Origin and significance of diamonds and other exotic minerals in the Dingqing ophiolite peridotites, eastern Bangong-Nujiang suture zone, Tibet

Abstract

We have recovered unusual mineral assemblages of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite from the peridotites of the Late Triassic Dingqing ophiolite along the Bangong-Nujiang suture zone in south-central Tibet. Diamond grains are 100-300 μm, pale yellow and reddish-orange to colorless, commonly anhedral in habit, and range in morphology from elongated to octahedral and subhedral crystals. They display a characteristic shift in the Raman spectra between 1325 cm-1 and 1333 cm-1, mostly at 1331.51 cm-1 or 1326.96 cm-1. Chromian spinels in the Dingqing harzburgites and dunites are high-Cr varieties with Cr# = 40.1-49.1 and Cr# = 65.3-69.6, respectively. Recovered diamonds, ultrahigh-pressure, and highly reduced minerals appear to have crystallized and then been encapsulated in chromian spinel grains over a wide range of depth in the diamond stability field near the mantle transition zone. These chromian spinels and their host peridotites were then brought up to shallow mantle levels by the convecting asthenosphere, and were subsequently trapped in a mantle wedge of an intraoceanic subduction zone. The peridotites underwent further partial melting processes and reactions with island arc tholeiitic and boninitic melts, significantly modifying their whole-rock and mineral compositions. The preexisting chromian spinel grains with the inclusions of diamonds and other exotic minerals coalesced to form podiform chromitites during this stage in a suprasubduction zone setting.