This study is focused on the geology and geochemistry of Midcontinent Rift-related intrusive and extrusive rocks present in northern Ontario, Canada. The study focuses on three sections in order to elucidate the geochemical characteristics of Midcontinent Rift-related rocks and investigate geochemical evolutionary signatures through time. The Coubran Lake basalts, the rocks of the Logan Basin and the intrusive rocks of the Nipigon Embayment were all included in this study to provide a sound spatial representation of Midcontinent Rift (MCR) intrusive and extrusive rocks present within Canada.
The Coubran Lake basalts within the Coldwell Complex, have provided new representation of Midcontinent Rift-related volcanism along the northeast shore of Lake Superior. Physical features, including proximity to surrounding syenites, as well as alteration features, suggest that the Coubran Lake basalts represent a pre-existing sequence into which the Coldwell Complex intruded. Geochemical evidence has linked the Coubran Lake basalt to the Two Duck Lake gabbro (TDLG) suggesting the basalts may represent the volcanic expression of the TDLG. Trace element and Nd isotope data suggest the Coubran Lake basalts are akin to the basalt type I composition, linking the unit to the Lower Siemens Creek volcanics, the basal units in the Ely's Peak and Grand Portage areas of the North Shore Volcanics, and to the lower suite of the Osler Group. The geochemical data presented here is consistent with the available paleomagnetic data suggesting the basalts were erupted early in the history of the Midcontinent Rift.
A re-evaluation of 2397 spatially referenced samples with associated whole-rock geochemistry has yielded previously unrecognized variation within the Midcontinent Rift-related Nipigon sills of the Nipigon Embayment. The whole rock major and trace element geochemistry of 796 Nipigon sill samples was re-examined to investigate the origin of the sills. This investigation revealed three distinct Nipigon sill types based on Th/Ybpm ratios and ?Nd1100 values: 395 Nipigon type I sill samples (Th/Ybpm = 1.97 to 3.4; εNd(t=1100Ma) = -0.5 to -1.5), 171 Nipigon type II sill samples (Th/Ybpm = 3.4 to 5.0; εNd(t=1100Ma) = -1.5 to -3.0), and 55 Nipigon type III sill samples (Th/Ybpm = 5.0 to 6.5; εNd(t=1100Ma) = -5.0 to -7.0). Spatially, the distribution of the more contaminated Nipigon sill types II and III suggest centres which are inferred to represent zones of more contamination.
Field work in the Logan Basin has delineated the following timing requence between the various units present in the Logan Basin: the Riverdale sill, the Devon Volcanics, the Logan sills, the Pigeon River dykes, the Cloud River dykes, the Mount Mollie dyke and the Crystal Lake gabbro. Geochemical source characteristics support this timing relationship as the Riverdale sill, the Devon volcanic and the Logan sills are geochemically similar to the ultramafic intrusions of the Nipigon Embayment which were emplaced early in MCR history. The three dyke sets of the Logan Basin have been determined to be emplaced in the following order: Pigeon River dykes, the Cloud River dykes and the Mount Mollie dyke. All three dyke sets display similar source characteristics yet show stronger crustal contamination signatures (i.e., higher Th/Ybpm and more negative εNd(t=1100Ma) ) through time consistent with each dyke set having spent more time in the magma chamber through the duration of the emplacement sequence.
Based on the plots of Th/Ybpm versus Nb/Thpm and Th/Ybpm versus Nb/Ybpm, previously unrecognized relationships between units have allowed for inferences into the timing and geochemical evolution of the Midcontinent Rift. The progression of magmatism for both the Logan Basin and the Nipigon Embayment suggests that source characteristics became more depleted through time. Evidence of a link between the magmatism of the Logan Basin and the Nipigon Embayment is also indicated by the relationship between the Riverdale sill, the Devon Volcanics and the Logan sills with the mafic and ultramafic units of the Nipigon Embayment (excluding the Nipigon sills). Furthermore, the spatial link between the two magmatically endowed areas is bridged as the dykes of Sibley Peninsula showing evidence to be potential feeders to Nipigon sills.
For more details about this thesis contact Dr. Peter Hollings
A copy of the thesis can be downloaded here