The dominant lithology at Kangimut sangmissoq is described as nebulitic tonalitic gneiss containing highly distended plagioclase phyric amphibolites. The gneiss amphibolite complex was intruded by Nuk gneiss between 3.05 and 2.90 Ga and later (2.6 to 2.7 Ga) by post granulite facies granitoid sheets. The amphibolites are though to be Ameralik dikes and the older gray gneiss are then Amitsoq by definition. The problem arises when the isotopic data are considered, none of which indicate rocks older that about 3.0 Ga

Bridgwater, D.

Mcgregor, V. R.

Nutman, A. P.

English

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-28551
REGIONAL VARIATION IN THE AMITSOQ GNEISSES RELATED TO CRUSTAL
LEVELS DURING LATE ARCHEAN GRANULITE FACIES METAMORPHISM, SOUTHERN WEST
GREENLAND. A. P. NUTMAN , D. BRIDGWATER , V.R. McGREGOR . 1 Department of
Earth Sciences, MUM, St. John's, Nfld., Canada. 2 Geological Museum,,
Copenhagen, Denmark. 3 Atammik, Green 1 and -V^i ( ( ' r T n £4~7£ ^Tttf
INTRODUCTION: Regional mapning by the Geological Survey of Greenland
has shown that large areas of southern West Greenland preserve middle to
late Archean granulite facies assemblages (Chronology, Table 1). From
mineralogical and textural evidence further large areas were affected by
granulite facies metamorphism but were subsequently retrogressed under
amphibolite facies conditions prior to the intrusion of the c. 2550 Ma
Qorqut granite complex (McGregor et al . , this volume). In Godthabsf jord
(Fig. 1) however, the rocks appear to have been only metamorphosed under
amphibolite facies conditions in the mid and late Archean.
Table 1. Relevant events in the regional chronology.
*?) Intrusion of post-tectonic Q3rqut granite complex, 2550 Ma
7) Deformation, retrogression of granulite facies rocks under amphibolite
facies conditions (A on Fig. 1) and intrusion of granitoid sheets,
2700-2600 Ma
6) Granulite facies metamorphism ( [&] on Fig. 1). In south, c. 2800 Ma,
in north, c. 3000 Ma.
5) Intrusion of the Nuk gneisses. 2900-3050 Ma.
4) Deposition of Malene supracrustal rocks.
3) Intrusion of the Ameralik dykes.
2) Intrusion of the Amttsoq gneisses (predominantly 3750-3700 Ma tonalites)
accompanied and followed by deformation, metamorphism and granite sheet
injection. 3750-3400 Ma.
1) Deposition of a supracrustal sequence, represented by the Akilia
association and the Isua supracrustal belt. 3800 Ma.
Airborne radiometric surveying (Secher, 1977, F, pers. comms.) shows
there is considerable areal variation in ftray (combined K-U-Th-Rbl
radioactivity, which persists even after 'stripping1 of anomalous
concentrations associated with descrete potassic granitoids of various ages,
such as the Oorqut granite complex. This leads to a relatively simple
regional pattern for radioactivity: Terranes that have never experienced
granulite facies metamornhism are more radioactive than those that have
(Fig. 1).
C-
https://ntrs.nasa.gov/search.jsp?R=19860019079 2020-03-20T14:46:58+00:00Z
CRUSTAL STRUCTURE & GRANULITE FACIES, W. GREENLAND
NUTMAN A.P. et.al. 79
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K A N G I M U T SANGMISSOQ
20 km 3JL
o o
Sampled area
Qorqut grani te
Low radioac t iv i ty - amph.
High radioactivity - amph.
Low radioac t iv i ty - gran.
Figure 1. Radioactivity/metamorphic facies zones of study area. {.&} -
affected by 3000-2800 Ma granulite facies metamorphism [A] - affected
by 3000-2800 Ma amphibolite facies metamorphism. A - amphibolite facies
metamorphic conditions between 2800-2550 Ma.
The northwestern terrane (Nordlandet) underwent granulite facies
metamorphism at c. 3000-2900 Ma, as opposed to at c. 2800 Ma for the
granulite rocks in the southeast (e.g. Taylor et al., 1980) This paper
comnares the southern [G] and the [A] zones (Fig. 1) to see if variation
in content of radioactive minerals can be related to the c. 2800 Ma
granulite facies event, ^his is done by examining Amttsoq grey gneisses,
mostly derived from tonalites, from three localities: (1) Kangimut
sangmissoq [G] , low radioactivity, (?) Akugdlerssuaq [/\] , high radio-
activity and (3) Isukasia [A] , moderate to low radioactivity (Fig. 1).
FIELD RELATIONS: Late Archean ductile deformation accompanied
retrogression of granulite facies rocks ( [G] followed by A )• Most
CRUSTAL STRUCTURE & GRANULITE FACIES, W. GREENLAND
NUTMAN, A.P. et al.
obvious are "straight zones' up to 1 km wide with a steeply dipping LS
fabric with amphibolite facies assemblages, and more or less contempor-
aneous non-cylindrical southerly-plunging folds with wavelengths of up to
15 km. In areas of low deformation granulite facies rocks are patchily
retrogressed, with amphibolite facies assemblages mimicking textures
developed under granulite facies metamorphism. These textures are
obliterated or strongly modified in the zones of strong deformation
described above. Granitoid sheets intruded during formation of these
structures are between 2700 and 2600 Ma old (McGregor et al., 1983), giving
the time of retrogression. The boundaries between the regions of different
metamorphic history (Fig. 1) are complicated on a 10 to 20 km scale due to
the folding, and in some places the boundary between the southeastern
granulite facies terrane and the amphibolite facies terranes of
Godth&bsfjord could be a tectonic break. This could explain why well
preserved, type Amltsoq gneisses of outer Ameralik are found juxtaposed
with granulite facies rocks slightly further into the fjord (Fig. 1).
However, the zonal facies arrangement appears valid on the regional scale
(?00km).
It is likely that 2700-2600 Ma deformation juxtaposed regions that
were at different crustal levels during the c. 2800 Ma granulite facies
event, so that 2800 Ma granulte facies rocks outcropping in the south were
probably overlain by amphibolite facies terranes equivalent to those
found in Godthabsfjord. Rb-Sr data on retrogressed granulites to the north
of Godthabsf jord (Garde et al., in press") shows that c. 3000 Ma gneisses
there were 'reset' under amphibolite facies conditions at c. 2850 Ma. This
suggests that during the c. 2800 Ma granulite facies event in the south,
that c. 3000 Ma granulite facies terrane to the north was not at great
depth, and formed a metastable granulite facies block in the middle-crustal
level amnhibolite facies terranes.
The character of the quartzofeldspathic components of the three areas
sampled in this study is outlined below.
Kangimut sangmissoq: The dominant lithology here is buff-coloured, medium
to coarse-grained, somewhat inhomogeneous to nebulitic tonalitic gneisses
that contain remnants of plagioclase-phyric amphibolite dykes that
lithologically and geochemically resemble poorly-preserved Ameralik dykes
from the type area, 10 km to the west. The gneisses contain granulite
facies assemblages but show widesnread retrogression under amphibolite
facies conditions. The Kangimut sangmissoq rocks are part of a large raft
intruded first by massive tonalites that resemble main phases of the Nuk
gneisses s. s. , emplaced between 3050 and 2900 Ma (Baadsgaard f: McGregor,
1981), and then by 2700-2600 Ma (post granulite facies) granitoid sheets
(Roberts, 1979; McGregor et al., 1983), The Kanigmut sangmissoq gneisses
are interpreted as containing an appreciable component of Amttsoq gneisses
(McGregor et al., this volume), Isotopic data on these rocks, when
considered in isolation, do not conclusively show that these rocks are
early Archean (S. Moorbath, P. Taylor f, H. Baadsgaard pers. comms. FT
Collerson et al., this volume),
Akugdlerssuaq: The gneiss complex is dominated by amphibolite facies
schlieric and nebulitic gneisses with disrupted amphibolite dykes (with
discordances only rarely preserved), and containing enclaves of banded
amphibolite and banded iron formation (A!:ilia association) These are cut
by coarse-grained granite and pegmatite sheets, that in places can be hard
to distinguish from, and grade into, in situ neosome. The complex was then
CRUSTAL STRUCTURE & GRANULITE FACIES, W. GREENLAND
NUTMAN A.P. et al.
cut by sharp-margined sheets of tonalitic to granitic composition, and.was
further deformed under amphibolite facies conditions (Nutman, 1982). The
gneisses are interpreted as containing an Amitsoq component, that was
affected by two episodes of late Archean migmatisation. Rb-Sr and Pb-Pb
isotopic studies from this and adjacent areas show that the isotopic
systematics of Amttsoq gneisses was strongly disturbed in the late Archean
and that the grey sheets are 2700-2600 Ma old (Robertson, 1983; S. Robertson
FT M. Brewer, pers. comms; Bridgwater, Nutman f, Larsen, unpublished data) .
Isukasia: The gneisses consist of 3750-3700 Ma tonalites veined by c. 3600
Ma granite sheets and comparatively rare c. 3400 Ma pegmatite dykes (Nutman
et al., 1983; Baadsgaard et al., in prep.). In the late Archean there was
heterogeneous deformation, but no significant migmatisation. The Amttsoq
gneisses of this area widely preserve their early Archean characters, whilst
in the other areas discussed above late Archean tectonometamorphic over-
printing and migmatisation have obliterated their early Archean characters.
GEOCHEMISTRY: This study considers grey gneisses that have undergone
different metamorphic histories. From field work the samples are identified
as derived from >3600 Ma tonalitic and banded Amitsoq grey gneisses. How-
ever from the point of view of element migration discussed here, this age
identification is unimportant since similar features are developed in
polymetamorphic suites of Nuk gneisses at localities adjacent to those
chosen for this study.
There is marked variation seen in the composition of these rocks in
areas of different metamorphic history (Table 2) For example
Akugdlerssuaq, where metamorphic grade was not above amphibolite facies
and which forms part of the belt of high radioactivity (Fig. 1), there is
a gain of LREE, Th, Rb, Pb, and K relative to both rocks from the low
radioactivity, granulite facies terrane (e.g., Kangimut sangmissoq) and the
well preserved, low radioactivity, amphibolite facies terrane to the north
(e.g. Isukasia).
Table 2. Significant regional variation in composition of Amitsoq grey
gneisses (expressed as average abundance - more than 15 samples per group),
Isukasia [A3 , moderate radioactivity
Akugdlerssuaq [Aj , high radioactivity
Kangimut sangmissoq [G] low radioactivity
K00
1.61
2.36
0.57
Rb
85
104
9
Pb
17
22
8
Th
4
11
1
Ce
37
141
29
There seems to be a good correspondence between metamorphic history and
regional variation of radioactivity and geochemistry. It is suggested that
the amphibolite facies, high radioactivity zone represented by Akugdlerssuaq
could be an enriched zone, containing LIL elements expelled from underlying
granulite facies terrane, represented here by Kangimut sangmissoq. The
amphibolite facies, low radioactivity rocks of Isukasia are interpreted as a
part of a crustal level above the enriched zone, not significantly
metasomatised and intruded by granitoids during the granulite facies
metamorphism at depth. Accepting this explanation implies redistribution
CRUSTAL STRUCTURE & GRANULITE FACIES, W. GREENLAND
NUTMAN A.P. et al.
of LIL elements on the scale of several kilometres during granulite facies
metamorphism. In a heterogeneous, polyphase gneiss complex, this must have
involved isotopic mixing between lithologies of different ages and origins,
leading to modification of the isotopic signatures of the units (McGregor
et al. fT Collerson et al., this volume) It is apparent that enrichment
of LIL elements in the gneisses from the enriched zone (Akugdlerssuaq) could
in part be introduced via the injected granitic and pegmatitic sheets, that
form at the most 10% of the gneisses complex at Akugdlerssuaq, and at other
comparable localities such as Nordafar (see Collerson et al., this volume).
However, the likely reason for concentration of these sheets is their
derivation as either melts or hydrous fluids from the area undergoing
granulite facies metamorphism (Bridgwater et al., in press; Schi^tte
et al., in press).
DISCUSSION: The regional variation of Amttsoq gneiss compositions
combined with structural interpretations suggest that the c. 2800 Ma crust
of the region was stratified, with granulite facies terrane at depth, (e.g.,
Kangimut sangmissoq) overlain by enriched amphibolite facies containing LIL
elements expelled from the granulite facies terranes (e.g., Akugdlerssuaq)
and capped by non-enriched, relatively undisturbed amphibolite facies
terrane (e.g., Isukasia). Such stratification has also been discussed for
other areas (e.g. Newton et al., 1981; Condie et al., 1982; Schitftte et al.,
in press). Development of such a layer cake structure with a zone rich in
LIL elements imposes important controls on subsequent anatexis and
granitoid generation.
RFFERENCES:
Baadsgaard, H. and McGregor, V.R., 1981: The U-Th-Pb systematics of
zircons from type Nuk gneisses, Godthabsfjord, West Greenland.
Geochim. Cosmochim. Acta., 45, 1099-1109.
Baadsgaard, H., Nutman, A.P. and Bridgwater, D. , in prep: Chronology and
isotope geochemistry of the early Archean Amitsoq gneisses of the
Isukasia area, southern West Greenland.
Bridgwater, D., Nutman, A.P., Schi0tte, L. and Ryan, B., in press: Field
and isotopic evidence for formation and reworking of early Archean
rocks, Saglek - Hebron area, Labrador, Geol. Surv. Canada Report.
Condie, K.C., Allen, P. and Marayana, B.L., 1982: Geochemistry of the
Archean low to high grade transition zone, southern India. Contr.
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Garde, A,A., Larsen, 0. and Nutman, A.P. in press: Dating of late Archean
crustal mobilisation north of Ougssuk, Godthabsfjord, southern West
Greenland. Gr^nlands geol. Unders. Rapp.
McGregor, V.R., Bridgwater, D. and Nutman, A.P., 1983: The Qarusuk acid
dykes: post-Nuk, pre-Qorqut magmatism in the Godthab region, southern
West Greenland. Grtfnlands geol. Unders. Rapp., 112, 101-112.
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CRUSTAL STRUCTURE & GRANULITE FACIES, W. GREENLAND
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quartzo-feldspathic rocks from Kangiussap nuna, Ivisartoq sheet,
southern West Greenland. Gr^nlands geol. Unders. Rapp., 115, 56-59.
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Regional variation in the Amitsoq gneisses related to crustal levels during late Archean granulite facies metamorphism: Southern west Greenland

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