Harzburgite xenoliths in alkali basalts from Kerguelen islands : refractory mantle affected by multi-metasomatic events above the Kerguelen plume

1. M. Gregoire1 (mgregoir@mq.edu.au) 2. S.Y. O'Reilly1 (sue.oreilly@mq.edu.au) 3. J.Y. Cottin2 (cottin@univ-st-etienne.fr) 4. A. Giret2 (giret@univ-st-etienne.fr)

1 National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia

2 Department of Geology, Jean Monnet University, URA CNRS 10, 42023 St Etienne, France.

Alkali basalts from the Kerguelen islands have entrained many mantle Ca-harzburgites. The Ca-harzburgites always contain small amount of clinopyroxene and spinel and attest to the existence of a refractory upper mantle beneath the Kerguelen islands.

The Ca-harzburgites can be divided into two distinct types : Cr-diopside-bearing protogranular Ca-harzburgites and Mg-augite bearing poikilitic Ca-harzburgites. They bear the imprint of two main types of processes : significant partial melting related to the South East Indian Ridge-Kerguelen plume interaction (45 Ma); and mantle metasomatic events. The first metasomatic event is evidenced by LREE-enriched patterns in both types of Ca-harzburgite and by the presence of the Cr-Na-rich Mg-augite sometimes associated with phlogopite in the poikilitic Ca-harzburgites. Mg-numbers of Ol, Cpx and Opx are offset from model partial melting trends in poikilitic Ca-harzburgites. The poikilitic Ca-harzburgites display a higher temperature range than those of the protogranular ones. These characteristics suggest reactions between previously depleted harzburgites and variable amounts of LREE-enriched basaltic melts. The second type of metasomatism is associated with the formation of the mineralogical association of feldspar + olivine (2) + Ti-chromite + rutile + ilmenite + loveringite + kennedyite (or armalcolite). These minerals occur in reaction zones close to Opx and spinel or as thin veins or dykelets cross cutting the olivines. The fact that the two types of Ca-harzburgites are affected, indicate that this second metasomatic event is relatively recent. The coronitic association feldspar + olivine (2) has been formed by the reaction opx + sp + fluid. The feldspar has variable compositions, but is alkali-rich (K2O : 1-10.5 wt%). It is compositionally similar to those of the lherzolite xenoliths from Hamar-Daban (Baikal-Russia, Ionov et al., 1995). As in this area, the formation of feldspar and other associated minerals is probably related to percolation of an alkali-rich, H2O poor fluid into harzburgites. It is concludes that the two types of metasomatism are related to the long-lived Kerguelen hotspot-activity which induced large amount of magma underplating in the vicinity of the thick oceanic crust-mantle boundary.

Ionov D. A. , O'Reilly S.Y. and Ashchepkov I.V. (1995), Contrib. Mineral. Petrol., 122, 174-190

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