Abstract
The origin of the assemblage of ultra-high pressure (UHP), super-reduced (SuR) and several crustally derived phases in ophiolitic chromitites is still hotly debated. In this paper, we report, for the first time, this assemblage of phases in ophiolitic chromitites of the Caribbean. We studied the Mercedita chromitite deposit in the eastern Cuban ophiolitic complexes. The mineral phases were characterized using microRaman spectroscopy, energy-dispersive spectroscopy with a scanning electron microscope (SEM-EDS), X-ray microdiffraction and electron microprobe analyses. Mineral concentrates were prepared using hydroseparation techniques. We have identified oriented clinopyroxene lamellae in chromite, oriented rutile lamellae in chromite, moissanite hosted in the altered matrix of the chromitite, graphite-like amorphous carbon, corundum and SiO2 hosted in healed fractures in chromite grains, and native Cu and Fe–Mn alloy recovered in heavy-mineral concentrates obtained by hydroseparation. This assemblage may correspond to UHP-SuR conditions, implying recycling of chromitite in the mantle or formation of the chromite grains at deep mantle depths, followed by emplacement at a shallow level in the mantle. However, the chromitite bodies contain gabbro sills oriented parallel to the elongation of the chromitite lenses, and these show no evidence of HP/UHP metamorphism. Therefore, the identified “exotic” phases may not be indicative of UHP. They formed independently as oriented clinopyroxene lamellae in chromite during cooling (clinopyroxene and rutile), in super-reduced microenvironments during the serpentinization processes, and by transference of subducted crustal material to the mantle wedge via cold plumes.
Highlights
IntroductionMinerals indicative of super-reducing (SuR) environments, e.g., native elements, alloys, carbides, nitrides and phosphides, and those formed at ultra-high pressure (UHP), e.g., diamond, TiO2 -II, pseudomorphs after coesite and stishovite, are increasingly being reported in peridotites and associated chromitites from ophiolite complexes, along with minerals that typically form in the continental crust, e.g., zircon, quartz, K-feldspar, almandine, andalusite, apatite and kyanite [1,2,3,4,5,6,7,8,9,10,11,12]
Crustal minerals are already present in the chromitite, which forms in a mantle wedge contaminated by subducted material with a crustal component after the ascent of cold plumes derived from the subducting slabs [12,16]
Olivine inclusions are more abundant near the contact with the dunite envelope (Figure 2d) and show the same optical orientation as olivine crystals in dunite, indicating dunite replacement by chromitite, previously documented by [44]
Summary
Minerals indicative of super-reducing (SuR) environments, e.g., native elements, alloys, carbides, nitrides and phosphides, and those formed at ultra-high pressure (UHP), e.g., diamond, TiO2 -II, pseudomorphs after coesite and stishovite, are increasingly being reported in peridotites and associated chromitites from ophiolite complexes, along with minerals that typically form in the continental crust, e.g., zircon, quartz, K-feldspar, almandine, andalusite, apatite and kyanite [1,2,3,4,5,6,7,8,9,10,11,12]. Rapid exhumation of the deep-seated chromitites and host peridotites carrying SuR, UHP and continental-crust-derived minerals is related to the generation of low-viscosity upwelling channels created during the opening of marginal basins in island arcs or continental arcs as a result of retraction of subducting slabs during the rollback process [14,15]. In this model, crustal minerals are already present in the chromitite, which forms in a mantle wedge contaminated by subducted material with a crustal component after the ascent of cold plumes derived from the subducting slabs [12,16]
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