Serpentinites in a Tertiary subduction complex in the northern Dominican Republic contain low concentrations of incompatible elements in bulk-rock compositions and high Mg in relict silicate minerals. The forsterite component in olivine ranges from 89.0% to 90.8%, and the enstatite component in orthopyroxene ranges from 89.4% to 91.1%, suggesting that they are mantle peridotites. Two different protoliths are identified for the serpentinites based on the bulk-rock compositions and spinel chemistry: abyssal peridotites and forearc mantle peridotites. Hydrated abyssal peridotites are voluminous and occur in ophiolite complexes in the northern terranes (Puerto Plata Complex and the northern part of the Rio San Juan Complex) and in serpentinite mélanges in the central part of the Rio San Juan Complex. The serpentinite mélanges contain fragments of high-pressure–low-temperature rocks and are interpreted to be tectonic mélanges, representing part of a serpentinite subduction channel. The serpentinites show moderate Al/Si weight ratios (0.026–0.081) in bulk rocks and moderate Cr# (atomic ratio of Cr/[Cr + Al] = 0.20–0.55) in spinel.
Hydrated forearc mantle peridotites occur along major strike-slip faults: the Camú fault zone, and the Septentrional fault zone. They show low bulk-rock Al/Si weight ratios (up to 0.021), high concentration in Ir-group platinum group elements (13.1–24.6 ppb total), and high Cr# (0.48–0.67) in spinel. Raman spectroscopy and X-ray powder diffraction indicate that lizardite is the predominant serpentine species. The absence of antigorite suggests that these serpentinites were derived from a shallow depth (<<35 km) in the mantle wedge. Their occurrence along the major strike-slip fault zones suggests that the faults allowed these serpentinites to protrude from the forearc mantle wedge during oblique transpressive collision of the Caribbean plate with the Bahamas Platform.