Age of the volcanoes of the Columbretes Islands (Western Mediterranean)

Abstract

The Columbretes Islands, compose the only volcanic archipelago in the western Mediterranean, and are located on the eastern edge of the Ebro continental shelf, 50 km to the east of the coast of the Iberian Peninsula, and constitute the emerging part of the Columbretes Volcanic Field. Only one volcanic structure is recognised on the surface, corresponding to Columbrete Grande, in which successive phases of hydromagmatic and strombolian volcanic activity are identified. The bathymetric analysis also identifies other monogenetic volcanoes, aligned according to two strikes: a main N170º E direction (formed by more than seven volcanic centres of phonolitic composition and which, at present, appears as small islets) and a second direction, N55º E (which belongs to the two volcanic centres of basic composition, one of which is Columbrete Grande). New K/Ar radiometric dating, along with the reinterpretation of the existing data and current volcanological criteria, allows two periods of main volcanic activity to be recognised: an older period with a basanitic composition (from 0.7 to 0.8 Ma) and a second period, of essentially phonolitic composition (from 0.2 to 0.3 Ma). The well-preserved morphology of submarine volcanoes indicates that volcanic activity has continued until much more recent times.

Resumen

Las islas Columbretes, el único archipiélago volcánico del Mediterráneo occidental, están situadas en el borde oriental de la plataforma del Ebro, a 50 km al este de la costa de la Península Ibérica, y constituyen la parte emergida del campo volcánico de Columbretes. En superficie sólo se reconoce una estructura volcánica, la correspondiente a Columbrete Grande, en la que se identifican fases sucesivas de actividad volcánica hidromagmática y estromboliana. El análisis batimétrico permite identificar además otros volcanes monogenéticos, alineados según dos directrices: una dirección principal N170ºE, formada por más de siete centros volcánicos, de composición fonolítica y que, en la actualidad, aparecen como pequeños islotes. A la segunda dirección, N55ºE, pertenecen los dos centros volcánicos de composición básica, uno de los cuales es Columbrete Grande. Nuevas dataciones radiométricas K/Ar, junto la reinterpretación de las existentes con criterios vulcanológicos actuales, permiten reconocer dos periodos de actividad volcánica principal: uno más antiguo de composición basanítica entre los 0.7 y los 0.8 Ma y uno segundo, esencialmente fonolítico de 0.2–0.3 Ma. La morfología bien conservada de los volcanes submarinos indica que la actividad volcánica ha continuado hasta tiempos mucho más recientes.

Appendix: sample site locations and rock descriptions

1.1 Navarrete islet

Sample 12,483. (39º 53′ 22′′ N–0º 40′ 00′′ E) Olivine-pyroxene basalt (Basanite TAS). Porphyritic rock, with micro to cryptocrystalline groundmass. Phenocrysts of hypidiomorphic olivine, slightly zoned clinopyroxene and idiomorphic plagioclase. The sample does not present palagonization nor alteration of any kind.

1.2 Bergantín islet

Sample 12,441. (39º 51′ 05′′ N–0º 40′ 18′′ E) Trachyte (Phonolite TAS). Porphyritic rock. Phenocrysts of sanidine-anortoclase and amphiboles with oxidized rim. Trachytic groundmass of feldspar and opaques. Small sphene crystals are also frequent. The sample does not present any kind of alteration.

1.3 Lobos islet

Sample 12,448. (39º 52′ 31′′ N–0º 40′ 18′′ E) Trachyandesite (Tephriphonolite TAS). Porphyritic rock, cryptocrystalline groundmass. Tabular phenocrysts of alkaline feldspar and crysts of amphibole with opacitic rims. There are xenocrysts of anhedral olivine, partially transformed into iddingsite and with rims reaction.

1.4 Ferrera islet

Sample 12,433. (39º 52′ 30″ N–0º 40′ 14″ E) Trachyte. Microcrystalline rock formed almost exclusively of anortoclase-sanidine crystals. Trachytic groundmass with the same minerals and opaques. The mafic minerals are in an accessory proportion.