UBC Theses and Dissertations
Petrology and geochemistry of the 25 MA Mt. Marion dufresne basaltic section on the Kerguelen Archipelago : constraining the transition from tholeiitic to mildly alkalic volcanism on a major oceanic island Annell, Heidi
Flood basalts on the - 6500 km² Kerguelen Archipelago in the southern Indian Ocean formed in an intraplate location on the Antarctic plate between 29-24 Ma as the Southeast Indian Ridge migrated from - 225 to - 400 km northeast relative to the Kerguelen hotspot. A - 700 m section of basalts exposed at Mt. Marion Dufresne, in the southern part of the Plateau Central, contains both transitional-tholeiitic and mildly alkalic lavas (A.I. -0.8 to 2.0) that are geochemically and isotopically variable, and captures an important period of time (-1 Myr) near the end of flood basalt volcanism on the Kerguelen Archipelago. The basal 300 m of the section consists of low-MgO (<5.2 wt. %) predominantly aphyric lavas characterized by a limited range of "enriched" isotopic compositions (¹⁷⁶Hf/¹⁷⁷Hf =.0.28282-0.28287; ⁸⁷Sr/⁸⁶Sr = 0.7048-0.7050; ¹⁴³Nd / ¹⁴⁴Nd = 0.5126-0.5127; ²⁰⁷Pb / ²⁰⁴Pb = 15.54-15.56) similar to compositions observed in the enriched 24-25 Ma mildly alkalic lavas to the east and southeast (e.g. Mt. Crozier). This aphyric sequence of basalts appears to represent a "steady state" magmatic system where differentiation occurred primarily in a solid-dominated (mush or slurry) environment. Highly calcic (>An ₈₀) plagioclase compositions observed in intercalated plagioclasephyric and ultraphyric basalts from the upper part of this interval signal a change in crystallization environments to relatively shallow (-5-6 km) magma storage and relatively hydrous conditions (>3 wt.% H₂0 ). The low-MgO basaltic lavas are overlain by a thick (-400 m) succession of olivine-phyric high-MgO basalts (7.1-11.4 wt.%) that is interpreted to represent an interval of increased magma supply 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. Unlike the low-MgO lavas, these high-MgO basalts span a range of "depleted" isotopic compositions (⁷⁶Hf/¹⁷⁷Hf = 0.28292-0.28300, ⁸⁷Sr/⁸⁶Sr = 0.7040-0.7046, ¹⁴³Nd / ¹⁴⁴Nd = 0.5127-0.5128, and ²⁰⁷Pb / ²⁰⁴Pb = 15.49-15.52) and represent the first documented occurrence of a depleted mantle component in mildly alkalic basalts from the Kerguelen Archipelago. The presence of this depleted mantle component, which is compositionally similar to that observed in the older 28-29 Ma basalts in the northern part of the archipelago, reflects a short term increase in the extent of melting beneath the oceanic lithosphere and renewed interaction between Southeast Indian Ridge MORB-type asthenosphere and the enriched component of Kerguelen mantle plume at 25 Ma.
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