Ti-poor high-Al chromitites of the Moa-Baracoa ophiolitic massif (eastern Cuba) formed in a nascent forearc mantle

Abstract

Podiform chromitite is an important archive recording detailed information on mantle magmatism, heterogeneity and dynamics. High-Al chromitites are believed to have formed in mid-ocean ridge, back-arc, and forearc basin environments. Correlation between the geochemical composition of high-Al chromitites and their geodynamic settings is not well explored. Here, we present the petrographic, microstructural, mineral chemical, and whole-rock platinum-group element (PGE) data of the high-Al podiform chromitites from the Moa-Baracoa ophiolitic massif (MBOM) (eastern Cuba), to investigate their petrogenesis and constrain their geodynamic settings. The chromitites occur in massive and semi-massive forms enveloped in dunite within the host harzburgite. The Cr-spinels in these chromitites contain inclusions of silicates (e.g., clinopyroxene, amphibole, and phlogopite), platinum-group minerals, and base-metal sulfides. These chromitites (Cr# = 42.5–54.7, Mg# = 64.1–75.5) are poor in TiO2 (0.02–0.15 wt%), and display an obvious enrichment in IPGE (Os, Ir, and Ru) over PPGE (Rh, Pt, and Pd). Cr-spinel grains usually show low-pressure characteristics illustrated by inclusions of amphibole and phlogopite. Additionally, high-pressure/high-temperature microstructures, such as densely exsolved needles of clinopyroxene, strong crystal-plastic deformation, and overgrowth, can be found in some Cr-spinels. The coexistence of high- and low-pressure characteristics implies that some Cr-spinel grains crystallized in a relatively deep mantle while others crystallized at a shallow depth. Mineralogical and chemical features indicate that the parental magma of the chromitites was hydrous and TiO2-poor, and had high oxygen fugacity. We propose that the Ti-poor high-Al chromitite of the MBOM originated from a nascent forearc mantle during subduction initiation, in which deep-crystallized Cr-spinel were carried upward by the parental melt and deposited together with the shallow-crystallized crystals.