Olesch, Martin
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Olesch, Martin
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Olesch, Martin
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Item type:Publication, Magmatic and volcanological evolution of the Desertas rift zone (Madeira Archipelago, NE Atlantic)(2004-06-24); ; ; Madeira and the Desertas Islands show two well-developed volcanic rift zones which intersect near the eastern tip of Madeira (São Lourenço peninsula). To examine a possible genetic link, volcanic structures, rock ages and rock compositions were investigated at the Desertas and São Lourenço. Barometric data derived from fluid inclusions and clinopyroxene-melt compositions indicate fractionation at multiple levels within the mantle beneath the rift zones, followed by temporary magma stagnation within the crust. Depths of crustal stagnation beneath São Lourenço and the Desertas differ significantly, and there is no evidence for a shallow magma reservoir feeding both rifts. This suggests that Madeira and the Desertas have separated magma supply systems. 40Ar/39Ar age determinations revealed that Desertas volcanism lasted from >5.1 to 1.9 Ma overlapping in age with the shield phase of Madeira. Eruptive quiescence between 4 and 2.5 Ma at São Lourenço coincides with the main period of volcanism on the Desertas. Major and trace elements and Sr-Nd-Pb isotope compositions of volcanics from São Lourenço and the Desertas overlap with those from Madeira, indicating a common magma source. São Lourenço samples include the most depleted Sr-Nd-Pb isotope ratios, whereas Desertas rocks of similar age show the most enriched ratios, not being compatible with mixing of Desertas and Madeira melts or with lateral magma transport from Madeira to the Desertas. Madeira and the Desertas rather represent two separate volcanoes that root in discrete sectors of the Madeira source. The Desertas volcanism was caused by flexural stresses within the lithosphere due to the loads of Madeira and Porto Santo, and an extensive stress field in-between the overlapping Madeira and Desertas edifices provoked the formation of the Desertas rift. The shift of volcanic activity between Madeira and the Desertas possibly reflects an irregularly shaped plume causing lateral variations in melt production.doctoral thesis273 118 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Geochemical and petrological evolution of La Palma (Canary Islands) and its rift zone during the last 1.0 Ma(2005-06-23); ; ; 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.doctoral thesis349 220