Paleomagnetic and 40Ar/39Ar results from the Grant intrusive breccia and comparison to the Permian Downeys Bluff sill — Evidence for Permian igneous activity at Hicks Dome, southern Illinois Basin

Igneous processes at Hicks dome, a structural upwarp at lat 37.5° N., long 88.4° W. in the southern part of the Illinois Basin, may have thermally affected regional basinal fluid flow and may have provided fluorine for the formation of the Illinois-Kentucky Fluorspar district. The timing of both igneous activity and mineralization is poorly known. For this reason, we have dated an intrusive breccia at Hicks dome, the Grant intrusion, using ⁴⁰Ar/³⁹Ar geochronometric and paleomagnetic methods. Concordant plateau dates, giving Permian ages, were obtained from amphibole (272.1±0.7 [1σ] Ma) and phlogopite (272.7±0.7 [1σ] Ma). After alternating-field (AF) demagnetization, specimens that contain titanomagnetite-bearing igneous rock fragments give a mean remanent direction of declination (D)=168.4°; inclination (I)=–8°; α95=8.6°; number of specimens (N)=10; this direction yields a virtual geomagnetic pole (VGP) at lat 54.8° N., long 119.0° E., δp=4.4°, δm=8.7°, near the late Paleozoic part of the North American apparent pole wander path. A nearly identical magnetization was found for the nearby Downeys Bluff sill (previously dated at about 275±24 Ma by the Rb-Sr method), in southern Illinois. Both AF and thermal demagnetization isolated shallow, southeasterly remanent directions carried by magnetite in the sill and from pyrrhotite in the baked contact of the Upper Mississippian Downeys Bluff Limestone: D=158.6°; I=–11.8°; α95=3.8°; N=15, yielding a VGP at lat 53.0° N., long 128.7° E., δp=2.0°, δm=3.9°. The paleomagnetic results, isotopic dates, and petrographic evidence thus favor the acquisition of thermal remanent magnetization by the Grant breccia and the Downeys Bluff sill during the Permian. The isotopic dates record rapid cooling from temperatures greater than 550°C to less than 300°C (the closure temperatures for diffusion of 40Ar in amphibole and phlogopite, respectively) after emplacement during the Permian. The results further indicate that individual clasts of the Grant breccia were emplaced at temperatures greater than about 550°C, the magnetization-blocking temperature of the titanomagnetite in the breccia, and that it cooled very rapidly, within less than 1–2 m.y. After cooling, the breccia was not affected by thermal perturbations greater than about 300°C.