Early Palaeozoic tectonothermal events in basement complexes of the eastern Greywacke Zone (Eastern Alps): evidence from U-Pb zircon data

Abstract

Field observations and U-Pb zircon data yield evidence that the Kaintaleck Complex represents a distinct crystalline basement unit of the eastern Greywacke Zone and contrasts partly with other pre-Alpine basement complexes of the Eastern Alps. The age data indicate possible Late Archean rock formation, several magmatic and metamorphic events in the Early Palaeozoic (ca. 520-490 and ca. 400-360 Ma), and low-grade metamorphic overprint in the Cretaceous. Zircons from a garnet gneiss layer in a plagioclase gneiss and amphibolite sequence yield an upper intercept age of ca. 2.55 Ga which is interpreted as a possible protolith age. The lower intercept age at 514 Ma represents either (1) a magmatic event or (2) a metamorphic overprint within amphibolite facies conditions. Magmatic zircons derived from granitic orthogneiss boulders of the Kalwang Conglomerate, which covers the crystalline basement, crystallised at ca. 500 Ma. The lower discordia intercepts of the orthogneiss boulders mark the Cretaceous low-grade metamorphic overprint. The lower intercept age of paragneiss zircons from another slice of the Kaintaleck Complex suggests a likely Devonian metamorphic event at ca. 390-400 Ma. The new data demonstrate that the Kaintaleck Complex experienced several stages of tectonothermal evolutions, which are in contrast to the evolution of the main mass of the Austroalpine basement. The data constrain linkages of this part of the Austroalpine basement to the Cordillere Ligérienne and Cadomian block within West-European Variscides.

Age constraints on the evolution of the Austroalpine basement to the south of the Tauern Window

“…Detrital zircon grains give ages around 2.0 Ga. Such data are comparable to Paleoproterozoic ages of inherited cores were found by Gebauer and Grü nfelder (1978), Thö ni (1999), Neubauer et al (2002), Doman (2002, and von Raumer et al (2003). These Svecofennian-BirimianAmazonian and older ages for inherited cores suggest that the ''Altkristallin'' is part of the Avalonia superterrane that was derived from the NW margin of Gondwana, i.e., present-day northern South America.…”

Conversion of carbonaceous material to graphite within the Greywacke Zone of the Eastern Alps

“…This zone is composed of four Alpine nappes (Neubauer et al 1994;Dallmeyer et al 1998). These are, from bottom to top, the Veitsch Nappe made up of Early Carboniferous to Permian carbonates and shales, the Silbersberg Nappe composed of a quartz-phyllitic basement and Verrucano-type cover sequences, the Kaintaleck Nappe including medium-grade metamorphic basement rocks (with Devonian cooling ages after metamorphism, Handler et al 1999;Neubauer et al 2002) which are overlain by low-grade metaconglomerates, and the Noric Nappe comprising Late Ordovician to Namurian shelf and slope sediments. The Noric Nappe is unconformably covered by Permian to Cretaceous sediments of the Northern Calcareous Alps.…”

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

“…The Ritting complex as part of the Kaintaleck slices is located below the low-grade Greywacke nappe (Figures 5 and 9). It consists of serpentinite, amphibolite, micaschist, quartzite, and marble boudins of Neoproterozoic to Early Paleozoic protolith age [122]. Amphibole from a retrogressed eclogite yields a Paleozoic 40 Ar/ 39 Ar age at 397 ± 7 Ma [123].…”