The 13 Ma Monts Ballons alkalic plutonic suite, Kerguelen Archipelago, Indian Ocean

The 12.955 ± 0.011 Ma (U-Pb zircon) Monts Ballons plutonic suite comprises several small alkalic intrusions (0.3 km²) that were emplaced into older (~25 Ma) flood basalts in the central region of the Kerguelen Archipelago, a major oceanic island in the southern Indian Ocean related to magmatic activity associated with the long-lived (130 million years) Kerguelen mantle plume. The Monts Ballons intrusions contain a suite of rocks ranging from amphibole gabbros to nepheline syenites (silica-undersaturated suite) and are amongst the most alkalic rock types on the archipelago [1]. They represent the products of small degrees of melting of the Kerguelen mantle plume source. Samples from the Monts Ballons suite are enriched in highly incompatible trace elements with sub-parallel normalized trace element patterns. They define a small range of radiogenic isotopic ratios, and lie at the low end of Hf-Nd-Pb isotopic fields in comparison to the majority of volcanic and plutonic rocks on the archipelago, with average values at 13 Ma of ɛ_Hf=-0.8±1.5, ɛ_Nd=-1.2±07 and ²⁰⁶Pb/²⁰⁴Pb=18.2. Values of ⁸⁷Sr/⁸⁶Sr at 0.7052±2 fall within the range of the composition of the enriched Kerguelen plume as defined by basalts from the Courbet Peninsula [2]. The Monts Ballons compositions bridge a gap between the radiogenic compositions of the 25-24 Ma mildly alkalic flood basalts from Mt. Crozier and Baie Charrier and the less radiogenic lavas of the young (<10 Ma) alkalic rocks of the Upper Miocene Series and the recent Mt. Ross stratovolcano [3]. The Monts Ballons chemistry does not show any evidence for continental input to account for these enriched signatures typical of the Kerguelen plume. The magmas that crystallized to form the high-level Monts Ballons plutonic suite derive from the source of the enriched Kerguelen mantle plume and subsequently interacted and assimilated some of the underlying Cretaceous Kerguelen Plateau during ascent. [1] Scoates et al. (2006) Contrib. Min. Petrol. 151, 582-599. [2] Weis et al. (2002) Geochem. Geophys. Geosyst. 7, doi: 10.1029/2001GC000251. [3] Mattielli et al. (2002) J. Petrol. 43, 1327-1339.