Geological Survey of Denmark and Greenland Bulletin
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Nr. 4, Review of Survey activities 2003, pp. 73-76 |
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One of the first detailed accounts of Precambrian supra-
crustal rocks in central West Greenland came from a small
group of islands and skerries a few kilometres north-east of
Aasiaat (Fig. 1). In 1948, K. Ellitsgaard-Rasmussen spent a
few days on the islands and published a metamorphic study
of their low-grade greenstones and aluminous clastic rocks
(Ellitsgaard-Rasmussen 1954). He observed a striking dis-
similarity between these supracrustal rocks and the grey
gneisses found in most of the Aasiaat region, although the lat-
ter were at that time also assumed to be of supracrustal ori-
gin. He furthermore noted that the regional significance of
the islands should be pursued, and that the island of
crustal rocks in central West Greenland came from a small
group of islands and skerries a few kilometres north-east of
Aasiaat (Fig. 1). In 1948, K. Ellitsgaard-Rasmussen spent a
few days on the islands and published a metamorphic study
of their low-grade greenstones and aluminous clastic rocks
(Ellitsgaard-Rasmussen 1954). He observed a striking dis-
similarity between these supracrustal rocks and the grey
gneisses found in most of the Aasiaat region, although the lat-
ter were at that time also assumed to be of supracrustal ori-
gin. He furthermore noted that the regional significance of
the islands should be pursued, and that the island of
Maniitsoq 4 km west of the small islands might hold a key to
their interpretation.
their interpretation.
More than 50 years were to elapse before the islands were
surveyed again in July 2003, during field work for the
Ikamiut map sheet in the northern Nagssugtoqidian orogen
(van Gool et al. 2002). The collision of two Archaean conti-
nents during the c. 1850 Ma Nagssugtoqidian orogeny caused
intensive structural and thermal reworking at up to granulite
facies grade in most of central West Greenland; see Connelly
et al. (2000) and van Gool et al. (2002). The small islands
north-east of Aasiaat are indeed regionally important, be-
cause they document a previously unrecognised low-grade,
Ikamiut map sheet in the northern Nagssugtoqidian orogen
(van Gool et al. 2002). The collision of two Archaean conti-
nents during the c. 1850 Ma Nagssugtoqidian orogeny caused
intensive structural and thermal reworking at up to granulite
facies grade in most of central West Greenland; see Connelly
et al. (2000) and van Gool et al. (2002). The small islands
north-east of Aasiaat are indeed regionally important, be-
cause they document a previously unrecognised low-grade,
73
Low-pressure metamorphism during Archaean crustal
growth: a low-strain zone in the northern
Nagssugtoqidian orogen, West Greenland
growth: a low-strain zone in the northern
Nagssugtoqidian orogen, West Greenland
Adam A. Garde, Mads Sylvest Christiansen, Julie A. Hollis, Stanislaw Mazur and
Jeroen A.M. van Gool
Jeroen A.M. van Gool
Fig. 1. Geological map of the low-grade, low-strain area north of Aasiaat. The well-preserved supracrustal rocks on the island groups around Isuamiut
and Equtiit Kangilleq were originally mapped by Ellitsgaard-Rasmussen (1954). Maniitsoq and the small island west of Manertooq (A) still preserve
intrusive contacts of (?) Archaean granodiorite and tonalite into the supracrustal rocks, whereas the intensity of Palaeoproterozoic (Nagssugtoqidian)
strain increases greatly towards the south. The inset map shows the location of the study area in the northern part of the Nagssugtoqidian orogen.
Geological Survey of Denmark and Greenland Bulletin 4, 7376 (2004) © GEUS, 2004
low-strain domain of presumed Archaean age that has largely
escaped the Nagssugtoqidian orogeny, and as predicted by
Ellitsgaard-Rasmussen (1954) a clue to their significance was
found on Maniitsoq.
escaped the Nagssugtoqidian orogeny, and as predicted by
Ellitsgaard-Rasmussen (1954) a clue to their significance was
found on Maniitsoq.
The low-grade supracrustal rocks
The islands form two different groups (Fig. 1). The eastern
islands, of which Equutit Kangilleq is the largest, consist of
pale grey, andalusite-staurolite-muscovite-biotite-garnet-
quartz-bearing metasedimentary rocks interspersed with
abundant fine-grained mafic sills. Graded bedding on a scale
of 510 cm is commonly preserved, and up to c. 5 cm large,
undeformed andalusite porphyroblasts are found in the alu-
minous bed tops (Fig. 2). Randomly orientated, centimetre-
sized staurolite porphyroblasts are also abundant. A 1 m thick
aluminous pegmatite with up to 15 cm sized, euhedral
andalusite crystals and even larger masses of cordierite were
also observed. Mafic sills reach thicknesses of a few tens of
metres. They are generally fine-grained, massive and dark
greenish in colour, and commonly display up to 10 cm thick
garnet-bearing reaction rims along their margins. The rocks
record a simple deformational history: open to tight folds
with amplitudes and wavelengths in the order of 1 km have
steeply SE- or SW-plunging axes and are associated with a
steep, centimetre-spaced cleavage that overprints bedding.
islands, of which Equutit Kangilleq is the largest, consist of
pale grey, andalusite-staurolite-muscovite-biotite-garnet-
quartz-bearing metasedimentary rocks interspersed with
abundant fine-grained mafic sills. Graded bedding on a scale
of 510 cm is commonly preserved, and up to c. 5 cm large,
undeformed andalusite porphyroblasts are found in the alu-
minous bed tops (Fig. 2). Randomly orientated, centimetre-
sized staurolite porphyroblasts are also abundant. A 1 m thick
aluminous pegmatite with up to 15 cm sized, euhedral
andalusite crystals and even larger masses of cordierite were
also observed. Mafic sills reach thicknesses of a few tens of
metres. They are generally fine-grained, massive and dark
greenish in colour, and commonly display up to 10 cm thick
garnet-bearing reaction rims along their margins. The rocks
record a simple deformational history: open to tight folds
with amplitudes and wavelengths in the order of 1 km have
steeply SE- or SW-plunging axes and are associated with a
steep, centimetre-spaced cleavage that overprints bedding.
The coastal exposures of the western islands (of which
Isuamiut is the largest, Fig. 1) consist of dark grey, massive
greenstones interspersed with smaller volumes of dark, very
fine-grained chlorite schist, mixed horizons of finely layered
chert and hematite-dominated, manganiferous banded iron
formation (Fig. 3) and calcareous layers less than 1 m thick
rich in actinolite. Most of the greenstones are sill complexes
up to c. 1 km thick with common columnar jointing and
locally preserved internal intrusive contacts and chilled mar-
gins. Irregular quartz and calcite veins up to c. 10 cm thick
are very common and may have formed by cementation of
open joint systems. On the north-west coast of Isuamiut a
small area of well-preserved pillow lavas was also found (Fig.
4), clearly demonstrating that the basic magmatism was con-
temporaneous with sedimentation, in contrast to the view of
Ellitsgaard-Rasmussen (1954) that the basic magmatism
occurred during folding. Pillow lava cusps, asymmetric dis-
tribution of gas vesicles and graded bedding in adjacent clas-
tic rocks, all point to north-west younging. The general style
of deformation, with a simple system of steeply SW- and SE-
plunging open to close folds, is very similar to that on the
eastern islands (Fig. 1). Steep pencil structures in fold hinges,
formed by intersection between cleavage and bedding,
demonstrate that the overall deformation was constrictional
with subvertical extension.
greenstones interspersed with smaller volumes of dark, very
fine-grained chlorite schist, mixed horizons of finely layered
chert and hematite-dominated, manganiferous banded iron
formation (Fig. 3) and calcareous layers less than 1 m thick
rich in actinolite. Most of the greenstones are sill complexes
up to c. 1 km thick with common columnar jointing and
locally preserved internal intrusive contacts and chilled mar-
gins. Irregular quartz and calcite veins up to c. 10 cm thick
are very common and may have formed by cementation of
open joint systems. On the north-west coast of Isuamiut a
small area of well-preserved pillow lavas was also found (Fig.
4), clearly demonstrating that the basic magmatism was con-
temporaneous with sedimentation, in contrast to the view of
Ellitsgaard-Rasmussen (1954) that the basic magmatism
occurred during folding. Pillow lava cusps, asymmetric dis-
tribution of gas vesicles and graded bedding in adjacent clas-
tic rocks, all point to north-west younging. The general style
of deformation, with a simple system of steeply SW- and SE-
plunging open to close folds, is very similar to that on the
eastern islands (Fig. 1). Steep pencil structures in fold hinges,
formed by intersection between cleavage and bedding,
demonstrate that the overall deformation was constrictional
with subvertical extension.
74
Fig. 2. Folded aluminous rocks with preserved graded bedding.
Andalusite porphyroblasts up to 5 cm across (arrows and inset) occur
in the bed tops. Scale shown by 2.8 cm coin left of centre. South side of
small island between Isuamiut and Equutit Kangilleq.
Fig. 3. Vertical layer of banded iron formation and chert, c. 1 m wide,
within massive greenstones. Northern Isuamiut.
Fig. 4. Pillow lava; horizontal surface of steeply dipping unit younging
north-west. Coin, 2.8 cm across, at cuspate pillow base for scale.
Northern Isuamiut.
Both the eastern and western islands represent the deposits
of a volcanic basin dominated by basic magmatism, with
associated chemical sediments now found as chert, banded
iron formation and calcareous rocks. The intercalated clastic
metasedimentary rocks are very fine-grained and were thus
deposited far from continental crustal sediment sources. The
origin of the aluminous metasediments on the eastern islands
is less clear, although they may also be pelagic sediments.
associated chemical sediments now found as chert, banded
iron formation and calcareous rocks. The intercalated clastic
metasedimentary rocks are very fine-grained and were thus
deposited far from continental crustal sediment sources. The
origin of the aluminous metasediments on the eastern islands
is less clear, although they may also be pelagic sediments.
Low-grade supracrustal rocks, reminiscent of those de-
scribed above, occur on Hunde Ejland and adjacent small
islands about 10 km north-west of Maniitsoq, but were only
briefly surveyed (inset map on Fig. 1). Layered basic volcanic
rocks and sills are intercalated with thin horizons of fine-
grained chemical and clastic metasedimentary rocks in which
chlorite and muscovite are the dominant phyllosilicates; the
metamorphic grade appears to have been too low for growth
of aluminosilicates. The deformation was sufficiently intense
to develop a penetrative schistosity, and bedding-cleavage
relationships are only rarely preserved in fold hinges.
islands about 10 km north-west of Maniitsoq, but were only
briefly surveyed (inset map on Fig. 1). Layered basic volcanic
rocks and sills are intercalated with thin horizons of fine-
grained chemical and clastic metasedimentary rocks in which
chlorite and muscovite are the dominant phyllosilicates; the
metamorphic grade appears to have been too low for growth
of aluminosilicates. The deformation was sufficiently intense
to develop a penetrative schistosity, and bedding-cleavage
relationships are only rarely preserved in fold hinges.
Relationships with the quartzo-feldspathic
gneisses in the Aasiaat area
gneisses in the Aasiaat area
Orthogneisses are absent from the small islands north-east of
Aasiaat, and their relationships with the low-grade supra-
crustal association therefore cannot be studied directly. How-
ever, a more strongly deformed and higher grade continuation
of the supracrustal association probably occurs along strike
some 4 km to the west, in the easternmost part of Maniitsoq
island and on a small island immediately to its east (A on Fig.
1). In this area, fine-grained amphibolite considered to be a
lateral continuation of the greenstones is intruded by a char-
acteristic unit of K-feldspar megacrystic granodiorite (on
Maniitsoq), or by grey tonalite (on the small island). The
intrusive contacts are weakly deformed but otherwise well
preserved (Fig. 5). Most of Maniitsoq is covered by the
megacrystic granodiorite, preserved in a weakly deformed
state close to its original magmatic appearance; the granodi-
orite is cut by several sets of flat-lying and inclined peg-
matites, and large angles between individual pegmatite
phases are still present. A similar, likewise un-migmatised, K-
feldspar porphyritic granodiorite was also observed on Kron-
prinsen Ejland and may be part of the same pluton.
Aasiaat, and their relationships with the low-grade supra-
crustal association therefore cannot be studied directly. How-
ever, a more strongly deformed and higher grade continuation
of the supracrustal association probably occurs along strike
some 4 km to the west, in the easternmost part of Maniitsoq
island and on a small island immediately to its east (A on Fig.
1). In this area, fine-grained amphibolite considered to be a
lateral continuation of the greenstones is intruded by a char-
acteristic unit of K-feldspar megacrystic granodiorite (on
Maniitsoq), or by grey tonalite (on the small island). The
intrusive contacts are weakly deformed but otherwise well
preserved (Fig. 5). Most of Maniitsoq is covered by the
megacrystic granodiorite, preserved in a weakly deformed
state close to its original magmatic appearance; the granodi-
orite is cut by several sets of flat-lying and inclined peg-
matites, and large angles between individual pegmatite
phases are still present. A similar, likewise un-migmatised, K-
feldspar porphyritic granodiorite was also observed on Kron-
prinsen Ejland and may be part of the same pluton.
More deformed outcrops of the megacrystic granodiorite
have been recognised on several small islands south of
Maniitsoq, and further south these give way to grey tonalitic
orthogneiss (Fig. 1). Both the granodiorite and orthogneiss
become increasingly strongly deformed southwards, and peg-
matites are tectonically thinned and lose their angular discor-
dance (Fig. 6). Southwards, towards Aasiaat, the rocks exhibit
an intense EW-trending vertical planar fabric and subhori-
Maniitsoq, and further south these give way to grey tonalitic
orthogneiss (Fig. 1). Both the granodiorite and orthogneiss
become increasingly strongly deformed southwards, and peg-
matites are tectonically thinned and lose their angular discor-
dance (Fig. 6). Southwards, towards Aasiaat, the rocks exhibit
an intense EW-trending vertical planar fabric and subhori-
zontal lineation related to upright, kilometre-scale, tight to
isoclinal folds.
isoclinal folds.
Metamorphism and regional significance
The occurrence of andalusite as the stable aluminosilicate
phase in staurolite-bearing pelitic rocks is consistent with low
pressure metamorphic conditions of
phase in staurolite-bearing pelitic rocks is consistent with low
pressure metamorphic conditions of
3 kbar. However, min-
eral assemblage constraints indicate significant variations in
temperature, from chlorite zone greenschist facies up to mid-
upper amphibolite facies. For example, on the western islands
chlorite-graphite and garnet-chlorite schists dominate the
metasedimentary assemblages; on the eastern islands stauro-
lite-biotite rocks are common, indicating up-temperature
crossing of the staurolite isograd (c. 520550°C).
temperature, from chlorite zone greenschist facies up to mid-
upper amphibolite facies. For example, on the western islands
chlorite-graphite and garnet-chlorite schists dominate the
metasedimentary assemblages; on the eastern islands stauro-
lite-biotite rocks are common, indicating up-temperature
crossing of the staurolite isograd (c. 520550°C).
The regional variation in metamorphic grade, coupled
with the intrusive relationships observed in the Maniitsoq
area, strongly suggest metamorphism during emplacement of
the granodioritic-tonalitic magmas into the upper crust. The
age of emplacement of these plutons is currently unknown;
based on regional age data (e.g. Connelly et al. 2000) it is pre-
sumed that the grey gneisses are late Archaean, and U-Pb
geochronology to confirm this is under way.
area, strongly suggest metamorphism during emplacement of
the granodioritic-tonalitic magmas into the upper crust. The
age of emplacement of these plutons is currently unknown;
based on regional age data (e.g. Connelly et al. 2000) it is pre-
sumed that the grey gneisses are late Archaean, and U-Pb
geochronology to confirm this is under way.
Both previous work in the northern Nagssugtoqidian oro-
gen and new observations of intensely deformed Palaeo-
proterozoic basic dykes in the Aasiaat region itself, indicate
that the intense EW structural grain in the Kangaatsiaq
Aasiaat region is due to the Nagssugtoqidian continent col-
lision, although almost all of the exposed rocks are of Ar-
chaean age. Contemporaneous deformation also took place
north-east of Disko Bugt, and it has recently been suggested
that the Nagssugtoqidian orogeny also incorporated the
Rinkian fold belt in northern West Greenland to form a com-
proterozoic basic dykes in the Aasiaat region itself, indicate
that the intense EW structural grain in the Kangaatsiaq
Aasiaat region is due to the Nagssugtoqidian continent col-
lision, although almost all of the exposed rocks are of Ar-
chaean age. Contemporaneous deformation also took place
north-east of Disko Bugt, and it has recently been suggested
that the Nagssugtoqidian orogeny also incorporated the
Rinkian fold belt in northern West Greenland to form a com-
75
Fig. 5. Fine-grained amphibolite intruded by weakly deformed tonalite.
Pen points at angular discordance. West side of island between Maniitsoq
and Manertooq (A on Fig. 1).
76
mon, more than 1000 km wide collisional belt extending
from West Greenland far into eastern Canada (Thrane et al.
2003).
from West Greenland far into eastern Canada (Thrane et al.
2003).
Other domains of low Palaeoproterozoic strain in the
NagssugtoqidianRinkian orogenic system have previously
been described, e.g. from the area north-east of Disko Bugt
(cf. Garde & Steenfelt 1999) and between Kangaatsiaq and
Attu (Piazolo et al. 2004); the former area is generally
assumed to consist of a tectonic block that was downthrown
along a major extensional shear zone (Garde & Steenfelt
1999). At Aasiaat, however, the strain increase is gradual, tak-
ing place over a width of several kilometres. Furthermore, the
intense subhorizontal lineation in the Aasiaat area is perpen-
dicular to the NS direction of increased strain; it therefore
indicates that the main tectonic transport was lateral and did
not include a significant vertical component.
been described, e.g. from the area north-east of Disko Bugt
(cf. Garde & Steenfelt 1999) and between Kangaatsiaq and
Attu (Piazolo et al. 2004); the former area is generally
assumed to consist of a tectonic block that was downthrown
along a major extensional shear zone (Garde & Steenfelt
1999). At Aasiaat, however, the strain increase is gradual, tak-
ing place over a width of several kilometres. Furthermore, the
intense subhorizontal lineation in the Aasiaat area is perpen-
dicular to the NS direction of increased strain; it therefore
indicates that the main tectonic transport was lateral and did
not include a significant vertical component.
Conclusions
Greenstones and aluminous metasediments of presumed
Archaean age crop out on a few small islands north of Aasiaat
and have been excellently preserved in a low-temperature and
low-strain window in the northern part of the Nagssug-
toqidian orogen in West Greenland. These relatively low-
grade rocks may well represent the oldest component of the
region, recording a history of metamorphism and deforma-
tion during Archaean crustal growth. They provide a unique
opportunity to study the primary lithological components of
Archaean age crop out on a few small islands north of Aasiaat
and have been excellently preserved in a low-temperature and
low-strain window in the northern part of the Nagssug-
toqidian orogen in West Greenland. These relatively low-
grade rocks may well represent the oldest component of the
region, recording a history of metamorphism and deforma-
tion during Archaean crustal growth. They provide a unique
opportunity to study the primary lithological components of
the Archaean supracrustal belts that are intercalated with the
regional grey gneisses. In addition, the state of preservation of
the supracrustal rocks provides support for an inhomoge-
neous Nagssugtoqidian orogenic overprint, where blocks
with intense thermal and tectonic reworking seem to alter-
nate with blocks of only weak reworking.
regional grey gneisses. In addition, the state of preservation of
the supracrustal rocks provides support for an inhomoge-
neous Nagssugtoqidian orogenic overprint, where blocks
with intense thermal and tectonic reworking seem to alter-
nate with blocks of only weak reworking.
Based on hornblende Ar-Ar cooling ages, Willigers et al.
(2002) proposed that the Nagssugtoqidian orogeny resulted
in uniform heating in most of the orogen (including its
northern part), followed by very slow cooling during isosta-
tic uplift. The preliminary observations reported here appear
to contradict this, but more work is required to substantiate
the new findings.
in uniform heating in most of the orogen (including its
northern part), followed by very slow cooling during isosta-
tic uplift. The preliminary observations reported here appear
to contradict this, but more work is required to substantiate
the new findings.
Acknowledgements
The authors thank Christian Knudsen, Mac Persson, Sandra Piazolo and
Thomas Rasmussen for their contributions to the study of the Aasiaat
region.
Thomas Rasmussen for their contributions to the study of the Aasiaat
region.
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Fig. 6. Intensely deformed granodiorite with several generations of peg-
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Authors' addresses
A.A.G., M.S.C., J.A.H. & J.A.M.v.G., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K,
Denmark. E-mail: aag@geus.dk
S.M., Institute of Geological Sciences, University of Wroclaw, Pl. Maxa Borna 9, 50-204 Wroclaw, Poland.
A.A.G., M.S.C., J.A.H. & J.A.M.v.G., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K,
Denmark. E-mail: aag@geus.dk
S.M., Institute of Geological Sciences, University of Wroclaw, Pl. Maxa Borna 9, 50-204 Wroclaw, Poland.
