Publication: 40Ar/39Ar laserprobe dating of mylonitic fabrics in a polyorogenic terrane of
NW Iberia
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Publication Date
2006
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Geological Society of London.
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Abstract
The tectonothermal evolution of a polyorogenic terrane in the Variscan belt of NW Spain has been
constrained by 40Ar/39Ar laserprobe incremental heating experiments on mylonitic fabrics developed in major
structures. Transitional levels between HP–HT and IP upper units in the O´ rdenes Complex where
metamorphic and structural records demonstrate two cycles of burial and exhumation were selected for dating.
Two groups of ages have been defined: (1) Silurian–Early Devonian, obtained from mylonites of the Forna´s
extensional detachment, here considered as the minimum age for the start of tectonic exhumation of the HP–
HT units and an upper age-limit for the HP–HT event itself; (2) Early to Mid-Devonian, from structures
related to the Variscan convergence in the area, which include top-to-the-east thrusts and extensional
detachments. A single, younger Carboniferous age obtained from the uppermost allochthonous sequences
possibly reflects the final stages of emplacement of the allochthonous complexes. Our data indicate a
polyorogenic character for a part of the Iberian allochthonous complexes, including Variscan (sensu stricto)
and Early Variscan convergence, as well as an older, Early Palaeozoic cycle.