Definition of Slave Craton

The Slave craton is an Archean granite-greenstone

terrane located in the northwestern part of the Canadian

shield. The Archean history of the craton spans the

interval from 4.03 billion years, the age of the world’s oldest

rocks, known as the Acasta Gneisses exposed in a basement

culmination in the Wopmay orogen, to 2.6–2.5 billion years

ago, the age of major granitic plutonism throughout the

province. The margins of the craton were deformed and loaded

by sediments during Proterozoic orogenies, and the craton

is cut by several Proterozoic mafic dike swarms.

Most of the volcanic and sedimentary rocks of the Slave

craton were formed in the interval between 2.7 billion and

2.65 billion years ago. Syntectonic to post-tectonic plutons

form about half of the map area of the province. The pre-late

granite geology of the Slave Province shows some broadscale

tectonic zonations. Greenstone belts are concentrated in a

narrow northerly trending swath in the central part of the

province, and the relative abundance of mafic volcanics, felsic

volcanics, clastic rocks, and gneisses are different on either

side of this line. The dividing line is coincident with a major

Bouger gravity anomaly, and with an isotopic anomaly indicating

that older crust was involved in granitoid petrogenesis

in the west but not in the east. Greenstone belts west of the

line comprise predominantly mafic volcanic and plutonic

rocks (except at Indian Lake), whereas volcanic belts to the

east contain a much larger percentage of intermediate and felsic

volcanic material. This is most evident in a large belt of

northwest-trending felsic volcanics extending from south and

east of Bathurst Inlet toward Artillery Lake. Quartzofeldspathic

gneisses older than the greenstones are rare throughout

the province and are confined to a line west of the central

dividing line.

In the middle 1980s, Timothy Kusky proposed that these

major differences in geology across the Slave Province reflect

that it is divided into a number of different tectonic terranes.

These ideas were initially debated but later largely accepted

and modified by further mapping, seismic surveys, and geo-

chemical analysis. The province is divided into an older

gneissic terrane in the west, known as the Anton terrane,

that contains the world’s oldest known rocks and is overlain

by a platform type sedimentary sequence. The Contwoyto

terrane and Hackett River arc represent an accretionary

prism and island arc that accreted to the Anton terrane in

the late Archean, uplifting the Sleepy Dragon terrane in a

basement culmination.

Gneissic rocks of the Anton terrane extend from Yellowknife

to the Anialik River. The name is taken from the

Anton complex exposed north of Yellowknife, which consists

of metamorphosed granodiorite to quartz diorite, intruded by

younger granitoids. The Anton terrane dips under the Wopmay

orogen in the west, and its eastern contact is marked by

a several kilometer thick, nearly vertical mylonite zone, best

exposed in the vicinity of Point Lake. The Anton terrane

includes the oldest rocks known in the world, the 4.03-billion-

year-old Acasta gneisses exposed in a basement culmination

along the border with the Wopmay orogen. Also,

3.48–3.21-billion-year-old tonalitic gray gneisses are exposed

in several locations, and similar undated old gray gneissic

rocks are preserved as inclusions and small outcrop belts

within a sea of younger granites in the western part of the

craton. Several different types of gneissic rocks are present in

these areas, including a variety of metamorphosed igneous

and sedimentary rocks. The oldest type of gneiss recognized

in most places includes tonalitic to granodioritic layers with

mafic amphibolite bands that are probably deformed dikes.

Younger orthogneisses have tonalitic, granodioritic, and

dioritic protoliths, and migmatization is common. Locally,

especially near the eastern side of the Anton terrane, the older

gneisses are overlain by a shallow-water sedimentary

sequence that includes quartz-pebble conglomerate, quartzite,

metapelite, and metacarbonates. These rocks are likely the

remnants of a thin passive margin sequence.

The Sleepy Dragon terrane extends from northeast of

Yellowknife to the south shore of the south arm of Point

Lake. This terrane includes intermediate to mafic quartzofeldspathic

gneiss complexes such as the 2.8–2.7-billionyear-

old Sleepy Dragon complex in the south, banded and

migmatitic gneisses near Beniah Lake, and 3.15-billion-yearold

chloritic granite on Point Lake. Isolated dioritic to gabbroic

bodies are found as inclusions and enclaves. The most

common protoliths to the gneisses are tonalites and granodiorites,

and rock types in the Sleepy Dragon terrane are broadly

similar to those in the Anton terrane. Sleepy Dragon

gneisses are locally overlain unconformably by shallow water

sedimentary sequences, notably along the southeastern margin

of the complex near Detour Lake. Here, a basal tonalite

pebble-bearing conglomerate grades up into metaquartzose

and calcareous sands, and then into a metacarbonate

sequence consisting of marbles and calc-silicate minerals.

From base to top this sequence is only 1,600 feet (500 m)

thick, but it has been shortened considerably. Several tens of

kilometers north at Beniah Lake in the Beaulieu River greenstone

belt up to 3,200 feet (1,000 m) of quartzite are recognized

between shear zones. There are thus several locations

where shallow water sediments appear to have been deposited

upon Sleepy Dragon gneisses. The similarities of the lithofacies

successions in these rocks to those found in

Phanerozoic rift and passive margin sequences are striking.

The Contwoyto terrane is composed of laterally continuous

graywacke mudstone turbidites exposed in a series of

westward-vergent folds and thrusts. Mapping in the Point

Lake area revealed westward-directed thrusts placing highgrade

metagraywackes over lower-grade equivalents. The

graywackes are composed of matrix, rock fragments (felsic

volcanics, mafic volcanics, chert, granite), and feldspars. Typically

only the upper parts of the Bouma sequence are preserved.

Black shales and iron formations are locally found,

especially near the structural base of the sequence. In many

places greenstone belts conformably underlie the sediments,

but the bases of the greenstone belts are either known to be

truncated by faults or are poorly defined, suggesting that they

are allochthonous. Ophiolite-like stratigraphy, including the

presence of sheeted dikes and cumulate ultramafics, has been

recognized in several greenstone belts. Other greenstone belts

of the Contwoyto terrane are composed predominantly of

basaltic pillow lavas and exhibit both tholeiitic and calc-alkaline

differentiation trends.

Rocks of the Contwoyto terrane thus include tectonic

slivers of ophiolite-like rocks, oceanic type sediments (shales,

iron formations), and abundant graywackes exhibiting both

volcanogenic and flysch-like characteristics. These rocks are

contained in westward-verging folds and are disrupted by

westward-directed thrusts. A series of granitoids intruded this

package of rocks at various stages of deformation. These relationships

are characteristic of an accretionary prism tectonic

setting. In such an environment graywackes are eroded from a

predominantly island arc source, as well as from any nearby

continents, and are deposited over ophiolitic basement capped

by abyssal muds and iron formations. Advance of the accretionary

prism scrapes material off the oceanic basement and

incorporates it in westward-vergent fold and thrust packages.

This material is accreted to the front of the arc and is intruded

by arc-derived magmas during deformation. Metamorphism is

of the low-pressure, high-temperature variety and is similar to

that of accretionary prisms that have experienced subduction

of young oceanic crust, or subduction of a ridge segment.

The Hackett River arc consists of a series of northweststriking

volcanic piles and synvolcanic granitoids, especially

in the south. Felsic volcanics predominate but a spectrum of

compositions including basalt, andesite, dacite, and rhyolite

is present. Volcanic piles in the Hackett River arc therefore

differ strongly from greenstone belts in the west, which consist

predominantly of mafic volcanic and plutonic rocks. In

the Back River area, cauldron subsidence features, rhyolitic

ring intrusions, tuffs, breccias, flows, and domes, with wellpreserved

subaerial and subaquatic depositional environments,

have been documented. Rhyolites from the Back River

complex have been dated at 2.69 billion years old, and the

volcanics are broadly contemporaneous with graywacke sedimentation

because the flows overlie and interfinger with the

sediments. Gneissic rocks in the area are not extensively

intruded by mafic dike swarms like the gneisses of the Anton

and Sleepy Dragon terranes, and they have yielded ages of

2.68 billion years, slightly younger than surrounding volcanics.

Since none of the gneisses in the Hackett River arc

have yielded ages significantly older than the volcanics,

deformed plutonic rocks in this terrane are accordingly distinguished

from gneisses in the western part of the Slave

Province. These gneisses are suggested to represent subvolcanic

plutons that fed the overlying volcanics. Another suite

of tonalitic, dioritic, and granodioritic plutonic rocks with

ages 60 to 100 million years younger than the volcanics also

intrude the Hackett River arc. This suite of granitoids is

equated with the late- to post-tectonic granitoids that cut all

rocks of the Slave craton.

A belt of graywacke turbidites in the easternmost part

of the Slave Province near Beechy Lake has dominantly eastward

vergent folds and possible thrusts, with some west vergent

structures. The change from regional west vergence to

eastward vergence is consistent with a change from a forearc

accretionary prism to a backarc setting in the Beechy

Lake domain.

The Hackett River terrane is interpreted as an island arc

that formed above an east-dipping subduction zone at 2.7

billion to 2.67 billion years ago. The mafic to felsic volcanic

suite, development of caldera complexes, and overall size of

this belt are all similar to recent immature island arc systems.

The Contwoyto terrane is structurally and lithologically

similar to forearc accretionary complexes; west vergent

folds and thrusts in this terrane are compatible with eastward

dipping subduction, as suggested by the position of the

accretionary complex to the west of the arc axis. The change

from west to east vergence across the arc axis into the

Beechy Lake domain reflects the forearc and backarc sides of

the system. Mafic volcanic belts within the Contwoyto terrane

are interpreted as ophiolitic slivers scraped off the subducting

oceanic lithosphere.

The Anton terrane in the western part of the Slave

Province contains remnants of an older Archean continent or

microcontinent including the world’s oldest known continental

crust. Quartzofeldspathic gneisses here are as old as 4.03

billion years, with more abundant 3.5–3.1-billion-year-old

crust. These gneissic rocks were deformed prior to the main

orogenic event at 2.6 billion years ago. The origin of the

gneisses in the Anton terrane remains unknown; many have

igneous protoliths, but the derivation of the rocks is not yet

clear. The Sleepy Dragon terrane might represent a microcontinent

accreted to the Anton terrane prior to collision with

the Hackett River arc, but more likely it represents an eastern

part of the Anton terrane uplifted and transported westward

during orogenesis. Studies at the southern end of the Sleepy

Dragon terrane have shown that the gneisses occupy the core

of a large fold or anticlinorium, consistent with the idea that

the Sleepy Dragon terrane represents a basement-cored

Alpine style nappe transported westward during the main

orogenic event. The distribution of pre-greenstone sediments

lying unconformably over the gneisses is intriguing. At Point

Lake, a few meters of conglomerates, shales, and quartzites

lie with possible unconformity over the Anton terrane gneiss,

whereas farther east, up to 1,600 feet (500 m) of sediments

unconformably overlie Sleepy Dragon terrane gneisses. These

include basal conglomerates and overlying sand, shale, and

carbonate sequences, and thick quartzites with unknown

relationships with surrounding rocks. These scattered bits of

preserved older sediments in the Slave Province may represent

remnants of an east-facing platform sequence developed on

the Anton–Sleepy Dragon microcontinent.

In a simple sense, the tectonic evolution of the Slave

craton can be understood in terms of a collision between an

older continent with platformal cover in the west with a

juvenile arc/accretionary prism in the east. The Anton terrane

experienced a sequence of poorly understood tectonomagmatic

events between 4.03 billion and 2.9 billion years

ago, then was intruded by a set of mafic dikes probably

related to lithospheric extension. After extension, the thermally

subsiding Anton terrane was overlain by an eastward

thickening shallow water platform sequence. To the east,

the Hackett River volcanic arc and Contwoyto terrane are

formed as a paired accretionary prism and island arc above

an east-dipping subduction zone. Numerous pieces of

oceanic crust are sliced off the subducting lithosphere, and

synvolcanic plutons intrude along the arc axis. Any significant

rollback of the slab or progradation of the accretionary

wedge will cause arc magmas to intrude the

accreted sediments and volcanics. Graywacke sediments are

also deposited on the backside of the system in the Beechy

Lake domain.

As the arc and continent collide at about 2.65 billion

years ago, large ophiolitic sheets are obducted, and a

younger set of graywacke turbidites are deposited as conformable

flysch. This is in contrast to other graywackes that

were incorporated into the accretionary prism at an earlier

stage and then thrust upon the Anton terrane. There are thus

at least two ages of graywacke sedimentation in the Slave

Province. Older graywackes were deposited contemporaneously

with felsic volcanism in the Hackett River arc, whereas

younger graywackes were deposited during obduction of the

accretionary prism onto the Anton continent. Synvolcanic

plutons along the arc axis became foliated as a result of the

arc-continent collision, and back thrusting shortened the

Beechy Lake domain. Continued convergence caused the

uplift and transportation of the Sleepy Dragon terrane as a

basement nappe, and strongly attenuated greenstone slivers.

Numerous late- to post-tectonic granitoids represent postcollisional

anatectic responses to crustal thickening, or pressure-

release melts formed during post-collisional orogenic

extension and collapse. These suites of granitoids have similar

intrusion ages across the province.

See also ARCHEAN; CRATONS; GREENSTONE BELTS.

sleet See PRECIPITATION.

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