Cambrian shallow-marine to emergent alkaline volcanism near Ouinguigui (Ougnat inlier, eastern Anti-Atlas, Morocco): Volcanic facies, geochemistry and geodynamic setting

Abstract

Various forms of volcanism occurred during the Middle Cambrian southeast of the Ougnat inlier in the eastern Anti-Atlas of Morocco. This study is the first to examine the formation of these volcanic deposits and interrelationships to the sedimentary host and tectonics on local and regional scales. The emplacement history of the volcanosedimentary succession of Ouinguigui has been characterised by a multi-disciplinary approach of field work, facies analysis of the volcanic rocks and their host and by geochemical analysis of coherent volcanic rocks. Additionally, we introduce a generic facies code system for subaqueous to emergent basaltic successions to make related deposits more comparable. The sedimentary host rock is composed of pre-transgressive conglomerates and sandstones of the Tikirt and Akerouz Member, which are interpreted as deposits of shelf-type fan-delta and shoreface deposits. The transgression is reflected by an abrupt decrease in grain size and the presence of packstones similar to the Micmacca Breccia Member. The post-transgressive clastic deposits of the Feijas Internes Group and Tabanit Group are the main host for the volcanic activity. They are composed of coarsening-upward sequences, which reflect gradual transitions from offshore to shoreface and foreshore facies.The alkaline volcanism comprises four main phases, which are induced by the SW-vergent tilting of a basement block during extensional tectonics. Phase I starts with the emplacement of diatremes within the Jbel Wawrmast Formation and Jbel Afraou Formation reaching an estimated depth of ca. 200 m. They are the result of phreatomagmatic eruptions caused by the rise of basaltic magma into the coastal environment of the host. Phase II is characterised by the emplacement of large volumes of basaltic lava through fissure eruptions and the formation of a lava delta in the eastern part as well as subaqueous to emergent phreatomagmatic volcanism in the middle and western part of the Ouinguigui area. Wave activity led to the formation of volcanogenic conglomerates and sandstones. Phase III and IV are reflected by the emplacement of subvolcanic intrusions consisting of trachy andesites and trachytes.Whole rock geochemical data of 13 samples confirm the alkaline character of the Ouinguigui magmatism, which derived from a garnet lherzolite source of within-plate volcanism.