GEMOC ARC National Key CentreAmphibole-phlogopite mantle xenoliths from the Kerguelen archipelago: Evidence of a major metasomatic event by alkaline silicate and carbonate-rich melts.
B.N. Moine1,2, M. Grégoire2, J-Y. Cottin1, S.Y. O'Reilly2 S.M.F. Sheppard3
1. Département de Géologie-Pétrologie-Géochimie, UMR-CNRS 6524, Université Jean Monnet
2. GEMOC Macquarie
3. Département de Sciences de la Terre, UMR-CNRS 8515, Ecole Normale Supérieure de Lyon
Alkali basaltic rocks from the Kerguelen Islands have entrained numerous phlogopite- and/or amphibole-bearing ultramafic to mafic xenoliths. The xenoliths can be subdivided into mantle wall-rocks (harzburgites, dunites, Type I xenoliths) and high pressure magmatic segregates (clinopyroxenites, hornblendites, glimmerites; type II xenoliths) that generally form composite xenoliths with mantle wall-rocks. A phlogopite megacryst-bearing lava was also incorporated in this study.
Chemical compositions of amphiboles and phlogopites from both xenolith groups are characterized by high titanium contents and are similar to those recognized in many ultramafic and mafic volatile-bearing xenoliths from kimberlites, alkali basalts and in peridotites or pyroxenites from orogenic massifs of lherzolites.
Phlogopites show homogeneous trace element compositions and important incompatible trace element fractionation processes characterized by strong enrichment in Rb, Ba, Nb, Ta, Pb, Sr, Ti and slightly in Zr, Hf. Amphiboles display compositional variation between samples but they are commonly high in LIL element and show systematic negative Zr, Hf, Pb and Sr anomalies.
Hornblendite dykelets from composite xenolith display major and trace element compositions closely similar with Kerguelen lamprophyric lavas.
Clinopyroxenes from type I, type II and composite hydrous Kerguelen xenoliths display tight similarities in their trace element contents (figure 1). These later display an enrichment in LREE (La/Yb)N=10 with large negative anomalies in Rb, Ba, Pb, Sr, Ti, Zr and Y. They are also similar to poikilitic harzburgite (anhydrous) clinopyroxene. The calculated liquids in equilibrium with clinopyroxene may be compared with ultramafic and alkaline lamprophyric melts. One sample of phlogopite-bearing dunite displays clinopyroxene with very large anomalies in Rb, Ba, Nb, Ta, Zr, Hf, Pb, Sr and Ti which is characteristic of an interaction with a carbonatitic melt.
The _D value of -86 to -70â versus SMOW for mica and amphibole of both types of Kerguelen xenoliths and of the phlogopite megacrysts are very homogeneous and within the common mantle range. These data are in agreement with neon isotopic data that indicate homogeneous and relatively primitive compositions of the volatile sources.
The phlogopite- and/or amphibole-bearing ultramafic to mafic xenoliths
from Kerguelen Islands may be all related to the circulation of highly
alkaline mafic silicate melts into the upper mantle. Interstitial amphibole
and phlogopite of harzburgites and dunites formed by a diffuse percolation
of such melts within the upper mantle (porous flow). Evidence from composite
xenoliths indicates channellized circulation of similar melts propagated
by hydraulic fracture resulting in a network of dykes in the Kerguelen
upper mantle. The high pressure magmatic xenoliths are evidenced of the
cristallisation of highly alkaline magmas into the upper mantle and the
lower crust. The mica-bearing lavas ("lamprophyres") are the surface expression
of this highly alkaline magmatic activity. The ubiquity of the volatile-rich
high alkaline magmatic activity and its isotopic homogeneity is inferred
to be the latest and largest metasomatic event related to the late intraplate
activity of the Kerguelen mantle plume.
Fig: Incompatible trace element patterns of clinopyroxene (Cpx)
from Kerguelen xenoliths.
