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TitleGeology of the mesoproterozoic pillar lake volcanics and inspiration sill, Armstrong, Ontario: evidence of early midcontinent rift magmatism in the northwestern Nipigon embayment
AuthorHollings, P; Smyk, M; Bleeker, WORCID logo; Hamilton, M; Cundari, R; Easton, W
SourceCanadian Journal of Earth Sciences vol. 58, issue 10, 2021 p. 1116-1131, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20210539
PublisherCanadian Science Publishing
Mediapaper; on-line; digital
File formatpdf; html
AreaArmstrong; Pillar Lake; Canada
Lat/Long WENS -89.2594 -87.5994 50.4603 49.1342
Subjectsgeochemistry; Science and Technology; geochronology
Illustrationslocation maps; photographs; tables; charts; diagrams
ProgramTargeted Geoscience Initiative (TGI-6) Magmatic Ore Systems
Released2021 05 14
AbstractThe Midcontinent Rift System of North America is a ~1.1 Ga large igneous province comprising mainly flood basalts and intrusive rocks. We present new data for the Pillar Lake Volcanics and Inspiration Sill from the northern edge of the Midcontinent Rift in the northwestern Nipigon Embayment. The Pillar Lake Volcanics comprise a ~20-40 m thick, flat-lying sequence of mafic pillowed and massive flows, pillowed flow breccia, and hyaloclastite breccia. They are characterized by SiO2 of 52-54 wt%, TiO2 of 1.2-1.3 wt%, and K2 O of 0.9-1.1 wt%. They are light rare earth element (LREE) enriched, with La/Smn of 3.0-4.4 with fractionated heavy rare earth elements (HREE) (Gd/Ybn = 1.4-1.7). The Inspiration diabase sill is = 50 m thick and is in direct contact with the underlying Pillar Lake Volcanics. Baddeleyite and zircon data from the Inspiration Sill yield a combined U-Pb upper intercept age of 1105.6 6 1.6 Ma. The Inspiration Sill is characterized by uniform SiO2 of 52-53 wt%, TiO2 of 1.1-1.2 wt%, and K2 O of 0.9-1.2 wt%. Inspiration Sill samples are LREE enriched with La/Smn of 3.2-3.3 and fractionated HREE (Gd/Ybn = 1.6). The Pillar Lake Volcanics are at least 1120 Ma, and perhaps as old as 1130 Ma, and represent an early, thin, and restricted mafic volcanic sequence, largely preserved below the younger Inspiration Sill. The Pillar Lake Volcanics and Inspiration Sill display a marked geochemical similarity, suggesting that they may represent magmatism associated with the earliest stages of Midcontinent rifting.
Summary(Plain Language Summary, not published)
This publication describes an occurrence of ancient, more or less flat-lying lava flows, the so-called Pillar Lake Volcanics, to the west of Lake Nipigon in northwestern Ontario, near the settlement of Armstrong. These ancient lavas are thought to be ca. 1.1 billion years old and they are intruded by a diabase sill, the Inspiration Sill. Here we date this sill in more detail at 1106 million years old, and the volcanic rocks must therefore be older. The geochemical composition of the volcanic lava flows and the sill are discussed in detail and compared to other rock units in the area.

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