Serpentinites and garnet peridotites from an oceanic subduction complex in northern Dominican Republic

Abstract

Subduction related rocks in northern Dominican Republic, Hispaniola, formed during the convergence of Proto-Caribbean oceanic lithosphere (part of North American plate) under the Caribbean plate between the Cretaceous and the mid-Eocene. Subduction was terminated by the oblique collision of the Bahamas Platform, which produced strike-slip faults, such as the Septentrional and Camu Faults. Serpentinites in two large inliers, the Puerto Plata Basement Complex (PPBC) and the Rio San Juan Complex (RSJC), were examined. Serpentinites are divided in three types based on their spatial distribution and their bulk rock and mineral chemistry. Serpentinites from the PPBC and the northern RSJC are slightly foliated and are composed of pseudomorphic lizardite and dusty magnetite. They have bulk rock compositions and Cr-spinel compositions similar to abyssal peridotites, and are interpreted to be hydrated abyssal peridotites. Serpentinites in central RSJC, which form melanges containing blueschist and eclogite blocks, are composed of blady antigorite and chlorite that overprint pseudomorphic lizardite. Bulk rock compositions of these serpentinites suggest that they are abyssal peridotites hydrated on the sea floor. They were subducted and exhumed along the subduction plane together with blocks of blueschist and eclogite in a serpentinite-rich subduction channel. Finally, serpentinites near the major strike-slip fault zones are composed of pseudomorphic lizardite and magnetite, and have low Al and Ti compared to abyssal peridotite. They contain Cr-spinels with relatively high Cr# (Cr/[Cr+Al] atomic ratio) between 0.48--0.70. The data suggest that they are forearc mantle peridotites hydrated by slab-derived fluids at the base of the mantle wedge. Their presence along late strike-slip fault zones suggests that the fault zones directly tap the mantle wedge and that buoyant serpentinites protruded along the fault zones in response to transpressive collision. Furthermore, the presence of high pressure and ultra-high pressure rocks near the fault zones suggests that serpentinites may have played a role in the late stages of exhumation of these rocks, either by incorporating them during the protrusion or by lubricating the fault zone. In addition, the serpentinite along the fault zones could have facilitated aseismic creep. Cr-spinels in serpentinites near the Septentrional Fault Zone are different from northern localities since they show gradational compositionally zoning; rims contain moderately high Fe2O3, FeO and Cr 2O3, and slightly low MgO and Al2O3 compared to cores. Furthermore, chlorite containing symplectic intergrowth of magnetite commonly surrounds chromite, forming partial to complete coronas. The ferritchromit and chlorite are interpreted as reaction products of spinel, magnetite and serpentine. Ferritchromit has been reported in rocks metamorphosed to amphibolite facies conditions, but ferritchromit from Dominican Republic contains less Fe and greater concentrations of Mg and Al than ferritchromit reported elsewhere. In addition, samples lack obvious evidence of thermal metamorphism: tremolite and cummingtonite are locally present in some samples, which suggests local heating, however ail samples are dominated by lizardite, which is metastable at high temperatures. Therefore, poorly developed ferritchromit in Dominican Republic likely formed at low temperatures. Alternatively, ferritchromit formed during a short lived heating event during the intrusion of the Rio Boba Gabbroic Suite north of the Septentrional Fault Zone or during the ascent of the serpentinites through the hot interior of the mantle wedge. Septentrional Fault Zone serpentinites are associated with garnet- and corundum-bearing wehrlites in the southern portions of the RSJC. Low concentrations in Ir-type platinum group elements and low Mg (Fo75--82) in olivine confirm that these rocks are cumulates. Clinopyroxene compositions are similar to those from other arc rocks, suggesting that clinopyroxene formed in a supra-subduction zone environment. Preliminary bulk rock data showing high concentrations of alkali and alkali-earth elements also suggest such an environment. Corundum occurs as inclusions in garnet, suggesting that it crystallized prior or during garnet formation during prograde metamorphism. Furthermore, textural evidence suggests that garnet, corundum, and clinopyroxene formed spinel and a liquid under ultrahigh pressure conditions. Preliminary results suggest that the garnet peridotites were recrystallized during prograde metamorphism.