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

Abstract

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