gins, 1990; Kalsbeek et al., 1990 containing undi- vided, typical Archaean tonalite, trondhjemite,
granodiorite TTG
suite gneisses
from the
Ketilidian boundary north to Frederiksha˚b Is- blink Fig. 1. Kalsbeek et al., 1990 considered
that the rocks north of Paamiut were similar to those north of Frederiksha˚b Isblink, and conse-
quently concentrated on describing the rocks be- tween Paamiut and Ivittuut. The gneiss protoliths
were thought to have formed between 3000 – 2850 Ma and to have been metamorphosed under am-
phibolite facies conditions in the late Archaean. No evidence for granulite facies metamorphism in
the region was reported e.g. Higgins, 1990; Kals- beek et al., 1990 and Refs. therein. Within the
TTG
gneisses, amphibolite
and gabbro-
anorthosite units
occur as
semi-continuous marker units or as trains of enclaves. Locally,
units of sillimanite-bearing rocks, interpreted to represent pelitic to semi-pelitic compositions have
been recognised e.g. Rivalenti and Rossi, 1972. The Tartoq Group Higgins and Bondesen, 1966;
Higgins, 1968 is a separate, thick sequence of greenschist and low amphibolite facies rocks
derived from pillow lavas, other volcanic and associated sedimentary rocks. On the basis of its
lower metamorphic grade and structural evidence the group was thought to be younger than the
main gneisses Berthelsen and Henriksen, 1975.
2
.
2
. Isotopic data Few isotopic data exist for the Paamiut region.
In a whole-rock Pb – Pb isochron study, Taylor and Kalsbeek 1986 examined three units. Am-
phibolite facies, grey, biotite-bearing gneisses from the south-west Vesterland Fig. 1 yielded a
date of 2784 9 53 Ma, interpreted as a minimum protolith age. At Kuummiut Fig. 1 grey, banded
gneisses gave 2985 9 115 Ma, and a younger, intrusive trondhjemite component was dated at
2769 9 110 Ma. The same trondhjemite was dated using SHRIMP U – Pb single zircons, with
dates of 2922 9 4 Ma 2s for prismatic grains with later zircon overgrowth at 2827 9 11 Ma.
These were interpreted to date the protolith and a high-grade metamorphic event respectively Nut-
man and Kalsbeek, 1994. Igneous zircons from a tonalitic sheet cross-cutting the Tartoq Group
were dated at 2944 9 7 Ma Nutman and Kals- beek, 1994. This demonstrates that at least parts
of the Tartoq Group is older than previously supposed c.f. Berthelsen and Henriksen, 1975
and appears to be different from the supracrustal units found further north as suggested by Higgins,
1990.
3. New field evidence
The new field evidence for the subdivision of the Paamiut region was presented by McGregor
and Friend 1997; only a summary is given here. The region between Frederiksha˚b Isblink and Ser-
miligaarsuk Fig. 1 was divided into four blocks based on: i the recognition of blocks where
rocks have been partially to totally retrogressed from granulite facies mineral assemblages, com-
pared with blocks containing amphibolite facies rocks which have never been to granulite facies
conditions; and ii the recognition of Archaean amphibolite facies ductile shear zones with my-
lonite between blocks of different metamorphic history. This division was possible because char-
acteristic
metamorphic ‘blebby
textures’ and
structures such as ‘spotty pegmatites’ partial melts produced by the growth of orthopyroxene
during granulite facies metamorphism e.g. Mc- Gregor and Friend, 1992, 1997, see below are
retained either during static retrogression or when retrogression is accompanied by only limited de-
formation. Rocks which have never been to gran- ulite facies do not show these textures and
structures e.g. McGregor and Friend, 1992, 1997. Gneisses with a blebby texture developed
from pseudomorphed orthopyroxene porphyrob- lasts McGregor et al., 1986 have been illustrated
from other parts of the West Greenland Archaean e.g. Garde, 1990, 1997; McGregor and Friend,
1992. They are also recognised elsewhere in the North Atlantic craton in Scotland e.g. Beach,
1974; Crane, 1978 and Labrador e.g. Collerson et al., 1982.
Between each of the blocks of different meta- morphic history there are either sharp or transi-
tional mylonitic boundaries with ductile shear
Fig. 2. Sketch cross section along the outer coast of the Paamiut region from Bjørnesund in the Tasiusarsuaq terrane to Tartoq in the Sermiligaarsuk block showing the relative structural positions of each of the identified blocks. For locations and legend see Fig.
1. Abbreviations used: p, protolith age; i, inheritance; af, amphibolite facies metamorphism; gf, granulite facies metamorphism.
zones. In these zones blebby textures and spotty pegmatites produced under granulite facies condi-
tions were deformed, indicating tectonic juxtapo- sition
occurred after
granulite facies
metamorphism. The
trace of
these tectonic
boundaries Figs. 1 – 3 show they developed prior to late Archaean folding under amphibolite facies
conditions. From north to south these blocks are:
3
.
1
. The Sioraq block This block occurs west and south of Frederik-
sha˚b Isblink Fig. 1 and is dominated by dioritic to tonalitic gneisses. Primary igneous features,
such as sharp intrusive contacts between different phases and relic coarse-grained igneous textures,
indicating derivation from plutonic rocks are rarely preserved. In some areas, more granodi-
oritic components appear to have undergone par- tial melting, before or during granulite facies
metamorphism, as they have copious leucosome veining. Supracrustal units are dominated by
mafic rocks, probably derived from volcanic pro- toliths, including homogeneous and layered am-
phibolite, meta-gabbroic amphibolites and some ultramafic pods. Occasional biotite-garnet-silli-
manite gneisses are derived from sediments. Or- thopyroxene-bearing rocks are preserved in the
north-west, but to the east and south, where post-granulite facies deformation under amphibo-
lite facies conditions becomes increasingly intense, partial to total retrogression has occurred Mc-
Gregor and Friend, 1997. Garnet is widespread in rocks that have been partly retrogressed from
granulite facies, but is generally absent in rocks that have been totally recrystallised.
The position of the northern boundary with the Tasiusarssuaq terrane is uncertain and was ex-
trapolated from its location on the coast by Mc- Gregor and Friend 1997, using information
from Dawes 1970, Steenfelt 1994 and struc- tural trends on maps 62 V.1N Bjørnesund and 62
V.1S Nerutussoq Fig. 1. Northwards, along the coast of Frederiksha˚b Isblink granulite facies
rocks pass northwards into totally retrogressed, blebby textured rocks which, with increasing
strain, become more and more mylonitic with a
Fig. 3. Sketch map of the geology south of Iterdlak in the vicinity of Nødre Storø see Fig. 1 for location, showing the
folded mylonitic contact between the ex-granulite facies Sioraq block and the amphibolite facies Paamiut block.
strong SSW-plunging linear fabric. About 2 km across strike to the west, an abrupt change in
metamorphic grade with the amphibolite facies gneisses and rocks of the Ravns Storø belt that
were never metamorphosed above amphibolite fa- cies conditions occurs. These observations suggest
that the contact zone between the amphibolite facies rocks and retrogressed granulite facies
rocks is tectonic and does not resemble the pro- grade amphibolite-granulite facies transition 25
km to the north in Bjørnesund within the Ta- siusarsuaq terrane McGregor and Friend, 1992.
This northern boundary of the Sioraq block lies on the northerly dipping limb of a large antiform
Andersen and Friend 1973 and implies that the amphibolite facies rocks of the Tasiusarsuaq ter-
rane are structurally above the Sioraq block Fig. 2. Gneisses similar to those in totally retrogressed
parts of the Sioraq block occur on Kangaarsup Nunaa but, on the northernmost of the Dalagers
Nunatakker Fig. 1, amphibolites with relic pil- low-lava structures occur which are equated with
pillow-structured amphibolites in the Ravns Storø belt Dawes 1970, and are thus interpreted to be
part of the Tasiusarsuaq terrane.
The southern boundary of the Sioraq block Fig. 1 is in places a 10 – 150 m wide zone of
folded, sub-mylonitic to ultramylonitic rocks with amphibolite facies assemblages Figs. 2 and 3.
Rocks recognised as retrogressed from granulite facies were followed into a high strain zone to
within 10 – 20 m from rocks on the other side that from textural evidenece never reached granulite
facies conditions. There is no indication of a prograde amphibolite- to granulite-facies transi-
tion and the contact is best interpreted as tectonic. The contact has been traced intermittently for ca.
30 km. Where observed there is an increasing strain gradient in the margin of the Sioraq block
towards the contact. Structurally, the ex-gran- ulite facies Sioraq block overlies the amphibolite
facies Paamiut block to the south which, from interpretation of published maps, occupies a
northeasterly-plunging domal structure with a large parasitic synform to the south Fig. 1.
Evidence supporting this interpretation comes from two independent sources. Rivalenti and
Rossi 1972 described the same textures and structures that are now understood to be blebby
textures characteristic of retrogressed granulites e.g. McGregor and Friend, 1997 and the rocks
inside the parasitic synformal structure have low K, Rb, and U, geochemical characteristics of
granulite facies rocks, as elsewhere in the Sioraq block Steenfelt, 1994; Steenfelt et al., 1994.
3
.
2
. The Paamiut block On the coast this block occupies the area be-
tween Nordre Storø and Vesterland, Fig. 1, whilst inland it envelops a major synform con-
taining the structurally higher Neria block and is then continuous with similar rocks to the north of
Neria Figs. 1 and 2. In the southern inland part of the region a contact is extrapolated using struc-
ture trends from published maps. The Paamiut block structurally underlies both the Sioraq and
Neria blocks but differs significantly in that it has never been metamorphosed above amphibolite fa-
cies, bearing none of the coarse-grained features found as a result of orthopyroxene growth and
retrogression McGregor and Friend, 1997. It is dominated by rather homogeneous tonalitic to
granodioritic biotite gneisses which, at any one locality comprise several different sheets with
sharp contacts and may preserve textures inter- preted to be secondary after plutonic igneous
textures. Minor trondhjemitic sheets and rare hornblende-bearing mafic tonalitic and dioritic
phases occur. The gneisses enclose thick units of likely supracrustal rocks dominated by amphibo-
lite, with minor ultramafic and biotite schists that are continuous over tens of kilometres outlining
major fold structures Fig. 1; e.g. Andrews, 1973; Higgins, 1990. Leucogabbroic and anorthositic
rocks have not been found. In some domal struc- tures the gneisses have undergone partial melting
to yield metatexites and diatexites.
3
.
3
. The Neria block The Neria block occupies the core of a major
synform Mı´sar, 1973 which runs east of Semilik Fig. 1, and then extends south-west through the
middle and outer parts of Sermilik to Neria, coming round the nose of a complimentary an-
tiform at the mouth of Neria Figs. 1 and 2. The north-western boundary of the block is truncated
by the Proterozoic Vesterland dextral shear zone of intense hydrous mylonitisation Watterson,
1968; Higgins, 1990. On the basis of strongly deformed blebby textures, the block is interpreted
to comprise rocks metamorphosed to granulite facies but later totally retrogressed to amphibolite
facies and moderately to strongly deformed Mc- Gregor and Friend, 1997. The northern part of
the block is dominated by hornblende-bearing tonalitic and dioritic phases enclosing abundant
enclaves, rafts and thin layers of mafic rocks, ultramafic
rocks and,
locally, gabbroic-
anorthositic lithologies. South of Neria Fig. 1 mafic lithologies are less abundant, the gneisses
being commonly finely pegmatite-layered and bi- otite-bearing and there are areas of polyphase,
leucocratic, trondhjemitic gneisses the white gneisses of Kalsbeek, 1970; Taylor and Kalsbeek,
1986. These trondhjemitic gneisses lack textural – mineralogical features indicating earlier granulite-
facies metamorphism, but appear to have had the same complex structural history as adjacent
gneisses that do preserve relic granulite-facies textures.
Field relations near the ice cap north of Sermili- gaarsuk Fig. 1 are unknown, though Masson
1967 recognised that hornblendic gneisses in the main synform were significantly different from the
rocks underneath. As one of several possible in- terpretations, he suggested a tectonic contact with
the hornblendic gneisses thrust over the biotite gneisses, though no recognition of such a contact
was documented. Late phases of biotite-bearing granitic gneiss on the south side of Neria appear
to post-date retrogression of the granulite-facies assemblages.
3
.
4
. The Sermiligaarsuk block Considerable modification of the rocks by
Proterozoic faulting has occurred on the south side of Neria obscuring Archaean relationships.
However, in contrast to the ex-granulite facies Neria block, the volcano-sedimentary Tartoq
Group to the south and associated gneisses have only been metamorphosed to greenschist and low-
ermost amphibolite facies, and are described as having had a much simpler structural history
Berthelsen and Henriksen, 1975; Higgins, 1990. It has been considered that the Tartoq Group was
tectonically emplaced and subsequently deformed into synformal structures Berthelsen and Henrik-
sen, 1975. These lower grade rocks are here named the Sermiligaarsuk block. The position of a
boundary has now been located on the outer coast where the Neria block overlies lower grade
rocks V.R. McGregor, pers. comm.. Retro- gressed granulite facies rocks of the Neria block
have been identified to within ca. 2 km of the much lower grade rocks of the Sermiligaarsuk
block Figs. 1 and 2 and there is no indication of any prograde transition preserved.
4. SHRIMP U – Pb zircon data
