Geochemistry of garnet peridotite massifs from lower Austria and the composition of deep lithosphere beneath a Palaeozoic convergent plate margin

In the southern Bohemian massif garnet-bearing high-temperature peridotite massifs have been exhumed during Carboniferous collision of the Baltic plate and rifted fragments of Gondwana. Geological and geochronological data suggest that the peridotites represent fragments of lithosphere derived from a destructive plate margin. Depleted garnet lherzolites, spinel and spinel-garnet harzburgites from lower Austria show major and trace element variations with MgO and HREE abundances which indicate that they underwent moderate (similar to 5%) to high (> 20%) degrees of partial melting in the spinel peridotite facies. Peridotites sampled away from pyroxenite layers have isotopic compositions typical of depleted mantle (Sr-87/Sr-86 = 0.7021 to 0.7033, epsilon(Nd) (335 Ma)= 8 to 12). Peridotites that occur close to LREE-enriched pyroxenites with negative Eu anomalies yield evidence for a multi-stage history, i.e. they have (Tb/Yb)(n) < 1 and (La/Sm)(n) > 1. Re-enrichment in incompatible elements, e.g., high (La/Sm)(n) in harzburgites and depleted Iherzolites, and the relatively high SiO2 contents of some of these rocks, can be explained by percolation of the LREE-enriched melts which were parental to nearby pyroxenite layers. Refertilization, bulk REE enrichment and isotope disequilibrium of minerals in some peridotites are related to late physical mixing with garnet pyroxenites during the exhumation process. A second group of peridotites comprises garnet Iherzolites with (La/Sm), < 1. These LREE-depleted garnet Iherzolites contain thin pyroxenite layers with flat to LREE-enriched REE patterns without Eu anomalies. Both, these peridotites and pyroxenites (Sr-87/Sr-86 UP to 0.7039, epsilon(Nd) (335 Ma) from 11 to -1.1) also appear to be isotopically distinct from the pyroxenites with negative Eu anomalies. A third group of Iherzolites has high Sr-87/Sr-86 (UP to 0.7049) and moderately high Nd isotopic compositions (epsilon(Nd) (335 Ma) from 7.9 to 4.4), and were probably also modified by the interaction with melts. The variability of isotopic compositions in the peridotites may indicate the possible variability in hangingwall lithospheric mantle beneath convergent plate margins. The average major and trace element composition of the peridotites from lower Austria is mon depleted than the average of continental spinel peridotite xenoliths, but less depleted compared with garnet peridotite xenoliths from cratonic settings.