Eclogites of the Speik Complex, Austria: Indicators of High-Pressure Metamorphism and an Early Variscan Subduction Zone

The Speik Complex, a meta-ophiolite predominantly exposed in the Gleinalpe region of the Eastern Alps, is part of the Austroalpine Silvretta-Seckau Nappe System and is primarily composed of mafic and ultramafic rocks. By applying petrological, geothermobarometric and geochronological techniques, this study tries to constrain the P-T-t path of the Speik Complex and reconstruct its tectono-metamorphic evolution.

The Speik Complex forms an E-W orientated belt in the Gleinalm and Stubalm mountain ridge and a SE-NW orientated belt at the southern side of the Seckau Mountains. The easternmost appearance of serpentinite is east of the Mur valley but the main ultramafic bodies occur near the village Traföß next to the river Mur, in the vicinity of the village Kraubath and at mountain Hochgrößen. The lithologies of the Speik Complex include serpentinite, abundant (garnet-)amphibolite and some occurrences of eclogite. Eclogites from two different sample localities within the Gleinalpe region are characterized by a mineral assemblage of garnet, clinopyroxene/omphacite, amphibole, zoisite, rutile and quartz. Garnets show homogeneous compositions with an increase in spessartine towards the core. Almandine, pyrope and grossular remain constant with compositional ranges of Alm_47-55, Py_4-6 and Grs_38-41. Results from geothermobarometry and thermodynamic modeling of eclogites suggest peak metamorphic conditions of ~600-620°C and ~1.5–1.7 GPa. Geochemical analyses of eclogites and (garnet-bearing) amphibolites suggest a tholeiitic differentiation trend derived from basaltic to andesitic protoliths. Their trace element compositions show MORB affinities. Sm-Nd whole rock (WR) - garnet ages from garnet-bearing amphibolites sampled near Kraubath in the vicinity of exposed eclogites gave WR-garnet isochron ages of 406 ± 4 Ma and 413 ± 5 Ma. Age calculations including amphibole yield slightly older ages with larger errors. U-Pb zircon dating of a coarse-grained amphibolite vein crosscutting massive serpentinites within the nearby Preg quarry yields a weighted mean age of 395 ± 1.5 Ma, consistent with a previously published ⁴⁰Ar/³⁹Ar amphibole cooling age (397.3 ± 7.8 Ma) from eclogitic relics at Hochgrößen (Faryad et al., 2002). According to the available data the Speik Complex represents an oceanic suture which initiated in the Early Devonian prior to the Variscan continental collision.

SW Faryad F Melcher G Hoinkes J Puhl T Meisel W Frank (2002) Relics of eclogite-facies metamorphism in the Austroalpine basement, Hochgrössen (Speik Complex), Austria Mineral Petrol 74 49–73