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> Forsiden > Publikationer > Geology of Greenland Survey Bulletin > Vol. 191 Geol. Greenl. Surv. Bull. > Review of Greenland Activities 2001, pp 13-23

GEOLOGY OF GREENLAND SURVEY BULLETIN 191

 
Precambrian geology of the northern Nagssugtoqidian orogen, West Greenland: mapping in the Kangaatsiaq area

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13
The Nagssugtoqidian orogen and its transition into the
Rinkian orogen to the north were the main focus of the
field activities of the Geological Survey of Denmark and
Greenland (GEUS) in West Greenland in the summer of
2001. This work was carried out within the framework
of the Survey's three-year programme of bedrock
mapping and mineral resource evaluation to enhance
the understanding of the Archaean and Palaeoprotero-
zoic crustal evolution in the transition zone between
the Nagssugtoqidian and Rinkian orogens (Fig. 1). The
work in the field season of 2001 comprised geological
mapping of the 1:100 000 Kangaatsiaq map sheet
described in this paper (Fig. 2), an investigation of the
supracrustal rocks at Naternaq / Lersletten (Østergaard
et al. 2002, this volume), a geochronological recon-
naissance of the southern Rinkian orogen in the north-
ern Disko Bugt region (Garde et al. 2002, this volume),
a resource evaluation of the Nagssugtoqidian orogen
(Stendal et al. 2002, this volume), a synthesis and inter-
pretation of geophysical data of the central part of the
Nagssugtoqidian orogen (Nielsen et al. 2002, this
volume) and a report on investigations of the kimber-
lites and related intrusive rocks in the southern
Nagssugtoqidian orogen and its foreland (Jensen et al.
2002, this volume).
The present investigations build on recent previous
activities in the region. The Disko Bugt project of the
former Geological Survey of Greenland investigated
the geology and evaluated the resource potential of
the southern part of the Rinkian orogen between
Nuussuaq and Jakobshavn Isfjord from 1988 to 1992
(Fig. 1; Kalsbeek 1999). The Danish Lithosphere
Centre (DLC) led a research project from 1994­1999
into the tectonic evolution of the Nagssugtoqidian
orogen concentrating on the southern and central
segments of the orogen between Sukkertoppen Is-
kappe and Nordre Strømfjord (Marker et al. 1995; van
Gool et al. 1996, in press; Mengel et al. 1998; Connelly
et al. 2000). Previous activity in the area between
Nordre Strømfjord and Jakobshavn Isfjord (Fig. 1)
included reconnaissance mapping by Noe-Nygaard &
Ramberg (1961), 1:250 000 scale mapping by Hender-
son (1969), and visits to key localities during the DLC
project (Marker et al. 1995; Mengel et al. 1998) from
which a few reconnaissance age determinations are
known (Kalsbeek & Nutman 1996). Most of this area
was known from coastal exposures, while map infor-
mation for large parts of the inland areas was based
only on photogeological interpretation. The mineralised
parts of the Naternaq supracrustal belt were investigat-
ed in detail by Kryolitselskabet Øresund A/S from
1962­1964 (Keto 1962; Vaasjoki 1965). Immediately
south of latitude 68°N the 1:100 000 scale Agto (Attu)
map sheet was published by Olesen (1984), and the
adjacent Ussuit map sheet to the east is in preparation
(Fig. 1). Mapping in 2001 concentrated on the Kan-
gaatsiaq map sheet area and the Naternaq area
(Østergaard et al. 2002, this volume), while mapping
activity for 2002 is planned between Naternaq and
Jakobshavn Isfjord (Fig. 1).
The field work in 2001 was supported by M/S
Søkongen as a floating base from which field camps
were established. The shoreline exposures are excel-
lent and the many islands and extensive fjord systems
in the map area provide easy access. Limited heli-
copter support was available for establishment of a
few inland camps and reconnaissance in areas far from
the coast.
The Nagssugtoqidian orogen
The Nagssugtoqidian orogen is a 300 km wide belt of
predominantly Archaean gneisses which were re-
worked during Palaeoproterozoic orogenesis. It is
characterised by E­W-trending kilometre-scale folds
and ENE­WSW-trending linear belts. It is divided into
three tectonic segments: the southern, central and
Precambrian geology of the northern Nagssugtoqidian
orogen,West Greenland: mapping in the Kangaatsiaq area
Jeroen A.M. van Gool, G. Ian Alsop, Uni E. Árting, Adam A. Garde, Christian Knudsen,
Adam W. Krawiec, Stanislaw Mazur, Jeppe Nygaard, Sandra Piazolo, Christopher W.Thomas and
Kristine Thrane
Geology of Greenland Survey Bulletin 191, 13­23 (2002) © GEUS, 2002
GSB191-Indhold 13/12/02 11:29 Side 13
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14
Svartenhuk
Halv
aigat
vik
Innerit
Itsaku
Simiuttap Kuua
Ubekendt
Ejland
Schade er
Qeqertarsuaq
Fig. 2
Eocene and y
Palaeogene basalts
Oil check in 1999
Intensive oil staining and seepage
Minor oil staining
Maastrichtian Paleocene
sediments
Albian Campanian
sediments
Precambrian basement
Extensional fault
Fault with lateral or
alternating displacements
Deep well
500 km
Inland Ice
Greenland
Inland
Ice
Greenland
Iceland
Canada
Granitic intrusions (s.l.)
Archaean gneiss reworked
in the Palaeoproterozoic
Supracrustal rocks
Orthogneisses
Intermediate to basic intrusions
Supracrustal rocks
(undifferentiated)
PROTEROZOIC
Anap Nunâ Group
(metasediments)
Granitic intrusions (s.l.)
Calc-alkaline intrusions
Arfersiorfik and Sisimiut suites
PHANEROZOIC
ARCHAEAN
Boye Sø anorthosite complex
Thrust
Sarfartoq Carbonatite Complex
Cretaceous­Palaeogene sediments
and volcanic rocks
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
0
50 km
K
D i s
k o
B u
g t
Sukkertoppen
Iskappe
t
t
Jakobshavn Isfjord
Ussuit
Nordre Strømfjord
Arfersiorfik
Aasiaat
Qeqertarsuaq
Ilulissat
Qasigiannguit
Kangaatsiaq
Attu
Sisimiut
Søn
dre
Strø
mfjor
d
Kangerlussuaq
Naternaq
Nuussuaq
Vaigat
A
B
C
68
°
66
°
51
°
70
°
Disko
Nag
ssugtoqidian o
r
ogen
Rinkian o
r
ogen
Nor
th
At
lantic
craton
SNO
CNO
NSSZ
ITZ
NNO
Inland
Ice
Fig. 1. Geological map of southern and central West Greenland, showing the divisions of the Nagssugtoqidian orogen and the bound-
aries with the North Atlantic craton to the south and the Rinkian orogen to the north. Outlined areas indicated A, B and C are, respec-
tively, the Kangaatsiaq, Agto (Attu) and Ussuit 1:100 000 map sheets. ITZ: Ikertôq thrust zone. NSSZ: Nordre Strømfjord shear zone.
SNO, CNO and NNO are, respectively, the southern, central and northern Nagssugtoqidian orogen. Modified from Escher & Pulvertaft
(1995) and Mengel et al. (1998).
GSB191-Indhold 13/12/02 11:29 Side 14
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15
68
°
15'N
53
°
30'W
54
°
30'W
52
°
30'W
52
°
30'W
68
°
15'N
53
°
W
Atan
eq
Ala
nng
ors
uup
Ima
a
Kangaatsiaq
Ik
erasarsuk
T
uttulik
Qeqer
tarsuatsiaq
Tunorsuup Nunaa
Tar
ajo
mit
soq
Aa
ppilattoq
Naternaq
g
g
g
g
Ik
erasak
53
°
52
°
30´
53
°
30´
Qeq
ert
ars
ua
tsi
aa
pT
un
ua
ta
Im
aa
d
d
d
d
Saq
qar
put
Arf
ers
iorf
ik
Tun
ors
uaq
Atarngup Nunaa
Quaternar
y
Metadolerite dyk
e
Granite or
granitic gneiss
Quar
tzo-f
eldspathic
or
thogneiss (tonalitic)
Mafic supracrustal
sequence
Mafic intrusiv
e
complex
Dioritic to quar
tz-dioritic
gneiss
Globule dyk
e
,
dashed wher
e inf
er
r
e
d
Quar
tzo-f
eldspathic
paragneiss
g
d
10 km
Mica schist and other
metasedimentar
y r
ocks
Fault
68
°
15´
Fig. 2. Simplified geology of the Kangaatsiaq map sheet. For location, see Fig. 1, frame A.
GSB191-Indhold 13/12/02 11:29 Side 15
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northern Nagssugtoqidian orogen (SNO, CNO and
NNO, Fig. 1; Marker et al. 1995). These segments are
interpreted by van Gool et al. (2002) as, respectively,
a southern parautochthonous foreland zone, a central
collisional core of the orogen and a northern transition
zone to the Rinkian orogen. Archaean granulite-facies
gneisses of the North Atlantic Craton, which forms the
southern foreland, were reworked in the SNO at
amphibolite facies during south-directed thrusting and
folding. The CNO comprises, besides Archaean gneisses,
two main bodies of Palaeoproterozoic calc-alkaline
intrusive rocks: the Sisimiut charnockite suite in the
south-west and the Arfersiorfik intrusive suite in the
north-east (Kalsbeek & Nutman 1996; Whitehouse et al.
1998), which are interpreted as remnants of magmatic
arcs associated with subduction (Kalsbeek et al. 1987).
Palaeoproterozoic metasedimentary rocks are known
from narrow belts in the CNO and in the northern part
of the SNO. In the northern part of the CNO they are
intruded by quartz diorite and tonalite of the Arfersiorfik
intrusive suite (Kalsbeek & Nutman 1996; van Gool et al.
1999). This association of Palaeoproterozoic intrusive
and supracrustal rocks was interleaved with Archaean
gneisses by NW-directed thrust stacking during early
stages of collision (van Gool et al. 1999, 2002; Connelly
et al. 2000). Thrust stacks and associated fabrics were
subsequently folded in several generations of folds,
the latest forming shallowly east-plunging upright
folds on the scale of tens of kilometres. The CNO is
largely at granulite facies, with the exception of its
north-eastern corner which is at amphibolite facies. Its
northern boundary is formed by the Nordre Strømfjord
shear zone (Fig. 1; Marker et al. 1995; Hanmer et al.
1997).
The NNO is the least known part of the orogen.
Tonalitic orthogneisses of Archaean age are interleaved
with supracrustal rocks of both volcanic and sedimen-
tary origin, most of which form belts up to 500 m wide
(Mengel et al. 1998). Supracrustal rocks are less common
than in the CNO, but the up to 2 km wide Naternaq
supracrustal belt in the north-east is one of the largest
coherent supracrustal belts in the orogen (Fig. 1). The
main deformational features are a regional foliation,
large-scale ENE­WSW-trending folds and several ductile
high-strain zones, both steeply and shallowly dipping.
The metamorphic grade is predominantly amphibolite
facies, but increases southwards to granulite facies
around Attu (Mengel et al. 1998; Connelly et al. 2000).
40
Ar/
39
Ar age determinations on hornblende from the
NNO indicate that Nagssugtoqidian metamorphic
temperatures of at least 500°C prevailed as far north as
Ilulissat (Willigers et al. 2002). Nagssugtoqidian defor-
mation in the Nordre Strømfjord shear zone at the
southern boundary of the NNO resulted in a penetrative
gneissic high-grade fabric, large-scale upright folds and
localised shear zones, as seen in the deformation of
Palaeoproterozoic intrusive and sedimentary rocks
(Hanmer et al. 1997; Mengel et al. 1998; van Gool et al.
2002). It is not clear to what extent the structures and
lithologies in the NNO can be correlated with those in
the Nordre Strømfjord shear zone or further south.
Geology of the Kangaatsiaq area
The Kangaatsiaq map sheet covers a large part of the
western half of the NNO (Figs 1, 2). Supracrustal rocks
were previously recognised in a zone trending from
the north-eastern to the south-western quadrant of the
map where they outline major fold structures. The
south-central and south-eastern parts were indicated
as homogeneous orthogneiss due to lack of observa-
tions (Escher 1971). A quartz-diorite body was distin-
guished in the south-eastern part of the map area by
Henderson (1969). A few minor occurrences of gran-
ite were known, of which that at Naternaq is the
largest (Figs 1, 2).
During field work in 2001, twelve lithological units
were distinguished, of which several were previously
unknown. Ten of these rock types are represented on
the map in Fig. 2, while occurrences of the others are
too small for the scale of the map. Relative age rela-
tionships were established for most of the rock types
but absolute ages are still largely unknown. The few
available geochronological data are discussed in a
separate section below. The Naternaq supracrustal
sequence is described by Østergaard et al. (2002, this
volume). The other lithological units are described
below from oldest to youngest.
Mafic intrusive complexes
Dismembered, layered mafic to ultramafic intrusive
complexes are dominated by medium- to coarse-
grained, massive to moderately foliated, homogeneous
amphibolite, but locally igneous layering is preserved
(Fig. 3). The rocks contain hornblende and plagio-
clase, with or without clinopyroxene, orthopyroxene,
biotite, quartz or garnet. The protolith rock types include
gabbro, gabbro-norite, ultramafic rocks (mostly pyrox-
enite and hornblendite), and rarely thin anorthosite
sheets occur. This association occurs within the domi-
16
GSB191-Indhold 13/12/02 11:29 Side 16
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nant tonalitic orthogneisses mainly as lenses up to tens
of metres in diameter, but also forms larger bodies up
to 2 km across. The rocks are cut by tonalitic and
granitic intrusive sheets and veins and occur often
strongly agmatised. The mafic lenses contain remnants
of a foliation and subsequent folding, which predate
the intrusion of the regional orthogneisses. The mafic
intrusive complexes are most abundant in the south-
ern part of the map area.
Mafic supracrustal sequences
Thinly layered mafic to intermediate sequences with
thin felsic intercalations are interpreted as supracrustal,
predominantly meta-volcanic sequences (Fig. 4). They
are layered on a millimetre- to centimetre-scale and
contain variable amounts of hornblende and plagio-
clase, with or without clinopyroxene, biotite, garnet
and quartz. Isolated, thin quartzo-feldspathic layers,
c. 5 to 20 cm thick, are interpreted as psammitic incur-
sions in which presumed granule and pebble-sized
detrital grains were observed north-east of Kan-
gaatsiaq. These rocks are intruded by the dominant
tonalitic gneiss and occur both as up to 500 m thick,
laterally extensive sequences and as smaller xenoliths.
In several cases the boundary between the tonalitic
gneiss and the supracrustal sequence is tectonically
reworked. The age relationship between the mafic
supracrustal and mafic intrusive rocks could not be
established. The mafic supracrustal rocks are common
in a c. 20 km wide belt that extends from the south-
17
Fig. 3. Well-preserved metamorphosed
layered gabbro in an outcrop of the
mafic intrusive complex on the island of
Ikerasak in the south-western corner of
the map area.
Fig. 4. Mafic supracrustal sequence consisting of layered (meta-
volcanic) amphibolite alternating with thin layers of psammite
and quartzite. Outcrop is located 18 km south-east of Kangaatsiaq.
Lens cap, centre, is 7 cm in diameter.
GSB191-Indhold 13/12/02 11:29 Side 17
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western to the north-eastern corner of the map area
(Fig. 2). The mafic supracrustal sequences contain rare,
up to 5 m thick layers of medium-grained, forsterite-
humite marble or medium- to coarse-grained, diopside-
rich calc-silicate rocks.
Mica schist
Sequences of mica-rich rocks vary from biotite-rich
semi-pelitic schists to biotite, garnet- and sillimanite-
bearing schists and gneisses, which are intercalated
with thin quartzo-feldspathic layers and some quart-
zite. In the northern part of the area the gneisses local-
ly contain muscovite, kyanite or cordierite. The schists
are generally associated with mafic supracrustal rocks,
and rarely form isolated occurrences. They are espe-
cially abundant in a belt in the central part of the map
area and in the Naternaq area (Østergaard et al. 2002,
this volume).
Quartzo-feldspathic paragneisses
Quartzo-feldspathic gneisses form 2­3 km thick se-
quences in the south-eastern part of the map area
where they are interpreted as metapsammitic rocks.
These grey, medium-grained paragneisses are rather
homogeneous, often quartz-rich and poor in biotite,
and may contain abundant small (1­2 mm) garnets.
Local rounded quartz and feldspar grains up to 1 cm
across are interpreted as pebbles. The quartzo-felds-
pathic paragneisses are interlayered with 5­100 cm wide
amphibolite layers, which are probably of volcanic
origin. Slightly discordant, deformed mafic dykes (see
below) have also been observed. Rare, biotite-rich
micaceous layers locally contain garnet and silliman-
ite. Contact relationships with the surrounding grey
orthogneisses and their relative ages are uncertain,
and locally these two litholigical units can be difficult
to distinguish in the field.
Dioritic to quartz-dioritic gneiss
This unit consists of medium-grained, uniform, dark-
grey migmatitic or agmatitic orthogneisses, containing
hornblende, plagioclase, quartz and minor biotite. It
occurs mainly as small lenses in the tonalitic ortho-
gneiss unit and only seldom forms larger, mappable
bodies in the south. The largest bodies and layers of
quartz-diorite are up to 50 m wide and occur in the
Arfersiorfik area (Fig. 2). Contact relationships with the
tonalitic gneisses are not clear everywhere, but a few
dioritic bodies occur as xenoliths. None of the quartz-
diorite bodies have so far been correlated with the
Palaeoproterozoic Arfersiorfik quartz diorite (Kalsbeek
et al. 1987) that occurs in the eastern end of Arfersiorfik
and Nordre Strømfjord (Fig. 1). However, this correla-
tion cannot be ruled out for at least some of the occur-
rences, and geochemical analyses and possibly
geochronology will be used to test this. The large body
of quartz-dioritic gneiss north and south of the fjord
Tarajomitsoq in the eastern part of the map area, indi-
cated by Henderson (1969), could not be confirmed.
18
Fig. 5. Tonalitic gneiss with amphibolite
lenses, presumed to be mafic dyke
remnants. Pink granitic veins give the
rock a migmatitic texture. Near
Alanngorsuup Imaa.
GSB191-Indhold 13/12/02 11:29 Side 18
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Tonalitic and associated quartzo-feldspathic
orthogneiss
The predominant orthogneiss unit comprises a wide
range of lithologies, which in most cases lack sharp
mutual contacts and cannot be mapped out as separate
units. Grey, fine- to medium-grained biotite-bearing
tonalitic gneiss predominates (Fig. 5). Tonalitic gneiss
with abundant medium-grained hornblende occurs
commonly in the proximity of mafic inclusions, and a
plagioclase-porphyric, hornblende-bearing tonalitic
gneiss, characterised by up to 2 cm large clusters of
hornblende occurs mainly in the north-western part of
the map area. In places, the orthogneiss is migmatitic,
containing up to 30% coarse-grained, K-feldspar-rich
melt veins up to 5 cm thick (Fig. 5). Another less
common melt phase intruding all varieties of the grey
orthogneiss consists of leucocratic, white, medium- to
coarse-grained granodiorite to granite and occurs pre-
dominantly in the south. It forms veins and larger
coherent bodies up to one metre wide and can local-
ly form up to 30% of the rock volume.
High-grade, mafic dyke relics
These metadolerite dyke relics are homogeneous,
fine- to medium-grained, and consist of hornblende,
plagioclase and clinopyroxene, with or without ortho-
pyroxene, biotite and quartz. Garnet is seen rarely at
the margins. Commonly the dykes are intensely
deformed, foliated and lineated, boudinaged, or trans-
formed to mafic schlieren which can be difficult to
identify as dykes (Fig. 5). The less deformed dykes are
commonly about 20 cm thick, but can reach 50 cm.
Discordant relationships can be preserved in areas of
low strain, but angles of discordance are always small.
The dykes are widespread and locally form up to 25%
of the rock volume, but they do not form a map unit
that can be depicted on the scale of Fig. 2. They were
commonly observed in the southern part of the map
area, where they form dense swarms in the grey
orthogneisses (Fig. 6).
Granite and granitic gneiss
Numerous small and large intrusive bodies of granite
with a wide range of lithological appearances and
different states of deformation were mapped. Coarse-
grained, homogeneous pink granite predominates and
may grade into megacrystic granite, sometimes with
rapakivi-textures, pink microgranite, or pegmatite.
White, leucocratic granite is also observed. Based on
their deformational state and contact relationships the
granite bodies fall into two main categories (not
distinguished on the map): foliated granites with
gradational boundaries to their host rocks, and rela-
tively undeformed granites with obvious intrusive
contacts. The contact zones between tonalitic ortho-
gneiss and the granites can be tens to hundreds of
metres wide, beginning with a few thin granitic vein-
lets in the orthogneiss, grading into granite or granitic
gneiss with abundant orthogneiss inclusions, and
ending with almost inclusion-free granite. The gneissic
fabric in the inclusions is commonly cut by the gran-
ites, which may nevertheless themselves be strongly
foliated.
Pegmatite
Several generations of pegmatite have been observed,
often cross-cutting and in different stages of deforma-
tion. They are commonly coarse-grained, rich in pink
K-feldspar, and contain quartz and plagioclase with or
without biotite. In general, two main types can be
distinguished. The older pegmatites are slightly discor-
dant, commonly irregular in shape and can be folded
and strongly sheared, resulting in porphyroclastic,
mylonitic textures. They appear to be associated with
the granitic gneisses described above. Some of these
pegmatites can be shown to be syn-kinematic with the
latest fold phase (see below). The second, younger
19
Fig. 6. Cliff exposing orthogneisses invaded by a dyke swarm which
is boudinaged and folded. Vertical dark streaks are caused by water
flowing over the cliff. Height of cliff is about 50 m. The outcrop
is located at the southern boundary of the map area, 6 km east
of the fjord Ataneq.
GSB191-Indhold 13/12/02 11:29 Side 19
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group consists of conjugate sets of late, straight-walled
pegmatites. They are undeformed and commonly
associated with steep brittle faults that have offsets
which are consistent with north­south compression.
These pegmatites may be younger than the metado-
lerite dykes described below.
Metadolerite dykes
Massive, 1­20 m wide metadolerite dykes occur main-
ly in the southern part of the map area. They cut the
regional gneissic fabric and most have E­W trends.
Foliation is only well developed in the dyke margins
although a weak linear fabric can be observed locally
in the unfoliated cores. The dykes have metamorphic
mineral assemblages of fine- to medium-grained horn-
blende, plagioclase and clinopyroxene, with or without
orthopyroxene, garnet and rarely biotite. In contrast to
the older foliated dyke remnants they always occur as
isolated bodies.
Globule dyke
A single, N­S-trending, 20­50 m wide composite dolerite
dyke with unusual globular structures was described
by Ellitsgaard-Rasmussen (1951). The name for the
dyke was based on the local presence of spheres with
igneous textures that comprise plagioclase and pyrox-
ene phenocrysts in the core, surrounded by glassy
mantles. Several locations were revisited and showed
the dyke to be undeformed and to consist of a c. 10 m
thick central dyke with thinner multiple intrusions on
both sides which have glassy, chilled margins. The
dyke is exposed in a few outcrops along a 60 km long
stretch from the entrance of the fjord Arfersiorfik
northwards to the coast east of Aasiaat. On the aero-
magnetic map of the NNO (Thorning 1993) this trace
is clearly visible, with several right-lateral steps as
depicted in Fig. 2.
Geochronology
U-Pb zircon age determinations have been carried out
on six samples from the map area (Kalsbeek &
Nutman 1996). Archaean ages in the range 2.7­2.8 Ga
were derived from four biotite orthogneiss samples. A
porphyric granite yielded zircons of c. 2.7 Ga, indis-
tinguishable in age from a gneiss which forms the host
rock at the same location. It is uncertain whether these
two lithologies are indeed of approximately the same
age, or whether the granite contains locally derived
inherited zircons. One of two samples from the
Naternaq supracrustal sequence contained Proterozoic
detrital zircons, suggesting that at least part of the
sequence is of Proterozoic age (Østergaard et al. 2002,
this volume).
Kalsbeek et al. (1984) derived an Archaean Pb-Pb
isochron age of 2653 ± 110 Ma for a granite that is
intrusive into the regional gneisses just south of the
map area. An undeformed granite sampled near
Aasiaat just north of the map area yielded an intrusive
age of 2778
+7
­3
Ma (TIMS U-Pb on zircon, Connelly &
Mengel 2000). Preliminary LAM-ICPMS Pb-Pb recon-
naissance analyses on detrital zircons from a felsic
layer of a dominantly mafic supracrustal sequence
north-east of Kangaatsiaq have yielded Archaean ages.
The available isotope data establish that the region-
ally dominant tonalitic gneisses have Archaean proto-
lith ages, and that at least some granites in the NNO
are also Archaean. The ages of the younger pegmatites
and of the metadolerite dykes are at present uncertain.
A regional dating programme of rocks in the northern
Nagssugtoqidian orogen and southern Rinkian orogen
is underway to establish the ages of the main lithologies
and tectonic events (Garde et al. 2002, this volume).
Metamorphism
The map area is dominated by upper amphibolite facies,
medium-pressure mineral assemblages, but has been
affected by granulite facies metamorphism south of the
fjords Arfersiorfik and Alanngorsuup Imaa (Fig. 2).
Mineral assemblages in metapelites include garnet-
biotite-sillimanite in most of the area, with minor kyanite
or cordierite observed locally north-east of Kangaatsiaq.
Muscovite is stable in the northernmost part of the
map area. Partial melt veins occur in most of the
region giving the gneisses a migmatitic texture. Relic
granulite facies rocks occur as patches in the south
within areas of amphibolite facies. The granulite facies
grade is reflected in the weathered appearance of the
rocks, but orthopyroxene is seldom visible in hand
specimen. It does, however, appear as relics in thin
section. The age of the granulite facies metamorphism
is uncertain, but Palaeoproterozoic rocks in the near-
by Nordre Strømfjord shear zone (Fig. 1) are also at
granulite facies, and were retrogressed to amphibolite
facies in high-strain zones during a late phase of
Nagssugtoqidian orogenesis.
20
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Structure
Detailed field observations combined with the map
pattern show that at least four generations of regional-
ly penetrative structures are recorded in the dominant
orthogneisses, while an even older penetrative planar
fabric and isoclinal folds are preserved in mafic inclu-
sions. The regional gneissosity dips to the NNW or SSE
at steep to moderate angles, and carries a subhorizon-
tal, ENE­WSW-trending mineral grain lineation or
aggregate lineation. It is a high-temperature fabric, and
commonly migmatitic veins are developed parallel to
it. Locally the gneissosity is axial planar to isoclinal,
often rootless, folds. The main gneissosity is a
composite fabric, heterogeneously developed either
progressively over an extended period of time or in
several phases before and after intrusion of the mafic
dyke swarm in the south.
At least two phases of folding affected the area. The
early isoclinal folds have no consistent orientation and
may represent several generations of folds, as report-
ed from the Attu area by Sørensen (1970) and Skjernaa
(1973). Map-scale isoclinal folds are most obvious in
the north-eastern and south-western map quadrants,
outlined by the supracrustal sequences. At several
locations the isoclinal folding resulted in interleaving
of ortho- and paragneisses. It is also possible that
some interleaving occurred by thrust repetition, as
reported from the Attu area (Skjernaa 1973) and from
south of the Nordre Strømfjord shear zone (van Gool
et al. 1999), but so far no unambiguous evidence for
thrust repetition has been found in the map area.
Shear zones are uncommon and of local extent, main-
ly associated with the reworking of intrusive contacts
between supracrustal rocks and orthogneisses and
lacking consistent kinematic indicators. Their relative
age with respect to the fold phases is uncertain.
Parasitic folds associated with the youngest, major
phase of upright folds are sub-horizontal to moderate-
ly plunging, with predominantly WSW-plunging axes.
Near the hinges of kilometre-scale folds the parasitic
folds are commonly steeply inclined, plunging to the
south. Mineral lineations are commonly parallel to
sub-parallel with the axes of parasitic folds. Sets of
late, steeply dipping conjugate fractures trend NE­SW
and NW­SE and some of these are filled with a
pegmatitic melt phase.
Summary and conclusions
The Kangaatsiaq region in the northern Nagssugtoq-
idian orogen predominantly consists of Archaean ortho-
gneisses. It includes a major ENE­WSW-trending belt
with abundant supracrustal rocks, which runs from
south of Kangaatsiaq to the southern part of Naternaq.
A second, previously unknown but extensive belt of
quartzo-feldspathic paragneisses, presumably of
Archaean age, occupies part of the south-eastern
corner of the map area.
The main events in the geological evolution of the area
comprise:
1. intrusion of mafic igneous complexes and deposi-
tion of mafic and associated supracrustal rocks;
2. formation of a foliation and isoclinal folds;
3. intrusion of the main tonalitic and associated gran-
itoid rocks;
4. formation of the regional gneissic fabric;
5. intrusion of a mafic dyke swarm in the south;
6. further deformation, probably associated with iso-
clinal folding and intensification of the regional
gneissosity;
7. intrusion of granite and pegmatite;
8. formation of a foliation and gneissosity in the gran-
ites, in part during their intrusion and associated
with upright folding;
9. intrusion of the E­W-trending, isolated metadolerite
dykes;
10. formation of upright brittle fractures during intrusion
of pegmatite.
At present, an evaluation of the tectonic evolution of
the Kangaatsiaq area in a regional perspective is diffi-
cult, since the absolute ages of several lithological
units and deformational events are still unknown. It is
uncertain to what extent the Palaeoproterozoic tecton-
ic events known from south of the Nordre Strømfjord
shear zone can be correlated with those of the
Kangaatsiaq area. The map area lacks the abundance
of Proterozoic supracrustal sequences intruded by
quartz-diorite and the shear zones associated with
their emplacement, that are typical for the central
Nagssugtoqidian orogen (van Gool et al. 1999). The
most likely candidate for a Palaeoproterozoic supra-
crustal sequence in the map area is the Naternaq
supracrustal belt. Furthermore, the shear zones in the
Kangaatsiaq area are not of regional extent. The lack
of consistent kinematic indicators in the shear zones
suggests that deformation may have been dominated
21
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background image
by pure shear. Coincidence of the orientation and style
of the youngest upright, E­W-trending folds in the
Kangaatsiaq area with similar structures of Palaeo-
proterozoic age to the south (Sørensen 1970; Skjernaa
1973; van Gool et al. 2002) and in the Disko Bugt area
to the north (several papers in Kalsbeek 1999) was
suggested by Mengel et al. (1998) as a possible indi-
cation for direct correlation.
Acknowledgements
Hans Myrup and Malene Weyhe of M/S Søkongen are acknowl-
edged for transport during the expedition and for generally
lending many helping hands. We thank helicopter pilot Jan
Wilken for efficient helicopter transport.
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23
Authors' addresses
J.A.M.v.G., A.A.G., C.K., J.N., S.P. & K.T., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K,
Denmark. E-mail: jvg@geus.dk
G.I.A., School of Geography & Geosciences, University of St. Andrews, St. Andrews, Fife, KY16 9AL, UK.
U.E.Á., Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
A.W.K., Department of Geological Sciences, University of Texas at Austin, Austin, TX, 78712, USA.
S.M., Institute of Geological Sciences, University of Wroclaw, Pl. Maxa Borna 9, 50­204 Wroclaw, Poland.
C.W.T., British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA, UK.
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