The Petrography of the Longnose Peridotite and Its Relationship to the Duluth Complex
1991-01
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The Petrography of the Longnose Peridotite and Its Relationship to the Duluth Complex
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1991-01
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Thesis or Dissertation
Abstract
The Longnose Peridotite is a funnel-shaped, ultramafic body intruded into middle Proterozic (1.1 b. y.) troctolitic series rocks of the Duluth Complex. The body is located approximately 83 kilometers north of Duluth, Minnesota. The geometry of the Longnose Peridotite is based on drill core logs and gravity data. In plan view, the body is tear drop in shape, measuring about 760 meters long and narrowing from 490 meters at the widest point to 76 meters at the narrowest point. In longitudinal section, it forms an asymmetrical funnel with a maximum thickness of 167 meters. A crude zonal arrangement of cumulate rocks exists. Going stratigraphically up section, they are oxide clinopyroxenite, oxide peridotite (wehrlite), oxide dunite, and feldspathic dunite. The marginal zone of the body is characterized by xenoliths of troctolitic rocks from the Duluth Complex and locally exhibits inversed stratigraphy. Discordant massive oxide zones composed of ilmenite and titanomagnetite are primarily hosted by oxide dunite and to a lesser extent by oxide clinopyroxenite. The zones vary from less than 0 .1 meter in thickness and perhaps to several meters in lateral extent to up to 30 m in thickness and 130 m in length. These zones are thought to have formed by filter pressing or by primary precipitation of ilmenite/titanomagnetite from a Fe-Ti rich ultramafic magma, and subsequent annealing at subsolidus oxide terperatures. Mineral chemical analyses of olivine, plagioclase, and augite show a similar mineralogy between the Longnose Peridotite and other troctolitic, gabbroic, and dunitic rocks in the Duluth Complex. The Longnose Peridotite was introduced soon after the intrusion of the troctolitic country rocks of the Duluth Complex based on the lack of alteration of the country rock and lack of chilled margins in the Longnose Peridotite. The stratigraphy of the Longnose Peridotite can be explained by multiple injections of magma. The inverted stratigraphy in the marginal zones and the sparcity of igneous foliation suggest that the injections may have been vigorous in nature.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Eric Kendall Linscheid in partial fulfillment of the requirements for the degree of Master of Science, January 1991. There are 2 supplementary files also attached to this record, which contain Plates 1 and 2 referenced in the thesis.
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