Circum-Superior Belt - Geology - Geologic Features

Geologic Features

The 1,600 km (990 mi) long Labrador Trough, extending from Ungava Bay through Quebec and Labrador, includes two volcano-sedimentary series, the first ranging from 2,170 to 2,140 million years old and second ranging from 1,883 to 1,870 million years old. This magmatism is considered to have formed as a result of back-arc volcanism. The youngest magmatic series (1883-1870 Ma) contains 1,880 million-year-old carbonatites and lamprophyres. 1,883 to 1,874 million-year-old mafic and a few ultramafic magmas comprise the Willbob and Hellancourt formations and Montagnais sills. The youngest magmatism of the youngest magmatic series is 1,870 million-year-old felsic and carbonatitic volcanics. These igneous rocks of the Labrador Trough cover an area of 30,000 km2 (12,000 sq mi).

To the northwest at the northern tip of Quebec near Hudson Strait, the Cape Smith Belt includes the 2,040 to 1,970 million-year-old Povungnituk volcano-sedimentary group and the 1,880 million-year-old Chukotat Group. The 5.6 km (3.5 mi) thick Chukotat Group is made of picritic and tholeiitic basalts. These basaltic lavas are intruded by narrow mafic and ultramafic sills. At least three different types of lava compositions exist in the Chukotat Group, including olivine phyric, pyroxene-phyric and plagioclase phyric. The upper unit of the Chukotat Group is 1,870 million years old whereas the lower unit is associated with the Katiniq Suite sills, which cut through the underlying Povungnituk Group. In 1989, the Katinq Suite sills were thought to be 1,918 million years old, but more recent dating and a reinterpretation of the original age in 2004 suggests that the Katinq Suite sills are closer to 1,880 million years old. Therefore, the age range for the Chutotat Group is 1,880 to 1,870 million years. Volcanism of the Chukotat Group might have originated from rifting of a microcontinent that now forms the southwestern portion of Baffin Island.

On the Belcher Islands of southern Nunavut, two volcanic groups are known as the Flaherty and Eskimo volcanics. The underlying Eskimo volcanics are related with the Richmond Gulf, Persillon, Pachi and Nastapoka Group volcanics. The 1,998 million-year-old Minto dikes are also interpretend to be related with the Eskimo volcanics. The overlying Flaherty volcanics remain undated apart from a lead-lead dating of 1,960 +/- 80 million years. However, the Flaherty volcanics are suggested to have relationships with the Stton Inlier and Haig sills. Geochemical indications suggest that the overlying Flaherty volcanics might also have relationships with the Povungnituk volcano-sedimentary group in the Cape Smith Belt, indicating the Flaherty volcanics might be 2,040 to 1,960 million years old. Sills on the Belcher and Snowy islands in Hudson Bay are dated to 1,870 million years old, indicating the 1,880 million-year-old magmatism also exists in this section of the Circum–Superior large igneous province.

The Fox River Belt in northern Manitoba is composed of sediments, volcanics and sills. Fox River Belt sills and Molsen dikes are 1,883 million years old, but the Molsen dikes at the northwestern Superior craton margin intrude an older, 2,090 to 2,070 millon-year-old dike swarm. The Ospwagan Group is younger than 1974 +/- 50 million years old and the 1,864 million-year-old Winnipegosis komatiite belt lies to the southwest. Numerous tectonic settings have been suggested for triggering magmatism in the Fox River Belt, including a marginal basin rifting event.

At the southern portion of the Circum-Superior Belt, a group of fragmental sediments composed of iron formation was formed during a period of magmatic activity in the Marquette Range Supergroup. Included in the Marquette Range Supergroup is the Hemlock Formation, a bimodal volcanic group that is estimated to have an age of 1874 +/- 9 million years. Another volcanic series in the Marquette Range Supergroup, known as the Gunflint Formation, has an age of 1878.3 +/- 1.3 million years. Further geologic units that are related to the Gunflint Formation include the Badwater volcanics and the Kienan sills. A number of origins have suggested for the Marquette Range Supergroup magmatism. This includes subsidence being driven ahead related to thrusting of the Penokean orogeny or a possible back-arc basin, and lateral flow through the Pickle Crow dikes transported magma from the Fox River Belt area through the Superior craton for placement in the Marquette Range Supergroup. This suggested long-range origin has been suggested for magmatism of the 2,215 million-year-old Nipissing sills as well, which the Ungava dike swarm could have transported magma to the Nipissing sills area.

Before 2003, all magmatism of the Circum-Superior Belt was thought to only occur along the outer margin of the Superior craton. However, the Pickle Crow dike was discovered in 2003 in the interior of the Superior craton and it has since been recognized as a magmatic event related to the Circum-Superior Belt. The dike is dated to 1,880 million years old and associated dikes are traced for at least 400 km (250 mi). These dikes likely extend over 700 km (430 mi) across the western portion of the Superior craton from near the Fox River Belt in the north to near Lake Superior in the south. This magmatic crossing creates a relationship with the 1,880 million-year-old magmatism in the northwestern portion of the Circum-Superior Belt and magmatism in the Marguette Range Supergroup on the southern portion of the Circum-Superior Belt. In addition, the north-northwest trend of the Pickle Crow dikes and the absolute north-northeastern trend of the Molson dike swarm combine to an area north of the northwestern portion of the Circum-Superior Belt. This might determine the zone of a mantle plume that was the source for the 1,880 million-year-old magmatism in the Circum-Superior Belt.