Relics of eclogite facies metamorphism in the Austroalpine basement, Hochgr�ssen (Speik complex), Austria

Abstract

Retrograde eclogites and serpentinites from the Hochgro Èssen massif, Styria, are parts of the Speik complex in the Austroalpine basement nappes of the Eastern Alps. They are in tectonic contact with pre-Alpine gneisses, amphibolites, and Permo-Triassic quartz phyllites (Rannach Series). The eclogites are derived from ocean-¯oor basalts with af®nities to mid-ocean ridge and back-arc basin basalts. Fresh eclogites are rare and contain omphacite with a maximum of 39 mol% jadeite content, garnet (Py 15±19 ) and amphibole. Retrograde eclogites consist of amphibole and symplectites of Na-poor clinopyroxene (5±8 mol% Jd) albite AE amphibole. Amphiboles are classi®ed as edenite, pargasite, tschermakite, magnesiohornblende and actinolite. In relatively fresh eclogite, edenite is a common amphibole and texturally coexists with omphacite and garnet. An average temperature of 700 C was obtained for eclogite facies metamorphism using garnet-pyroxene thermometry. A minimum pressure of 1.5 GPa is indicated by the maximum jadeite content in omphacite. Thermobarometric calculations using the TWEEQ program for amphibole in textural equilibrium with omphacite and garnet give pressures of 1.8±2.2 GPa at 700 C. The equilibrium assemblage of Na-poor clinopyroxene, albite, amphibole and zoisite in the symplectites gives a pressure of about 0.6±0.8 GPa at 590±640 C. 40 Ar/ 39 Ar radiometric dating of edenitic amphibole in textural equilibrium with omphacite gave a plateau age of à Present address: Federal Institute of Geosciences and Natural Resources, Hannover, Federal Republic of Germany 397.3 AE 7.8 Ma, and probably indicates retrograde cooling through the closure temperature for amphibole ($ 500 C). The age of the high-pressure metamorphism thus must be pre-Variscan and points to one of the earliest metamorphic events in the Austroalpine nappes known to date.

An early Palaeozoic supra-subduction lithosphere in the Variscides: new evidence from the Maures massif

“…In the Cabo Ortegal area (NW Iberian massif), U-Pb zircon SHRIMP ages obtained for spinel harzburgites (391 ± 8 Ma), garnet pyroxenites (383 ± 8 Ma), granite pegmatite within a peridotite (387 ± 1 and 388 ± 2 Ma, Santos et al 1996), eclogites (380 ± 14 Ma), mafic granulites (383 ± 8 Ma), and metasediments (394 ± 11 Ma) (Ordóñez Casado et al 2001) reflect a Lower Devonian high pressure metamorphism. In the Speik complex of the Austroalpine basement, metabasalt intercalated with ultramafic rocks underwent HP metamorphism (18-22 kbar, 700°C) dated Silurian (397 ± 8 Ma; Faryad et al 2002). In the Rouergue area of the Massif Central region, the transition from norites to eclogites in the Lower Gneiss Unit have been dated Lower Devonian (410-390 Ma, zircon U-Pb and whole-rock SmNd) (Paquette et al 1995).…”