Interpretation of a gravity study of the Laramie anorthosite complex limits petrogenetic models. Bouguer gravity anomalies within the anorthosite average about −155 mgal, decrease by 12 mgal over younger granite, and increase up to 20 mgal over bordering syenite bodies. The gravity field over the anorthosite is low with respect to the metamorphic rocks on the northern border. Computed models indicate that the anorthosite mass is a plate 4 km thick with no apparent feeder pipes and is not underlain by a high-density mass. High gravity gradients and ubiquitous surface association of gravity highs over norite-syenite suggest that norite floors these syenite bodies at a depth as shallow as 0.5 km. Computed thickness of norite is about 4 km. From the gravity data, the norite-syenite seems to rim the anorthosite, except where younger intrusions may have disturbed this relation. Minimum calculated mass for respective rock types are: anorthosite, 9.7 × 1018 g; norite, 2.3 × 1018 g; syenites, 6.0 × 1017 g. The parent magma is estimated to have been of noritic anorthosite composition. Petrogenetic models derived from these data suggest that (1) in situ differentiation of gabbroic magma is unlikely, and (2) anorthosite initially differentiated from the parent magma followed by norite and syenites, then this sequence rose into the present domal configuration as a partly consolidated mush.
Present Address: (Owen) Department of Geology, the Ohio State University, Columbus, Ohio