Abstract
New whole-rock geochemical and coupled U–Pb and Lu–Hf LA-ICP-MS zircon data of metasedimentary rocks of the Austroalpine, South Alpine and Penninic basement domains are presented, to disentangle the pre-Variscan tectonic evolution of the proto-Alps. The studied units seem to record distinct stages of protracted Late Ediacaran to Carboniferous tectonosedimentary processes prior to the Variscan collision. In the case of Austroalpine and South Alpine units, nevertheless, no major differences in terms of provenance are observed, since most detrital zircon samples are characterized by a major Pan-African peak. Their detrital zircon spectra record a provenance from the northeastern Saharan Metacraton and the Sinai basement at the northern Arabian-Nubian Shield, being thus located along the eastern Early Paleozoic northern Gondwana margin, whereas sources located further west are inferred for the Penninic Unit, which might have been placed close to the Moldanubian Unit of the Bohemian Massif. In any case, it is thus clear that the Alpine basement remained in a close position to the Gondwana mainland at least during the Early Paleozoic. The Late Ediacaran to Silurian tectonic evolution, which includes Cadomian and Cenerian tectonometamorphic and magmatic processes, seem thus to record a continuum related to a retreating-mode accretionary orogen, with diachronous back-arc basin opening and possibly discrete compressional/transpressional pulses linked to changes in subduction zone dynamics. On the other hand, it is inferred that the Alpine basement essentially comprises Pan-African metasedimentary and subordinate metaigneous rocks, possibly with very few Early Neoproterozoic relics. This basement was significantly reworked during the protracted Paleozoic orogenic evolution, due to anatexis and/or assimilation by mantle-derived juvenile magmatism.
Highlights
Convergent margins are the locus of accretionary orogens, which can be divided into advancing and retreating types depending on whether they undergo crustal thickening or thinning, respectively (Cawood et al 2009)
The Innsbruck Quartzphyllite Complex shows a well-defined Late Ediacaran maximum sedimentation age based on YGC2 (568 ± 3 Ma) and YGC1 (568 ± 3 Ma), though the YSG indicates an Early Ordovician age (480 ± 3 Ma; Table 1)
Though Late Ediacaran maximum sedimentation ages are constrained by most YGC2 values for the Ötztal–Stubai Complex, several samples exhibit younger Cambrian ages indicated by YGC1 and YSG (Table 1; PZ 17-03, PZ 17-05, WINN 16–1), together with Ordovician to Devonian metamorphism
Summary
Convergent margins are the locus of accretionary orogens, which can be divided into advancing and retreating types depending on whether they undergo crustal thickening or thinning, respectively (Cawood et al 2009). The Ediacaran to Cambrian evolution is intimately related to the last stages of the Pan-African Orogeny, leading to the final assembly of Gondwana, and the nearly coeval Cadomian Orogeny, associated with a retreating accretionary margin, back-arc basin development and a magmatic flare-up (Linnemann et al 2007, 2014; Murphy et al 2013; Garfunkel 2015; von Raumer et al 2015; Moghadam et al 2017, 2019; Oriolo et al 2021). Further subduction and coeval retroarc extension triggered the opening of the Rheic Ocean by the Ordovician, leading to the separation of Avalonia from the Gondwana margin and favoring crustal reworking of Cadomian domains
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