Precambrian intrusive bodies in the northern Bighorn Mountains, Wyoming, were formed during at least two distinct episodes of igneous injection. An older group of metadolerite and metapyroxenite intrusions was emplaced prior to the last regional metamorphism (2.75 b.y. ago) to affect the metasomatic, granitic country rock of the area. A younger group of dolerite and dolerite porphyry dikes has formed since that event. Chemical analyses indicate that both groups of intrusive rocks are of tholeiitic basalt affinity.

As a group, the dolerite and dolerite porphyry appear to represent a differentiated sequence dominated by iron enrichment similar to the trend defined by metadolerite and related rocks in the nearby Beartooth Mountains.

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