Sm-Nd Isotope Systematics in Metamorphic and Magmatic Garnets: Age and P-T Constraints on the Eo-Alpine Subduction/Exhumation History (Koralpe-Saualpe, Austria)

Martin Thöni Institut für Geologie, Geozentrum, Althanstraße 14, A-1090 Wien, Austria

Christine Miller Institut für Mineralogie und Petrographie, Innrain 52, A-6020 Innsbruck, Austria

Within the Austroalpine basement nappe system, the Koralpe and Saualpe complexes range among the highest grade regions that experienced eo-Alpine (Cretaceous) metamorphism of eclogite to amphibolite facies conditions. This metamorphism was accompanied by pervasive deformation producing high-temperature mylonites on a kilometre-scale. Sm-Nd and P-T data on garnets from micaschists, basic eclogites and variously mylonitized pegmatites have yielded a) new constraints on the tectonothermal history of this polymetamorphic basement and b) some additional hints on the behaviour of the Sm-Nd system at high P-T conditions and intense deformation.

In the southern Koralpe (localities Bärofen, Gressenberg), mostly olivine-free Pl-Cpx gabbros of Permian age and with typical DM (depleted mantle) isotopic signatures are recognized as the former protoliths of some of the basic eclogites. The transformation of this relictic oceanic crust into eclogites during eo-Alpine subduction occurred at P-T conditions of c. 20 kbar/ „ 600 ¾ 700°C. Isotopically, the complex mineral assemblage Grt+Omp+ Zo+Qtz±Amp±Ky±Phe±Rut shows often slight disequilibrium. Additionally, precise garnet dating in these rocks is an analytical challenge, since these minerals have very low Nd concentrations of 20-100 ppb. Despite of this, Grt-Cpx (+WR, +Amp) mineral pairs from different localities of the southern Koralpe (Hohl, Mautnereck, Bärofen) give near-concordant Sm-Nd ages of close to 100 Ma. In accordance with the homogeneous element distribution in the garnets the ages are interpreted to date the eo-Alpine high-P evolution stage in the Koralpe at about 100±15 Ma (mean age range). Metapelite garnets from the eclogite host rocks in the Koralpe with ages of close to 80 Ma record partly younger stages of evolution.

Pyrope-rich garnets from St-Ky-bearing micaschists of the eclogite host rocks in the Saualpe yielded precise Grt-WR Sm-Nd ages of close to 90 Ma. Based on phengite geothermobarometry these ages are interpreted to record maximum Alpine P-T conditions of 20 kbar/680°C. It is therefore concluded that eclogite facies conditions prevailed in the central Saualpe (Kupplerbrunn) until c. 90±2 Ma.

Mn-rich garnets from Late Hercynian pegmatites that were intensely mylonitized during Alpine reworking of the basement show even element profiles and Sm-Nd ages of close to 260 Ma. These data are interpreted as primary magmatic ages that completely survived high Alpine P-T conditions. In other samples, however, the Sm-Nd system of pegmatite garnets seems to have been completely reset during the Alpine event.

On a regional scale, the data record Cretaceous subduction of the pre-Alpine continental basement and Late Paleozoic oceanic crustal fragments, probably at the northwestern end of the former Tethys-Vardar oceanic domain. The final stages of this evolution are obviously characterized by a very rapid exhumation. Metapelite and eclogite garnets with ages of c. 80-100 Ma may record different stages of the early decompression/exhumation path.