Origin of the late Paleozoic plutonic massifs in Morocco

Throughout northwestern Africa are small (less than 200 km²) late Paleozoic massifs. These are shallowly emplaced into unmetamorphosed Paleozoic sediments and always produce a well-defined contact metamorphic aureole. The Tichka massif is the only well-exposed massif with over 2,000 m of vertical relief developed. In contrast, the other massifs have been barely unroofed.

The Tichka massif contains basic rocks surrounding teardrop-shaped granitic pods. At the contact of the massif with the overlying sediments, large masses of granite occur. On the basis of field relationships, two models can be proposed: the rock types are related to a common parent, or the dioritic and granitic magmas are independent melts. A least-squares approximation test, using all available chemical data, is consistent with the latter model.

The granitic massifs approximate a “minimum-temperature melt” composition, which suggests that these rocks originate by fractional fusion of the crustal rocks. The heat source is proposed to be caused by emplacement of mantle-derived gabbroic melts. In some massifs the granitic material was generated and emplaced without intermingling with the gabbroic melts, and it is these massifs that are associated with relatively large negative gravity anomalies (−40 mgal). The chemistry and mineralogy of these massifs are statistically identical. In contrast the Tichka and Midelt massifs are not associated with negative gravity anomalies, and the granitic rocks from these two massifs are statistically identical; but each is statistically different from the massifs with negative gravity anomalies. The Tichka massif contains abundant, possibly mantle-derived, basic rocks and it is inferred that the Midelt massif does also.

There is no evidence that supports a subduction zone model for the origin of these late Paleozoic massifs. It is proposed that the massifs of Morocco may be a response of the continental crust to initial spreading and rifting of North America from Africa. The origin of the granitic rocks is suggested to be the result of fractional fusion of the lower crust, with the heat source caused by sporadic upwelling of mantle-derived gabbroic material associated with the initial extension and thinning of the lithosphere. On the basis of the ages of these massifs, the initial activity began about 320 m.y. B.P.