HIGH-PRESSURE MICRO-INCLUSIONS AND DEVELOPMENT OF AN INVERTED METAMORPHIC GRADIENT IN THE SANTIAGO-SCHISTS (ORDENES-COMPLEX, NW IBERIAN MASSIF, SPAIN) - EVIDENCE OF SUBDUCTION AND SYNCOLLISIONAL DECOMPRESSION

The Santiago Schists are located in the Basal Unit of the Ordenes Complex, one of the allochthonous complexes outcropping in the inner part of the Hercynian Belt in the north-west of the Iberian Peninsula. Their tectonothermal evolution is characterized by the development of an eo-Hercynian metamorphic episode (c. 374 Ma) of high-P, low- to intermediate-T. The mineral assemblage of the high-P episode is preserved as a very thin Si=S1 foliation included in albite porphyroblasts, being composed of: albite + garnet-I + white mica-l + chlorite-l + epidote + quartz + rutile +/- ilmenite. The equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 +/- 10 degrees C and 14.7 +/- 0.7 kbar (probably minimum pressure). The later evolution (syn-D2) of the schists defines a decompressive and slightly prograde P-T path which reached its thermal peak at c. 525 +/- 10 degrees C and 7 kbar. Decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient, so that the zones of garnet-II, biotite (with an upper subzone with chloritoid) and staurolite developed from bottom to top of the formation. The estimated P-T path for the Santiago Schists suggests that the Basal Unit, probably a fragment of the Gondwana continental margin, was uplifted immediately after its subduction at the beginning of the Hercynian Orogeny. It also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension, probably related to the continued subduction of the Gondwana continental margin and the contemporaneous development of compensatory extension above it. The inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin, when it came into contact with the upper parts of the schists along a detachment, probably of extensional character. The general metamorphic evolution of the Santiago Schists, with the development of high-P assemblages with garnet prior to decompressive and prograde parageneses with biotite, is unusual in the context of the European Hercynian Belt, and shows a close similarity to the tectonothermal evolution of several high-P, low- to intermediate-T circum-Pacific belts.