Petrology of the Bald Eagle Intrusion and associated rocks and its relevance to crystallization in dynamic magma chambers in the Midcontinent Rift (Minnesota).

摘要

The purpose of my PhD thesis research has been to advance the understanding of the role that fractional crystallization plays in producing the diversity of igneous rocks in intracontinental rifts. The specific subjects investigated were the genetic relationships between the flow-layered Bald Eagle Intrusion (BEI) and associated intrusive rocks in close proximity in the Midcontinent Rift.; The concept that the crystallization of mapped units in the Duluth Complex occurred in dynamic, growing magma chambers in a rift environment is supported by the funnel-shaped structure, the internal layering, and the foliated textures of the Bald Eagle Intrusion and the variations in mineral chemistry of all the units studied.; Documentation for sample locations, structural data, descriptions from field notes and geophysical data were compiled in a digital format consistent with the current standard used by the Minnesota Geological Survey (MGS) in ArcView shapefiles.; The scope of data collection was restricted to the determination of the major and minor element chemistry of the minerals. There is only limited exposure of the rocks associated with the Bald Eagle Intrusion, but fortunately seven drill cores were available for this study. Over 50 polished thin-sections were studied petrographically, and over 8,000 electron microprobe analyses were made. The results provide a new extensive database on mineral compositions of rocks in intracontinental rifts and provide a basis for selection of samples for future major, minor, and trace element whole rock studies.; The petrographic studies and the electron microprobe analyses showed that every rock unit is characterized by a certain mineral assemblage and a specific range of mineral compositions. Every unit corresponds to some portion of the crystallization sequence: Cr Sp+OL+PL->CPx->Fe-Ti-Oxide, the common sequence for the Duluth Complex.; Computer modeling indicates that units of the BEI can be produced from a common parent magma composition (that of a primitive olivine tholeiite from the North Shore Volcanic Group, referred to as P-melt). Parent magma for more evolved rock units spatially associated with the BEI (D-melt) can be produced by fractional crystallization of P-melt at 2–3 kilobars total pressure.