In the southern Bighorn Mountains two groups of mafic dikes occur within a terrain dominated by quartzofeldspathic gneiss. Un-metamorphosed dolerite dikes possess ophitic or subophitic texture and consist primarily of plagioclase (average composition, Ans60 and augite (average Ca:Mg:Fe = 38:44:18).
Metadolerite dikes in the same area consist of two distinctive facies. In each dike a granoblastic margin facies of clear plagioclase (average composition, An38) and hornblende grades into an interior facies possessing relict ophitic or subophitic texture and consisting of clouded plagioclase (average composition, An49) and granular augite (average Ca:Mg:Fe = 42:45:13). Many of the equant augite grains are peripherally replaced by hornblende.
The metadolerites, similar in bulk chemical composition to the dolerites of the area, formed by recrystallization of comparable dolerites during the last regional metamorphism (low amphibolite facies) which affected the gneiss country rock. The dikes were metamorphosed under water-poor conditions which allowed only their margins to be completely recrystallized. Dike interiors underwent partial recrystallization accompanied by diffusion of iron from augite to relict plagioclase, causing clouding, and diffusion of calcium from plagioclase to form slightly more calcic augite and hornblende.
Present Address (Nelson): Geology Department, Homestake Mining Company, Lead, South Dakota 57754