Resume of the Geology of the Laramie Anorthosite Mass

The Laramie Anorthosite underlies the southern part of the Laramie Range in south- eastern Wyoming. Two bodies of anorthosite comprise the mass, the larger northern body and the southern body. Several small marginal exposures, no more than a few thousand feet in extent, rest upon the later hypersthene syenite near the contact. The extent of the mass is 30 miles north-south and 6 to 10 miles east-west. All of the rock types discussed in this paper are of Precambrian age, but Paleozoic and younger rocks border the mass on the east and we st and overlle small areas. The Precambrlan rock types, in order of Increasing age, include quartzofeldspathic dikes, granite, hornblende syenite, hyper sthene syenite, norite, olivine gabbroic anorthosite, granulated anorthosite, quartz anorthosite, gabbroic anorthosite, anorthosite, and older gneisses and schists. Older gneisses and schists underhe the western, northwestern, and northern portlons and also separate the north- ern anorthosite body from the southern anorthosite body. In the area of separation, the gneisses overlie the anorthosite and are in may places intruded by it. The anorthositic rocks underlie over 80 percent of the area. In the northern body the gabbroic anorthosite forms a border facies, and is exposed along the western, n orthwestern, and northern margins of the anorthosite mass. Quartz anorthosite, exposed in the south-east part, is.believed to be an hybrid rock, formed by the partial assimilation of quartzite by anorthosite. Granulated anorthosite is the result of shearing, and is exposed in many parts of the mass. Olivine gabbroic anorthosite intrudes the anorthosite, prmcipally in the western and northern parts. Norite dikes intrude the anorthositic rocks, with one large elongate mass underlying the north-center of the area. Hypersthene syenite intrudes the marginal areas of the anorthosite in the west, northwest, and south-east, and also the older