Ti-poor high-Al chromitites of the Moa-Baracoa ophiolitic massif (eastern Cuba) formed in a nascent forearc mantle
Hui-Chao Rui, Jingsui Yang, Angélica I. Llanes Castro, Jianping Zheng, Dongyang Lian, Weiwei Wu, Yurisley Valdes Mariño
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.