The island of Réunion in the Indian Ocean consists of two volcanic massifs: the active volcano, Piton de la Fournaise; and Piton des Neiges, which is now extinct. During the shield-building stage (Phases I and II) of Piton des Neiges, olivine-phyric, transitional alkali basalts were erupted. The products of Phase I and II magmatism constitute 90% of the edifice by volume. The focus of magmatism migrated to Piton de la Fournaise c. 350,000 years ago. Simultaneously, Piton des Neiges entered Phase III when it started to erupt evolved magmas, producing the first Differentiated Series lavas. Some 220,000 years ago Piton des Neiges magmas became sufficiently evolved to erupt explosively; these eruptions mark the base of Phase IV, the subject of this study. The products of Phase IV consists of mugearite to quartz trachyte lavas , intrusives, and pyroclastic deposits; the most recent explosive eruption may have occurred as little as 12,000 years ago. There are two categories of pyroclastic formation: (i) heterogeneous deposits (Salazie Formation, St Louis Formation, Dalle Soudée Formation), which show evidence for mixing between mugearite or benmoreite and quartz trachyte magmas; and (ii) homogeneous deposits (St Gilles Formation, St Pierre Formation), containing aphyric, geochemically homogeneous, benmoreite to quartz-trachyte pumices, with little or no evidence for magma mixing. Compositions of juvenile material from the heterogeneous deposits lie on a coherent whole-rock fractionation line. The Salazie Formation is zoned, with a mixed benmmoreite-trachyte lower flow-unit and a mugearite upper flow-unit. Evidence for magma mixing in the heterogeneous deposits includes: co-existence of fiamme/pumices with different compositions and phenocryst populations; reverse zoning in phenocrysts; and banded pumice. The most evolved material in any deposits is enriched by a factor of 2 to 5 in incompatible elements (REE, LILE, REE, K and Na), and is similarly depleted in compatible elements (e.g. Mg, Ca, transition metals). Post-magmatic alteration has affected pumices from the homogeneous deposits, and accounts for much of the observed within-deposit variation. The homogeneous deposits have lower Ca, P, Mn, Fe, Na, Ba, Sc, and Zn; and higher Al, Sr, Rb, Pb, Cu, and Zr for the same SiO<SUB>2</SUB> than those in the heterogeneous deposits.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:660285 |
| Date | January 1994 |
| Creators | Page, Barbara Anne |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/15571 |
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