Pyroxenite–harzburgite sequences in the Dazhuqu ophiolite (Southern Tibet) formed through hydrous melt infiltration and melt–peridotite reaction

• Published in: Contributions to mineralogy and petrology Vol. 178; no. 12; p. 92
• Main Authors: Zhang, Zhen-Yu, Liu, Chuan-Zhou, Liang, Yan, Liu, Tong, Zhang, Chang, Liu, Bo-Da, Lin, Yin-Zheng, Zhang, Wei-Qi, Ji, Wen-Bin
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.12.2023; Springer Nature B.V
• Subjects: Aluminum oxide; Asthenosphere; Depletion; Dunite; Earth and Environmental Science; Earth Sciences; Fractures; Geology; Lithosphere; Melting; Mineral Resources; Mineralogy; Ophiolites; Original Paper; Peridotite; Petrology; Sequences; Trace elements; Veins (geology); Tibet; Melt–rock reaction; Pyroxenite; The Yarlung-Tsangbo ophiolites; Hydrous melt infiltration; Concentration gradient
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-023-02076-y

Pyroxenite veins are commonly observed in the mantle section of ophiolites, reflecting a variety of melts that percolate through the mantle, react, and finally crystallize in the lithosphere. To better understand the formation mechanism of the pyroxenite veins and associated peridotites, we conducted an integrated petrological and geochemical study of a suite of orthopyroxenite, websterite, and composite clinopyroxenite–orthopyroxenite veins in residual peridotites from the Dazhuqu ophiolite (Southern Tibet, China). Both orthopyroxene and clinopyroxene in pyroxenites are characterized by high Mg#, low Al 2 O 3 concentrations, and depleted patterns of incompatible trace elements. This suggests that parental melts of the pyroxenites could be formed by re-melting of a previously depleted mantle source. We observed systematic variations in the major and trace element compositions of clinopyroxene and orthopyroxene across the pyroxenite–harzburgite sequences. Through trace element modeling, we have established a model for the formation of pyroxenite veins and associated harzburgites. According to this model, hydrous and aggregated melts were expelled from the tip of the dunite channels and subsequently injected into the lithosphere via fractures. The infiltrating melts then underwent diffusional loss of water into the residual harzburgites within the asthenosphere, which in turn promoting partial melting of the harzburgite. The pyroxenite veins were formed by mixing the infiltrating channel melt and matrix melt derived from partially molten harzburgite. This formation model can be applied to explain the pyroxenite–harzburgite and dunite–pyroxenite–harzburgite sequences in the Yarlung-Tsangbo ophiolite and other ophiolites around the world.