DIACHRONOUS VARISCAN TECTONOTHERMAL ACTIVITY IN THE NW IBERIAN MASSIF- EVIDENCE FROM AR-40 AR-39 DATING OF REGIONAL FABRICS/

Multigrain concentrates of hornblende and muscovite together with whol e-rock slate/phyllite samples have been dated (27 analyses) using Ar-4 0/Ar-39 incremental-release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwes tern sectors of the Variscan Iberian Massif. Hornblende concentrates f rom amphibolites in the allochthonous Ordenes Complex yield plateau is otope-correlation ages of 425 Ma and 377 Ma. Muscovite concentrates an d whole-rock slate/phyllite from this and the Cabo Ortegal Complex yie ld 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 kinemati c granitic stocks yield plateau ages between 309 Ma and 274 Ma. At lea st seven of the Ar-40/Ar-39 analyses from metamorphic rocks record var iable thermal rejuvenation of intracrystalline argon systems associate d with emplacement of proximal granitic stocks, The remaining analyses may be used to constrain the local age of various Variscan tectonothe rmal events. The oldest fabric ages are recorded in allochthonous unit s whereas the youngest fabric ages occur along the boundary between in ternal and external zones. Middle Devonian ages are recorded in the al lochthon and suggest a chronological continuity with deformational eve nts in the relative autochthon, where Variscan deformation initiated i n the Upper Devonian and diachronously prograded eastward. The first d eformational events recorded in the limit within the internal and exte rnal zones occurred ca. 20-25 Ma later (lower Namurian). Variscan defo rmation systematically prograded diachronously eastward across the oro gen as new crustal material was added along the front of the developin g orogenic wedge, However, the entire orogen remained tectonically act ive with different structural features forming at different times and at different places, An average propagation rate of ca, 5 km/m.y. is s uggested by consideration of a 20-25 Ma difference in correlative fabr ic ages end present separations.