Grenville Boundary in Ohio

Abstract

Basement rocks penetrated in deep wells in Ohio can be sharply divided by a boundary trending slightly west of north into high-grade metamorphic rocks on the east and unmetamorphosed igneous and sedimentary rocks on the west. Micas from the metamorphic rocks give Rb-Sr ages between 900 and 1,000 million years, indicating that they are part of the Grenville orogenic belt. The sharp lithologie contrast across the boundary suggests that it is truly the Grenville boundary. The unmetamorphosed igneous rocks west of the boundary are similar to those from basement wells in Illinois and Indiana and from outcrops in southern Wisconsin, the St. Francis Mountains, and the Arbuckle Mountains. Micas and feldspars from the outcropping rocks give Rb-Sr ages of 1,350 to 1,450 million years. The writer thinks that all these rocks west of the boundary comprise a non-orogenic igneous province, and therefore the rocks west of the boundary in Ohio will prove to be older than the rocks of the Grenville orogenic belt.