Geology and petrogenesis of the Tuscarora Intrusion of the Duluth Complex, Gillis Lake 7.5’ quadrangle, northeastern Minnesota

Abstract

The Tuscarora Intrusion is a layered mafic intrusion located at the base of the 1.1 Ga Duluth Complex in northeastern Minnesota. Detailed field mapping (1:12,000) and follow-up petrographic and geochemical analyses were conducted to evaluate the troctolitic igneous stratigraphy of the Tuscarora Intrusion, as well as to understand the petrologic relationship between the troctolitic cumulates and the plagioclase-rich gabbros of the overlying Anorthositic Series. 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 Tuscarora Intrusion. 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 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 Anorthositic Series. No field evidence was found for an interlayered relationship between the two lithologies and geochemical studies imply distinct parent magma compositions. This study concludes that the Tuscarora Intrusion and Anorthositic Series are two separate intrusive suites, as found in other areas of the Duluth Complex. Furthermore, the lower and upper zones of the Tuscarora are interpreted to have formed successively from major emplacement episodes of moderately evolved tholeiitic mafic magma. The emplacement model proposed has plagioclase porphyritic magmas intruding at some level above the base of the North Shore Volcanic Group to create the Anorthositic Series. 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 Anorthositic Series cap, which contributed anorthositic inclusions.