A peri-Gondwanan arc in NW Iberia I: Isotopic and geochemical constraints on the origin of the arc—A sedimentary approach

Abstract

The arc-derived upper terrane in the NW Iberia Variscan belt contains a 3000 m thick turbiditic formation at
its structural top. Most of the sandstones are feldspathic greywackes with a framework of quartz and weakly
altered plagioclase. Rock fragments of vitric and microgranular texture are common in polymictic
conglomerates and coarse-grained greywackes, together with slates, cherts and bipyramidal volcanic quartz
fragments. Although recrystallization under greenschists facies conditions (chlorite and biotite zones) and the
presence of two cleavages hinder detailed textural analysis, the sandstones appear to be typically immature,
first-cycle sandstones. The metagreywackes have average major and trace element compositions similar to
PAAS (Post Archean Australian Shale), which is considered to reflect the composition of the upper continental
crust. Their trace element composition is very consistent and records deposition within a convergent tectonic
setting, probably in an intra-arc basin located in a volcanic arc built on thinned continental margin. Detrital
zircon populations suggest a Middle Cambrian maximum depositional age (530–500 Ma) and a Gondwanan
provenance located at the periphery of the West African Craton. Nd isotope data suggest mixing Ediacaran and
Paleoproterozoic sources for the provenance of the greywackes, with TDM ranging between 720 and 1215 Ma
with an average of 995 Ma (n=20)—an age range unrepresented in the detrital zircon population. The Nd
model ages are similar to those exhibited by West Avalonia, Florida or the Carolina terrane, but younger than
those of Cambrian and Ordovician sandstones and shales from the autochthonous realm. These data suggest
a westernmost location along the Gondwanan margin for the upper terrane of NW Iberia relative to other
terranes located in the footwall of the Variscan suture, consistent with several previously proposed
paleogeographic models for the NW Iberia terranes.