P-T-t constraints and geodynamic implications from blueschists and eclogites of the north Gondwanan margin in Iberia (Malpica-Tui complex, Galicia)
Résumé
The Malpica-Tui complex (MTC) in the NW Iberian Massif consists of rocks of Gondwanan affinity. It preserves evidence of late Devonian high-pressure (HP) metamorphism varying from eclogite (P~ 26 kbar and T~ 650 °C) to blueschist facies conditions (19-22 kbar and 460-560 °C). Thermodynamic modelling on the HP rocks reveals a P-T path characterised by a sub-isothermal decompression to ~10 kbar, at 480°C in the blueschist-facies rocks and 650 °C in the eclogites, followed by cooling to ~5 kbar at 380°C and 500 °C, respectively. New 40 Ar/ 39 Ar data corroborate a minimum age of ~ 370 Ma for the subduction-related HP metamorphism. Subsequent decompression to pressures of about 10 kbar started at ~ 360 Ma and was contemporaneous with thrust-and-fold nappe tectonics and intrusion of early Variscan granodiorites dated at ~ 350-340 Ma. Final exhumation due to the late orogenic gravitational collapse of the orogenic pile, associated with the main detachment system, has been constrained from ~ 340-335 Ma to 320±5 Ma, which is the age of the syntectonic leucogranites constrained by U-Pb (López-Moro et al., this volume), emplaced in the autochthon of the MTC. Peak P-T conditions correspond to an approximate depth of 70-80 km and a geothermal gradient of 6-7°C km -1 , attributable to a cold subduction zone. Age differences between the HP event and the beginning of the post-nappe tectonics indicate that the exhumation of the MTC lasted about 15-20 Ma. The nearly isothermal decompression from ~ 26 to 10 kbar provides a vertical component exhumation rate of about 2-2.5 mm/year from ~ 80 to 30 km depth. The last stages of exhumation from 8 to 5 kbar occurred within a period of about 10-15 Ma (from 350 to 340-335 Ma) and from ~ 480-380°C, indicating a cooling rate of 7°C Ma -1 (or a geothermal gradient of 10°C km -1). These rates suggest that exhumation took place in two stages: (1) a fast, almost isothermal one, followed by a (2) slower episode with substantial cooling once the rocks reached the upper crust.