The Dongbo ultramafic massif: A mantle peridotite in the western part of the Yarlung Zangbo suture zone, Tibet, with excellent prospects for a major chromite deposit.
东波超镁铁岩体:西藏雅鲁藏布江缝合带西段一个甚具铬铁矿前景的地幔橄榄岩体

The Dongbo ultramafic massif is located in the western part of the Yarlung Zangbo suture zone, where it is in fault contact with the surrounding Cretaceous sedimentary and volcanic rocks. The massif has a surface exposure of about 400km2, and extends to a certain depth referred by air magnetic pole. It consists of depleted mantle peridotites, mainly high-Mg harzburgite and dunite with minor lherzolite. Both the olivine (Ol) and orthopyroxene (Opx) in the peridotites are highly magnesian (Fo=89.5~91.5) and (Mg#=90~91.5), respectively. The orthopyroxenes in the lherzolite have somewhat higher Al2O3 and CaO than those in the harzburgites. Clinopyroxenes (Cpx) in the harzburgite have Mg#s of 92~95, whereas those in the lherzolite range from 92~93. The Cpx in the lherzolites also has higher Al2O3 and CaO contents than that in the harzburgite. These are typical features of Alpine mantle peridotites. Olivine in dunite has Fo ranging from 92 to 93.2, whereas Opx and Cpx both have Mg#s of ~93 but lower Al2O3 and CaO contents in harzburgite than in lherzolite. The regular variations among the three rock types suggest formation by different degrees of partial melting in the mantle. Chromium spinels in the lherzolite have Cr#s of 20~30, showing an affinity with abyssal peridotites and indicating a MOR setting. In contrast, chrome spinels in the harzburgite have Cr#s ranging from 20 to 75, indicating both MOR and SSZ settings. Primitive mantle-normalized REE patterns and trace element spider diagrams support these inferences. The petrological features of the Dongbo massif are similar those of the Luobusa peridotite massif in the eastern part of the Yarlung Zangbo suture zone, which contains numerous podiform chromitites. These similarities, along with the high content of Cr2O3 (56%~59%) in the massive chromitites of the Dongbo massif, suggest that it may also contain a significant large chromite deposit.