ORIGIN OF LREE-DEPLETED AMPHIBOLES IN THE SUBCONTINENTAL MANTLE

Ion-microprobe analyses of interstitial kaersutite and Ti-pargasite grains from orogenic peridotites and Iherzolite xenoliths reveal that LREE-depleted amphiboles are common in the subcontinental mantle samples. Incompatibility diagrams for the investigated amphiboles show that REEs almost parallel those of coexisting clinopyroxenes, whereas Sr, Zr, and Ti show variable anomalies (i.e., Sr/Sr* and Ti/Ti* > 1 and Zr/Zr* < 1). In the chondrite-normalized incompatibility diagrams, Sr lies almost a factor of two above Ce and Nd and is usually depleted relative to HREEs. Average amphibole/clinopyroxene partition coefficients for spinel-bearing assemblages range from 1.4-1.8 for LREEs and from 1.8-2.2 for HREEs. Corresponding D values for Zr, Sr, and Ti are about 1, 3, and 5, respectively. Present data apparently contrast with the conventional wisdom that the presence of amphibole in mantle rocks is related to the introduction of melt or fluids enriched in incompatible elements. In the absence of experimental evidence that aqueous fluids in equilibrium with deep mantle are LREE-, Sr-depleted, we propose either a diffusive redistribution (near solidus or at subsolidus) or a chromatographic process to account for the formation of depleted amphibole from LREE-, Sr-enriched fluids. The crystallization of LREE-, Sr-depleted kaersutite and Ti-pargasite has important geodynamic implications, since it refers, at least for some peridotite massifs (i.e., Zabargad, External Ligurides, Eastern Pyrenees) to the steady-state equilibration under spinel facies conditions and is related to the early evolution of peridotites. This stage is broadly related to the timing of lithospheric accretion.