Characteristics of trace elements of clinopyroxenes in podiform chromitites of the Yarlung−Zangbo ophiolite zone and its genetic significances

The tectonic setting of podiform chromitites remains controversial. This study systematically compares the geochemical characteristics of clinopyroxenes from the Luobusa, Zedang, and Dazhuqu intrusions in the Yarlung-Zangbo ophiolite zone. The results show that clinopyroxenes in the mantle peridotites are commonly influenced by the late-stage melt, whereas clinopyroxenes in the dunites and chromitites were formed through the magmatic crystallization differentiation. Based on the estimated results of trace elemental compositions of clinopyroxenes, the parental magma for the low-Cr# dunite is a kind of melt with highly depleted LREE, the parental magma for the intermediate-Cr# dunite is a transitional basaltic melt, while the parental magma for the high-Cr# dunite is a kind of melt similar to the boninitic andesitic melt. Continuous variations of Al2O3, TiO2, and Ga contents and V/Mn ratios of clinopyroxenes from low- to medium- to high-Cr# samples suggest that the mantle sources had undergone a transition from a reduced mantle with a low degree of partial melting to an oxidized mantle with a high degree of partial melting. Furthermore, the evolutionary characteristics of compositions of the clinopyroxenes in these chromitite bodies indicate that they were formed in different stages of the initial subduction process.