Chemical evolution of magmas in the proterozoic terrane of the St. Francois Mountains, southeastern Missouri: 1. Field, petrographic, and major element data

Abstract

Major element chemical data for plutonic and volcanic rocks of the eastern St. Francois Mountains indicates that ring plutons of the Butler Hill caldera are low‐silica granites and chemically distinct from the high‐silica Butler Hill granite and Grassy Mountain ignimbrite. Additionally, the ring plutons (Silvermine, Stono, Knoblick, and Slabtown granites) are characterized by abundant mafic inclusions, whereas these are almost completely absent in Butler Hill granite. Silvermine granite is mostly of a medium‐grained variety, but cliffs along the St. Francis River expose gradations upward to a high‐silica, fine‐grained ‘cap rock’. Least squares mixing calculations indicate that the cap rock could have formed by the fractional crystallization of plagioclase, hornblende, biotite, magnetite, and sphene, all of which are liquidus phases in the medium‐grained Silvermine granite. The chemical similarity of the fine‐grained cap rock to Butler Hill granite and Grassy Mountain ignimbrite suggested that Silvermine granite might be a sample of the parental magma from which the high‐silica‐rocks were derived. Least squares mixing calculations indicate that Butler Hill granite magma could have been derived from the composition of the average medium‐grained Silvermine by fractional crystallization of the same liquidus phases indicated above. The major element composition of Grassy Mountain ignimbrite can be derived from the average Butler Hill granite magma by fractional crystallization of plagioclase, alkalic feldspar, quartz, biotite, and magnetite, all of which are liquidus phases in Butler Hill granite. Textural and chemical variations in Butler Hill granite from northeast to southwest suggest that the body was internally differentiated and that the present distribution of rock types is due to tilting and erosion. The internal zonation of Butler Hill granite can be accounted for by fractional crystallization of plagioclase, alkalic feldspar, quartz, biotite, sphene, and magnetite. The chemical data and calculations indicate that the primary magma emplaced at the beginning of igneous activity was of Silvermine type. The magma crystallized fractionally so that the magma chamber became compositionally zoned, with high‐silica magma in the upper part. The top‐most part of this material was erupted to form Grassy Mountain ignimbrite during formation of the Butler Hill caldera. Subsequently, more of the high‐silica magma was emplaced into the volcanic material in the caldera during resurgent doming. After formation and resurgent doming of the Butler Hill caldera, some of the parental magma from deeper in the chamber leaked upward into the ring fracture or into arcuate faults within the caldera, forming the ring plutons. The abundant mafic inclusions in the ring plutons may be earlier crystallized magma carried upward during intrusion. One of the ring plutons, Silvermine granite, apparently formed a sill‐like intrusion and underwent in situ differentiation.A body of foliated granodiorite exposed as a giant inclusion in granite at Hawn State Park, east of the principal exposures of the St. Francois Mountains, has the mineralogy, major element chemistry, and trace element chemistry to be a sample of the cumulate resulting from the processes in the magma chamber.