A Petrologic and Geochemical Study of Granophyric Granite in the Roof of the Keweenawan Duluth Complex, Eagle Mountain, Cook County, Minnesota
1991-04
Title
A Petrologic and Geochemical Study of Granophyric Granite in the Roof of the Keweenawan Duluth Complex, Eagle Mountain, Cook County, Minnesota
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1991-04
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Thesis or Dissertation
Abstract
Field work in the roof zone of the Keweenawan (1.1 b.y.) Duluth Complex in the area of Eagle Mountain, Cook County, Minnesota, resulted in the identification of eight major rock units. These are metabasalt, metagabbro, ophitic diabase, syenogabbro, ferrodiorite, rhyolite, and two distinct granophyre units. On the basis of field relations and petrography, the rhyolite appears to be the oldest unit in the area and to have formed the roof rocks below which the intrusive units were emplaced. The metabasalt unit is composed of xenoliths of basalt that were carried and slightly metamorphosed by the magma now represented by the metagabbro unit. This metagabbro was emplaced beneath the rhyolite and later slightly metamorphosed by the intrusion of the magma that formed the Eagle Mountain granophyre. The Eagle Mountain granophyre shares a gradational contact with the ferrodiorite, and these two units appear to be genetically related. The relationship of the syenogabbro to other units is unknown. The miarolitic granophyre was intruded beneath the rhyolite, but its age relation to other units is ambiguous. The ophitic diabase crosscuts other units and appears to be the youngest unit in the field area. The Eagle Mountain granophyre (EMG) is red-brown, fine-grained, porphyritic, and locally granophyric and spherulitic. It is composed of 60-75% feldspars, 20-30% quartz, 5-10% ferroaugite and hornblende, and <5% Fe-Ti oxides. Probe analysis shows average compositions of An40Ab52Or8 for andesine phenocrysts and Mg16Fe41Ca43 for ferroaugite phenocrysts. Chemically the EMG contains 69-71 wt% SiO2, 1.4-1.8 wt% CaO, and K2O/Na2O ~ 1.1. The miarolitic granophyre (MG) is brick-red, medium-grained, and contains approximately 5% miarolitic cavities. It is composed almost entirely of feldspar and quartz in well-developed granophyric intergrowths that radiate outward from phenocrysts of nearly pure albite (An2Ab95Or3) and pure orthoclase (An0Ab5Or95). Chemically the MG contains 73-74 wt% SiO2, 0.2-0.4 wt% CaO, and K2O/Na2O ~ 2.3. The MG has an unusual REE pattern, with HREEs enriched over LREEs. Mass balance calculations and trace element models support formation of the EMG by fractional crystallization of a tholeiitic parent liquid comparable to local basaltic andesites. Chemical and mineralogical evidence also supports silicate liquid immiscibility as a possible fractionation mechanism. For the formation of the MG, mass balance calculations and trace element models do not clearly support either fractional crystallization or partial melting as a mode of origin. This unit may have undergone deuteric or metasomatic alteration that makes modeling impossible. Based on the estimated total thickness of the overlying volcanic pile, the granophyres crystallized at a maximum pressure of 2.0 kb; pressure could have been much lower, depending on the thickness of the volcanic pile at the time of intrusion. Based on experimental work on the granite system, both units crystallized at temperatures near the 990° C thermal minimum for the Ab-Or-Qz system at 1 kb pressure.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Nancy Nelson in partial fulfillment of the requirements for the degree of Master of Science, April 1991. There is 1 supplementary file also attached to this record, which contains Plate 1 referenced in the thesis.
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