METASOMATIZED REFRACTORY UPPER MANTLE AND DEEP MAGMATIC INTRUSIONS BENEATH THE KERGUELEN ISLANDS REVEALED BY ULTRABASIC AND BASIC XENOLITHS

摘要

The Kerguelen plateau, the second largest in the world, is ihe result of an interaction between the South East Indian Ridge and the Kerguelen plume. Alkaline lavas of ihe kerguelen archipelago contain exceptional suites of ultrabasic and basic xenoliths. Three types of xenoliths are distinguished. Type Ⅰ consists of refractory mantle spinel bearing harzburgites and dunites which are variably metasomatized. U-shaped REE pattems of the prologranular and Mg augite bearing poikilitic harzburgites reflect mantle metasomatism. The dunites have similar chemical characteristics to poikilitic harzburgites and result from interactions between the harzburgitic mantle and the percolating basaltic melts. Type Ⅱ are deep basaltic cumulates of "tholeiitic-lransitional" or alkaline affinities, which have been reequilibrated in the granulite facies, i.e. in the P-T conditions of the oceanic lower crust and upper mantle. Type Ⅲ are genuine crustal magmatic cumulates with either hydrous alkaline or anhydrous tholeiitic characteristics. All the characteristics of the ultrabasic-basic xenolites from the Kerguelen archipelago are in accordance wiui the existence of a thickened crust, proposed on the basis of seismic studies (14-24 km). That thickening of the oceanic erust may be generated by intrusions of magmas at different levels of the lithosphere, particularly near the Moho discontinuity (underplating) and it may be responsible for the oceanic granulitic metamorphism observed in the type Ⅱ metacumulates.