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In this chapter we describe the petrogenesis of aplitic segregations in the fluorine-rich Proterozoic Butler Hill and Graniteville granites of the St. Francois Mountains volcano-plutonic terrane, southeastern Missouri. Both plutons contain an early coarse-grained type of granite that grades into or is crosscut by fine-grained aplitic segregations. The aplitic segregations are generally enriched in fluorine and alkalis, have more pronounced negative Eu anomalies in their rare earth element (REE) patterns and higher concentrations of heavy REEs compared to their coarse-grained high-silica counterparts. In the Butler Hill granite, the distinction in fluorine concentrations between the two rock types was obliterated in part by subsolidus hydrothermal alteration, which is indicated by complete chloritization of biotite, sericitization of feldspar, and recrystallization or reequilibration of muscovite with water-rich fluids.

These chemical relations suggest that the aplitic segregations in both the Butler Hill and Graniteville plutons are the products of crystal-liquid fractionation in which fluorine played a significant role. Initially, the slightly peraluminous to metaluminous compositions of the fluorine-containing magmas were near the composition of a minimum melt in the Q-Ab-Or system. However, early crystallization of quartz and feldspars resulted in enrichment of the remaining melt in fluorine, causing the pseudoternary minimum to move away from quartz. As a consequence of the enlargement of the quartz field, quartz became the sole crystallizing phase, yielding the silica-enriched composition of the coarse-grained granites. The aplitic rocks crystallized from the relatively alkalic residual melt, which separated from the crystalline assemblage as the melt’s viscosity decreased due to an increase in the fluorine and water content. These results are in accord with published experimental data that show that the effect of fluorine is to decrease the silica content of residual liquids, contrary to normally observed fractionation trends in igneous rocks.

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