Origin of chromitites and dunites of the Dazhuqu ophiolite in the Yarlung-Zangbo ophiolite zone, Tibet

Abstract

The origin of chromitites in ophiolites remains a topic of debate. The Dazhuqu ophiolitic massif located in the central segment of the Yarlung-Zangbo ophiolite zone contains podiform chromitites. The massive chromitites exhibit significantly low Cr# (38–47) in chromite. In contrast, the disseminated chromitites have relatively wide range of Cr# (16–64) in chromite. The dunites can be classified into two group: low-Cr# chromite-bearing dunites (Cr# = 19–51) and high-Cr# chromite-bearing dunites (Cr# = 74–83). The chromite of harzburgites show similar Cr# range (27–55) to that of the disseminated chromitites and low-Cr# chromite-bearing dunites. The negative correlation between Ti and Cr# along with the positive correlation between Ni and Mg# in chromite suggests that the massive chromitites formed through the physical sorting and accumulation of chromite grains in the melt conduits. The compositional patterns of chromite in the disseminated chromitites and low-Cr# chromite-bearing dunites resemble those of the harzburgites, implying that their chromite may have inherited compositional signatures from the surrounding harzburgites. In contrast, the chromite in the high-Cr# chromite-bearing dunites exhibits compositional patterns associated with boninitic melts. The coexistence of the chromitites and dunites with lower Cr# of chromite and high-Cr# chromite-bearing dunites indicate that the Dazhuqu massif was once modified by both MORB-like and boninitic melts, causing highly variable Cr# of chromite. A model of subduction initiation accounts for the magmatic evolutionary history of the chromitites and mantle sequence of the Dazhuqu massif.