THE GEOLOGY AND PETROLOGY OF THE LEVEAUX PORPHYRITIC DIORITE, COOK COUNTY, MN: INVESTIGATING POSSIBLE MAGMATIC RELATIONSHIPS TO THE ANORTHOSITIC SERIES OF THE DULUTH COMPLEX

The Leveaux porphyritic diorite (LPD) is a 72-m thick, discontinuous, hypabyssal, sheet-like intrusion that is part of the Mesoproterozoic (1.1 Ga) Beaver Bay Complex of NE Minnesota. The LPD makes up five prominent, cuesta-like ridges that dip gently southeast and trend parallel to Lake Superior over a 20 km distance. The LPD is composed of an upper porphyritic zone, which contains 1-4 cm plagioclase megacrysts in a fine-grained ferrodiorite matrix, and a lower aphyric zone composed of fine-grained ferrodiorite with rare megacrysts. The aphyric and porphyritic zones are separated by a 20-30 cm thick gradational contact.

Geochemical and petrographic characteristics are nearly identical in the aphyric zone and the matrix material of the porphyritic zone. Mineralogically, the ferrodiorite is composed of plagioclase, augite, K-feldspar, Fe-Ti oxides, and minor accessory minerals of quartz, inverted pigeonite, calcite, and apatite, which collectively display an intergranular, nonfoliated texture. Modally, the ferrodiorite varies between diorite and quartz monzonite. Based on XRF analyses, the normative An composition in the matrix is 39, while ranging from 19-64 as determined by microprobe analyses. Matrix augite contains a constant composition (mg # 55-61) throughout the intrusion. Microprobe traverses and Nomarski DIC microscopy of plagioclase megacrysts indicate subtle zoned cores (~An 65) and strongly zoned rims with a similar An to matrix plagioclase.

Based on these observations, we conclude that the LPD formed by the shallow emplacement of a single Pl-phyric ferrodiorite magma. Density calculations indicate the possibility of plagioclase megacryst flotation in the host ferrodiorite magma. Moreover, we conclude that the LPD is likely a hypabyssal equivalent to the anorthositic series of the deeper-seated Duluth Complex (DAS), which is thought to have formed from plagioclase crystal mushes derived from deep crustal magma chambers. This genetic link is implied by similarities in 1) the average composition, structural state, and zonation patterns of LPD plagioclase megacrysts and cumulus DAS plagioclase and 2) the evolved compositions of the LPD ferrodiorite and estimated DAS trapped liquid compositions.