Origin of podiform chromitites during subduction initiation

Abstract

The Luobusa ophiolite in Southern Tibet and Kop ophiolite in NE Turkey are two fragments of the Neo-Tethyan forearc lithospheres. Both of them contain a dunitic paleo-Moho transition zone (MTZ) and an upper harzburgitic mantle sequence. Peridotites and high-Cr chromite deposits in them records multiple events of melt/fluid activities and post-magmatism deformation during subduction initiation. Olivines from the Luobusa mantle sequence display large variations of primary δ7Li values from ~ 4 to 13‰ in harzburgites and 1 to 10‰ in chromitites. Clinopyroxenes in harzburgites have 2-8 ppm Li and light δ7Li values from -15 to -10‰, beyond those of clinopyroxene in equilibrated mantle peridotites (< 1 ppm and 4± 2‰, respectively). These data suggest that the mantle sequence was firstly penetrated by shallow slab-derived fluids, and later refertilized by exotic melts which generated the light-δ7Li clinopyroxenes. The parental magmas of high-Cr chromitites originated from regions straddling the hydrated-normal mantle boundaries. Clinopyroxenes in cpx-bearing harzburgites and harzburgite-cutting pyroxenite veins of both ophiolites are compositionally close to pure diopside, but their REE patterns are similar to those of clinopyroxene in MORB mantle undergone 5-10% fractional melting. Modeling show that the parental melts of these clinopyroxenes have similar to the Izu-Bonin-Mariana transitional lavas. The HREE concentrations of chromitites and their associated dunites are 1-2 orders of magnitude lower than chondrite. Their parental melts were generated by ~ 15-30% degrees of partial melting in the mantle. Given the olivine, chromite and clinopyroxene associations in them, their parental magmas possibly range from low-Ca boninitic to arc picritic melts. Large Fe isotopic variations were observed for chromite (-0.14 to 0.06‰) and olivine (-0.07 to 0.14‰) from chromitites and dunites in the Kop MTZ. Both chromite and olivine display increasing δ56Fe trends across chromitite-dunite profiles with gradual lithological transitions. These data can be explained with continuous fractional crystallization, which resulted in heavier Fe isotopes concentrated in the evolved magmas. In each cumulate cycle, dunite should be formed from evolved melts after massive precipitation of chromite. Mixing of relatively evolved and replenished more primitive melts in magma chambers was a potential mechanism for causing the oversaturation of chromite. Luobusa nodular chromitites are composed of well-oriented chromite nodules in dunite matrices. Olivines in dunite matrices of the underwent prevalent dislocation and pressure-solution creeps, and have abnormal high Fo and δ56Fe values, obviously beyond the range of olivine in normal ophiolitic dunites. Well-developed Fe-rich stripes and fluid inclusion zones are found parallel to the kink bands of these olivines. These features suggest that plastic deformation had promoted the Mg-Fe exchange between dunite matrices and chromite nodules, and deformation is a requirement for the formation of nodular textures of podiform chromitites. Formation of high-Cr chromitites during subduction initiaiton are achieved in the context of asthenospheric upwelling rather than directly triggered by addition of slab-derived fluids into refractory peridotites, but combined effects of these processes facilitate the generation of Mg-rich parental magmas of high-Cr chromitites. Effective mantle flow plays an important role in modifying the compositions and textures of podiform chromitites.