During the drilling of the McClure-Sparks well, in the Michigan Basin, two metadiabase units were encountered at about 5.3-km depth, after boring through a sequence of clastics underlying the Cambrian Mt. Simon sandstone. These metadiabases belong the the basement of the Michigan Basin and are probably late Precambrian in age and therefore contemporaneous with the basalts and the gabbros of the Keweenawan Peninsula, Michigan. No evidence exists of the intrusive or extrusive nature of the diabases. The presence of igneous titaniferous clinopyroxene denotes the alkaline affinity of the original diabase. The six studied samples from the lower metadiabase unit disply metamorphic mineral parageneses which are characteristic of the lower greenschist facies. The remarkable freshness of the igneous clinopyroxene could not be satisfactorily explained. The chemical analysis of one noncataclastic metadiabase appears to indicate that the rock has been enriched in MgO and depleted in CaO during the metamorphism when compared with the average Keweenawan basalt. The scarcity of sheared zones and deformation features indicate that the pressure during the metamorphic process was essentially hydrostatic. Temperatures not in excess of 400¿ C and pressures ranging from less than 2 to 7 kbar probably prevailed during the formation of the lower metadiabase. These physical conditions correspond to depth in the continental crust of 6--20 km. The absence of secondary minerals corresponding to the 125¿ C temperature measured at the bottom of the well and the lack of retrograde alteration imply that the paragenesis of the metadiabase is a metastable assemblage.