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> Forside > Publikationer > Geology of Greenland Survey Bulletin > Vol. 191 Geol. Greenl. Surv. Bull. > Review of Greenland Activities 2001, pp 33-38

GEOLOGY OF GREENLAND SURVEY BULLETIN 191

 
A coastal survey in the southern part of the Palaeoproterozoic Rinkian fold belt, central West Greenland

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A brief but potentially important part of the 2001 field
investigations in the Precambrian of West Greenland (van
Gool et al. 2002, this volume) was devoted to the south-
ernmost part of the Palaeoproterozoic Rinkian fold belt
east of Disko Bugt (Fig. 1). From 9­17 August the five
authors carried out a reappraisal of critical Archaean and
Proterozoic relationships and collected samples for pre-
cise geochronological studies. The principal aims are to
date the main Rinkian tectonic and metamorphic events
in this region as precisely as possible and compare
them with the evolution of the Nagssugtoqidian oro-
gen to the south (see van Gool et al. 2002, this volume,
fig. 1). The vessel M/S Søkongen provided logistic sup-
port; a helicopter provided transport to Nunatarsuaq.
Geological background
The Archaean continental terrains in West and East
Greenland, north-eastern Canada, Scotland and
Scandinavia were gradually amalgamated during a series
of major Palaeoproterozoic orogenic events to form
one of the earliest large continents on Earth. Recent struc-
tural and geochronological studies have significantly
improved the correlation between the individual oro-
gens and the understanding of the overall plate-tec-
tonic framework (e.g. Clowes et al. 1999), but there
remain significant geographical, chronological and struc-
tural uncertainties. One of these concerns the bound-
ary zone between the Nagssugtoqidian and Rinkian
orogenic belts in central West Greenland (van Gool et al.
2002, this volume, fig. 1). It is quite possible that the
two belts formed contemporaneously within a com-
mon, or at least related, plate-tectonic setting. However,
the structural styles of the two belts are different; at least
one part of the contact region between the two belts
only displays very weak Palaeoproterozoic reworking,
and only a rudimentary geochronological data base is
currently available for the Rinkian belt.
Recent geochronological and structural studies in the
southern and central Nagssugtoqidian orogen by the
Danish Lithosphere Centre have established an accu-
rate and robust time-frame for the accretion of its
Archaean magmatic elements, the Palaeoproterozoic
magmatic arc component, its subsequent tectonic accre-
tion, and final stabilisation (Connelly et al. 2000; Willigers
et al. 2002). These insights are being largely corrobo-
rated by ongoing work in the northern Nagssugtoq-
idian orogen (van Gool et al. 2002, this volume).
Such constraints are not yet available for the Rinkian
fold belt (Henderson & Pulvertaft 1987; Grocott &
Pulvertaft 1990; Kalsbeek et al. 1998). During the most
recent investigations by the Survey conducted in
1988­1991 (Kalsbeek 1999), it was established that
Rinkian structural reworking was strong in eastern
Nuussuaq, southern Arveprinsen Ejland and areas to the
east. In contrast, the intervening Ataa domain was largely
unaffected by reworking (Fig. 1; Kalsbeek et al. 1988;
Escher et al. 1999; Garde & Steenfelt 1999a, b; Grocott
& Davies 1999). An Ar-Ar and K-Ar geochronological
study confirmed this general interpretation but with a
broad spread of ages (Rasmussen & Holm 1999). A few
ion probe U-Pb zircon ages, whole-rock Pb-Pb and Rb-
Sr ages and model Sm-Nd ages showed that both the
Atâ tonalite and adjacent supracrustal lithologies in the
Ataa domain were magmatically accreted at 2800 Ma
(Kalsbeek & Taylor 1999; Nutman & Kalsbeek 1999).
However, the age histories of the reworked, suppos-
edly Archaean basement of Nuussuaq and the Palaeo-
proterozoic reworking north and south of the Ataa
domain have yet to be unravelled.
In this contribution, we present a summary of our
main objectives in the southern part of the Rinkian fold
belt and relate them to the geological problems outlined
above. An overview of the southern part of the Rinkian
belt can be found in Garde & Steenfelt (1999a).
33
A coastal survey in the southern part of the
Palaeoproterozoic Rinkian fold belt, central West
Greenland
Adam A. Garde, James N. Connelly, Adam W. Krawiec, Sandra Piazolo and Kristine Thrane
Geology of Greenland Survey Bulletin 191, 33­38 (2002) © GEUS, 2002
GSB191-Indhold 13/12/02 11:29 Side 33
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34
Rodebay granodiorite
Discordant granitic bodies (c. 2750 Ma)
Atâ tonalite (c. 2800 Ma)
Augen granite (Saqqaq, Nuussuaq)
Dioritic gneiss
Tonalitic­granodioritic orthogneiss
Acid metavolcanic rocks
Itilli diorite (Nuussuaq)
Metagabbro
Pillow lava and undifferentiated amphibolite
Archaean
Garnet-mica schist
Anorthosite
Muscovite schist
Anap nunâ Group: marble
Dolerite dykes and sills
Anap nunâ Group: sandstone, siltstone
Quaternary deposits
Cretaceous­Palaeogene province
Albitised rocks
Ductile thrust
Shear zone
Palaeoproterozoic
N
R
Greenland
70
°
Torsuka
ttak
Anap
Nunaa
Boye Sø
Puiattup Qaqqaa
Puiattup Qaqqaa shear zone
Itilliarsuup Nuua
Saqqaq
Inussuk
69
°
69
°
50
°
52
°
Ataa
domain
N u u s s u a q
Itilli
Oqaatsut
Ataa
Arveprinsen
Ejland
Ilulissat
Nunatarsuaq
Laksebugt
Paakits
oq
Rodebay
'Falcon
cliff'
Ikorfat
Qitermi-
unnguit
10 km
Inland Ice
Fig. 1. Geological sketch map of the area north-east of Disko Bugt with place names used in the text, modified from Garde (1994).
Dash lines: domain boundaries. Filled circles: towns, settlements. Open circle: abandoned settlement. Dash-dot lines: faults.
N and R on the inset map of Greenland denote the Nagssugtoqidian and Rinkian belts, with the map area at the arrow (also shown
on van Gool et al. 2002, this volume, fig. 1).
GSB191-Indhold 13/12/02 11:29 Side 34
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Archaean acid metavolcanic rocks and
the Torsukattak shear zone, south coast
of Nuussuaq
The Survey's field work in Nuussuaq in 1988­1991 was
largely restricted to helicopter reconnaissance; due to
difficult ice conditions and lack of a suitably large boat,
the south coast was not mapped. Nevertheless, a pre-
liminary lithostratigraphy of the main, supposedly
Archaean supracrustal sequences in southern Nuussuaq
was established (Garde & Steenfelt 1999a), and two
major Proterozoic shear zones were proposed: (1) the
well-exposed, NW­SE-trending Puiattup Qaqqaa shear
zone in southern Nuussuaq, and (2) the Torsukattak
shear zone, a largely unexposed younger extensional
shear zone along the fjord Torsukattak with oblique
downthrow to the south-east (Fig. 1). The existence of
the latter shear zone was mainly inferred from numer-
ous inland observations of intense SE-plunging exten-
sion lineations, apparent tectonic reworking of the
Puiattup Qaqqaa shear zone towards Torsukattak, and
an abrupt decrease in the overall metamorphic grade
between southern Nuussuaq and the Ataa domain (for
details see Garde & Steenfelt 1999b).
In 2001, it was possible to study the outcrops along
the coast of Nuussuaq by boat in spite of much ice
(Fig. 2). An acid metavolcanic rock with pale, fine-
grained volcanic clasts (Fig. 3) from a previously reported
supracrustal sequence north of Itilliarsuup Nuua (Fig. 1;
Garde 1994) was sampled in order to obtain a deposi-
tional age for the metavolcanic rock by U-Pb dating of
its igneous zircons (using the conventional thermal ion-
isation method, TIMS); this will also provide a minimum
age for the underlying Nuussuaq gneisses, and will
allow us to compare the timing of volcanism at the
continental margin of southern Nuusuaq with the arc-
type volcanism in the Ataa domain, which was previ-
ously dated at c. 2800 Ma (Kalsbeek & Taylor 1999). In
addition, the zircon U-Pb system might also provide a
discordia age for the Torsukattak shear zone.
During a boat traverse of the excellently exposed,
SSE-dipping acid metavolcanic and associated metased-
imentary rocks we observed asymmetric volcanic clasts
and garnet porphyroclasts within the intense, SE-plung-
ing LS fabric. The asymmetric fabric elements clearly
indicate relative downthrow of the southern hanging
wall towards the south-east along the steep lineation
(Fig. 4), corroborating Garde & Steenfelt's (1999b) inter-
pretation that the Torsukattak structure is a major,
oblique extensional shear zone. It was previously noted
in inland areas that the intensity of the LS strain fabric
35
Fig 2. M/S Søkongen in the ice-filled waters of Torsukattak, north-
west of Anap Nunaa.
Fig. 3. Fragmental acid metavolcanic rocks at Itilliarsuup Nuua.
Pencil for scale.
Fig. 4. Asymmetric fabric elements (volcanic clasts) at Ililliarsuup
Nuua, indicating relative downthrow of the southern (right-hand)
side of the Torsukattak shear zone. View eastwards along the south
coast of Nuussuaq.
GSB191-Indhold 13/12/02 11:29 Side 35
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increases towards the south. We observed that the strain
intensity reaches a maximum about 50 m from the point
of Itilliarsuup Nuua, where ultramylonitic rocks occur.
Further south, towards the point itself, the intensity of
the LS fabric decreases significantly. It is concluded that
the central part of the Torsukattak shear zone is prob-
ably not hidden in the fjord as hitherto believed, but
may be located close to Itilliarsuup Nuua.
Other supracrustal rocks of supposed
Archaean and Palaeoproterozoic age
Garnet- and biotite-rich metasedimentary rocks at
Inussuk near the head of Torsukattak, Oqaatsut and on
Nunatarsuaq were sampled for U-Pb ion probe studies
of detrital zircons and microtextural and metamorphic
studies. In addition, dating of metamorphic minerals may
allow determination of the cooling path (U-Pb: titan-
ite, apatite; Rb-Sr: biotite, muscovite). Samples of low-
grade sedimentary rocks from the Anap nunâ Group
were also collected on northern and south-eastern Anap
Nunaa.
Archaean basement of southern
Nuussuaq and in the
Ilulissat/Jakobshavn area
Representative samples of orthogneiss and granitic rocks
were collected from various parts of the Archaean base-
ment in the southern Rinkian belt and the border region
to the Nagssugtoqidian orogen as part of an ongoing
regional age characterisation of the Archaean basement
in West Greenland. The sampled units include grey
tonalitic orthogneiss and associated younger granitoid
phases at the embayment 2 km west of Ikorfat, grano-
dioritic augen gneiss c. 5 km west of Saqqaq, Itilli dior-
ite north of inner Torsukattak, grey tonalitic orthogneiss
at western Arveprinsen Ejland at Laksebugt, Ilulissat
harbour and Ilulissat airport, and Rodebay granite at
Rodebay (Fig. 1).
North- to west-directed Proterozoic
thrusting of Archaean orthogneisses at
Paakitsoq
In the area south of the Ataa domain, which was affected
by significant Palaeoproterozoic reworking, Escher et al.
(1999) reported major low-angle ductile imbrication of
Archaean orthogneisses in the vicinity of Paakitsoq.
The main thrusting is envisaged to have occurred dur-
ing an early phase of Proterozoic deformation, and
according to Escher et al. (1999) the predominant move-
ment direction of the thrusts was westwards. The thrust-
ing event was followed by open to tight folding, and
the thrusts were reactivated during the emplacement of
a suite of up to c. 100 m thick mafic sills. These were
commonly emplaced along thrust planes and were sub-
sequently boudinaged and their margins deformed
during continued movement along the thrusts (Escher
et al. 1999, table 1). The sills and their host rocks are
cut by dolerite dykes as well as thin lamprophyre dykes
and sills; the latter were dated at c. 1750 Ma by Larsen
& Rex (1992).
It is of critical importance for the correlation between
the Rinkian and Nagssugtoqidian belts that a compar-
ison can be made between the timing of the Rinkian
west-directed thrusting and the main crustal shorten-
ing event in the central Nagssugtoqidian orogen at c.
1860­1820 Ma (Connelly et al. 2000). Therefore, it is
important to determine as precisely as possible when
the main episode of thrusting at Paakitsoq prior to the
sill emplacement took place. Our attention was focused
on the orthogneisses at Qitermiunnguit and on the coast
of north-eastern Paakitsoq (Fig. 1) which are readily
accessible and in structural continuation with those at
the `Falcon cliff' of Escher et al. (1999, fig. 8). At both
localities subhorizontal, upper greenschist to lower
amphibolite facies high-strain zones up to a few metres
36
Fig. 5. Mylonitised orthogneiss with asymmetric porphyroclasts
and extension lineation, north coast of Qitermiunnguit.
GSB191-Indhold 13/12/02 11:29 Side 36
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thick, which are commonly ultramylonitic, are sepa-
rated by up to c. 100 m thick zones of much less
deformed rocks. In the high-strain zones there are abun-
dant, well-developed, asymmetric
- and
-shaped K-
feldspar and plagioclase porphyroclasts within the LS
fabric (Fig. 5), which is dominated by an intense, shal-
lowly east-plunging extension lineation. It was easy to
confirm the main westerly transport direction previ-
ously reported from other localities, e.g. at `Falcon cliff'.
Several orthogneiss samples were collected for U-Pb zir-
con geochronology: we hope to date the intense duc-
tile deformation by means of precise lower concordia
intercepts.
The c. 1650 Ma Melville Bugt dyke
swarm at inner Torsukattak
In order to close the present gap in the existing data
set of palaeomagnetic poles used to constrain Palaeo-
proterozoic to Mesoproterozoic plate reconstructions of
Laurentia and Baltica (Buchan et al. 2000, 2001), a c.
100 m wide dyke belonging to the Melville Bugt dyke
swarm of North-West Greenland and its contact rocks
were sampled in inner Torsukattak. This particular dyke
has a strike length of 400 km and was previously stud-
ied by Kalsbeek & Taylor (1986), who obtained a Rb-
Sr age of 1645 ± 35 Ma. In addition to a sample set for
the palaeomagnetic studies, a very large sample was col-
lected from the dyke centre in the hope of retrieving
magmatic zircon or badelleyite for precise dating.
Concluding remarks
The survey north-east of Disko Bugt initiated in 2001
is a geochronological, structural and metamorphic study
that aims to update understanding of the Rinkian oro-
gen and its position in the contemporaneous frame-
work of Palaeoproterozoic orogens in Greenland and
eastern Canada. In 2002, with support from the Carlsberg
Foundation, the survey will be extended into the cen-
tral part of the Rinkian fold belt.
Acknowledgements
We thank the owners of M/S Søkongen, Hans Myrup and Malena
Weyhe, as well as Jesper W. Andersen and Jakob Lautrup for their
excellent support on board the ship.
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38
Authors' addresses
A.A.G., S.P. & K.T., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail:
aag@geus.dk
J.N.C. & A.W.K., Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712, USA.
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Review of Greenland Activities 2001, pp 33-38