Permian to Triassic protolith ages of type locality eclogites in the Eastern Alps: Implications for the opening of the Meliata back-arc basin

Abstract

Abstract The Austroalpine domain contains the type locality of eclogites, but their protolith age is unknown except for a Permian metagabbro. Therefore, we studied the non-gabbroic eclogites from the Saualpe-Koralpe Complex (SKC) representing meta-basalts within a coherent continental rift fragment subducted during the Cretaceous. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating revealed protolith ages of 283 ± 5 Ma, 255 ± 3 Ma, 251 ± 3 Ma, and 241 ± 3 Ma. Magmatic zircons exhibit 176Hf/177Hf ratios of 0.282935–0.283090 and juvenile εHf(t) values of +10 to +17.4. Typical normal mid-oceanic ridge basalt (N-MORB) geochemistry is established. The SKC host metasedimentary rocks are interpreted as Variscan continental crust close to the margin of the oceanic Meliata basin and were affected by Permian metamorphism. Metamorphic zircon of one SKC eclogite yields an age of 87–93 Ma. These results combined with previous data are used to present a new model for the tectonic evolution of the distal Austroalpine unit associated with the Meliata Ocean in a Wilson cycle: The SKC represents a distal continental margin during Permian to Middle Triassic rifting, which was intruded by few gabbro bodies and numerous basaltic sills and mostly Permian pegmatites. In contrast, structurally separated thick Triassic sedimentary cover successions of the Austroalpine domain lack any magmatism, excluding SKC as Triassic basement of the sedimentary cover successions. The present eclogite-bearing piece of continental crust adjacent to the Meliata oceanic lithosphere was subducted to mantle depth during Late Cretaceous and then exhumed.