Podiform chromitites enclosed in depleted harzburgites of the Luobusa massif (Tibet) contain diamonds and a highly reduced trace-mineral association that suggests that the chromitites formed at ultra-high pressure (UHP) corresponding to the Transition Zone (>400 km; >12.5 GPa). However, trace-element signatures of the chromites are indistinguishable from those of typical ophiolitic chromitites (e.g. Antalya Complex, Turkey), implying primary crystallization from typical arc-type melts at shallow depths. New data on geochronology and Fe oxidation state may explain this conundrum.
The authors propose that a lithospheric mantle slab containing the crystallized chromitite was subducted to the Transition Zone, where chromite inverted to a high-pressure polymorph and reacted locally with reducing fluids to form the highly reduced trace-mineral assemblage. Thermo-mechanical modeling suggests a rapid (≲10 m.y.) rise of the buoyant harzburgites from >400 km depth during the early Tertiary and/or Late Cretaceous rollback of the Indian slab.
This process may occur in other collision zones; mantle samples from the transition zone may be more widespread than currently recognized.
Citation: Nicole M. McGowan et al., Tibetan chromitites: Excavating the slab graveyard, Geology, http://dx.doi.org/10.1130/G36245.1.