KERGUELEN - CONTINENTAL FRAGMENT OR OCEANIC ISLAND - PETROLOGY AND ISOTOPIC GEOCHEMISTRY EVIDENCE

The Rallier du Baty peninsula in the southwestern part of the Kerguelen archipelago contains five ring complexes composed of rocks of syenitic and granitic affinities. These ring complexes are mutually intrusive and intrude into a series of basaltic flows of about 20-25 m.y. in age. Geologic studies show that these complexes progressively become younger to the north. The main rock types of the ring complexes consist of amphibole syenites and biotite syenites representing two magmatic suites, an peralkaline and an peraluminous type. The isotopic study of the southern syenite-granite ring complex of the Rallier du Baty peninsula is presented. A very precise chronology of the intrusions (A, outermost ring, to H, innermost ring) is given: outer gabbros: K-Ar age = 15-13 m.y. ring A: K-Ar age = 12 m.y. ring B: whole rock Rb-Sr age = 9.7 ± 0.2 m.y. (2σ) ring C: whole rock Rb-Sr age = 8.6 ± 0.1 m.y. (2σ) rings D,E,F,G,H: whole rock Rb-Sr age = 7.9 ± 0.2 m.y. (2σ). The close grouping of initial 87Sr/86Sr ratios (0.7057) indicates that different ring complexes have the same source regions. Furthermore, the time interval between production of melt and final crystallization can be estimated at several 105 years. As the age differences between ring complexes are about 1 m.y., the different liquids cannot be differentiates of the same magma. The initial ratios of basic plutonic and volcanic rocks from the archipelago range from 0.7041 to 0.7077 with strong grouping around 0.7057. When plotted in a 207Pb/204Pb vs. 206Pb/204Pb diagram, the lead isotope ratios of all igneous rocks from these islands form a linear array similar to the results from other oceanic islands. Strontium and lead isotope data for all Kerguelen rocks suggest that most of the basic plutonic and volcanic rocks as well as the syenite-granite plutons are probably derived from a similar source located in an undepleted mantle. © 1979.