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PETROLOGY OF FLOOD BASALTS AT THE THOLEIITIC–ALKALIC TRANSITION AND PHENOCRYST COMPOSITIONS, MT. MARION DUFRESNE, KERGUELEN ARCHIPELAGO, SOUTHERN INDIAN OCEAN

Heidi Annell^1,¶, James S. Scoates^1,, Dominique Weis¹ and André Giret²

¹ Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada
² Laboratoire de Géologie-Pétrologie, CNRS–UMR 6524, Université Jean-Monnet, F–42023 Saint-Etienne, France

E-mail address: jscoates{at}eos.ubc.ca

We report the compositions of phenocrysts, xenocrysts, and reaction coronas from basaltic lavas sampled from the ~700- m-thick Mt. Marion Dufresne section, located in the southern part of the Plateau Central region of the Kerguelen Archipelago, southern Indian Ocean. Compositional variations are used to constrain environments of phenocryst crystallization, temporal changes in magma activity, and magma-mixing relationships. The basal 300 m of the section consists of predominantly aphyric, mildly alkalic basaltic lavas that are increasingly intercalated with plagioclase-phyric to plagioclase–ultraphyric (up to 60 vol.% phenocrysts ranging from An₆₃ to An₈₆) basalt flows with increasing stratigraphic height. Growth of the plagioclase phenocrysts occurred in relatively shallow (<5–6 km) crystal-rich magma reservoirs, and mixed populations of phenocrysts were collected during periods of renewed magmatic activity and erupted within evolved magmas. Above an elevation of 400 m, there is a thick succession of tholeiitic olivine-phyric (up to 20 vol.%) high-MgO basalts that is interpreted to represent an interval of increased supply of magma and eruptive flux. Olivine – whole-rock Fe–Mg relations indicate that olivine phenocrysts with ~Fo_80–86 are in equilibrium with parental magma compositions of 8–10 wt.% MgO, which represents the maximum MgO content of the melt erupted as lavas on the Kerguelen Archipelago. Three quartz-bearing basaltic andesites that occur within this upper high-MgO succession contain olivine, plagioclase, and quartz phenocrysts. These phenocrysts exhibit extreme disequilibrium textures, including rounded, resorbed quartz surrounded by fine-grained pyroxene coronas, and rounded, reversely zoned, and sievetextured plagioclase, which are consistent with incorporation of a quartz-bearing magma into a high-MgO basaltic magma. The presence of evolved quartz-saturated magmas requires sporadic decreases in magmatic activity to allow for locally extensive fractionation. The phenocryst textures and compositions of basaltic lavas from the Marion Dufresne section documented in this study demonstrate the important role of changing magma-flux conditions in magma-conduit systems beneath the Kerguelen Archipelago at ~25 Ma.

Keywords: ocean-island basalt, phenocryst, olivine, clinopyroxene, high-An plagioclase, plagioclase–ultraphyric basalt, tholeiitic–alkalic transition, Mt. Marion Dufresne, Kerguelen Archipelago.