Petrological and geochemical evolution of the basaltic growth stages of La Gomera, Canary Islands

La Gomera is the only island of the canaries without Quaternary volcanism, and compared to the others it is the geologically least known, specially regarding its subaerial volcanic growth stages. In order to reconstruct its volcanic evolution we report a comprehensive data set including new field observations, mineral chemistry, whole rock major and trace element composition for lavas from the two subaerial volcanic edifices, the Old Edifice and the Young Edifice. The Old Edifice was built up in two main stages, starting with the Lower Old Edifice (LOE; 10.5- 8.7 Ma) represented by a large basaltic shield and followed by the growth stage of the Upper Old Edifice (UOE; 8.6- 6.2 Ma), composed of a thick succession of basalt and trachybasalt lava flows and pyroclastics, with two important trachytic to phonolitic episodes associated. The Young Edifice was built up in two main stages too: the Young Edifice I (YE-I: 5.9-4.9 Ma) and the Young Edifice II (YE-II: 4.7-2.3 Ma). The first stage is represented by lava flows, flowing essentially south and south-eastwards and ranging between basalts and trachyandesites (Young Basalts I). The second stage is mainly made up by a thick sequence of horizontal picrobasalt-trachyandesite lava flows covering the central area and parts of the NE and SW sectors (the Young Basalts II), but also by felsic domes, and the Trachytic Unit, composed of basaltic trachyandesites, trachyandesites and trachytes lava flows. The subaerial volcanic activity of La Gomera is characterised by alkaline rocks ranging from picrites to trachytes as a whole. However, each of the main growth stages shows specific compositional characteristics. As for the petrographic and geochemical characteristics, the LOE is the less variable suite (MgO=15.4-4.5 %). The other units display a greater variation range: 18.6-2.9% MgO in the UOE mafic suites; 13.2-1.2% MgO in the Young Basalts I; 14.4-1.5% MgO in the Young Basalts II and 3.6-0.6% MgO in the Trachytic Unit. The mafic rocks are alkaline OIB basalts coming from a very similar mantle source, that is enriched in incompatibles elements (Ta, Nb) at least two or three times the primordial mantle concentration, and less in Th, U and LREE, being the basalts of the LOE the most enriched and the UOE ones the least enriched. Most of the incompatible elements ratios are typical of HIMU mantle reservoir, as the ratios of Ba/Th vs Th/Nb. We detected differences in the rate of partial melting and in the fractioned crystallization processes. The rate of partial melting decreases from LOE to UOE basalts, whereas it increases from YE I to YE-II basalts, with the maximum degree during the LOE eruption. In addition, the graphic and numerical modelling of the fractioned crystallization processes allows us to detect the differences between the Old basalts and the Young basalts. The data presented support the important differences between the basaltic growth stages of La Gomera, including the both Young Edifice ones, and point out that its volcanic evolution is most complex that the until now accepted.