Christian F. J. Carl HBSc thesis abstract
The Sibley Peninsula is an approximately 300km2 land mass that projects southwest into Lake Superior, approximately 30km east of Thunder Bay, Ontario, Canada. The geology of the peninsula consists of Southern Province assemblages, including intrusive rocks of the ~ 1.1Ga Midcontinent Rift. Due to their location within Sleeping Giant Provincial Park, the Sleeping Giant Sill (SGS) and numerous dikes on the Sibley Peninsula have not been geochemically classified. With permission from Ontario Parks, this study was undertaken to 1) classify the SGS and dikes of the Sibley Peninsula based on their geochemistry and 2) determine relative ages of the SGS and dikes of various orientations.
Thirty-four dike samples and five SGS samples were collected and analyzed for whole rock geochemistry at the Geoscience Laboratories. Dikes with three different orientations were sampled: 1) ~70˚ striking dikes, which represent the majority of the dikes on the peninsula, 2) two 110˚ striking dikes and 3) two 29˚ striking dikes. The ~70˚ striking dikes were classified as Pigeon River Dikes based on Mg# versus TiO2and Gd/Ybn versus La/Smn discrimination diagrams. The 110˚ striking dikes plotted near or within fields defined by Hollings et al. (2007a) for ultramafic intrusions of the MCR. Despite geochemical similarities, the 110˚ strike directions of these dikes make them spatially unsuitable as feeders to any of the ultramafic assemblages found in the Nipigon Embayment, northeast of the study area. The 29˚ striking dikes plotted in the field defined for mafic / ultramafic sills and intrusions on the Mg# versus TiO2 plot and in the field defined for Nipigon Sills on the Gd/Ybn versus La/Smn diagram. Although these dikes were geochemically similar to Osler volcanic rocks of the neighbouring Black Bay Peninsula, they were just 18 and 20 cm wide and tapered eastward, making them unlikely candidates as feeders to Osler volcanic rocks. Using Mg# versus TiO2 and Gd/Ybn versus La/Smn discrimination diagrams, the SGS was determined to be of Logan affinity and now represents the easternmost recognized Logan Sill. The thickness of the SGS was found to be 194 metres, thicker than any other previously recognized Logan Sill.
Cross-cutting relationships indicate that the ~70˚ striking dikes are younger than both the SGS and the 29˚ striking dikes. The SGS and the 29˚ striking dikes had lower silica and higher magnesium weight percentages than the ~70˚ striking dikes, which may suggest that more primitive magmas were locally emplaced earlier. The ~70˚ striking dikes displayed the largest negative niobium anomalies which could indicate an increase in contamination of magma sources over time, since these dikes were the youngest intrusions observed in this study.
The SGS was exclusively observed on the Sibley Peninsula's southern tip where it has a lowermost contact with Rove Formation shale. The SGS is unexpectedly absent in drill core taken from the peninsula's northeastern shoreline from which the thickness of the Rove Formation was determined to be ~ 610 metres. This can be explained by the timing of the Silver Islet Fault and the glacial erosion that removed the SGS from the northern portion of the peninsula and shaped the Sleeping Giant landform on the peninsula's southern terminus.