RECENT ADVANCES IN GEOCHEMISTRY OF ~1.1 GA MIDCONTINENT RIFT RELATED INTRUSIVE ROCKS AROUND THUNDER BAY, ONTARIO, CANADA

Recent geochemical geochronological and paleomagnetic data from Midcontinent Rift (MCR)-related rocks have provided new insight into the discrimination of intrusive units north of the border and new implications for source characteristics and rift dynamics. Based on La/Sm­­_n vs. Gd/Yb_n and Mg# vs. TiO₂, 4 different mafic sills (Nipigon, Logan, Inspiration and McIntyre), 3 different mafic to ultramafic sills (Jackfish, Shillabeer and Riverdale) and 4 ultramafic intrusions (Disraeli, Kitto, Seagull and Hele) are recognized. The mafic and mafic to ultramafic units can be broken into 2 broad groups; a high-TiO₂ group consisting of the Logan sills to the south as well as the spatially restricted Jackfish, McIntyre and Shillabeer sills to the north, and a low-TiO₂ group consisting of the dominant Nipigon sills, the Inspiration sill to the north and the Riverdale sill to the south. All units display a fractionation trend of decreasing TiO₂ with decreasing Mg#, as well as distinct REE signatures. The Disraeli, Kitto, Seagull and Hele intrusions make up the ultramafic group, all emplaced along north-trending, crustal-scale faults within the Nipigon Embayment. The ultramafic intrusions display the highest Mg#, as well as a strongly depleted HREE signature, indicated by elevated Gd/Yb_n.

Three dyke sets along the northern flank of the MCR are currently recognized based on geochronology and orientation: the east-northeast to northeast-trending Pigeon River dykes, the east-northeast-trending Mount Mollie dyke and the northwest-trending Cloud River dykes. Geochemically, these units display broadly similar signatures to those of Nipigon sills, likely having tapped the same long-lived reservoir; this idea is supported by their similar Sr and Nd isotopic signatures. Age dates from Pigeon River dykes (1141±20 and 1078±3 Ma) and other units indicate that MCR magmatism was more prolonged than previously thought (~60 m.y.). This is considerably longer than other LIP events and suggests that the MCR may represent a plume-cluster event or melting of mafic underplating at the base of the Superior Province during a passive rifting event. Work is ongoing to decipher the relationships between intrusive units and investigate the geochemical evolution of this prolonged magmatic event in order to better constrain the tectonic model.