Neogene Evolution of Canary Island Volcanism Inferred from Ash Layers and Volcaniclastic Sandstones of DSDP Site (Leg 47A)

Miocene to Pleistocene volcaniclastic sediments were analyzed from Holes 397 and 397A, northwest of Cape Bojador (Northwest Africa) and south of the Canary Islands.These sediments are of two types: (1) ash-fall deposits of silicic and alkalic (trachytic-phonolitic-rhyolitic) composition, and (2) deposits of submarine volcaniclastic mass flows (debris flows, turbidity currents) of basaltic composition.Air-fall ash layers (mostly 0.5 to 5 cm thick) occur at about 19 m.y.B.P. but most are found in sediments ranging from about 14 to 0.3 m.y.They consist of colorless to brown tricuspate and pumice shards, feldspar, and mafic phenocrysts, and small felsic rock fragments.Refractive indexes of about 1.52 to 1.54 suggest rhyolitic to phonolitic compositions.Glass is replaced by palagonite, clay minerals, zeolites etc. in rocks older than about 14 m.y., while the younger shards probably all have been hydrated and chemically changed to some degree.Some ash layers are completely zeolitized (phillipsite, clinoptilolite).Two middle Miocene volcaniclastic sandstones, a lower 7.8-meter-thick turbidite (V-3) and an upper 4.5-meter-thick debris flow (V-l), are indistinctly graded.The rocks of flow V-3 are fine to medium sand-sized hyaloclastites with dense to moderately vesicular, palagonitized "sideromelane" shards, minor igneous rock fragments (microgabbro, basalt, tachylite), and small amounts of nonvolcanic rock fragments, detrital quartz, and biogenic debris.The matrix is dominated by clay minerals (smectite) with minor admixtures of carbonate and zeolite (analcime).V-4 is a 20 to 30 meter thick early Miocene hyaloclastite-rich sandstone.The upper unit (V-l) is very poorly sorted and coarse grained.It consists chiefly of rounded to angular, generally non-vesicular tachylite to fully crystallized basalt and trachyandesitic rock fragments.Admixtures are trachyte, microgabbro, highly vesicular basaltic pumice shards (now replaced by clay and carbonate), and minor non-volcanic clastic and biogenic debris.The partly recrystallized matrix consists of brown clay and, less commonly, carbonate.Clinopyroxene (titanaugite) is common as phenocrysts in the rock fragments and as single crystals; clinopyroxene shows only minor replacement.Olivine, next in abundance, is always replaced by layered silicates or carbonate.Plagioclase is the most common groundmass constituent of the rock fragments and is mostly albitized or replaced by clay minerals.Mineral composition (Ti-augite!) and paragenesis show that alkali basalts and related derivative rock types typical of the volcanic rocks of the Canary Islands are the dominant or only source rock for the igneous components.Good rounding, oxidation textures in olivine, and a high ratio of crystalline relative to tachylitic and glassy basaltic fragments indicate that most or all volcanic fragments in the sandstone of the upper flow (V-l) are of subaerial, epiclastic derivation.Age and mineralogical composition of rock fragments strongly suggest derivation from the eastern (Fuerteventura) rather than central (Gran Canaria) Canary Islands.The older (ca.19 m.y.B.P.) ash fall might have been supplied by uplifted emergent plutonic and Cretaceous sedimentary rocks), and (b) the subaerial shield-building stage of Fuerteventura between about 16 to 17 m.y.B.P. (unit V-l: subaerial erosional detritus andtachylite from lavas advancing into the sea).Volcanic episodes following the shield-building stages were much less voluminous on all of the Canary Islands, although development of highly differentiated magmas resulted in abundant explosive activity producing numerous ash layers particularly during the middle to late Miocene (14 to 9 m.y.), early Pliocene (4 m.y.), and late Pliocene to Pleistocene (3.3 to 0.3 m.y.B.P.).