LPHT metamorphism in a late orogenic transpressional setting, Albera Massif, NE Iberia: implications for the geodynamic evolution of the Variscan Pyrenees

During the Late Palaeozoic Variscan Orogeny, Cambro-Ordovician and/or Neoproterozoic metasedimentary rocks of the Albera Massif (Eastern Pyrenees) were subject to low-pressure/high-temperature (LPHT) regional metamorphism, with the development of a sequence of prograde metamorphic zones (chlorite-muscovite, biotite, andalusite-cordierite, sillimanite and migmatite). LPHT metamorphism and magmatism occurred in a broadly compressional tectonic regime, which started with a phase of southward thrusting (D1) and ended with a wrench-dominated dextral transpressional event (D2). D1 occurred under prograde metamorphic conditions. D2 started before the P-T metamorphic climax and continued during and after the metamorphic peak, and was associated with igneous activity. P-T estimates show that rocks from the biotite-in isograd reached peak-metamorphic conditions of 2.5 kbar, 400 degrees C; rocks in the low-grade part of the andalusite-cordierite zone reached peak metamorphic conditions of 2.8 kbar, 535 degrees C; rocks located at the transition between andalusite-cordierite zone and the sillimanite zone reached peak metamorphic conditions of 3.3 kbar, 625 degrees C; rocks located at the beginning of the anatectic domain reached peak metamorphic conditions of 3.5 kbar, 655 degrees C; and rocks located at the bottom of the metamorphic series of the massif reached peak metamorphic conditions of 4.5 kbar, 730 degrees C. A clockwise P-T trajectory is inferred using a combination of reaction microstructures with appropriate P-T pseudosections. It is proposed that heat from asthenospheric material that rose to shallow mantle levels provided the ultimate heat source for the LPHT metamorphism and extensive lower crustal melting, generating various types of granitoid magmas. This thermal pulse occurred during an episode of transpression, and is interpreted to reflect breakoff of the underlying, downwarped mantle lithosphere during the final stages of oblique continental collision.