GEMOC ARC National Key CentreTRACE ELEMENTS AND D/H GEOCHEMISTRY OF PHLOGOPITE AND AMPHIBOLE-BEARING ULTRAMAFIC AND MAFIC XENOLITHS FROM KERGUELEN ARCHIPELAGO (TAFF): CHARACTERISTICS OF MELTS PERCOLATING INDIAN OCEANIC LITHOSPHERIC MANTLE
B.N. Moine1,2, J.Y. Cottin1, S.M.F. Sheppard3, M. Gregoire2, S.Y. O'Reilly2 AND A. Giret1
1. Dept. de Géologie UMR 6524 du CNRS, Université J. Monnet,
23 rue Dr. P. Michelon, F-42023 cedex 02 Saint-Etienne, France
2. GEMOC Macquarie
3. Lab. des Sciences de la Terre, ENS-Lyon, 46 allée d'Italie,
F-69364 Lyon cedex 07
Detailed petrological, mineralogical and geochemical studies of
hydrous silicates bearing ultramafic-mafic xenoliths from Kerguelen Island,
have been undertaken for two main reasons :
* The Kerguelen plume is remarkable among mantle plumes because its
voluminous and long lived (115 Ma) activity and of its occured in various
geotectonic settings related to the spreading of the Indian Ocean and
* mantle metasomatism processes play an important role in the concentration,
the distribution and the fractionation of incompatible trace element in
the upper mantle. Among xenoliths we can recognize phlogopite-bearing and
phlogopite-amphibole-bearing dunites, harzburgites and various type of
composite xenoliths rich in phlogopite and/or amphibole (lherzite, phlogopite-bearing
clinopyroxenite, hornblendite).
Amphibole displays higher LILE content than clinopyroxene. It is characterized by large Zr, Hf, Pb and slight Ta negative anomalies. Amphibole in the veinlets commonly has much higher HFSE content than amphibole disseminated in mantle dunitic wall-rock and increase in smaller veinlets.
Phlogopite displays very low REE, Th, U and Y and very high RB, Ba, Pb, Sr and Ti contents. Its shows very large positive Nb, Ta anomalies and slight positive Zr and Hf anomalies.
The dD values of ?65 to ?96â vs SMOW for mica and amphibole are within the accepted mantle range. These Deuterium data are in agreement with neon isotopic data which seem to indicate a relatively primitive and homogeneous composition of miscible or immiscible fluids in percolating melts.
Amphibole and/or phlogopite-bearing mantle xenoliths from Kerguelen archipelago show that the most important metasomatic agent affecting the Kerguelen upper mantle are high alkaline melts which have affinities with oceanic lamprophyric magmas.
