Abstract

Most of what we know about basement rocks in Kansas and Missouri is derived from cores and cuttings from deep drilling. These rocks may be divided into a northern terrane, underlain by rocks consisting of abundant granite commonly showing cataclastic textures and by metavolcanic to metasedimentary rocks; and a southern terrane, underlain almost exclusively by rhyolitic flows and ash-flow tuffs and epizonal granite plutons. The northern terrane is interrupted in central Kansas by mafic igneous rocks and flanking arkosic sedimentary rocks of the Central North American Rift System.

Data from 63 Rb-Sr whole-rock analyses yield ages ranging from 1,153 to 1,748 m.y., but these are considered to represent only the minimum ages of the rocks. Ages derived from U-Pb analyses of suites of cogenetic zircons from 22 rock samples indicate that some rocks of the northern terrane were formed 1,610 to 1,650 m.y. ago. These are apparently intruded by younger granite plutons formed 1,450–1,470 m.y. and 1,340–1,380 m.y. ago. Rocks of the southern terrane were formed 1,460–1,480 m.y. ago in the St. Francois Mountains terrane of southeastern Missouri and its buried equivalents, but these are about 1,380 m.y. old in southwestern Missouri and southern Kansas. Rocks of the Central North American Rift System in Kansas are assumed to be about 1,100 m.y. old by geophysical and drill-hole extension to their outcrop in the Lake Superior region, where they have been dated.

Both the northern and southern terranes are notable for the great abundance of granitic rocks and the scarcity of intermediate to mafic igneous rocks. Quartzite is the most abundant metamorphic rock. Although the decrease in ages from north to south in the mid-continent region suggests their sequential accretion at the edges of a pre-existing continent, the rock assemblages are not consistent with convergent plate-margin suites of the Andean type. The great volumes of rhyolite and epizonal granite of the southern terrane may represent melting of thickened, somewhat older crust following accretion at the continental margin.

Major-element chemical data are presented for basement rocks from Kansas, Oklahoma and Texas, and these are compared with the compositions of similar rocks from the St. Francois Mountains. Names and locations are given for wells from which samples were obtained.

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