The Nipissing sills, also called the Nipissing diabase, is a large 2217– to 2210–million year old group of sills

Sill (geology)

In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...

 in the Superior craton

Superior craton

The Superior craton forms the core of the Canadian Shield at the heart of the North American continent. It extends from Quebec in the east to eastern Manitoba in the west...

 of the Canadian Shield

Canadian Shield

The Canadian Shield, also called the Laurentian Plateau, or Bouclier Canadien , is a vast geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton. It is an area mostly composed of igneous rock which relates to its long volcanic history...

 in Ontario

Ontario

Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....

, Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, which intrude the Huronian Supergroup

Huronian Supergroup

The Huronian Supergroup is a Proterozoic geologic feature of the Superior craton of the Canadian Shield in Ontario and Quebec. It extends from the city of Sault Ste. Marie in the west to the Ontario-Quebec border to the east....

. Nipissing sills intrude all the Huronian sediments and older basement rocks in the northern margin of the Sudbury Basin

Sudbury Basin

The Sudbury Basin, also known as Sudbury Structure or the Sudbury Nickel Irruptive, is a major geologic structure in Ontario, Canada. It is the second-largest known impact crater or astrobleme on Earth, as well as one of the oldest....

; they were emplaced after the faulting and folding

Fold (geology)

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

 of Huronian rocks, and are hornblende

Hornblende

Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....

 gabbro

Gabbro

Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....

 of tholeiitic basalt composition. In the Sudbury–Elliot Lake area the Nipissing diabase is deformed; outcrops are parallel to the fold axes of the Huronian sedimentary rocks. Nipissing diabase intrusions are east-northeast trending and are no wider than 460 m (1,509.2 ft).

The Nipissing sills in the Southern Province of the Superior craton are thought to originate from a radiating dike swarm

Dike swarm

A dike swarm or dyke swarm is a large geological structure consisting of a major group of parallel, linear, or radially oriented dikes intruded within continental crust. They consist of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event and are...

 area 1300 km (807.8 mi) to the northeast. The mantle source for the Nipissing sills did not come from the mantle beneath the Southern Province that had generated the 2500– to 2450–million year old Matachewan dike swarm

Matachewan dike swarm

The Matachewan dike swarm is a large 2,500 to 2,450 million year old Paleoproterozoic dike swarm of Northern Ontario, Canada. It consists of basaltic dikes that were intruded in greenschist, granite-greenstone, and metamorphosed sedimentary terrains of the Superior craton of the Canadian Shield...

. The 2217– to 2210–million year old Ungava magmatic event

Ungava magmatic event

The Ungava magmatic event was a widespread magmatic event that began about 2.22 billion years ago during the Proterozoic Eon.- Extent :With an area of , the Ungava magmatic event consists of a large igneous province. Magmatic features that were formed during the Ungava magmatic event include...

 – located under the Labrador Trough

Labrador Trough

The Labrador Trough or the New Quebec Orogen is a long and wide geologic belt in Canada, extending south-southeast from Ungava Bay through Quebec and Labrador....

 – fed the Nipissing sills; evidence shows the sills were laterally fed from a mantle plume

Mantle plume

A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...

center 1500 km (932.1 mi) away via the 2216–million year old Senneterre dikes which form part of the radiating dike swarm.