ORIGIN AND TECTONIC SIGNIFICANCE OF EXHUMED SERPENTINITES IN THE ANDES

Serpentinites exposed in modern continental realms represent fragments of ancient ocean floor or mantle wedge exhumed through different processes. While common in suture zones following continental collisions, their origin and tectonic significance in non-collisional (i.e., accretionary) orogens are not fully understood.

The Andes are widely considered the best example of a modern non-collisional orogen. However, serpentinite- and ultramafic-bearing HP-LT complexes along the mountain belt are common. Exhumation of these rocks is often related to accretionary processes in the forearc, or to collisional events that have left behind discrete suture zones in the core of the Andes.

Thus, to assess the origin and tectonic significance of Andean serpentinites, we studied serpentinite boulders from five river basins in a segment of the Ecuadorian Andes (northern Cordillera Real). All samples are fully serpentinized with antigorite as the main polymorph, indicative of high temperature serpentinization, while spinel-group minerals are the only relic phase. Whole-rock rare earth element data suggest an abyssal setting for the protoliths. Fluid-mobile elements (WR), and in-situ B isotope data on antigorite and chlorite favor serpentinization by crust-derived metamorphic fluids in a mantle wedge setting. Our results suggest four different mantle wedge-derived serpentinite slivers draining into the northern Cordillera Real stream network. These are likely associated with Triassic, Jurassic-Early Cretaceous, and potentially Late Cretaceous-Paleocene high pressure-low temperature metamorphic sequences in the study area.

Throughout the orogen, regional phases of slab rollback have been reported since the mid-Paleozoic to the Late Cretaceous, favoring exhumation of underplated subduction interface slivers into the forearc and opening of back-arc basins. Subsequent compressional phases trigger short-lived basin inversion that culminates with ophiolite obduction and associated blueschist exhumation. We propose that serpentinites in the Andes are sourced from underplated slab slivers that undergo intense serpentinization by crust-derived metamorphic fluids in the mantle wedge prior to their exhumation during extensional phases.