Abstract
Detrital zircons from pre-orogenic Upper Ordovician to Devonian low-grade metasedimentary rocks have been dated by laser ablation–inductively coupled plasma mass spectrometry (LA–ICP-MS) to: (1) constrain the variations of zircon age populations within the autochthonous sequences of NW Iberia; (2) compare U–Pb detrital zircon ages with previous data; (3) test the hypothesis of the Armorica microplate as a peri-Gondwanan terrane separated from Gondwana. The similarity in the ages of detrital zircons found in the Palaeozoic samples studied here to those published for the Upper Proterozoic, Lower Cambrian and Lower Ordovician sediments of NW Iberia argues against the separation of Armorica, and points to a common source area for all of the Palaeozoic detrital formations of NW Iberia, the West African craton and the surrounding Pan-African belts. LA–ICP-MS U–Pb dating of zircons from a synorogenic flysch preserved in klippen in the core of a syncline establishes a maximum depositional age for this deposit as Early Namurian. This age is also a maximum age limit for thrusting and refolding in this part of the Iberian Massif. Correlation of the zircon age populations with published ages confirms the link between the emplacement of the allochthonous complexes of NW Iberia and synorogenic sedimentation.
Highlights
Palaeomagnetic investigations on the position of the Variscan belt of central and western Europe during the Palaeozoic led to the formulation of the Armorica microplate hypothesis in the early 1980s
Detrital zircons from pre-orogenic Upper Ordovician to Devonian low-grade metasedimentary rocks have been dated by laser ablation–inductively coupled plasma mass spectrometry (LA–ICP-MS) to: (1) constrain the variations of zircon age populations within the autochthonous sequences of NW Iberia; (2) compare U–Pb detrital zircon ages with previous data; (3) test the hypothesis of the Armorica microplate as a peri-Gondwanan terrane separated from Gondwana
The similarity in the ages of detrital zircons found in the Palaeozoic samples studied here to those published for the Upper Proterozoic, Lower Cambrian and Lower Ordovician sediments of NW Iberia argues against the separation of Armorica, and points to a common source area for all of the Palaeozoic detrital formations of NW Iberia, the West African craton and the surrounding Pan-African belts
Summary
Palaeomagnetic investigations on the position of the Variscan belt of central and western Europe during the Palaeozoic led to the formulation of the Armorica microplate hypothesis in the early 1980s. Fernandez-Suarez et al (1999, 2000b, 2002b) have shown that the zircon age populations of Lower Cambrian and Lower Ordovician sedimentary rocks in Iberia are similar to those of Neoproterozoic sedimentary rocks, which suggest the same source area, the West African craton and the surrounding Pan-African belts (Fernandez-Suarez et al 2002c) This is consistent with the assertion of Noblet & Lefort (1990) that, accepting the Armorica microplate hypothesis, pointed out that the time of the alleged drift of Armorica was unconstrained by palaeomagnetism. Their assertion was based on the large volume of clastic material transported and the consistent current directions, indicating that the sediment source was to the south of the present-day Sahara
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