Physical volcanology, geochemistry and basin evolution of the Ediacaran volcano-sedimentary succession in the Bas Draa inlier (Ouarzazate Supergroup, Western Anti-Atlas, Morocco)

摘要

New geologic mapping, lithofacies and granulometric analysis, and geochemistry from the volcano-sedimentary successions of the central part of the Bas Draa inlier, Western Anti-Atlas, constrain the Ediacaran Ouarzazate Supergroup evolution during the post-collisional stage of the Pan-African orogeny. Volcanosedimentary facies analysis is the key aspect of the present contribution. We distinguished sixteen terrestrial volcanosedimentary lithofacies in the Bas Draa succession (BDS), which reaches a total thickness of 2000 m. BDS evolution can be grouped into four units (Aouinet Aiet Oussa Ⅰ to Ⅳ, AO Ⅰ-AO Ⅵ). The earliest volcanic activity produced rhyolitic ignimbrite sheets (AO Ⅰ), which had been considered as lava flows by previous workers, and which were presumably related to caldera system(s). During AO Ⅱ, a complex of high-silica andesitic and rhyolitic lavas formed, punctuated by the explosive eruption of a high-temperature silica-rich magma leading to the formation of parataxitic ignimbrite. AO Ⅲ consists of basalt and andesite lava fields and small explosive, in parts phreatomagmatic volcanic vents. It is dissected by fluvial systems depositing external non-volcanic and local volcanic debris. BDS evolution terminated with the formation of a large SiO_2-rich lava dome complex (AO Ⅳ), accompanied by small basalt effusive event. Volcanosedimentary facies analysis infers that the BDS evolved in a continental extensional setting developing in a low topography under humid paieoclimatic conditions. Alteration textures are dominated by a piemontite-calcite-albite-quartz (+ iron oxides) assemblage. Chemical analysis of BDS volcanic and subvolcanic rocks belongs to high-k calc-alkaline and alkali-calcic to alkaline magmatic trend typical for a post-collision setting. Trace elements spidergrams show a pattern typical for subduction-related suites of orogenic belts. REE patterns show moderate enrichment in LREE relative to flat HREE, with strong negative Eu anomaly in rhyolitic rocks. Geotectonic diagrams suggest an affinity transitional between subduction- and within-plate settings. BDS magmatism can be explained by a magma source enriched in inherited components from Pan-African subduction. Volcanosedimentary facies, structural features, geochemical composition and radiometric data of BDS suggests correlation with the upper Ouarzazate Supergroup of the Anti-Atlas. Geodynamic evolution of the BDS was an expression of an extensional setting that operated in northern Gondwana margin precursory to the Cambrian anorogenic volcanic activity.