Publication: Petrography and provenance of beach sands from volcanic oceanic islands: Cabo Verde, Atlantic Ocean
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Publication Date
2021-03-12
Authors
Le Pera, Emilia
Morrone, Consuele
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Abstract
Volcaniclastic deposits have been extensively analyzed in several settings in the Pacific and circum-Pacific area. Recent volcaniclastic products from Atlantic oceanic islands offer another opportunity to add new data and be an important key to a better understanding of volcanic imprints on the sedimentary record. The Cabo Verde archipelago is an Atlantic Oceanic plateau with late Oligocene to Holocene volcanism. Outcrops consist mainly of mafic and strongly alkaline and ultra-alkaline volcanic (pyroclastic and lava flows) and less abundant intrusive rocks with minor carbonatites and carbonate sedimentary rocks, constituting a multiple-provenance assemblage for the sandy beaches surrounding the islands. Currently, climate is semiarid to hyperarid with ephemeral and intermittent streams. Thirty-six samples of beach sand from six principal Cabo Verde Islands were selected for petrographic inspection. On average, beach sands constitute a volcanolithic petrofacies. A relative increase in carbonate limeclasts and bioclasts dilutes the pure volcaniclastic contribution mainly on the older island beaches (Sao Vicente, Sal, and Boa Vista). The major components of Cabo Verde beach sands are highly variable; in general, composition is a function of island morphological evolution and age. Thus, beaches of the younger islands (Sao Nicolau, Santiago, and Fogo) consist mainly of volcanic lithic fragments, and monomineralic grains of dense minerals such as olivine, pyroxene, and amphibole, and single grains of plagioclase and anorthoclase. By contrast, beaches of older eastern islands (Sal, Boa Vista, and Sao Vicente) contain more calcareous bioclasts, micritic and/or sparitic sedimentary lithic grains. The presence of carbonate grains suggests provenance from shallow carbonate platforms developed during periods of volcanic quiescence. Cabo Verde volcanic sandy fractions are composed mostly of black, brown, and orange glassy volcanic particles exhibiting microlitic, lathwork, and vitric textures. Volcanic particles with lathwork textures are linked to mafic provenance assemblages (nephelinites, basanites, and tephrites). The content of glassy particles is nearly constant in all beaches, and both hydroclastic and epiclastic processes are reflected in these populations of glassy grains. Boa Vista, Sao Vicente, and Santiago beaches contain higher proportions of sideromelane, linked to recent coastal volcanism, and lower proportions of orange and black glassy particles. The concentration of orange glass particles in the beaches of Santiago Island is higher than in the other island beaches. These orange glassy textures have been preserved even if they were sourced from the intensely altered Ancient Eruptive Complex, representing the pre-Miocene seamount stage of Santiago Island. A very small percentage of altered labile monocrystalline grains such as olivine and the paucity of altered volcanic components reflect the weathering-limited erosion regime of the islands. The exposed phonolitic lava flows that occupy only a minor surface part of the inland source produce particles with microlitic texture in sand beaches. Thus, this texture is not exclusive to andesitic, basaltic, and basaltic andesites sources, suggesting the need for a review of these particles as source-sensitive provenance signals.