Protolith and metamorphic ages of eclogites from the Eastern Alps: Implications for the Permian to Cretaceous Wilson cycle of the Austroalpine mega-unit

The Austroalpine mega-unit contains the type locality of eclogites (Haüy, 1822) but their protolith age is largely unknown except that of the Permian Bärofen eclogite, for which three Sm-Nd ages between 275 ± 18 and 275 ± 18 Ma have been reported (Thöni & Jagoutz, 1993, Geochim. Cosmochim. Acta 56, 347–368; Miller & Thöni, 1997, Chem. Geol. 137, 283–310). Therefore, we studied the non-gabbroic eclogites from the Saualpe-Koralpe and Sieggraben Complexes, which are considered to represent a previously coherent subducted and then exhumed fragment of a continental rift, which led to the formation of the late Middle Triassic Meliata oceanic basin. A combined zircon U–Pb and Hf isotopic study, whole-rock geochemistry of two complexes revealed a protolith age of 242.3 ± 2.6 Ma (Middle Triassic) in the Sieggraben Complex, and 283 ± 5 Ma, 255 ± 3 Ma (Early and Late Permian), 251 ± 3 Ma, and 241 ± 3 Ma (Early to Middle Triassic) in the Saualpe-Koralpe Complex. Magmatic zircons from the Sieggraben eclogites have ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.283067–0.283174, εHf(t) values of +15.7 to +19.4, and that from Saualpe-Koralpe eclogite have ¹⁷⁶Hf/¹⁷⁷Hf ratios of εHf(t) 0.282935–0.283090, εHf(t) values of +10 to +17.4 showing their juvenile mantle source rather than significant crustal assimilation. In both complexes, N-MORB geochemical characteristics are established. Associated ultramafic rocks of Sieggraben eclogites as part of oceanic or Permian subcontinental mantle lithosphere suggest a depleted mantle source and a deep subduction environment. Two zircon grains of Sieggraben eclogites with low Th/U ratios yield ages of 113 ± 2 Ma and 86 ± 4 Ma and represent the approximate age of eclogite metamorphism during the Cretaceous. A trondhjemite dike cutting through the eclogite gives a crystallization age of 82.19 ± 0.4 Ma and is formed by partial melting of likely eclogite during decompression. The host metasedimentary rocks of Sieggraben and Saualpe-Koralpe Complexes are interpreted as old continental crust close to the margin of the Meliata basin and were affected by Permian migmatitic metamorphism. Metamorphic zircons of one eclogite from the Saualpe-Koralpe Complex give an age of 87–93 Ma (peak at 91 ± 1.2 Ma). The results of this study combined with previous results 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 Austroalpine Sieggraben and Saualpe-Koralpe Complexes represent a location on the distal continental margin during Permian to Middle Triassic rifting. The mafic rocks are associated with numerous Permian and potential Triassic acidic pegmatites, whereas structurally separated thick Triassic sedimentary cover successions lack any magmatism, likely excluding the present-day eclogite-bearing units as Triassic basement of the sedimentary cover successions.

The now eclogite-bearing piece of continental crust adjacent to the Meliata oceanic lithosphere subducted during Early Cretaceous times to mantle depth. The subducted continental crust was then exhumed incorporating even ultramafic mantle rocks. During exhumation and decompression of mafic rocks, partial melting took place forming the trondhjemite dike in Late Cretaceous times.

Acknowledgment： The study is financially supported by NSFC (91755212).