
cut by shear zones, up to several metres wide, which dip to
the SE. Mafic dykes tend to follow the same trend and are
thenlselves strongly sheared in most cases. Ultramylonite
zones exhibiting a variety of trends are also present in the
pluton. Both the shear zones and the ultramylonites are
post-emplacement phenomena.
Petrography
The Heilhornet Pluton is generally an
Fig.4. Conlact rclltionships in BuIsvikbotn (UTM coord:
36665 722520). Small xeooliths ofbiotitc rich schist and calc silicatc
rocks hirvc sharp contacts with the granitc. l he corn lor scale is
25 mm in diameter.
equigranular,
medium-grained monzogranite. Plagioclase, microcline,
quartz, biotite and in cases hornblende are thc major
minerals. Accessory phases comprisc white mica. titanite,
apatite, zircon, epidote, allanite. tourmaline and opaques.
Fine grained epidote. white mica, calcite and chlorite are
HEILHORNET PLUTON, NORTH,CENTRAL NORWAY
Horndalen, the granite is finer grained than elsewhere. ln its
northern and central parts the pluton is relatively rich i.
mafic minerals including hornblende. The eastern and
southeastern parts of the pluton have been less extensively
sampled, but appear to represent an intermediate
g.ain-sized variety.
Plagioclase forms lath-shaped subhedral to anhedral
grains. Microprobe analyses show it to be an oligoclase
(Ann-Anrr) with weak normal zoning (up to 10 An-units).
Albite twinning is common. Alteration is variable but
commonly limited to crystal cores, where minute grains of
epidote, white mica and calcite grow randomly, sometimes
accompanied by a dark dusting.
The K-feldspar has cross-hatch twinning and is always a
microcline with a small Ab component. It forms either fairly
large, subhedral crystals, or more commonly smaller,
anhedral grains between plagioclase and quartz. The large
grains show Carlsbad twinning and may enclose other
0
10 20 30 40 50 60 70 80 SO
minerals such
10O
Fig.5. Modal Q-A-P plot for ten
selected samples from the
Heilhornet Pluton employing the classification of Streckeisen
(1976). The data were obtained by point counting on thin sections
(800 points).
alt€ration products. Point-counting of ten selected thin
sections (Fig.5) shows lhal the plulon is a monzogranite
according to the classification of Streckeisen (1976).
There are obvious variations in grain-size and texture
throughout the pluton. In the topographically highest areas
in the western part, the granite is coarse-grained and
contains relatively little biotite, while hornblende is usually
absent. Along the western and northern contacts and in
Fig.
6. Map showing sample localities.
as plagioclase and biotite. Patch-
and
string-perthite are very common, and myrmekitic texture is
often observed.
Quartz is extensively recrystallized. The new grains have
with gently curved or irregular grain
boundaries and show undulose extinction.
Biotite is usually brownish-green and occurs as isolated
flakes or grouped in aggregates of small, randomly oriented
grains. Local alteration to chlorite is observed. Close to the
westen and northwestern contact, parts of the granite,
including dykes, carry a red-brown biotite which in cases is
variable size,
partly altered to chlorite and white mica with
exsolved
opaque phases. Hornblende is not present in all samples and
always less abundant than biotite. It is present as
anhedral, dark green to bluish green crystals, and is
ftequently associated with biotite and accessory phases.
is
444
@. NORDGULEN
&
B. SCHOUENBORG
o.0
1A
At2
At203
03
Na2 O
":,ii.'li+
12
6.0
Fe2O3
Fe
2O3
;i.:.*n..
o.o
'|.4
t\4nO
Mso
o.o2
o.25
P205
P205
7a%
Fig.7. Harker
some homblende-bearing samples. Tourmaline is a
common species forming tiny blue green to brownish
blue-green needles and prisms. Allanite is usually present
as quite large, zoned, euhedral crystals. Commonly it is
in
rimmed by epidote which is euhedral against mafic minerals,
but has very irregular grain shape in contacts towards quartz
and feldspar. The textural evidence points to a magmatic
origin for the epidote.
Geochemistry
The geochemical data of the Heilhomet Pluton are based on
49 samples which have been analysed for major and 20 trace
elements. Chemical analyses were done by X-ray fluorescence on glass discs for the major elements. The sample
localities were chosen to ensure a representative selection of
petrographic varieties ftom the pluton (Fig. 6). The detailed
geochemistry and genetic models will be discussed in a
separate paper.
SiO, varies between 61.5 and'72.5 wrEo but most samples
have more than 68wtEa SiOr. In Harker diagrams (Fig. 7),
PrO5 decrease regularly with increase
and
in SiO, content. K:O
A single analysis of
aplite (SiO,:76.8wt%) is also included in the diagrams
and plots on the extension of the trend observed for the
tends to increase with increasing SiO..
granite.
oo
60
68
diagrams for major elements.
Titanite occu6 as large euhedra, but may also be present
as small, anhedral grains on or close to biotite. Irregular
aggregates of titanite/leucoxene with an ilmenite core occur
Tio,, Al,oi, Fe,o. (total Fe), Mgo, cao, Na.o, Mno
."t r.lflr .
The Heilhornet Pluton is metaluminous. and the
high alkali contents, low CaO contents and rather high
Fe/Mg ratio indicate that it is transitional towards alkaline
compositions.
U-Pb dating
Analytical procedures
About 50 60kg ol freshly blasted, undeformed granite
were
sampled from ft road cuttin8 in Bursvikbotnet in the northern part
of the pluton (Fig. 6). The U Pb analysis was carried out at the
Laborabry of Isoiope Geology at the Museum of Natural History
in Stockholm, Sweden. Zircon separntion followed slandard
procedures (e.9. Schouenborg 1981t). The zircons were split into five
size-fractions which were finally purified by h.rnd picking under the
microscope. Chemical methods mainly followed the methods
described by Krogh (1973), with slight modification according to
Christiansson (1982). Uranium isotope ratios were measured with
an AVCO 901 A mass spectrometer, while the lcad isotopc ratios
were measured with a MAT 261 mass spectrometer.
Calculation of the age is based on the procedure of Ludwig
(1980). The aoalyticxl errors at lhe 2(' level are estimatcd to be
lower than 0.67, for the '?'bPb/'?33u ratio and abou! 1-2a/o lot the
'"'Pb/'r5u and '?o?PbFo6Pb ratios. The decay constanrs recommended by Steiger & Jeger (1977) were used- Corrections for mass
frrcrionation were 0.10olo AMU for U and 0.72o/o AMU for Pb.
Total lead blank was 1.2 ng. Common lcad corrections according k)
the model of Stacey & Kramcrs (1975) were: '?06Pb/'?o4Pb: 1l'l 0,
for 450 Ma old Pb.
'?o7pb/'?oapb = 15.6, '?o3pb/'zuPb = 37.8,
.
HEII-HORNET PLUTON. NORTH-CENTRAL NORWAY
model of Ludwig (1980) gives an upper intercept age of
444:t 11 Ma (MSWD :0.4), stated within the 20 limits of
analytical error. Considering the igneous habit of the zircons
and the lack of cores and overgrowths, this age most likely
reflects the time of granite crystallization The employed
model also yields a lower intercept age of -250t482Ma'
is meaningless and only reflects the poor spread
between the five size-fractions combined with a high degree
which
of concordancy. This results in a very high extrapolation
uncertainty.
Rb-Sr-isotope analysis
Analytical procedure
The Rb-Sr analysis was performed
at the Mineralogical
Geological Museum, Oslo, Norway. The chemical preparation of
the rock samples mainly followed the technique described by
Pankhurst & O'Nions (1973), including HF-HCIOa dissolution and
HCI ion exchange. The Rb/Sr ratios were determined by XRF
spectrometry, and the Sr_isotopes were measured on a VG
3tRb decay constant of
Micromass 30 mass spectrometer. A
7.42x70 1t a was used, and the 3tsr/3osr isotope ratios were
normalized to a ousr/""Sr of 0.1194. The least square method oI
York (1969) was used in the calculation ofthe Rb Sr isochron Age
and intercept errors are noted at the 20 level.
'
Fig.
t,
Resulx
Scanning electron microscope photographs of typical zircons
fro=m the Heilhornet Pluton, sample N86 98 The scale bar is
100 !m. (A) Shows prismatic igneous morphology. (B) Back-scatter
image of a polished zircon grain, showing inclusions and a weak
Rb Sr-isotope data are available to 17 whole-rock samples.
Sample localities are shown in Fig 6; the results of the
chemical zonation.
the diagram it can be seen that all the samples taken
together scatter too much to define an isochron. The
samples that appear to contribute most to the spread were
collected in the northern and southern parts of the pluton
As mentioned above, inclusions of metasedimentary rocks
are frequent at Bursvikbotn (Fig. 2). In addition, secondary
volatile activity is indicated by the local presence of white
Zircon description
A 25 pm thick polished thin section with zircons was studied
using the polarizing microscope and a scanning electron
microscope. The zircon grains are euhedral with sharp
crystal edges and pyramidal or blunt terminations
(Fig 8A).
analyses are listed in Table 2 and plotted in Fig. 10. From
Length to width ratios are around 3. The crystals are
transparent, colourless and commonly show a weakly
developed zonal structure and numerous inclusions (Fig
88).
Resuls
The analytical data and isotopic ages are shown in Table 1.
All five zircon size-fractions plot very close to the upper
intercept with the concordia (Fig 9). The age-calculation
Tsble
l-
Pb rad
U
Fraction (!m) sample (ppm) (ppm)
<45
106- r50
150 210
(Fig.6) generally plot below the reference line. Xenoliths
are virtually absent, and only a few quartz veins with some
sulphides occur in the sampled ateas. However, some local
alteration of these rocks cannot be ruled out although we
found no lield evidence to support this. Seven samples
collected in the central part of the pluton define a fairly
large variation in Rb/Sr (Fig. 10). Only these analyses were
used to draw the 'line of best nt' corresponding to an age of
zircon analytical data
mg
45-',74
74 106
mica in the granite as well as veins and shear zones carrying
quartz and sulphides. The seven samples from Sorf.jorden
5.2
7.2
4.5
5.6
:r 1
1349
1091
10U9
1073
632
93
78
'74
'74
45
':o6Pb/'z04Pb '?o?Pb/'?o6Pb
a
b
t3,920
0.0559
0.0559
0.0563
0.0561
40,921)
83,420
4083
2'/42
0 0553
1.6 A using a Franz isodynamic separator.
(a) Atomic ratios corrected for discrimination and lead blank of l-2 ng.
(b) Corrected for discrimination, Iead blank and common lead.
All zircons are non-magnetic at
':''PbFrsu
,06Pb/,]3u
b
b
0.5198
0.5427
0.5208
0 5262
0.5347
0.067 4
0.0704
0.0671
0 068{l
0.0701
Isotope ages in Ma
,06PbFr3u'o?PbFrsu,(,?Pb/,o6Pb
42t
439
418
424
43',7
425
440
426
429
435
448
44',7
465
457
425
& B. SCHOUENBORG
@. NORDGULEN
446
0.07
Fig.9. U-Pb concordia diagram with
2
oerror boxes. The numbers corespond
0.4
0.5
+ 14 Ma. The age overlaps the error limit in the U Pb
age and raises the possibility that the Rb Sr whole-rock
system closed somewhat after the U Pb zircon system. This
may be due to a prolonged cooling history for the pluton.
The initial Sr-ratio (c.0.7070) shows that isotopically
evolved material was present in the source region- For the
pluton as a whole, it is possible that the initial Sr-ratios are
not constant- This could at least partly explain the scatter in
the isotope data (Fig. 10). The Sr data do not indicate that
significant upper crustal contamination has occurred.
However, the fact that samples collected away from the
contacts show a comparatively small scatter indicates that
the large scatter near the contacts was caused by some
disturbance of the Rb-Sr-system du ng or after
emplacement.
Regional considerations
The Helgeland Nappe Complex, which is part of the
Uppermost Allochthon in the Scandinavian Caledonides,
Rb-*
represents an exotic teriane with respect to the Baltic Shield
(Sturt & Roberts 1987). The most striking feature of this
terrane is a protracted tectonomagmatic history (see below).
Although it has been suggested that the Helgeland Nappe
Complex and nappes of similar nature have been derived
from a microcontinent or from the Laurentian margin
(Stephens et al. 1985b), the origin remains enigmatic.
The Heilhornet Pluton transects the boundary between
the nappe complex and underlying tectonostratigraphic units
(Fig. 1). The dating of the pluton is therefore important in
terms of reconstructing tectonic events in central Scandinavia. Recent work in the southwestern part of the nappe
complex has shown the presence of mafic and ultramafic
fragments with unconformably orerlying cover sequences in
Velfjord (Ldseth 1985; Thorsnes 1985), and on the islands
of Rodoy (Bang 1985) and Bolvar (Heldal 1987). The
relationships in these localities are reminiscent of those
desc bed ftom Leka, where the Skei Group (Sturt er 41.
1985) lies unconformably on the Leka Ophiolite Complex
dan)
Rb/Sr l sigma
Sample
HH06
HH08
HHrl
HH12
HH13
HH14
HH16
HH268
N86 98
HH345
IJH2I4
HH215
HH216
HH218
HH391
HH396
HH397
:
0.6
430
Tthle 2-
to the following size fractions: 1 = <
45 pm;2:45-74 pm,3:'74-106 unr
4= 106 150pm; 5 150-210 pm.
239.3
242.4
244.1
258.I
216.1
250.6
268.8
t9s.2
1',72.1
.0
25t.3
252.t
234.t
290.2
318.8
189.2
254.5
2t'7
154.3
157.1
165.7
152.9
140.3
153.6
146.2
209.9
316.s
289.6
156.4
150.6
t59.'7
131.0
123.s
227.5
155.5
1.5503
1.s435
1.472'7
1.6878
1.9686
1.6313
1.8391
0.9296
0.5437
0.'7 494
1.6069
)..6'746
1.4658
2.2151
2.5816
0.8314
L.6366
0.016
0.015
0.015
0.017
0.020
0.016
0.018
0.009
0.005
0.007
0.016
0.017
0.015
0.022
0.46
0.008
0.016
37Rb/37Sr
4.49'7
4.4'76
1.210
4.896
5.',713
4.'732
5.33',7
2.694
t.5'74
2.171
4.662
4.859
4.251
6.43,1
7.502
2.409
4.'748
"tsr/"usr
0.73339
0.13245
0.72940
0.73494
0.'73940
0.'73334
0.73919
0.72303
0.'71122
0.72465
0.73500
0.73'741
0.73267
0.'/459'7
0.75298
0.72t13
0.73s72
1 sigma
0.0(XXr1
0.00002
0.0{D01
0.00m1
0.00002
0.00002
0.00001
0.00001
0.00005
0.00003
0.00001
0.00001
0.00001
0.00001
0.00003
0.00003
0.00003
HEILHORNET PLUTON, NORTH.CENTRAL NORWAY
760
B7
Sr
B6
.750
Sr
740
.730
Fig.l0. Rb
Heilhor-
Sr diagram for the
net Pluton. Seven samples are from
Sorfjorden (fi tled triangles); three
samples are from Bursvikbotnet
terisks);seven samples (filled circles) arc
from various localities in tbe interior of
the pluton (see Fig. 6 for sample
locations).
(as-
.720
-
MSWD
0 70696
=
Ma
t
0.0008
87
Rb
86
Sr
2.0
.700
with metasedimentary rocks. The
mafic rocks on Leka and SkSlver are considered to
represent preserved parts of dismembered ophiolite
complexes (Gustavson 1978; Prestvik 1980; Furnes e, .?1.
1985). Recent work (Furnes et al. 1988) shows that the
formation of the Leka Ophiolite Complex was related to
greenstone associated
subduction zone and back-arc environments.
The other mafic fragments mentioned above have also
been interpreted as remnants of deformed and eroded
ophiolitic rocks (Husmo, pers. comm. 1983; Sturt 1984;
Loseth 1985; Thorsnes 1985; Bang 1985; Heldal 1987).
Considering the close spatial relationship between these
ophiolite fragments, it is plausible that they represent rocks
which originated in a similar tectonomagmatic environment
and that they have similar ages. A trondhjemite in the Leka
a U Pb zircon
Fig. 11. Summary of tectonic development in north central Norway. The
oceanic and continental terranes present
in the Helgeland Nappe Complex (lower
part of the figure) had a separate
evolution prior to their amalgamation in
the Ordovician- Thrusting of the Helgeland Nappe Complex across metasedimentary rocks belonging to the vestranden sequence also pre-dates the intrusion of the Heilhornet Pluton. but the
time interval between construction of
the Helgeland Nappe Complex and the
thrusting of the composite terrane is not
known- The amount of middle Silurian
to early Devonian, Scandian deformation is not well constrained in the
Helgeland Nappe Complex but has had
a strong influence in the Vestranden
region (Mitller 1988: Schouenborg
1988). The pegmatite dated at 401 +
3 Ma may also have been deformed in
the ScaDdian event. Devonian deformation resulted in upright, NE trending
folds.
t"
.7 10
(Prestvik 1980; Furnes el al. 1988). Further north, on
Skilver, Gustavson (1978) described occurrences of
Ophiolite Complex has yielded
Age = 430 114
o--*t^
age of
49'7 +2Ma (Dunning & Pedersen 1988). This age is among
the oldest obtained for ophiolites in the Caledonian
Appalachian belt (Dunning & Krogh 1985; Dunning &
Pedersen 1988). There is therefore little reason to assume
that other ophiolite fragments in the Helgeland Nappe
Complex should be significantly older. ln SOrfjorden (Fig.
2), one of these ophiolite fragments and the unconformably
overlying metasedimentary rocks form an integral part of an
imbricate zone at the base of the nappe complex (Husmo &
Nordgulen 1988). Deformation and imbrication in this zone
pre-date intrusion of the HP 444 t 11 Ma ago.
If one accepts the correlation of mafic complexes and
corresponding cover sequences as outlined above, this shows
that uplift and erosion, and later metamorphism and
deformation including imbrication, must have occurred
during the Ordovician (Fig. 11). The date would also
provide a minimum age for construction of the imbricate
zone between Bindalsfjorden and Kongsmoen (cf. Sigmond
NORTH-CENTRAL NORWAY
JOU
DEV
408
SIL
438
ORD
DEVONIAN DEFORIVATION
PEGMATITE (401*/ -
3
Ma)
? SCANDIAN DEFORMATION
?
HEILHORNET GRANITE Q44* / - 11 MA)
DE
FO R MAT I ON/I M BR I CAT I ON
UPLIFT, EROSION
505
CAM
AND DEPOSITION
POLYDEFORMED MARBLE AND
OCEANIC FRAGMENTS
IN HELGELAND
(ISLAND-ARC?)
MIGMATITIC GNEISS CUT
BY CAMBRO-ORDOVICIAN
GRANITOIDS
O. NORDGIJLEN & B. SCHOT]ENBORG
dl. l.9tl4). Since this zone is considered to be the presentjy
exposed western margin of the Helgeland Nappe Complex
(Kollung 1967; Roberts el dl. 1983; Nissen 1986; Nordgulen
& Bering 1987; Husmo & Nordgulen 1988), thrusting of the
et
complex across the Vestranden supracrustal sequence must
have occurred prior to or during the late Ordovician.
In the Helgeland Nappe Complex, the ophiolite
fragments and associated cover sequences described above
are juxtaposed with rock units dominated by partly
migmatitic gneisses and thick marble sequences (e.g. in
Tosen; cf. Fig. 1 in Thorsnes 1987). The gneisses and marble
were penetratively deformed prior to intrusion of large
tonalite and granodiorite plutons dated by th€ Rb-Sr
method to 503t23Ma and 526t10Ma, respectively
(Nissen 1986). The gneisses and marble as well as the
plutons are therefore older than the ophiolite fragments
(Fig. 11). Another consequence of this is that in the nappe
complex, amalgamation
of rocks of ophiolitic affinity with
older, continental type gneisses and marble must have
occurred in the Ordovician (Fig. 11). Age determinations
from the Mosjoen area are indicative of some middle
Silurian to early Devonian, Scandian, deformation (Torudbakken & Mickelson 1986). However, the data so far
available suggest that this Scandian event had only limited
effects in the nappe complex. This resembles the tectonic
evolution on Smola and Hitra (Fig. 1) where fossiliferous
metasedimentary and associated volcanic rocks of Arenig
Llanvirn age (Bruton & Bockelie 1979), were deformed
prior to emplacement of large granitoid plutons and dykes
yielding ages mainly between 450 and 430Ma (Roberts
1980; Gautneb 1988; Tucker 1988). Scandian deformation
has not been reported from this area (Hall & Roberts 1988).
Thus, in the granitoid-dominated terranes represented by
the nappe complex and the Smola Hitra area, important
tectonothermal events took place in the Ordovician.
Deformation in the Ordovician or earlier has also been
reported from elsewhere in the Upper Allochthon (Kullerud
et al. 1988) and the Uppermost Allochthon (Claesson 1979;
during the middle Silurian to the earliest Devonian (Gee
1978; Sturt 1984; Brekke et al. 1984; Stephens e1 al. 1985b;
Dallmeyer & Gee 1986). At this time the metasedimentary
rocks that are cut by the Heilhornet Pluton in Vestranden,
were already stitched to the Helgeland Nappe Complex, and
consequently they cannot be correlated with the Seve
Nappes (Schouenborg 1989). Whether they can
be
corelated with higher units in the Upper Allochthon is still
a matter of discussion.
The Scandian collisional event was followed by uplift,
erosion and sedimentation in the early to middle Devonian
(Steel er a/. 1985; Lux 1985; Tucker et al 1987). 'fhe
Devonian rocks are affected by regional folding and
low-grade metamorphism (Torsvik er al. 1988). ln the
Trondheimsfjord region, large fold structures are post lower
middle Devonian in age (Boe et al. 1989). These folds are
NE-trending and can be followed into the Grong-Olden
region. A pegmatite, which yielded a U-Pb zircon age of
401 + 3
Ma (Schouenborg 1988), occurs in Vestranden rocks
of the Heilbornet Pluton. The pegmatite is rather
strongly deformed and folded by upright folds with
NE-trending axes. This provides evidence that the
west
large-scale, NE-trending folds which are common through-
out Vestranden, are at least partly Devonian in
age.
Structures having similar trend and age are probably also
represented by the late folds in the Helgeland Nappe
Complex.
Conclusions
(1) The Heilhornet Pluton is a biotite t hornblende
of c.0.7070 indicates
contribution from continental material during magma
formation. A U-Pb zircon date of 444 + 11 Ma represents
rhe cr)\tallizalion age ol lhe glanite.
(2) The pluton intrudes the imbricate thrust zone
granite whose initial s?Sr/365r-ratio
separating the Helgeland Nappe Complex in the east from
supracrustal rocks of the underlying Vestranden sequence in
Cribb 1981).
the west. Mafic rocks in the imbricate zone are corelated
It should be noted that in contrast to what is found in the
Helgeland Nappe Complex, recent work in Vestranden
shows clear evidence for a Silurian tectonothermal event
(Johansson pers. comm. 1988; Schouenborg in prep.).
system (Dunning
The present work also has
consequences
for
the
correlation of tectonostratigraphic units in central Scandinavia. The metasedimentary rocks which lie structurally
above orthogneisses in Vestranden (Fig. 1) have been
assigned to the Seve Nappes of the Upper Allochthon (e.g.
Gee et al. 1985). Juxtaposition of these metasedimentary
rocks and the nappe complex must pre-date the intrusion of
the Heilhornet Pluton at 444t11Ma. To the east, the
Helgeland Nappe Complex overlies the Kdli Nappes (Fig.
1). Both the Lower and Middle Kitli Nappes show evidence
for island-arc development in the Tremadoc with subsequent
rift-related igneous activity and deposition in a marginal
basin environment (Claesson e/ a/. 1983; Stephens & Gee
1985; Claesson et d/. 1988). The sedimentary sequences in
the Lower Kdli Nappes are of Ordovician to early Silurian
age (Stephens et a/. 1985b). They became strongly deformed
and imbricated, and were thrust across the underlying Seve
Nappes during Scandian continent continent collision
(Stephens e, al. 1985b; Claesson et al. 1988; Dallmeyer &
Gee 1988).
ln Scandinavia. this is considered
to have taken
place
with the Leka Ophiolite complex, which is thought
to
represent an early Ordovician island-arc/arc basin type of
&
Pedersen 1988; Furnes er
a/.
1988)
Similar mafic rocks. as well as their associated cover
sequences, are also present within the Helgeland Nappe
Complex.
(3) This terrane was juxtaposed with
continental
metasedimentary rocks which had Sone through an early
tectonometamorphic cycle post-dated by granite intrusion at
526 1 10 Ma and 503 t Ma (Nissen 191i6).
(4) Combining lleld and isotopic data, it is concluded
that an important part of the tectonomagmatic development
of the Helgeland Nappe Complex took place in the
Ordovician. Thrusting and amalgamation of this composite
terrane with metasedimentary rocks in northem Vestranden
were also Ordovician events.
(5) The final accretion of the Helgeland Nappe Complex
and part of the underlying supracrustal rocks of the
Vestranden sequence onto the Baltoscandian continent
probably occurred during the middle Silurian to early
Devonian continent continent collision. ln addition, deformation during the Devonian affected central Scandinavia.
B.S. wishes to ackoowledgc support from a linancial granl (no.
3559-139) from the Swedish Natural Science Research Council.
HEILHORNET PLUTON, NORTH-CF,NTR
B.S. would also like to thaok Hans Schdberg and Eric Welin at the
Swedish Museum of Natural History for help during U Pb zircon
analysis. O.N. wishes to acknowledge financial support from the
Geological Survey of Norway and from project MT0020.20343
(Ores associated with Caledonian Batholiths) funded by the
Norwegian Council for Scientific and lndustrial Research (NTNF).
@.N. is indebted to B. KjOsnes (Trondheim) and T. Enger, A.
Stabel and B. Sundvoll (Oslo) for valuable help with Rb-Sr-isotope
analyses. J. Gersner (Lund) is thanked for technical assistance- THusmo provided detailed maps and took part in sampling in the
Slrfjorden area and is also thanked for many constructive
discussions on the geology of Helgeland. Significant improvement to
the manuscript resulted from comments by T. B. Andersen (Oslo),
H.
Furnes (Bergen),
D. Gee and R.
Gorbatschev (Lund), D.
Ramsay (Dundee), and B. Sturt and T. Thorsnes (Trondheim)
N. 1985. The snotigrcpht and structwal deuelophent of
Bergen, Noruay.
H,.
FURNES,
H,, NORDis. J. &
HERrcAEN,
J. 1984. LOWET
Palacozoic conv€rgent plate margin volcanism on Bomlo. Sw NoNay,
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scqucnce
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