Detrital zircons and the interpretation of palaeogeography, with the Variscan Orogeny as an example

Abstract

Analysis of the distribution of detrital zircon grains is one of the few parameters by which Precambrian palaeogeography may be interpreted. However, the break-up of Pangea and the subsequent dispersal of some of its fragments around the Indian Ocean demonstrate that zircon analysis alone may be misleading, since zircons indicate their original derivation and not their subsequent plate-tectonic pathways. Based on analysis of Precambrian–Ordovician zircon distributions, the presence of microcontinents and separating oceans in the north Gondwanan realm has been rejected in favour of an undivided pre-Variscan continental northwards extension of Africa to include Iberia, Armorica and neighbouring southern European terranes, based on analysis of Precambrian–Ordovician zircon distribution. However, contrasting views, indicating the presence of three peri-Gondwanan oceans with complete Wilson cycles, are reinforced here by a critical reappraisal of the significance of that Variscan area detrital zircon record together with a comparison of the evolution of the present-day Indian Ocean, indicating that Iberia, Armorica and other terranes were each separate from the main Gondwanan craton during the early Palaeozoic Era.

Pangea B and the Late Paleozoic Ice Age

“…Several studies of detrital zircon provenance and regional tectonostratigraphy have attempted to place Iberia (and other Armorican units) relative to the West African craton in the Neoproterozoic-Paleozoic, ranging from a position similar to today's (e.g., Diez Fernández et al, 2010;Pastor-Galán et al, 2013;Stephan et al, 2019), and hence more consistent with Pangea A sensu assumptions of Correia and Murphy (2020), to a position closer to the Africa-South America embayment (Linnemann et al, 2004) that we observe would be more compatible with Pangea B. The concept that Iberia as part of the Armorican domain was attached to Africa for much of the Paleozoic has, however, been questioned by Franke et al (2019), who cite geological evidence pointing to the rifting of Armorican units (including Iberia) from peri-Gondwana in the Early Paleozoic, postdating the time range of nearly all of the zircon data included in the recent and comprehensive review of Stephan et al (2019) and before main Variscan coalescence starting in the Devonian. The 'missing link' between Iberia and the Appalachians found by Correia and Murphy (2020) would thus no longer be able to resolve the Pangea A versus Pangea B controversy.…”

The Stavelot-Venn Massif (Ardenne, Belgium), a rift shoulder basin ripped off the West African craton: Cartography, stratigraphy, sedimentology, new U-Pb on zircon ages, geochemistry and Nd isotopes evidence

“…This intricate story has been enlightened particularly by a combination of T paleomagnetic and paleontological studies (e.g. Torsvik, 2002, 2005;Franke et al, 2017;Franke et al, 2019), and by the study of detrital zircon in Paleozoic sediments (e.g. Pereira et al, 2012;Linnemann et al, 2014;Orejana et al, 2015;Siegesmund et al, 2018 among many others).…”