Chromitite genesis based on chrome-spinels and their inclusions in the Sartohay podiform chromitites in west Junggar of northwest China

Abstract

Primary high-Al [Cr# = 100 × Cr/(Cr + Al) of 48.2–54.7] chromitites coupled with high-Cr (Cr# = 70.9–80.7) chromitite fragments are identified in the podiform chromitites hosted by the Sartohay ophiolitic mélange in west Junggar, northwest China. New petrographic and mineralogical data for chrome-spinels in both types of chromitites and their host harzburgites, as well as inclusions within chrome-spinels, are reported here to reveal the genesis of the Sartohay chromitites. Abundant primary monomineralic and polymineralic mineral inclusions including Na-phlogopite, enstatite, diopside, pargasite, olivine, rutile, ilmenite, sulphide, and occasionally zircon are found in the high-Al chromitites. Chemical compositions of these mineral inclusions indicate that the trapped melts are rich in H2O, Na2O, and TiO2. Temperatures and pressures estimated based on the equilibrium of mineral phases within inclusions are 812–992 ℃ and ~10.6 kbar, respectively. The calculated parental melt compositions (Al2O3 = 15.0–16.1 wt%; TiO2 = 0.30–0.96 wt%; Co = 37.0–48.2 ppm; Ga = 11.2–13.3 ppm) in equilibrium with high-Al chromitites suggest that these chromitites probably formed by the interaction between MORB-like melts and intermediate depleted harzburgites. In contrast, the high-Cr chromitite fragments with needle-shaped clinopyroxene lamellaes are entirely free of globular hydrous silicate inclusions. The parental melt compositions (Al2O3 = 10.3–12.2 wt%; TiO2 = 0.11–0.26 wt%; Co = 55.8–60.4 ppm; Ga = 13.4–17.2 ppm) in equilibrium with high-Cr chromitites show boninitic affinity. We propose that the high-Cr chromitites initially formed at an arc-related environment, then the deep recycling and uplift events occurred and resulted in high-Cr chromitites occurring as residual fragments in a back-arc basin spreading ridge. Subsequently, high-Al chromitites were formed and mechanically juxtaposed with the pre-existing high-Cr ones in the Sartohay ophiolitic mélange.