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 Inspiration Sill from northern edge in northwestern Nipigon Embayment. comprise ?20–40 m thick, flat-lying sequence mafic pillowed massive flows, flow breccia, hyaloclastite breccia. They are characterized by SiO 2 52–54 wt%, TiO 1.2–1.3 K O 0.9–1.1 wt%. light rare earth element (LREE) enriched, with La/Sm n 3.0–4.4 fractionated heavy elements (HREE) (Gd/Yb = 1.4–1.7). diabase sill ? 50 thick direct contact underlying Volcanics. Baddeleyite zircon yield combined U–Pb upper intercept age 1105.6 ± 1.6 Ma. uniform 52–53 1.1–1.2 0.9–1.2 samples LREE enriched 3.2–3.3 HREE 1.6). at least 1120 Ma, perhaps as old 1130 represent an early, thin, restricted volcanic sequence, largely preserved below younger Sill. display marked geochemical similarity, suggesting that they may magmatism associated earliest stages rifting.