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.

ABSTRACT The Saint Francois Mountains are the physiographic expression of the central part of the Ozark Dome of southeastern Missouri. The mountains are made up of a quaquaversal-dipping series of Paleozoic units cored by the Mesoproterozoic-aged rocks of the broader Saint Francois Mountains terrane. The Saint Francois Mountains terrane lies within the Eastern Granite-Rhyolite province along the eastern margin of Laurentia and contains at least four mapped caldera complexes (Eminence, Lake Killarney, Butler Hill, and Taum Sauk), associated volcanic and volcaniclastic rocks, and four distinct types of intrusive units. The Mesoproterozoic rocks represent two major pulses of magmatic activity: (1) an older 1.48–1.45 Ga episode of caldera-forming volcanism and associated subvolcanic to massif-type granitic intrusions; and (2) a younger 1.33–1.28 Ga episode of bimodal intrusions. Volcanism included primarily high-silica rhyolite and volcaniclastic sediments associated with caldera-forming volcanism with lesser amounts of basalt and basaltic andesite that formed as flows and subvolcanic intrusions. The older (ca. 1.4 Ga) intrusive rocks can be divided into three broad categories: (1) granite massifs including the Butler Hill/Breadtray massif-type granites, (2) caldera ring–type granites such as the Silvermine Granite, and (4) mafic- to intermediate-composition intrusive rocks such as the Silver Mines Mafic Series. The younger (ca. 1.3 Ga) bimodal intrusions are represented by the highly evolved felsic Graniteville-types granites and the gabbros of the Skrainka Mafic Group. This field guide provides an overview of the magmatic history of the Mesoproterozoic rocks exposed in the eastern Saint Francois Mountains. Field-trip stops are divided into two days, highlighting well-known stops and lesser-known localities that illustrate the magmatic activity of one the premier igneous locations in the midcontinent region. The field trip is focused on two main areas. Day 1 focuses on the rhyolite sequence and associated caldera-forming eruption of the Taum Sauk caldera. Day 2 focuses on the volcanic rocks and granitic intrusions related to the Butler Hill caldera and ends with a visit to one of the youngest granitoids in the terrane, the Graniteville Granite. The field guide presents a summary of the volcanic history and petrogenesis of the Saint Francois Mountains rhyolites and granites.