The Steepledge Intrusion is an irregularly shaped mafic to ultramafic layered intrusion located north of Thunder Bay, Ontario. It is recognized as part of the early intrusive phase of the 1.1 Ga Midcontinent Rift. Light rare earth element enrichment and fractionation of the heavy rare earth elements is consistent with other early Midcontinent Rift-related intrusions and is interpreted to indicate a primitive mantle source.
The stratigraphy of the Steepledge Intrusion can be divided into seven zones on the basis of internal variations in petrology and geochemistry. The Hybrid Zone is a quartz leucogabbro containing quartz rich inclusions assimilated from the Quetico Basin country rocks. The Upper Layered Zone is a weakly layered sequence comprised of cm-scale layers of alternating troctolite and olivine gabbro. A gradational contact exists between the Upper Layered Zone and the underlying Upper Gabbroic Zone. The Upper Gabbroic Zone comprises a sequence of unlayered olivine gabbro containing oikocrystic clinopyroxene which poikilitically encloses fine-grained cumulate olivine. The Middle and Lower Layered Zones comprise a 70m thick unit of olivine gabbro to olivine melagabbro with increasing olivine content at depth. Centimetre-scale layering within the Middle and Lower Layered Zones are composed of alternating bands of olivine-cumulate peridotite and pyroxene-cumulate-pyroxenites. A transition from fine-grained olivine melagabbro to coarse-grained lherzolite marks the upper contact of the Coarse Peridotite Zone. The Coarse Peridotite Zone is characterized by coarse-grained cumulate olivine, intercumulate plagioclase and megacrystic orthopyroxene that enclose the cumulate olivine. The Basal Peridotite Zone is an approximately 130m thick zone of monotonous, very-fine grained to fine-grained peridotite. Geochemical analysis has identified the presence of cryptic layering throughout the entirety of the intrusion and is interpreted to be the resulted of multiple pulses of more primitive magma causing cyclic variations in the magma composition within the intrusion.
Sulphur isotope values for the Steepledge Intrusion range from -3.0 to -0.8 and are consistent with a mantle-derived magma with no external input of sulphur. Localized contamination by the host Quetico metasiltstones is concentrated in the Hybrid Zone, however, the overall negative Nb anomaly observed may represent an earlier, more pervasive contamination of the parent magma at depth.