The Back River volcanic complex is an

Archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...

stratovolcano A stratovolcano, also known as a composite volcano, is a typically conical volcano built up by many alternating layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with ...

spanning the

Northwest Territories The Northwest Territories is a federal Provinces and territories of Canada, territory of Canada. At a land area of approximately and a 2021 census population of 41,070, it is the second-largest and the most populous of Provinces and territorie ...

–

Nunavut Nunavut is the largest and northernmost Provinces and territories of Canada#Territories, territory of Canada. It was separated officially from the Northwest Territories on April 1, 1999, via the ''Nunavut Act'' and the Nunavut Land Claims Agr ...

border in

Northern Canada Northern Canada (), colloquially the North or the Territories, is the vast northernmost region of Canada, variously defined by geography and politics. Politically, the term refers to the three Provinces_and_territories_of_Canada#Territories, terr ...

. It is located northwest of

Yellowknife Yellowknife is the capital, largest community, and the only city in the Northwest Territories, Canada. It is on the northern shore of Great Slave Lake, about south of the Arctic Circle, on the west side of Yellowknife Bay near the outlet of t ...

and to the northwest of the Back River from which it takes its name. The

volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

constitutes the Back Group of the Yellowknife Supergroup and is somewhat anomalous in the Slave craton because it has undergone only a low degree of deformation and is subhorizontal. The southern half of the complex is exposed at the crest of a small dome. This is the eroded portion of the stratovolcano that has been preserved in an upright position. The complex comprises four volcanic sedimentary sequences (Innerring, Thlewyco, Boucher-Regan, Kelsh) that correspond to the phases of growth and destruction of this stratovolcano.

Innerring

The Innerring sequence, which constitutes the oldest rocks of the complex, represents the upper part of an eroded early phase of the volcano (U-Pb zircon igneous age of 2.708

Gigaannum A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) are more e ...

(Ga)).

Thlewyco

The Thlewyco sequence represents the main construction phase of the volcano and forms an outward dipping, annular succession around the Innerring sequence, with an aggregate thickness of -. Its stratigraphy changes dramatically around the crater, varying from five cycles of

andesitic Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

and rhylotic

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

, followed by succession of volcanistic debris on the north side, to 30 subaerial andesitic flows and rare

pyroclastic Pyroclast, Pyroclastic or Pyroclastics may refer to: Geology * Pyroclast, or airborne volcanic tephra fragments * Pyroclastic rock, rock fragments produced and ejected by explosive volcanic eruptions * Pyroclastic cone, landform of ejecta fro ...

and epivolcaniclastic units on the eastern side, to interlayered dacitic and andesitic lava and

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...

overlain by a thick succession of voluminous, nonwelded, ash-flow tuff and volcaniclastic rocks on the south side. Volcanism in this sequence ended with the eruption of large

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

and

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

dome-flow complexes (U-Pb zircon dated at 2.692 Ga). The Innerring and Thlewycho sequences represent a complex history of explosive eruptions from numerous eruptive centers.

Boucher-Regan

The Boucher-Regan sequence, with its predominance of pillowed lava flows, suggests that this northern flank of the volcano was submerged during deposition.

Kelsh

The Kelsh sequence on the northwestern side comprises epiclastic volcarenite, rhyolite-dacite block

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

from lava domes, polymict breccia, and a conglomerate of andesite, dacite-rhyolite clasts, and andesitic tuff. The exposed succession consists of iron-formation, oolitic-stromatolitic carbonate, sulphidic volcaniclastic rocks and graphitic slate that marks the end of volcanism. The Kelsh sequence forms a broad apron that is interpreted as a shallow, submarine to subaerial, clastic fan derived by degradation of the volcanic pile (U-Pb zircon dated at 2.586 Ga).

Age

Age determinations in the Black River complex are similar to the ages of

volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...

found in the upper

Kam Group The Kam Group is a thick Archean volcanic group in the Yellowknife greenstone belt of the Northwest Territories, Canada. It consists of tholeiitic mafic and subordinate felsic volcanic rocks that were erupted in a submarine environment about 270 ...

of the

Yellowknife greenstone belt The Yellowknife greenstone belt, also called the Yellowknife Volcanic Belt, is an Archean greenstone belt in the southern Slave craton, Northwest Territories, Canada. It is mostly made of mafic volcanic rocks (basalt and andesite) and is bordered ...

as well as other volcanic centers in the western Slave province.

Detrital zircon geochronology Detrital zircon geochronology is the science of geochronology, analyzing the age of zircons deposited within a specific sedimentary rock, sedimentary unit by examining their inherent radioisotopes, most commonly the uranium–lead dating, uranium ...

on turbiditic sequences confirm a secondary deposition of

turbidite A turbidite is the geologic Deposition (geology), deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing ...

units at ca. 2.62-2.60 Ga that was widespread throughout the Slave province.

References

* {{coord, 65, N, 108, W, region:CA-NU_type:mountain_dim:500km_source:GeologicalSurveyofCanada, display=title Volcanoes of the Northwest Territories Volcanoes of Nunavut Stratovolcanoes of Canada Archean volcanoes

Innerring

Thlewyco

Boucher-Regan

Kelsh

Age

See also

References