Records of fluid-rock interaction in an oceanic subduction complex

Abstract

The recycling of tectonic plates at subduction zones plays a vital role in maintaining Earth’s tectonic organization and magmatic systems over geologic time. The progressive release of aqueous fluids from subducting lithosphere is a significant Earth process: it provides a mechanism for mass transfer between recycled lithosphere and the overlying mantle, causes rheological changes in the residual plate, influences styles and locations of seismicity, and is responsible for the generation of arc volcanoes and the creation of continents, for instance. Globally, subduction complexes provide the primary material record for studying fluid-related processes at depth and represent critical grounding sites for informing numeric models and seismic imaging of active subduction zones. The island of New Caledonia exposes one such complex, and the locality is well-known for its exceptional preservation of lawsonite blueschist- through eclogite-facies rocks. It has been used as a case study locality to investigate mass transfer between the slab and mantle, and the production of high-pressure vein systems, which reveal insights into intra-slab plumbing systems and physical devolatilization processes. This dissertation contributes to knowledge of the fluid history of this complex, presenting directed studies of its most hydrous rocks: serpentinites (Ch. 2) and “hybrid” rocks (chlorite and talc schists, Ch. 3). We examine their composition, textures, and spatial distribution, and integrate our findings within a study that refines the tectonostratigraphic distribution of lithologic components in the complex (Ch. 4). Our work reveals the existence of hydrated mantle from the downgoing plate and overriding mantle, identifying New Caledonia as a rare locality that preserves evidence for exhumation of both types of mantle. Examination of Fe-Ti oxides and accessory apatite in hybrid rocks reveals at least two stages and conditions of growth for oxides, and low-temperature thermochronology of the same minerals places new temperature-time constraints on a previously unstudied portion of the complex’s exhumation history. Finally, results of integrated mapping underscore the existence of hydrated shear zones within the complex, proposing an intimate link between the location and type of serpentinites, the formation of hybrid rocks, the channelization of fluids, and the mechanisms that juxtaposed distinct tectonic units and facilitated exhumation of these deeply subducted rocks.