Abstract: Presses Universitaires de Bordeaux

Abstract: The Rallier-du-Baty Peninsula forms the southwestern part of the Kerguelen Archipelago (Indian Ocean), whose magmatic activity is related to the long-lived 115-Ma Kerguelen plume. The peninsula is mostly made of alkaline rocks constituting two well-defined ring complexes. This paper focuses on the northern ring complex, which is not yet known. Recent field studies have revealed seven discrete syenitic ring dykes ranging in age from 6.2 to 4.9 Ma, and two later volcanic systems. 40Ar/39 Ar dating of a trachytic ignimbrite linked to the Dôme Carva volcano complex yields an age of 26 ± 3 Ka. This represents the last major eruptive event on the Kerguelen Archipelago. The volcanism is bimodal with trachybasalts and trachyandesites constituting the mafic lavas and trachytes and rhyolites constituting the felsic lavas. The volume of erupted felsic magma is by far the larger, and is represented by abundant pyroclastic deposits and lava flows. Boulders of plutonic rocks are found to the northwest of Dôme Carva, and represent intermediate rocks (i.e. monzogabbros and monzonites) that are not present at the surface. Basic rocks are mostly trachybasalts and trachyandesites, while true basalts are scarce. Their mineralogy consists chiefly of plagioclase, olivine, diopside and oxides. Sieve-textured plagioclase is common, as well as corroded olivine and diopside phenocrysts. Peralkaline commenditic trachytes are the most abundant type of acid volcanic rocks. They consist of abundant sanidine, augite and magnetite phenocrysts and interstitial quartz, aegerinic pyroxenes and Na-amphiboles. Ring dykes of quartz-poor alkali feldspar syenites display the same mineralogy, except hornblende is common and replaces diopside. Hornblende is particularly abundant in intermediate monzogabbros. Major and trace element variations of volcanic rocks emphasise the predominant role of fractional crystallisation with a general decrease of MgO, CaO, P2O5, TiO2, FeO. Ba, Sr and Ni from basic to felsic rocks. However, the scattering of the data from the basic rocks indicates that other processes have operated. The overall evolution from trachyte to rhyolite is in agreement with the fractionation of sanidine as the major control. An increase of incompatible elements from trachyte to rhyolite is observed. The felsic lavas display an increase of 87Sr/86Sr(i) without any significant variations in the Nd isotopic composition. The genesis of the basic rocks is complex and reflects concomitant processes of fractional crystallisation, mixing between different basic magmas and probable assimilation of Ba-rich oceanic crust. Major and trace element modelling confirms the possibility of producing the trachytes through continuous differentiation from a basaltic alkaline parent. Discrepancies observed for some trace elements can be explained by the crystallisation of amphibole at an intermediate stage of magma evolution.