Eisele, Steffen

Freundt, Armin

Kutterolf, Steffen

Ramalho, R.

English

OceanRep

The phonolitic Cão Grande Plinian volcanism on Santo Antão, Cape Verde Islands:When, Where, HowSteffen Eisele1, Armin Freundt1, Steffen Kutterolf1, Ricardo Ramalho21GEOMAR Helmholz Centre for Ocean Research Kiel, Germany, 2Lamont-Doherty Earth Observatory,Columbia University, USAE-mail: seisele@geomar.deSanto Antão is an ocean island shield volcano in an early post-erosional stage. At this stage, mafic Strombolianvolcanic activity had been replaced by Pleistocene highly explosive eruptions of evolved, phonolitic magmas. Twophonolitic tephra successions, Cão Grande Pumice units I and II (CGI and CGII) had been previously identified anddescribed (Holm et al. 2006; Mortensen et al. 2009) but our tephrostratigraphic fieldwork on Santo Antão identifieda third, older explosive phonolitic phase (CG 0) and two phonolitic domes. CG 0 comprises eight pumice falloutlayers and two ignimbrites.Using major and trace element glass compositions, mineral compositions and bulk rock analyses, we correlate CãoGrande Group units with ash beds in 14 sediment gravity cores from the ocean floor up to 400 km away from theisland. These correlations demonstrate that CG0, CGI and CGII all comprised large magnitude Plinian eruptionsthat produced tephra blankets dispersed over more than 105 km².While the exact source-vent locations on Santo Antão for CG0 and CGI remain unknown, we found that the Morrode Figueira dome has the same composition as the CGII tephra and thus marks the source vent of that youngestCão Grande tephra, an interpretation supported by the observed lateral variations in thickness and grain size of theCGII fallout tephra.Our combined on-land and marine tephrostratigraphy combined with geochemical and petrological analyses andongoing 40Ar/39Ar-dating of sanidines will allow us to reconstruct in detail the post-shield history of evolved magmaevolution and explosive eruptions of Santo Antão.

The phonolitic Cão Grande Plinian volcanism on Santo Antão, Cape Verde Islands: When, Where, How

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