88 Extended abstracts of Conference Proceedings: What drives metamorphism and metamorphic reactions: heat production, heat transfer, deformation and kinematics? Convenors Treloar, P.J. and O’Brien, P. Kingston University. 1996 2: Kyanite-bearing belts in the Variscan, low-pressure orogen in NW Spain. Their relation with late normal faulting. J.Reche, F.J. Martínez & M.L. Arboleya Dept. de Geologia, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain Low-pressure metamorphism has been considered characteristic of the Variscan Chain in Europe, although exotic local remnants of older higher-pressure episodes occur sporadically (f.i. Saxonian, Galician, Brittanian and French Central Massif granulites and eclogites). The low-pressure metamorphism is a thermal event that may be locally related to multiple intrusions during the latest, locally extensional, deformation stages of the chain. Nevertheless a more careful study reveals this scheme to be too simplistic since in Galicia, NW Spain, medium-pressure, ky-bearing assemblages occur in Ordovician and Silurian pelite belts which constitute F2 synforms that rim low-P, high-T redressed F2 antiforms cored by older outcropping materials. The eastern boundary of one of the belts is a transtensional ductile shear zone (the Vivero fault) that sinks the belt and rises the eastern older materials, bringing into contact Silurian black slates with Cambrian psamitic schists. Kyanite-muscovite±cloritoid aggregates pseudomorphic after quiastolite occur in favourable Al-rich lithologies within this pelite belt. The quiastolite is clearly postectonic and overprints the first penetrative slaty cleavage S1, axial planar to isoclinal F1 folds, and is pretectonic with respect to an S2 crenulation related to the redressed F2 synforms. The prograde assemblages observed in black, graphite-rich Ordovician and Silurian pelites, when approaching from the west the transtensional shear zone (Vivero fault) that separates the kyanite-bearing pelite belt form the higher-T antiform are: (1) kyanite-chloritoid-chlorite-muscovite; with sporadic strained andalusite remnants and zincoan (ZnO = 4.13 wt%) staurolite. (2) kyanite-staurolite- chlorite -muscovite; staurolite often rims kyanite. (3) andalusite-staurolite-biotite-muscovite; kyanite relics occur within st. Conditions for these three assemblages have been deduced for two bulk compositions: graphitic, Al-richer and psamitic, Al-poorer slates, using the most recent Thermocalc 2.4 version (Powell and Holland, 1988). Two KFMASH pseudosections have been generated, one for each bulk composition, plus one Mn-KFMASH pseudosection for Al- poorer, garnet-bearing slates, with MnO = 0.13 wt%. The joint figure corresponds to the Al-rich compositions. As- suming a prograde path, the starting conditions with post-F1, stable quiastolite previous to assemblage 1, should be in the andalusite field at a maximum T given by the core of prograde chloritoids; this would set starting conditions around 400 and 2.6 kbar . Chloritoids cores from two outcrops containing assemblage 1 are sistematically Fe-richer than rims. Chlorite-chloritoid thermometry (Perchuck, 1991) implies an increase in T from core to rim, and, given the attitude of chloritoid isopleths calculated with Thermocalc, this prograde path must be also one of increasing P. This P increase is texturally confirmed since chloritoid occurs inside kyanite-mucovite pseudomorphs after quiastolite. Univariant reaction kyanite+chloritoid = staurolite+chlorite separates assemblages 1 and 2. This reaction produces staurolite that rims kyanite. Passage from assemblage 2 to 3 conditions is observed in a tenths-of-meters wide rock strip adjacent to the Vivero fault hanging-wall, and is syn- to post-tectonic with respect to F2. Within these rock strip st occurs partially resopted into andalusite-biotite aggregates, evidencing a prograde path across the staurolite-biotite-andalusite divariant field. Assemblage 3 represents the later, higher-T conditions reached near the fault due to heat transferred from the hotter, ascending, footwall material to the east plus multiple sill-like granite intrusive bodies along the Vivero fault zone itself. The P-T paths deduced indicate that it would be too simplistic to ascribe the low-P Variscan metamorphism to an almost isobaric T-increase in an extensional regime. Actually the close relation between the location of late-kyanite belts resulting from and to kyanite inversion in F2 synforms indicates that subvertical re-folding played a role in the metamorphic evolution down-dragging the and-bearing materials and bringing them into the kyanite stability field. Although the overall attitude of the P-T paths deduced reflects a T increase, dP/dT is not constant as locally previous low-P gradients (~45°C/km) evolve to intermediate-P ones (~ 27-°C/km). The localised change in dP/dT is related to the F2 subvertical planar, coaxial, synformal refolding of F1 fabric. One of the F2 synform limb later evolves into a transtensional shear zone (Vivero fault zone) with abundant syn-shear zone granitoid elongated bodies.