Geological Survey of Denmark and Greenland Bulletin 13, 2007
|
|
Review of Survey Activities 2006, 53-56 |
Please use the pdf-file for printing: nr13_p53-56.pdf (~1.3 Mb)
PDF-files requires a pdf-reader, Acrobat Reader GSview or similar
53
During the field campaign in the Nuuk
region, one of the objectives was to
describe Archaean primary geological
environments (Hollis et al. 2006). On
Nunatak 1390, which is part of the
Tasiusarsuaq terrane (Figs 1, 2), a bi -
modal volcanic succession is preserved
and interpreted as former ocean floor.
The field investigation included geologi-
cal mapping and sampling of the volcanic
sequence comprising mafic to ul tra mafic
rocks, and associated acid volcanic rocks
and granite intrusions.
region, one of the objectives was to
describe Archaean primary geological
environments (Hollis et al. 2006). On
Nunatak 1390, which is part of the
Tasiusarsuaq terrane (Figs 1, 2), a bi -
modal volcanic succession is preserved
and interpreted as former ocean floor.
The field investigation included geologi-
cal mapping and sampling of the volcanic
sequence comprising mafic to ul tra mafic
rocks, and associated acid volcanic rocks
and granite intrusions.
The Tasiusarsuaq terrane
The Tasiusarsuaq terrane is dominated by
mafic rocks (amphibolite), tonalitic gneiss
and granodiorite yielding ages of 2.92-
2.86 Ga (Fig. 1; Schiøtte et al. 1989;
Friend & Nutman 2001; Crowley 2002).
Metamorphic grade ranges from green-
schist to granulite facies conditions, with
peak metamorphism dated at ~2.79 Ga
(Pidgeon & Kalsbeek 1978). The mafic
rocks comprise greenschist facies mafic
rocks with pillow structures, metagab-
broic and ultramafic pods, and dykes.
The thicknesses of the mafic to ultra-
mafic sequences vary from 50 m up to
more than 1000 m. The Tasiusarsuaq ter-
rane rocks are cross-cut by brown-weath-
ering E-W-trending dolerite dykes (up to
30 m wide) with well-developed chil led
margins. Alterations such as calc-silicate
formation are common within the mafic-
ultramafic rocks. The pillowed mafic se -
quences contain intercalations of 1-2 m
wide, rusty, sulphide-bearing layers and
tourmalinites (exhalites). The sulphides
recorded include pyrite, pyrr hotite, chal-
copyrite and arsenopyrite.
mafic rocks (amphibolite), tonalitic gneiss
and granodiorite yielding ages of 2.92-
2.86 Ga (Fig. 1; Schiøtte et al. 1989;
Friend & Nutman 2001; Crowley 2002).
Metamorphic grade ranges from green-
schist to granulite facies conditions, with
peak metamorphism dated at ~2.79 Ga
(Pidgeon & Kalsbeek 1978). The mafic
rocks comprise greenschist facies mafic
rocks with pillow structures, metagab-
broic and ultramafic pods, and dykes.
The thicknesses of the mafic to ultra-
mafic sequences vary from 50 m up to
more than 1000 m. The Tasiusarsuaq ter-
rane rocks are cross-cut by brown-weath-
ering E-W-trending dolerite dykes (up to
30 m wide) with well-developed chil led
margins. Alterations such as calc-silicate
formation are common within the mafic-
ultramafic rocks. The pillowed mafic se -
quences contain intercalations of 1-2 m
wide, rusty, sulphide-bearing layers and
tourmalinites (exhalites). The sulphides
recorded include pyrite, pyrr hotite, chal-
copyrite and arsenopyrite.
A well-preserved bimodal Archaean volcanic succession
in the Tasiusarsuaq terrane, South-West Greenland
in the Tasiusarsuaq terrane, South-West Greenland
Henrik Stendal and Anders Scherstén
© GEUS, 2007.
Geological Survey of Denmark and Greenland Bulletin
13, 53-56. Available at:
www.geus.dk/publications/bull
Fig. 1. Geological map of the Nuuk region and location of the Nunatak 1390 study area (modified
from Escher & Pulvertaft 1995).
Nunatak 1390
Nunatak 1390 is located within the Inland Ice east of
Alangorlia and was first described by Escher & Pidgeon
(1976; Figs 1, 2). The entire volcanic package (Fig. 3; Table
1) is north-east-striking and dips steeply to the north-west.
Although slightly to moderately deformed with lineations,
folding, faulting and shearing, the rocks show well-preserved
primary textures.
Alangorlia and was first described by Escher & Pidgeon
(1976; Figs 1, 2). The entire volcanic package (Fig. 3; Table
1) is north-east-striking and dips steeply to the north-west.
Although slightly to moderately deformed with lineations,
folding, faulting and shearing, the rocks show well-preserved
primary textures.
Stratigraphy
The lower mafic pillow sequence shows large deformed pil-
low structures (50-100 cm across) and pillow breccias with
calc-silicate alteration in the matrix between the pillows and
in the centre of some pillows. The calc-silicate minerals include
epidote, diopside and carbonates and make up to 20 vol.% of
the rock. The pillowed sequence is cut by a slightly deformed
E-W-trending swarm of mafic dykes (1-5 m thick). The
dykes are fine- to medium-grained gabbroic or noritic rocks.
low structures (50-100 cm across) and pillow breccias with
calc-silicate alteration in the matrix between the pillows and
in the centre of some pillows. The calc-silicate minerals include
epidote, diopside and carbonates and make up to 20 vol.% of
the rock. The pillowed sequence is cut by a slightly deformed
E-W-trending swarm of mafic dykes (1-5 m thick). The
dykes are fine- to medium-grained gabbroic or noritic rocks.
Ultramafic greenstones and soapstones occur between the
upper and lower pillow lava sequences. These magnetite-
bearing rocks were probably originally sills. The upper pillow
sequence contains very well-preserved primary structures in
pillows, lava flows and ash layers (Fig. 4). The least deformed
pillow lavas and flows contain relic vesicles. Way-up can read-
ily be determined from the pillow structures and consistently
youngs to the south. The upper mafic pillow sequence is
overlain by a unit of acid volcanic and pyroclastic rocks,
including ignimbrites, 80 m in thickness (Fig. 5). Fine-
grained, grey to light-coloured porphyritic dykes (0.3-0.8 m
wide) cut the volcanic rocks and are interpreted as feeder
dykes to the acid rocks. Mafic flows and ash layers are inter-
calated with ultramafic sills, and mafic rusty layers contain
sulphides and tourmalinites. The tourmalinite forms an up to
one metre thick layer.
bearing rocks were probably originally sills. The upper pillow
sequence contains very well-preserved primary structures in
pillows, lava flows and ash layers (Fig. 4). The least deformed
pillow lavas and flows contain relic vesicles. Way-up can read-
ily be determined from the pillow structures and consistently
youngs to the south. The upper mafic pillow sequence is
overlain by a unit of acid volcanic and pyroclastic rocks,
including ignimbrites, 80 m in thickness (Fig. 5). Fine-
grained, grey to light-coloured porphyritic dykes (0.3-0.8 m
wide) cut the volcanic rocks and are interpreted as feeder
dykes to the acid rocks. Mafic flows and ash layers are inter-
calated with ultramafic sills, and mafic rusty layers contain
sulphides and tourmalinites. The tourmalinite forms an up to
one metre thick layer.
A prominent hydrothermal zone, strongly silicified and
epidotised, strikes parallel with the mafic ash layers. It follows
54
Fig. 2. Geological map of Nunatak 1390. Profile A-B is described in
Table 1 and shown in Fig. 3.
PD
, Palaeoproterozoic dyke. For location
see Fig. 1.
a fault lineament, is up to 50 m wide and can be recognised
from the light brownish surface colour of the altered rocks.
The hydrothermal zone is overlain by a thick sequence
(700-800 m) of finely laminated tuff layers. Granite intru-
sions in the tuffs increase in abundance upwards and pass
upwards into porphyritic granite with tuff xenoliths.
from the light brownish surface colour of the altered rocks.
The hydrothermal zone is overlain by a thick sequence
(700-800 m) of finely laminated tuff layers. Granite intru-
sions in the tuffs increase in abundance upwards and pass
upwards into porphyritic granite with tuff xenoliths.
Two phases of granite occur: one is porphyritic with K-
feldspar phenocrysts up to several centimetres in length; the
other is more homogeneous, medium-grained, slightly foli-
ated and muscovite-bearing. Parts of the granites, especially
in the western part of the exposure, are altered and have a dis-
tinct pink coloration due to hematite formation.
other is more homogeneous, medium-grained, slightly foli-
ated and muscovite-bearing. Parts of the granites, especially
in the western part of the exposure, are altered and have a dis-
tinct pink coloration due to hematite formation.
On a regional scale, it should be noted that western and
southern parts of the Tasiusarsuaq terrane preserve remnants
of volcanic rocks at several localities, probably of similar age
to that on Nunatak 1390 (Escher & Myers 1975).
of volcanic rocks at several localities, probably of similar age
to that on Nunatak 1390 (Escher & Myers 1975).
Geochemistry
Twenty samples representing most rock types found on
Nunatak 1390 were analysed by Actlabs, Canada (Research
Package 4E). Altered samples were screened using e.g.
K2O/P2O5, and those with anomalous ratios are not con-
Nunatak 1390 were analysed by Actlabs, Canada (Research
Package 4E). Altered samples were screened using e.g.
K2O/P2O5, and those with anomalous ratios are not con-
sidered any further. Melanocratic-ultramafic pillow lavas,
flows and ash have komatiitic and Mg- and Fe-rich basalt
compositions and plot along a well-defined tholeiitic trend
(Fig. 6A). The acid rock, which intercalates with the tholei-
ites, forms a loosely defined calc-alkaline group of andesitic
to dacitic composition (Fig. 6A).
flows and ash have komatiitic and Mg- and Fe-rich basalt
compositions and plot along a well-defined tholeiitic trend
(Fig. 6A). The acid rock, which intercalates with the tholei-
ites, forms a loosely defined calc-alkaline group of andesitic
to dacitic composition (Fig. 6A).
The tholeiites are characterised by near chondritic relative
REE abundances (mean La/SmN = 1.2 ± 0.4 1
; La/YbN =
1.3 ± 0.3 1
, n = 9), while the acid rocks are more enriched
and varied in their LREE (mean La/SmN = 5 ± 1 1
; La/YbN
= 24 ± 17 1
, n = 5; Fig. 6B). The different incompatible
trace element abundances in the tholeiites and acid rocks per-
sist through all the elements, albeit with more scatter in the
mobile elements. Important immobile, incompatible ele-
ment ratios are indicative of potentially different tectonic set-
tings for the tholeiites and the acid rocks. Nb/La ratios do not
vary with Nb or other incompatible elements, which implies
insig nif icant effects of fractional crystallisation and insignifi-
cant amounts of crustal contamination (Fig. 6C). The tholei-
ite mean Nb/La ratio is 0.6 ± 0.1 (1
sist through all the elements, albeit with more scatter in the
mobile elements. Important immobile, incompatible ele-
ment ratios are indicative of potentially different tectonic set-
tings for the tholeiites and the acid rocks. Nb/La ratios do not
vary with Nb or other incompatible elements, which implies
insig nif icant effects of fractional crystallisation and insignifi-
cant amounts of crustal contamination (Fig. 6C). The tholei-
ite mean Nb/La ratio is 0.6 ± 0.1 (1
; n = 9), while the acid
rocks have a substantially lower ratio of 0.14 ± 0.03 (1
; n = 5).
The ratios of the tholeiites are reminiscent of lower crust,
while the low ratios of the acid rocks are typical of volcanic
arc related rocks (Hawkesworth & Kemp 2006). Even
though metamorphic element mobility may disturb e.g.
Ce/Pb ratios, the consistent and low ratios of the acid rocks
corro borate an arc origin (Fig. 6D), which we postulate for
these rocks. The tholeiites are more ambiguous and are akin
to MORB or island arc tholeiites. The spatial relationship
while the low ratios of the acid rocks are typical of volcanic
arc related rocks (Hawkesworth & Kemp 2006). Even
though metamorphic element mobility may disturb e.g.
Ce/Pb ratios, the consistent and low ratios of the acid rocks
corro borate an arc origin (Fig. 6D), which we postulate for
these rocks. The tholeiites are more ambiguous and are akin
to MORB or island arc tholeiites. The spatial relationship
55
Fig. 3. Central part of the stratigraphy of profile A-B on Nunatak 1390
(see Table 1).
Fig. 4. Pillow lava structures in the upper pillow lava sequence. Hammer
(50 cm) for scale.
Fig. 5. Laminated acid volcanic rock (ignimbrite). Inset is an enlarge-
ment (inset is about 5 cm wide).
between the acid rocks and the tholeiites seems to support a
common origin, in which case an arc setting seems most
plausible. By analogy with modern arc systems, the two com-
ponents may reflect input from trench-side (tholeiites) and
back arc-side (calc-alkaline) volcanoes respectively, or tempo-
ral shifts in the petrogenetic processes.
common origin, in which case an arc setting seems most
plausible. By analogy with modern arc systems, the two com-
ponents may reflect input from trench-side (tholeiites) and
back arc-side (calc-alkaline) volcanoes respectively, or tempo-
ral shifts in the petrogenetic processes.
References
Crowley, J.L. 2002: Testing the model of late Archean terrane accretion
in southern West Greenland: a comparison of the timing of geological
events across the Qarliit nunaat fault, Buksefjorden region.
Precambrian Research 116 , 57-79.
events across the Qarliit nunaat fault, Buksefjorden region.
Precambrian Research 116 , 57-79.
Escher, J.C. & Myers, J.S. 1975: New evidence concerning the original
relationships of early Precambrian volcanics and anorthosites in the
Fiskenæsset region, southern West Greenland. Rapport Grønlands
Geologiske Undersøgelse 75 , 72-76.
Fiskenæsset region, southern West Greenland. Rapport Grønlands
Geologiske Undersøgelse 75 , 72-76.
Escher, J.C. & Pidgeon, R.T. 1976: Field mapping of nunatak 1390 m, east
of Alángordlia, southern West Greenland. Rapport Grønlands Geolo -
giske Undersøgelse 80 , 84-87.
giske Undersøgelse 80 , 84-87.
Escher, J.C. & Pulvertaft, T.C.R. 1995: Geological map of Greenland,
1:2 500 000. Copenhagen: Geological Survey of Greenland.
Friend, C.R.L. & Nutman, A.P. 2001: U-Pb zircon study of tectonically
bounded blocks of 2940-2840 Ma crust with different metamorphic
histories, Paamiut region, South-West Greenland: implications for the
tectonic assembly of the North Atlantic craton. Precambrian Research
105 , 143-164.
histories, Paamiut region, South-West Greenland: implications for the
tectonic assembly of the North Atlantic craton. Precambrian Research
105 , 143-164.
Hawkesworth, C.J. & Kemp, A.I.S. 2006: Evolution of the continental
crust. Nature
443
, 811-817.
Hollis, J.A., Schmid, S., Stendal, H., van Gool, J.A.M. & Weng, W.L. 2006:
Supracrustal belts in Godthåbsfjord region, southern West Greenland.
Progress report on 2005 field work: geological mapping, regional
hydrothermal alteration and tectonic sections. Danmarks og Grøn
Progress report on 2005 field work: geological mapping, regional
hydrothermal alteration and tectonic sections. Danmarks og Grøn
-
lands Geologiske Undersøgelse Rapport
2006/7
, 171 pp.
Pidgeon, R.T. & Kalsbeek, F. 1978: Dating of igneous and metamorphic
events in the Fiskenaesset region of southern West Greenland.
Canadian Journal of Earth Sciences 15 , 2021-2025.
Canadian Journal of Earth Sciences 15 , 2021-2025.
Schiøtte, L., Compston, W. & Bridgwater, D. 1989: U-Pb single-zircon age
for the Tinissaq gneiss of southern West Greenland: a controversy
resolved. Chemical Geology (Isotope Geoscience Section) 79 , 21-30.
resolved. Chemical Geology (Isotope Geoscience Section) 79 , 21-30.
56
Authors' address
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
E-mail:
hst@geus.dk
Fig. 6.
A
: FeO
t
/MgO versus SiO
2
variation diagram, and Jensen cation plot; rhyolite (
r
), dacite (
d
), andesite (
a
), basalt (
b
), Mg tholeiite basalt (
m
), Fe
tholeiite basalt (
f
), komatiitic basalt (
kb
) and komatiite (
k
).
B
: REE distribution of the tholeiitic suite (
green
) and calc-alkaline suite (
red
).
C
: Nb ver-
sus Nb/La of the tholeiitic suite (
green
) and calc-alkaline suite (
red
).
D
: Ce/Pb ratios variation versus Ce of the tholeiitic suite (
green
) and calc-alka-
line suite (
red
).
