Jordan Seaby's Honours Thesis Abstract
The Nipigon Embayment encompasses ~10,000 km3 of magmatism in an area of 200,000 km2 that stretches from Lake Nipigon to the southwestern side of Thunder Bay. The majority of the intrusive units within the embayment have been classified as Nipigon Sills. Diabase sills at Hele, Tease Lake and two at Thread Lake exhibit textures typical of Nipigon sills. All four sills contained three distinct textures throughout; a chill zone at contacts, a medium-grained to coarse-grained subophitic zone, and a coarse-grained ophitic texture, with Hele and Tease Lake also displaying of a fourth type of texture in the form of a megacrystic ophitic zone. The crude layering that is seen throughout the sills is evidence of a multiple pulse system. All sills maintained consistent mineral modal abundances down hole; Hele averaged 55% plagioclase and 45% pyroxene; Thread Lake 60% plagioclase and 40% pyroxene; and Tease Lake 55% plagioclase and 45% pyroxene.
Plots of Th/Ybpm versus Nb/Thpm subdivide the sills into distinct subclasses. Hele, Thread Lake 2 and Tease Lake all plot within the Nipigon 1 suite whereas Thread Lake 1 represents a Nipigon 2 Suite. This classification is based on the emplacement and contamination history of the sills. Hele, Thread Lake 2, and Tease Lake represent a more primitive magma, whereas Thread Lake 1 is derived from a less primitive magma. Thus giving evidence of differing contamination histories.