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One of the most prominent negative Bouguer gravity anomalies in the Canadian shield follows the northeastern part of the Grenville front for almost 1 200 km. It is attended along its southeastern flank, within the Grenville province, by a belt of positive anomalies that are significantly more positive than the general level of the gravity field along the opposite flank, over the older Superior, Churchill, and Nain provinces. Together, the belts of negative and positive anomalies constitute a typical example of a particular kind of paired gravity anomaly that signifies the presence of collisional sutures produced by convergent plate tectonics. The tectonic significance of the gravity signature first became apparent by way of the Grenville example when it was noted that the steep gradient separating negative and positive components, ascribed to a steep discontinuity penetrating the entire crust, was located within a zone defined on paleomagnetic grounds as one likely to contain a suture; the discontinuity was subsequently equated with a cryptic suture. Studies of other paired gravity anomalies in both Phanerozoic and Precambrian terranes have provided a variety of evidence supporting a collisional origin for the signature.

In the northeastern half of the Grenville province, gabbroic areas of terranes formed of rocks of the anorthositic suite correlate with large positive gravity anomalies. Gravity models suggest that these are related to saucer- or funnel-shaped gabbroic masses extending, in cases, to mid-crustal depths. These masses are interpreted as dense mafic cumulates produced by gravity-assisted differentiation of gabbroic magma, and as such represent the lower levels of former magma chambers. Associated anorthosites are believed to have developed by flotation of plagioclase to higher levels.

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