Contrasting metamorphic evolution of metapelites from the Malpica-Tuy unit and the underlying so-called parautochthon at the coast of NW Spain

Two garnet-chloritoid bearing metapelites from the northern Malpica-Tuy unit (MTU) and a staurolite-andalusite bearing metapelite from the underlying parautochthon were studied to decipher their pressure-temperature (P-T) evolution, mainly based on chemically zoned minerals, such as phengite, using P-T, P-O2, and T-O2 pseudosections. In addition, monazite ages were determined with the electron microprobe in the metapelite from the parautochthon. The chloritoid schists from the MTU experienced peak P-T conditions around 21 kbar at 570 °C and a subsequent isothermal exhumation. Thus, two high pressure rock types can be distinguished in the MTU: orthogneiss (peak pressures: 13-16 kbar) and eclogite + metapelite (peak pressures: 20-23 kbar). The latter rock types were exhumed in a subduction channel environment. The staurolite-andalusite bearing schist from the parautochthon experienced only peak pressures around 8 kbar at a temperature around 570 °C since garnet and potassic white-mica with Si contents above 3.2 per formula unit are lacking. This rock represents sediments that were originally at the surface of the downgoing continental plate and metamorphosed in early Variscan times (around 364 Ma) similar to the MTU rocks. Our mean monazite ages of 339 and 320 Ma might be related to backthrusting of the MTU rocks from deeper crustal levels onto the parautochthon.