The Taitao Ophiolite: Snapshot of Nascent Oceanic Mantle Emplaced Via Ridge-Trench Collision

Abstract

The Taitao ophiolite is a sliver of oceanic lithosphere that formed at the Chile ridge at ~6 Ma and was emplaced shortly afterwards onto the Chilean continental margin during a ridge-trench collision. Located southeast of the modern-day Chile Triple Junction, the ophiolite provides a unique window into the mantle beneath an intermediate-spreading ridge axis, from which few peridotites have been recovered globally. We analyzed the compositions of harzburgites, plagioclase harzburgites, and clinopyroxenites from the mantle section of the Taitao ophiolite to investigate the history of partial melting, melt addition, and thermal evolution. Most Taitao harzburgites have spinel major element and clinopyroxene rare earth element contents that are consistent with residues after 12–15% melting of a depleted MORB mantle source. The compositional modifications due to melt addition are generally limited and spatially confined to plagioclase harzburgites near gabbroic intrusions and clinopyroxenites associated with dunites.

To constrain the thermal history of the Taitao ophiolite from formation at the Chile ridge to emplacement at the Chilean continental margin, we calculated closure temperatures by applying pyroxene- and olivine-based geothermometry to the Taitao peridotites. We compare these results with models for closure temperature as a function of initial temperature and cooling rate. Our results suggest that the Taitao peridotites underwent faster cooling (1–10°C/yr) than ophiolites globally, most of which cooled with rates <1°C/yr. This agrees with our calculation that the peridotites remained at high temperature for a relatively short time (<0.5 My) based on timescale estimates of emplacement using a ridge thermal model and mineral diffusive re-equilibration. Our rapid cooling rates for the Taitao peridotites exceed predictions from conductive cooling models, even those that account for hydrothermal effects, indicating the involvement of additional mechanisms. We interpret the fast cooling to result from ridge-trench collision and rapid obduction. This tectonic configuration juxtaposed hot sub-ridge mantle against a cold continental margin, leading to a significant temperature contrast and, therefore, faster average cooling rates during ophiolite emplacement. Other ophiolitic peridotites, in contrast, typically remain part of the oceanic lithosphere for longer and experience prolonged conductive cooling prior to obduction, resulting in slower cooling rates. Altogether, our results demonstrate that the Taitao ophiolite is an ideal archive for studying tectonic processes at intermediate-spreading ridges and during ridge-trench collision.