BEDROCK GEOLOGIC MAP OF THE TUSCARORA INTRUSION OF THE DULUTH COMPLEX, NORTHEASTEN MINNESOTA: AN INVESTIGATION INTO THE PETROGENETIC RELATIONSHIP BETWEEN THE LAYERED SERIES AND THE ANORTHOSITIC SERIES

The Tuscarora Intrusion (TI) is a layered mafic intrusion located at the base of the 1.1 Ga Duluth Complex in northeastern Minnesota. The Duluth Complex is a multiphase system emplaced into the base of comagmatic volcanics related to the development of the Midcontinent Rift. Detailed field mapping (1:12,000) and follow-up petrographic and geochemical analyses were conducted to evaluate the contact relationships between the troctolitic rocks of the TI and plagioclase-rich gabbros of the overlying Anorthositic Series (AS). Previous studies have interpreted the two lithologies as being interlayered, a relationship that would be unique within the Duluth Complex.

Field mapping has identified two lithologically distinct stratigraphic zones within the TI. The lower zone is composed of olivine gabbro to augite troctolite that is typically heterogeneous in mode and texture and locally displays modal layering and foliation. Most notably, it contains abundant, large (>100m) basaltic hornfels inclusions. The upper zone is predominately composed of homogenous, foliated troctolite to leucotroctolite, which grades upward from melatroctolite at the base of the zone. Inclusions in the upper zone are mostly large, often elongate blocks of anorthositic rocks and are especially abundant near the contact with the AS. No field evidence was found for an interlayered relationship between the two lithologies and geochemical studies imply distinct parent magma compositions.

We conclude that the TI and AS are two separate intrusive packages, as found in other areas of the Duluth Complex. Furthermore, the upper and lower zones of the TI are interpreted to have formed from successive injections of moderately evolved tholeiitic mafic magma that overplated each other. The emplacement model proposed has plagioclase porphyritic magmas intruding at some level above the base of the volcanic pile to create the anorthositic rock of the AS. This was followed by emplacement of lower zone magmas closer to base of the volcanic pile. This resulted in the incorporation of abundant volcanic inclusions and in strong contamination of the magma by interaction with pre-rift rocks of the footwall. Emplacement of upper zone magmas occurred above the newly crystallized Lower Zone and below the AS cap, the latter which contributed anorthositic inclusions.