CHARACTERIZING SUBDUCTION OF ACTIVE SPREADING RIDGES IN THE ANCIENT GEOLOGIC RECORD. A CASE STUDY FROM THE VARISCAN OROGENY

The kynematics involved in plate tectonics imply that oceanic spreading ridges and subduction zones must necessarily interact during their evolution and, accordingly, several active or recent examples of this phenomenon are found in the most important present-day subduction setting, the circum-Pacific area. In contrast, few ridge subduction events are well-documented in the ancient geologic record, suggesting that this process has been underestimated in the studies dealing with the evolution of orogens. Yet, analyses of fossil examples of ridge-trench interactions would provide important constraints on the long-term and deep-crustal effects of this tectonic process. The only case of ridge-trench interaction of Paleozoic age proposed up to now is the Aracena metamorphic belt (AMB), in SW Iberian Peninsula, which marks the suture between Armorica and Avalonia in the Iberian Massif (European Variscan Belt).

The AMB is characterized by (1) the presence of MORB-derived metabasites (a remnant of the former Rheic ocean) that were affected by an inverted HT/LP metamorphism, related to SW-verging thrusting (in the present geographic coordinates) whose thermal peak (ca. 800 ºC at pressures below 4 kbar) diachronously migrated eastwards along the belt; (2) a complex imbricate or fan-shaped unit interpreted as an accretionary prism that was overthrust by the MORB-derived metabasites through a transpressional shear zone; (3) the occurrence of a HT/LP metamorphism, related to an extensional collapse occurred between two contractional deformation phases, affecting continental rocks belonging to the forearc of the Armorican plate; this metamorphism followed a nearly isobaric heating path, reached maximum temperatures of ca. 975 ºC (i.e., 150 ºC than in the oceanic metabasites) at less than 6 kbar at the contact with the oceanic metabasites, and shows an abrupt temperature descent away from it; and (4) the presence, in the forearc of the Armorican plate, of near-trench syn-tectonic noritic intrusives that have a high-Mg andesite composition. These features suggest that the AMB formed as a consequence of the migration of a triple-junction resulted from the subduction of an active oceanic ridge (the responsible for the opening of the Rheic Ocean) beneath the Armorican microplate.