Geology of the Mesoproterozoic Pillar Lake Volcanics and Inspiration Sill, Armstrong, Ontario: evidence of early Midcontinent Rift magmatism in the northwestern Nipigon Embayment

The 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 K₂O of 0.9–1.1 wt%. They are light rare earth element (LREE) enriched, with La/Sm_n of 3.0–4.4 with fractionated heavy rare earth elements (HREE) (Gd/Yb_n = 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 ± 1.6 Ma. The Inspiration Sill is characterized by uniform SiO₂ of 52–53 wt%, TiO₂ of 1.1–1.2 wt%, and K₂O of 0.9–1.2 wt%. Inspiration Sill samples are LREE enriched with La/Sm_n of 3.2–3.3 and fractionated HREE (Gd/Yb_n = 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.