Geology Reference
In-Depth Information
domain probably represents the
high-grade equivalent of the Sashigo
granite-greenstone assemblage.
The narrow
Uchi domain
con-
sists of a series of thin, tightly-folded,
E-W-trending belts of metavolcanic
and metasedimentary rocks intruded
by elongate granodioritic to tonalitic
intrusions.
The
English River domain
is an
E-W linear belt up to 60 km wide con-
taining high-grade, 2700-2650 Ma-old
metasedimentary, metavolcanic and
granitic gneisses. The metasediments
include metagreywackes believed to
result from the erosion of the neigh-
bouring granite-greenstone domains.
The
Winnipeg River domain
is a
granite-greenstone belt similar to the
Uchi but dominated by granitoid rocks.
Highly metamorphosed
supracrus-
tal
rocks (i.e. sediments and volcan-
ics) are intruded by granodioritic to
dioritic plutons emplaced between
2710 and 2660 Ma. 3319 Ma-old ton-
alitic gneisses are also present.
The
Quetico domain
is a metasedi-
mentary belt consisting predominantly
of 2700-2690 Ma-old metagrey-
wackes believed to be derived from
the adjoining volcanic domains.
The
Wawa domain
is the western
extension of the
Abitibi
granite-green-
stone domain of the eastern Superior
Province. It is regarded as a '
super-
terrane
' and consists of a subduction-
accretion complex of several separate
intra-oceanic and volcanic island-arc
belts formed during the period 2720-
2680 Ma and amalgamated in a colli-
sional event with the adjoining terranes
to the north and south at 2690-2670 Ma.
The
Minnesota River domain
is
located in the extreme south-western
corner of the Superior Province and
lies wholly within the USA. It con-
sists of ~2700 Ma metavolcanic and
metasedimentary rocks on a base-
ment of granitoid gneisses up to
3600 Ma old - the oldest rocks pres-
ently known within the Province.
Although the Superior Province has
been thought of as a ~2700 Ma col-
lisional orogen, it is now bounded by
younger orogenic belts on both sides,
and neither the nature of the original
bordering continental plates, if they
existed, nor the original dimensions of
the orogen, can now be established. It
may be more realistic to visualise the
orogen as a collection of accretionary
terranes. However, the existence of gran-
itoid gneisses ranging in age from 3600
to 3000 Ma indicates that several dis-
tinct continental fragments have been
caught up in the orogen and may repre-
sent pieces of pre-existing continents.
be considered to be a craton-wide
orogeny. Because of the length of time
represented by the Archaean, it has
been found convenient to subdivide
it into early, middle and late periods
-
Eo-archaean
(4000-3500 Ma),
Meso-
archaean
(3500-3000 Ma) and
Neo-
archaean
(3000-2500 Ma) respectively.
Granitoid rocks, generally gran-
itic to tonalitic in composition, are
the dominant rock type and are typi-
cally deformed into gneisses. These
contain elongate sheets and lenses from
around one metre to over a kilometre
in width, of supracrustal rocks that
are often strongly folded. The suprac-
rustal units include basic to ultrabasic
igneous rocks, including both volcanics
and intrusives, which are comparable
geochemically both with the green-
stone belts of the Superior Province
and with modern oceanic crustal
sequences. The supracrustal units also
include mica-schists and marbles,
whose association with the oceanic
volcanics suggests that they were prob-
ably laid down on oceanic crust.
According to the chronology estab-
lished in the early mapping of the
western NAC, the earliest rocks in the
NAC, a group of supracrustal volcanic
and sedimentary rocks of Eo-archaean
age (~3900 Ma) known as the
Isua
supracrustals
, were invaded by gran-
itic gneisses (the
Amitsoq gneisses
).
After the resulting Eo-archaean complex
had been formed, a second set of
supracrustal rocks, of Meso-archaean
age (the
Malene supracrustals
), were
deposited, and involved in a major
deformation that resulted in the tec-
tonic interleaving of Amitsoq and
Malene units at ~3000 Ma ago. These in
turn were invaded by further granitic
The North Atlantic craton (NAC)
This region, often taken as the 'type
example' of a high-grade gneiss terrain,
lies partly in south Greenland and
partly in eastern Labrador, and forms
the north-eastern foreland of the
Trans-Hudson orogen described above
(
see
Figure 12.9). It is much smaller
than the Superior craton to the south
- only about 500 km at its widest on
the western coast of Greenland, and
much of it is obscured by the Green-
land icecap. However, the exceptional
coastal exposures have been inten-
sively studied. The craton reveals a long
history of geological activity spanning
around 1500 million years that includes
at least three major tectonic events that
could be analogous to more modern
orogenies; only the last of these might
Search WWH ::
Custom Search