Report of Investigations

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Browsing Report of Investigations by Author "Boerboom, Terrence J"

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    Characterization of the Philbrook Intrusion, Central Minnesota
    (Minnesota Geological Survey, 2017) Boerboom, Terrence J; Geary, Jesse
    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.
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    GEOPHYSICAL AND PETROLOGIC ADVANCES IN UNDERSTANDING THE TAUNTON-BELT WEDGE, AN ARCHEAN TERRANE WITHIN THE MINNESOTA RIVER VALLEY SUBPROVINCE OF THE SUPERIOR CRATON, NORTH-CENTRAL U.S.A.
    (2023) Southwick, David L; Chandler, V.W.; Jirsa, Mark A; Boerboom, Terrence J
    The Taunton-belt wedge, defined herein, is an unexposed fault-bounded Archean terrane located within the Morton block of the Minnesota River Valley (MRV) subprovince of the Superior Craton. It is bounded on the north by the Yellow Medicine shear zone (YMSZ), a prominent regional structure that extends from the west margin of the Paleoproterozoic Penokean orogen in Minnesota to the east margin of the Paleoproterozoic Trans-Hudson orogen in South Dakota. The south boundary of the wedge is a less distinct curving fault zone that splays west–southwest from the YMSZ in south-central Minnesota and rejoins it in eastern South Dakota. The interior geology of the wedge is poorly known, owing to continuous cover of the Precambrian basement by Phanerozoic strata and Pleistocene glacial deposits. Regional aeromagnetic and ground gravity mapping indicate a broad belt of mafic metavolcanic and related rocks in roughly the northern half of the wedge, and many granitoid intrusions in the southern half. Geophysical signatures characteristic of layered gneiss, such as those observed over known gneiss elsewhere in the Morton block, are minor to absent.