Chronological evolution of the Kerguelen Islands syenite–granite ring complex

Abstract

ALKALINE syenites in the Rallier-du-Baty peninsula of the Kerguelen Islands1,2 intrude and metamorphose the basaltic lava flows which constitute the major part of the islands3. Recent work4 has revealed the existence, in the peninsula, of five ring complexes consisting mainly of syenites containing from 2 to 15% normative quartz (Figs 1 and 2). The southern complex is the largest, and consists of nordmarkites (ring dykes a, b, c) and of quartz-rich syenites and alkaline granite (ring dykes d, e, f). Some minor intrusive bodies of gabbros (G) and nordmarkites (Σ) are located on the margins of the southern centre (Fig. 2). Chilled margins and enclaves in each individual ring dyke clearly indicate that the sequence of intrusions is centripetal. Furthermore, the contact relationships between the centres show that the magmatic activity has migrated northwards. Previous K–Ar data5 have indicated that a syenite from the southern ring complex was emplaced 8.7±0.9 Myr ago, whereas basaltic flows were erupted during the late Oligocene. An additional K–Ar age of 11.5±0.2 Myr was obtained for a metabasalt collected near the contact with the syenites and was interpreted3 as the minimum value for the time of eruption of the basalts.