Geochemical and petrological evolution of La Palma (Canary Islands) and its rift zone during the last 1.0 Ma

Abstract

The aim of this thesis is to better understand the geochemical and petrological evolution of a rift zone in space and time focusing on La Palma (Canary Islands) as a case study. In order to model the magmatic evolution of the rift zone. Major, trace elements and radiogenic isotope compositions of representative rocks from the last 1.0 Ma were determined. This timespann encompasses the volcanic activity of the Taburiente shield volcano (1.2 - 0.4 Ma) and the Cumbe Nueva ridge (830 560 ka) as well as the short Bejenado volcanic phase (560 490 ka) which followed the Cumbre Nueva collapse (~560 ka), and the presently active Cumbre Vieja rift phase (125 ka to present).Cpx-melt thermobarometry and microthermometry of fluid inclusions were applied to constrain magma pathways and main levels of magma stagnation. The combined data indicate that two separate magma stagnation levels existed for each rift phase: (1) a main fractionation level within the upper mantle, and (2) intermittent stagnation close to the Moho for the Taburiente volcano and above the Moho for Bejenado, Cumbre Nueva and Cumbre Vieja. A possible relationship of the magma plumbing systems of the extinct Taburiente shield volcano and the active Cumbre Vieja rift can be excluded. Relative depletion in Rb and Ba on mantle-normalized diagrams of fractionation-corrected La Palma basalts require the presence of residual amphibole in their melting region. Model calculations illustrate that the observed rare earth element ratios and the high contents of P, Sr, Th, U and LREE of Cumbre Vieja lavas cannot be derived by a single-stage melting process from amphibole-bearing garnet lherzolite. The compositions require assimilation of ~1% apatite by Cumbre Vieja melts during their ascent through the lithosphere. The geochemical data indicate that there is no progressive geochemical evolution of La Palma magmas but a compositional break between Taburiente volcanism in the north and Cumbre Vieja volcanism in the south.