MS Defense Seminar of Damian Donoso-Tapia

Exhumed serpentinites and their tectonic significance in non-collisional orogens

Exhumed serpentinites represent fragments of ancient ocean floor or mantle wedge that record fluid-rock interactions and metasomatism by seawater- or sediment-derived fluids. While common in suture zones after closure of ocean basins, in non-collisional orogens their origin and tectonic significance are not fully understood. We study serpentinite boulders from five river basins in a segment of the non-collisional Andean orogen in Ecuador (central Cordillera Real). All samples are fully serpentinized, with spinel being the only relic phase. Antigorite is the dominant serpentine polysome, indicative of high-temperature serpentinization. Spinel chemistry and rare earth element (REE) data point to a common supra-subduction zone origin for the protoliths. Whereas bulk-rock and mineral trace element data, and in-situ B isotope on antigorite (11B = -15.9‰ to +5.7‰) and chlorite (11B = -6.3‰ to -3.6‰) favor serpentinization by sediment-derived metamorphic fluids in a mantle wedge setting with negligible influence of seawater-derived fluids. Our data suggest four different mantle wedge-derived serpentinite slivers draining into the Cordillera Real stream network, associated with Triassic, Jurassic-Early Cretaceous, and potentially Late Cretaceous-Paleocene HPLT metamorphic sequences. We propose that exhumation of serpentinites in non-collisional orogens is controlled by extensional-compressional cycles. Regional phases of slab rollback have been reported throughout the orogen since the mid-Paleozoic to Late Cretaceous, favoring exhumation of underplated subduction interface slivers into the forearc and opening of back-arc basins. Subsequent compressional phases may trigger short-lived basin inversions that culminate with ophiolite obduction and associated blueschist exhumation.