Age and significance of Precambrian Basement Samples from northern Illinois and adjacent states

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Abstract
Geochronologic results are reported for 12 localities where deep drilling has penetrated Precambrian basement in Iowa, Illinois, Indiana, Michigan, and Kentucky. For the northern Illinois deep drill holes UPH 1 and UPH 3, a zircon U‐Pb age of 1463±10 m.y. was obtained. Rb‐Sr data for these samples indicate mild disturbance of this system but are not interpreted in terms of any specific younger event. U‐Pb ages for other localities fall in the interval 1450–1510 m.y. and indicate that this was a major period of igneous activity throughout the southern Great Lakes region. Rb‐Sr results indicate that alteration was ubiquitous and that most ages in the range 1100–1400 m.y. probably represent disturbed systems. The results obtained so far tend to support interpretations in which Kentucky, Indiana, and Illinois are primarily underlain by a rhyolite and epizonal granite terrane that is 1450–1500 m.y. old; older Proterozoic rocks may be absent or more deeply buried.