High-Al and high-Cr podiform chromitites from the western Yarlung-Zangbo suture zone, Tibet: Implications from mineralogy and geochemistry of chromian spinel, and platinum-group elements

Abstract

On the basis of their mineral chemistry, podiform chromitites are divided into high-Al (Cr#=20–60) (Cr#=100∗Cr/(Cr+Al)) and high-Cr (Cr#=60–80) varieties. Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs that can have different chromitite compositions. We report here the occurrence of both high-Cr and high-Al chromitite in a single massif in China, the Dongbo mafic-ultramafic body in the western Yarlung-Zangbo suture zone of Tibet. This massif consists mainly of mantle peridotites, with lesser pyroxenite and gabbro. The mantle peridotites are mainly composed of harzburgites and minor lherzolites; a few dike-like bodies of dunite are also present. Seven small, lenticular bodies of chromitite ores have been found in the harzburgites, with ore textures ranging from massive through disseminated to sparsely disseminated; no nodular ore has been observed. Individual chromitite pods are 1–3m long, 0.2–2m wide and strike NW, parallel to the main trend of the peridotites. Chromitite pods 3, 4, and 5 consist of high-Al chromitite (Cr#=12–47), whereas pods 1 and 2 are high-Cr varieties (Cr#=73 to 77). In addition to chromian spinel, all of the pods contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. The mineralogy and geochemistry of the Dongbo peridotites suggest that they formed originally at a mid-ocean ridge (MOR), and were later modified by suprasubduction zone (SSZ) melts/fluids. We interpret the high-Al chromitites as the products of early mid-ocean ridge basalt (MORB) or arc tholeiite magmas, whereas the high-Cr varieties are thought to have been generated by later SSZ melts.