INFILTRATION METASOMATISM AT LHERZ AS MONITORED BY SYSTEMATIC ION-MICROPROBE INVESTIGATIONS CLOSE TO A HORNBLENDITE VEIN

Systematic investigations (electron- and ion-microprobe, and X-ray str ucture-refinement) of pyroxenes and amphiboles close to a cm-wide horn blendite vein have been carried out on a composite peridotite sample f rom the Lherz massif with the aim of constraining the processes of mel t infiltration in the subcontinental mantle. Vein amphiboles have ligh t rare earth element (LREE)-enriched patterns with the maximum at Nd a nd Sm, and positive Ba, Nb, Sr, and Ti anomalies. The liquid flowing i n the vein had an alkaline geochemical signature (i.e., large-ion lith ophile element (LILE)-enriched without significant high field strength element (HFSE) anomalies). Modal metasomatism, repre sented by crysta llization of amphibole and consumption of clinopyroxene, occurred in a 2-cm thick layer of wall-rock. Newly formed amphiboles, i.e., those i n the vein and in the modally metasomatised layer, have a lower ([6]) Al disorder than the pre-existing disseminated amphiboles, which forme d during an older metasomatic event. Cryptic metasomatism is recorded beyond the modally altered layer in the cores of clinopyroxenes and in amphiboles. Moving away from the vein towards the peridotite, amphibo les and pyroxenes display systematic compositional variations, and app roach equilibrium with the wall-rock within a few cm. Porphyroclastic clinopyroxenes record chromatographic separation of elements: less inc ompatible elements reach equilibrium with the wall-rock Iherzolite at shorter distances than the more incompatible ones. The inverse correla tion between the depth of the metasomatic effect and partition coeffic ients in porphyroclastic clinopyroxenes indicates that different conce ntration fronts developed for different elements during liquid/rock in teraction, suggesting that liquid percolation along grain boundaries w as responsible for clinopyroxene metasomatism. As in the clinopyroxene , abundances of incompatible elements in the amphibole decrease in the peridotite with distance from the vein. However, a less marked separa tion of elements is recorded by the amphiboles. Elements that are mark edly more compatible in amphibole than in other minerals of a Iherzoli te, such as K, Ba, and Rb, have abrupt gradients which are confined to the modally-metasomatised region. The lack of metasomatic enrichment in these elements in the farther, disseminated amphiboles, indicates t hat amphibole crystallization was synchronous with melt infiltration a nd acted as a buffer for such elements. Micro-analytical investigation s indicate that the geochemical effects associated with small-scale li quid infiltration can be successfully described by models which take i nto account both the chromatographic fractionation of elements and the influence of mineral-assemblage variations.