Superior Craton

The Superior Province is the largest Archean craton preserved on the planet, with an area of 625,000 square miles (1.6 million km2). Most rocks within the Superior Province resemble those found at younger subduction zones and in island and continental margin arc sequences, suggesting that the processes involved in forming the Superior Province were the result of plate tectonics. Contrary to popular geological myth that Archean cratons do not show linear tectonic zonations, the Superior Province exhibits strong linear tectonic zones with individual belts extending for thousands of miles (km) in a southwest-northeast arrangement. Some belts are dominated by metasedimentary rocks, others by mafic volcanic and plutonic rocks, high-grade gneisses, or granitoids. Most of these belts are separated from other belts by major fault zones, many of which have long tectonic histories. In general these different belts are older and were joined with each other at older ages in the north, extending back to 3.0 billion years ago, and get progressively younger to the south until the ages are about 2.7 billion years along the Canadian/United States border. The final belt to the south, however, is the Minnesota River Valley Province, with ages of old gneisses that extend back to 3.66 billion years. This old terrane probably represents an older continental fragment that the rest of the Superior Province was accreted to around 2.7 billion years ago.

The Minnesota River Valley Province occupies the southwest corner of the Superior Province and includes an assemblage of granitic gneisses and amphibolites with ages that go back to 3.66 billion years ago. These rocks experienced their first deformation event at 3.6 billion years ago, with additional deformation and metamorphic events at 3.05 and 2.7 billion years ago. This older continental block is separated from the Wawa terrane to the north by a major structure called the Great Lakes tectonic zone, interpreted to be a suture along which the terranes to the north were thrust southward over the Minnesota foreland.

The Wawa terrane consists of a thick assemblage of volcanic and sedimentary rocks, and plutons that largely represent the deeper level equivalent to the volcanic rocks. Most of the volcanic and plutonic rocks are 2.75 to 2.71 billion-year-old tholeiitic basalts intercalated with tholeiitic to calc-alkaline mixed mafic and felsic volcanics. These are intruded by a suite of tonalitic plutons, then a series of post-deformational plutons with ages of 2.68 billion years. The Wawa terrane is therefore interpreted as an island arc that formed at 2.75-2.71 billion years ago and was thrust over the Minnesota foreland by 2.68 billion years ago. In the eastern part of the province in Quebec, rocks of the Wawa terrane are tilted upward on a major crustal-scale shear zone known as the Kapuskasing structure that formed in the Early Proterozoic around 1.9 billion years ago, and uplifts from as deep as 12 miles (20 km) to the surface. The origin of the uplift is thought to be from a distant continent-continent collision such as along the Trans-Hudson orogen, forming an intracontinental uplift similar to the Tien Shan north of the contemporaneous India-Asia collision.

To the east of the Kapuskasing uplift, the Abitibi belt forms a wide area of island arc type rocks similar in aspect and age to the Wawa belt, with 2.73 to 2.70 billion year old basaltic to komatiitic lava complexes intruded by diorite-tonalite-granodiorite plutons and intruded by 2.7 to 2.67 billion-year-old suites of more silicic volcanic rocks. The Abitibi province is the site of many gold mines, with many of them hosted in quartz veins in shear zones. Other massive sulfide deposits are directly associated with the volcanic rock sequences. A group of strongly deformed turbidites called the Pontiac belt on the south side of the Abitibi belt is interpreted to be an accretionary prism associated with the Abitibi arc.

The Quetico Province is a large metasedimentary belt located north of the Wawa arc, consisting of strongly folded and sheared graywacke turbidites, conglomerates, and other rocks derived from a mixed mafic and felsic volcanic source. The belt is metamorphosed more strongly in the center of the belt than along its margins. The most reasonable interpretation of the Quetico is that it is a large accretionary prism terrane composed of trench-fill turbidites that formed the forearc to the Wabigoon arc to the north. Late stage deformation of the Quetico belt reflects dextral oblique subduction beneath the Wawa belt to the south.

The Wabigoon terrane probably formed as a south-facing island arc terrane. This arc is similar in terms of rock types to the other arcs in the south, but the age of deformation (2.71 to 2.70 billion years) is about 10 million years older than farther south in the Wawa and Abitibi belts and about 20 million years younger than in the arcs to the north. These relationships support a general north to south accretion of arc and accretionary prism terranes in the Superior Province during the Archean. The Wabigoon terrane is unusual though, in that it has some older gneissic fragments with ages of about 3.0 billion years old, and these are overlain by locally thick (~1,600 feet; 500 m) limestone and dolostone sequences with shallow-water stromatolites, interpreted to be remnants of a passive margin sequence later overthrust by the arc sequence.

The English River accretionary prism lies north of the Wabigoon arc and is similar in aspect to the Pontiac and Quetico belts. The English River belt is located between the Wabigoon arc and the uchi-Sachigo arc to the north. Its northern boundary is a major dextral/south over north thrust, and deformation can be shown to have ended by 2.66 billion years ago based on the ages of cross-cutting unde-formed granitic rocks. The uchi-Sachigo island arc shows three distinct periods of magmatism at 2.93, 2.83, and 2.73 billion years ago, and deformation at 2.73 to 2.72 billion years ago, about 20-30 million

English Wabigoon River Map

Map of the western Superior Province showing division into plutonic, volcano-plutonic, metamorphic, and metasedimentary subprovinces. The volcanic and volcano-plutonic subprovinces are interpreted to represent ancient island arc and continental arc terranes, whereas the metasedimentary subprovinces are interpreted to represent accretionary prisms that formed on the margins of these arcs. The metamorphic terranes include fragments of older continents. Collision of the arcs, accretionary prisms, and continental fragments formed the Superior Province. Large strike-slip faults currently separate many of the individual subprovinces. (Map modified after Ken Card and John Percival.)

years before deformation in the southern superior Province.

The northwest part of the superior Province consists of the Pikwitonei uplift and Thompson belt, consisting largely of high-grade granulites that are interpreted to represent a very deeply eroded section of the sachigo arc terrane. The metamorphism and late deformation is related to the Proterozoic events in the 1.9 to 1.8 billion-year-old Trans-Hudson belt that borders the superior Province just to the north and west of this location.

The northeastern part of the superior Province is called the Minto block, and it was largely regarded as one of the world's largest granulite facies terranes until mapping in the late 1990s and 2000s by

Geological survey of Canada geologists led by John Percival divided the block into a number of different terranes of different character. These include the Inukjuak terrane, Tikkerutuk terrane, and Lake Minto domains in the west that consist largely of granitic and granulitic gneisses, the Goudalie and Qal-luviartuuq belts in the center of the block, consisting of oceanic assemblage ophiolitic and arc rocks tec-tonically mixed with metasediments, and in the east another group of granitoid and granulitic rocks in the utsalik, Lepelle, and Douglas harbour terranes. In a general way these are interpreted as two arc/continental fragments that collided across a closed ocean basin preserved in the Goudalie and Qalluviartuuq belts.

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