Significance of detrital zircons in Siluro-Devonian rocks from Iberia

Abstract

Seven samples of Siluro-Devonian sedimentary rocks from the Cantabrian and Central Iberian zones of the Iberian
Variscan belt have been investigated for provenance and contain four main age populations in variable relative proportion:
Ediacaran–Cryogenian (c. 0.55–0.8Ga), Tonian–Stenian (0.85–1.2Ga), Palaeoproterozoic (c. 1.8–2.2Ga) and Archaean (c.
2.5–3.3Ga). Five samples contain very minor Palaeozoic (Cambrian) zircons and six samples contain minor but significant
zircons of Middle and Early Mesoproterozoic (Ectasian–Calymmian, 1.6–1.8) age. These data highlight the transition from
an arc environment to a stable platform following the opening of the Rheic Ocean. Variations in detrital zircon populations
in Middle–Late Devonian times reflect the onset of Variscan convergence between Laurussia and Gondwana. The presence
of a high proportion of zircons of Tonian–Stenian age in Devonian sedimentary rocks may be interpreted as (1) the existence
of a large Tonian–Stenian arc terrane exposed in the NE African realm (in or around the Arabian–Nubian Shield), (2) the
participation, from the Ordovician time, of a more easterly alongshore provenance of Tonian–Stenian zircons, and (3) an
increase in the relative proportion of Tonian–Stenian zircons with respect to the Ediacaran–Cryogenian population owing to
the drift of the Avalonian–Cadomian ribbon continent, or the progressive burial of Ediacaran–Cryogenian rocks coeval with
the denudation of older source rocks from the craton interior.