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The K-feldspar in all major rock units exposed in the Middle Proterozoic Butler Hill Caldera, St. Francois Mountains, south- eastern Missouri, shows a high, but not maximum, degree of structural order. The structural state parameter Z for phenocryst material from the felsic volcanic rock units averages 0.89, with a standard deviation of 0.06 ( Z would be 1 in fully ordered microcline). K-feldspar extracted from the matrix of these same samples yields an average Z value of 0.86, with a standard deviation of 0.07. The Z parameter for the K-feldspar in the ring pluton granites averages 0.87, with a standard deviation of 0.06. The structural state of the K-feldspar in these ring pluton granites is virtually identical to that reported by our group for t he Butler Hill - Breadtray Granite, the subvolcanic granite massif exposed in the center of the Butler Hill Caldera. There is no statisti cally significant variation of K-feldspar structural state with inferred structural elevation, present topographic elevation, or prox imity to the Simms Mountain Fault. We interpret the homogeneity of the K-feldspar structural state throughout the Butler Hill Caldera complex, and especially the unexpectedly high degree of structural order in the volcanic K-feldspar, as evidence for a pervasiv e alteration event, which re-ordered the K-feldspar throughout the entire volcanic-plutonic complex. The most likely cause of the homogenization of the K-feldspar structural state is the late-stage alkaline magmatism inferred to have occurred throughout the St. Francois Mountains at approximately 1.36 Ga.

the canadian mineralogist

STRUCTURAL STATE OF K-FELDSPAR IN THE FELSIC VOLCANIC ROCKS AND RING PLUTON GRANITES OF THE BUTLER HILL CALDERA, ST. FRANCOIS MOUNTAINS, SOUTHEASTERN MISSOURI