Diachronous Variscan tectonothermal activity in the NW Iberian Massif: Evidence from 40Ar/39Ar dating of regional fabrics

摘要

Multigrain concentrates of hornblende and muscovite together with whole-rock slate/phyllite samples have been datedud(27 analyses) using 40Ar/39Ar incremental-release methods along a systematic traverse across the various lithotectonicudstructural elements which comprise northwestern sectors of the Variscan Iberian Massif. Hornblende concentrates fromudamphibolites in the allochthonous Ordenes Complex yield plateau isotope-correlation ages of 425 Ma and 377 Ma.udMuscovite concentrates and whole-rock slate/phyllite from this and the Cabo Ortega1 Complex yield plateau ages whichudrange from 367 Ma to 295 Ma. Analyses of similar material from the relative autochthon yield plateau ages between 359udMa and 275 Ma. Muscovite concentrates from three late- to post-kinematic granitic stocks yield plateau ages between 309udMa and 274 Ma.udAt least seven of the 4oAr/39Ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystallineudargon systems associated with emplacement of proximal granitic stocks. The remaining analyses may be used to constrainudthe local age of various Variscan tectonothermal events. The oldest fabric ages are recorded in allochthonous units whereasudthe youngest fabric ages occur along the boundary between internal and external zones. Middle Devonian ages are recordedudin the allochthon and suggest a chronological continuity with deformational events in the relative autochthon, whereudVariscan deformation initiated in the Upper Devonian and diachronously prograded eastward. The first deformationaludevents recorded in the limit within the internal and external zones occurred ca. 20-25 Ma later (lower Namurian).udVariscan deformation systematically prograded diachronously eastward across the orogen as new crustal material wasudadded along the front of the developing erogenic wedge. However, the entire orogen remained tectonically active withuddifferent structural features forming at different times and at different places. An average propagation rate of ca. 5 km/m.y.udis suggested by consideration of a 20-25 Ma difference in correlative fabric ages and present separations.