Origin of the Zedang and Luobusa Ophiolites, Tibet

The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt, and they share similar geological tectonic setting and age. Thus, an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean. There is no complete ophiolite assemblage in the Zedang ophiolite. The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks, with some blocks of olivine-pyroxenite. The mantle peridotite mainly consists of Cpx-harzburgite, harzburgite, some lherzolite, and some dunite. A suite of volcanic rocks is mainly composed of calc-alkaline pyroclastic rocks and secondly of tholeiitic pillow lavas, basaltic andesites, and some boninitic rocks with a lower TiO2 content (TiO2 < 0.6%). The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched (compared to HFSE) spider diagram, demonstrating an island-arc origin. The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted (compared to HFSE) spider diagram, indicative of an origin from MORB. The boninitic rock was generated from fore-arc extension. The Luobusa ophiolite consists of mantle peridotite and mafic-ultramafic cumulate units, without dike swarms and volcanic rocks. The mantle peridotite mainly consists of dunite, harzburgite with low-Opx (Opx < 25%), and harzburgite (Opx > 25%), which can be divided into two facies belts. The upper is a dunite-harzburgite (Opx < 25%) belt, containing many dunite lenses and a large-scale chromite deposit with high Cr2O3; the lower is a harzburgite (Opx >25%) belt with small amounts of dunite and lherzolite. The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower. Many mantle peridotites are highly depleted, with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite. The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite, of the P-P-G ophiolite series. This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite, a high-Cr2O3 chromite deposit, and cumulates of the P-P-G ophiolite series. We conclude that the evolution of the Eastern Tethys Ocean involved three stages: the initial ocean stage (formation of MORB volcanic rock and dikes), the fore-arc extension stage (formation of high-Cr2O3 chromite deposits and P-P-G cumulates), and the island-arc stage (formation of calc-alkaline pyroclastic rocks).