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 ...

Revisiting the paleomagnetism of the 1.476 Ga St. Francois Mountains igneous province, Missouri

“…Lower Paleozoic strata overlying the Butler Hill batholith are nearly flat lying at, and between, our sites 1 -6, and dikes intruding the granites at site 3 are near vertical, as is the dike at site 4. We therefore have applied a regional tilt correction to sites 1 -6 where we see no obvious bedding with a strike of 150°and a dip of 10°in accordance with the estimates of Sides [1980] and Bickford et al [1981]. We applied the tilt test using data from 18 of 22 sites, and the results are shown in Figure 9a -9c.…”

The Midcontinent of the United States: Permissive terrane for an Olympic dam-type deposit?

“…Diabase dikes and gabbro bodies were emplaced late in the regional magmatic history (Amos and Desborough, 1970;Sylvester, 1984). Several caldera complexes and associated ring plutons have been recognized both on the surface and in the subsurface Bickford, Sides, and Cullers, 1981;Cullers and others, 1981;Sides and others, 1981). The epizonal granitic plutons are penecontemporaneous with the rhyolites and represent frozen magma chambers of calderas (Sides and others, 1981).…”

“…These exposures are a small part of much larger anorogenic granite-rhyolite terranes in the Midcontinent subsurface (Bickford, Sides, and Cullers, 1981;Denison, 1981;Van Schmus and Bickford, 1981;Hoppe and others, 1983;Thomas and others, 1984;Bickford and others, 1986). Sm-Nd studies DePaolo, 1982, 1985) on basement rocks in the Midcontinent indicate that these Middle Proterozoic rocks were formed rapidly by partial melting of 1.9-1.7-billion-year-old crust.…”

“…Data on the basement rocks of the United States ( fig. 8) are mostly derived from drill core and cuttings Bickford, Sides, and Cullers, 1981;Van Schmus and Bickford, 1981;Thomas and others, 1984;Sims, 1985;Bickford and others, 1986;.…”

“…Granitic rocks of similar age, composition, and origin are found in the St. Francois Mountains in Missouri and throughout the Midcontinent area ( fig. 8) (Bickford, Sides, and Cullers, 1981;Cullers and others, 1981;.…”