EVOLUTION OF THE EASTERN ALPS FROM THE PALEOZOIC ONWARD: CONSTRAINTS AND INSIGHTS FROM THE (ULTRA)BASIC ECLOGITIC COMPLEX OF MILCHENKAR ÖTZTAL-STUBAI COMPLEX (EASTERN ALPS, TYROL, AUSTRIA)

The Ötztal-Stubai Complex (OSC) is one of the widest tectonic units of the Alps, belonging to the Eastern Austroalpine domain and extending between the Engadine and Tauern windows, the Northern Calcareous Alps and the Vinschgau/Venosta valley. Together with other polycyclic crystalline basements of the Alps, the OSC preserves various evidences of Late Ediacaran to Ordovician events attributed to the peri-Gondwanan Cadomian orogeny (e.g., Neubauer et al., 2022), particulary in the form of metaigneous bodies ranging from (ultra)basic intrusions to acidic, peraluminous granitoids. The Northern Eclogite Zone (NEZ) of the Central Metabasite Zone comprises abundant garnet amphibolites hosting variably retrogressed eclogites and subordinate occurrences of their gabbroic protoliths, often showing igneous cumulitic textures, with crystallization ages around 530 Ma (Miller & Thöni, 1995). In the Milchenkar area, petrological and geochemical analyses on preserved olivine gabbro, anorthosite and pegmatoid gabbro samples allowed to infer the main crystallization processes, conditions of intrusion and main geochemical features of the parental melt. Olivine gabbros are the most primitive products and are characterized by the predominant fractionation of cumulus olivine and by the appearance of clinopyroxene before plagioclase; in all the samples analysed, orthopyroxene is a minor constituent and does not impact differentiation trends. Thermobarometric estimates on the igneous protoliths yield a temperature of 1200 ◦C for the initial crystallization of clinopyroxene at a minimum pressure of 5 kbar. Trace element compositions of the main igneous minerals and modeled equilibrium melts suggest the generation of a tholeiitic parental melt from a depleted mantle source possibly at conditions of garnet stability, and the intrusion in continental crust in an extensional context, with some similiarities to the Oligocenic Red Sea rift. Additionally, the relatively alkalic nature of the melt suggested by mineral chemistry and whole rock compositions is consistent with the inferred source. These results point to the propagation of a continental rifting context, already postulated for the Central European Cadomian domain (Linnemann et al., 2008a) in Cambrian times, to the Proto-Alpine terranes located further east along the peri-Gondwana margin. The subsequent metamorphic evolution of the NEZ is characterized by a pervasive, although partially retrogressed, high pressure event resulting in the eclogitization of the (ultra)basic complex completely affecting the more evolved gabbroic rocks and involving the aforementioned protoliths in part. Textural and mineralogical variations of the mafic eclogites suggest the occurrence of at least two high pressure stages, the first of which is characterized by the previously unreported occurrence of corundum and Mg-staurolite. In contrast with Variscan age estimates for the high pressure metamorphism (Miller & Thöni, 1995), the newly discovered occurrence of eclogite-hosted leucosomes preliminary dated to 484 Ma (in situ U-Pb on zircon magmatic rims, Milchenkar area) suggests the possible existence of an eclogitic event in the OSC pre-dating the widespread anatectic plutonism of Ordovician age. This would be consistent with a protracted active margin scenario for the Proto-Alpine Cadomian terranes (e.g., von Raumer et al., 2013) in those times, also supported by subduction-related magmatism and further, although scattered, evidences of high pressure metamorphism. Moreover, U-Pb radiometric dating performed on zircon separates of the S-type Suztal Granite, neighbouring the NEZ, yields a crystallization age of 463.94 ± 0.65 Ma, confirming previous estimates, while in the Vallelunga/Langtaufers area, preliminary petrological analyses of calc-silicate nodules hosted in metasedimentary rocks suggest the occurrence of a high temperature metasomatic event pre-dating the Variscan metamorphism.