GEOCHEMISTRY OF THE LOGAN IGNEOUS SUITE AND IMPLICATIONS FOR THE MAGMATIC EVOLUTION OF THE NORTHERN PART OF THE MIDCONTINENT RIFT

Recent field work and interpretation of geochemistry has focussed on defining the emplacement sequence of Midcontinent Rift (MCR)-related units and evaluating their geochemical sources and contamination characteristics so as to better understand the magmatic evolution of the Logan Igneous Suite. Based on a mixing model between mantle-derived melts and crustal sources evaluated on plots of Nb/Yb_pm versus Th/Yb_pm and Th/Yb_pm versus Nb/Th_pm, four distinct trends are observed: the Nipigon sill trend; the Jackfish, McIntyre, Inspiration and Logan trend (JMIL); the Dyke trend (including the Pigeon River dykes, Cloud River dykes, Mount Mollie dyke and the Crystal Lake gabbro); and the Devon volcanics and the Riverdale sill trend (DR).

Field work in the Logan Basin has delineated the following timing sequence between units, from oldest to youngest: Riverdale sill; Devon volcanics; Logan sills; Pigeon River dykes; Cloud River dykes; Mount Mollie dyke and Crystal Lake gabbro. Based on these relative ages, the mantle source characteristics can be seen to become more depleted in incompatible elements as rift development progresses, each trend having been derived from a geochemically distinct mantle source. Furthermore, although all three dyke sets display similar source characteristics, younger dykes display progressively stronger crustal contamination signatures consistent with the magma having spent more time in the magma chamber.

A similar incompatible element depletion trend in mantle sources, akin to that in the Logan Basin units, is also apparent in the Nipigon Embayment. Geochemical source characteristics for the Nipigon sills and the older ultramafic units show that the ultramafic units were derived from a more enriched source when compared to a more depleted source for the Nipigon sills. Logan sills of the Logan Basin and ultramafic units of the Nipigon Embayment all show similar source characteristics, suggesting an overlapping magmatic history. From the interpretation of the geochemistry, in conjunction with known emplacement sequences, it is proposed that magma which produced units of the Logan Igneous Suite were derived from geochemically distinct mantle sources showing a progressive depletion in incompatible elements as MCR development progressed.