Mineralogy and geochemistry of the peridotites and high-Cr podiform chromitites from the Tangbale Ophiolite Complex, West Junggar (NW China): Implications for the origin and tectonic environment of formation

The Tangbale Ophiolite Complex is located in southwestern part of the West Junggar, NW China, which is a major component of the Central Asian Orogenic Belt (CAOB). The Tangbale Ophiolite Complex consists of voluminous dunites, harzburgites and high-Cr podiform chromitites. Harzburgite contains chromian spinel of low Cr# (100 x Cr/(Cr + Al) = 38-64) but high Mg# (100 x Mg/(Mg + Fe2+) = 55-61), whereas dunite contains spinel of higher Cr# (56-82) and lower Mg# (41-55). Numerical modeling of heavy rare earth element (HREE) contents in harzburgites are consistent with derivation from similar to 15% to 20% partial melting of a fertile mantle source, and dunites with low concentrations of HREE are suggested to be generated by higher degrees (similar to 20%-25%) melting of the same source. The peridotites have REE signatures with enrichments in LREE, indicating the involvement of metasomatism and interaction by subduction-related melts in their formation. The chromite in podiform chromitite displays the highest Cr# (80-84) and Mg# (65-76), as well as moderate TiO2 contents, suggesting crystallization from hydrous, high-Si and high-Mg melts in supra-subduction zones, which is consistent with the calculated compositions of parental melts for spinels in chromitites. The genesis of high-Cr chromitites is likely to be triggered by the metasomatism and interaction between high-Mg boninitic-like melts and mantle peridotites. The addition of SiO2 and chrome in boninitic-like melts via interaction with peridotites trigger high-Cr chromite to precipitate and separate from parental magma, and finally concentrate into chromitites with boninitic-like signature at SSZ zone. The calculated parental melts for spinels in peridotites show hybrid compositions between MORB (mid-ocean ridge basalt) and arc melts, suggesting a two-stage formation in an evolving from MOR (mid-ocean ridge) to SSZ (supra-subduction zone) setting. The peridotites in the Tangbale body were most likely formed at a MOR setting and trapped above an intra-oceanic subduction zone as part of the mantle wedge, where the upper peridotites were metasomatized and partially re-melted under fluid-saturated and high temperature conditions to generate boninitic melts. The interaction between upward-migrating boninitic melts and/or rising asthenospheric mantle-derived melts with the peridotites beneath the Moho triggered the generation of high-Cr podiform chromitites and associated dunite envelopes.