The Wechsel Gneiss Complex of Eastern Alps: an Ediacaran to Cambrian continental arc and its Early Proterozoic hinterland

Abstract

Many metamorphosed basement complexes in the Alps are polymetamorphic and their origin and geological history may only be deciphered by detailed geochronology on the different members including oceanic elements like ophiolites, arc successions, and continental passive margin successions. Here we present a case study on the Lower Austroalpine Variegated Wechsel Gneiss Complex and the overlying low-grade metamorphosed Wechsel Phyllite Unit at the eastern margin of Alps. The Wechsel Gneiss Complexes are known to have been overprinted by Devonian metamorphism, and both units were affected by Late Cretaceous greenschist facies metamorphism. New U–Pb zircon ages reveal evidence for two stages of continental arc-like magmatism at 500–520 Ma and 550–570 Ma in the Variegated Wechsel Gneiss Complex. An age of ca. 510 Ma of detrital zircons in metasedimentary rocks also constrain the maximum age of metasedimentary rocks, which is younger than Middle Cambrian. The overlying Wechsel Phyllite Unit is younger than 450 Ma (Late Ordovician) and seems to have formed by denudation of the underlying Variegated Wechsel Gneiss Complex. We speculate on potential relationships of the continental arc-type magmatism of the Variegated Wechsel Gneiss Complex and potential oceanic lithosphere (Speik complex) of Prototethyan affinity, which is also preserved in the Austroalpine nappe complex. The abundant, nearly uniform 2.1 Ga- and ca. 2.5 Ma-age signature of detrital zircons in metasediments (paragneiss, quartzite) of the Variegated Wechsel Gneiss Complex calls for Lower Proterozoic continental crust in the nearby source showing the close relationship to northern Gondwana prominent in West Africa and Amazonia.

Reconstructing the Variscan Terranes in the Alpine Basement: Facts and Arguments for an Alpidic Orocline

“…Nevertheless, since this metamorphic complex yields Paleoproterozoic detrital zircons with Gondwanian signatures, its place close to the Grobgneiss complex appears likely. Furthermore, the development of a magmatic arc in the Wechsel Gneiss Complex accounts well for the formation of the Speik back-arc basin [125,136].…”

“…The Wechsel gneiss complex, exposed in an Alpine tectonic window, is the lowermost part of this group of Variscan units (Figure 9). According to Neubauer et al [136], it is subdivided into several litho-tectonic units [136], and enclosed references). Namely, from bottom to top, the Wechsel Gneiss Complex, in turn subdivided in lower Monotonous, and upper Variegated Gneiss complexes, followed by the Wechsel Phyllite Unit that consists of chlorite-muscovite-albite paragneiss, micaschist, blackschist, and quartz-phyllite with subordinate actinolite-epidote amphibolite.…”

The birth of the Alps: Ediacaran to Paleozoic accretionary processes and crustal growth along the northern Gondwana margin

“…Particularly for the Ordovician, this is also in line with the common occurrence of peraluminous magmatism, indicative of assimilation or anatexis of (meta)sedimentary rocks, which is further reinforced by the presence of inherited Proterozoic to Cambrian zircon xenocrysts (Sect. 4.3;Schulz and Bombach 2003;Schulz et al 2004Schulz et al , 2008Siegesmund et al 2007;Bussien et al 2011;Cavargna-Sani et al 2014;Bergomi et al 2018;Mandl et al 2018;Neubauer et al 2020).…”

The role of the proto-Alpine Cenerian Orogen in the Avalonian- Cadomian belt

“…3). An example for such a "transferred terrane" could be the Alpine Wechsel Gneiss Unit, for which Neubauer et al (2020) credibly proposed a west African ancestry based on detrital zircon data (note the green point labelled W in Fig. 2).…”