The Early-Cambrian Boho volcano of the El Graara massif, Morocco: petrology, geodynamic setting and coeval sedimentation.
Résumé
A major volcanic episode is recorded across the Neoproterozoic-Cambrian transition in the Moroccan Anti-Atlas. Several volcanic cones are still preserved in the El Graara massif, laterally correlatable with volcanic flows dated as Early Cambrian (U/Pb date of 534 ± 10 Ma). Volcanic ashes and flows are interbedded with the uppermost part of the Adoudou dolostones, whereas the best-preserved volcano (the Boho Jbel) is onlapped by the overlying Lie-de-vin strata. Available petro-geochemical data from the Boho volcano suggest an alkaline magmatism probably derived from low-grade melting of a garnet–lherzolite mantle source, followed by fractional crystallization. The silica-undersaturated basaltic liquid evolved to form oversaturated rocks: the fractionation of a ferromagnesian phase with high-Ca and low-Al contents is suggested as the main process to cut across the critical plane of silica undersaturation in this geochemical series. Although the Boho geochemical patterns are similar to those of some rift emplacements, more data from other coeval magmatic eruptions are necessary to constrain their geodynamic setting. Erosion of the Boho volcano favoured formation of a slope-apron composed of four sedimentary facies belts: chaotic megabreccia (related to downslope mass movements of rigid blocks), amalgamated breccia sheets (emplaced by viscous debris flows), a heterogeneous terrigenous belt (representing offshore substrates interrupted by channels intersected by cross-bedded shoals), and variegated shales and stromatolitic dolostones (typical of the Lie-de-vin Formation). Sharp changes in sedimentation rate were associated with modifications in paleorelief sloping and transport mechanisms from subaerial (?) rock fall at the foot of the cone escarpment to sheet-like debris flow on the slopes, and the replacement by sedimentation under wave and storm influence. The presence of an active carbonate productivity, recorded in the primary porosities of the Boho slope-apron, is suggested by widespread development of a robust, coelobiontic, microbial carbonate factory resilient to poisoning by terrigenous influx. Diagenesis in these deposits includes marine, meteoric and deep-burial cementation of calcite, dolomite, iron oxides, quartz, feldspar, and celestine, the latter indicating precipitation from hypersaline pore fluids.
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