Publication: Detrital zircon ages of Neoproterozoic sequences of the Moroccan Anti-Atlas belt
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Publication Date
2010
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Elsevier Science B.V., Amsterdam
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Abstract
Detrital zircon dating from Neoproterozoic successions in the Sirwa inlier of the Anti-Atlas belt in Morocco
confirms that the maximum depositional age of the main stratigraphic groups is significantly younger
than has been previously proposed in lithostratigraphic correlations. This can probably be extended
to the whole Anti-Atlas according to other recent data from the Saghro inlier. Although the relative
stratigraphic position of the different units remains valid as published previously, a crucial implication
of the new ages is that the sequences believed to be contemporaneous with oceanic crust and island
arc formation during the rifting and break-up of the northern margin of the West African Craton (WAC),
and believed to be involved in the first phases of the Pan-African orogeny, are actually late to postorogenic.
The age of the main deformation associated with the collision of the oceanic- and arc-derived
terranes to the WAC, allegedly affecting the sediments of the Saghro Group, has been estimated at around
663–640 Ma. However, the youngest zircon populations of sediments of the Saghro and Bou Salda Groups,
obtained in this study, cluster around 620–610 Ma, constraining the maximum age of deposition. This age
of sedimentation is indistinguishable from the age of intrusive high-K calc-alkaline plutons of the Assarag
Suite, suggesting a very rapid cycle of magmatism, relief formation, erosion and sedimentation in an active
geodynamic scenario. Moreover, the proportion of the 610Ma detrital zircons becomes less with respect
to the Paleoproterozoic zircons at higher stratigraphic levels, suggesting that the source of young zircons
was progressively eroded and more extensive cratonic areas, that probably underlie the Neoproterozoic
rocks, were exposed. We interpret these data in terms of the development of a ca. 610Ma magmatic
arc, built upon WAC basement, and its progressive dismantling. This arc can be correlated with the
voluminous late Neoproterozoic (ca. 640–570 Ma) arc magmatism characteristic of the north Gondwana
margin and the peri-Gondwanan terranes. The diamictite beds that appear in the Imghi Formation of the
Saghro Group have been correlated with the Sturtian glacial period ca. 700 Ma. However, zircons from one
sample of these diamictites indicate that this correlation cannot be longer maintained, and instead they
should be correlated with the Marinoan glacial period ca. 630–610 Ma, with a widespread distribution
of glaciogenic deposits in West Africa. In addition, around 375 U–Pb concordant analyses obtained from
Paleoproterozoic zircons from six samples represent a statistically significant population of this area of
theWACbasement, which can be a useful database for comparison with the detrital zircon populations of
the peri-Gondwanan terranes of Europe and North America, as the WAC margins were one of the major
sediment suppliers for these terranes.