High-Pressure Tourmaline Formation and Fluid Activity in Fe-Ti-rich Eclogites from the Kreuzeck Mountains, Eastern Alps, Austria

A tourmaline-bearing eclogite from the Kreuzeck Mountains, Eastern Austria, is described in this study as part of an eclogite suite situated SE of the Tauern Window. These rocks formed during Eoalpine high-P metamorphism of the Austroalpine basement. They have high and variable Fe, Ti, P, Zr and Sr contents and unlike other Eoalpine eclogites their Ti-V-Zr-P-Nb-Y systematics indicates a within-plate rather than mid-ocean ridge basalt type environment for the formation of their protoliths. Major and trace element variations are consistent with progressive differentiation of a mafic magma with Fe-Ti oxide and apatite as important igneous phases and with metasomatic addition of K and Sr. The eclogites contain garnet + omphacite + quartz + epidote-group phases + calcic amphibole + K-feldspar +/- phengite + rutile + apatite + zircon formed under peak metamorphic P-T conditions of similar to 2.1GPa and similar to 650 degrees C. In situ U-Pb dating of two zircons from the tourmaline-bearing sample yields concordant ages of 86 and 109 Ma. Inclusions of K-feldspar + omphacite in garnet, and the very rare presence of phengite as a matrix phase only, indicate very limited availability of fluid during the early stages of the high-P event. Tourmaline formed by fluid influx under eclogite-facies P-T conditions as evidenced by the presence of omphacite and K-feldspar inclusions, and its growth may have continued during uplift and decompression. Tourmaline shows variable and in part extremely high Sr contents of up to similar to 5000 mu g g(-1), which are amongst the highest reported so far for this phase. A possible source of B is the metapelitic country rocks that were pre-enriched in B by the intrusion of tourmaline-bearing pegmatites during a Permian metamorphic event. A crustal source of B is consistent with high Li concentrations in zircons intergrown with the tourmaline. The flow of B-Li-(K-Sr)-bearing fluids into and within the eclogites must have been very localized, as evidenced by the rare occurrence of tourmaline, significant variations in the halogen content of apatite on a thin-section scale and irregular vein-like compositional zoning on a micrometer scale in tourmaline and garnet, which is thought to mark former fluid pathways. Direct evidence for at least two types of fluid present under eclogite-facies P-T conditions is provided by texturally primary H2O-NaCl-N-2-CH4-CO2 fluid inclusions in tourmaline and by H2O-NaCl-CaCl2 fluid inclusions in coexisting apatite and quartz. Laser ablation inductively coupled plasma mass spectrometry analysis shows that the primary fluid inclusions in quartz are rich in Ca (1.6-2.7wt %), K (3500-4900 mu g g(-1)), B (250-370 mu g g(-1)), Sr (490-1080 mu g g(-1)) and Pb (8-16 mu g g(-1)), with total solute concentrations of up to 9.3 wt %. Indirect evidence for the presence of fluids rich in high field strength elements and large ion lithophile elements under amphibolite-and/or greenschist-facies conditions is provided by texturally late light rare earth element-Nb-Ta-rich titanite with a positive Eu-anomaly.