The St. Francois Mountains batholith of southeastern Missouri appears to be a shallow composite batholith that has intruded a roof of its own volcanic ejecta. The complex is composed mostly of silicic intrusive units and rhyolitic pyroclastic rocks that are mostly about 1,500 m.y. old. Field, petrographic, and chemical data suggest that the batholith has been tilted to the southwest and beveled by erosion, thus exposing a 6-km stratigraphic section of the volcanic roof and a 4-km cross section of the underlying intrusive units.
The Butler Hill Granite is the largest single intrusive unit in the St. Francois Mountains batholith and probably controlled the development of most of the batholith. The Butler Hill Granite is characterized by small but systematic chemical variations, with SiO2 and K2O most abundant in the southwest and Fe2O3, TiO2, MgO, and CaO most abundant in the northeast. Moreover, the pluton is characterized by striking gradational changes in mineralogy, with plagioclase and perthitic orthoclase most abundant in the northeast and southwest, respectively. These modal and chemical changes support the tilted-batholith hypothesis, which predicts that the Butler Hill Granite should be structurally highest and most differentiated to the southwest and deeper and less differentiated to the northeast. The large increase in plagioclase to the northeast is due to increased fluid pressure resulting from the greater original depth of this magma. High fluid pressure (about 3 kb) has resulted in partial elimination of the one-feldspar field, so that albite to sodic oligoclase formed rather than a mixed feldspar. These modal data, along with volcanic stratigraphic evidence and chemical relationships expressed in the quartz-albite-orthoclase-water system, suggest that the depth of crystallization of Butler Hill Granite now exposed in the northeasternmost part of the pluton was about 10 km.