A large positive gravity anomaly (amplitude >600 g.u.) coincides with the E part of the Lac Fournier Massif, an anorthosite complex in the Quebec part of the eastern Grenville Province. A more mafic phase of anorthosite than found elsewhere in the complex is present in the anomalous region, and density sampling indicates that the anomaly is probably caused by a large subsurface body of gabbro. The residual gravity anomaly indicates that the body’s anomalous mass is at least 2.6 × 1015 kg, corresponding to a minimum gabbro volume of 1.0 × 104 km3. Negative residual anomalies flanking the positive appear to be caused by bodies of relatively low density anorthosite.
The preferred three-dimensional interpretation of the causative body of the positive anomaly is of an uppermost flat inverted cone extending to a depth of c. 5 km with two roots extending to c. 12 km. A model with outward sloping contacts can also simulate the observed anomalies, but is considered unlikely on petrogenetic grounds and by comparison with other basic intrusions whose geometry is known. Bodies of anorthosite flank the inferred gabbro body and extend to depths of c. 5 km.
The gabbro represents the lowest level of a differentiated magma body initially emplaced by forcible intrusion. During differentiation mafic minerals sank in the magma chamber while plagioclase feldspar separated at a higher level, perhaps by flotation. The present flanking locations of anorthosite may have been accomplished by buoyant ascent and lateral spreading.
The interpreted gabbroic body would have significant gravitational effects even at great depth. This places constraints on the origin of anorthosites in the western Grenville Province as no such positive gravity anomalies have been observed over these complexes.