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Title: The evolution of the early volcanic rocks of Reunion
Author: Laux, Stephen John
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1976
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The island of Reunion lies in the western Indian Ocean, 700 km. east of Madagascar and approximately 1200 km. west of the mid-Indian Ocean Ridge. With the islands of Mauritius 170 km. R. and Rodriguez 800 km. S., it forms the geographic group known as the Mascarene Islands. Reunion appears to have developed away from the mid-Indian Ocean Ridge and independently from the other Mascarene Islands and the Mascarene Plateau to the north-east. The island is built of two volcanoes; Piton des Neiges (3,069 m.) an extinct volcano forms the north-western two-thirds of the island, while the active volcano of Piton de la Fournaise (2,631 m.) occupies the south-eastern part. Deep erosion on Piton des Neiges has exposed an older series of faulted, altered rocks termed the Older Oceanite Series, unconformably underlying mainly fresh, gently dipping lavas of the Younger Oceanite Series. Both series consist of predominantly olivine basalts and constitute the main shield-building phase of Piton des Neiges, which is overlain by a capping of more varied products ranging from basalt to trachyte, constituting the differentiated series. The Older Oceanite Series extrusive rocks, consisting of breccias and subordinate 'lobate' lavas, were erupted into a shallow submarine environment at depths of less than 500 m. However, much of the magma was intruded at shallow levels as parallel sheet swarms and dyke networks from fissures trending in a mid - S to N.N.L. - S.S.W. direction. Following and partly contemporaneous with this activity, there was rapid uplift of at least 1900 m. in the late Pliocene/Pleistocene, associated with vertical movement along rejuvenated, N.S. - S.W. trending, late Cretaceous transform fault lines, passing north and south of Reunion. Rapid uplift was succeeded by deposition of the Younger Oceanite Series malta on to faulted platforms of the older rocks. In spite of their different environments of eruption, the Older and Younger unite Series are believed to have been originally virtually indistinguishable in their range of compositions and mineralogy and can thus be treated as a whole in any petrogenetic model. Studies of trapped silicate melt inclusions in olivine reveal that primititmas ancestral to the Oceanite Series were probably picritic liquids. Their K, 2-rich character could be explained by previous high-pressure crystal fractionation e.g. eclogite, or more probably by small amounts of partl-:1 meltin E 3: mantle material containing minor amounts of a o, Ti-rich phase, possibly biogopite. The picritic, primitive magmas accumulated at depths of circa 15 km. and nuer, in a system of small magma chambers and ramifying channels. Crystallise-ti rid fractionation of olivine with small amounts of chromite, immiscible Ni-Fe nlphide, clinopyroxene and plagioclase formed on eruption, a suite of transitior boleiitic olivine basalts, containing associated dunite and occasional wehrlite and ga bbro inclusions. A limited development of hydrous conditions in the subvolcanic zone led to crystallisation of amphibole gabbros, which were subsequently triated as inclusions in basalts during explosive eruption. Periodically, small quantities of main migrated into the volcanic superstructure and differentiated to form relatively evolved gabbroic bodies. Eventually, the main magma supply was diverted south-east from Piton des Neiges to Piton de la Fournaise. A large body of magma has trapped about 50 - 1' below the surface of Piton des Neiges and underwent olivine-clinopyroxene fractition to produce a magma parental to the Differentiated Series eruptives. During this period of quiescence, the Older Oceanite Series and nasal part S the lounger Oceanite Series near to the volcanic centre were metamorphosed in the zeolite and lower greenschist faciee. Zones of zeolitisation were developing around the centre of the volcano. The distribution of these was dependent on the burial proximity to volcanic conduits and intrusive centres and the circulating friction of hot fluids. The onset of metamorphism may have been linked with the establishment of a large magma chamber below Piton des Neiges.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available