U-Pb detrital zircon ages in synorogenic deposits of the NW Iberian Massif, Variscan belt : interplay of Devonian-arboniferous sedimentation and thrust tectonics

摘要

Detrital zircons from Devonian and Carboniferous synorogenic flysch deposits occurring in animbricate stack have been dated by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS)to: (1) obtain a maximum depositional age to constrain the maximum age limit for thrusting of exoticterranes in the NW Iberian Massif; (2) correlate the zircon age populations with published ages in nearbyunits to establish their possible source areas. The maximum depositional ages are Late Devonian for rockshigh in the structural nappe pile (Gimonde Formation), in accordance with palynomorph dating, and aroundthe Devonian–Carboniferous boundary for structurally lower samples (San Vitero Formation). Used inconjunction with previously published ages, the new ages are interpreted in terms of the advance of the thrustsystem responsible for the emplacement of exotic terranes upon the Iberian autochthon during the Variscancollision. Early Variscan zircon population ages indicate the exotic terranes as the source of synorogenicsediments, whereas their scarcity suggests derivation from the Iberian autochthon. One of the samplesanalysed lacks Variscan detrital zircons; this feature, together with the absence of an Early Palaeozoic zirconage population, puts into question its synorogenic character and suggests that the sample may be representativeof the preorogenic parautochthon.