THE EVOLUTION OF PRE-VARISCAN ECLOGITES OF THE TAUERN WINDOW (EASTERNALPS) - A SM ND, CONVENTIONAL AND LASER ICP-MS ZIRCON U-PB STUDY/

Geochronological and geochemical analyses were carried out in order to characterize the geological evolution of the pre-Variscan basement of the Tauern Window (Eastern Alps). The Penninic units of the Lower Sch ist Cover from the central part of the Tauern Window host meta-basalts , ortho-and paragneisses and subordinate quartzites and graphite schis ts. These units were intruded by several granitoid plutons in Variscan time. A chronology and isotopic signatures for the Lower Schist Cover (LSC) evolution and magmatism was established dating various eclogiti c amphibolites by conventional and laser ablation microprobe (LAM-ICP- MS) U-Pb zircon techniques and Sm-Nd mineral analyses. Zircons of the eclogitic amphibolites yielded an age of 488 +/- 12 Ma interpreted as the igneous protolith emplacement age similar to those from the northe rn part of the Lower Schist Cover. Ages of 418 +/- 18 Ma (U-Pb-zircon) , 415 +/- 18 Ma (LAM ICP-MS, zircon) and 422 +/- 16 Ma (Sm-Nd) are the first evidence of a Silurian metamorphic overprint within the Eastern Alps. The new LAM-ICP-MS technique results in concordant U/Pb zircon ages in contrast to the discordant conventional U/Pb zircon age. Analy sed zircons did not include an old lead component; an episodic Pb-loss of Variscan and Alpine age causes the discordance of the zircon fract ions. The high-pressure metamorphism converted these mafic rocks into eclogites during the Silurian subduction.Sm-Nd whole rock analyses def ine a reference line at 845 +/- 26 Ma with an initial epsilon-Nd value of +5.3. Such old protolith ages are known from other parts of the Al ps, but in this case no geological significance is attributed to this age. No old intrusion ages are detectable by U-Pb zircon techniques. T hus, the epsilon-Nd T-500 signatures along with different REE pattern and trace element discrimination diagrams are interpreted to reflect a slightly enriched mantle source. A detailed Sm-Nd whole rock study wi thin one eclogite body demonstrates that the Nd systematics were distu rbed after the Cambrian intrusion and partly re-homogenized during the Variscan and/or Alpine metamorphism.