PETROGRAPHIC AND MICROPROBE ANALYSIS OF THE COTTAGE GROVE DIKE (SALINE COUNTY, ILLINOIS)

Alkaline ultramafic dikes a few centimeters to over 30 meters wide are reported along with sills, explosion breccia pipes, and diatremes associated with Hicks Dome and the Tolu Arch in southern Illinois and western Kentucky, respectively. Paleozoic sedimentary rocks in the region host world-class Mississippi Valley-type mineral deposits. Although these ore-bearing rocks have been the subjects of numerous investigations, much remains unknown in regard to the associated alkaline igneous intrusions. The focus of our investigation concerns a dike encountered in an underground coal mine in Saline County, Illinois near the former community of Cottage Grove. This dike, which we refer to as the Cottage Grove Dike, is nearly vertical, 10 meters wide, trends 5 to 10 degrees west of north, and has been traced for several kilometers by exploration drilling and exposures in coal mines. The dike, although unweathered, is pervasively altered and exhibits complex magmatic reactions among some phases. These alteration effects have hindered the identification of specific primary minerals and textures. The dike consists of phenocrysts of olivine (pseudomorphed by serpentine) and mottled phlogopite with inclusions of apatite set in a groundmass of phlogopite, andradite (Ti-rich), apatite, perovskite, clinopyroxene, magnetite (Ti-rich), chromite (Mg-rich), a Ca-Fe-Ti silicate, calcite, serpentine, and chlorite. A completely replaced lath-shaped mineral present in the groundmass and as inclusions in phlogopite is tentatively regarded as melilite based on its habit and reported occurrence in similar rocks from the region. The Ca-Fe-Ti silicate comprises up to 30 % of the groundmass in some samples and occurs as reaction rims on primary Ti-rich andradite and magnetite. It has the general composition (wt.% oxides) of Ca-35%, Si-32%, Fe (total)-15%, Ti-15%, Al-1%, Mg-1%, and minor Mn and Cr. A plot of the Al2O3 versus TiO2 variations of groundmass phlogopite is similar to published evolutionary trends of minettes and alnöites. This compositional similarity and the inferred presence of melilite (>10%) suggest the Cottage Grove Dike may be considered an olivine phlogopite melilitite. Continuing research on the rock and mineral compositions should further elucidate the petrology of this igneous complex.