Much of the Precambrian basement of the mid-continent region of the United States is underlain by silicic volcanic rocks and related silicic epizonal granite bodies. In the St. Francois Mountains of southeastern Missouri and in the buried basement of Illinois, Indiana, and eastern Ohio, these rocks mainly yield ages of 1,450 to 1,480 m.y. Previously published U-Pb ages from zircons showed that rhyolite and granite in the subsurface of southwestern Missouri, southern Kansas, and parts of Oklahoma were formed 1,350 to 1,400 m.y. ago. New U-Pb age data from zircons presented here show that this terrane makes up much of the basement of eastern and central Oklahoma, where it includes both rhyolite and granite, and extends into the Texas Panhandle, where the Panhandle Rhyolite Terrane, which was thought to be ∼1,200 m.y. old, yields zircon U-Pb ages of 1,350 to 1,400 m.y. Zircons from the San Isabel batholith of the southern Wet Mountains, Colorado, also yield U-Pb ages of 1,360 m.y. and are thus coeval with the granite and rhyolite of the southern mid-continent region. The San Isabel batholith intruded crust formed at least 1,700 m.y. ago.
A possible model is that the granite and rhyolite of the mid-continent are essentially a veneer emplaced upon and within an older, possibly 1,700-m.y.-old crust; the mesozonal San Isabel batholith is a body formed during this igneous activity that is exposed in contact with older crust because of later uplift in the southern Rocky Mountains. The tectonic setting of the emplacement of these rocks is obscure but appears to be continent-marginal, because only younger rocks are known to the south.