New geochronological constraints on the Ordovician plutonism and Cenerian orogeny in the Ötztal Complex, Eastern Alps

This study presents new U–Pb–Hf isotopic data and zircon ages from the Ediacaran to Ordovician Ötztal Complex of the Eastern Alps in Austria to provide new constraints on the evolution of the northern Gondwana active margin in the “proto-Alpine” realm. The multistage tectonic evolution of the complex started with siliciclastic deposition presumably in an accretionary wedge that may have lasted from ca. 600 Ma to ca. 517 Ma. The age spectra are dominated by Neoproterozoic zircon grains indicating that the complex was most likely sourced from the Arabian–Nubian shield, with a contribution of older Proterozoic and Archean grains from the more westerly Saharan metacraton. The deposition was partly overlapping in time with Cambrian to Early Ordovician mafic magmatism that formed either as mafic underplate below the accretionary wedge or outboard, being later accreted as part of the lower plate. The wedge was then intruded by compositionally diverse granitoids from ca. 500 Ma until ca. 440 Ma. By comparing the Ediacaran and Early Paleozoic evolution of the Ötztal Complex with originally more westerly Cadomian-basement terranes (e.g., those now found in the Bohemian Massif), we concluded that the Cenerian orogeny was generally younger in the proto-Alps than elsewhere in the former Cadomian belt. This was possibly due to a significantly curved geometry of the northern Gondwana margin and/or due to an eastward ridge–trench–transform triple point migration. Arrival of a warmer part of the oceanic plate then may have caused mantle melting, mafic underplating, and voluminous granitic plutonism in the forearc, perhaps finally terminated by ridge–trench interaction and slab break-off.