The Salazie Cirque Ignimbrite (Piton Des Neiges Volcano, Reunion Island)- Chronostratigraphy, Description And Significance Of Lithic Fragments And Eruptive Mechanisms

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Journal Article: The Salazie Cirque Ignimbrite (Piton Des Neiges Volcano, Reunion Island)- Chronostratigraphy, Description And Significance Of Lithic Fragments And Eruptive Mechanisms

Abstract
An ignimbrite sequence in the Salazie Cirque is characterized by a pyroclastic-flow unit composed of basal breccias (layers A1 and A2), a welded tuff (layer B), and an upper ashy layer (layer C). This eruption began a new eruptive cycle in the Piton des Neiges volcanic complex, the second stage of a differentiation series characterized by the emission of undersaturated lava. The paleogeography of the volcano's eastern flank is reconstituted at about 150,000 y. B.P. from the distribution of ignimbrite deposits that partially fill two large erosion cirques, the Salazie protocirque and the Marsouins protocirque. The total volume of these deposits is only a few km3, and no evidence indicates a related caldera collapse. Basal breccias are composed of: (1) juvenile magmatic clasts yielding a mugearitic composition and normative nepheline; and (2) xenoliths representative of the different units in a 1500-2000-m-thick volcanic pile which include differentiated lavas (first stage of differentiated series) basaltic lava flows and plutonic rocks ("oceanite" series). The appearance of hydrothermal parageneses only in subsurface facies emphasizes the striking role of volcanotectonic structures in mineralized-fluid circulation. The common features of magma mixing (abnormal mineralogical associations, reverse zonings, disequilibrium features) in layer B are interpreted as resulting from the assimilation of highly differentiated foreign rocks by the mugearite magma during and after the course of the eruption. The Salazie Cirque ignimbrite is of the overflowing type (non-Plinian) characterized by high emplacement temperatures. A model is proposed for its formation: (1) rising of a mugearitic eruption column; (2) vesiculation of magma at shallow depth beneath the plug; (3) increase of magma pressure and eruption of a mixture of gas, magmatic blocks and xenoliths down slope; deposition of fines-depleted heterogenous lithic-rich flow (layer A1); (4) explosion (flashing) of the gas-rich upper part of the magma column and eruption of the main welded tuff unit with segregation of a basal non-fines-depleted lithic concentration zone during flow (layer A2) and elutriation of an ash-cloud fall (layer C); (5) effusive phase.

Authors 
Philippe Rocher and Denis Westercamp








Published Journal 
Journal of Volcanology and Geothermal Research, 1989





DOI 
Not Provided
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Citation

Philippe Rocher,Denis Westercamp. 1989. The Salazie Cirque Ignimbrite (Piton Des Neiges Volcano, Reunion Island)- Chronostratigraphy, Description And Significance Of Lithic Fragments And Eruptive Mechanisms. Journal of Volcanology and Geothermal Research. (!) .