New U-Pb zircon and monazite ages on Ordovician magmatism and migmatization in the Paleozoic basement of the Eastern Alps (Ötztal Nappe, N Italy)

The Austroalpine Domain in the Eastern Alps consists of Mesozoic (meta)sedimentary cover units and polymetamorphic crystalline basements. The latter were heterogeneously overprinted by the Alpine metamorphism, with some of them still preserving traces of their Paleozoic magmatic and metamorphic evolution. One of the most prominent examples is the Ötztal–Stubai Complex, which extends over approximately 50 × 20 km across northern Italy and western Austria. Despite this complex experienced extensive Variscan high- to medium-grade metamorphism, some relicts of older events related to previous orogenies (i.e. Cadomian) are still preserved. As such, it represents a key natural archive for reconstructing the Paleozoic evolution of the Austroalpine Domain.

Within this regional framework, a small area of about 25 km² near Reschen Pass (South Tyrol, NE Italy) is of particular interest, as it hosts potential pre-Variscan geological features that escaped younger metamorphism and deformation. These include the Klopaier Pluton, its contact metamorphic aureole, and associated migmatites, all enclosed within country gneiss characterized by Variscan mineral assemblages and fabrics. The coexistence of these elements offers is key to investigate the timing and relationships between magmatism, metamorphism, and deformation during the early Paleozoic.

New detailed field mapping indicates that the Klopaier Pluton is largely undeformed, locally behaving as a rigid body and locally preserving primary intrusive contacts. Relicts of cordierite-bearing assemblages are locally found at the pluton margins and may represent remnants of a contact metamorphic aureole. The surrounding migmatites, predominantly metatexites, are interpreted as the result of syn-intrusion partial melting of the host rocks, later affected by Variscan metamorphism. Pegmatitic dikes are widespread both within the pluton and in the surrounding country rocks. These dikes are likely related to highly evolved fluid-rich melts derived from the main body of the Klopaier Pluton and have previously yielded U–Pb ages between ca. 490 and 413 Ma. These ages suggest that the observed structural configuration was already established during the Ordovician. New U–Pb zircon dating of the Klopaier Tonalite constrains its emplacement to approximately 460 Ma, in good agreement with the ages obtained from the associated pegmatites.

Despite these constraints, the relative timing of pluton emplacement and migmatite formation remains unresolved, posing a classic geological “chicken-or-egg” problem: did the pluton intrude into pre-existing, already cooled migmatites, or did its emplacement and associated heat supply trigger partial melting in the surrounding rocks? To address this question, new U–Pb zircon and monazite ages from the migmatites, combined with their geochemical characterization, indicate crystallization and melt-related processes between ca. 460 and 450 Ma. These new data provide crucial constraints on the early Paleozoic tectonometamorphic evolution of the Austroalpine Domain and contribute to a better understanding of pre-Alpine crustal processes in the Eastern Alps.