Corundum-bearing garnet peridotite from northern Dominican Republic: A metamorphic product of an arc cumulate in the Caribbean subduction zone

Abstract

Garnet peridotite in oceanic subduction complexes has been reported only in two locations in the world. One of these examples occurs in the late Cretaceous to early Tertiary subduction complex in northern Dominican Republic. The garnet peridotite (wehrlite and olivine-bearing clinopyroxenite) occurs as large (≤ 4 m) boulders together with boulders of eclogites and serpentinites along a narrow (< 10 m) stream of the Rio Cuavas in the southern part of the Rio San Juan Complex. The peridotite is composed of garnet, diopsidic clinopyroxene (partially altered to magnesiohornblende), olivine (extensively altered to serpentine), Al-spinel and minor corundum; the latter two are mostly enclosed in garnet. Coarse-grained garnet also encloses small grains of early-formed garnet that contain Ca- and Al-rich cores and Mg-rich rims. The garnet peridotite contains low Cr, Ni, and Ir-group platinum group elements in bulk rock compared to primitive mantle values, low Mg (Mg#; 0.74–0.83) and NiO (< 0.1 wt.%) in olivine, and elevated concentrations of fluid-mobile elements (Sr, Pb and U) in clinopyroxene and bulk rock. Combined with the rare earth element data of bulk rocks and clinopyroxene, these data suggest that the peridotite originally solidified as a plagioclase-bearing cumulate of an arc melt at a shallow depth, < 35 km, in the mantle wedge. The cumulate was later dragged by mantle flow from the subarc mantle towards the subduction plane. Subsequent downward movement along the subduction plane resulted in the crystallization of corundum and Ca- and Al-rich garnet at the expense of plagioclase, at a depth of ~ 50 km. The garnet peridotite continued to be subducted to a depth of ~ 120 km, causing a progressive increase in pressure and temperature that resulted in the crystallization of Mg-rich garnet. The garnet periodotite was then exhumed in the serpentinite subduction channel. These events likely took place during the very early stages of the subduction system, where a strong mantle corner flow was likely produced by poor lubrication along the interface between the subducting plate and the overlying Caribbean Plate.