Geochronological and geochemical study of the Muhatayi ophiolitic mélange in West Junggar, Xinjiang, and its tectonic implication

Abstract: Located in the northeast Darbute ophiolitic mélange, West Junggar, Xinjiang, the Muhatayi ophiolitic mélange consists mainly of harzburgite with serpentinization and silica-carbonate alteration, basalt and chert, together with some intrusive gabbro veins. In this study, petrographic and geochemical analysis, U-Pb dating and Hf isotope study were conducted for ophiolitic mélange. Both of basalt and gabbro in ophiolitic mélange belong to low-potassium tholeiitic series, display typical characteristics of E-MORB and N-MORB, and are enriched in LILE (Ba, U, K, La, Ce) and depleted in HFSE（Nb, Ta,Th）. In combination with the discrimination diagrams of immobile trace elements and oxides, the authors hold that the Muhatayi ophiolitic mélange was produced in an arc-basin setting. The LA-ICP-MS U-Pb isotope dating of zircons from basalt yielded an age of (392.5 ± 2.9) Ma（n=26，MSWD=1.3）, which represents the age of ophiolitic mélange. εHf (t) values of zircons range from +12.2 to +15.1 and t(DM)（407-543 Ma）values are close to the ages of zircons, which indicates that the basaltic magmas were derived from depleted mantle without crustal contamination. The chemical composition of Cr-spinel of harzburgite and the Dy/Yb versus La/Yb of basalt reveal that the harzburgite was the residual after the 25%-30% partial melting of depleted spinel-lherzolite, the mafic melt was the parental magmas of basalt and gabbro. It is thus considered that the west Junggar Basin represented by Darbute ophiolitic mélange was developed in an arc-basin setting resulting from the northwestward subduction of Junggar Ocean，the west Junggar Ocean was spread before Middle Silurian and remained existent in early Carboniferous, and then the oceanic basin experienced the stage of subdution and closure.