Large silicic magma systems

Abstract

U–Pb LA-ICP-MS analyses on zircon extracted from felsic ignimbrites and sandstones of the Bas Draâ succession (BDS) constrain the post Pan-African evolution in the Anti-Atlas (AA) at the northern margin of Gondwana. The 2000-m-thick Ediacaran BDS consists of four volcanic units including: (i) rhyolitic ignimbrite sheets (AO I); (ii) high-silica andesitic and rhyolitic lavas (AO II); (iii) silica-poor lava and pyroclastics (AO III); and (iv) rhyolitic lava dome complexes (AO IV). AO III is dominated by basaltic and andesitic lava and it contains at least three sandstone lenses. The BDS is capped by an angular unconformity, which is defined by sedimentary rocks of the Adoudou Formation of the Taroudant Group.The U–Pb data set (99 zircon grains with 90–110% concordance) from the ignimbrite yields an age of 565 ± 6 Ma (564.8 ± 5.6 Ma), interpreted as the crystallization age, which represents the early volcanic activity in the BDS. For the upper BDS, maximum ages of ca. 552 and ca. 541 Ma are indicated by U–Pb ages of detrital zircons extracted from sandstone. These age constraints confine the BDS to the Upper Ouarzazate Supergroup.Most detrital-zircon ages (three sandstone samples with 367 zircon grains analyzed) fall in the range of ca. 606−541 Ma. This main Ediacaran peak is interpreted to have been derived from the post Pan-African magmatic complexes surrounding the West African Craton (WAC). Secondary age peaks at ca. 652 Ma and ca. 655 Ma are inferred to originate from Early to Late Pan-African magmatism of the Anti-Atlas region. Due to the complete absence of pre-Neoproterozoic zircon ages, our data set is interpreted to indicate a large voluminous volcanic episode that covered the Palaeoproterozoic and Cryogenian basement or the river system was confined to the BDS basin. Facies and provenance analyses of BDS sandstones indicate that the detrital input was mostly locally derived, with a minor amount from an external source. In addition, large peaks of older zircon ages are recognized only on the lower sandstone lens (BD 86, AOIII). Thus, the drainage system was large within the lower part and became more localized at the end of BDS evolution.The maximum depositional age for the zircons separated from the uppermost AO III sandstone lens (541 Ma) shifts the Precambrian–Cambrian boundary down from the Lower Adoudou Formation, as proposed by previous studies, to the uppermost Ouarzazate Supergroup or to the hiatus between both units.The volcanic activity within the BDS is linked to a transtensional regime that operated along the Gondwana margin during the Ediacaran. The BDS volcanic activity is contemporaneous with the widespread volcanic, volcanoclastic and siliciclastic rocks of Upper Ouarzazate Supergroup.