Geology Reference
In-Depth Information
1. The
Snowbird phase
(~1920-
1890 Ma): convergence and colli-
sion of the Hearne and Rae cratons
together with the deformation of the
intervening volcanic-arc deposits;
development of the oceanic volcanic
arc terranes in the ocean between the
Hearne, Sugluq and Superior cratons.
volcanic arc terranes, accompanied by
a widespread tectono-thermal over-
print. Collision started in the south and
migrated around the Superior craton,
ending in the New Quebec belt.
Archaean cratons
) are therefore frag-
ments of larger areas that have been dis-
rupted by younger events, and it is rare
to find evidence of boundaries between
Archaean areas that have demonstrably
experienced orogenesis and adjoining
areas that might represent a contem-
porary stable foreland. Most Archaean
cratons were formed in the period
2700-2600 Ma and are thought by some
to represent pieces of a late Archaean
supercontinent. Evidence of earlier
Archaean continental cratons of any
size is sparse. However, Archaean rocks
of the
Pilbara craton
in NW Australia
and the
Kaapvaal craton
in South Africa
had become stabilised at 2800 Ma and
2900 Ma respectively, and could be said
to represent the continental forelands
of younger Archaean orogenic belts.
For many years there was consid-
erable debate about whether or not
'modern' plate-tectonic processes oper-
ated in the Archaean, but this debate
has been largely resolved. Increasing
knowledge of the detailed composi-
tion of several Archaean cratons has
indicated that they can be explained by
the processes of accretion of continen-
tal, oceanic and volcanic arc material
in much the same way as more recent
orogenic belts. Moreover, the compo-
sition of the igneous rocks is similar,
though not identical, to the magmatic
products of modern subduction zones.
Differences include a preponderance
of
greywackes
in the sedimentary
sequences compared to typical platform
sequences, a lack of high-pressure
blue
schists
and Phanerozoic-type ophiolite
assemblages, a more
tonalitic
compo-
sition of the granitic plutons, and the
prevalence of
komatiites
(ultrabasic
volcanics). Some of these differences
The Trans-Hudson orogen is one of
the earliest examples of an orogenic belt
where 'modern' plate tectonic processes
can plausibly be adduced to explain the
overall structure and sequence of tec-
tonic events. This is due to the fact that
separate terranes and their intervening
sutures, together with subduction-
related magmatism, can be identified.
The preservation of identifiable shelf
sequences, such as the Piling Group,
on bordering cratons, well-developed
ophiolites (the Purtuniq ophiolite),
foreland thrust belts (e.g. along the
Superior craton margin) and foreland
basins (e.g. the Penrhyn-Piling basin)
all point to the operation of plate tec-
tonic processes similar to those of later
periods. The same cannot be said of the
Archaean, which we shall now discuss.
2. The
Reindeer-Foxe phase
(~1880-1865 Ma): accretion of the La
Ronge-Lynn Lake oceanic arc terrane
to the Hearne craton; collision of the
Meta Incognita craton with the Rae
craton; and consequent closure and
deformation of the Penrhyn-Piling
basin. In the Eastern Churchill Prov-
ince, collision between the NAC and
the Core zone to form the Torngat belt.
3. The
Wathaman phase
(~1865-
1840 Ma): accretion of the Parent-
Spartan and Flin-Flon-Glennie volcanic
arc terranes to the Sugluq and Hearne
cratons; collision of the Sask terrane
with the Flin-Flon-Glennie volcanic
arc; emplacement of granite batho-
liths, incuding the Wathaman and
Cumberland batholiths, along the
south-west margins of the Hearne,
Sugluq and Meta Incognita cratons;
formation of a magmatic arc (the
Snow Lake belt
) at the north-western
margin of the Superior craton.
The Archaean
The
Archaean Eon
extends from
4000 Ma ago until the beginning of the
Proterozoic at 2500 Ma, a time span
three times as long as the Phanerozoic.
However, evidence of tectonic pro-
cesses becomes increasingly difficult to
establish, the further back in time we
seek it. Rocks of Archaean age make up
a large proportion of the Precambrian
cores of all the continents, but in many
cases have been intensively reworked in
subsequent orogenies, as can be seen,
for example, in the Archaean terranes
incorporated into the Early Proterozoic
Trans-Hudson orogen just discussed.
Most areas of Archaean outcrop (i.e. the
4. Main
Hudsonian orogeny
(~1840-1800 Ma): emplacement of
continental-arc plutons along the
margin of the Meta Incognita craton
and in the Core zone; (from ~1830 Ma)
terminal collision of the Superior craton
with the previously accreted collage
(the Hearne, Rae, Sugluq and NAC-
Core zone cratons plus the intervening
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