Missouri River Breaks Diatremes

A number of diatremes extend from Haystack Butte (described briefly in No. 29), northeast of the Highwood Mountains, and Eagle Buttes, close to the Missouri River, eastwards for 160 km through the Missouri River Breaks to the Little Rocky Mountains. Dykes and plugs at either end of the belt appear to be wholly intrusive, but those towards the middle are diatremes which have an extrusive component. The intrusive rocks are petrographically relatively uniform consisting of phenocrysts of forsteritic olivine in a groundmass of monticellite and/or melilite, nepheline, interstitial or poikilitic phlogopite and a little perovskite and opaques. The diatremes are filled generally with breccias and bedded pyroclastic rocks with suites of xenoliths in some of them. Rock types represented are monticellite peridotites and alnoites. A map and section of the Black Butte diatreme southwest of the Little Rocky Mountains are given by Hearn (1968, Figs 2 and 3). Palaeomagnetic data are available in Diehl et al (1983).