EVOLUTION OF A KYANITE-BEARING BELT WITHIN A HT-LP OROGEN - THE CASE OF NW VARISCAN IBERIA

Kyanite replaces andalusite in a belt of Ordovician and Silurian pelit ic rocks that form a narrow synform pinched between high-grade antifor ms in NW Variscan Iberia. Kyanite occurs across the belt in Al-rich, b lack pelites in assemblages I: kyanite-chloritoid-chlorite-muscovite a nd II: kyanite-staurolite-chlorite-muscovite. In I, kyanite occurs in the matrix and in kyanite-muscovite aggregates that pseudomorph earlie r andalusite porphyroblasts. The aggregates are found across the belt and can still be recognized in assemblage II and even in III: andalusi te-staurolite-biotite-muscovite, this latter being a hornfelsic Siluri an schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed inside new massive pleochroic andalusite. KFMASH and M nKFMASH pseudosections have been constructed using Thermocalc for Al-r ich and Al-poorer compositions from the belt. Chloritoid zoning in Al- rich rocks containing assemblage I, plus chloritoid-chlorite thermomet ry complemented with garnet-chlorite thermometry in Al-poorer litholog ies, mean that the path is one of increasing pressure and temperature. Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite-chloritoid-chlorite field as testified by chl oritoid enclosed in andalusite porphyroblast rims. The passage from as semblage I to II implies a prograde path within the kyanite field. Ass emblage III represents peak conditions, indicating a prograde stauroli te-consuming reaction across a KFMASH field, leading eventually to a l ocally found andalusite-biotite-muscovite hornfels. The lowest pressur e stages are recorded by cordierite-biotite in Al-poor pelites. Garnet -bearing MnKFMASH assemblages in Al-poorer pelites record conditions s imilar to assemblages II and III. The replacement of andalusite by kya nite in assemblage I is attributed to downdragging of andalusite-beari ng rocks into a synform as testified by the strained andalusite porphy roblasts affected by a subvertical crenulation cleavage. Prograde meta morphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the Vivero fault.