Cadomian metasediments and Ordovician sandstone from Corsica: detrital zircon U–Pb–Hf constrains on their provenance and paleogeography

Abstract

The island of Corsica, which belongs to the southern Variscan realm, was detached from southern France in the Tertiary. Alongside Alpine and Variscan edifices, it carries outliers of Neoproterozoic metasediments and Lower Paleozoic siliciclastics including Ordovician sandstone and conglomerates as well as Hirnantian diamictites in its NW sector. We investigated the U–Pb–Hf of detrital zircons of basement metasediments and overlying Ordovician sandstone and channeling conglomerate to constrain their provenance, the timing of their deposition, and to deduce the late Ediacaran to Ordovician paleogeography. The youngest detrital zircons in the metasediments are 0.55–0.53 Ga indicating their maximum age of deposition is Late Ediacaran to Early Cambrian, thus classifying the Corsica basement metasediments as Cadomian. The U–Pb analyses revealed that a preponderance of the detrital zircons in the basement micaschist and quartzite portray Neoproterozoic ages concentrating between 0.55 and 0.65 Ga. This is partly consistent with derivation from Pan-African terranes of north Africa yet the presence of detrital zircons younger than ~ 0.6 Ga indicates significant input from Cadomian magmatic arcs that resided within or at the margin of the Cadomian basin in which the metasediments of Corsica were deposited. eHf values of the Ediacaran zircons varies between samples, indicating the provenance comprised both, juvenile arcs and magmatic arcs that involved various degrees of mixing with old crustal components. The Hf-TDM ages of many of the Ediacaran-aged zircons point to a plausible involvement of Meso-Paleoproterozoic crust in the generation of these Cadomian arcs. The presence of small but fairly distinguished populations of Mesoproterozoic-aged (1.0–1.6 Ga) as well as 2.0–2.2 Ga and 2.4–2.6 Ga detrital zircons in the Cadomian metasediments farther indicates the presence of such crust in the provenance. Although Mesoproterozoic detrital zircons are usually considered the hallmark of Avalonian terranes, the presence of Hirnantian glacial sediments at the Corsican sequence indicates it resided in the vicinity of Gondwana. We therefore postulate that the Pre-Neoproterozoic zircons have sourced from exotic crustal vestiges that were entrained and accreted within the Cadomian realm itself. The transition into the overlying Ordovician sandstone and conglomerate marks a major change in the provenance, possibly pointing to lateral motions along the strike of the peripheral Cadomian domain. The youngest concordant detrital zircon in the Ordovician (“Ciuttone”) sandstone yielded an age of 0.48 Ga. The detrital zircon ages define an overwhelming concentration at 0.55 Ga, indicating the source of the Ordovician sandstone was cut off from the Gondwana hinterland and that sand was exclusively derived from a latest Ediacaran arc. In view of the sharp detrital zircon age peak, the Corsica Ordovician sandstone cannot be straightforwardly correlated with Armorican sandstone because the detrital zircon spectra of the latter are generally broader, indicating derivation from various sectors of the North Gondwana crust. eHf(t) values of the 0.55 Ga zircons are mostly positive and the corresponding TDM ages at 0.7–1.2 Ga indicating derivation from a juvenile island arc. While TDM ages of this type are common for late Ediacaran Avalonian rocks, the presence of Hirnantian diamictite in Corsica further substantiates that the aforementioned 0.55 Ga island arc evolved in the peripheral Cadomian realm. As a whole, the U–Pb–Hf zircon data from the Corsica sequences reveal the presence of juvenile Cadomian arcs alongside Cadomian arcs that recycled ancient (pre-Neoproterozoic) crust. Along strike variations of this type are known from the Japanese islands, in line with the peripheral Cadomian orogeny being an ancient analog of a Western-Pacific type plate boundary.