Petrology and geochemistry of ultramafic and mafic rocks in the late Silurian-early Devonian Darbut ophiolitic mélange of west Junggar (NORTHWESTERN CHINA): implications for petrogenesis and tectonic evolution

ABSTRACT

New petrological and geochemical data for lherzolite, harzburgite, and gabbros in the Darbut ophiolitic mélange of west Junggar are combined to constrain the geological evolution of the Darbut ophiolite. Lherzolite, consisting of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with low Cr# values (34–39), is analogous to fertile abyssal peridotite. Harzburgite, composed of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with relatively high Cr# values (48–55), is similar to the supra–subduction zone (SSZ) peridotite. Isotropic gabbro, characterized by a flat rare earth element (REE) pattern as well as low Nb/Yb and high Ti/V ratios, is comparable to mid–ocean ridge basalt (MORB). Hornblende gabbro, displaying relative enrichments of fluid-soluble elements and elevated Th/Yb ratios, is similar to that of fore–arc basalt. Geochemical modelling of partial melting suggests that lherzolite samples are compatible with their formation after relatively low-degree (11–16%), anhydrous dynamic melting of the primitive mantle, while harzburgite samples have undergone 5–10% secondary-stage partial melting based on the already 16% depleted primitive mantle. These data suggest that the Darbut ophiolite was generated in a forearc setting. The upwelling asthenosphere triggered by the subduction initiation of the Junggar oceanic lithosphere led to low-degree, anhydrous decompression melting, producing lherzolite as well as the MORB–like melts at Late Silurian period. Increasing slab–derived fluids influx, accompanied by the progressively sinking slab, largely enhanced the partial melting degrees of the depleted mantle, and formed refractory harzburgite.

Graphical Abstract

KEYWORDS:

• Lherzolite
• harzburgite
• gabbro
• partial melting
• Darbut ophiolitic mélange

Acknowledgments

This study is financially supported by the National Natural Science Foundation of China (Grant No. 42072077, 41802113). Dr. Wanyi Feng and Huichao Zhang are thanked for the comments and suggestions that helped improve the paper. We also particularly appreciate Dr. Robert J. Stern and three anonymous reviewers for providing suggestions and critical comments, which greatly improved this paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

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Highlights

1. Isotropic and hornblende gabbros represent the extracted magmas during subduction initiation.

2. Lherzolite represents residue after low-degree (11–16%), anhydrous melting of upwelling asthenosphere.

3. Harzburgite was produced as a result of increasing infiltration of slab–derived fluids with subduction proceeded.

The Darbut ophiolitic mélange records the incipient to mature stages of subduction.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [42072077; 41802113].