An interpretation of the gravity field of the Morin Anorthosite Complex, southwest Quebec

Abstract

The Morin Anorthosite Complex of the western Grenville province is composed of an igneous sequence of anorthosite-leucogabbro–jotunite–quartz mangerite which intrudes granulitic gneisses and metasedimentary rocks. Separation of the gravity field observed over the complex into regional and residual gravity anomalies by a graphical method reveals that the negative and positive gravity anomalies associated with the Morin Complex are superimposed upon a regional negative anomaly that may be caused by a low-density mass no deeper than the lower crust. There is no evidence of a complementary dense mafic body underlying the complex. The negative residual gravity anomalies over the anorthosite-leucogabbro are attributed to an underlying body of relatively pure anorthosite of density 2.69 g/cm 3 intruded into a region whose average density is approximately 2.78 g/cm 3 . The body of anorthosite is divided by a basement ridge into two lobes, which may represent different intrusions. These two lobes have the form of irregular sheets 2 to 4 km thick, underlain by thin pipes, which may represent feeders, extending to depths of at least 12 km. The anorthosite may extend subsurface to the northwest, northeast, and south of the main outcrop. Positive residual gravity anomalies are associated with jotunite, quartz mangerite, and metasedimentary rocks. Jotunite forms a thin surface sheet within the anorthosite-leucogabbro and possibly a more substantial body beneath its southern margin. Quartz mangerite forms an extensive body to the south of the massif and may extend beneath the Paleozoic cover of the St. Lawrence Lowlands.