A medium- and coarse-grained granite has been encountered in the lower 914 m of deep drill-holes in northwestern Illinois. The mineralogy and chemistry of both types of granite suggest they are anorogenic A-type granites after the definition of Loiselle and Wones (1979). The major oxide composition for both types is nearly identical. Trace element data (Rb, Y, Th) indicate the medium-grained granite crystallized from a more evolved melt than did the coarse-grained granite. Trace element data (Y, Th, Ba, Sr) for the coarsegrained granite show it to be slightly zoned from the bottom upward to the contact with the medium-grained granite at 731 m.
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