PETROLOGIC AND GEOPHYSICAL CHARACTERIZATION OF THE PRECAMBRIAN IN CENTRAL ILLINOIS: VOLCANISM AND MAFIC IGNEOUS INTRUSION ALONG THE ANCIENT LAURENTIAN CONTINENTAL MARGIN

The Precambrian basement rocks of the Eastern Granite-Rhyolite Province (EGRP) in central Illinois (midcontinent region of North America) exhibit a complex history of early volcanism followed by shallow intrusion by deeper seated mafic igneous rocks. A comprehensive suite of dedicated petrographic analysis, geophysical-logs, and drill core from four basement-penetrating wells, two-dimensional and three-dimensional seismic reflection, and isotopic age data from the Illinois Basin–Decatur Project (IBDP) and Illinois Carbon Capture Storage Project (ICCS) site provide unprecedented constraints for interpreting the development of Precambrian basement beneath the Illinois Basin. These new data reveal the basement to be compositionally and structurally complex, having typical EGRP felsic rocks interlayered with atypical volcanic rocks, including the first recorded in situ gabbros in Illinois. Zircons within the EGRP rhyolites give a U-Pb isochron date typical of EGRP (ca. 1.55–1.35 Ga), whereas the gabbro gives a date correlative to Keweenawan Rifting and extensional Grenville-age tectonism (ca. 1.3–0.9 Ga). A high-resolution three-dimensional seismic reflection volume, coincident with the four wells, provides a context for interpreting the petrological data and implies a high degree of heterogeneity for basement rocks at the IBDP-ICCS site, as also shown by the drill cores. The occurrence of Keweenawan-age equivalent mafics intruding into typical EGRP felsics at the IBDP-ICCS site is suspected to relate to major bowl-like structures observed on local two-dimensional seismic reflection profiles and interpreted as a deep-seated mafic-sill complex (McBride et al., 2016). Furthermore, heterogeneities such as brecciated EGRP felsics and later intruded mafics observed in the basement lithology at the IBDP-ICCS may reflect a structurally active Precambrian basement along the suspected margin of Laurentia.