Petrology and Geochemistry of Anorthosite Xenoliths from the North Shore of Lake Superior

Purple and green anorthosite xenoliths occur along the North Shore of Lake Superior. A comparative study was conducted of their mineralogy, texture, and geochemistry to explore color-correlated differences. Samples contain >95% plagioclase and primary Fe-Ti oxides, with secondary minerals indicating hydrothermal alteration. One sample includes trace olivine and pyroxene. Major and trace element abundances are broadly similar among samples. Purple samples on average contain more oxides, some of which have altered to hematite. Green samples contain serpentine that has replaced mafic minerals. The best distinguisher of color difference is the presence and abundance of Fe-Ti oxides (ilmenite, magnetite, and hematite). These most likely reflect differences in the pressure, temperature, or oxidation state during anorthosite crystallization.