Tourmalinites, Nelsonites, and Related Rocks (Early Proterozoic) near Philbrook, Todd County, Minnesota
1987-02
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Tourmalinites, Nelsonites, and Related Rocks (Early Proterozoic) near Philbrook, Todd County, Minnesota
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1987-02
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Abstract
Lower Proterozoic rocks near Philbrook, Minnesota consist of tourmaline-rich metasedimentary rocks, ultramafic to mafic igneous rocks, and metaconglomerates. The metasedimentary rocks have been intersected by drilling only; the igneous rocks have been intersected by drilling and also occur in outcrop; and the metaconglomerates occur in outcrop only. The metasedimentary rocks intersected by drill holes west of Philbrook are a minimum of 230 m thick, strike N-NE, dip steeply east, and are believed to be part of the Mille Lacs Group. The metasedimentary rocks have been regionally metamorphosed to the upper greenschist facies. Regional geologic relations and mineralogy indicate that these metasedimentary rocks are Lower Proterozoic in age and in close proximity to Archean crustal rocks. The general stratigraphy of the metasedimentary rocks from bottom to top, consists of a lower arkose or feldspathic arenite, laminated quartz-magnetite or hematite iron-formation, upper arkose, and 'metagraywacke' (tourmaline- and magnetite-bearing micaceous schist). Neither the top nor bottom of the stratigraphic section has been penetrated by drilling. The high feldspar content of the arkosic rocks indicates a predominance of chemical weathering and a high relief source area. A submarine-fan depositional environment is indicated by the presence of very thin silt and sand beds intercalated with laminated iron-formation, with periodic slumping of clean sand from a shallow-water staging area upslope in the basin. Tourmaline occurs in the arkosic rocks as disseminated crystals and as laminated tourmalinite, and in the micaceous schist as disseminated crystals. The chemical composition of the tourmaline (schorl-dravite_ is similiar to that of tourmaline which is associated with several syngenetic, stratiform massive sulfide deposits elsewhere in the world, indicating a potential for sulfide mineralization at Philbrook. The metaconglomerates occur in two poorly exposed clusters of outcrops along the edge of the Long Prairie River. Although less than 300 m apart, the two groups of conglomerate outcrops are quite different in mineralogy, suggesting a basal deposit of local provenance. One of the conglomerates is comprised mostly of quartz-rich pelitic clasts, and the other contains predominantly coarse-grained plutonic clasts. The clasts in the latter are mineralogically similiar to a nearby hypabyssal, porphyritic tonalite which is believed to be Archean in age, providing evidence that the conglomerates are the basal part of an Early Proterozoic sequence which unconformably overlies Archean crustal rocks. The ultramafic to mafic igneous rocks, based on limited outcrop and drill hole data and aeromagnetic signature, are part of an intrusion which is approximately 2.4 km in diameter, circular in shape with steep sides, and concentrically zoned. Rock types in the intrusion range from ultramafic oxide-apatite rock (nelscnite), pyroxenite, and hornblendite (penetrated by drill holes) to apatite-rich melanocratic diorite and mesocratic diorite (in outcrop). Pegmatitic and anorthositic zones occur in the latter rock type. Chemically, this entire suite of igneous rocks is high in P2D5, TiO2, and Fe2D3, and low in SiO2, MgO, K2O, and Na2O. Abundant primary and deuteric hornblende (after pyroxene), net veins, pegmatitic patches, and local saussuritization of plagioclase supply evidence for a high H2O content of the intrusion. An immiscible split model is employed as the origin for the ultramafic parts of the intrusion. The melanocratic diorite may be a possible parent material which has split into an Fe-Ti-P-rich fraction and a Si-Al-Na-K-rich fraction. Some cumulus mineralogical layering may have further modified the splits, producing the zonal arrangement of the intrusion. Late brittle shear zones and some weakly developed cleavages are the only observable structures in the intrusion, indicating that it has not been severely affected by the Penokean Orogeny. It is believed that the Philbrook intrusion is similiar in age, and probably related to, Penokean granites of the St. Cloud type.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Terrence John Boerboom in partial fulfillment of the requirements for the degree of Master of Science, February 1987.
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