Geological Survey of Denmark and Greenland Bulletin 13, 2007
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Review of Survey activities 2006, 49-52 |
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The Nordre Strømfjord shear zone is a 1.8 Ga zone of large-
scale, transcurrent and sinistral ductile shear (Sørensen et al .
2006) within the Nagssugtoqidian Mobile Belt (NMB) of
central West Greenland. It has been hypothesised that the
NMB is a suture between two Archaean continental masses
(Kalsbeek et al . 1987). During field work in 2005 along the
Nordre Strømfjord shear zone in the fjord Ataneq (Fig. 1),
some unusual rock types were discovered that preserve evi-
dence of magmatic and metamorphic processes not previ-
ously reported in the area. These observations include the
first indication of high-pressure (HP) metamorphism in West
Greenland and the first reported occurrence of a cumulate of
giant orthopyroxene. The tectonic telescoping of these fea-
tures together within the Nordre Strømfjord shear zone has
scale, transcurrent and sinistral ductile shear (Sørensen et al .
2006) within the Nagssugtoqidian Mobile Belt (NMB) of
central West Greenland. It has been hypothesised that the
NMB is a suture between two Archaean continental masses
(Kalsbeek et al . 1987). During field work in 2005 along the
Nordre Strømfjord shear zone in the fjord Ataneq (Fig. 1),
some unusual rock types were discovered that preserve evi-
dence of magmatic and metamorphic processes not previ-
ously reported in the area. These observations include the
first indication of high-pressure (HP) metamorphism in West
Greenland and the first reported occurrence of a cumulate of
giant orthopyroxene. The tectonic telescoping of these fea-
tures together within the Nordre Strømfjord shear zone has
important implications for reconstructing the Palaeopro-
terozoic history of this region, and provides evidence that
processes typical of Phanerozoic continent-continent colli-
sion zones (e.g. the Caledonian and Alpine systems) operated
at least as far back as 1.8 Ga ago.
terozoic history of this region, and provides evidence that
processes typical of Phanerozoic continent-continent colli-
sion zones (e.g. the Caledonian and Alpine systems) operated
at least as far back as 1.8 Ga ago.
High-pressure enclave
On the north side of inner Ataneq fjord an approximately 1.2
m wide and 4 m long lens of ultramafic rock occurs within
strongly foliated garnet-sillimanite gneisses and schists, and
garnet-bearing calc-silicate rock (Fig. 2). The pale yellowish
green, ultramafic rock is moderately foliated with its long axis
parallel to the fabric in the enclosing gneisses. This core of the
m wide and 4 m long lens of ultramafic rock occurs within
strongly foliated garnet-sillimanite gneisses and schists, and
garnet-bearing calc-silicate rock (Fig. 2). The pale yellowish
green, ultramafic rock is moderately foliated with its long axis
parallel to the fabric in the enclosing gneisses. This core of the
© GEUS, 2007.
Geological Survey of Denmark and Greenland Bulletin
13, 49-52. Available at:
www.geus.dk/publications/bull
Two tectonically significant enclaves in the Nordre
Strømfjord shear zone at Ataneq, central West Greenland
Strømfjord shear zone at Ataneq, central West Greenland
William E. Glassley, John A. Korstgård and Kai Sørensen
Fig. 1. North-eastern part of the Agto map sheet (Olesen 1984), with the localities of Figs 2, 3 and 4 marked. The Nordre Strømfjord shear zone of the
map area is characterised by vertically oriented supracrustal units alternating with quartzofeldspathic units, as also described by Sørensen et al. (2006)
from the area to the east. The regional amphibolite to granulite facies transition occurs over the eastern part of the map.
50
enclave consists of anthophyllite with a few minor additional
phases. It is surrounded by a dark rim of dense, fine-grained
rock approximately 30 cm thick (Fig. 2) that is conformable
to the shape of the ultramafic lens. The rim appears to be the
result of a reaction between the silica-poor ultramafic rocks
and the enclosing aluminium- and silica-rich metasediments,
and it consists of olivine-orthopyroxene-clinopyroxene-spinel-
garnet-amphibole. The fine-grained nature of the rim rock
and its complex textural characteristics make it difficult to
unambiguously decipher all aspects of its petrogenetic his-
tory. However, certain key observations show that the rim
rock records an unusual history involving high-pressure
metamorphism. The olivine occurs as remnant crystals that
are occasionally seen to be in optical continuity but separated
by pyroxene and spinel. Garnets occur as isotropic areas that
are nearly completely overgrown by spinel and pyroxene.
Garnet also occurs as inclusions in spinel. All combinations
of grain-to-grain contacts have been observed, with the
exception of garnet-olivine. There are also textural features
phases. It is surrounded by a dark rim of dense, fine-grained
rock approximately 30 cm thick (Fig. 2) that is conformable
to the shape of the ultramafic lens. The rim appears to be the
result of a reaction between the silica-poor ultramafic rocks
and the enclosing aluminium- and silica-rich metasediments,
and it consists of olivine-orthopyroxene-clinopyroxene-spinel-
garnet-amphibole. The fine-grained nature of the rim rock
and its complex textural characteristics make it difficult to
unambiguously decipher all aspects of its petrogenetic his-
tory. However, certain key observations show that the rim
rock records an unusual history involving high-pressure
metamorphism. The olivine occurs as remnant crystals that
are occasionally seen to be in optical continuity but separated
by pyroxene and spinel. Garnets occur as isotropic areas that
are nearly completely overgrown by spinel and pyroxene.
Garnet also occurs as inclusions in spinel. All combinations
of grain-to-grain contacts have been observed, with the
exception of garnet-olivine. There are also textural features
suggesting that two generations of orthopyroxene and clino-
pyroxene may be present.
pyroxene may be present.
These mineralogical features document a petrogenetic his-
tory in which the oldest mineral assemblage preserved in the
rim of the enclave is garnet-olivine-orthopyroxene-clinopy-
roxene (i.e. garnet peridotite). The occurrence of garnet +
olivine in ultramafic rocks and the occurrence of eclogite
minerals in mafic compositions are the diagnostic mineral
assemblages for HP metamorphism. Defined in this way HP
metamorphism is intermediate between granulite facies meta-
morphism and ultra high-pressure metamorphism (UHP) in
which diamond and coesite are stable phases. In the HP
enclave, the olivine + garnet-bearing assemblage is replaced,
via reaction between olivine and garnet, by the assemblage
spinel-orthopyroxene-clinopyroxene (i.e. spinel peridotite).
Olivine and garnet are preserved because the reaction was
arrested before it went to completion. This metamorphism
took place at a very low thermodynamic activity of water.
Replacement of a garnet peridotite mineral assemblage by
that of spinel peridotite is the hallmark of recrystallisation
during decompression from minimum pressures of about
18-20 kilobars (> 60 km) and temperatures> 750°C (Schmä -
dicke & Evans 1997; Fumagalli & Poli 2005).
rim of the enclave is garnet-olivine-orthopyroxene-clinopy-
roxene (i.e. garnet peridotite). The occurrence of garnet +
olivine in ultramafic rocks and the occurrence of eclogite
minerals in mafic compositions are the diagnostic mineral
assemblages for HP metamorphism. Defined in this way HP
metamorphism is intermediate between granulite facies meta-
morphism and ultra high-pressure metamorphism (UHP) in
which diamond and coesite are stable phases. In the HP
enclave, the olivine + garnet-bearing assemblage is replaced,
via reaction between olivine and garnet, by the assemblage
spinel-orthopyroxene-clinopyroxene (i.e. spinel peridotite).
Olivine and garnet are preserved because the reaction was
arrested before it went to completion. This metamorphism
took place at a very low thermodynamic activity of water.
Replacement of a garnet peridotite mineral assemblage by
that of spinel peridotite is the hallmark of recrystallisation
during decompression from minimum pressures of about
18-20 kilobars (> 60 km) and temperatures> 750°C (Schmä -
dicke & Evans 1997; Fumagalli & Poli 2005).
Preliminary electron microprobe analyses of all of the
mineral phases have been conducted. Clinopyroxene-ortho-
pyroxene geothermometry and orthopyroxene-garnet geobar -
ometry (Brey & Köhler 1990) intersect at 785°C and 21 kb.
However, uncertainty in identifying cogenetic minerals, as
well as the fact that these rocks have experienced extensive
recrystallisation during decompression and cooling make it
likely that these P-T conditions are a minimum; modifica-
tions are to be expected as further analyses are conducted.
pyroxene geothermometry and orthopyroxene-garnet geobar -
ometry (Brey & Köhler 1990) intersect at 785°C and 21 kb.
However, uncertainty in identifying cogenetic minerals, as
well as the fact that these rocks have experienced extensive
recrystallisation during decompression and cooling make it
likely that these P-T conditions are a minimum; modifica-
tions are to be expected as further analyses are conducted.
The electron microprobe data provide support for the
argument that the high density rim around the ultramafic
rock is, in fact, a metasomatic feature reflecting steep chemi-
cal potential gradients between the metasediments and the
ultramafic rock. In particular, the very high modal abun-
dance of the spinel (> 20%) and the absence of detectable Cr
in any of the minerals are inconsistent with primary crystalli-
sation from an ultramafic composition. Rather, these charac-
teristics suggest limited metasomatic reaction between the
enclave and the surrounding metasediments into which we
envisage the enclave to have been tectonically emplaced.
rock is, in fact, a metasomatic feature reflecting steep chemi-
cal potential gradients between the metasediments and the
ultramafic rock. In particular, the very high modal abun-
dance of the spinel (> 20%) and the absence of detectable Cr
in any of the minerals are inconsistent with primary crystalli-
sation from an ultramafic composition. Rather, these charac-
teristics suggest limited metasomatic reaction between the
enclave and the surrounding metasediments into which we
envisage the enclave to have been tectonically emplaced.
Giant orthopyroxene cumulate with inter -
stitial anorthosite and associated rocks
stitial anorthosite and associated rocks
Approximately 3 km west of the HP site a series of gabbroic
anorthosite and coarse-grained orthopyroxenite lenses occur
that are metres to tens of metres in size (Fig. 3). This series of
lenses is traceable along the coast over a distance of 1 km. The
anorthosite and coarse-grained orthopyroxenite lenses occur
that are metres to tens of metres in size (Fig. 3). This series of
lenses is traceable along the coast over a distance of 1 km. The
Fig. 2. Lens (boudin?) of yellowish green, ultramafic rock within garnet-
biotite-sillimanite gneiss and calc-silicate rock. The hammer (1 m) rests
on the ultramafic rock and is just to the right of a dark, 30 cm thick rim
(indicated by arrow) that completely encloses the ultramafic lens. The
dark rim is the source of the garnet-spinel-olivine-orthopyroxene-
clinopyroxene sample. For location see Fig. 1.
51
margins of these lenses are tectonised at their contact with the
enclosing quartzofeldspathic gneisses.
enclosing quartzofeldspathic gneisses.
The orthopyroxenites were observed in two distinct forms.
One of these is a monomineralic lens of thumb-sized, equant,
euhedral to subhedral orthopyroxene crystals. The lens is
approximately two metres by four metres in size and exhibits
no internal fabric. The other form is a spectacular giant ortho-
pyroxene cumulate containing crystals more than 30 cm long
and 15 cm wide that have a strong preferred orientation, with
long axes parallel to each other in a classic cumulate texture.
The crystals exhibit striking macroscopic kink banding (Fig.
4). Anorthosite is found as discontinuous films along the
edges of the orthopyroxene crystals and as cuspate pockets
where triple junctions of orthopyroxene crystals occur.
euhedral to subhedral orthopyroxene crystals. The lens is
approximately two metres by four metres in size and exhibits
no internal fabric. The other form is a spectacular giant ortho-
pyroxene cumulate containing crystals more than 30 cm long
and 15 cm wide that have a strong preferred orientation, with
long axes parallel to each other in a classic cumulate texture.
The crystals exhibit striking macroscopic kink banding (Fig.
4). Anorthosite is found as discontinuous films along the
edges of the orthopyroxene crystals and as cuspate pockets
where triple junctions of orthopyroxene crystals occur.
In thin section the orthopyroxenites are seen to have pre-
served detailed evidence of a complex magmatic history and
metamorphic recrystallisation, even though field evidence
unequivocally shows these rocks to have been tectonically
emplaced into their present setting. The primary magmatic
mineral assemblage consists of remnant forsteritic olivine
incompletely resorbed by orthopyroxene, green spinel and
plagioclase with chromite, rutile, phlogopite, apatite and zir-
con as additional phases, either primary or a result of exsolu-
tion. All of these minerals are observed as inclusions within
the orthopyroxene, as well as phases interstitial to orthopy-
roxene in pockets of anorthosite. Secondary minerals associ-
ated with metamorphic recrystallisation are amphibole (as
trains of small grains within orthopyroxene, occurring along
crystallographically controlled planes) and quartz.
metamorphic recrystallisation, even though field evidence
unequivocally shows these rocks to have been tectonically
emplaced into their present setting. The primary magmatic
mineral assemblage consists of remnant forsteritic olivine
incompletely resorbed by orthopyroxene, green spinel and
plagioclase with chromite, rutile, phlogopite, apatite and zir-
con as additional phases, either primary or a result of exsolu-
tion. All of these minerals are observed as inclusions within
the orthopyroxene, as well as phases interstitial to orthopy-
roxene in pockets of anorthosite. Secondary minerals associ-
ated with metamorphic recrystallisation are amphibole (as
trains of small grains within orthopyroxene, occurring along
crystallographically controlled planes) and quartz.
Reconnaissance electron microprobe analyses of the pla-
gioclase show that its composition is affected by its environ-
ment: plagioclase grains within the anorthositic pockets are
close to An
ment: plagioclase grains within the anorthositic pockets are
close to An
60
, while those contained within the orthopyro-
xene, which generally are associated with amphibole, are
approximately An
approximately An
40
. The amphibole is nearly pure cumming-
tonite. Other observations made with the electron micro-
probe showed the presence of Fe-Ni sulphides and pure Cu
probe showed the presence of Fe-Ni sulphides and pure Cu
spherules. In addition, the spinels and phlogopites are Ti-
and Cr-rich.
and Cr-rich.
These characteristics of the orthopyroxenites suggest that
the cumulates formed by gravitational settling of giant
orthopyroxenes in a magma chamber. The presence of pla-
gioclase, clinopyroxene and rutile exsolution lamellae sug-
gests that the orthopyroxenes crystallised at high pressure
(> 10 kb), which is consistent with the co-existence of
orthopyroxene-olivine-plagioclase-spinel.
orthopyroxenes in a magma chamber. The presence of pla-
gioclase, clinopyroxene and rutile exsolution lamellae sug-
gests that the orthopyroxenes crystallised at high pressure
(> 10 kb), which is consistent with the co-existence of
orthopyroxene-olivine-plagioclase-spinel.
It has been postulated that anorthositic massifs form via
fractionation of orthopyroxene from magmas of appropriate
compositions at or near the base of the continental crust
(Emslie 1985). However, such cumulates have never been
observed before, and the slivers of cumulate orthopyroxenite
observed in Ataneq may be the remnants of such a system
that has been tectonically dismembered
compositions at or near the base of the continental crust
(Emslie 1985). However, such cumulates have never been
observed before, and the slivers of cumulate orthopyroxenite
observed in Ataneq may be the remnants of such a system
that has been tectonically dismembered
Fig. 3. Two 3 m long lenses of gabbro
anorthosite approximately 2 km west of the
ultramafic lens shown in Fig. 2. Note the duplex
structure within the lens. For location see Fig. 1.
Fig. 4. Giant orthopyroxene crystals (dark olive green except where
reflecting) separated by grey and white intercumulus anorthosite. Glove
above the large single crystal in the centre of the photograph is approx-
imately 20 cm long. Note kink banding in the central crystal (arrow
points towards the kink band).
opx
, orthopyroxene. For location see
Fig. 1.
Conclusions
The high-pressure rocks and orthopyroxenite cumulates
observed in Ataneq attest to tectonic telescoping of rocks that
originated from profoundly different geological environ-
ments. They provide evidence that within the Nordre
Strømfjord shear zone, samples of the deepest levels of conti-
nental crust and the upper mantle are present. These rocks
occur within contrasting lithologies of the Nordre Strøm-
fjord shear zone: the HP lens within a supracrustal unit ( sensu
Sørensen et al . 2006) and the pyroxenite-anorthosite assem-
blage within a quartzofeldspathic gneiss unit. Further to the
east, a complex of lenses of ultramafic rocks, pillow lavas and
unusual tourmaline-phlogopite rocks (interpreted to be the
metamorphosed remnant of submarine hot-spring exhalations;
Sørensen et al . 2006) have been observed enclosed in supra-
crustal units. These rock types provide compelling evidence
that upper mantle, deep continental crust and oceanic crust
were tectonically juxtaposed, probably during continent-con-
tinent collision, later to be deformed within the Nordre Strøm-
fjord shear zone. The tectonism responsible for the emplace-
ment of these rocks within continental rocks may be thrust
stacking as described by van Gool et al . (1999) and Sørensen
et al . (2006) south of the shear zone near the Inland Ice.
observed in Ataneq attest to tectonic telescoping of rocks that
originated from profoundly different geological environ-
ments. They provide evidence that within the Nordre
Strømfjord shear zone, samples of the deepest levels of conti-
nental crust and the upper mantle are present. These rocks
occur within contrasting lithologies of the Nordre Strøm-
fjord shear zone: the HP lens within a supracrustal unit ( sensu
Sørensen et al . 2006) and the pyroxenite-anorthosite assem-
blage within a quartzofeldspathic gneiss unit. Further to the
east, a complex of lenses of ultramafic rocks, pillow lavas and
unusual tourmaline-phlogopite rocks (interpreted to be the
metamorphosed remnant of submarine hot-spring exhalations;
Sørensen et al . 2006) have been observed enclosed in supra-
crustal units. These rock types provide compelling evidence
that upper mantle, deep continental crust and oceanic crust
were tectonically juxtaposed, probably during continent-con-
tinent collision, later to be deformed within the Nordre Strøm-
fjord shear zone. The tectonism responsible for the emplace-
ment of these rocks within continental rocks may be thrust
stacking as described by van Gool et al . (1999) and Sørensen
et al . (2006) south of the shear zone near the Inland Ice.
Acknowledgement
The work of W.E.G. was funded by the University of Aarhus.
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Authors' addresses
W.E.G. & J.A.K.,
Geological Institute, University of Aarhus, Høegh-Guldbergsgade 2, DK-8000 Århus C, Denmark.
E-mail:
geobg@nf.au.dk
K.S.,
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
52
