Characterization of the Philbrook Intrusion, Central Minnesota

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Characterization of the Philbrook Intrusion, Central Minnesota

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Minnesota Geological Survey




The Philbrook intrusion is an informal name for a small, sub-circular mafic pluton that is exposed at the surface along the southeast side of the Long Prairie River near the town of Philbrook, in northeastern Todd County, central Minnesota. An 40Ar/39Ar age of 1,854 ± 4 Ma has been obtained from a sample of what is interpreted to be magmatic hornblende from the intrusion. The pluton is composed primarily of melanocratic to mesocratic diorite, along with substantial proportions of pyroxenite, hornblendite, oxide-apatite rock (nelsonite), and rare anorthosite as inclusions in the mesocratic diorite. The Philbrook intrusion contains abundant hornblende interpreted to be both primary-magmatic and secondary-deuteric, which coupled with local net-veined pegmatitic textures and pervasively saussuritized plagioclase, imply that the magma was rich in hydrous components. Even though primary textures are commonly overprinted by secondary hornblende, relict primary cumulate textures are present in most of the different rock types. Chemically, the entire suite of rocks is high in phosphorous, iron, titanium, and vanadium, and low in silica, magnesium, and potassium. The Philbrook intrusion was emplaced into a suite of interlayered sedimentary, volcanic, and hypabyssal intrusive rocks that have been inferred to be Paleoproterozoic, but have more recently been interpreted to be possibly Archean, in age. The country rocks are schistose and were regionally metamorphosed to the upper greenschist/lower amphibolite facies, either during or before the Penokean Orogeny, or possibly both, depending on their age, which is unknown.


The Philbrook intrusion is likely a funnel-shaped • intrusion with steep walls. The extent of the pluton is delineated by a pronounced positive aeromagnetic signature, by which the pluton is estimated to be approximately 2 mi2 (5 km2) in surface area. The origin of the pluton remains enigmatic; • however, it is postulated that it may be similar to iron-rich mafic rocks and oxide-apatite rocks elsewhere associated with large anorthosite massifs.

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Boerboom, Terrence J; Geary, Jesse. (2017). Characterization of the Philbrook Intrusion, Central Minnesota. Retrieved from the University Digital Conservancy,

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