Journal of Petrology

Crustal Trace Element and Isotopic Signatures in Garnet Pyroxenites from Garnet Peridotite Massifs from Lower Austria

Harry Becker

Max-Planck-Institut fur Chemie, Abt. Geochemie, Postfach 3060, D-55020 Mainz, Germany
Now at: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N. W., Washington D. C., 20015, USA

Abstract

Garnet-bearing high-temperature peridotite massifs in lower Austria were exhumed during Carboniferous plate convergence in the Bohemian massif. The peridotite massifs contain garnet pyroxenite layers, most of which are high-pressure cumulates that crystallized in the deep lithosphere during ascent and cooling of hot asthenospheric melts. Many of the pyroxenites have negative Eu anomalies and high LREE abundances in pyroxenes and bulk rocks and 87Sr/86Sr (335 Ma) as high as 0.7089, and epsilon-Nd (335 Ma) as low as -4.8 (leached clinopyroxenes and garnets). These pyroxenites also show strong depletions in Rb, K, Ta, P and Ti compared with the REE. Equilibrium melt compositions calculated from the cumulate compositions have very high LREE abundances (La[n] = 300-600) and show strong LREE fractionation (La/Sm[n] = 7-47). Trace element abundances, the Ca-Al rich composition of the cumulates, and possible Ti saturation in the melts suggest that these melts were of primitive carbonatitic-melilitic or lamprophyre-like composition. Other garnet pyroxenites such as Al-rich garnet-kyanite clinopyroxenites with positive Eu anomalies probably represent metamorphosed crustal rocks which were subducted and accreted to the lithospheric mantle. The high 87Sr/86Sr, low epsilon-Nd (335 Ma), and negative Eu anomalies of the high-pressure cumulates can be explained if their equilibrium melts contained a component derived from subducted upper crustal rocks. The high equilibration pressures of the host peridotites (3-3.5 GPa) and the high equilibration temperatures of the pyroxenites (1100-1400°C) indicate that these melts are likely to be derived from the sub-lithospheric mantle. There, melting may have been triggered by small amounts of melt or fluids derived from a subducting slab at greater depth.

Keywords:

Garnet Pyroxenites/Geochemistry/Lower Austria/Ultramafic Massifs/Subduction.

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