F
.-) (;
t-\4t
Journal of the Geological Society, London,
Vol. L50,
1993, pp. 771-783,11 figs. Printed
in Northern Ireland
U-Pb zircon ages from the Bindal Batholith, and the tectonic history of the
Helgeland Nappe Complex, Scandinavian Caledonides
O. NORDGULENl, M.E. BICKFORD2, A. L. NISSENl & G. L. WORTMAN'
l
Geological Suruey of Norway, PO Box 3006-Lade, N-7002 Trondheim, Norway
2Department of Geology, Heroy Geology Laboratory, Syracuse Uniuersity, Syracuse, New York, 13244, USA
Abstract: The Helgeland Nappe Complex forms part of the Uppermost Allochthon in the central
Scandinavian Caledonides. It consists of metasupracrustal rocks of assumed Precambrian age which
are juxtaposed with metasupracrustal rocks present as non-conformable cover sequences to subjacent
fragments of Early Ordovician ophiolitic rocks. All of these rock units have been intruded by the
Bindal Batholith, which ranges in composition from mafic gabbro to leucogranite. In this paper, U-Pb
zircon ages from five plutons in the southern part of the batholith are presented. These are a
porphyritic granite of the Andalshatten pluton (447 +7 Ma), a tonalite from the Krikfjellet pluton
(443 17 Ma), a granodiorite west of Gisvassfjellet (437 X4Ma), the Kalwatnet monzonite (435+
10Ma), and a monzodiorite near Tosbotn (430+7 Ma). The dates are essentially concordant and are
considered to reflect the crystallization ages of the intrusions.
Based on regional correlations and the age of the oldest of the dated plutons, it is concluded that
internal accretion and polyphase tectonometamorphism of the Helgeland Nappe Complex took place
in Ordovician times. The final Silurian (Scandian) thrusting of the nappe stack across the continent of
Baltica appears to post-date emplacement of the Bindal Batholith.
The Bindal Batholith and its regional context
The Scandinavian Caledonides are characterized by a series
of generally westward-dipping nappe complexes commonly
referred to as the Lower, Middle, Upper and Uppermost
Allochthons (Roberts & Gee 1985; Gee et al. 1985). 'fhe
Lower and Middle Allochthons consist mainly of Proterozoic crystalline and sedimentary rocks which can be related
to the continental margin of Baltica. The Seve Nappes of
the Upper Allochthon are thought to represent a transition
zone between this continent and Iapetus (Gee
The Bindal Batholith is located within the Helgeland Nappe
Complex (Fig. 1), which is part of the Uppermost
Allochthon in the central Scandinavian Caledonides
(Ramberg 1967; Gustavson 1978, 1981, 1988; Gee et al.
1985). In the east, the Helgeland Nappe Complex lies
structurally above low-grade rocks (Kcili Nappe Complex) of
the Upper Allochthon (Foslie & Strand 1956; Lutro 1979;
Dallmann 1986, 1987). In the west it lies above Proterozoic
orthogneisses and medium-grade metasedimentary rocks of
uncertain age; this region constitutes the northern part of
the Western Gneiss Region and is commonly referred to as
Vestranden (Fig. i; Kollung 1967; Roberts et al . 1983;
Husmo & Nordgulen 1988; Schouenborg 1989).
1975),
whereas the Uppermost Allochthon and the Kcili Nappes of
the Upper Allochthon contain rock units which are exotic to
Baltica (Roberts 1988; Stephens & Gee 1989). The present
configuration of the nappe pile is a result of episodic
orogenic processes starting in the Late Cambrian to Early
Ordovician and culminating with Scandian collision in the
Silurian to Early Devonian (Gee 1975; Sturt 1984; Roberts
The rocks of the Helgeland Nappe Complex
& Gee 1985; Stephens & Gee 1985).
Granitoid plutonism of Caledonian age has been
recorded in several parts of the orogen, but is essentially
confined to the Upper and Uppermost Allochthons
(Stephens et al. 1985a). The rocks in the nappes are
commonly intensely deformed and metamorphosed, and
are
commonly highly deformed and generally record mediumgrade metamorphic assemblages. In a few areas, however,
greenschist-facies rocks are present (Gustavson 1975;
T6rudbakken & Mickelson 1986; Bucher-Nurminen 1988).
Thorsnes (1987), Nordgulen & Schouenborg (1990) and
Thorsnes & Loseth (1991), have identified two types of
metasedimentary complexes, which occur in different nappe
units. One of these comprises migmatitic gneiss, marble and
calc-silicate gneiss which may correlate with rocks of
Precambrian age further north in the Uppermost Allochthon
(e.g. Riis & Ramberg 1981; Brattli et al.1982). These rocks
contrast with sequences of conglomerate, schist, psammite,
calc-silicate schist and marble which are interpreted as an
unconformable cover sequence to variably-sized, mafic to
ultramafic lensoid bodies of ophiolitic rocks (Fig. 1). For
reasons discussed by Nordgulen & Schouenborg (1990) and
Thorsnes & Loseth (1991), these deposits are interpreted as
fossils are extremely scarce. Consequently, lithostratigraphic
correlation is severely hampered, and the dating of rocks
and tectonometamorphic events must largely be based on
isotopic age determinations.
U-Pb zircon dates on five plutons of the Bindal Batholith
in north-central Norway are presented here. Together with
earlier age determinations, the data suggest that a major
part of the batholith was emplaced in the period from Late
Ordovician to Early Silurian. The data also provide a basis
upon which the tectonic evolution of the Helgeland Nappe
Complex can be assessed.
771
Z. NORDGULEN ET AL.
772
Fig. 1. Geological setting of the Bindal
Batholith. The locations of Figs 2, 4 and
5 are shown on the map. Abbreviations:
HNC, Helgeland NaPPe ComPlex;
KNC, Kdli Nappe ComPlex; RNC,
Rcidingsfjiillet Nappe Complex; HP,
Heilhornet pluton; OM, Oksdal Massif;
VM: Visttindane Massif; R: ROdOY.
Early Ordovician or younger in age. The rocks suffered
polyphase deformation and metamorphism, including
imbrication and juxtaposition with the older gneisses. prior
being cut by Late Ordovician plutons of the Bindal
Batholith (see below).
The Bindal Batholith embraces a wide spectrum of rock
types ranging from mafic gabbro to leucogranite (Kollung
to
1967; Myrland 1972; Nissen 1974; Gustavson
& Mitchell
1988;
al. 1992; Nordgulen
in press). Equigranular and porphyritic granites and
granodiorites are the most common rock types, but
Nordgulen
1988; Barnes et
tonalites, monzonites and more basic rocks are also notable.
In the western part of the batholith, anatectic granitoids and
tourmaline granites are present.
The great majority of the rocks in the batholith are
metaluminous and high-K calc-alkaline (Nordgulen in
press). There is a general westward increase in 8tsr/86sr
initial ratios from fairly low values in the east (0.704-0.705)
to intermediate (0.705-0.71) and high values (>0.71) in the
west (Nordgulen & Sundvoll 1992). A combined isotopic
in feldspar) shows that
isotopically different crustal and sub-crustal source regions
were involved in the generation of the batholith (Birkeland
et al. 1993) . Rb-Sr whole-rock dates from various rock types,
and a U-Pb zircon date on the Heilhornet Pluton (Fig. i),
indicate ages between Late Cambrian and Silurian (Priem el
al. 1975; Gustavson & Prestvik 1979; Nissen 1986, 1988;
Torudbakken & Mickelson 1986; Nordgulen & Schouenborg
1990). However, the U-Pb dates presented here suggest that
some of the Rb-Sr dates are not reliable and that the main
intrusive activity took place in the Late Ordovician to Early
study (Sm-Nd, Rb-Sr and Pb-Pb
Silurian.
The geology of the dated plutons
The Andalshatten pluton
The Andalshatten pluton (see Appendix) is located in the
region between Velfjord in the south and Visten in the
AGE OF THE BINDAL BATHOLITH
It consists of porphyritic granodiorite to
granite and has a fairly wide range in composition from
mafic hornblende-biotite granodiorite (c. 58Vo SiOr) in the
northeast, to biotite granite (c.70Vo SiOr) in the south and
southwest. Chemical data (Nordgulen, in press) show
smooth variation trends. Sr-isotope data from a number of
samples plot on a Rb-Sr errorchron (MWSD:29.5) ol
north (Fig. 2).
+ 48 Ma with an initial ratio of 0.70855 + 0.00031
(Nordgulen & Sundvoll 1992). Nd-isotope data give similar
€"n-values between -4 and -5 for one fairly basic and one
evolved sample (Birkeland et al. 1993). These data are
consistent with the interpretation that the pluton constitutes
a single magmatic unit in which internal chemical variation
is probably controlled by crystal-liquid processes.
The pluton contains small mafic enclaves which are
elongate parallel to the mineral alignment. Granitic and
aplitic dykes are rare and mostly occur adjacent to or cutting
rafts of metasedimentary rocks. A number of N-S-trending,
grey, microporphyritic granitic dykes, which may be up to
448
several metres wide,
cut the pluton. The dykes
are
chemically and isotopically related to their host (Nordgulen
& Sundvoll 1992). Different types of mafic dykes also cut
the Andalshatten pluton.
A striking feature of the pluton is the abundance of
inclusions ranging in size from small xenoliths to large
mappable rafts usually orientated parallel to the foliation in
the host (Fig.2). In the west, the inclusions
consist
of rusty,
banded, calc-silicate schists and
psammites with local horizons of polymict conglomerates
predominantly
strong, polyphase, ductile deforrnation. In areas of
comparatively low strain, an approximately layer-parallel
pressure-solution cleavage (S1) is present in calcareous
metasandstone. Generally, this structure has been obliter-
ated by a strong NE-SW, steeply dipping foliation (S2)
which is axial planar to close or tight, high-amplitude folds
with sharply defined closures. The long axes of inclusions
are generally parallel to 52 and to transposed layering in the
metasedimentary rocks, and these structures as well as tight
F2 folds, are cut by dykes related
to the Andalshatten
pluton (Fig. 3). Thus, the structures in the metasedimentary
inclusions, in the western part of the intrusion, show that
they were subjected to a polyphase tectonometamorphic
evolution prior to being incorporated in the Andalshatten
pluton.
The metasedimentary rocks can be followed along strike
to the Horn area south of Velfjord (Fig. 2), where similar
rocks are present as part of the Br@nn0ysund Group
(Heldal & Hjelmeland 1987). This latter is considered to be a
cover sequence lying unconformably upon the Bolvaer Ophiolite (Heldal 1987). As outlined by Nordgulen & Schouenborg
(1990) and Thorsnes & LOseth (1991), the ophiolite fragments
occurring within the Helgeland Nappe Complex probably have
ages similar to that of the Leka Ophiolite Complex, where
quartz keratophyres have been dated at 497 L 2 Ma (Dunning
& Pedersen 1988). The cover sequence, including rocks
assigned to the Bronnoysund Group and the Tosen Group in
Velfjord and Bindal, may thus be no older than Early
Ordovician.
and marble. The metasedimentary rocks have suffered
L:]
-t
aONGLOMEFATE MAFBLE aALaAREOUS
SCHST GAFNETMICASCHST
I
a
N
eruz
4471 7Ma
3. Isoclinal F2 fold in a xenolith of calc-silicate schist with thin
bands of grey metasandstone. The folded banding and foliation in
the xenolith are cut by an undeformed aplite sheet. The hammer
handle is 55 cm long. (Locality: UTM 37875-727805, Andalshatten
Fig.
Fig. 2. The geology of the Andalshatten pluton and its immediate
surroundings (simplified from Nordgulen et al. 1992). Locality of
dated sample N87-02: UTM 37880-727800.
pluton).
Z. NORDGULEN ET AL.
In the southwestern part of the Andalshatten pluton, a
large area is occupied by dioritic to monzodioritic rocks with
smaller bodies of gabbro, olivine gabbro and peridotite
(Myrland 1972). The diorites are conspicuously heterogeneous with medium-grained, foliated, equigranular or
hornblende t plagioclase porphyritic texture. In most areas,
they are veined by an irregular network of plagioclase *
microcline porphyritic granodiorite to diorite which in places
appears to grade into the normal granodiorite of the
Andalshatten pluton. The contacts between the Andalshat-
ten pluton and the diorite complex are
generally
transitional, and over large areas scattered xenoliths of
diorite occur adjacent to the contact (see Myrland 1972, fig.
7).
In the south-central part of the Andalshatten pluton,
a zone of medium- to fine-grained, foliated tonalitic and
granodioritic rocks occurs as a steep wedge-shaped inclusion
(Fig. 2). These granitoids and the spatially associated zones
of marble and migmatitic gneiss are continuous along strike
to the area east of Velfjord in the south.
The
metasedimentary rocks continue northwards as prominent
rafts in the Andalshatten pluton. Along the central zone of
the pluton, south of Tasklivatn, a number of variably sized
inclusions of serpentinitic ultramafic rocks are present, the
largest of which is shown on Fig. 2. These lie approximately
along strike from ultramafic rocks in Velfjord (Figs 1 & 2)
and may represent the continuation of the zone of small
fragments of ultramafic rocks marking different tectonostratigraphic levels within the Helgeland Nappe
Complex (Thorsnes & Lgseth 1991). A similar relationship
is observed to the west of the pluton where small bodies of
ultramafic rocks as well as highly deformed metagabbro
occur together with mica schist west of the granitic gneisses
at Hamnpya (Fig. 2). These mafic bodies are considered to
be part of a discontinuous NE-SW-trending belt of ophiolite
fragments from Leka and Bolvr in the southwest, via
Hamnoya to Rodoy in the northeast (Fig. 1).
In the eastern part of the pluton the inclusions comprise
marble, calc-silicate rocks and migmatitic gneiss. These
rocks are also present east of the Andalshatten pluton as
well as on islands northwest of the pluton (Nordgulen el a/.
1992), and are interpreted to be part of an older, possibly
Precambrian sequence
of
metasedimentary rocks
in
the
Helgeland Nappe Complex.
The Krfrkfjellet pluton
The Krikfjellet pluton (see Appendix) is a NW-SE- to
N-S-trending elongate intrusion situated in the southwestern part of the Bindal Batholith (Figs 1 & 4). It consists of
medium to coarsegrained tonalite and granodiorite in the
north and medium- to fine-grained tonalite to quartz diorite
in the south (Nordgulen 1984). The pluton has a range in
show that most elements plot along well-defined arrays
(Nordgulen 1984). Six samples from the central part of the
pluton yielded a Rb-Sr date of 464
l30Ma (MSWD:3.74)
t 0.00005 (Nordgu-
with an "Sr/"usr initial ratio of 0.70549
len & Sundvoll 1992).
Elongate mafic enclaves, biotite-rich schlieren and, in
places, extremely abundant rafts of metasedimentary rocks
are generally orientated parallel to the fabric in the pluton.
Xenoliths of diorite and hornblende gabbro are also present;
southeast of Tosen, a substantial area is occupied by
Fig.
4. Geological map of the central part of the Krikfjellet pluton
al. 1989).
and its surroundings (simplified from Nordgulen et
Locality of dated sample N88-3: UTM38225-722215.
hornblende gabbro (Fig. a). The Krikfjellet pluton is cut by
aplite, pegmatite, granite and a variety of basic
and
composite dykes.
The contacts of the pluton are largely concordant with
the strong regional 52 schistosity in the wall-rocks, which
consist of migmatitic gneisses, schists, calc-silicate rocks and
banded marbles. The contact towards the Terrikfjellet
pluton is also concordant and fabrics in both intrusions are
contact parallel. The pluton cuts a strong 52 foliation in
metasedimentary inclusions, and detailed structural studies
led Nordgulen (1984) to suggest that the pluton
was
emplaced during the waning stages of D2 deformation. This
would imply that ductile strains were imposed on the rocks
no later than in Late Ordovician times. Granitoid
dykes
which cut the pluton and its inclusions are weakly deformed,
indicating that post-emplacement deformation was of
relatively little importance.
Granodiorite west of Gdsuassfj ellet
In the southeastern part of the batholith (Majafjellet
and
1& 5), tonalitic to granitic
intrusions (see Appendix) occur in migmatitic garnet
gneisses in steep belts parallel to the NE-SW- to
Gisvassfjellet area, Figs
N-S-trending foliation. The gneisses are also intruded by
small bodies of gabbro and diorite, and xenoliths of the
AGE OF THE BINDAL BATHOLITH
The Kaluuatnet monzonite
The Kalvvatnet monzonite (see Appendix) is
located
between Namsskogan and Tosbotn in the southeastern part
of the Helgeland Nappe Complex (Figs 1 & 5). It is a fairly
large pluton consisting of medium- to coarse-grained quartz
monzonite (Nissen 1988). Elongate, mafic enclaves are
common and are orientated parallel to the mineral fabric in
the pluton, which is cut by numerous granitic, aplitic and
pegmatitic dykes. The Kalwatnet monzonite has a limited
range in composition (58-63% SiOr) with high K'O (>4%)
and Rb (>150ppm). Ba and Sr consistently show values
greater than 0.17o, whereas high-field-strength elements
have moderate abundances (Nordgulen in press). Sr isotope
data do not yield a meaningful age (Nissen 1988), however,
based on the U-Pb zircon age of 435 Ma, the initial ratios for
the analysed samples range between0.7067 and0.7073.
The Kalvvatnet monzonite is partly surrounded by
porphyritic granitoids (Fig. 5). Contacts are sharp, and large
rafts of porphyritic to equigranular granitic rocks are present
in the northern and southern parts of the pluton. The
contact towards porphyritic granite at Kalvfjellet (Fig. 5) in
the south dips away from the monzonite. Taken together,
these observations lead to the conclusion that the latter is
younger than the porphyritic rocks.
Migmatitic garnet gneisses and marble with small bodies
of gabbro and diorite occur to the west of the Kalvvatnet
monzonite (Fig. 5). The metasupracrustal rocks have
transitional contacts towards granitoids in the OksdalenFinnlifiellet area (Fig. 5). Most of the Oksdal Massif (Fig. 1)
consists of granite which can be followed northwards to the
Fig. 5. Geological map of the Tosen-Majafjellet area (simplified
and revised from Gustavson 1981 and Nordgulen et a|.7990).
Sample localities: N87-03: UTM 40180-724475; N488: UTM
40610-7 22185 ; N89-61 : UTM 41 185-721680.
metasedimentary rocks are common in the granitoids
(Nordgulen et al. t990). The granitoids occur along strike
from tonalites and granodiorites near Namsskogan (Nissen
1986, 1988; see Fig. 1). Geochemically, the rocks in this part
of the batholith contain 60-70% SiOr. Dykes of
trondhjemite and tourmaline granite, which are present in
the gneisses near Namsskogan (Fig. 1), are more evolved,
with up to 75Vo SiO'.
At Gisvassfjellet, two main types of granitoids are
distinguished (Fig. 5). The intrusive rocks assigned to the
tonalite-granodiorite zone ate grey to dark grey and
medium- to coarse-grained with a fairly strong foliation
which is cut by numerous granitic and pegmatitic dykes and
veins. Rafts and xenoliths of migmatitic gneiss are oriented
parallel to the fabric in the intrusive rocks, and the contact
towards migmatitic gneiss in the west is partly transitional.
The granitoids to the east of the tonalite-granodiorite zone
(Fig. 5) are grey, medium-grained and equigranular; they
contrast with those to the west in having finer grain size and
lower abundances of biotite.
Nissen (1986, 1988) obtained Rb-Sr whole-rock dates of
526+I0Ma (MSWD:0.57) for granodiorite and 506t
26Ma (MSWD:3.93) for tonalite from near Namsskogan.
However, field relations indicate that the dark tonalites to
in the area
a sample of fairly mafic,
foliated granodiorite was collected for Zircon-dating
granodiorites are the oldest plutonic rocks
(Nissen 1986). From this unit,
(N89-61; Fig. s).
Tosbotn atea where it cuts monzodiorite dated at
Ma (see below). This would imply that the granites
younger than the
Kalwatnet monzonite.
430
+
5
of the Oksdal Massif are probably
The monzodiorite near Tosbotn
This is located on the northern shore of Tosen, c. 3km
southwest of Tosbotn (Figs 1 & 5). It is a coarse-grained,
mafic monzodiorite with c. 52Vo SiO. (see Appendix). The
high KrO content (2.8Vo) shows that it is part of the high-I(
monzodioritic to monzonitic suite occurring in the southern
part of the Bindal Batholith.
The geology in the Tosbotn area is characterized by
migmatitic gneisses with subordinate calc-silicate gneisses,
amphibolites and marble which are cut by a number of
intrusive rocks (Fig.
5). Small bodies of
serpentinite,
hornblendite and gabbro are present in the gneisses. The
porphyritic rocks south of Tosbotn are similar to those north
of the Kalwatnet monzonite. In Tosbotn and along the
southern shore of the fjord towards the southwest, mediumto coarse-grained rocks ranging from clinopyroxene-hornblende monzodiorite
to biotite-quartz monzonite
are
present. Chemical data from the Tosbotn area show that
these monzonites are very similar to the Kalwatnet
monzonite, and it is highly likely that they have a similar
age. The youngest intrusive rock in the area is a medium- to
fine-grained granite which has been mapped into the
Visttindane Massif to the north and the Oksdal Massif to the
south (Fig. 1). In the area to the west of the sampling
locality, the monzodiorite cuts folded migmatitic gneisses.
The monzodiorite contains inclusions of porphyritic granite,
but occurs as inclusions in the granite immediately
southwest of the sample locality. Thus, the sequence of
Z. NORDGULEN ET AL
776
intrusion established in the Tosbotn area is, with decreasing
age, porphyritic granite, monzodiorite and monzonite, and
granite.
U-Pb zircon age determinations
Analyses were corrected for blank using the composition of Pb
measured in the laboratory. Initial Pb was assumed to have the
composition given by the model of Stacey & Kramers (1975) for the
age of the sample. All samples contained enough radiogenic Pb that
a reasonable uncertainty in the composition of the non-radiogenic
Pb component does not contribute significantly to the uncertainty in
the calculated age. The decay
constants used were it38U:
x 10-ea-rand 123s1): o.SS+SS x 10 ua ' lsteiger & Jiiger
1977). Uncertainties (1o) in measured Pb ratios were typically
abolt 0.05Vo or less for 2o7Pbf2o6Pb and 208Pb/2o6Pb, and 1.0Vo or
less for 2oaPbl2o6Pb. The U-Pb ratios are considered accurate to
0.15513
Analytical methods
The U-Pb isotopic measurements reported in this paper were
performed by M.E.B. and G.L.W. in the Isotope Geochemistry
Laboratory at the University of Kansas, USA. Zircons from the
samples to be dated were separated by standard methods at the
Geological Survey of Norway. In Kansas, the zircons were further
purified by magnetic separation and hand picking to remove
residual impurities. The final, purified zircons were fractionated on
the basis of their magnetic susceptibilities; for two of the samples
the least magnetic fraction was air-abraded (Krogh 1982) to
improve concordance. For one sample (N88-3), monazite was also
separated and analysed.
Isotopic compositions and concentrations were determined by
standard methods for isotope dilution analysis following, with minor
modifications, the method of Krogh (1973); the data are given in
Table 1. Mass spectrometry was carried out using a VG Sector
multicollector instrument. During the period of this work analytical
blanks were 250 pg 2o6Pb and less than 200 pg 238U.
1.57o at 2o.
The regression and error analysis methods of Ludwig (19t30,
1983) were used. For four of the samples reported here the
analytical points are not collinear, but cluster about concordia (Figs
6-10). In this situation, the regression methods
commonly
employed are neither appropriate nor useful. We have, therefore,
made reasonable interpretations of the age and its uncertainty by
inspection of the computed ages for the fractions analysed (Table
1). Sample N89-61 (Fig. 8) yielded somewhat discordant data; these
were regressed to determine the age.
Results of age determinations
Five plutons from the southwestern part of the Bindal
Batholith were studied. A brief description of the zircons
Table 1. Analytical data for zircon fractions
Concentrations2
Fraction
r
U (ppp-) Pb (ppm)
N 87-02 (Andalshatten
NM (-2)
NM (-1)
NM (o)
NM (+1)
NM (+2)
M (+2)
Meas.3
2061204
Radiogenic Pba
207
1206
81.6
4831
22 573
t497
92.5
99.7
96.6
105.6
1483
t04.4
t6754
41.1
1502
59.5
90.9
1348
I 131
t321
1426
1398
N88-03 (Krdkfjellet pluton)
548
795
M (-1)
t0 526
0.05584
0.0s577
0.05587
17 499
0.0ss95
20 816
0.05581
0.05568
M (0)
M (1)
1247
t24r
95.8
2822
1176
Monazite
1813
570.6
7852
0.05613
0.05581
0.05611
0.05568
0.05588
N89-61 (Granodiorite west
N
(-1)
(-1)
AA
(0)
(+2)
2081206
2061238 207 1235
0.10185
0.10311
0. 1070.5
0.10879
0.12357
0.07178
2061238
Age
(Ma)
2071235
0.07012
0.06968
0.06888
0.06938
0.5526
0.5392
0.5368
0.5313
Age
(Ma)
(Ma)
0.5339
0.t2242
0.06926
0.5318
0.13000
0.13076
0.12528
0.14764
4.20395
0.07099
0.07055
0.07042
0.07113
0.06903
0.5494
0.5429
0.08510 0.06747 0.5172
0.5448
0.5461
0.5319
+
+
+
+
442.1
439 .5
438.7
+6.6 444.6+6.7
t 6.5 440.3 + 6 -6
+6.5 44t.6+6.6
r
+6.4
446.7
437.9
436.3
432.7
434.4+6.5
445.0+1..2
433.0 +
439.8+1.2
56.0
63.4
61.4
14234
16069
0.05560
0.05560
23 105
0.05562
63.8
22337
0.05563
315
24.7
1090
0.0557s
290
22.5
20.6
946
0.05522
418
0.05495
9s0
925
964
0.08284
0.08323
0.08780
6.5
457.3+4.t
444 .9 X
456.7
4.6
+2.4
443.0+6.6 442.4+6.7 439.5+5.2
12
430.3
+
420.9
L2.0
433.0
+
t2
447 .7
*
423.3
+2.0
436.3
+1.3
of Gdsuassfjellet)
846
+6.7 446.0+3.0
+6.5 443.3 +1.2
+6.5 447.2+2.0
+6.4 450.2+1.2
446.9
436.9
434.2
429.4
432.4
431.7
6.6
6.5
6.5
6.4
6.4
2071206
Age
pluton):
NM (-1)
NM
NM
NM
NM
Pb/U Ratios
0.06811 0.5222 424.8+2.0
0.06780 0.5199 422.9 L2.0
0.06729 0.5162 419.8 +2.0
1
.4
426.6+2.0 436.6+1.2
425.1
+2.0
422.6+2.0
437.2+1.2
437.9
+1.2
488: (Kaluuatnet monzonite)'.
NM (-1
M (-1)
M (0)
N 87 -03 (M o nzo dio
NM
NM
NM
NM
(-3) AA
(-3)
(-1)
(+1)
t
241
rite near
Tos
102
113
106
135
b o
tn)'.
8.3
9.0
8.8
1r.1
2098
4823
1343
1984
0.05470
0.05538
0.05547
0.05534
0.21127 0.06893 0.5298 429.7 +6.4
0.20543 0.06846 0.52t2 426.8 +6.4
0.20936 0.06931 0.5251 432.0 +6.4
0.30910
0.30558
0.06866 0.5178
o.0679'7
0.5190
0.33144
0.32643
0.06883
0.06821
0
.5264
0.5205
6.5
431.7 +
425 .9 + 6
428.5 +'7
442.2 + 5.6
.5 421 .0 + 6.6
.1 410.1 + 15
+6.4 423.7 +6.3 399.8 + 1.6
+6.3 424.5 +6.3 427.7 +1.7
6.4 429 .5 + 6.5 431 .4 + 4.8
6 .3 425 .s + 6.3 425 .9 + .5
428.1
423.9
429 .1 +
425 .4 +
NM, nonmagnetic; M, magnetic; numbers in parentheses indicate side tilt used on Franz separator at 1.5 amp power; AA, air abraded.
and Pb, corrected for analytical blank.
Measured ratios, uncorrected for blank or non-radiogenic Pb.
4Pb corrected for blank and non-radiogenic Pb; ages given in Ma.
U
'Total
3
1
AGE OF THE BINDAL BATHOLITH
is provided in the Appendix.
Analytical data and calculated ages based upon the ratios
tntPb/'noPb are given in Table 1.
^upblrrrlJ,2o7pbl235lJ, and
777
separated from the samples
KRAKFJELLET PLUTON
The Andalshatten pluton. We analysed 6 fractions of zircon
from a sample of the Andalshatten pluton (Fig. 2; N87-02;
Table 1). The data do not yield a linear array on the
concordia diagram (Fig. 6), although they do show a
206Pb/238U
and 't'Pb/tttu
normal distribution of decreasing
0.071
206p0
238
U
ages with increasing magnetic susceptibility, whereas the
tn'Pbl'*Pb ages are the same within t5 Ma. The variation
in 'ntPb/'noPb ages may reflect minor inheritance of older
zircon.The ages calculated for the least magnetic fraction
are concordant at 447 t7 Ma, and we interpret this as the
best estimate of the crystallization age of the pluton.
o
The Krdkfjellet pluton. Four zircon fractions and one monazite fraction were analysed from the Krikfjellet pluton (Fig.
4; N88-3; Table 1). The data plot near concordia (Fig. 7)'
but do not form a linear array. Two fractions (NM(-1) and
M(0); Table 1) have 'n'Pbl'*Pb ages that are significantly
greater than their U/Pb ages, possibly indicating inheritan-ce
6f small amounts of older zircon. We believe the 'nuPb/tttu
and 2o7Pbl23-5u ages (443.0 t 6.6 and 442.4 + 6.7 Ma, respectively) of fraction M(1), the most concordant fraction' best
determine the crystallisation age for the pluton, noting that
its 2o7Pb/2o6Pb age is 439.5 * 5.2 Ma and the 'ntPb/t*Pb ages
of fraction M(-1) and monazite are 444.9 t4.6Ma and
447.7 + 1.4Ma, respectively. On the basis of these data we
assign an age of 443 t7 Ma to this pluton.
Granodiorite west of Gdsuassfiellet A sample of the granodiorite west of Gisvassfjellet (Fig. 5) yielded four fractions
of these (N89-61, Table 1) yielded
of zircon. Analysis 2o6Pbl238I)
and'"'Pbft3sU ages but
slightly discordant
n7Pbf2(j6Pb ages that are identical within analytical uncertainties (Table 1). These data were regressed, yielding an
upper intercept age of 434.4 t 4.8 Ma and an unrealistic
lower intercept of -118 I214Ma. Regression of the data
with the lower intercept forced through zero yields a precise
upper intercept age of 437 Ma (Fig. 8). Noting that there are
somewhat greater uncertainties associated with individual
o
0.069
057
1-
l,4onazite
0.54
0.52
050
0.56
0.58
247 p61235 g
Fig.7. U-Pb concordia diagram for the Krikfjellet pluton.
206Pb/23tiu
4
and
t"'Pbf"tu
ages, we assign an age of 437 r.
Ma to this pluton.
The Kaluuatnet monzonite. Three fractions of zircon were
analysed from a sample of the Kalwatnet monzonite (Fig. 5;
N488; Table 1). Fig. 9 shows that the data points are not
collinear, with two points (M(1) and M(0)) plotting slightly
above concordia. This behaviour indicates U-loss relative to
Pb and may have been caused by incomplete dissolution of
the zircon fractions. The'nuPb/"tu and'ntPb/"tu ages vary
around 430 Ma, but individually have uncertainties of t
7Ma. The 2o7Pbl2o6Pb age of fraction NM(-1) is 442.2t
5.6 Ma. Because the data allow considerable uncertainty, we
assign an age of 435 * 10 Ma to this pluton.
Monzodiorite in Tosbotn. Fov zircon fractions were
analysed from a monzodiorite body near Tosbotn (Fig. 5;
GASVASSFJELLET GRANODIORITE
436
ANDALSHATTEN PLUTON
E,
432
0.069
428
206
206
pb
238
g
Pb
J
424
0.068
/,.a
238 g
4
a
.a
0.065
0.4s
-- 0.50
0.067
-
0.52
0.54
201
Fig.6.
p6l23s
0.56
U
U-Pb concordia diagram for the Andalshatten pluton.
0.50
0.51
0.52
0.53
2a7 p6l 235 l)
0.58
Fig.8. U-Pb concordia diagram for the granodiorite west of
Gisvassfjellet.
0.54
Z. NORDGULEN ET AL.
778
the published Rb-Sr ages of these bodies and similar to the
U-Pb age obtained for the Heilhornet pluton (444 A 11 Ma)
in the southwestern part of the batholith (Nordgulen &
Schouenborg 1990). The gabbroic and dioritic rocks which
are cut by these plutons are older, but at present it is not
possible to provide further constraints on the age of this
earlier mafic plutonism.
KALVVATNET MONZONITE
206 pb
238
435
Ll
Monzonitic and monzodioritic rocks gave ages of
+ 10 Ma for the Kalwatnet Monzonite and 430 a 7 Ma
for monzodiorite near Tosbotn. The geology of the Tosbotn
area (see above) shows that the Visttindane Massif, granitic
rocks east of Tosen, and the major part of the Oksdal
Massif are probably younger than the monzodiorite near
a substantial part of the Bindal
Batholith is Early Silurian in age.
Rb-Sr whole-rock data from the Bindal Batholith indicate
a broad age span and include Late Cambrian (Nissen 1986,
1988), Early Ordovician (Gustavson & Prestvik 1979) and
Silurian (Torudbakken & Mickelson 1986) dates. However,
the U-Pb age for the granodiorite west of Gisvassfjellet
@37 f 4 Ma) strongly suggests that the Late Cambrian dates
are incorrect. A possible explanation may be that the Sr
isotope analyses were carried out on samples with somewhat
variable initial Sr ratios. Regarding the U-Pb dates as more
reliable indicators of emplacement age, the results so far
obtained from the Bindal Batholith suggest that most of the
intrusive activity took place in the Late Ordovician to Early
Tosbotn. Consequently,
0.064
0.50
0.48
a.52
207
Fig.9.
0.54
0.56
0.58
p6l23s g
U-Pb concordia diagram for the Kalvvatnet monzonite.
N87-03; Table 1). Fig. 10 shows that three of the analytical
points plot in a quasiJinear group along concordia whereas
one point (NM(-3)AA) plots slightly above concordia. As
noted above, this behaviour may have resulted from incom-
plete dissolution of the zircon fraction. We prefer to interpret the age from the quasi-linear artay of points, noting
that fraction NM(-l) is essentially concordant at 430 1
7 Ma. We believe that this age provides the best indication
of the age of the pluton.
Discussion
The age determinations reported in this paper have
consequences for understanding the evolution of intrusive
activity in the southern part of the Bindal Batholith. The
oldest dates have been obtained from the Andalshatten
(447 +7 Ma) and the Krikfjellet (4$ r 7 Ma) plutons, which
are probably Late Ordovician in age according to the time
al. (1990). The dates are concordant with
TOSBOTN
Late Ordovician to Early Silurian intrusive activity
is
also a characteristic of the Smola-Hitra Batholith in central
Norway (Gautneb 1988; Tucker 1988; Gautneb & Roberts
1989) and the Sunnhordland Batholith in southwest Norway
(Andersen & Jansen 1987; Fossen & Austrheim 1988).
Although most of the dates are based on Rb-Sr whole-rock
data, it may be concluded that, at this time, important parts
The age of the Bindal Batholith
scale of Tucker et
Silurian.
I,4ONZODIORITE
of the Upper and Uppermost Allochthons of
the
Scandinavian Caledonides were characterized by extensive
granitoid plutonism.
Ordouician tectonometamorphism in the Helgeland
Nappe Complex
The Helgeland Nappe Complex, as well as the Uppermost
Allochthon in general, has been considered to represent an
exotic terrane with respect to the continent of Baltica
(Roberts 1988; Stephens & Gee 1989). Recent work in the
southwestern part of the nappe has confirmed the composite
nature of the terrane, and it is now considered to consist of
at least two different sequences of rocks, the older of these
probably being Precambrian
206 pn
238
I
0.064
0.48
0.50
0.52
0.54
247 p6l 235
0 56
U
Fig. 10. U-Pb concordia diagram for the monzodiorite near
Tosbotn.
in age (Thorsnes 1987;
Nordgulen & Schouenborg 1990; Thorsnes & LOseth 1991).
The younger sequence consists essentially of metasedimentary rocks which were derived from rocks of continental
affinity as well as from a substrate which is currently
preserved as fragments of variably tectonized ophiolitic
rocks (shown as greenstone, gabbro and ultramafic rocks in
Fig. 1).
From studies of the Heilhornet pluton (Fig. 1) and its
surroundings (Husmo & Nordgulen 1988; Nordgulen &
Schouenborg 1990), it was concluded that the initial
thrusting of the Helgeland Nappe Complex, and probably
also its internal amalgamation, took place prior to intrusion
of the pluton at 444|llMa. As described above, the
Andalshatten pluton intrudes both types of metasedimen-
AGE OF THE BINDAL BATHOLITH
semblages in the cover sequences to the ophiolitic rocks are
WGF
z
:
NE-SW UPRIGHT FOLDS
z
DEPOSITION OF
DEVONIAN ROCKS
o
IJJ
,i'i'i
o
z
:
E
:f
J
(t)
z
I
o
441, , 1
'i
+1lJ
ITT Ttori
PEGMATITES (WGR)
Bindal Batholith.
DEFORMATION &
METAMORPIIISM
Helgeland Nappe Complex has consequences for the
reconstruction of the Caledonian mountain belt. The
(scaNDrAN THRUSTTNG)
'
oT .l
HNC:
a
2t
o
OEFORMATION &
METAMORPHISi,l
ASSEMBLY OF THE
NAPPE COMPLEX
o
tc
o
ISLAND.ARC FORMATION
(LEKA OPHIOLITE)
+
Fig. 11. Summary diagram showing relevant age determinations
and the main geological events in the Uppermost Allochthon and
the adjacent part of the Western Gneiss Region (WGR); commonly
referred to as Vestranden. Notice that the exact timing and duration
of tectonic and intrusive events are uncertain. In the Ordovician.
juxtaposition of oceanic and continental terranes formed the
Helgeland Nappe Complex; Ordovician tectonometamorphism has
not been documented from the gneisses of the Western Gneiss
Region. The U-Pb zircon dates suggest that most of the Bindal
Batholith was emplaced in the Late Ordovician to Early Silurian.
The intrusive activity may overlap in time with Ordovician
deformation in the Helgeland Nappe Complex as well as with
deformation related to the initiation of the Scandian orogeny (Early
Silurian to Early Devonian). The Scandian orogeny culminated with
the collision between Laurentia and Baltica and led to high-P
metamorphism in the Western Gneiss Region (Griffin et al. 1985;
Johansson et al. 7990). The already-assembled Helgeland Nappe
Complex was thrust eastwards across low-grade rocks of the Upper
Allocthon.
The time scale used is essentially that of Tucker et al. (1991,).Filled
square, U-Pb zircon age; filled diamond: Rb-Sr whole-rock age;
filled circle, Rb-Sr mineral age; filled triangle, Sm-Nd mineral age.
The dates from the Uppermost Allochthon are from: (1) Dunning
& Pedersen 1988; (2) Gustavson & Prestvik 1979; (3) Nordgulen &
Schouenborg 1990; ( -8) this work; (9 & 10) Tdrudbakken &
Mickelson 1986; (11) Claesson 1979; (12) Trudbakken & Brattli
1985. The dates from the Western Gneiss Region are from: (13 &
16) Johansson et
al. 1990; (14) Schouenborg
The Ordovician tectonometamorphic history of
the
within the Helgeland Nappe
Complex, including the Leka Ophiolite Complex, have been
assigned to the so-called Group I ophiolites in the
Scandinavian Caledonides (Furnes et al. 1985). These are
Early Ordovician in age (Dunning & Pedersen 1988) and
originated in a supra-subduction zone environment (Pedersen et al. 1988). Evidence from elsewhere in the
Scandinavian Caledonides shows that the ophiolites were
accreted to continental rocks, and subsequently uplifted an<!
fragmented ophiolites
INTRUSIVE ACTIVITY
(BINDAL BATHOLITH)
,rl
related to the deformation during amalgamation of the
nappe complex (Thorsnes & LOseth 1991). Since the
Andalshatten pluton clearly cuts S2-fabrics (Fig. 3), it also
provides a minimum age for this tectonometamorphic
episode. Consequently, the rocks of the Helgeland Nappe
Complex went through a polyphase, Ordovician, tectonometamorphic evolution prior to emplacement of the
et al . 1991; 15)
Schouenborg 1988.
tary sequences present within the Helgeland Nappe
Complex as well as serpentinitic rocks similar to those of the
ophiolite fragments. Thus, the age of the pluton
(447 +7 Ma) confirms the conclusion of Nordgulen &
Schouenborg (1990) that the various elements of the
Helgeland Nappe Complex had been juxtaposed no later
than in the Late Ordovician (Fig. 11). The dominant
S2-foliation and the medium-grade metamorphic as-
eroded in the Early to Mid-Ordovician (e.g. Sturt 1984).
There are diverse opinions as to the nature and
palaeogeographic position of the continental block onto
which the ophiolites were juxtaposed; both Baltica and
Laurentia, as well as a microcontinent have been suggested
(Stephens & Gee 1985, 1989; Pedersen et al. 1988; Sturt &
Roberts l99I). Applying this general model to the
Helgeland Nappe Complex, the metasedimentary sequences
overlying the ophiolites may be regarded as a response to
initial accretion of an island arc to a continental block
consisting of Precambrian paragneisses. Subsequently, the
different rock complexes were imbricated and strongly
deformed to form the composite Helgeland Nappe Complex
(see also Stephens et al. 1985b). The palaeogeographic
position of the continental block cannot be determined,
however, since the Ordovician tectonometamorphic history
of the Helgeland Nappe Complex is distinctive both in
timing and character compared to that along the margin of
Baltica, it is unlikely that the ophiolite fragments in the
nappe complex interacted with rocks of Baltic affinity during
initial juxtaposition with continental crust.
Bucher-Nurminen (1988) pointed out that boundaries of
metamorphic grade cross nappe contacts in several areas in
the central Scandinavian Caledonides, indicating that
prograde metamorphism reached its peak during or shortly
after Scandian thrusting in the Early to Middle Silurian.
However, as argued above, the Andalshatten pluton cuts 52
and post-dates the peak of medium-grade metamorphism in
the Helgeland Nappe Complex.This apparent contradiction
may be resolved
if one allows for an initial amalgamation of
the Helgeland Nappe Complex to subjacent rocks prior to
final translation during Scandian collision. Such a situation
appears to obtain along the southwestern part of the nappe
where metasedimentary rocks of the Vestranden sequence
were juxtaposed with the Helgeland Nappe Complex before
the intrusion of the Heilhornet pluton (Nordgulen &
Schouenborg 1990). It has also been suggested that parts of
the Upper Allochthon were accreted to the Uppermost
Allochthon during the Early Ordovician (Stephens & Gee
1989). Thus, the upper part of the nappe pile in central
Scandinavia may preserve the record of a complex history of
Ordovician accretion which led to juxtaposition of different
tectonic units.
Z. NORDGULEN ET AL
780
Post-Ordouician tectonic deuelopment
The strong N-S to NE-SW foliation in the granodiorite
west of Gisvassfjeller (437 + 4 Ma) shows that deformation
in the southeastern part of the Helgeland Nappe Complex
took place near the Ordovician-Silurian boundary or later,
which is in agreement with the conclusion from structural
studies in the Koli Nappe Complex south of Rossvatn (Fig.
1; Dallmann 1986). However, the monzonitic to mon-
zodioritic rocks (435410Ma and 430+7Ma), which are
the youngest dated piutons of the Bindal Batholith (Fig. 11),
exhibit little evidence of having been subjected to strong
regional deformation. Considering the internal structure of
the Helgeland Nappe Complex, the foliation in the central
to western part strikes predominantly N-S to NW-SE' and
lineations are weakly developed. In contrast, the rocks of
the Vefsnfjord and Ranafjord areas (Fig. 1)
are
characterized by a steeply dipping NE-SW foliation and a
pronounced lineation with shallow NE or SW plunge' The
general NE-trending structures in the NW and SE parts of
the Helgeland Nappe Complex are similar to that of the
Vestranden area (Fig. L), where important deformation
took place in the Silurian (see below). This may suggest that
the structural patterns in parts of the Helgeland Nappe
Complex are a result of fairly strong Silurian modification,
whereas the central parts of the nappe complex appear to
have been less affected by such strains.
Late two-mica granite dykes, which are common
throughout the batholith, have been deformed by upright,
open, N-S- to NE-Sw-trending, regional folds commonly
regarded as D3-structures. A pegmatite, which occurs in
Vestranden rocks SW of the Heilhornet pluton, is folded by
the NE-SW-trending upright folds. A U-Pb date of
t 3 Ma for the pegmatite (Schouenborg i988) indicates
that the folding took place no earlier than in the Early
Devonian. A Devonian age for the folding is supported by
structural studies in Vestranden by Piasecki & Clitr (1988)
401
and also by the fact that Devonian sedimentary rocks in
central Norway are affected by similar folds (e.g. BOe et al1e89).
Batholith (Nordgulen & Sundvoll 1992). In this model, the
intrusion of Early Silurian plutons, being among the latest
products of igneous activity, would broadly have coincided
with destruction of the subduction system and the early
stage of Scandian collision and thrusting (Fig. 11). The
nature of the collision process is extremely diffrcult to assess
from the present complicated nappe stack. Nevertheless, the
apparent lack of plutons of similar age in the tectonostratigraphic units adjacent to and underlying the
Uppermost Allochthon strongly suggests that the final
positioning of the nappe post-dates intrusive activity. The
tectonometamorphic history of the southwestern Helgeland
Nappe Complex, exemplified by the metasedimentary rocks
described above, shows that the rocks preserved in the
upper part of the nappe stack did not reach deep crustal
levels during the collisional stage.
The salient features of the development of
the
Uppermost Allochthon, with some additional information
from the adjacent part of Vestranden, are shown
schematically in Fig. 11. Although the durations of
tectonometamorphic and magmatic events are uncertain, it
is clear that, in the Helgeland Nappe Complex, a significant
Ordovician tectonometamorphic cycle pre-dates the emplacement of the Bindal Batholith. In contrast to the Upper
Allochthon, there is no evidence for a Late Cambrian to
Early Ordovician high-P event as recorded by the presence
of eclogites of this age (Mork et al. 1988) and Early to
Mid-Ordovician uplift dates (e.g. Dallmeyer & Gee 1986,
1988; Dallmeyer & Stephens 1991).
A polyphase Ordovician tectonometamorphic history
followed by extensive Late Ordovician to Early Silurian
granitoid plutonism, and generally less penetrative Scandian
deformation are features which the Helgeland Nappe
Complex has in common with the Smola terrane in Central
Norway (Roberts 1988; Nordgulen & Schouenborg 1990). In
contrast, the Western Gneiss Region is characterized by the
absence of Ordovician deformation and a pronounced
Silurian to Early Devonian tectonometamorphic evolution
(Tucker et al . 1987, 1991; Mciller 1988; Piasecki & Clitr
1988; Johansson et
al.
1990; Schouenborg et a/. 1991). Thus,
the Helgeland and Smola terranes appear to have
a
Regional considerations
The final eastward thrusting of the Helgeland Nappe
Complex across the margin of Baltica took place during
Scandian continent-continent collision in Mid-Silurian to
Early Devonian time (Gee 1975; Roberts & Gee 1985;
Stephens et al. 1985b; Dallmann 1986, 1987; Stephens &
Gee 1989). From studies of metamorphism in basement
tectonometamorphic and tectonomagmatic history unlike
that of the terranes with which they are juxtaposed. More
geochronological data are required to obtain a better
understanding of this complex evolutionary history.
Ma ago. Southwest of the Helgeland Nappe Complex, in
the Vestranden part of the Western Gneiss Region, the
maximum P-T event (14kbar, c.850"C) has been dated at
432t6Ma (Sm-Nd mineral isochron, Fig. 11), with
subsequent near-isothermal uplift in the Late Silurian and
Devonran (Moller 1988; Johansson e/ a/. 1990). Highpressure metamorphism in the Silurian, which has been
reported from abundant eclogites in the Western Gneiss
Region, has been interpreted in terms of imbrication and
northwestward subduction of the Baltic margin beneath
Laurentia (Griffin et al.1985).
have been dated by the U-Pb method on zircons. The dates
windows, Lindqvist (1990) concluded that Scandian
thrusting in central Scandinavia occurred later than 430 to
Conclusions
Five plutons from the southern part of the Bindal Batholith
435
range between c. 447 and 430 Ma, indicating that a
substantial part of the Bindal Batholith is Late Ordovician
to Early Silurian in age. Although more age determinations
are needed, it appears that during this time period, granitoid
magmatism was common in the Upper and Uppermost
A Late Ordovician to Silurian westward-dipping
subduction zone would be compatible with an east to west
increase
in initial Sr isotope ratios within the
Bindal
Allochthons of the Scandinavian Caledonides.
Internal accretion
of the
Helgeland Nappe Complex
occurred at low- to medium-grade conditions and involved
rocks of assumed Precambrian age, as well as Early
Ordovician ophiolitic rocks and their non-conformable cover
sequences. The dated plutons are essentially younger than
these tectonometamorphic events, and pre-date the final
Scandian thrusting of the Helgeland Nappe Complex across
AGE OF THE BINDAL BATHOLITH
Baltica. Penetrative Silurian strains are probably present
only in parts of the Helgeland Nappe Complex.
The granitoid-dominated Helgeland and Sm@la terranes
have a geological history characterized by polyphase
Ordovician deformation and metamorphism followed by
extensive calcalkaline plutonism prior to thrusting onto
Baltica during Scandian continent-continent collision.
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 Industrial Research (NTNF). H.Hatling
and
B.Johansen are thanked for skilful separation of zircons. We thank
P.M.Ihlen. D.Roberts and B.A.Sturt for constructive comments.
Thorough reviews by F.Corfu and M.B.Stephens significantly
improved the manuscript and are gratefully acknowledged.
Appendix: Petrographic description of the dated
plutons and description of zircons
The Andalshatten plnton consists of porphyritic granodiorite to
granite with prominent grey to white, subhedral microcline
The groundmass is medium grained and consists of
plagioclase and microcline with aligned, lenticular clusters of
quartz. Biotite is mostly present in elongate aggregates, commonly
associated with stubby prisms of hornblende. Euhedral titanite and
epidote with allanite cores are common in mafic varieties- In
addition, apatite, zircon and opaques are present. In the western
part of the pluton, there is a moderate NE-SW to NW-SE mineral
alignment defined by oriented megacrysts, lenticular quartz and
aggregates of mafic phases. Towards the east, the fabric becomes
stronger, and the pluton has a penetrative N-S foliation with
sub-parallel microcline augen in a strongly deformed groundmass.
Zircon description: The sample contains abundant zircon. The
crystals are euhedral, equant to prismatic with length:width ratios
up to 5. They are colourless transparent or pale yellowish grey,
transparent to translucent. A few grains contain inclusions of
megacrysts.
opaques.
-fhe Krfrkfjellel pluton consists predominantly of tonalite and
granodiorite. Tabular, subhedral laths of pale grey plagioclase,
glassy grey quartz and variably sized flakes of biotite are the most
important rock-forming minerals. Hornblende is present in the
more mafic varieties, but is always subordinate to biotite. Grey
microcline is usually interstitial, but larger grains may include
plagioclase, biotite, quartz and hornblende. In some samples,
epidote is an important mafic phase, reaching up to 2-37o of the
mode. Common accessory minerals include allanite with epidote
rims, titanite, apatite, zircon, monazite, opaques, white mica and
chlorite. The pluton exhibits a well-developed, flattening-type,
mineral alignment defined by tabular plagioclase, elongate clusters
of quartz, and oriented single crystals and clusters of mafic
minerals. This fabric is considered to be probably igneous in origin
and grades into zones of a penetrative foliation in which the
minerals are partly or completely recrystallized.
Zircon description: Small, euhedral, transparent and colourless
crystals with aspect ratios up to 1:5 are present together with
broken fragments of colourless to light grey, translucent grains.
Monazite is present as irregular, angular and rounded grey crystals.
The granodiorite west of Gfisuassfjellet is a dark grey, mediumto coarse-grained, foliated granodiorite. Generally, this rock type
contains subhedral plagioclase and lenticular clusters of grey quartz.
Small grains of biotite constitute 15-20 % of the rock. Microcline
may occur as small megacrysts (<10mm), and the accessory
minerals present are hornblende, clinopyroxene, epidote, allanite,
titanite, apatite, zircon, opaques and chlorite (Nissen 1986).
Zircon description: Euhedral to subhedral, transparent crystals
of fairly uniform size with length:width ratios about 2 to 3.
781.
'fhe Kaluuatnel monzonite consists of medium- to coarse-grained
quartz-monzonite. Lath-shaped plagioclase and microcline and
euhedral prisms of hornblende are the dominant minerals. Small
crystals of biotite and quartz are present, and the accessories
comprise titanite, epidote, apatite, zircon and opaques. In a small
area in the northwestern part of the pluton, the texture
is
characterized by prismatic needles of amphibole in a mediumgrained groundmass. The pluton generally exhibits a moderately- to
well-developed flattening-type fabric defined by a preferred
orientation of hornblende prisms and tabular feldspar. The fabric is
approximately parallel to the N-S foliation in the wall-rocks and
has steep to intermediate dips to the east.
Zircon description: The sample is very rich in zircon. The
to yellowish grey, non-transparent, rounded and
ellipsoidal with length:width ratios about 1.5. A number of grains
crystals are grey
contain opaque inclusions.
The monzodiorite near Tosbotn consists of elongate, pale pink,
euhedral, twinned tablets of plagioclase (<20 mm) intergrown with
euhedral, prismatic hornblende (<15 mm) on which small flakes of
biotite (c. 2mm) occur. K-feldspar and quartz are interstitial, and
accessories include apatite, titanite, zircon and opaques. A weak
fabric in the rock is defined by crudely aligned crystals of
plagioclase and hornblende.
Zircon description: The sample contains abundant zircon' The
crystals are translucent, colourless to grey, and subhedral to
rounded with length:width ratios roughly between 1 and 2. Opaque
inclusions occur in some grains.
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