INDUCED MELTING IN A LOW PRESSURE AUREOLE IN THE VARISCAN NE IBERIAN BASEMENT

A Thermocalc 2.75 model for rare andalusite-bearing, partially molten granulitic hornfelses has been constructed. It suggests that the Caradocian (Upper Ordovician) metapelites of the pre-Pyrenean, Variscan basement of NE Spain underwent multiple LP thermal pulses due to syntectonic magma injections during the late Variscan D2 deformation. The intrusions form a main subconcordant sheet-like hectometric body of intermediate rocks grading in the outer part into a swarm of metric-sized layers. The contact rocks away from the aureole are regionally metamorphosed schists in the andalusite-cordierite zone recording conditions of 550-575 °C and 2.5-3 kbar. Three main host rock types are distinguished, i) pelitic hornfelses containing both andalusite and sillimanite+cordierite+biotite+quartz+plagioclase±garnet+muscovite occur in the outer part of the aureole, ii) the assemblage andalusite+sillimanite+cordierite+biotite+spinel±corundum±quartz occurs in Al-rich, Si-depleted selvedges around abundant quartz veins. The selvedges seem to be due to local metasomatism which removed silica near prograde fluid conduits represented now by the veins, iii) K-poor, Al-rich restites occurring near the more basic intrusions in the inner part of the aureole are cordierite-rich and contain garnet, ilmenite and relic orthopyroxene. Garnet-cordierite thermometry gives temperature up to 820 °C for the restites. Milimetric to decimetric trondhjemitic to tonalitic veins occur in the aureole, and are interpreted as melts migrating from the melting sites. The trondhjemitic veins probably indicate H2O-saturated melting conditions and/or K-poor bulk compositions melt sources; however, the norm of these veins gives a granodioritic composition. Actually, secondary muscovite and myrmekites could have formed from previous K-feldspar by the retrograde reaction K-felspar+H2O+andalusiteýquartz+muscovite+plagioclase. The crystallization of the induced melts caused a fluid pressure peak in the country rocks. The presence in the hornfelses of irregular microscopic quartz pods surrounding idiomorphic andalusite and cordierite are also indicative of microfracturing due to high fluid pressure.