Gravity Study of a Hypersthene Syenite in the Laramie Anorthosite Complex, Wyoming

Abstract

On the northwest border of the Laramie anorthosite complex, 236 gravity stations were located approximately 1 per square km over a hypersthene syenite body and country rock. The hypersthene syenite has a maximum width of about 6 km, extends about 18 km, and is bordered principally by anorthosite, hornblende syenite, and granite gneiss. Three 15 to 18-mgal anomalies with maximum gradients of about 4 mgal/km are centered over the hypersthene syenite. Bouguer anomaly values range from a mean of about —150 mgal in the anorthosite to a high of —128 mgal over the hypersthene syenite to Bouguer mean values of —150 mgal over granite gneisses to the west. Strikingly, isoanomaly lines locally crosscut geologic contacts. Computed minimum density contrasts of about δσ = .23 indicate that a subsurface mafic mass, possibly norite, is the principal disturbing mass and not the hypersthene syenite. Models indicate that this mafic mass is within .5 km of the surface in places and extends to a depth of about 5 km. The geometry of the hypersthene syenite is difficult to ascertain.