Evidence of early Archean crust in northwest Gondwana, from U–Pb and Hf isotope analysis of detrital zircon, in Ediacaran surpacrustal rocks of northern Spain

Abstract

The Mora Formation (Narcea Group) is one of the oldest Precambrian supracrustal successions in northern Spain. Here, we use U–Pb and in situ Hf isotope analysis on detrital zircon to determine its age and provenance. The youngest U–Pb dates constrain the maximum depositional age of the Mora Formation at 565 ± 11 Ma. Results indicate: (1) a dominant Ediacaran zircon population (33%; 565–633 Ma, Cadomian) within a spectrum of Neoproterozoic ages (40%; 636–996 Ma); and (2) smaller Mesoproterozoic (5%; 1004–1240 Ma), Palaeoproterozoic (11%; 1890–2476 Ma) and Archean (11%; 2519–3550 Ma) populations. Results here do not point to one specific cratonic source area; instead, detritus may have been derived from the West African craton and Amazonia, or even the concealed Iberian basement. The lack of 1.3–1.8 Ga grains suggests exclusion of the Sahara Craton as a major source, but this is not certain. This mixed composition favours a complex source history with reworking of detritus across terrane/craton boundaries. Hafnium isotope compositions indicate a range of crustal and juvenile sources, with initial eHf values between −15.8 and 11.1, and Hf model ages from 0.8 to 3.7 Ga. For Neoproterozoic zircons (80%), juvenile components (eHf(i) ~+10) may be related to Rodinia fragmentation and the onset of an active margin setting leading to the Cadomian orogeny. Palaeoproterozoic to Paleoarchean grains (20%) all have negative eHf values and Meso- to Eoarchean Hf model ages. This indicates an early (Archean) sialic crustal component for northwestern Gondwana.