Upper proterozoic ophiolites of the Siroua Massif (anti-atlas, Morocco) a marginal sea and transform fault system

Abstract

The panafrican ophiolites of Siroua (Khzama and N'Kob) are similar to recent ophiolitic suites and represent oceanic crust fragments combined, at least in Khzama region, with part of oceanic lithosphere. The petrographical characteristics of these ophiolites, mainly their mineral crystallisation order and the presence of primary homblende in the cumulates, especially in N'Kob, are typical of the so-called plagioclase series (type IIA). The geochemistry of the lavas and hypabyssal formations, the composition of primary phases such as pyroxenes, show that the corresponding magmas are oceanic tholeites of N-MORB and T-MORB affinities. The relations between the different parts of the ophiolites are mainly tectonic, so they are typical dismembered ophiolites. In Khzama region, crops out a conglomerate with pebbles formed by the various rocks of the ophiolitic suite, so it is characteristic of an intraoceanic faulting similar to present day transform zones. So we can say that the Siroua ophiolites were created in a near island arc transform zone for Khzama, and in a marginal sea for N'Kob. They represent another remnant of a rather narrow ocean included in very long mobile belt, with the nearest Bou Azzer massif and more remote Tilemsi (Mali), of same age, scattered in many basins during Upper Proterozoic along the eastern margin of the West African craton.