Timing and conditions of UHP metamorphic and metasomatic processes in the subducting Farallon plate revealed by eclogite and omphacitite xenoliths of the Navajo volcanic field (USA)

Abstract

Eclogite and omphacitite xenoliths of the Navajo Volcanic Field (NVF) provide a unique opportunity to study processes in an oceanic slab down to subarc depths. However, ambiguities remained about protolith origin, prograde metamorphic conditions, and metasomatic processes, which we address here with new geothermobarometric and in-situ U-Pb data. The earliest garnet (53.4 ± 4.8 Ma), omphacite, and phengite generations yielded conditions of 2.4–3.3 GPa, 400–540 °C. Zircon cores (87−37 Ma ²⁰⁶Pb/²³⁸U dates) yielded higher Th/U ratios (0.1–0.6) than zircon mantles (65−30 Ma and Th/U < 0.1). Rutile dates cluster at 32.3 ± 1.2 to 28.4 ± 1.9 Ma. The Th/U ratios suggest magmatic growth of the zircon cores, which we consider a strong argument that the NVF eclogites and omphacitites were at least partly derived from Cretaceous Farallon oceanic crust. Rare relic Proterozoic zircon can be explained by inheritance, or by derivation of some xenoliths from the Proterozoic North American lithosphere. Regardless of protolith origin, the rocks were brought to depth by the Farallon slab and resided at lawsonite eclogite facies conditions for a ca. 20–25 Myr interval bracketed by the garnet and rutile dates, during which they experienced two (likely ultra-high pressure) metasomatic events. Firstly, Na-Si-rich fluids, likely derived from metasedimentary rocks, caused growth of Na-rich omphacite. A second metasomatic episode through serpentinite-derived fluids happened just prior to ~ 30 Ma xenolith exhumation as part of the “Great Hydration Event” that affected the Colorado Plateau.