The 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 x Cr/(Cr + Al)] and TiO2 contents similar to 35-50 and 0.05-0.30 wt.%, respectively, intermediate between those of chromite in typical MORB-like lavas (Cr#, similar to 20-60; TiO2, similar to 0.6-1.7 wt.%) and boninites (Cr#, similar to 70-85; TiO2, <0.4 wt.%). Olivine grains in the dunites have delta Li-7 values varying from similar to-2 parts per thousand to +21 parts per thousand with most between +10 parts per thousand and +15 parts per thousand, beyond that of normal mantle (+4 +/- 2 parts per thousand) but comparable to those of some arc lavas (up to +12 parts per thousand). 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 degrees C/yr in order to preserve the large delta Li-7 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.

Plain Language Summary Subduction initiation (SI) is a prerequisite for starting plate movement. It is featured by rollback of subducted oceanic slabs in the mantle and upward flow of deep mantle materials into the shallow mantle wedges, resulting in mid-ocean ridge-like spreading settings above the retreating slabs and mid-ocean ridge basalt-like (MORB-like) lavas. Heated by surrounding hot mantle, slabs gradually release fluids into the overlying mantle, inducing H2O-rich magmatism such as boninitic ones. Although MORB-like and boninitic lavas are accepted to originate from H2O-poor and H2O-rich mantle sources, respectively, few details were revealed on how transition between the two contrasting types of magmatism is achieved. This work did chemical analyses and modeling on some SI-related rocks that show affinity to transitional MORB-boninitic magmatism. Our results show the transitional magmas originated from mantle sources chemically similar to those of boninites, but formed under cooler conditions than both MORB-like and boninitic lavas. Development of such transitional magmatism reveals that the sources of SI-related magmas became increasingly rich in fluids, and there was a cooling period between the MORB and boninitic magmatism, possibly due to slow slab rollback and sluggish upwelling of deep hot mantle below the SI-induced spreading centers during the transitional period.

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