Sluggish Slab Rollback at the Early Stage of Flux Melting During Subduction Initiation: Li Isotopic Evidence From the Coto High‐Al Chromite Deposit, Zambales Ophiolite, Philippines

Abstract

AbstractThe compositions of chromitites and dunites from Moho transition zone (MTZ) of the Coto block of the Zambales ophiolite, Philippines, are used to investigate the geodynamic transition from anhydrous to hydrous magmatism during subduction initiation (SI). Chromite grains in the chromitites have Cr# values [100 × Cr/(Cr + Al)] and TiO2 contents ∼35–50 and 0.05–0.30 wt.%, respectively, intermediate between those of chromite in typical MORB‐like lavas (Cr#, ∼20–60; TiO2, ∼0.6–1.7 wt.%) and boninites (Cr#, ∼70–85; TiO2, <0.4 wt.%). Olivine grains in the dunites have δ7Li values varying from ∼−2‰ to +21‰ with most between +10‰ and +15‰, beyond that of normal mantle (+4 ± 2‰) but comparable to those of some arc lavas (up to +12‰). The data set indicates that parental magmas of the high‐Al chromitites originated from hydrated harzburgitic mantle sources and formed temporally between MORB‐like and boninitic magmatism during SI, resulting from the early stage of flux melting in the Zambales proto‐forearc mantle. Modeling of Li diffusion reveals that the MTZ cooled down at a minimum rate of 0.1°C/yr in order to preserve the large δ7Li variation of olivine in the dunites, comparable to the thermal conditions below ultra‐slow to slow spreading ridges. Such a stage of transitional magmatism, although displaying notable slab contributions, took place at a sluggish period of slab rollback and asthenospheric upwelling, leading to a trough level of heat flow and magma production during the entire course of SI.